Appendix Chromatography Lexicon

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APPENDIX

CHROMATOGRAPHY LEXICON -AAbsolute Retention Time. This name is sometimes used for the time elapsed between the injection of the sample and the appearance of the maximum concentration of the band of the compound. This is more often and properly called the Retention Time. In some cases, it is defined as the mass center or first moment of the concentration distribution. The two definitions are different for non-symmetrical peaks. See p. 13. Symbol: t,. Absolute Retention Volume. The net retention volume. This is NOT the volume of gas corresponding to the absolute retention time. For this reason the two terms should be avoided. See p. 13. Symbol: V,. Activation of Adsorbent. Many adsorbents used in gas chromatography adsorb readily water and, possibly, some laboratory pollutants during storage. The adsorbent surface is then much less polar and the adsorption energy is considerably decreased. The product gives small retention volumes and exhibits a low degree of selectivity. Furthermore, because the carrier gas is usually dry, the sorbates are slowly desorbed and the column retention properties drift, preventing the achievement of reproducible results. Before using it in gas-solid chromatography, it is necessary to eliminate these sorbates from the adsorbent under dry atmosphere or vacuum. This is especially true with molecular sieves, silica gel and alumina. Adjusted Retention Volume (Time). Retention volume (time) less the dead volume (time). The adjusted retention time is the time spent in the stationary phase by a retained compound. It is more properly called the true retention volume (time). See p. 13. Symbols: V i (t;). Adsorbents. Materials, usually exhibiting a large specific surface area, which adsorb organic vapors more strongly than the carrier gas. The value of the adsorption constant depends on the structure, molecular weight, polarizability and dipole moment of the vapor, so it differs from compound to compound, making separation possible. Adsorbents most useful in GC are: graphitized carbon black, silica gel, zeolites (molecular sieves), alumina, porous polymers and activated carbons. See p. 182.

Adsorption. Physico-chemical process by which there is a difference in concentration at equilibrium in a bulk phase, gas or liquid, and at the interface between this phase and another one. There can be adsorption at a gas-solid, a liquid-solid or a

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gas-liquid interface. In all cases, such an adsorption results in a contribution to the retention of a compound. This is the essential or only contribution in gas-sdid chromatography, but *adsorptionat either the gas-liquid or the liquid-solid interface, or at both, may often constitute an important contribution to retention in gas-liquid chromatography. See pp. 77-82. Adsorption Chromatography. An implementation of chromatography using an adsorbent as stationary phase. Also called Gas-Solid Chromatography. See p. 77. Adrorption Coefficient.Henry’s law coefficient of proportionality between the amount of vapor sorbed on an adsorbent and the partial pressure of the compound in the gas phase. See p. 77. Symbol: K or KH. Adsorption Isotherm. The composition of a gas-solid system at equilibrium is a function of the amount of material involved. There is a relationship between the partial pressure of the vapor in the gas phase and the surface coverage of the adsorbent by the sorbed molecules. This relationship, which is not linear, is called the distribution or adsorption isotherm. In all cases, when the partial pressure of the vapor becomes close to its vapor pressure, the amount sorbed increases indefinitely, by a process known as capillary condensation, where the small pores fill up with liquid. At very low partial pressures, the curvature of the isotherm can be either positive (towards the surface coverage axis) or negative (towards the pressure axis). In the first case the amount adsorbed increases faster than the partial pressure and the chromatographic peak will start to front or lead ( t R increases with increasing sample size) when the column is overloaded. In the second case the amount adsorbed increases more slowly than the partial pressure and the peak starts to tail ( t R decreases with increasing sample size). The most classical isotherm is the Langmuir isotherm, with a negative curvature. See Chapter 5. Aerogels. Many of the adsorbents used in GC are prepared in the liquid phase, from a dispersed gel system. If the solvent contained in the gel can be removed without significant shrinkage of the gel matrix and the dry structure does not collapse, the product is called an aerogel. This is the case with silica gels and with the glass gels obtained by alkaline etching of borosilicate glasses, followed by a heat treatment, which produces regular-sized pores of somewhat controllable dimensions. Gels which shrink or collapse upon removal of the dispersing agent are called “xerogels”. Air Retention Time. Obsolete term for gas hold-up time. See p. 13. Alkali Metal Flame Ionization Detector. See Thermoionic Detector. Alumina. Precipitated alumina (from aluminum salt solutions) can be dried to give a porous adsorbent with a large specific surface area (ca 200 m2/g). The adsorption energy depends much on the amount of residual water (measured by the degree of activation in the Brockman scale; the grade I is the most active and the driest). By

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contrast with silica gel, whose surface is mildly acidic, the surface of alumina is basic. Hence it presents a different selectivity towards polar compounds, especially those carrying oxygen and/or nitrogen atoms. Apiezon. Originally prepared as vacuum greases, these products are supposed to be high molecular weight saturated hydrocarbons, obtained as residues of molecular distillation of heavy petroleum cuts. They are markedly more polar as a stationary phase than squalane, and contain significant amounts of polar products (oxidation products?) which can be removed by simple liquid chromatography on Florid. The most popular is L; M and N have also been used. Their maximum temperature of use is around 250°C. Argon Ionization Detector. This detector was described by Lovelock in 1958. It is more sensitive than the flame ionization detector and has a shorter time constant. It is, however, more sensitive to pollution and has a lower dynamic linear range. The principle of the detector uses the reaction of organic vapors with excited, metastable argon atoms, leading to the ionization of these vapors, with the formation of electrons which are collected. The current obtained is a measure of the mass flow of compounds into the detector. The metastable argon atoms are formed by collision of the argon atoms with accelerated secondary electrons produced by irradiation of the gas contained in the detector cell by the P-rays emitted by a radioactive foil (wSr, 63Ni, 3H) placed on the detector wall. He, Ne, Kr, H,, N,, O,, CO, CO,, CH,, halogens, and fluorocarbons, all of which have an ionization potential larger than the excitation potential of argon, give no response. The detector is quenched by water. See pp. 472-476. Asymmetry. Ratio of the front and the tail half-widths of the peak. Sometimes the ratio of the front and tail widths at a fractional height, such as 0.10. See p. 19. Symbol: As. Auerage Currier Gas Velocity. Column length divided by the gas hold-up time of the column. See p. 41. Symbol: ii. Axial Molecular Diffurion. Mass transfer process by diffusion along the axis of the column. It results from the very existence of a compound band, which creates a concentration gradient parallel to the column axis. Molecular diffusion proceeds after Fick’s law and produces a band spreading effect which increases with increasing time spent in the column. See p. 96. Symbol in the HETP equation: B.

