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the column

the column may be glass or metal. If it is to be packed, the internal diameter should be 2 to 6 mm; and in a versatile apparatus it should be possible to vary the column length. Soft copper tubing may be used for metal columns but it can cause trouble in certain separations owing to the possibility of the formation on its inner surface of a reactive oxide that will catalyse changes in some reactive molecules. When this is not a danger, 'bright annealed seamless' copper tubing, 6 mm external diameter is suitable; it may be packed straight and then coiled round a mandrel about 20 cm diameter. It is unwise to coil more tightly owing to the possibility of disturbance of the packing and hence serious reduction of the column efficiency.

Stainless steel is a more popular material for metal columns owing to its lower reactivity as compared with copper. It may be packed and coiled in a similar way, but it is much tougher and does not work so easily. Packing is usually carried out in the already coiled column in the manner described on p. 225.

Glass columns are used in a similar way to stainless steel, although coiling is nearly always carried out before packing since high temperatures are required. The main disadvantage of glass is that it is easily broken, and its main advantage is its low reactivity. It is also possible to check the column packing visually; the evenness of the packing in metal columns must be taken on trust.

Column temperature control

The column operating temperature that is chosen will depend on the nature of the sample to be separated, but owing to the tendency of liquid stationary phases to 'bleed', that is, to be very slowly eluted from the column, it is wise to use the lowest temperature consistent with a good separation.

Operating temperatures of columns may lie anywhere between liquid nitrogen temperature (about --196°C) and about 500°C, although temperatures below 0°C are not often employed except for the separation of the more volatile material such as permanent gases. For very low temperatures the coiled column may be immersed in a refrigerant such as liquid nitrogen, various slushes or dry-ice-acetone mixture contained in a wide mouthed Dewar flask. Otherwise, low temperature thermostat baths may be used.

Most separations by gas chromatography are carried out at temperatures above ambient and in these circumstances an air oven with forced circulation is easily the most convenient arrangement. Astbury, Davies and Drinkwater[18] described a very efficient air thermostat in which rapid temperature changes could be made at the rate of about 20°C per minute, if required. The apparatus was, however, very elaborate and modem commercial chromatographs usually employ much simpler ovens that are, nevertheless, very efficient both for isothermal operation and when temperature programming is employed.

Ovens with linear-programmed temperature control are very useful for the separation of complex mixtures contain­ing compounds with a wide range of boiling points. In an isothermal oven maintained at the temperature necessary for good resolution of the lower boiling components the reten­tion times of the higher boiling materials may be un-acceptably long, and the peaks very broad, whereas at the higher temperature necessary for the rapid elution of the less volatile materials the more volatile materials are incompletely resolved.

When an oven is operated isothermally it is important that the temperature does not vary more than 2°C, particularly if retention volumes are being measured, for example, for identification purposes. Variations in temperature may also affect the response of certain detectors such as the thermal conductivity cell.

For very accurate measurements of retention times such as are necessary in thermodynamic studies, thermostat bathscontaining water or oil, as appropriate, may be used because of the very close temperature control that is possible with them. For certain fixed temperatures columns may be enclosed in vapour jackets in which liquids of suitable boiling points are being refluxed.

Metal columns, particularly of stainless steel, may be heated by passing high current at low voltage directly through them, although it is doubtful if very constant temperatures can be maintained in this way without elab­orate insulation.

Detectors

The purpose of detectors is to monitor the column effluent, measuring variations in its composition. They do not, except in the case of the so-called 'specific' detectors; usually identify the components of a mixture. So vital are detectors in gas chromatography instrumentation that almost as much time must have been devoted to their development as to that of all the other components put together.

Most detectors are of the so-called 'differential' type, that is, they give zero signal when pure carrier gas is passing through them, but when a component of a mixture is detected the signal is proportional to the concentration or mass of that component (see Fig. 4.8). 'Integral' detectors, on the other hand, give a continuous signal which is proportional to the total amount of substances which have been eluted. Stepped records will be obtained as shown in Fig. 4.9. Such a record is given by the Janak absorption device or the gravimetric integral device described by Bevan and Thorbum (p. 204); each step corresponds to a different component and the step height is proportional to the amount of the particular constituent. This record may be confused with the frontal analysis or displacement development records mentioned in Chapter 1 (p. 11) which, though of similar appearance, are obtained with differential detectors.

Apart from a brief description of the Janak absorption method on page 203, only differential detectors will be considered. There are many kinds each relying on some physical property of the gas such as:

thermal conductivity,

gas density,

flame ionisation,

ß-ray ionisation,

                cross-section

                argon,

                helium,

                electron-capture,

                electron-mobility,

photo-ionisation,

microwave emission,

flame emission,

flame conductivity [ 19],

rapid scanning infrared or ultraviolet absorption,

dielectric constant.

heat of sorption.

this list is not exhaustive and only (i), (ii), (iii), and (iv) a)-d) will be described in any detail.

