Thin film transistors [TFT]

Introduction

Flat-panel displays (FPDs) are becoming increasingly commonplace in today's commercial electronic devices. FPDs are finding widespread use in many new products, such as cellular phones, personal digital assistants (PDAs), camcorders, and laptop personal computers (PCs). This generation of handheld electronics places stringent demands on their displays. FPDs in these devices are expected to be lightweight, portable, rugged, low-power and high-resolution. Displays having all these attributes will enable a wide variety of commercial applications in the future.

A thin film transistor liquid crystal display (TFT-LCD) is a variant of liquid crystal display (LCD) which uses thin film transistor (TFT) technology to improve image quality (e.g. addressability, contrast). TFT LCD is one type of active matrix LCD, though all LCD-screens are based on TFT active matrix addressing. TFT LCDs are used in television sets, computer monitors, mobile phones and computers, personal digital assistants, navigation systems, projectors, etc.

Definitions of Terms:

• Liquid-Crystal Displays (LCD's):
  LCD's are currently the leading flat-panel display technology. Liquid crystals change orientation under an applied electric field and can thereby block or pass light.
  
• Active-Matrix Liquid-Crystal Displays (AMLCD's):
  LCD technology that incorporates an active-matrix, as opposed to a passive-matrix or "dual-scan" technology.
  
• High Mobilities:
  Mobility is the proportionality constant that relates the drift velocity to the electric field strength in a semiconductor. Mobility essentially gauges how easily current carriers (i.e. electrons, holes) can move through a piece of silicon. Electrons move most easily through single-crystalline silicon because of the uniform arrangement of the atoms. Unfortunately, single-crystalline films are difficult to deposit due to the low melting point of glass.
• Low Leakage Currents:
  Leakage current refers to the small amount of current that flows (or "leaks") through a transistor when it is "turned off." In an ideal transistor, leakage current would be zero, but in practice, leakage current always has a finite value. Leakage current causes the voltage in the pixel capacitor to drop between each frame refresh, and thus changes the pixel brightness. Leakage current most significantly affects the fineness of the display's grayscale. With a low leakage current, finer levels of grayscale can be achieved.
  
• Threshold Voltages:
  The voltage necessary to turn on a transistor. Threshold voltages should be low so that it takes lower voltages to charge and discharge the display's pixels (thereby turning them on and off).
  
• Integration of Driver Circuitry:
  A display needs row and column drivers to properly read the image data into the pixels. Most displays are "dumb" and have external IC drivers that require bonded connections to the rows and columns.
  
• Amorphous Silicon TFT's:
  TFTs that are made using a thin layer of amorphous silicon. Atoms in amorphous silicon have no short- or long-range order. When a film of silicon is deposited at low-temperature on glass or plastic, the atoms are normally arranged in this amorphous state. High temperatures are required if films are to crystallize into poly-Si.
  
  
• Gate-Dielectric Materials:
  In a transistor, we want the current to flow from the source to the drain, and not into the gate. Thus, we must put an insulating material between the gate and the channel of the transistor. The most common gate-dielectric material is silicon dioxide.
  
  
• High-Voltage Bias Stressing:
  This refers to a testing procedure in which newly fabricated electronics are tested for reliability by applying high voltages to them.
  
  
• Physically Based Model:
  A model of transistor operation based on measured data, as opposed to a theory-based model.

Construction:-

The circuit layout of a TFT-LCD is very similar to that of a DRAM memory. However, rather than fabricating the transistors from silicon formed into a crystalline wafer, they are made from a thin film of silicon deposited on a glass panel. Transistors take up only a small fraction of the area of each pixel; the rest of the silicon film is etched away to allow light to pass through.

The silicon layer for TFT-LCDs is typically deposited using the PECVD process from a silane gas precursor to produce an amorphous silicon film. Polycrystalline silicon (frequently LTPS, low-temperature poly-Si) is sometimes used in displays requiring higher TFT performance. Examples include high-resolution displays, high-frequency displays or displays where performing some data processing on the display itself is desirable. Amorphous silicon-based TFTs have the lowest performance, polycrystalline silicon TFTs have higher performance (notably mobility), and single-crystal silicon transistors are the best performers.

Types:-

There are many types of TFT available in the markets.

· TN+Film :- The TN display suffers from limited viewing angles, especially in the vertical direction. For colour representation many panels use 6 bits per colour, instead of 8, and are consequently unable to display the full 24-bit truecolor (16.7 million colour shades) available from modern graphics cards.

· IPS :- IPS (in-plane switching) was developed by Hitachi in 1996 to improve on the poor viewing angles and color reproduction of TN panels. Most panels also support true 8-bit per channel color.Their are few types of IPS in market.

1. AS-IPS.

2. AS-TW-IPS

· MVA :- MVA (multi-domain vertical alignment) was originally developed in 1998 by Fujitsu as a compromise between TN and IPS. It achieved pixel response which was fast for its time, wide viewing angles, and high contrast at the cost of brightness and color reproduction.

· PVA :- PVA (patterned vertical alignment) and S-PVA (super patterned vertical alignment) are alternative versions of MVA technology offered by Samsung.

· CVA :- CPA (Continuous Pinwheel Alignment) was developed by Sharp.

Electrical interface:-

External consumer display devices like a TFT LCD mostly use an analogue VGA connection, while newer, more expensive models mostly feature a digital interface like DVI, HDMI, or DisplayPort as well.

Inside an external display device there is a controller board that will convert VGA, DVI, HDMI, CVBS etc. to digital RGB at native resolution that the display panel can make use of. In a laptop the graphics chip will directly produce a signal suitable for connection to the builtin TFT. A control mechanism for the backlight is usually included on the same controller board.

