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magnetic separation described

magnetic separation method

magnetic separation method

Magnetic separation is a process used to separate materials from those that are less or nonmagnetic. All materials have a response when placed in a magnetic field, although with most, the effect is too slight to be detected. The few materials that are strongly affected (magnetised) by magnetic fields are known as Ferromagnetics, those lesser (though noticeably) affected are known as Paramagnetics.

Ferromagnetics require relatively weak magnetic fields to be attracted and devices to separate these materials usually have magnets that are permanently magnetised (Permanent magnets do not require electricity to maintain their magnetic fields). Paramagnetics require stronger magnetic fields and these can only be achieved and maintained by electro magnets (large wire coils around an iron frame current is continuously passed through the coils creating the magnetic field within the iron. The field is concentrated across an air gap in the circuit).

Both ferromagnetic (low intensity) and paramagnetic (high intensity) separation devices (Laboratory Magnetic Separator) may be operated with dry solids or with solids in pulp form. (A complete classification of magnetic separating devices is given in Wills Mineral Processing Technology, pp. 338-356).

(*The units given are kilogauss (kG). These are the units most commonly used. The equivalent S.I. unit is the Tesla (T) * 1 Tesla = 10 kilogauss). The extremes of field strength used are based on experience from a magnetic separation testing laboratory over many years.

magnetic separation - an overview | sciencedirect topics

magnetic separation - an overview | sciencedirect topics

Magnetic separation takes advantage of the fact that magnetite is strongly magnetic (ferromagnetic), hematite is weakly magnetic (paramagnetic), and most gangue minerals are not magnetic (diamagnetic).

The current research and development initiatives and needs in magnetic separation, shown in Fig. 7, reveal several important trends. Magnetic separation techniques that have been, to a greater extent, conceived empirically and applied in practice, such as superconducting separation, small-particle eddy-current separation, and biomedical separation, are being studied from a more fundamental point of view and further progress can be expected in the near future.

In addition, methods such as OGMS, ferrohydrostatic separation, magnetic tagging, and magnetic flocculation of weakly magnetic materials, that have received a great deal of attention on academic level, are likely to enter the development and technology transfer stages.

The application of high-Tc superconductivity to magnetic separation, and novel magnetism-based techniques, are also being explored, either theoretically or empirically. It can be expected that these methods, such as magnetic flotation, magnetic gravity separation, magnetic comminution, and classification will take advantage of having a much wider control over these processes as a result of the presence of this additional external force.

Magnetic separation takes advantage of the fact that magnetite is strongly magnetic (ferromagnetic), hematite is weakly magnetic (paramagnetic), and most gangue minerals are not magnetic (diamagnetic). A simple magnetic separation circuit can be seen in Figure 1.2.5 [9]. A slurry passes by a magnetized drum; the magnetic material sticks to the drum, while the nonmagnetic slurry keeps flowing. A second pass by a more strongly magnetized drum could be used to separate the paramagnetic particles from the gangue.

Magnetic separation can significantly shorten the purification process by quick retrieval of affinity beads at each step (e.g., binding, wash, and elution), and reduce sample dilution usually associated with traditional column-based elution. The method can be used on viscous materials that will otherwise clog traditional columns and can therefore simplify the purification process by eliminating sample pretreatment, such as centrifugation or filtration to remove insoluble materials and particulates. The capability of miniaturization and parallel screening of multiple conditions, such as growth conditions for optimal protein expression and buffer conditions for purification, makes magnetic separation amenable to high-throughput analysis which can significantly shorten the purification process (Saiyed et al., 2003).

Paramagnetic particles are available as unmodified, modified with common affinity ligands (e.g., streptavidin, GSH, Protein A, etc.), and conjugated particles with specific recognition groups such as monoclonal and polyclonal antibodies (Koneracka et al., 2006). In addition to target protein purification, they can also be used to immobilize a target protein which then acts as a bait to pull down its interaction partner(s) from a complex biological mixture. See Chapter 16.

Magnetic separation of cells is a simple, rapid, specific and relatively inexpensive procedure, which enables the target cells to be isolated directly from crude samples containing a large amount of nontarget cells or cell fragments. Many ready-to-use products are available and the basic equipment for standard work is relatively inexpensive. The separation process can be relatively easily scaled up and thus large amount of cells can be isolated. New processes for detachment of larger magnetic particles from isolated cells enable use of free cells for in vivo applications. Modern instrumentation is available on the market, enabling all the process to run automatically. Such devices represent a flexible platform for future applications in cell separation.

IMS play a dominant role at present but other specific affinity ligands such as lectins, carbohydrates or antigens will probably be used more often in the near future. There are also many possibilities to combine the process of cell magnetic separation with other techniques, such as PCR, enabling the elimination of compounds possibly inhibiting DNA polymerase. New applications can be expected, especially in microbiology (isolation and detection of microbial pathogens) and parasitology (isolation and detection of protozoan parasites). No doubt many new processes and applications in other fields of biosciences and biotechnologies will be developed in the near future.

Magnetic separation methods are widely used for isolation of a variety of cell types. Magnetic particles with immobilized antibodies to various antigens have been employed for the rapid isolation of populations T-(CD4 +, CD3 +, CD8+) and B- (CD19+) of lymphocytes, NK cells, and monocytes. Similarly, immobilization of glycoconjugates on magnetic beads allows the isolation of cell populations expressing a particular carbohydrate-recognizing molecule [19, 20]. Glycosylated magnetic beads can be prepared by loading biotinylated probes onto streptavidin-coated magnetic beads. The glycoparticles are then incubated with a cell suspension and the subpopulation of interest is fished out by means of a magnetic device [20].

When these materials are used in the biological field, special restrictions should be considered and all possible reactions with the biological materials should be predicted. Magnetic properties should be maintained for a specific time during the test. Some applications can be classified as follows:

Magnetic separation is used for clinical application, such as in the separation of proteins, toxemic materials, DNA, and bacteria and viruses. This is also used for real time detecting of viruses. The most important stage in this field is the labeling of molecules with magnetic materials by a reliable connection. Magnetic beads from iron oxide are typically used for biological separation. The main properties of iron oxide are super paramagnetic properties (Meza, 1997).

Effective drug delivery can greatly improve the process of treatment and reduce side effects. In this method, while the amount of drug decreases, the concentration of the drug in the target area increases. Protecting the drug before its gets to the target area is one of the most important factors, because after releasing the drug in the blood stream, white cells detect the drug and swallow them in a short time. An ideal nanoparticle for drug delivery should have the potential to combine with a relatively high-weight drug and disperse uniformly in the blood stream (Shultz et al., 2007).

Also, while chemotherapy is one of most effective methods for cancerous tissues, many of the other healthy cells are destroyed in the process. So the conventional thermotherapy has many side effects. In hyperthermia treatment, after delivering the drug to the target area, an AC magnetic field is used to generate controllable energy and increase temperature. Heat transfer in this process is a balance between blood flow, heat generation, and tissue porosity and conductivity (Sellmyer and Skomski, 2006).

Magnetic Resonance Imaging (MRI) is considered a great help in the diagnoses of many diseases. The advantages of this imaging are high contrast in soft tissue, proper resolution, and sufficient penetration depth for noninvasive diagnosis. In fact, in MRI imaging magnetization of protons is measured when exposed to the magnetic field with radio frequency (Corot, 2006).