The requirements of a detector for gas chromatography are exacting. Among the more important are (a) sensitivity to very small concentrations of one gas in the presence of a high concentration of another (the carrier), (b) a rapid response which is also directly proportional to concentration, (c)a small 'dead' volume, and (d) a high signal-to-noise ratio.

ionisation detectors are usually more sensitive than other types; for example, the detection limit of the thermal conductivity  cell and gas density balance is about 10-6 moles, whereas that of the flame ionisation and ß-ray (argon) detector is about 10-12 to 10-13 moles. In fact an alternative classification of detectors might be: high sen­sitivity —  ionisation detectors, low sensitivity — others.

More recently development work has been carried out on so-called 'specific' detectors, which are instruments that respond selectively to a particular compound or class of compounds. The electron capture detector was an early example of this kind, where compounds containing strongly electron capturing atoms such as oxygen or a halogen cause a much greater response than compounds without them. The flame photometer detector is specific for compounds con­taining sulphur or phosphorus. When such specificity exists extremely low concentrations of the compounds sought may be detected, and the important role played by the electron capture detector in determining very low levels of chlorinated pesticide residues in wildlife is well known. Thus yet another classification is possible — specific detectors and non-specific detectors. Of the following detectors described in any detail only the electron capture and microwave plasma detectors fall in the former class.

One other classification of detectors is worth mentioning namely: concentration sensitive detectors and mass flow sensitive detectors[20]. This classification is of importance for quantitative measurements[21] (p. 215).

Thermal conductivity cell (katharometer)

In spite of frequent predictions that the thermal conductivity cell will disappear from the gas chromatography scene it still stubbornly refuses to do so, and valuable papers are still being published on its performance[22]. Certainly as more and more different detectors are developed so the katharometer will become less and less important, but it now seems doubtful if it will ever disappear completely.

The response of a katharometer depends on the thermal conductivity of the gas stream passing through it. Essentially the katharometer is a filament of a metal such as platinum (Fig. 4.10) which has a high temperature coefficient of resistance. This filament is heated by a small electric current from a six-volt accumulator. The temperature of the filament, and hence its resistance, is determined by the current and the thermal conductivity of the ambient gas. Changes in constitution of the gas change the temperature, and hence the resistance, of the hot filament. When the filament is part of a Wheatstone bridge (Fig. 4.10(b)) the out-of-balance currents due to changes in the conductivity of the filament  may be fed into a recorder.

The katharometer was one of the earliest detectors to be used, and for a while was one of the most popular. Provided that its limitations are realised, it is capable of satisfactory performance. In spite of its seeming simplicity, however, it is not a particularly easy instrument to make and it may prove more convenient to buy a commercial version. A number of highly satisfactory makes are available. A simple cell is described in the Appendix.

There are several excellent accounts {e.g. references 21 and 23) of the characteristics of katharometers. A few dis­advantages are:

(1) They are often non-linear in their response and hence require calibration.

(2) They are sensitive to changes in ambient temperature and gas flow.

(3) Their effective volume or 'dead' volume is large and they are thus unsuitable for use with capillary col­umns, where very small samples are used.

(4) They are not easily made sufficiently sensitive for use with very small samples and their response time is usually too long to detect substances that are eluted very rapidly.

For ordinary packed columns most of these objections are not important or can be overcome; for example, sensitivity to changes in temperature and gas flow can be minimised by incorporating the analysis and reference cells in the same metal block (see Fig. Al and Appendix) and by making sure that the gas which flows through the two cells has passed through identical systems. This requires duplication of the apparatus between positions 3 and 10 in Fig. 4.1, one portion to be used for the separations and the other for pure carrier-gas for the reference cell.

The roles of the two portions may be reversed. It is only necessary to reverse the signal from the bridge incorporating the two thermal conductivity cells in order to give the correct response on the recorder. If the columns in the different portions are packed with different stationary phases a choice of two columns in the same apparatus is obtained.

The use of thermistors in place of wire filaments has made it possible to reduce the 'dead' volume of cells below 0.2 cm3. Provided that they are operated at temperatures not greater than 100°C such katharometers may be used in conjunction with capillary columns because of their im­proved sensitivity and response-time compared with filament-type cells.

Katharometers depend for their operation on the thermal conductivity of the gases passing through them. It will thus be seen that the nature of the carrier-gas plays a large part in determining their performance; for example, sensitivity de­pends on the difference between the thermal conductivity of the carrier-gas and that of the component being sensed. Hydrogen and helium permit the highest sensitivities because the differences between their thermal conductivities and those of most of the vapours encountered in gas chromatography are substantially greater than for such carrier-gases as nitrogen and carbon dioxide.