The lowlevel interface of STN, DSTN, or TFT display panels use either single ended TTL 5V or TTL 3.3V that transmits Pixel clock, Horizontal sync, Vertical sync, Digital red, Digital green, Digital blue in parallel. Some models also feature input/display enable, horizontal scan direction and vertical scan direction signals.

New and large (>15") TFT displays often use LVDS or TMDS signaling that is the same as the parallel interface but will put control and RGB bits into a number of serial transmission lines synchronized to a clock at 1/3 of the data bitrate.

Backlight intensity is usually controlled by varying a few volts DC to the backlight highvoltage (1.3kV) DC-AC inverter. It can also be controlled by a potentiometer or be fixed. Some models use PWM signal for intensity control.

The bare display panel will only accept a video signal at the resolution determined by the panel pixel matrix designed at manufacture. Some screen panels will ignore colour LSB bits to ease interfacing (8bit->6bit/colour).

Applications

Organics have long been attractive for use in electronics because of their light weight, flexibility, and low cost compared with their Si counterparts. Recent increases in performance, however, have rapidly expanded organic FETs from niche markets, making them targets for a wider range of applications.

Organics offer potential advantages in displays, where TFTs are implemented as switches to activate individual pixels. Hand-held devices (cell phones, PDAs, etc.) with ultrathin displays can achieve higher resolution and information content, while new technologies, such as flexible displays and electronic paper, are potentially revolutionary advancements. Integrated smart pixels, with an OTFT switching an organic light-emitting diode (OLED) pixel, have been demonstrated, even though actual OTFT active-matrix OLED displays are yet to be demonstrated.

An alternative to active-matrix flexible displays is an innovative example by E-ink utilizing an OTFT backplane with a laminated electronic ink frontplane, consisting of a layer of electrophoretic microcapsules on a transparent electrode. The OTFT backplane controls the contrast of the display by moving charged black and white pigments to the transparent layer. In late 2000, E-Ink presented the world's first flexible (16 cm × 16 cm) electronic ink display using an OTFT backplane circuit created by Lucent, consisting of an array of 256 transistors fabricated using a low-cost, rubber stamp printing process. The printed transistors from Lucent and a typical flexible. Plastic Logic, a company actively developing ink-jet-printed plastic TFTs, subsequently has demonstrated a bistable reflective display driven by an ink-jet-printed active-matrix backplane together with Gyricon Media, the provider of SmartPaper™ reusable display material. This first experimental prototype is a display featuring 3024 pixels (63 × 48) at 50 dpi on a glass substrate. More recently, Philips and E-Ink jointly demonstrated a similar electronic ink display driven by OTFTs with 320 × 240 pixels, a diagonal length of 127 mm, a resolution of 85 dpi, and a bending radius of 2 cm. Philips has also announced that it had formed a technology incubator company, Polymer Vision, to partner with other companies interested in ultrathin, rollable displays that could double as electronic paper.

Schematic of an E-ink display. An OTFT backplane addresses each element, with encapsulated, charged pigments shifting to the transparent electrode surface.

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A 256-transistor array produced by Lucent using a rubber stamp printing process. (Reprinted with permission from.

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The world's first flexible electronic ink display driven by organic transistors. (Reprinted with permission fromal Academy of Sciences USA.)

Safety Measures:-

The liquid crystals inside the display are poisonous and must not be ingested or brought into contact with skin. Spills from a cracked display should be washed off immediately with soap and water.

The leading manufacturer of liquid crystal materials for display applications states as follows:

Merck KGaA has committed itself to not introduce into the market liquid crystal materials which are either acutely toxic or mutagenic.

The complete report "Toxicological and Ecotoxicological Investigations of Liquid Crystals; Disposal of LCDs" is available from Merck KGaA

HARD & SOFT SUPER CONDUCTORS

Normal electronic conductors have electrical resistance to the motion of electrons whenever a current flows through the material. A voltage must be applied in order to replace this energy lost as heat. A superconductor, however, has no resistance at all. Many metals, but not all, show electrical resistance at ordinary room temperatures but turn superconductive when refrigerated near to absolute zero.

This behaviour of superconductors is exciting today for a variety of commercial applications and in research because the limits of superconductors are a long way from being reached.

. In 1911 superconductivity was first observed in mercury by Dutch physicist Heike Kamerlingh Onnes of Leiden University (shown above). When he cooled it to the temperature of liquid helium, 4 degrees Kelvin (-452F, -269C), its resistance suddenly disappeared. The Kelvin scale represents an "absolute" scale of temperature. Thus, it was necessary for Onnes to come within 4 degrees of the coldest temperature that is theoretically attainable to witness the phenomenon of superconductivity. Later, in 1913, he won a Nobel Prize in physics for his research in this area.

Resistance is classically due to collisions of free electrons with thermally displaced ions with impurities and defects in the metal. This approach can not explain superconductivity as electrons always suffer some collisions so resistance can never be zero. This is put to good use in light bulbs.

The best normal conductors have weak interactions between the electrons and the lattice which is why they are good conductors, but this prevents them from becoming superconductors.

The only way to describe superconductors is to use quantum mechanics. The model used is the BSC theory (named after the 3 men who derived it, Bardeen, Cooper and Schrieffer), which was first suggested in 1957^[4]. It states that lattice vibrations play an imp

Type 1 and Type 2 Superconductors

The first superconductors were of little use in a practical sense, because they could not carry a significant amount of current. These are known as type 1 or “soft” superconductors^[10]. They require the coldest temperatures (to slow down molecular vibrations sufficiently to allow unimpeded electron flow in accordance with BCS Theory) to become superconductive and exhibit a very sharp transition to a superconducting state and “perfect” diamagnetism.