-BBackflush. A technique for the GC analysis of complex mixtures or mixtures containing heavy compounds of little specific interest. After the separation of the light components has been performed and their elution achieved, the flow of carrier

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gas is reversed and the heavy components are eluted from the column inlet. Provided backflush lasts longer than direct flow, the heavy, but still volatile, components do not accumulate in the column. Alternatively, a precolumn can be used and backflushed while the analysis is performed on the main column. A significant time saving is achieved. In some cases a quantitative estimate of the total amount of the heavy components can be obtained from the area of the composite backflushed zone. See pp. 344-346. Backpurging. A technique for the routine quantitative analysis of complex mixtures containing heavy compounds considered as of no analytical interest. Differs from backflushing in that the compounds are vented without being detected. See pp. 341-343 and 352. Band. A zone of mobile phase containing a compound. It is often used synonymously with peak or zone. Usually peak conveys an idea of a symmetrical or quasi-symmetricalband, i.e. rather narrow, well behaved. A band or zone will often be highly unsymmetrical. Band Broadening. A process which takes place under the combined influence of axial diffusion and radial resistance to mass transfer, and which leads to the elution of zones which are markedly wider than the injection band of the sample. See Chapter 4. Band Width. See peak width. Measured at half-height or on the base line. See pp. 16-20. Symbol: w . Base Line. Signal of the detector when no compound is eluted of the column. This represents the detector background signal, offset to place the base line on the zero of the recorder chart. Measurements of peak height or area are done with respect to the base line. Ideally it should be an almost straight line with minor tremors, showing the background noise. The instabilities of the base line are drifts and noise. Base Line Drift. Any low frequency change in the detector signal. It often arises from flow rate changes, sometimes associated with temperature drift of the column oven. Also attributed to column bleeding, in temperature programming analysis. May be due to the elution of large amounts of very strongly retained material injected long before the current analysis was started. See pp. 639-644. Bleeding. Loss of stationary phase. It can occur either by decomposition of the phase or by evaporation in the carrier gas stream. As the speed of these two phenomena increases rapidly with increasing temperature there is a maximum temperature above which the stationary phase should not be operated. This temperature depends on the amount of phase in the column and on the nature of the solid support used. Bleeding results in (i) a progressive decrease of the retention volumes, (ii) an increased background current, which becomes temperature-dependent, and

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an increased base line noise, (iii) a marked drift following an exponential rise in temperature programming and (iv) sometimes a significant decrease in column efficiency. When silicone phases are used, bleeding results in the deposition of silica flakes on the burner of flame ionization detectors, which creates noise and may alter the response. See pp. 694-695. Bonded Stationary Phases. A new type of stationary phases, used mostly in HPLC, but employed also in GC. They derive from silica or glass adsorbents, by reaction of the surface with a substituted chlorosilane. HC1 is eliminated and the rest of the silane molecule is bonded to the surface. A large variety of silanes have been used, leading to the fixation on the silica surface of methyl-, butyl-, octyl-, dodecyl-, octadecyl-, aminopropyl-, phenylalkyl-, cyanopropyl-, alkyldiol-, etc. These phases have properties intermediate between those of coated liquids (in thin films) and regular adsorbents. Brockman Scale. A series of six dyes for which adsorption on alumina (i.e. retention volumes) increases in a given order. Adsorption is a function of the water content of the adsorbent. Various samples of alumina can be compared as potential stationary phases for a separation by the retention data (i.e. R , in TLC) of these dyes, or by the determination of which dyes stay on top of the column, which ones move to the bottom, and which ones are eluted rapidly. Unfortunately, the original Brockman scale uses carbon tetrachloride as an eluent and is dangerous to use. Bulk Property Detector. Any detector which measures the change in a physical property of the mobile phase when a compound is eluted. The thermal conductivity detector, spectrophotometric detectors, etc., are bulk property detectors. A differential method must be used, to determine small changes of the corresponding property. Accordingly, these detectors are sensitive to drifts of any parameter, such as temperature, which may change the value of the measured property. They tend to be less sensitive than solute property detectors. See pp. 397-411. By-puss Injector. A type of injector using a gas chamber which can be isolated from the main stream of carrier gas, to be filled with the sample, and then, by actuating valves, can be placed in the main stream of mobile phase.

-CCapacity Factor or capacity ratio of the column. Ratio of the times spent by the compound in the stationary and the mobile phase. The most convenient parameter to characterize the retention. See pp. 15 and 57-70. Symbol: k'. Capillary Column. See Open Tubular Column. The name is improper, because tubes of any diameter, large or small can be used to carry out GC separations. See Chapter 8.

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Carbowax. Trade name for polyethyleneglycols. The number associated with the name is related to the average molecular weight. These compounds are rather polar and well suited to the analysis of compounds which have several oxygen functions and a rather small saturated chain or skeleton. They are usually terminated by OH groups, which adds to their polarity and decreases their thermal stability. Kovats has described a process of methyl termination. Carrier Gas. The mobile phase in Gas Chromatography. Catharometer (also written Katharometer, notably in German). See Thermal Conductivity Detector. Celite. Diatomaceous earth, used for the preparation of supports for GC. The diatomite is fused with a small amount of a flux (sodium carbonate), at 900°C. Depending on the conditions of the treatment, it is white or pink. The material has a large porosity but a small specific surface area (3-8 mz/g). So it can hold a large amount of liquid phase if needed, but does not retain most solutes by adsorption. Chromathermography. A chromatographic process using a temperature gradient moving slowly along the column and obtained by moving an oven along the column at a constant speed. This is a form of displacement gas chromatography, in which the zones stabilize at the temperature at which their migration rate is equal to the speed of the oven. Although zones tend to be narrow and spreading is limited, the resolution power obtained by this method is not very large. This method seems to be inferior to temperature programming in nearly all cases. Chromatogram. Plot of the detector signal versus time. It is usually supplied by a recorder, sometimes by a computer. Chromatograph. The instrument used to separate substances by chromatography. Chromatography. A separation process discovered by Tswett. Separates the components of a mixture based on the difference between their equilibrium constants between a stationary phase and a mobile phase which percolates across the bed of stationary phase. The stationary phase is either an adsorbent or a liquid spread over an inert support. The mobile phase is a fluid, gas, high density gas or supercritical fluid or liquid. Chromosorb. A popular trade name for (a) supports for gas chromatography (packed columns); (b) porous polymers, used as adsorbents for the analysis of light polar molecules. The supports are designated by letters: A, G, P, T and W, or by a number, and come in different mesh sizes. A is similar to P, but is supposed to have a larger porosity and be able to carry a large coating ratio. G is a hard diatomaceous earth with a low specific surface area and a low porosity, supposed to have a relatively inert surface and carry low coating ratios. P is Celite treated with an