The gas density balance

Designed by Martin[24], the gas density balance seems at first the ideal detector for gas chromatography, but diffi­culties of construction and lack of sensitivity compared with the ionisation detectors have prevented its more general adoption. There are now available, however, several com­mercial gas chromatographs which use the gas density balance or simplified versions of it. It depends for its operation on a very sensitive anemometer for detecting minute gas flows. The anemometer is located in a channel joining two other channels drilled in a copper block, one connected to the column effluent, and the other to the reference-gas supply which is pure carrier gas. While pure carrier gas emerges from the column the densities of the two streams (reference and column effluent) are the same and there is no gas flow through the anemometer; if, however, a constituent of the sample emerges from the column the density of the column effluent is changed and there will be a flow of gas through the anemometer which is proportional to the change in density. By an ingenious arrangement of the channels only pure carrier gas flows through the anemometer. It follows, from the mode of operation of this device, that the response will be a linear function of concentration and also of the relative molecular mass of the constituent. The instrument therefore requires no calibration, and can, in fact, be used to determine relative molecular masses [25 ].

Flame ionisation and ß-ray ionisation detectors

Both the flame[26] and ß-ray ionisation[27] detectors depend on the increase in current produced when eluted substances passing through them are ionised. Suitably de­signed, they are almost ideal for use with capillary columns and are eminently satisfactory for use with packed columns, although their high sensitivity (except the cross-section ionisation detector) may prove an embarrassment. Their 'dead' volume may be extremely small, their response is almost instantaneous, they are very sensitive and they are stable to fluctuations in the flowrate of the gas, and to changes of temperature. Finally, they give a response which is a linear function of concentration - at least for the con­centrations normally met in gas chromatography. A com­prehensive review of ionisation methods for the analysis of gases has been given by Lovelock [ 28 ].

Flame ionisation detector

In the flame ionisation detector shown schematically in Fig. 4.11 hydrogen is used as the carrier gas, or is added to it, and is burned in a small metal jet, such as a hypodermic needle cut square. The jet forms one electrode — usually the negative — of the cell and the other, which may be a piece of brass or platinum wire, is mounted at some point near the tip of the flame. The potential difference across the electrodes is about 200 V. Pure hydrogen or hydrogen/carrier gas mixture gives rise to a small background signal which may be offset electrically, and the signal which is recorded arises from ionisation of substances in the flame, including flammable as well as non-flammable compounds. The mechanism of ion production is not completely clear, but for organic sub­stances, ions may originate from carbon aggregates which ionise  relatively  easily   (approximately 415kJ mol-1 [4.3 eV] ). Response of the detector to organic substances also depends on the 'carbon number' of the compound. A design for a flame ionisation detector is included in the Appendix.

According to Condon [29] the following substances are not detected because of their high ionisation potentials:

the noble gases, H2 , Q2, N2, SiCl4, SiHCl3, SiF4, H2S, SO2,. COS, CS2, NH3, NO, NO2, N2O, CO, CO2, H2O.

About the only organic compound not detected is formic acid. An advantage of the flame ionisation detector is that it does not indicate the presence of water thus enabling the analysis of aqueous solutions to be performed.

Cross-section ionisation detector

The cross-section detector was the first ionisation method to be used and is possibly the least sensitive. A source of ionising radiation such as 90Sr is contained in a brass vessel similar to (though not identical with) that shown in Fig. 4.12. The potential across the electrodes is 300-1000 V. Production of ions in the cell, and hence the current through it, is proportional to the product of the total mole fraction and the ionisation cross-section of the gas mixture.

Any gas may be used as the carrier but the best are hydrogen or helium since both give relatively few ions on account of their small cross-sections. Large molecules such as are likely to be separated by gas chromatography are much more readily ionised and therefore increase the current through the detector.

Advantages of the detector are its ability to respond to all gasses and vapours, in any concentration and in any carrier-gas, but the lower limit of detection is about 10-5 moles.

Argon ionisation detector

The argon ionisation detector (Fig. 4.12) is in many respects similar to the cross-section ionisation detector, but is far more sensitive. The cell is a stainless steel or brass vessel of small volume, which acts as the cathode; the anode is of similar material. The applied potential is usually between 600 and 1200V; the higher the. potential the greater the sensitivity. A ß-emitter such as 90Sr,85Kr or tritium is sealed I into the vessel.