Magnetic separation: based on the generation of magnetic forces on the particles to be separated, which are higher than opposing forces such as gravity or centrifugal forces. This principle is used to separate ferromagnetic particles from crushed scrap mixtures.

Eddy current separation: is a particular form of magnetic separation. An alternating magnetic field induces electrical eddy currents on a metal particle. This results in a magnetic field whose direction is opposite to the primary magnetic field. The exchange interactions between the magnetic fields result in a repulsive force on the metallic particle; the net effect is a forward thrust as well as a torque. This force and hence the efficiency of separation is a function of the magnetic flux, or indirectly of the electrical conductivity and density and the size and shape of the metallic particles.

Air separation/zigzag windsifter: Air-based sorting technique, which separates the light materials from the heavier. The most prominent application is in shredder plants producing the shredder light fraction, or in fridge recycling, removing among others the polyurethane (PUR) foam from the shredded scrap.

Screening: Separation of the scrap into different particle size classes is performed to improve the efficiency of the subsequent sorting processes and/or to apply different processing routes for different size fractions (based on material breakage and hence distribution over various size fractions).

Fluidized bed separation: A fluidized bed of dry sand is used to separate materials based on density. This technology is in principle a dry sink-float separation, which is still hampered by several difficulties (tubular or hollow particles filling up with sand and tend to sink; formation of unsteady current due to the use of high velocity air, etc.). The fluidized bed could also be heated for simultaneous de-coating and combustion of organic material.

Image processing (including colour sorting): Colour sorting technologies, which sense the colour of each particle and use computer control to mechanically divert particles of identical colour out of the product stream (red copper, yellow brass, etc.). A complicating issue is that shredding results in mixtures of particles that show a distribution in composition, size, shape, texture, types of inserts, coatings, etc. The variance of these properties complicates identification that is solely based on this principle.

X-ray sorting: Dual energy X-ray transmission imaging (well known for luggage safety inspections at airports) identifies particles based on the average atomic number, particle shape, internal structure (e.g. characteristic variations of thickness) and presence of characteristic insert material. It is rather sensitive to particle thickness and surface contaminations.

LIBS (laser induced breakdown spectroscopy) sorting: A series of focused ablation laser pulses are delivered to the same spot on each particle. A pulse of an ablation laser vaporizes only the first nanometres of the surface, i.e. the first pulses are necessary to clean the surface of oxide layers (different composition than the mother metal), the last pulse vaporizes a tiny amount of metal generating a highly luminescent plasma plume. The light from the plasma is collected and analysed to quantitatively determine the chemical composition. This determines to which bin the particle is directed (e.g. by air pulse).

Iron ore processors may also employ magnetic separation for beneficiation of classifier output streams. Wet high-intensity magnetic separators (WHIMS) may be used to extract high-grade fine particles from gangue, due to the greater attraction of the former to the applied magnetic field.

In addition to beneficiating the intermediate middlings streams from the classifier, WHIMS may be used as scavenger units for classifier overflow. This enables particles of sufficient grade to be recovered that would otherwise be sacrificed to tails.

Testwork has been performed on iron ore samples from various locations to validate the use of magnetic separation following classification (Horn and Wellsted, 2011). A key example was material sourced from the Orissa state in northeastern India, with a summary of results shown in Table 10.2. The allmineral allflux and gaustec units were used to provided classification and magnetic separation, respectively.

The starting grade of the sample was a low 42% Fe. It also contained significant ultrafines with 58% passing 20m. This is reflected in the low yield of allflux coarse concentrate; however, a notable 16% (abs) increase in iron grade was eventually achieved. The gaustec results for the middlings and overflow streams demonstrate the ability to recover additional high-grade material. With the three concentrate streams combined, an impressive yield of almost 64% was achieved with minimal decline in iron grade.

The automatic separation system, developed by Magnetic Separation System of Nashville, Tennessee, uses X-ray, IR, and visible spectra sensors for separating the post-consumer recyclate bottles or flakes into individual plastics and into different color groups. X-ray sensors, used for separating PVC, are very accurate and can operate at as high as 99% or better efficiency. IR and visible sensors are used to separate the colored bottles into individual polymers and color groups.

The separation system (Figure 4) essentially consists of a metering inclined conveyer, air knife, special disk screen, singulating infeed conveyor, and sensor module. A motor control system provides operator interface screens which control the sorting functions, including the number of bottles sorted into each fraction, ejection timing, and sort positions. Individual systems currently in use in Germany, Switzerland, and the United States are described in a paper by Kenny and Vaughan.16 The systems are customized, based on the composition of the post-consumer recyclate and the end application of the separated streams. Some systems use X-ray and IR sensors in two locations to achieve better separation. In addition to sorting equipment, some systems also use equipment for breaking the bales and splitting the bottles into more than one stream for smooth operation. Grinders are used when the bottles have to be ground into flakes for further processing. Whereas PVC separation is accomplished at 99%. HDPE and PET separation is between 80 and 90%, depending on the level of contamination.

Automated separation provides two advantages: improved quality and lower labor cost for sorting. The automatic separation system at Eaglebrook Plastics uses the Magnetic Separation System (MSS), which detects and separates the bottles into different categories based on the type of the resin and color, and eliminates impurities such as broken pieces of plastics, rocks, aluminum cans, and other contaminants.17 Metering the feed is critical to obtain maximum throughput at Eaglebrook. This is accomplished by a special debaling device and an incline metering system. Factors contributing to proper operation include clear height, width, spacing, belt speed, and incline angle. Proper presentation of the bottle to the sensor is critical. The bottles are split into four streams and two to three bottles are presented to the sensor per second, one at a time.

The primary identification sensor uses a multibeam, near-IR array to identify the bottles into three classes: Class 1, PVC, PET; Class 2, natural HDPE, PP; Class 3, mixed color HDPE and opaque containers. This sensor is also capable of separating colored PET from clear PET and PP from milk jug HDPE. The X-ray sensor identifies PVC, and a machine vision sensor system provides up to seven color classifications of the plastic bottles. After identification, the containers are ejected from the conveyors into appropriate collection stations using high-speed pulsed air nozzles. The motor control center (MCC) of the separation system controls motor protection, sequential slant up for the system, fault indication, and operation control. In addiiton, a touch screen input panel allows the operator to select any available sort to be directed to any ejection station. Visible light color sensors have been added which sort pigmented HDPE into different colors. The system also includes a decision cross-checking device between the primary sensor and the color sensor. This compares the decisions of the two sensors by comparing them with a logic file. The latter then provides correct identification in case there are discrepancies between the two decisions. The system has successfully operated for the last three to four years at a capacity of 5000 bottles h1.

The debaling system designed for Eaglebrook requires that the bales be presented to the debaling equipment in the same orientation as the original compression. This design feature requires less horsepower, reduces bottle clusters, and requires minimum energy. The debaling and declumping system incorporates a surge bin and metering conveyor to feed the screening system. The improved capacity and higher separation accuracy, due to increased metering efficiency, reduces bottle clusters and provides a more uniform feeding system. The separation efficiency depends on several factors. Timing and catcher bounceback accounts for 12% accuracy loss; contamination, container distortion, and loose labels contribute to about 34%, and nonsingulation of the bottles 510% of accuracy loss.