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alkaline flux at high temperature, to reduce the specific surface area and the chemical activity; it is pinkish. T is a screened Teflon powder, for use as a support for the analysis of extremely polar or aggressive gases. It is difficult to pack properly. W is a white diatomaceous earth (“Celite”). Chromosorb A, G, P and W are also available acid washed, or silanized, to increase their chemical inertness. The porous polymers are designated by a number (101 to 108), depending on their composition. Most are crosslinked polystyrenes, some contain polar monomers such as acrylonitrile or acrylic esters. Column. Chromatography is carried out by percolating a mobile phase through a bed of stationary phase. In gas chromatography the bed must be contained in a leakproof tube, so the gas phase cannot escape. In gas chromatography there are three different kinds of columns: the traditional packed column, the capillary or open tubular column, and the wall coated or support coated or porous layer open tubular column. See Open Tubular Column (OTC), Packed Column (PC) and Support Coated Open Tubular Columns (SCOT). See Chapters 6-8. Column Performance. Number of theoretical plates of the column. See p. 18 and Chapter 4. Column Switching. Procedures permitting the use of several columns to perform the analysis of a difficult or complex mixture. Often used for routine trace analysis. Preferred to temperature programming in process control analysis and in many cases in routine laboratory analysis. Today called multidimensional chromatography. See pp. 340-384. Compressibility Correction Factor. Corrective factor derived by James and Martin, to correct retention volumes, flow rate and flow velocity for the effect of the compressibility of gases and of the pressure drop. Because of the viscosity of gases, it is necessary to apply a certain pressure at column inlet, and a given mass of gas there occupies a smaller volume than at column exit. The effect of the passage of this mass on the migration of a band depends on the exact position of the band. See p. 41. Symbol: j. Concentration Sensitive Detector. A type of detector the response of which is proportional to the concentration of analyte in the eluent. See pp. 397-401. Continuous Chromatography. Chromatography is usually a batch separation process. A certain amount of mixture is injected in the column and, after a certain time,

purified fractions are collected. Another batch can then be processed. A number of attempts, not entirely successful so far, have been made to run preparative chromatography in a continuous mode. The column must be moved, either by moving the packing material in the direction opposite to the carrier gas, or by rotating the column. Considerable technological difficulties have been met in both approaches and have not yet been fully solved.

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Corrected Retention Volume (Time). The retention volume (time) corrected for the effect of the compressibility of the carrier gas. See p. 14. Symbol: OR). Correction Factor. (i) See Compressibility Correction Factor. (ii) Also means the coefficient which is used to correct peak areas for the difference between the response factor of the detector for different compounds (see Response Factor). Coulometric Detector. A selective detector principle used in gas chromatography. The column effluent is heated at 800 O C and reacted so that compounds containing nitrogen, sulfur and halogens give ammonia, sulfur dioxide and halohydric acids, respectively. These gases are measured coulometrically in an electrolytic cell were they are absorbed. The eluites can also be oxidized to carbon dioxide which is measured in a coulometer. The detector is then non-selective. This last implementation has been less successful than the first one. Craig Machine. An automatic separation machine using a large number of interconnected tubes, for liquid-liquid extraction. The separation depends on the values of the distribution constants of the different components of the mixture between the two immiscible liquids. There is a superficial analogy between this process and chromatography, which is often used to explain the concept of theoretical plates. See pp. 7-10. Cross-section Detector. The first type of ionization detector described. The column eluent flows through a cell containing a P-ray radioactive source. A potential is applied across the cell and a current is collected which is a function of the ionization cross-section of the gas contained in the cell. When a gas or a vapor is eluted from the column, the ionization current changes. The sensitivity is modest, the detection limit being one or two orders of magnitude larger than for the TCD. The principle was abandoned very early on. Cut and Weigh. A procedure for the determination of the peak area in quantitative analysis. The peaks of each component of the mixture on the chromatogram are cut with scissors and the pieces of paper are weighed, together with a square of paper having a known side length. Errors arise from incorrect drawing of the base line, incorrect cutting of the peaks and lack of homogeneity of the paper. The method is very economical in terms of investment and very costly in manpower. See p. 634. Cutting. A switching valve technique which permits a large reduction of the analysis time by shortening the column and letting the heavy components elute from a short column section while the important components have been separated on a long column. See pp. 347-349.

-DDeactioation. A treatment of the stationary phase support and possibly of the column wall and gas lines, to remove active adsorption sites having a strong

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adsorption energy and possibly a slow desorption kinetics and being responsible for band tailing or even for the partial or total loss of some sample components. Deactivation is mainly carried out by coating with nonvolatile strongly polar compounds (e.g., detergents) or with appropriate polymers, by chemical treatment of the surface (e.g., washing with an acid or even with aqua regia), by silanization or by using a polar vapor mixed with the carrier gas (e.g., steam or ammonia). Dead Volume (Time). The retention volume (time) of an inert, i.e. non-retained, compound (often improperly called ‘air’). Also called the gas hold-up of the column, the retention volume of an inert or a non-retained compound or of ‘air’. See p. 13. Symbol: t , or to. Deferred Standard. Pure compound or, rarely, a simple mixture injected independently from the sample, using a different valve and at a different time, so the peaks of the deferred standard do not interfere with those of the sample. Provides a continuous check of the reliability of the chromatograph and permits easy calibration and increased accuracy. See pp. 703-718. Deriuatization. Formation by a suitable chemical reaction of a selective derivative which is less polar or more volatile and thus is more readily amenable to gas chromatographic analysis. E.g., transformation of fatty acids into their methyl esters or of sugars into their per(trimethylsily1)ethers. Detection Limit. The smallest amount or concentration of a compound which may be detected with a given detector or using a given analytical procedure. It is typically taken as the amount of product giving a signal three times as high as the background noise. This does not guarantee detection of the corresponding compound at this concentration every time. A ratio of five at least is necessary. See pp. 402-405. Detector. A device which monitors the composition of the column eluate, by measuring a property of the carrier gas or of the eluates or analytes. See Chapter 10. Diatomaceous Earth. The most common starting material for the preparation of liquid phase support in gas chromatography. Also known as Celite, Kieselguhr, or under commercial names such as Chromosorb. They are fossiles, remains of microscopic, single-cell microorganisms. They are formed mainly of silica and contain various metal oxides in small amounts. See pp. 181-193. Differential Detector. Detector which measures the change in property of the eluent as a function of time. It gives a signal proportional to the concentration or mass flow of analyte. The total amount of material eluted from the column is related to the integral of the signal during the elution. See p. 397. Diffusion. A process of slow spatial drift of molecules, due to their constant, random motions, known as Brownian motion. Molecules drift away from their original

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position in space. As a statistical result, there will be a net drift from the regions where the concentration is large to those where it is small. Diffusion is governed by Fick's law which states that the diffusive flux or net drift (number of molecules. crossing a surface area in unit time) is proportional to the concentration gradient. The proportionality coefficient is the diffusion coefficient. See p. 95. Symbol: Dg or 4 . Unit: cm2/sec. Displacement. A form of chromatography in which the mobile phase is replaced just after injection of the sample by a fluid more strongly retained than the last component of the sample to be eluted. There has been very little application of this method in gas chromatography, probably under the influence of the erroneous belief that the displacer must be much more strongly sorbed or solved than the most retained compound of the analyzed mixture. It just needs to be somewhat more retained. An a value of 1.2 is certainly large enough. See p. 7. Distribution Coefficient. See Partition Coefficient. Drift. See Base Line Drift. Dynamic Linear Range. Ratio of the largest amount of a component for which the detector response remains linear (change in the response factor smaller than 5%), to the detection limit. See p. 407.