Diagram 6: Type 1 superconductivity showing a sharp transition

For a type 1 superconductor the critical current is a consequence of the critical magnetic field, H_c^[11]. H_c is low in type 1 superconductors along with their critical current densities (important in wire manufacturing) and therefore they have been of little interest to magnet builders or the electric utilities^[12].

Type 2 or “hard” superconductors are comprised of metallic compounds and alloys such as “perovskites” (metal oxide ceramics). They achieve higher T_c than type 1 by a mechanism that is still unclear^[13]. They differ from type 1 in that their transition from a normal to a superconducting state is gradual across a region of “mixed state” or vortex behaviour. They admit the magnetic field into their interiors while still remaining superconducting. It has been these type 2 superconductors that contemporary scientific and commercial superconducting magnets are wound.

States of Superconductors

In both types of superconductors the electrons combine in pairs under the critical temperature to form macroscopic material waves.

In type 1, the conventional metals and metalloids, the electrons interact with the lattice vibration, whereby both electrons in the Cooper Pair have S= 0 and L=0 (quantum numbers) and can be described by an s-wave function. The wave-function, therefore has the same characteristics along every axis of the lattice.

In type 2 superconductors, the unconventional ceramic compounds, the electron pair processes for an S=0 state; L= 0, 2h, 4h etc. due to quantum mechanic restrictions. The compound always takes on the lowest possible energy and therefore, in most superconductors the (S=0, L=0) state occurs.

7 Paul Brown, Heidelberg University, 2004

High Temperature Superconductors (HTS)

For over 75 years superconductivity remained a low temperature phenomenon, and it was theoretically shown and widely believed that high temperature Superconductivity was impossible, that the highest transition temperature, or critical temperature (T_c), could not go above 30K (according to BCS theory). This changed in 1986, when J.G.Bednorz and K.A.Müller discovered the barium-doped structures of LaCuO₄^[14], which broke the 30K limit.

With the 30K barrier broken the race was on to find still higher transition temperatures. The first was via strontium substitution: La2-xSrxCuO4 giving a T_c of 38K^[15]. It was also found that under extreme pressure the critical temperature could be increased to 50K^[16].

The next step was to simulate pressure via chemical substitution. This was done by adding yttrium to the perovskite structure of BaCuO₃. Surprisingly the compound (YBa₂Cu₃O₇) went superconducting at 92K^[17].

This was important as it put superconductivity in the range of liquid nitrogen and so hundreds of labs joined the race.

It was found that nearly any of the rare earth metals could be substituted for yttrium without any significant change to the transition temperature^[18].

The structure of the compound is that of a sandwich with planes of copper oxide in the centre, where the superconducting current flows. The other elements act only as spacers.

The record T_c today is owned by HgBa₂Ca₂Cu₃O₈, which by room pressure has a T_c of 135K and under pressure can reach 164K^[19]. One of today’s theories predicts an upper limit of 200K for superconductivity, while others predict no limit.

All of these HTSs were brittle ceramic compounds. This is surprising as ceramics are normally insulators. The theory behind this is still not fully understood^[20]. This brittleness causes drawbacks in practical applications, such as drawing out wires. Another drawback is the magnetic properties of these materials.

Most HTSs are produced form metastable materials; this means that the thermodynamically stabile reactants are forced into forming the compound either by high pressure and temperature or by doping. This method of synthesis, however, does not represent in any way an absolute criterion for HTS synthesis.

Type II superconductors are, for the most part, comprised of metallic compounds and alloys. This class of superconductors generally has a much higher critical temperature than those in Type I. They achieve a higher critical temperature than Type 1 superconductors by a mechanism that is still not completely understood. It is believed that it relates to the planar layering within the crystalline structure. The highest critical temperature reached is currently 138 K. Debates still arise as to whether or not an upper limit exists for a critical temperature to be found.

REVIEW OF LITERATURE:

1)TITLE:Hard superconductivity of a soft metal in the

quantum regime

MUSTAFA M. ÖZER1, JAMES R. THOMPSON1,2 AND HANNO H. WEITERING

Submitted on:27 january 2006

Superconductivity is inevitably suppressed in reduced dimensionality. The thin superconducting wires or films can be before they lose their

superconducting properties have important technological ramifications and go to the

heart of understanding coherence and robustness of the superconducting state in

quantum-confined geometries. Here, we exploit quantum confinement of itinerant

electrons in a soft metal to stabilize superconductors with lateral dimensions of the

order of a few millimeters and vertical dimensions of only a few atomic layers10.These extremely thin superconductors show no indication of defect- or fluctuationdriven suppression of superconductivity and sustain supercurrents of up to 10% of

the depairing current density. The extreme hardness of the critical state is attributed

to quantum trapping of vortices. This study paints a conceptually appealing, elegant

picture of a model nanoscale superconductor with calculable critical state properties.

It indicates the intriguing possibility of exploiting robust superconductivity at the

nanoscale.