Asoma Instrument of Austin, TX, is a leading manufacturer of automated bottle sorting equipment. The company uses an X-ray fluorescence spectrophotometer sensor. The identification is completed in 10ms and the separation takes about 20s per bottle. The sorted PET streams have less than 50ppm PVC. National Recovery Technology of Nashville, TN, uses a proprietary electromagnetic screening process which can handle the bottles either in crushed or whole form and does not require any special positioning or orientation of the bottle to achieve high efficiency. Chamberlain/MCR, Hunt Valley, MD, and Automation Industrial Control of Baltimore, MD, offer a paysort bottle sorting system, which uses a sophisticated video camera and color monitor incorporating a strobe to detect and distinguish colors of post-consumer bottles following a near-IR detection system which also determines the primary resin found in each bottle.

A substantial amount of research is focused on microseparation techniques and on techniques which can reject bottles with trace amounts of harmful contaminant. Near-IR spectrometry is being used to separate bottles for household chemicals and ones with hazardous waste residues.

Sorting of automotive plastics is more difficult than sorting of plastics from packaging recyclates. Whereas only five to six polymers are used for packaging, post-consumer automotive plastics contain large numbers of engineering and commodity plastics, modified in various ways, including alloying and blending, filling, reinforcing, and foaming. Hence, sorting of automotive plastic recyclate poses several challenges. Recently, a systematic study, PRAVDA, was undertaken by a German car manufacturer and the plastic suppliers in Europe to investigate the potential of various analytical techniques in separating post-consumer automotive plastics.18

The techniques examined in this study include near-IR spectroscopy (NIR), middle-IR spectroscopy (MIR), Fourier transform Raman spectroscopy (FTR), pyrolysis mass spectrometry (PY-MS), pyrolysis IR spectroscopy (PYIR), and laser-induced emission spectral analysis (LIESA). X-ray methods were excluded because they have insufficient sensivitity to polymers, other than ones containing chlorine. Since commercial spectrophotometers were not available for most techniques except NIR, either laboratory models (MIR, FTR) or experimental stage instruments (PY-MS, PY-IR, and LIESA) were used in this study. A large number of parts (approximately 7000) were analyzed. The techniques were compared in respect to their success in identification, fault rate, time for identification, degree of penetration, and sensitivity to surface quality. The fault rate is the number of wrong identifications, given as percent. If the sum of the identification and fault rate is less than 100, the difference gives the rate of incomplete correct identification. The biggest stumbling block was the identification of black samples which could not be analyzed by NIR and FTR. MIR is the only technique which not only identified the black samples, but gave the highest identification rate. Some difficulties were experienced, however, in MIR analysis in the case of blends of two similar polymers such as PP/EPDM or nylon 6/nylon 66. The pyrolytic methods showed poorer identification rates and higher fault rates. The LIESA method is very fast and a remote technology, particularly for fast identification of heteroatoms. It is therefore suitable for identifying fillers, minerals, reinforcing fibers, pigments, flame retardants, and stabilizers specific to the individual plastic. The difficulty with MIR is that it is sensitive to surface micro-roughness and, hence, the samples need to be very smooth. Also, paint or surface coats on the part have to be removed for correct identification of the resin used for making the parts. Further, at this stage, no fiber optic or separated probe is available with MIR technology and, hence, the part has to be brought close to the spectrophotometer instead of the probe reaching the part. Another method of measuring efficiency is the level of contamination. Contamination of parts sorted by the MIR method was less than 1%, whereas contamination of parts sorted manually, using a Car Parts Dismantling Manual, is greater than 1015%. When the level of contamination is high, further separation by swim-sink or hydrocyclone techniques are necessary.

The cost of a MIR spectrophotometer is approximately DM 100000. The cost calculated for small dismantlers (dismantling less than 25 cars per day) is approximately DM 0.34 per kg and that for large dismantlers is somewhat less than DM 0.19. Manual sorting, on the other hand, would cost DM0.71 and DM0.23 per kg for small and large dismantlers, respectively. Spectrophotometric identification of plastics in automotive plastics waste therefore makes substantial economic sense.

magnetic separator - an overview | sciencedirect topics

magnetic separator - an overview | sciencedirect topics

As magnetic separators progress toward larger capacity, higher efficiency, and lower operating costs, some subeconomic iron ores have been utilized in recent years. For example, magnetite iron ore containing only about 4% Fe (beach sands or ancient beach sands) to 15% Fe (iron ore formations) and oxidized iron ore of only about 10% Fe (previously mine waste) to 20% Fe (oxidized iron ore formations) are reported to be utilized. They are first crushed and the coarse particles pretreated using roll magnetic separators. The magnetic product of roll magnetic separators may reach 2540% Fe and then is fed to mineral processing plants.

As shown in Figure5, slurry is fed from the top of an inclined screen in a low-intensity magnetic field, with the mesh size of screen sufficiently larger than those of particles in slurry. As the slurry flows down the above surface of screen, magnetic particles agglomerate with the size of agglomerations increasingly growing and roll down as magnetic concentrate at the lower end of screen. The less- or nonmagnetic particles pass through the screen as tailings. Figure5 shows the operation of screen magnetic separators for cleaning of magnetite.

Commercial magnetic separators are continuous-process machines, and separation is carried out on a moving stream of particles passing into and through the magnetic field. Close control of the speed of passage of the particles through the field is essential, which typically rules out free fall as a means of feeding. Belts or drums are very often used to transport the feed through the field.

As discussed in Section 13.4.1, flocculation of magnetic particles is a concern in magnetic separators, especially with dry separators processing fine material. If the ore can be fed through the field in a monolayer, this effect is much less serious, but, of course, the capacity of the machine is drastically reduced. Flocculation is often minimized by passing the material through consecutive magnetic fields, which are usually arranged with successive reversals of the polarity. This causes the particles to turn through 180, each reversal tending to free the entrained gangue particles. The main disadvantage of this method is that flux tends to leak from pole to pole, reducing the effective field intensity.

Provision for collection of the magnetic and nonmagnetic fractions must be incorporated into the design of the separator. Rather than allow the magnetics to contact the pole-pieces, which then requires their detachment, most separators are designed so that the magnetics are attracted to the pole-pieces, but come into contact with some form of conveying device, which carries them out of the influence of the field, into a bin or a belt. Nonmagnetic disposal presents no problems; free fall from a conveyor into a bin is often used. Middlings are readily produced by using a more intense field after the removal of the highly magnetic fraction.

Conventional magnetic separators are largely confined to the separation or filtration of relatively large particles of strongly magnetic materials. They employ a single surface for separation or collection of magnetic particles. A variety of transport mechanisms are employed to carry the feed past the magnet and separate the magnetic products. The active separation volume for each of these separators is approximately the product of the area of the magnetised surface and the extent of the magnetic field. In order for the separators to have practical throughputs, the magnetic field must extend several centimetres. Such an extent implies a relatively low magnetic field gradient and weak magnetic forces.

To overcome these disadvantages HGMS has been developed. Matrices of ferromagnetic material are used to produce much stronger but shorter range magnetic forces over large surface areas. When the matrices are placed in a magnetic field, strong magnetic forces are developed adjacent to the filaments of the matrix in approximately inverse proportion to their diameter. Since the extent of the magnetic field is approximately equal to the diameter of the filaments the magnetic fields are relatively short range. However, the magnetic field produced is intense and permits the separation and trapping of very fine, weakly magnetic particles (Oberteuffer, 1979).