-EEddy Dif@ion. Contribution to band broadening due to the unevenness of the flow velocity distribution around the packing particles in a packed column. Molecules travel along the column following paths of different lengths, at different velocities. This results in a contribution to the variance of their residence time in the column, which is independent of the nature of the compound and its retention, but depends on the particle size and size distribution, and probably on the packing quality, although this last factor has never been properly elucidated. See pp. 97-100. Effective Peak Number. Number of peaks with a resolution of unity that can be placed between two successive n-alkanes. Characterizes the separation power of a column. See p. 26. Symbol: EPN. Effective Plate Number. Number of theoretical plates of a column calculated with the adjusted or true retention time, instead of the absolute retention time. It is mainly used with open tubular columns. See p. 18. Symbol: Neff. Efficiency of a column. Ability of a column to easily separate a complex mixture or a group of closely related compounds. It is measured by the number of theoretical

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plates, the effective plate number, the peak capacity or the separation number. See p. 18 and Chapter 4. Symbol for the plate number: N . Electrolytic Conductivity Detector. (Also Coulson or conductivity detector.) A detector in which the eluite is oxidized in an excess of oxygen at column outlet, the eluate is extracted with water and the conductivity of the solution measured continuously. The detector responds to halogens, nitrogen and sulfur. It has been used for pesticide analysis. Electron Capture Detector. Electrons from a radioactive source are used to ionize the carrier gas and form very low energy electrons in the detector cell. Some compounds with high electron affinity (halogen derivatives, highly conjugated molecules, etc.) can capture these electrons and give negative ions which are much less mobile than electrons and react much faster with positive ions. The charge carriers in the detector cell are collected periodically (10 to 100 kHz). The current obtained decreases with increasing concentration in the eluent of compounds which have some electron affinity. It is a measure of the concentration of these compounds in the sample. See pp. 447-457. Electronic Integrator. Electronic device which provides the area of the peaks recorded during a chromatographic analysis. It replaced the manual methods of integration and the mechanical integrators during the late 'sixties. Modern instruments include a microprocessor and can achieve sophisticated tasks. Unfortunately, some of them have been programmed by computer technicians not fully aware of analytical problems. See pp. 635-638. Electron Mobility Detector. An ionization detector used mainly for the analysis of permanent gases. It is based on the variation of the mobility of thermal electrons with the composition of the gas. Eluate. The fluid at column outlet. Usually the mobile phase or a mixture of mobile phase and the vapor of the analyzed compounds. Eluent. The mobile phase in chromatography. Eluite. The vapor of analyte eluted off the column. Elution. The classical form of chromatography in which the sample is injected as a narrow plug, the different components of the sample move at different speed and are separated into a series of bands, under the influence of the flow of mobile phase. External Standard. Pure compound or calibration mixture injected from time to time, between analytical sequences of samples of the stream controlled. See p. 653.

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-FFlame Ionization Detector. Detector based on the formation of ions during the oxidation of organic compounds in an oxygen-rich hydrogen flame. These ions are collected and the current measured. The combustion of hydrogen gives very few ions, so the hydrogen flame gives a low background current with little noise. The elution of almost all organic compounds (exceptions, H,C=O, CCl,, etc.) gives a current corresponding to around 10 to 20 mC/g of carbon. The detection limit is very low. See pp. 437-447. Flame Photometric Detector. Detector based on the emission of photons during the oxidation of organosulfur and organophosphorus compounds. Two photomultipliers protected by two different filters collect the photons. This permits the selective detection of either P- or S-derivatives. The detector response is linear for phosphorus, quadratic for sulfur. See pp. 463-466. Flash Pyrolysis. A mode of sampling nonvolatile compounds for gas chromatographic analysis. See Pyrolysis Gas Chromatography. Flow Meter. A device permitting the determination of the volume flow rate of the carrier gas or the gas streams used for the detectors. The most common devices used are the soap bubble meter, which measures the rate at which a soap bubble rises in a calibrated glass tube, and the bead flow meter in which the gas stream raises a bead or cone in a slightly conical, calibrated, vertical glass tube, up to a height that is a function of the flow rate. The accuracy of these devices is poor. Flow Rate. The volume of carrier gas passing through the column per unit time. Usually it is measured at standard temperature and pressure, sometimes at column temperature and/or outlet pressure. See Chapter 2. Symbol: F. Flow Rate Programming. A technique used sometimes in gas chromatography for the elution of strongly retained compounds. The inlet pressure, hence the carrier gas flow rate is increased, either stepwise or progressively, after a first, isorheic elution period, during which most of the compounds of interest are eluted. Most bulk property detectors give a drifting signal during flow rate programming. Only mass flow detectors give a useful response. See p. 52. Frontal Analysis. A form of chromatography in which the mobile phase is suddenly replaced by a stream of a dilute solution of sample in the mobile phase. Each component breaks through the column at a time which depends on the strength of its interaction with the stationary phase. See p. 6. Frontal Ratio. Ratio of the gas hold-up time to the compound retention time (uncorrected or corrected). See p. 15. Symbol: R,.

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Fronting. The term used to characterize unsymmetrical peaks with a slow rising front and a very sharp tail, such as results from an anti-Langmuir isotherm, when the amount of analyte solved or sorbed by the stationary phase increases faster than its concentration or partial pressure in the mobile phase. The opposite is tailing (q.v.). See Chapter 5.

Gas-Adsorption Layer Chromatography, Mode of chromatography that uses as stationary phase an adsorbent modified by coating of a small amount of a low vapor pressure liquid, usually one monolayer or a fraction of a monolayer. Also called modified gas-solid chromatography or modified adsorption chromatography. See Chapter 7. Gas Density Balance. A detector measuring the difference between the density of the pure carrier gas and that of the column eluent. This is the only detector for which relative response factors of two compounds can be calculated exactly from first principles. See pp. 411-423. Gas Hold-up Time (Volume). Retention time (volume) of an inert or non-retained compound on a chromatographic column. In GC the gas hold-up volume is practically equal to the volume of the column available to the gas phase. Symbol: t , or to. See p. 13. Gas-Liquid Chromatography. The form of chromatography that uses a gas as mobile phase and a non-volatile liquid as stationary phase. Gas-Solid Chromatography. The form of chromatography that uses a gas as mobile phase and an adsorbent as stationary phase. GC, GLC, GSC. Gas chromatography, gas-liquid chromatography, gas-solid chromatography. GC-MS. An analytical instrument and the techniques which make use of it. It is the combination of a gas chromatograph, separating the components of a mixture, and a mass spectrometer, analyzing the column eluate and generating mass spectra for the compounds resolved. See pp. 543-557. Glass Beads. A non-porous support for the liquid phase in GLC. The coating ratio must be small enough to prevent excessive accumulation of the liquid in pools around the contact points between beads. The beads are preferably etched to hold the liquid phase in surface pores. Golay Column. See Open Tubular Column. Golay Equation. T h eplate height expression for open tubular columns.