2)Title:		Nonlinear diffusion in hard and soft superconductors
Authors:		Gilchrist, John; van der Beek, C. J.
Publication Date:		09/1994
Bibliographic Code:		1994PhyC..231..147G

We discuss the diffusion of magnetic flux in a field-cooled (``hard'') superconducting slab in a creep regime in which E ~ |J|<sup>σ</sup> J. Bryksin and Dorogovtsev recently discussed flux diffusion in a pinningless (``soft'') superconductor in which E ~ |B|J. This problem is closely related to the flux-creep one with σ=1, and provides additional insight into the possible types of behaviour. We list a series of possible long-term asymptotic solutions of a scaling form, which are either analytically exact or accurately calculated. We check numerically that the relevant long-term solution is approached after various initial conditions. Amongst other conclusions we find S=d(In|M|)/d(Int)-->-1/σ or -1/2σ, after application and removal of an additional field, aJump to main content

3)Limited flux jumps in hard superconductors

R G Mints and A L Rakhmanov
Inst. of High Temperatures, Moscow, USSR

Limited flux jumps in superconductors are investigated under the conditions when the heating of the sample is not too high. The surface temperature rise, electric field and magnetic flux change associated with the instability development are calculated. The theory is compared with experiments, and a satisfactory agreement is found.

Print publication: Issue 12 (14 December 1983)

4)Magnetic instabilities in hard superconductors

R G Mints and Aleksandr L Rakhmanov

The magnetic instabilities in hard and combined Type II superconductors in detail give the criteria for stability of the critical state with respect to magnetic-flux jump.Then the total effect of magnetic and thermal diffusion, as well as that of the structure of a combined superconductor, on the magnetic-field value for a flux jump. The theoretical results will be compared with the existing experimental data.

Print publication: Issue 3 (1977)

Superposition of currents in hard superconductors placed into crossed AC and DC magnetic fields

FISHER L. M. (1) ; KALINOV A. V. (1) ; VOLOSHIN I. F. (1) ; BALTAGA I. V. ; IL'ENKO K. V. ; YAMPOL'SKII V. A. ;

Publishing year:1996

The superposition of currents in YBCO melt-textured samples placed into crossed ac and dc magnetic fields is predicted and observed. This superposition is a direct consequence of the critical state model. The dc magnetic field distribution is shown to become uniform wherever the ac field has penetrated. Owing to this nonlinear process, the area of the dc magnetization loop diminishes and eventually disappears completely with an increase of the ac field magnitude. This means that under the action of the external ac field, the static magnetic properties of hard superconductors change and tend to the well known properties of soft ones.

SUMMARY:

Superconductors conduct electricity with little or no resistance. Organic superconductors contain carbon and are less dense than their ceramic or metallic counterparts; they also offer unusual potential for fine-tuning of electrical properties. Argonne National Laboratory long has carried out the major U.S. effort to synthesize and identify organic superconductors. Nearly 100 new superconductors of this type have been produced, with critical temperatures (at which a superconductor loses all electrical resistance) as high as -260 degrees C, or -434 degrees F. Recently, the first superconductor composed entirely of organic components (with no metal atoms) was synthesized, with a transition temperature in this range. Although this remains far lower than the highest known transition temperature for ceramics, scientists still expect that a high-temperature organic superconductor may be possible, such that liquid nitrogen (at -196 degrees C, or -321 degrees F) could be used as the coolant instead of the more costly liquid helium, thus making practical applications more feasible. The new compound has a two-dimensional, layered structure, which may provide significant insight into the nature of superconductivity.

These advances will help scientists develop a theory of how organic superconductors work and contribute to the design of new materials with higher transition temperatures. The all-organic material is ideal for studies of magnetic and charge transport properties because there is no possibility of contamination from metallic impurities.

APPLICATIONS:

Superconductivity already has important applications, such as medical diagnostic equipment, and many more uses are possible if transition temperatures are high enough. The availability of purely organic superconductors greatly expands the possibilities, especially for applications in which weight is a factor

Superconducting high speed train system comprising a rail including at least one elongated hard superconducting member disposed horizontally along the running direction of the train and having a hollow or gap portion extending in the elongated direction, and a train body including a superconducting magnet for generating a magnetic field perpendicular to the hard superconducting member, thereby floating the body from the rail by the magnetic force acting between the superconducting magnet and the hard superconducting member.

LIMITATIONS:

Limitations on performance of Superconductor oversampling ADCs
For the development and optimization of superconductor oversampling modulators, We highlight the importance of specially engineered and parasitic components of the feedback loop. In particular, LR circuits operating as low-pass filters are capable of providing a noticeable SNR improvement and dramatically reducing the dynamic range requirements for used SFQ comparators. On the other hand, the feedback loop delay and time-jitter in timing circuits are able to spoil the potentially extremely high performance of superconductor oversampling ADCs. We also developed a simple formula describing time-jitter in superconductor

BIBLOGRAPHY:

1):	arXiv:cond-mat/0601641v1

2)www.iop.org/EJ/abstract/0022-3727/16/12/026

3)http://www.freepatentsonline.com

4)www.sciencedirect.com/science

5)http://physics.aps.org/articles

6) http:/www.msd.anl.go

Chromatography-2

CHROMATOGRAPHY

ABSTRACT

Few methods of chemical analysis are truly specific to a particular analyte. It is often found that the analyte of interest must be separated from the myriad of individual compounds that may be present in a sample. As well as providing the analytical scientist with methods of separation, chromatographic techniques can also provide methods of analysis.

Chromatography involves a sample (or sample extract) being dissolved in a mobile phase (which may be a gas, a liquid or a supercritical fluid). The mobile phase is then forced through an immobile, immiscible stationary phase. The phases are chosen such that components of the sample have differing solubilities in each phase. A component which is quite soluble in the stationary phase will take longer to travel through it than a component which is not very soluble in the stationary phase but very soluble in the mobile phase. As a result of these differences in mobilities, sample components will become separated from each other as they travel through the stationary phase.