The transport medium for HGMS can be either liquid or gaseous. Dry HGMS processing has the advantage of a dry product although classification of the pulverised coal is required to ensure proper separation. Small particles tend to agglomerate and pass through the separator. It has been shown that individual particles of coal in the discharge of a power plant pulveriser flow freely and hence separate well only if the material below about 10 m is removed (Eissenberg et al., 1979). Even then drying of that part of run of mine coal to be treated by HGMS may be required to ensure good flow characteristics.

A schematic representation of a batch HGMS process is shown in Figure 11.5 (Hise, 1979, 1980; Hise et al., 1979). It consists of a solenoid, the core cavity of which is filled with an expanded metal mesh. Crushed coal is fed to the top of the separator. Clean coal passes through while much of the inorganic material is trapped to be released when the solenoid is later deactivated.

Data from a batch HGMS process of one size fraction of one coal are plotted in Figure 11.6 as weight per cent of material trapped in the magnetic matrix, the product sulphur and the product ash versus the independent variable of superficial transport velocity. At low superficial transport velocities the amount of material removed from the coal is high partly due to mechanical entrapment. As the velocity is increased the importance of this factor diminishes but hydrodynamic forces on the particles increase. These hydrodynamic forces oppose the magnetic force and the amount of material removed from the coal decreases (Hise, 1979).

For comparison, Figure 11.7 shows data from a specific gravity separation of the same size fraction of the same coal. While the sulphur contents of the products from the two separation processes are similar the ash content of the HGMS product is considerably higher than that of the specific gravity product. It should be emphasised that this comparison was made for one size fraction of one coal.

More recently dry HGMS has been demonstrated at a scale of 1 t/h on carousel type equipment which processes coal continuously (Figure 11.8; Hise et al., 1981). A metal mesh passes continuously through the magnetised cavity so that the product coal passes through while the trapped inorganics are carried out of the field and released separately.

Wet HGMS is able to treat a much wider range of coal particle sizes than dry HGMS. The efficiency of separation increases with decreasing particle size. However, depending on the end use a considerable quantity of energy may have to be expended in drying the wet, fine coal product. Wet HGMS may find particular application to the precleaning of coal for use in preparing coal water mixtures for subsequent combustion as both pulverising the coal to a fine particle size and transporting the coal in a water slurry are operations common to both processes.

Work at Bruceton, PA, USA has compared the pyrite reduction potential of froth flotation followed by wet HGMS with that of a two stage froth flotation process (Hucko and Miller, 1980). Typical results are shown in Figures 11.9 and 11.10. The reduction in pyritic sulphur is similar in each case although a greater reduction in ash content is achieved by froth flotation followed by HGMS than by two stage froth flotation. However, Hucko (1979) concludes that it is highly unlikely that HGMS would be used for coal preparation independently of other beneficiation processes. As with froth flotation there is considerable variation in the amenability of various coals to magnetic beneficiation.

In the magnetic separator, material is passed through the field of an electromagnet which causes the retention or retardation of the magnetic constituent. It is important that the material should be supplied as a thin sheet in order that all the particles are subjected to a field of the same intensity and so that the free movement of individual particles is not impeded. The two main types of equipment are:

Eliminators, which are used for the removal of small quantities of magnetic material from the charge to a plant. These are frequently employed, for example, for the removal of stray pieces of scrap iron from the feed to crushing equipment. A common type of eliminator is a magnetic pulley incorporated in a belt conveyor so that the non-magnetic material is discharged in the normal manner and the magnetic material adheres to the belt and falls off from the underside.

Concentrators, which are used for the separation of magnetic ores from the accompanying mineral matter. These may operate with dry or wet feeds and an example of the latter is the Mastermag wet drum separator, the principle of operation of which is shown in Figure 1.43. An industrial machine is shown in operation in Figure 1.44. A slurry containing the magnetic component is fed between the rotating magnet drum cover and the casing. The stationary magnet system has several radial poles which attract the magnetic material to the drum face, and the rotating cover carries the magnetic material from one pole to another, at the same time gyrating the magnetic particles, allowing the non-magnetics to fall back into the slurry mainstream. The clean magnetic product is discharged clear of the slurry tailings. Operations can be co- or counter-current and the recovery of magnetic material can be as high as 99.5 per cent.

An example of a concentrator operating on a dry feed is a rotating disc separator. The material is fed continuously in a thin layer beneath a rotating magnetic disc which picks up the magnetic material in the zone of high magnetic intensity. The captured particles are carried by the disc to the discharge chutes where they are released. The nonmagnetic material is then passed to a second magnetic separation zone where secondary separation occurs in the same way, leaving a clean non-magnetic product to emerge from the discharge end of the machine. A Mastermagnet disc separator is shown in Figure 1.45.

The removal of small quantities of finely dispersed ferromagnetic materials from fine minerals, such as china clay, may be effectively carried out in a high gradient magnetic field. The suspension of mineral is passed through a matrix of ferromagnetic wires which is magnetised by the application of an external magnetic field. The removal of the weakly magnetic particles containing iron may considerably improve the brightness of the mineral, and thereby enhance its value as a coating or filler material for paper, or for use in the manufacture of high quality porcelain. In cases where the magnetic susceptibility of the contaminating component is too low, adsorption may first be carried out on to the surface of a material with the necessary magnetic properties. The magnetic field is generated in the gap between the poles of an electromagnet into which a loose matrix of fine stainless steel wire, usually of voidage of about 0.95, is inserted.

The attractive force on a particle is proportional to its magnetic susceptibility and to the product of the field strength and its gradient, and the fine wire matrix is used to minimise the distance between adjacent magnetised surfaces. The attractive forces which bind the particles must be sufficiently strong to ensure that the particles are not removed by the hydrodynamic drag exerted by the flowing suspension. As the deposit of separated particles builds up, the capture rate progressively diminishes and, at the appropriate stage, the particles are released by reducing the magnetic field strength to zero and flushing out with water. Commercial machines usually have two reciprocating canisters, in one of which particles are being collected from a stream of suspension, and in the other released into a waste stream. The dead time during which the canisters are being exchanged may be as short as 10 s.

Magnetic fields of very high intensity may be obtained by the use of superconducting magnets which operate most effectively at the temperature of liquid helium, and conservation of both gas and cold is therefore of paramount importance. The reciprocating canister system employed in the china clay industry is described by Svarovsky(30) and involves the use a single superconducting magnet and two canisters. At any time one is in the magnetic field while the other is withdrawn for cleaning. The whole system needs delicate magnetic balancing so that the two canisters can be moved without the use of very large forces and, for this to be the case, the amount of iron in the magnetic field must be maintained at a constant value throughout the transfer process. The superconducting magnet then remains at high field strength, thereby reducing the demand for liquid helium.

Micro-organisms can play an important role in the removal of certain heavy metal ions from effluent solutions. In the case of uranyl ions which are paramagnetic, the cells which have adsorbed the ions may be concentrated using a high gradient magnetic separation process. If the ions themselves are not magnetic, it may be possible to precipitate a magnetic deposit on the surfaces of the cells. Some micro-organisms incorporate a magnetic component in their cellular structure and are capable of taking up non-magnetic pollutants and are then themselves recoverable in a magnetic field. Such organisms are referred to a being magnetotactic.

where mpap is the inertial force and ap the acceleration of the particle. Fi are all the forces that may be present in a magnetic separator, such as the magnetic force, force of gravity, hydrodynamic drag, centrifugal force, the friction force, surface forces, magnetic dipolar forces, and electrostatic forces among the particles, and others.