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-HHeadspace Analysis. An analytical procedure for the volatile compounds contained in a complex matrix, such as body fluids, foods or beverages. It consists in analyzing an aliquot of the gas phase in equilibrium with the sample contained in a closed vessel. It avoids rapid pollution and destruction of the column by the repetitive injection of nonvolatile materials contained in these samples. It requires less experimental work than solvent extraction and involves a lower risk of pollution. Calibration is a critical problem, because the solubility of the analytes in the matrix is often very different from that in pure water. Heartcutting. A technique for the routine quantitative analysis of complex mixtures. A fraction of the eluate containing the compounds of interest, usually trace components, is trapped and reinjected in another column on which the components can be separated and quantitized. Also used to eliminate the band of a major component. See pp. 346-350 and 355-360. Height Equivalent to a Theoretical Plate. A measure of column efficiency. Value obtained by dividing the column length by the number of theoretical plates. See p. 19 and Chapter 4. Symbol: HETP or H. Helium Detector. An argon (or Lovelock) ionization detector working with helium as carrier gas. Since metastable helium has a higher energy than the ionization potential of all molecules, except He and Ne, it permits the sensitive detection of all gases and vapors. It is exceedingly sensitive to pollution. See pp. 472-477. HETP. See Height Equivalent to a Theoretical Plate. Hold-up Time (Volume). See Gas Hold-up Time (Volume). Hot Wire Detector. See Thermal Conductivity Detector.

-1Inert Compound Retention Time. See Gas Hold-up Time. Injection Port. The device through which the sample is injected into the carrier gas stream with a syringe. It is closed with a septum, held tighly by a metal nut, and heated at the appropriate temperature to permit rapid vaporization of the analytes. Inlet Splitter. See Splitter. Inlet System. See Sampling System.

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Integral Detector. A detector which gives a signal proportional to the accumulation of sample mass eluted through the column. Total condensation of the analytes on a piezoelectric quartz crystal gives an integral detector. Interface. Device placed between a gas chromatograph and another analytical instrument, such as a mass spectrometer or an infrared spectrophotometer. The purpose of the interface is to make the flow rate, pressure and temperature of the gas stream compatible with proper operation of the spectrometer. Internal Standard. Compound added in a known amount to an aliquot of the. mixture to be analyzed. After proper calibration, the quantitative composition of the mixture is derived from the peak area ratio of the compounds of interest and the internal standard. The compound(s) used as internal standard should be chemically similar to those analyzed, eluted rather closely but must be well resolved from all other components. See p. 653. Interstitial Volume. Part of the column volume which is available to the gas phase. Also called dead volume or gas hold-up. It contains the interparticle and intraparticle porous volumes. Ionization Detector. Any detector which transforms the analyte into ions which are collected and counted. Isorheic. Mode of operation of a GC column in which the gas flow rate is kept constant. The term is little used because flow rate programming has not proven to be a useful technique. In truth, the carrier gas flow velocity changes in temperature programming, which is not an isorheic mode of GC. Isothermal Chromatography. The mode of elution of a gas chromatographic column in which the column temperature is kept constant. Opposite: temperature programming (4.v.).

-JJames and Martin Correction Factor. Correction factor relating the outlet carrier gas velocity to the average velocity and the apparent retention volume to the corrected retention volume. Takes into account the influence of the column pressure drop and the ideal gas compressibility. See pp. 41-44.

-KKatharometer. See Thermal Conductivity Detector. Kieselguhr. A brand of diatomaceous earth used to prepare inert support for gas-liquid chromatography. See Diatomaceous Earth and Chromosorb.

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Kouats Index. Retention index. The normal alkanes are used to establish a scale of retention. See pp. 486-490 and 508-515. Symbol: RZ or I . -L-

Lungmuir Isotherm. The most simple isotherm of adsorption, resulting from the finite surface area of the adsorbent. It assumes the formation of a monolayer of sorbate on the surface. The rate of desorption is proportional to the fraction of the surface area occupied by the sorbate molecules. The rate of adsorption is proportional to the partial pressure of the sorbate and to the fraction of the surface area which is free. As a result the amount sorbed is related to the partial pressure by: m = aP/(l+ bP). Leading. See Fronting. Limit of Detection. See Detection Limit. Linear Gas Velocity. Parameter related to the carrier gas flow rate. Usually the flow rate measured at column outlet divided by the surface area of the column cross section available to the gas phase (outlet carrier gas velocity). Also the ratio of column length to the dead retention time (average carrier gas velocity). These two velocities are related by the compressibility correction factor. See p. 12 and Chapter 2. Symbols: uo and u. Linearity. (i) Situation in which the equilibrium isotherm between the mobile and stationary phases is linear (linear chromatography). (ii) Behavior of a detector whose response is proportional to the concentration of compound in the mobile phase (linear detector). See Dynamic Linear Range. Linear Range. Amount of sample (sometimes concentration) above which the detector response is not linear but exhibits the onset of saturation. Often taken as the amount (concentration) at which the response is 58 lower than extrapolated for a linear response. see p. 405. Liquid Loading. See also Phase Ratio. Amount of stationary phase contained on 100 g of packing material (in gas-liquid chromatography, or gas-adsorption layer chromatography). See pp. 195-199. Liquid Phase. The high-boiling or thermostable liquid used to impregnate a porous support and which is used as solvent or stationary phase in gas chromatography. Logarithmic Dilution Method. A calibration procedure using a special device composed of a flask with a fast agitator and a flow rate controller. A known amount of a volatile compound is injected in the flask, while a constant flow rate stream of gas

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is used to constantly dilute the vapor. The concentration of the vapor in the gas effluent decreases exponentially with increasing time. The time constant of the exponential decay is the ratio of the flask volume to the gas stream flow rate. See pp. 591-595. Lovelock Detector. See Argon Ionization Detector. Lower Detection Limit. See Detection Limit.

-M-

Muss Sensitive Detector. A type of detector the response of which is proportional to the mass flow rate of analyte to the detector cell. See p. 399.

Muss Transfer Resistance. Term of the plate height equation accounting for the kinetics of exchange between the mobile and the stationary phases. It contains several contributions, ascribed to the radial diffusion in the mobile phase (between the center of gas streamlets and the particule surface), diffusion in the stagnant mobile phase contained inside the particles, the kinetics of adsorption-desorption (GSC) or the diffusion across the droplets of stationary liquid phase (GLC). See pp. 97-102. Matrix. Usually the whole bulk of the sample, when one or a few components are analyzed. McReynolds Constants. The set of differences between the retention indices of a selection of probe solutes on a certain stationary phase and on squalane. These constants characterize the selectivity of the phase for compounds of a certain type. They permit the calculation of the retention indices of other compounds on the corresponding stationary phase. See p. 518. Mesh. The dimension of the particles of support or adsorbent used to pack a column. Refers to the size of the screen mesh used to sort the particles. Methyl Silicone. Silicone polymers of high molecular weight, used as general purpose stationary phases in GLC. They have a low polarity and are very stable. Mobile Phase. The camer gas in gas chromatography. Molecular Dijfwion Term. See Axial Molecular Diffusion. Molecular Sieves. Synthetic silico-aluminate crystals, similar to the natural mineralogical compound zeolites, which incorporate water molecules in narrow, communicating channels. The water molecules may be eliminated by heating under vacuum, without modifying the crystal network. The specific surface area is very large. These adsorbents have an extremely large adsorption capacity for the molecules which are

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small enough to penetate inside the channels, a very small one for the molecule: which are too large or bulky. Product used in GSC to separate argon and oxygen. krypton and nitrogen. Multidimensional Chromatography. Term improperly used for various implementations of Column Switching (q.v.) using columns made with different stationary phases.