Techniques such as H.P.L.C. (High Performance Liquid Chromatography) and G.C. (Gas Chromatography) use columns - narrow tubes packed with stationary phase, through which the mobile phase is forced. The sample is transported through the column by continuous addition of mobile phase. This process is called elution. The average rate at which an analyte moves through the column is determined by the time it spends in the mobile phase.

INTRODUCTION

Pictured is a sophisticated gas chromatography system. This instrument records concentrations of acrylonitrile in the air at various points throughout the chemical laboratory.

Chromatography is the collective term for a family of laboratory techniques for the separation of mixtures. It involves passing a mixture dissolved in a "mobile phase" through a stationary phase, which separates the analyte to be measured from other molecules in the mixture and allows it to be isolated.

Chromatography may be preparative or analytical. Preparative chromatography seeks to separate the components of a mixture for further use (and is thus a form of purification). Analytical chromatography normally operates with smaller amounts of material and seeks to measure the relative proportions of analytes in a mixture.

History

The history of chromatography spans from the mid-19th century to the 21st. Chromatography, literally "color writing", was used—and named— in the first decade of the 20th century, primarily for the separation of plant pigments such as chlorophyll. New forms of chromatography developed in the 1930s and 1940s made the technique useful for a wide range of separation processes and chemical analysis tasks, especially in biochemistry.

Some related techniques were developed in the 19th century (and even before), but the first true chromatography is usually attributed to Russian botanist Mikhail Semyonovich Tsvet, who used columns of calcium carbonate for separating plant pigments in the first decade of the 20th century during his research on chlorophyll.

Chromatography began to take its modern form following the work of Archer John Porter Martin and Richard Laurence Millington Synge in the 1940s and 1950s. They laid out the principles and basic techniques of partition chromatography, and their work spurred the rapid development of several lines of chromatography methods: paper chromatography, gas chromatography, and what would become known as high performance liquid chromatography. Since then, the technology has advanced rapidly. Researchers found that the principles underlying Tsvet's chromatography could be applied in many different ways, giving rise to the different varieties of chromatography described below. Simultaneously, advances continually improved the technical performance of chromatography, allowing the separation of increasingly similar molecules

Chromatography terms

• The analyte is the substance that is to be separated during chromatography.
• Analytical chromatography is used to determine the existence and possibly also the concentration of analyte(s) in a sample.
• A bonded phase is a stationary phase that is covalently bonded to the support particles or to the inside wall of the column tubing.
• A chromatogram is the visual output of the chromatograph. In the case of an optimal separation, different peaks or patterns on the chromatogram correspond to different components of the separated mixture.

On the x-axis is the retention time and plotted on the y-axis a signal (for example obtained by a spectrophotometer, mass spectrometer or a variety of other detectors) corresponding to the response created by the analytes exiting the system. In the case of an optimal system the signal is proportional to the concentration of the specific analyte separated

• A chromatograph is equipment that enables a sophisticated separation e.g. gas chromatographic or liquid chromatographic separation.
• Chromatography is a physical method of separation in which the components to be separated are distributed between two phases, one of which is stationary (stationary phase) while the other (the mobile phase) moves in a definite direction.
• The effluent is the mobile phase leaving the column.
• An immobilized phase is a stationary phase which is immobilized on the support particles, or on the inner wall of the column tubing.
• The mobile phase is the phase which moves in a definite direction. It may be a liquid (LC and CEC), a gas (GC), or a supercritical fluid (supercritical-fluid chromatography, SFC). A better definition: The mobile phase consists of the sample being separated/analyzed and the solvent that moves the sample through the column. In one case of HPLC the solvent consists of a carbonate/bicarbonate solution and the sample is the anions being separated. The mobile phase moves through the chromatography column (the stationary phase) where the sample interacts with the stationary phase and is separated.
• Preparative chromatography is used to purify sufficient quantities of a substance for further use, rather than analysis.
• The retention time is the characteristic time it takes for a particular analyte to pass through the system (from the column inlet to the detector) under set conditions.
• The sample is the matter analysed in chromatography. It may consist of a single component or it may be a mixture of components. When the sample is treated in the course of an analysis, the phase or the phases containing the analytes of interest is/are referred to as the sample whereas everything out of interest separated from the sample before or in the course of the analysis is referred to as waste.

The solute refers to the sample components in partition.

Techniques by chromatographic bed shape

Column Chromatography

• The solvent refers to any substance capable of solubilizing other substance, and especially the liquid mobile phase in LC.
• The stationary phase is the substance which is fixed in place for the chromatography procedure. Examples include the silica layer in Chromatography & Thin layer chromatography

Column chromatography is a separation technique in which the stationary bed is within a tube. The particles of the solid stationary phase or the support coated with a liquid stationary phase may fill the whole inside volume of the tube (packed column) or be concentrated on or along the inside tube wall leaving an open, unrestricted path for the mobile phase in the middle part of the tube (open tubular column). Differences in rates of movement through the medium are calculated to different retention times of the sample.

In 1978, W. C. Still introduced a modified version of column chromatography called flash column chromatography (flash). The technique is very similar to the traditional column chromatography, except for that the solvent is driven through the column by applying positive pressure. This allowed most separations to be performed in less than 20 minutes, with improved separations compared to the old method. Modern flash chromatography systems are sold as pre-packed plastic cartridges, and the solvent is pumped through the cartridge. Systems may also be linked with detectors and fraction collectors providing automation. The introduction of gradient pumps resulted in quicker separations and less solvent usage.