Workable models of particle motion in a magnetic separator and material separation must be developed separately for individual types of magnetic separators. The situation is complicated by the fact that many branches of magnetic separation, such as separation by suspended magnets, magnetic pulleys, or wet low-intensity drum magnetic separators still constitute highly empirical technology. Hesitant steps have been taken to develop theoretical models of dry separation in roll and drum magnetic separators. Alternatively, open-gradient magnetic separation, magnetic flocculation of weakly magnetic particles, and wet high-gradient magnetic separation (HGMS) have received considerable theoretical attention. A notable number of papers dealing with the problem of particle capture in HGMS led to an understanding of the interaction between a particle and a matrix element. However, completely general treatment of the magnetostatic and hydrodynamic behavior of an assembly of the material particles in a system of matrix elements, in the presence of a strong magnetic field, is a theoretical problem of considerable complexity which has not been completed, yet. Detailed description of particle behavior in various magnetic separators can be found in monographs by Gerber and Birss (1983) and Svoboda (1987, 2004).

The brick material ratio was: Slag(1.0mm<): Grog (3.0mm<): Ceramic Gravel (1.0mm<): Clay (1.0mm<) at 20 : 35 : 25 : 20. To this mixture, 2% of pigment were added. Kneading and blending was done by a Mller mixer for 15 minutes. Molding was done by a 200 ton friction press, and the bricks were loaded onto the sintering truck.

This paper presents preliminary results using the Magnetic Micro-Particle Separator, (MM-PS, patent pending) which was conceived for high throughput isothermal and isobaric separation of nanometer (nm) sized iron catalyst particles from Fischer-Tropsch wax at 260 oC. Using magnetic fields up to 2,000 gauss, F-T wax with 0.30.5 wt% solids was produced from 25 wt% solids F-T slurries at product rates up to 230 kg/min/m2. The upper limit to the filtration rate is unknown at this time. The test flow sheet is given and preliminary results of a scale-up of 50:1 are presented.

Most loads for flap valves, conveyors, vibrating feeders, crushers, paddle feeders, magnetic separators, fans and trash screens generally are supplied at 415 V three-phase 50 Hz from the 415 V Coal Plant Switchboard, although 3.3 kV supplies may be used when the duty demands. Stacker/reclaimer machines are supplied at 3.3 kV. Electrical distribution is designed to safeguard the independent operational requirements of the duplicated coal plant facilities and to ensure that an electrical fault will not result in the total loss of coal supplies to the boilers.

The first step in any form of scrubbing unit is to break the lumpy materials and remove tramp elements by a magnetic separator. The product is then led into the scrubbing unit. The dry scrubbing principle is to agitate the sand grains in a stream of air so that the particles shot-blast each other. A complete dry scrubbing plant has been described in a previous book of this library in connection with sodium silicate bonded sands.* For clay-bonded sands the total AFS clay content in the reclaimed sand varies from 05% to 25% clay depending on the design of the plant.

magnetic particles for the separation and purification of nucleic acids | springerlink

magnetic particles for the separation and purification of nucleic acids | springerlink

Nucleic acid separation is an increasingly important tool for molecular biology. Before modern technologies could be used, nucleic acid separation had been a time- and work-consuming process based on several extraction and centrifugation steps, often limited by small yields and low purities of the separation products, and not suited for automation and up-scaling. During the last few years, specifically functionalised magnetic particles were developed. Together with an appropriate buffer system, they allow for the quick and efficient purification directly after their extraction from crude cell extracts. Centrifugation steps were avoided. In addition, the new approach provided for an easy automation of the entire process and the isolation of nucleic acids from larger sample volumes. This review describes traditional methods and methods based on magnetic particles for nucleic acid purification. The synthesis of a variety of magnetic particles is presented in more detail. Various suppliers of magnetic particles for nucleic acid separation as well as suppliers offering particle-based kits for a variety of different sample materials are listed. Furthermore, commercially available manual magnetic separators and automated systems for magnetic particle handling and liquid handling are mentioned.

Magnetic separation is an emerging technology that uses magnetism for the efficient separation of micrometre-sized para- and ferromagnetic particles from chemical or biological suspensions. Enrichment of low-grade iron ore, removal of ferromagnetic impurities from large volumes of boiler water in both conventional and nuclear power plants, or the removal of weakly magnetic coloured impurities from kaolin clay are typical examples of magnetic separation in traditional industries. The application of these techniques in biosciences had been restricted and of limited use up to the 1970s. The idea of using magnetic separation techniques to purify biologically active compounds (nucleic acids, proteins, etc.), cells, and cell organelles led to a regrowing interest over the last decade. New magnetic particles with improved properties were developed for the partly complicated separation processes in these fields [see reviews: Olsvik et al. 1994; Safarik and Safarikova 1999; Franzreb et al. 2006].

Magnetic separation of nucleic acids has several advantages compared to other techniques used for the same purpose. Nucleic acids can be isolated directly from crude sample materials such as blood, tissue homogenates, cultivation media, water, etc. The particles are used in batch processes where there are hardly any restrictions with respect to the sample volumes. Due to the possibility of adjusting the magnetic properties of the solid materials, they can be removed relatively easily and selectively even from viscous sample suspensions. In fact, magnetic separation is the only feasible method for the recovery of small particles (diameter approx. 0.051m) in the presence of biological debris and other fouling material of similar size. Furthermore, the efficiency of magnetic separation is especially suited for large-scale purifications (Safarik et al. 2001; Franzreb et al. 2006).

These upcoming separation techniques also serve as a basis of various automated low- to high-throughput procedures that allow to save time and money. Centrifugation steps can be avoided and the risk of cross-contamination when using traditional methods is no longer encountered. Various types of magnetic particles are commercially available for nucleic acid purification, magnetic separators working in the manual and automated mode are offered. A short description of traditional and magnetic separation methods for nucleic acid isolation, together with a short overview of batch and automated separators, will be given below.

The isolation of DNA or RNA is an important step before many biochemical and diagnostic processes. Many downstream applications such as detection, cloning, sequencing, amplification, hybridisation, cDNA synthesis, etc. cannot be carried out with the crude sample material. The presence of large amounts of cellular or other contaminating materials, e.g. proteins or carbohydrates, in such complex mixtures often impedes many of the subsequent reactions and techniques. In addition, DNA may contaminate RNA preparations and vice versa. Thus, methods for the efficient, reliable and reproducible isolation of nucleic acids from complex mixtures are needed for many methods that are used today and rely on the identification of DNA or RNA, e.g. diagnosis of microbial infections, forensic science, tissue and blood typing, detection of genetic variations, etc.

A range of methods are known for the isolation of nucleic acids in the fluid phase, but they are generally based on complex series of precipitation and washing steps and are time-consuming and laborious to perform. Thus, classical methods for the isolation of nucleic acids from complex starting materials such as blood or tissues, involve the lysis of the biological material by a detergent or chaotropic substance, possibly in the presence of protein-degrading enzymes, followed by several processing steps applying organic solvents such as phenol and/or chloroform or ethanol, which in general are highly toxic and require special and, hence, expensive disposal. For example, the complete removal of proteins from nucleic acids can be achieved by the addition of sodium perchlorate (Wilcockson 1973). The separation of RNA from DNA requires selective precipitation steps with LiCl or a specific nuclease-free isolation with guanidinium hydrochloride or guanidinium thiocyanate, combined with phenol extraction and ethanol precipitation (Bowtell 1987). Such methods are not only cumbersome and time-consuming, but the relatively large number of steps required increases the risk of degradation, sample loss or cross-contamination of samples especially when several samples are processed simultaneously. In the case of RNA isolation, the risk of DNA contamination is comparatively high.