-NNet Retention Time. The retention time corrected for pressure drop along the column and for the gas hold-up of the column. Adjusted retention time corrected for pressure drop. Corrected retention time less the dead time. See p. 14. Symbol: t N . Net Refention Volume. The retention volume corrected for pressure drop along the column and for the gas hold-up of the column. See p. 14. Symbol: VN. Noise. High frequency fluctuations of the base line signal of the detector.

-0Open Tubular Column. A chromatographic column made with an empty tube, whose walls are coated by a layer of stationary phase. Usually the tube has a narrow diameter, hence these columns are often called capillary columns. See Chapter 8. Symbol: OTC.

-PPacked Columns. Long tube packed with an Adsorbent or a Solid Support coated with a liquid phase and used as a Chromatographic column. See Chapters 6 and 7. Packing. Material contained by the column. It is responsible for the retention. Partition. Equilibrium between a solvent and a gas phase. Responsible for the retention in gas-liquid chromatography. See Chapter 3. Partition Coefficient. Equilibrium constant in a partition phenomenon. Usually the ratio of the concentration at equilibrium in the gas and stationary phases. See p. 60. Symbol: KR or KR,. Peak. Concentration profile of a compound at column outlet. Trace of the detector signal on a recorder chart during the elution of one compound. Synonymous with band and zone, but often implies a better behavior. Peak Area. Area enclosed between the peak profile and the base line on the recorder trace. More generally integral of the difference between the detector signal and the

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interpolation of the base line. See Chapter 15. Symbol: A. Peak Base. Sometimes used to designate the interpolation of the base line under the peak, between its extremities. Peak Capacity. Number of peaks with a resolution of one which can be placed between two peaks or which can be eluted in a given range of capacity ratio (between k; and k;, usually between 0 and 7.4 or between two successive n-alkanes). Symbol: TZ. Peak Height. The maximum difference between the detector signal and the background during the elution of a peak. Distance between the maximum of the peak and its base, measured parallel to the signal axis. See p. 27. Symbol: h. Peak Leading. Deviation from symmetry characterized by a rise from base line slower than the return to base line. Usually due to column overloading. Peak Symmetry. Characterizes the shape of the profile and the deviation of the elution behavior from a totally linear one. Deviation from symmetry may originate in mixed retention mechanism, with slow kinetics, in column overloading or in poor injection technique. The asymmetry, or ratio of the peak front and tail half width, is often used as a measure. See p. 19. Peak Tailing. Deviation from symmetry characterized by a return to base line slower than the peak rise. Peak Width. The segment of the peak base which is intercepted by its two inflexion tangents. See p. 18. Symbol: w . Peak Width at Half-Height. The distance intersected by the peak profile on a line parallel to base line and bisecting the peak height. Symbol: w ~ , ~ . Phase Ratio. The ratio of the volumes available to the mobile and stationary phases. The gas hold-up corrected for gas compressibility divided by the volume of liquid in the column. Phenyl Silicones. A very popular type of stationary phases for the analysis of compounds with very low vapor pressure. They are stable above 300OC. They include many commercial products, among others the SE‘s, OV’s and DC‘s silicone oils, greases and rubber. The ratio of phenyl to methyl groups varies from 0 to ca 25%.

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Photoionization Detector. An ionization detector using irradiation by a beam of high energy, far UV photons to ionize the eluate vapor. See pp. 466-471. Photometric Detector. See Flame Photometric Detector. Planimeter. A device used to measure the surface areas enclosed by irregularly shaped curves. Used long ago to measure the peak areas of chromatograms. Slow, tedious and not very accurate. See p. 635. Plate Height. Length of the column corresponding to one theoretical plate. Height equivalent to a theoretical plate. See Chapter 4. Symbol: H. Plate Theory. A theory of chromatography which describes the separation as a step by step process involving successive equilibria between mobile and stationary phases in an imaginary series of containers. PLOT or Porous tayer Open Tubular Column. Variety of open tubular columns where the stationary phase is a layer of adsorbent coated on the inner wall or a layer of support impregnated with liquid phase. See pp. 278-279. Polarity. A loose term employed to characterize the electronic properties of molecules. A molecule which has a strong dipole moment is polar. Often molecules which have large quadrupole moments or even large polarizability have been characterized as being polar. A universal polarity scale of stationary phases has long been an elusive quest. See p. 521. Polyesters. A very popular type of liquid phase used for the analysis of a wide variety of mixtures. They include poly(ethyleneglyco1) adipate, succinate and sebacate, poly(neopenty1)glycol adipate and succinate. Polyglycols. A very popular type of liquid phase for the selective analysis of alcohols or other compounds capable of forming hydrogen bonds. They include Carbowaxes (polyethyleneglycols)and Ucons (polypropyleneglycols). Polystyrene Gels. Porous particles prepared by copolymerization of styrene and divinylbenzene, with the possible addition of some polar substituted derivatives. These products have a large specific surface area. Small molecules of gases or volatile compounds may penetrate the gel by diffusing between the chains. Most used are the products known under the names Chromosorb 100 and Porapak. Porupak. Reticulated polystyrene-polydivinylbenzene copolymers used as stationary phase in gas-solid chromatography. Their composition and structure are qualitatively analogous to those of the products of the Chromosorb 100 series. Pressure Drop. Difference between inlet and outlet column pressures. Symbol: 6P or p .

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Pressure Gradient. Difference between inlet and outlet column pressures divided by the column length. Differential of the pressure with respect to the abscissa. See Chapter 2. Pressure Programming. See Flow Rate Programming. Programmed Temperature Gas Chromatography. See Temperature Programming. Pyrolysis Gas Chromatography. A method of analysis of nonvolatile samples by gas chromatography, using flash pyrolysis. The nonvolatile sample (usually a polymer) is heated very rapidly to a high temperature (800 to 1,000O C). During this phase the sample degrades thermally. The result of the GC analysis of the mixture evolved can be used as a fingerprint to identify the original material.