A spreadsheet that assists in the successful development of flash columns has been developed. The spreadsheet estimates the retention volume and band volume of analytes, the fraction numbers expected to contain each analyte, and the resolution between adjacent peaks. This information allows users to select optimal parameters for preparative-scale separations before the flash column itself is attempted.

In expanded bed adsorption, a fluidized bed is used, rather than a solid phase made by a packed bed. This allows omission of initial clearing steps such as centrifugation and filtration, for culture broths or slurries of broken cells.

Planar Chromatography

Thin layer chromatography is used to separate components of chlorophyll

Planar chromatography is a separation technique in which the stationary phase is present as or on a plane. The plane can be a paper, serving as such or impregnated by a substance as the stationary bed (paper chromatography) or a layer of solid particles spread on a support such as a glass plate (thin layer chromatography). Different compounds in the sample mixture travel different distances according to how strongly they interact with the stationary phase as compared to the mobile phase . The specific Retardation factor (R_f) of each chemical can be used to aid in the identification of an unknown substance.

Paper Chromatography.

Paper chromatography is a technique that involves placing a small dot of sample solution onto a strip of chromatography paper. The paper is placed in a jar containing a shallow layer of solvent and sealed. As the solvent rises through the paper, it meets the sample mixture which starts to travel up the paper with the solvent. This paper is made of cellulose, a polar substance, and the compounds within the mixture travel farther if they are non-polar. More polar substances bond with the cellulose paper more quickly, and therefore do not travel as far.

Thin layer chromatography

Thin layer chromatography (TLC) is a widely-employed laboratory technique and is similar to paper chromatography. However, instead of using a stationary phase of paper, it involves a stationary phase of a thin layer of adsorbent like silica gel, alumina, or cellulose on a flat, inert substrate. Compared to paper, it has the advantage of faster runs, better separations, and the choice between different adsorbents. For even better resolution and to allow for quantitation, high-performance TLC can be used.

Techniques by physical state of mobile phase

Gas chromatography

Gas chromatography (GC), also sometimes known as Gas-Liquid chromatography, (GLC), is a separation technique in which the mobile phase is a gas. Gas chromatography is always carried out in a column, which is typically "packed" or "capillary" (see below) .

Gas chromatography (GC) is based on a partition equilibrium of analyte between a solid stationary phase (often a liquid silicone-based material) and a mobile gas (most often Helium). The stationary phase is adhered to the inside of a small-diameter glass tube (a capillary column) or a solid matrix inside a larger metal tube (a packed column). It is widely used in analytical chemistry; though the high temperatures used in GC make it unsuitable for high molecular weight biopolymers or proteins (heat will denature them), frequently encountered in biochemistry, it is well suited for use in the petrochemical, environmental monitoring, and industrial chemical fields. It is also used extensively in chemistry research.

Liquid chromatography

Liquid chromatography (LC) is a separation technique in which the mobile phase is a liquid. Liquid chromatography can be carried out either in a column or a plane. Present day liquid chromatography that generally utilizes very small packing particles and a relatively high pressure is referred to as high performance liquid chromatography (HPLC).

In the HPLC technique, the sample is forced through a column that is packed with irregularly or spherically shaped particles or a porous monolithic layer (stationary phase) by a liquid (mobile phase) at high pressure. HPLC is historically divided into two different sub-classes based on the polarity of the mobile and stationary phases. Technique in which the stationary phase is more polar than the mobile phase (e.g. toluene as the mobile phase, silica as the stationary phase) is called normal phase liquid chromatography (NPLC) and the opposite (e.g. water-methanol mixture as the mobile phase and C18 = octadecylsilyl as the stationary phase) is called reversed phase liquid chromatography (RPLC). Ironically the "normal phase" has fewer applications and RPLC is therefore used considerably more.

Specific techniques which come under this broad heading are listed below. It should also be noted that the following techniques can also be considered fast protein liquid chromatography if no pressure is used to drive the mobile phase through the stationary phase. See also Aqueous Normal Phase Chromatography.

Affinity chromatography

Affinity chromatography^[4] is based on selective non-covalent interaction between an analyte and specific molecules. It is very specific, but not very robust. It is often used in biochemistry in the purification of proteins bound to tags. These fusion proteins are labelled with compounds such as His-tags, biotin or antigens, which bind to the stationary phase specifically. After purification, some of these tags are usually removed and the pure protein is obtained.

Supercritical fluid chromatography

is commonly used to purify proteins using FPLC. Supercritical fluid chromatography is a separation technique in which the mobile phase is a fluid above and relatively close to its critical temperature and pressure.Ion exchange chromatography utilizes ion exchange mechanism to separate analytes. It is usually performed in columns but the mechanism can be benefited also in planar mode. Ion exchange chromatography uses a charged stationary phase to separate charged compounds including amino acids, peptides, and proteins. In conventional methods the stationary phase is an ion exchange resin that carries charged functional groups which interact with oppositely charged groups of the compound to be retained. Ion exchange chromatography

TECHNIQUES BY SEPARATION MECHANISM

Ion exchange chromatography

Ion-exchange chromatography (or ion chromatography) is a process that allows the separation of ions and polar molecules based on the charge properties of the molecules. It can be used for almost any kind of charged molecule including large proteins, small nucleotides and amino acids. The solution to be injected is usually called a sample, and the individually separated components are called analytes. It is often used in protein purification, water analysis, and quality control.

PRINCIPLE

Stationary phase surface displays ionic functional groups (R-X) that interact with analyte ions of opposite charge. This type of chromatography is further subdivided into cation exchange chromatography and anion exchange chromatography. The ionic compound consisting of the cationic species M+ and the anionic species B- can be retained by the stationary phase.