Apart from laborious and time-consuming traditional methods, alternative separation techniques have been developed. Sorption processes based on (a) hydrogen-binding interaction with an underivatised hydrophilic matrix, typically silica, under chaotropic conditions, (b) ionic exchange under aqueous conditions by means of an anion exchanger, (c) affinity and (d) size exclusion mechanisms were used for DNA purification. Solid-phase systems which adsorb DNAsilica-based particles (Vogelstein and Gillespie 1979; Boom et al. 1990, 1999; Melzak et al. 1996; Tian et al. 2000; Breadmore et al. 2003), glass fibres, and anion-exchange carriers (Ferreira et al. 2000; Endres et al. 2003; Teeters et al. 2003)are used in chromatographic separation columns [e.g. DE 41 43 639 C2 (Qiagen GmbH)] for example.

These carriers are applied for DNA isolation or purification together with highly concentrated chaotropic salt solutions (e.g. sodium iodide, sodium perchlorate, guanidinium thiocyanate). In US 5,075,430 (BioRad), for instance, usage of diatomaceous earth as a carrier material is described. Again, bonding takes place in the presence of a chaotropic salt. Other approaches are based on detergence together with a nucleic-acid-binding material (EP 0 796 327 B1, Dynal) or on the usage of a solid carrier with DNA-binding functional groups combined with polyethylene glycol and salts at high concentrations (WO/1999/058664, Whitehead Institute for Biomedical Research).

The increasing use of magnetic solid carriers in biochemical and molecular biology processes has many advantages compared to other non-magnetic separation processes. The term magnetic means that the support obtains a magnetic moment when placed in a magnetic field. Thus, it can be displaced. In other words, particles having a magnetic moment may be removed readily by the application of a magnetic field, e.g. by using a permanent magnet. This is a quick, simple and efficient way to separate the particles after the nucleic binding or elution step (see Fig.1) and a far less rigorous method than traditional techniques, such as centrifugation, that generate shear forces which may lead to the degradation of the nucleic acids. It is also possible to isolate components of the cell lysate, which inhibit for example the DNA polymerase of a following PCR reaction like polysaccharides, phenolic compounds or humic substances (Demeke and Adams 1992; Watson and Blackwell 2000).

Usually, it is sufficient to apply a magnet to the side of the vessel containing the sample mixture for aggregating the particles near the wall of the vessel and pouring away the remainder of the sample (see Fig.1).

Magnetic carriers with immobilised affinity ligands or prepared from a biopolymer exhibiting affinity to the target nucleic acid are used for the isolation process. Many magnetic carriers are commercially available and can also be prepared in the laboratory. Such materials are magnetic particles produced from different synthetic polymers, biopolymers, porous glass, or magnetic particles based on inorganic magnetic materials such as surface-modified iron oxide. Especially suited are superparamagnetic particles, which do not interact among each other in the absence of a magnetic field. These particles will magnetise under a strong magnetic field, but retain no permanent magnetism once the field is removed. When magnetic aggregation and clumping of the particles are prevented during the reaction, easy suspension of the particles and uniform nucleic acid extraction are ensured.

The diameter of the particles is approximately between 0.5 and 10m. Materials with a large surface area are preferred for binding the nucleic acids. Without going into theoretical details, the nucleic-acid-binding process may be assisted by the nucleic acid wrapping around the support. Such supports generally have an irregular surface and may be porous for example. Particulate materials, e.g. beads and in particular polymer beads, are generally preferred due to their larger binding capacity. Conveniently, a particulate solid support used will comprise spherical beads.

In the laboratory, colloidal magnetite Fe3O4 (or similar magnetic material such as maghemite Fe2O3 or ferrites) particles usually are surface-modified by silanisation. Naked iron oxide (Fe3O4) has the capacity of adsorbing DNA (Davies et al. 1998), but aggregates due to attractive forces reduce the surface area that can be used for adsorption. Silane compounds coupled to magnetite derivatised with carboxyl groups are known to have a DNA extraction ability in solutions containing PEG (Hawkins et al. 1994). Modified bacterial magnetite particles in the presence of amino silane compounds and hyperbranched polyamidoamine dendrimer are used for DNA extraction by Yoza et al. (2002, 2003). Modified magnetic cobalt ferrite particles have been investigated for DNA isolation under high sodium chloride and PEG concentrations by Prodelalova et al. (2004).

Surface modification of magnetic nanoparticles with alkoxysilanes (Bruce et al. 2004; Tan et al. 2004; Bruce and Sen 2005) or polyethyleneimine (Chiang et al. 2005; Veyret et al. 2005) is also useful. The above-mentioned magnetic colloids are not easy to separate using classical magnets. This is due to a small particle size, at which Brownian motion forces are higher than the exerted magnetic force. To enhance phase separation, various magnetic latexes that may interact with nucleic acids were prepared.

Magnetic micro-beads can be prepared in a number of ways, but usually magnetically susceptible particles (e.g. iron oxide) are coated with synthetic or biological polymers. Elaissari et al. (2003) describe the interaction of nucleic acids and different polymers. Biopolymers such as agarose, chitosan, -carrageenan, and alginate, can be prepared easily in a magnetic form (Levison et al. 1998; Prodelalova et al. 2004). In the simplest case, the biopolymer solution is mixed with magnetic particles and, after bulk gel formation, the magnetic gel formed is broken into fine particles. Alternatively, the biopolymer solution containing dispersed magnetite is dropped into a mixed hardening solution or a water-in-oil suspension technique is used to prepare spherical particles. Basically, the same process can be used to prepare magnetic particles for nucleic separation from synthetic polymers such as hydrophobic polystyrene (Ugelstad et al. 1992) and hydrophilic polyacryl amide (Elaissari et al. 2001) or poly(vinyl alcohol) (Oster et al. 2001). Genomic DNA was also successfully isolated from cell lysate on weak acid derivatives of magnetic P(HEMA-co-EDMA) and P(HEMA-co-GMA) microparticles in the presence of PEG and sodium chloride (Horak et al. 2005).

The first approach to synthesising micro-sized particles was published by Ugelstad et al. They developed an interesting methodology leading to monosized polystyrene magnetic microspheres, which were studied in various biomedical applications (Ugelstad et al. 1993). These particles have an excellent size distribution and spherical shape, but their surface is very hydrophobic and results in a high amount of unspecific protein binding on the particle surface.

Another possibility consists in combining different polymer matrix materials with silica components (Grttner et al. 2001; Mller-Schulte et al. 2005) that specifically interact with the nucleic acids.