-RRadial Diffusion. See Mass Transfer Resistance. Recorder. Apparatus which transforms the detector signal into a plot of detector signal versus time: the chromatogram. Reduced Plate Height. A dimensionless number expressing the column efficiency, as the ratio of the plate height to the average particle size of the packing. See pp. 111-113. Reduced Velocity. A dimensionless number expressing the carrier gas velocity, as the product of the actual gas velocity by the average particle size, divided by the diffusion coefficient of the solute in the carrier gas. Also known in chemical engineering as the Peclet number. See pp. 111-113. Relative Response Factor. Ratio of the detector response factors for two compounds. See p. 401. Relative Retention. Ratio of the adjusted or net retention volumes or times of two compounds, or of their capacity ratios. See pp. 20 and 65. Symbol: a. Resistance to Mass Transfer. The phenomena which cause equilibrium between two phases to proceed at a finite rate. Their kinetics control the extent to which the concentration in one phase lags behind its equilibrium value, and therefore the degree of band broadening that takes place during elution (see pp. 97-102). There are three main sources of resistance to mass transfer in a chromatographic column, usually referred to as the diffusion through the mobile phase stream, the diffusion through the stagnant mobile phase in the particles and the diffusion through the liquid phase.

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Resolution. Degree of separation between the bands of two compounds. Usually the ratio between the difference in their retention time and their average band width. See pp. 22-25. Symbol: R. Response Factor. Ratio between the amount of a certain compound and the peak area obtained. Depends on the detector and its settings. Usually referred to the amount injected. See p. 401. Response Time. Delay between the time when a concentration change (step) occurs in the detector cell and the moment the signal reaches a certain fraction of the corresponding value. Time constant of the detector response, assuming it is a first order system. See p. 408. Retention Factor. Logarithm of the retention of a compound relative to n-nonane, Rx,9.Can be calculated easily by measuring the relative retention of the compound studied to the most closely eluted n-alkane and the relative retention of two successive n-alkanes, since beyond propane there is a linear relationship between the logarithm of the corrected retention time and the number of carbon atoms. This system is much less popular than the retention indices. Retention Index. See Kovats Retention Index. A system of retention data which uses the alkanes to define a scale. See pp. 20 and 486-490. Retention Time. (i) Absolute, or uncorrected: time elapsed between injection of the sample and elution of the peak maximum. (ii) Adjusted: absolute retention time less the dead time or 'air' retention time. (iii) Corrected: absolute retention time corrected for the gas compressibility. (iv) Net: absolute retention time corrected for both the compressibility of the mobile phase and the gas hold-up of the column; also called totally corrected retention time. (v) Specific: the net retention time at standard temperature and pressure divided by the amount of liquid phase in the column or the total surface area of adsorbent. (vi) True: synonym of adjusted. (vii) Uncorrected: cf absolute. See pp. 11 and 13-15. Retention Volume. The volume of mobile phase or carrier gas which flows through the column during a time equal to the corresponding retention time. There exist accordingly absolute, adjusted, corrected, net, specific, true and uncorrected retention volumes (cf Retention Time). See pp. 13-15. Reversing. A technique of column switching similar to storing (see Storing) where there is no need for a compensation column. See p. 349.

-SSample Loop. Part of a sampling valve which is usually replaceable. Tube loop in which an aliquot of a gas sample is placed prior to its transfer to the sampling port and the carrier gas stream. See pp. 327-339.

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Sampling Port. Often synonymous with injection port. Also the volume at the top of the column where a gas sample is mixed with the carrier gas stream. Sampling System. The system permitting the selection of a volume of sample representative of the mixture to be analyzed and its transfer to the carrier gas stream, just on the top of the column. See pp. 286-300 and 327-339. Sampling Value. Two-position valve used to introduce a sample for analysis into the chromatograph. In one position, the carrier gas flows directly to the column, while a stream of sample sweeps the sampling loop. In the other position, the sample stream is sent directly to waste, while the carrier gas sweeps the loop and carries the sample to the column through the sampling port. Sampling valves are automatic (electric or pneumatic) or manual, rotary or sliding, and inject gas or liquid samples. Most have six ports. More complex ten and twelve port valves are available permitting simultaneous column switching and sampling. See pp. 327-339. SCOT or Support Coated Open Tubular Column. Synonymous with porous layer open tubular column. Selective Detector. A detector which gives a different response factor for different compounds. Since all detectors are'selective to a certain degree, the term tends to be used to qualify a detector which gives a large response for compounds belonging to some chemical classes and a very small response for the other compounds. See Chapter 10. Selectiuity. Name sometimes given to the relative retention or ratio of adjusted retention times or of capacity ratios of two compounds. See p. 400. Sensitiuity. The response factor of a detector, especially in relation to the influence of ambient parameters. Since detectors may have a large response factor in some unit, but this fact is unrelated to the intensity of the signal noise, a detector with a large sensitivity may not be very sensitive, i.e., may not detect small concentrations of analytes. There is some ambiguity in the words sensitivity and sensitive. Detection limit is a well-defined term the use of which should be preferred. See p. 401. Separation Factor. Alternative definition of the resolution. See p. 26. Separation Number. Ratio of the difference of the retention times of two compounds and their average peak widths at half height. Septum. Thin disk of a self-sealing elastomer used in the design of an injection port, on the top of which it is held tightly. Permits the injection of gas or liquid samples with a syringe. The septum keeps a leakproof seal around the syringe needle and closes behind the needle when it is withdrawn. Septa are usually made of silicone rubber, sometimes of high-temperature resistant Viton or similar material. See p. 332.

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Signal to Noise Ratio. In chromatographic data collection, usually the ratio of the signal at peak maximum to the noise range. Silica Gel: Porous gel of hydrated silica. Used as adsorbent in gas-solid chromatography. Silicone. A polymer with an -[Si-01- backbone chain and a large majoiity of CH, groups bonded to the Si atoms. All groups can be methyl, or some can be phenyl, heptafluoropropyl, cyanoethyl, etc. Usually they have a high thermal stability. They decompose by giving products which have a small response on the FID, which is why they are often unnoticed. They give strong peaks at certain characteristic masses in GC-MS. Silylation. A common method of derivatization where a hydroxyl, a primary or secondary amine or some other group with a reactive hydrogen atom is reacted with an appropriate silicon derivative, usually a chlorotrimethylsilane, another substituted chlorosilane, a substituted disilazane, etc., to give a silyl ether. The most popular derivative is the trimethylsilyl ether. These ethers are much more volatile and more stable than the parent compounds, and much easier to analyze by gas chromatography. Slurry. A thick dispersion of a GC support or adsorbent in a solvent or a solution of stationary phase or modifier. See p. 200. Solid Support. Finely divided solid material, whose particles are usually but not necessarily porous, coated by the liquid used as stationary phase.This permits rapid mass transfer between the liquid and the gas phase and prevents convective mixing of the stationary phase. See pp. 181-193. Solute. This word is very often used as a synonym for sample or analyte, even in gas-solid chromatography, where there is no solution. Solute Property Detector. Detector that does not respond to the mobile phase. Its background signal does not depend on the changes of properties of the mobile phase due to fluctuations of temperature or pressure. The signal depends only on the concentration of analytes. Such a detector is selective. Specific Retention Volume. The net retention volume at standard temperature and pressure divided by the amount of liquid phase in the column or the total surface area of adsorbent. See pp. 14 and 61-62. Symbol: Vg. Splitter. A device used to inject an extremely small sample on an open tubular column. Typical samples injected with a syringe are 1 pL in volume. A standard