Cation exchange chromatography retains positively charged cations because the stationary phase displays a negatively charged functional group:

Anion exchange chromatography retains anions using positively charged functional group:Note that the ion strength of either C+ or A- in the mobile phase can be adjusted to shift the equilibrium position and thus retention time.The ion chromatogram shows a typical chromatogram obtained with an anion exchange column.

Size exclusion chromatography

Size exclusion chromatography (SEC) is also known as gel permeation chromatography (GPC) or gel filtration chromatography and separates molecules according to their size (or more accurately according to their hydrodynamic diameter or hydrodynamic volume). Smaller molecules are able to enter the pores of the media and, therefore, take longer to elute, whereas larger molecules are excluded from the pores and elute faster. It is generally a low resolution chromatography technique and thus it is often reserved for the final, "polishing" step of a purification. It is also useful for determining the tertiary structure and quaternary structure of purified proteins, especially since it can be carried out under native solution conditions.

SPECIAL TECHNIQUES

Reversed-phase chromatography

Reversed-phase chromatography is an elution procedure used in liquid chromatography in which the mobile phase is significantly more polar than the stationary phase. Reversed-phase chromatography (RPC) includes any chromatographic method that uses a non-polar stationary phase. The name "reversed phase" has a historical background. In the 1970s most liquid chromatography was done on non-modified silica or alumina with a hydrophilic surface chemistry and a stronger affinity for polar compounds - hence it was considered "normal". The introduction of alkyl chains bonded covalently to the support surface reversed the elution order. Now polar compounds are eluted first while non-polar compounds are retained - hence "reversed phase". All of the mathematical and experimental considerations used in other chromatographic methods apply (ie separation resolution proportional to the column length). Today, reversed-phase column chromatography accounts for the vast majority of analysis performed in liquid chromatography.

Stationary Phases

* Silica Based Stationary Phases

Any inert non-polar substance that achieves sufficient packing can be used for reversed-phase chromatography. The most popular column is a C18 bonded silica (USP classification L1) with 297 columns commercially available ^[3] This is followed by C8 bonded silica (L7 - 166 columns), pure silica (L3 - 88 columns), cyano bonded silica (L10 - 73 columns) and phenyl bonded silica (L11 - 72 columns). Note that C18, C8 and phenyl are dedicated reversed phase packings while cyano columns can be used in a reversed phase mode depending on analyte and mobile phase conditions. It should be noted at this point that not all C18 columns have identical retention properties. Surface functionalization of silica can be performed in a monomeric or a polymeric reaction with different short-chain organosilanes used in a second step to cover remaining silanol groups (end-capping). While the overall retention mechanism remains the same subtle differences in the surface chemistries of different stationary phases will lead to changes in selectivity.

* Mobile Phase Considerations

Mixtures of water or aqueous buffers and organic solvents are used to elute analytes from a reversed phase column. The solvents have to be miscible with water and the most common organic solvents used are acetonitrile, methanol or tetrahydrofuran (THF). Other solvents can be used such as ethanol, 2-propanol (iso-propyl alcohol). Elution can be performed isocratic (the water-solvent composition does not change during the separation process) or by using a gradient (the water-solvent composition does change during the separation process). The pH of the mobile phase can have an important role on the retention of an analyte and can change the selectivity of certain analytes. Charged analytes can be separated on a reversed phase column by the use of ion-pairing (also called ion-interaction). This technique is known as reversed phase ion-pairing chromatography.

Two-dimensional chromatography

In some cases, the chemistry within a given column can be insufficient to separate some analytes. It is possible to direct a series of unresolved peaks onto a second column with different physico-chemical (Chemical classification) properties. Since the mechanism of retention on this new solid support is different from the first dimensional separation, it can be possible to separate compounds that are indistinguishable by one-dimensional chromatography.

Simulated Moving-Bed Chromatography

In chromatography, the simulated moving bed (SMB) technique is a a variant of high performance liquid chromatography; it is used to separate particles and/or chemical compounds that would be difficult or impossible to resolve otherwise. This increased separation is brought about by a valve-and-column arrangement that is used to lengthen the stationary phase indefinitely.

In the moving bed technique of preparative chromatography the feed entry and the analyte recovery are simultaneous and continuous, but because of practical difficulties with a continuously moving bed in the simulated moving bed technique instead of moving the bed the sample inlet and the analyte exit positions are moved continuously, giving the impression of a moving bed.

True moving bed chromatography (MBC) is only a theoretical concept. Its simulation, SMBC is achieved by the use of a multiplicity of columns in series and a complex valve arrangement, which provides for sample and solvent feed, and also analyte and waste takeoff at appropriate locations of any column, whereby it allows switching at regular intervals the sample entry in one direction, the solvent entry in the opposite direction, whilst changing the analyte and waste takeoff positions appropriately as well.

Ref 3 explains that the advantage of the SMBC is high speed, because a system could be near continuous, whilst its disadvantage is that it only separates binary mixtures. It does not say, but perhaps it can be assumed that this is equivalent with the separation of a single component from a group of compounds. With regard to efficiency it compares with simple chromatography technique like continuous distillation does with batch distillation.

Advantages

When affinity differences between molecules are very small, it is sometimes not possible to improve resolution via mobile- or stationary-phase changes. In these cases, the multi-pass approach of SMB can separate mixtures of those compounds by allowing their small retention time differences to accumulate.

At industrial scale an SMB chromatographic separator is operated continuously, requiring less resin and less solvent than batch chromatography. The continuous operation facilitates operation control and integration into production plants.