Depending on the support and the nature of the subsequent processing required, it may or may not be desirable to release the nucleic acid from the support. The direct use of magnetic beads, e.g. in PCR or other amplifications, without eluting the nucleic acid from the surface is not trivial. The enzymatic detection and amplification methods will be inhibited by the magnetic beads, their stabilisers, or their metal oxides (Spanova et al. 2004), which decrease PCR sensitivity or lead to false negative PCR results. For many DNA detections or identification methods, elution is not necessary. Although the DNA may be randomly in contact with the bead surface and bound at a number of points by hydrogen binding or ionic or other forces, there generally will be sufficient lengths of DNA available for hybridisation to oligonucleotides and for amplification. If desired, however, elution of the nucleic acid may be achieved using known methods, e.g. higher ionic strength, heating or pH changes.

Commercially available magnetic particles that are suited for nucleic acid separation can be obtained from a variety of companies. Mostly, the matrixes are based on silica, porous glass, cellulose, agarose, polystyrene and silane (see Tables1 and 2). Moreover, some important patents exist that describe the synthesis of magnetic carriers not only for nucleic separation:

One of the first patents for particle synthesis is the Ugelstad polymerization process, which is described, for example, in EP 0 003 905 B2, US 5,459,378, and US 4,530,956 (SINTEF). It leads to monodisperse magnetic particles by several swelling and polymerisation steps. WO/1992/016581 (Cornell Research Foundation) also describes the preparation of monodisperse particles, particularly macroporous polymer beads. The process proposed uses a three-phase emulsion containing soluble polymer particles, a monomer phase and water. Nucleic acid separation using magnetic beads is described in (Alderton et al. 1992) and in WO/1991/012079 as well as in US 5,523,231 (Amersham). These magnetic beads are able to absorb the nucleic acid after a salt-ethanol precipitation. The approaches are not nucleic-acid-specific, i.e. the magnetic beads adsorb other bio-substances in parallel. Of course, this is a drawback of these approaches.

In the declaration WO/1996/041811 (Boehringer; Roche) mainly non-porous glass particles comprising mica and magnetite particles are described (Bartl et al. 1998). During their production, magnetic particles and a surrounding glass coating are superimposed on a mica core. The disadvantage of these products is their affinity to sedimentation. Furthermore, the production process is time-consuming and based on a complex spray process. Another approach to the production of particles from spherical magnetite kernels with a surface coating of silicon dioxide is covered by the European patent application EP 1 468 430 A1.

Monodisperse magnetic beads are described in WO/1998/012717 (Merck). They consist of a SiO2 core, which is given magnetic properties by a ferric-oxide coating. After a subsequent silanisation of the ferric-oxide coating, the particles can bind nucleic acids.

Many patents concerning nucleic acid separation are from the Dynal company. They developed monodisperse polymer magnetic particles with different sizes (coefficient of variation less than 5%) (see EP 0 796 327 B1), which are sold with a polystyrene matrix under the name of Dynabeads. The small-size distribution ensures reproducible separation properties. Protocols for nucleic acid separation with these particles are described by EP 0 512 439 B1 and with oligonucleotide-linked particles for specific nucleic acid separation in US 5,512,439.

Magnetic beads based on mica or polystyrene and coated by a magnetic oxide reach a high specific density, which leads to a fast sedimentation. Thus, additional mechanical mixing is necessary. The main drawback of the coated particles consists in the fact that the metal oxides may be in direct contact with the analytical solutions despite silanisation. All state-of-the-art approaches to the production of magnetic beads are laborious; the production process time amounts to several hours. To overcome this problem, the US patents 6,204,033 and 6,514,688 (chemagen Biopolymer Technologie AG) describe spherical, magnetic polymer particles based on polyvinyl alcohol particles, which can be produced in short terms using inverse suspension polymerisation. The polymer particles contain reactive hydroxyl groups to which other molecules can be coupled. Due to their hydrophilic surface, the particles exhibit small unspecific bindings only. Together with an at least partly silanised surface (DE 100 13 955 A1 and EP 1 274 745 A1) or a germanium-containing compound (DE 101 03 652 A1), they can be used for specific nucleic acid separation.

The inverse suspension process for the separation of nano- and micro-sized silica particles is suggested in WO/2002/009125 (Fraunhofer-Gesellschaft). The main idea is the dispersion of aqueous silica-sole containing magnetic colloids, which are hardened to spherical hydrophilic gel particles by adding a suited base. These particles can be used for nucleic acid separation with high binding capacities (WO/2005/50 52 581 A3, MagnaMedics GmbH).

Both total DNA and RNA are separated by the same magnetic beads. For the purpose of removing RNA from DNA, the RNA is destroyed before the DNA separation step. Adding of an RNAse or an alkali such as NaOH is an appropriate process. Vice versa, RNA can be separated if the DNA is degraded with DNAse.

The primary method considered for plasmid purification is the separation of plasmid DNA (pDNA) from the chromosomal DNA and cellular RNA of the host bacteria. Stadler et al. (2004) show that even in the case of a high copy plasmid, pDNA represents not more than 3% of the cleared lysate and that most of the critical contaminants are negatively charged (RNA, cDNA, endotoxin) and similar in size (cDNA, endotoxins) and hydrophobicity (endotoxins). A number of methods have been developed to generate a cleared lysate, but they are not able to remove proteins and lipids. Alkaline lysis of harvested bacterial cells with a subsequent neutralisation, as originally described by Birnboim and Doly (1979), is the process of choice. Cleared lysate protocols may vary slightly from each other as regards salt concentrations, volume, pH, temperature, and process step durations (Hirt 1967; Holmes and Quigley 1981; Birnboim 1983). These techniques make use of the differences in denaturation and renaturation characteristics of covalently closed circular plasmid DNA and chromosomal DNA fragments.

Table1 shows some commercially available magnetic particles used for DNA, RNA and pDNA isolation. Many magnetic particles are available with optimised buffers and protocols for small lab scale and automated systems. There are also some companies offering particles for nucleic acid purification without any further information.

The magnetic carrier is provided with binding solutions to assist in the selective capture of nucleic acids. For example, complementary DNA or RNA sequences (Satokari et al. 2005) or DNA-binding proteins may be used as well as viral proteins binding to viral nucleic acids. In this review, a short overview of eukaryotic mRNA and viral DNA/RNA will be given.

There are several companies (see Table2) offering oligodeoxythymidine immobilised with magnetic particles, which can be used effectively for the rapid isolation of highly purified mRNA from eukaryotic cell cultures or total RNA preparations (Jacobsen et al. 2004). These procedures are based on the hybridisation of the oligonucleotide dT sequence with the stable polyadenylated 3 termini of the eukaryotic mRNA. The length of the complementary sequence differs between 20 and 30 oligonucleotides. This sequence is directly bound covalently to the particle surface or indirectly by biotinylated oligonucleotides and the interaction of streptavidin-coated particles. CPG and Dynal (now Invitrogen) offer MPG and Dynabeads with already immobilised biotinylated oligonucleotide, but also other companies offer streptavidin-modified particles, which can be used for mRNA isolation, as described, e.g. by the mRNA isolation kit with MagneSphere from Promega. Nearly all magnetic particles (except for MagaCell oligo-dT30 and Sera-Mag oligo-(dT)30) are available together with an optimised buffer system and helpful protocols.

Automated extraction of viral RNA and DNA from the plasma mini-pool is performed by the chemagic Viral DNA/RNA Kit and chemagic Magnetic Separation Module I (Hourfar et al. 2005a,b; Pichl et al. 2005).