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OTC accepts sample loads 2 to 3 orders of magnitude smaller. The sample is injected in a conventional injection port and the gas stream split between the OTC and a side stream which is vented through an adjustable needle valve. The trouble with these devices is that it is difficult to achieve fast vaporization, a small vaporization chamber volume and a thorough mixing of the vapors to the carrier gas. Some segregation takes place and the split ratio depends on the molecular weight of the component. Accordingly, the sample injected to the column is not an aliquot of the mixture to be analyzed. Various designs have been suggested to remedy this situation. See pp. 286-300. Squalane. A polyisoprenic saturated hydrocarbon, 2,6,10,15,19,23-hexamethyltetracosane. Boiling point 35OoC, upper limit temperature in GC 120OC. A widely known, often used stationary phase which is considered to be totally non-polar. It has been taken as the reference in most polarity scales. It is well suited for the separation of hydrocarbons and weakly polar compounds, such as halogen substituted hydrocarbons. Standard Addition. A procedure for quantitative analysis which involves the preparation of additional samples by adding known amounts of the component to be quantitized to the original mixture. If the detector is linear, no calibration is necessary. The method is applicable only to mixtures with low vapor pressure and is very tedious and time consuming. See p. 652. Standard Deviation. The standard deviation of a Gaussian curve characterizes its width. The profile of a chromatographic peak is often very close to Gaussian. See p. 16. Symbol: a. Start. The time when the sample is injected on the column. Stationary Phase. Packing material contained in the column. The active solid (adsorbent) or the support coated or impregnated by the liquid. Storing. A technique of column switching which permits keeping the bands of a number of components immobilized in a column while the rest of the mixture is eluted from another column. This permits the use of several columns and a single detector to perform an analysis. See pp. 347, 349 and 352-355. Support. See Solid Support. Support Coated Open Tubular Column (SCOT column). See PLOT or Porous Layer Open Tubular Column. Switching Value. Valve used to change the pathway of gas streams during an analysis. It permits the selective transfer of some compounds from one column to

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another, a change in the order in which the carrier gas flows through a series of columns, the transitory trapping of some compounds, etc. See p. 341. Syringe. The most popular device for the injection of samples on chromatographic columns. They are high precision instruments, with an accuracy and precision depending essentially on the skill of the analyst. They are available for the injection of gases and liquids, in a wide range of capacities. See p. 332.

-TTailing. A form of peak asymmetry in which the band front is sharp and the rear part returns slowly to the base line. Peak tailing can be ascribed to three types of causes. (i) A dead volume accessible only by diffusion in the mobile phase line, e.g. in the injection device or in the detector, or a large detector response time; (ii) a slow kinetics of mass transfer between the mobile and stationary phases; (iii) too large a sample volume, resulting in a non-linear equilibrium isotherm. See Chapter 5.

Temperature Programming. A mode of gas chromatography in which the column temperature is raised progressively during the course of the analysis. Most often the temperature is raised linearly. This permits the elution of low boiling compounds at a low temperature where they can be separated, and the elution of high boiling compounds at a high temperature where their retention is not prohibitively long. See pp. 83-88. Theoretical Plate. Imaginary element of the column characterizing the speed of equilibrium between the two phases. Distance along the column over which equilibrium takes place. Measure of the peak broadening or of the column efficiency. see p. 94. Thermal Conductivity Detector. A very popular detector based on the variation of the thermal conductivity of gases with their composition. Generally a high thermal conductivity gas such as hydrogen or helium is used as mobile phase. When organic vapors are eluted the thermal conductivity decreases. As a result the temperature of two heated resistors increases. This unbalances a Wheatstone bridge containing these resistors. The bridge potential is a measure of the concentration of eluite in the carrier gas. See pp. 423-437. Thermistor. A semiconductor, the resistance of which decreases rapidly with increasing temperature. Used in thermal conductivity detector. Thermoionic Detector. A selective detector for gas chromatography. A hydrogen flame is burned along a pellet of an alkaline metal (Rb or Cs) salt. The response is greatly enhanced for derivatives of phosphorus, nitrogen, sulfur and halogens. See pp. 457-463.

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Trap. Glass or metal device connected to the detector exit and cooled down to a proper low temperature, at which the components separated by the column can be condensed and collected at a high degree of purity. Triangulation. A procedure for the determination of the area of the peaks recorded on a chromatogram. The area is derived from the product of the peak height by its area at a certain fractional height. See pp. 631-633. True Retention Time (Volume). The time elapsed (the volume of carrier gas flowing through the column) while the solute is in the stationary phase. This is equal to the retention time (volume) corrected for the column gas hold-up (but not for the pressure drop along the column). See p. 13. Symbol: t; (V,').

-UUltrasonic Detector. A detector for gas chromatography based on the variation of the sound velocity in a gas with its composition. The detector measures the beat frequency between two ultrasonic beams, one passing through a reference cell filled with pure camer gas, the other one through a cell swept by the column eluent. In spite of interesting qualities, this detector has not been successful. Uncorrected Retention Time. Time elapsed between injection of the sample and elution of the band maximum. See p. 11. Symbol: t,.

-VVan Deemter Equation. Equation relating the plate height of a gas chromatography column to the experimental conditions. Especially designed to relate the plate height to the mobile phase flow velocity. It gives the plate height as the sum of three terms, accounting for the contributions of molecular axial diffusion, eddy diffusion and resistance to mass transfer. This last contribution is broken down into the sum of several contributions, accounting for diffusion in the mobile gas phase, in the stagnant gas phase (inside the particles) and in the liquid phase. See pp. 105-110. Van der Wads Forces. Responsible for molecular interactions between all kind of molecules. They result from the fundamental imbalance between the positive charges (protons) that are highly localized in the nucleus of the atom and the negative charges (electrons) that are distributed over all the space occupied by a molecule. This asymmetry results in an attraction between neutral species, even when there is no other electric force. Vapor Phase Chromatography. Ancient term used for gas-liquid chromatography.

768

Variance. For a Gaussian profile, this is equal to the square of the standard deviation. For any distribution, it is related to the second moment of this distribution. See p. 17. Void Volume. Dead volume, retention time of an inert compound or gas hold-up of the column. Should be corrected for gas compressibility. see p. 13.

-WWall Coated Open Tubular Column (WCOT Column). See Open Tubular Column. Watson-Biemunn Interface. An interface between a gas chromatograph and a mass spectrometer in which the eluent mass flow rate is reduced and the proper gas pressure for entering the MS source is achieved by selectively leaking the carrier gas through a sintered glass tube, terminated at both ends by a narrow capillary. Stream splitting is achieved at the same time as a 10 to 100 fold enrichment in solute vapor. See p. 552. Wheatstone Bridge. An electric circuit with two arms, supplied by a common power source. It is used in many GC detectors, especially the thermal conductivity detector and the gas density balance. When the bridge is balanced, no signal is recorded in the diagonal between the two arms. A change in one of the four resistances of the bridge results in a current passing through the diagonal. -Z-

Zeolites. See Molecular Sieves. Zone. Synonym of band.