Drawbacks

The drawbacks of the SMB are higher investment cost compared to single column operations, a higher complexity, as well as higher maintenance costs. But these drawbacks are effectively compensated by the better yield and a much lower solvent consumption as well as a much higher productivity compared to simple batch separations.

For purifications, in particular the isolation of an intermediate single component or a fraction out of a multicomponent mixture, the SMB is not suited in general. It can only separate two fractions from each other and it does not implement linear solvent gradients as required for the purification of biomolecules.

Applications

In size exclusion chromatography, where the separation process is driven by entropy, it is not possible to increase the resolution attained by a column via temperature or solvent gradients. Consequently, these separations often require SMB, to create usable retention time differences between the molecules or particles being resolved. SMB is also very useful in the pharmaceutical industry, where resolution of molecules having different chirality must be done on a very large scale.

For the production of Fructose e.g. in High fructose corn syrup or amino-acids, biological-acids, etc. industrial scale chromatography is used.

Fast protein liquid chromatography

Fast protein liquid chromatography (FPLC) is a term applied to several chromatography techniques which are used to purify proteins. Many of these techniques are identical to those carried out under high performance liquid chromatography, however use of FPLC techniques are typically for preparing large scale batches of a purified product.Basically Fast Protein Liquid Chromatography, usually referred to as FPLC, is a form of column chromatography used to separate or purify proteins from complex mixtures. It is very commonly used in biochemistry and enzymology. Columns used with an FPLC can separate macromolecules based on size, charge distribution (ion exchange), hydrophobicity, or biorecognition (as with affinity chromatography).

The system setup is very similar to that of an HPLC, although the materials, buffers, and pressures used are usually different.

Technically, FPLC is the trade name for the protein chromatography system developed by Pharmacia (now GE Healthcare), and is now sold under the ÄKTA brand. However, it is often used as a genericized trademark to describe high-pressure chromatography purification of proteins.

Countercurrent chromatography

Countercurrent chromatography (CCC) or partition chromatography is a category of liquid-liquid chromatography techniques.^[1] Chromatography in general is used to separate components of a mixture based on their differing affinities for mobile and stationary phases of a column. The components can then be analyzed separately by various sorts of detectors which may or may not be integrated into an apparatus. Partition chromatography is based on differences in capacity factor,k, and distribution coefficient,Kd.of the analytes using liquid stationary and mobile liquid phase.In liquid-liquid chromatography, both the mobile and stationary phases are liquid. In contrast, standard column chromatography uses a solid stationary phase and a liquid mobile phase, while gas chromatography uses a liquid stationary phase on a solid support and a gaseous mobile phase. By eliminating solid supports, permanent adsorption of the analyte onto the column is avoided, and a near 100% recovery of the analyte can be achieved. The instrument is also easily switched between various modes of operation simply by changing solvents. With liquid-liquid chromatography, researchers are not limited by the composition of the columns commercially available for their instrument. Nearly any pair of immiscible solutions can be used in liquid-liquid chromatography, and most instruments can be operated in standard or reverse-phase modes. Solvent costs are also generally cheaper than for HPLC, and the cost of purchasing and disposing of solid adsorbents is completely eliminated. Another advantage is that experiments conducted in the lab can easily be scaled to industrial volumes interface between them has a large area, and the analyte can move between the phases according to its partition coefficient. A partition coefficient is a ratio of the amount of analyte found in each of the solvents at equilibrium and is related to the analyte's affinity for one over the other. The mobile phase is mixing with then settling from the stationary phase throughout the column. The degree of stationary phase retention (inversely proportional to the amount of stationary phase loss or "bleed" in the course of a separation) is a crucial parameter. Higher quality instruments have greater stationary phase retention. The settling time is a property of the solvent system and the sample matrix, both of which. When GC or HPLC is done with large volumes, resolution is lost due to issues with surface-to-volume ratios and flow dynamics; this is avoided when both phases are liquid.

CCC can be thought of as occurring in three stages: mixing, settling, and separation (although they often occur continuously). Mixing of the phases is necessary so that the greatly influence stationary phase retention

Chiral chromatography

Chiral chromatography involves the separation of stereoisomers. In the case of enantiomers, these have no chemical or physical differences apart from being three dimensional mirror images. Conventional chromatography or other separation processes are incapable of separating them. To enable chiral separations to take place, either the mobile phase or the stationary phase must themselves be made chiral, giving differing affinities between the analytes. Chiral chromatography HPLC columns (with a chiral stationary phase) in both normal and reversed phase are commercially available.

CONCLUSIONS

In any chemical or bioprocessing industry, the need to separate and purify a product from a complex mixture is a necessary and important step in the production line. Today, there exists a wide market of methods in which industries can accomplish these goals. Chromatography is a very special separation process for a multitude of reasons! First of all, it can separate complex mixtures with great precision. Even very similar components, such as proteins that may only vary by a single amino acid, can be separated with chromatography. In fact, chromatography can purify basically any soluble or volatile substance if the right adsorbent material, carrier fluid, and operating conditions are employed. Second, chromatography can be used to separate delicate products since the conditions under which it is performed are not typically severe. For these reasons, chromatography is quite well suited to a variety of uses in the field of biotechnology, such as separating mixtures of proteins

REFERENCES

1. http://en.wikipedia.org/wiki/Chromatography

2. http://www.rpi.edu/dept/chem-eng/Biotech-Environ/CHROMO/chromintro.html

3.http://teaching.shu.ac.uk/hwb/chemistry/tutorials/chrom/chrom1.htm