A rapid diagnosis of enterovirus infection by magnetic bead extraction has been established by Muir et al. (1993). Enterovirus RNA can be separated from large-volume water samples using the NucliSens miniMAG System (Rutjes et al. 2005). Hei and Cai (2005) developed a system for purifying SARS coronavirus RNA by a hybridisation of a specific oligonucleotide sequence, which is immobilised on the magnetic bead surface.

A variety of magnetic separators are available on the market, ranging from very simple concentrators for one tube to complicated fully automated devices. The isolation of nucleic acids is mostly performed in the batch mode using commercially available lab-scale magnetic separators (particle concentrators). Separators are usually made of strong rare-earth permanent magnets designed to hold various amounts of micro-tubes or tubes.

Particles with a diameter larger than 1m can be separated easily using simple magnetic separators, while separation of smaller particles (magnetic colloids with a particle size ranging from ten to hundreds of nanometres) may require the use of high-gradient magnetic separators.

The racks are designed to hold various amounts of micro-tubes or tubes. Test tube magnetic separators allow to separate magnetic particles from volumes between approximately 5l and 50ml. There are many combinations with other features like a mixing function (Ademtech) or a possibility to turn the separator over for the removal of the supernatant (chemagen Biopolymer-Technologie AG). Other devices are applied for the separation of magnetic particles from the wells of standard micro-titration plates. In some of them the temperature can be pc-controlled (AGOWA), other devices may be inserted into automated separation devices.

Laboratory automation is increasingly important in molecular biology and biotechnology. Constantly increasing numbers of analyses of different sources and sample volumes have resulted in an enormous importance of flexible robots or automated systems. Automation is also required for handling a large number of samples without human errors.

Many instruments have been developed to automate PCR amplification, the sequencing reaction and the detection of nucleic acids, but automating DNA extraction by traditional methods with centrifugation and vacuum steps still is difficult. A complete separation of the solid carrier matrix by centrifugation is not possible. Supports filled with carrier materials cannot be used, as the ineluctable dead volumes of the support lead to sample material loss in case of small amounts of sample materials. Another drawback is the danger of mutual contamination of different biological samples, especially if directly neighbouring supports are emptied by the vacuum. However, the last decade shows that DNA purification using magnetic bead technology is suitable for automation systems, and several automated instruments for handling magnetic beads have been developed (Alderton et al. 1992; Wahlberg et al. 1992; Rolfs and Weber 1994; Fangan et al. 1999; Obata et al. 2001; Akutsu et al. 2004; Vuosku et al. 2004).

More and more vendors offer commercially automated devices for the handling of magnetic particles, e.g. for the purification of nucleic acid (see Table4). Most systems are offered together with system-specific optimised particles, buffer systems and protocols.

The devices are able to process between six and 96 samples in parallel and commonly customised for small buffer volumes. For larger volumes, the chemagic Magnetic Separation Module I (<10ml) (see Fig.2) or the Magtration System 8l(7ml) can be used.

chemagic Magnetic Separation Module I consisting of (A) separation head with magnetizable rods [here 12-well format for large (50ml) volumes; 96-well format for MTPs also available], (B) electro magnet, (C) chemagic dispenser for parallel filling of all required buffer solutions (accessory) and (D) tracking unit. The principle functionality regarding separation and resuspension of magnetic beads is shown in the scheme

The present review has shown that the separation of nucleic acid is a highly dynamic field of research and development. An increasing number of commercial vendors offer magnetic particles, also in the form of a kit that is optimally suited for the application desired. The increasing number of publications shows that magnetic particles of higher potential are currently under research. Materials with more specific-binding properties and a better separability are promising approaches. A higher degree of automation leads to systems analysing a larger number of samples and higher sample volumes at the same time.

Akutsu J-I, Tojo Y, Okochi M, Yohda M, Segawa O, Obata K, Tajima H (2004) Development of an integrated automation system with a magnetic bead-mediated nucleic acid purification device for genetic analysis and gene manipulation. Biotechnol Bioeng 86:667671

Boom R, Sol C, Beld M, Weel J, Goudsmit J, Wertheim-van Dillen P (1999) Improved silica-guanidinium thiocyanate DNA isolation procedure based on selective binding of bovine alpha-casein to silica particles. J Clin Microbiol 37:615619

Breadmore MC, Wolfe KA, Arcibal IG, Leung WK, Dickson D, Giordano BC, Power ME, Ferrance JP, Feldman SH, Norris PM, Landers JP (2003) Microchip-based purification of DNA from biological samples. Anal Chem 75:18801886

Fangan BM, Dahlberg OJ, Deggerdal AH, Bosnes M, Larsen F (1999) Automated system for purification of dye-terminator sequencing products eliminates up-stream purification of templates. Biotechniques 26:980983

Hourfar MK, Schmidt M, Seifried E, Roth WK (2005) Evaluation of an automated high-volume extraction method for viral nucleic acids in comparison to a manual procedure with preceding enrichment. Vox Sang 89:7176

Jacobsen N, Nielsen PS, Jeffares DC, Eriksen J, Ohlsson H, Arctander P, Kauppinen S (2004) Direct isolation of poly(A)(+) RNA from 4M guanidine thiocyanate-lysed cell extracts using locked nucleic acid-oligo(T) capture. Nucleic Acids Res 32:e64

Muir P, Nicholson F, Jhetman M, Neogi S, Banatvala JE (1993) Rapid diagnosis of enterovirus infection by magnetic bead extraction and polymerase chain-reaction detection of enterovirus RNA in clinical specimes. J Clin Microbiol 31:3138

Obata K, Segawa O, Yakabe M, Ishida Y, Kuroita T, Ikeda K, Kawakami B, Kawamura Y, Yohda M, Matsunaga T, Tajima H (2001) Development of a novel method for operating magnetic particles, Magtration Technology, and its use for automating nucleic acid purification. J Biosci Bioeng 91:500503

Rutjes SA, Italiaander R, van den Berg HHJL, Lodder WJ, de Roda Husman AM (2005) Isolation and detection of enterovirus RNA from large-volume water samples by using the nucliSens miniMAG System and real-time nucleic acid sequence-based amplification. Appl Environ Microbiol 71:37343740

Vuosku J, Jaakola L, Jokipii S, Karppinen K, Kamarainen T, Pelkonen VP, Jokela A, Sarjala T, Hohtola A, Haggman H (2004) Does extraction of DNA and RNA by magnetic fishing work for diverse plant species? Mol Biotechnol 27:209215

DE 101 03 652 A1 Magnetische Polyvinylalkoholpartikel mit modifizierter Oberflche zur Isolierung und Reinigung von Nukleinsuren (2002) Brassard L, Parker J, Smets H, Oster J; chemagen Biopolymer-Technologie AG, Germany

US 4,336,173 Process for preparing an aqueous emulsion or dispersion of a partly water-soluble material, and optionally further conversion of the prepared dispersion or emulsion to a polymer dispersion when the partly water-soluble material is a polymerizable monomer (1980) Ugelstad J; SINTEF, Norway

US 4,530,956 Process for the preparation of aqueous dispersions of organic materials and possible further conversion to a polymer dispersion when the organic material is a polymerizable monomer (1985) Ugelstad J, Berge A; SINTEF Norway

WO/2002/009125 Spherical, magnetic SiO2 particles with an adjustable particle and pore size and an adjustable magnetic content. Method for producing them and use of SiO2 particles of this type (2001) Mller-Schulte D, Fischer R; Fraunhofer-Gesellschaft zur Frderung der angewandten Forschung e.V. Germany

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