Ionisation of substances in the stream of argon carrier-gas occurs mainly as a result of collisions with metastable argon atoms. Production of metastable atoms appears to occur mainly as follows: electrons from the radioactive source ionise argon atoms (and, to some extent any other atoms or molecules which happen to be present) and the electrons thus released are further accelerated by the applied field so that other argon atoms with which they collide are excited to their metastable state. A single electron can produce several excited atoms. Metastable argon atoms have an excitation level of 1129 kJ mol-1; hence, collision of substances with lower ionisation potentials results in the further production of ions and this increases the conductivity of the gas. Clearly, substances   with   ionisation   potentials  greater  than 1129 kJ mol-1 will not be detected, or only give rise to a poor signal; among them are H2, N2, O2, CO2, CO(CN)2, H2O and fluorocarbons, which are not detected, and CH4 C2H6, CH3CN and C2H5 CN, which give a small response. The difficulty of detecting these substances can be overcome to some extent by an ingenious method described by Willis[30]. Advantage is taken of the fact that such sub­stances reduce the ion current produced in the detector by an organic vapour such as ethylene, and they can therefore bel detected in the presence of such a vapour by their 'negative' peaks.                                                 I

Helium ionisation detector

Berry [31] has described an ionisation detector which de­pends on the use of highly purified helium as the carrier gas. The construction and mode of operation are similar to those of the argon detector and the sensitivity is comparable. However, helium has an excitation level of 1910 kJ mol-1 which means that gases not sensed in the argon detector are ionised and therefore give good signals. Even neon (ionisation potential 2074 kJ mol-1) is ionised to some extent in this detector, possibly by a two-stage process.

Electron-capture detector

The electron-capture detector resembles the argon and helium detectors, but the geometry of the cell is different and the signal depends on the capture of electrons by the various substances being sensed which causes a reduction in the ion current. The applied voltage across the electrodes is only about 20 V. Particular advantages of this detector are its very high sensitivity and specificity for molecules such as oxygen, halogens, oxygen-containing and halogen-containing compounds, which have high electron affinities. The response of weakly electron-capturing compounds, such as hydro­carbons and ethers, can be eliminated by increasing the applied potential. This means that the detector can be made selective in its response.

Microwave plasma detector

The microwave plasma detector, or MPD, is a fairly recent development and may be regarded as a specific detector, but with the considerable advantage that it may be tuned to any element - or number of elements - including those most frequently sought by chromatographers, namely, C, H, N, P, S and halogens. It is basically an atomic emission device, the emission resulting from the passage of eluted components through a microwave-sustained helium plasma discharge that causes them to be completely dissociated into their con­stituent atoms.

In a particular commercial instrument the column effluent is divided into two equal streams, one going to a flame ionisation detector, the other to the plasma tube. The 'interfacing' with the chromatograph is thus relatively simple.

Although expensive, the MPD would seem to offer substantial advantages over most other detectors; in addition its sensitivity is comparable to that of the flame ionisation detector.

E integrating detectors

The first detector described in association with gas-liquid chromatography was an integral detector devised by Martin and James [71] in which volatile fatty acids were titrated against sodium hydroxide solution as they emerged from the column by an automatic burette. This device was clearly of limited application and hence was not generally adopted, but in common with the other instruments mentioned below, it did directly measure the amount of substance - other than the carrier gas — passing through it and therefore was particularly suitable for quantitative work.

In the gravimetric integral detector devised by Bevan and Thorbum [32] the column effluent is led into an adsorption cell where it is taken up by the material lining the cell and, hence increases its weight. The cell is supported by an auto-recording gravimetric balance with a sensitivity of about 10-7 g; thus separated substances are detected and measured absolutely according to their mass.

The Janak[33] absorption method for the detection and estimation of the separated components of a mixture is simple and relies on the measurement of gas volumes. It is suitable for gases with relatively low boiling points such as the low molecular weight hydrocarbons. The column is operated in the usual way but the carrier gas is carbon dioxide, which, after passage through the column, is absorbed by a strong solution of potassium hydroxide. Any non-absorbed gases pass into a gas burette where their volume is measured at constant pressure. The non-absorbed gases are, of course, the components of the original mixture. If the volume of gas in the burette is plotted against time a stepped curve is obtained which is similar to that shown in Fig. 4.9 This is an 'integral' record as already mentioned, and might be confused with the record obtained during a frontal or displacement development analysis. In Fig. 4.9 the heights of the steps are proportional to the amounts of the particular constituent in the original mixture. The identity of a particular component can often be inferred from the time taken for it to emerge from the column, measured from the injection time of the original mixture.

Recorders

Choice of recorder depends very largely on the funds available. Practically any potentiometric recorder (range about 1 mV) will do for most of the methods of detection including thermal conductivity, flame ionisation, beta-ray ionisation and gas density, only minor modifications being necessary. An additional voltage amplifier is necessary with the gas-density balance, and the ionisation detectors de­scribed require electrometer amplifiers.

Identification of the components of a mixture

The most reliable way of identifying the components of a  mixture separated by chromatography is to trap them as they emerge from the column, and to identify each separately by techniques such as infrared and mass spectroscopy, but see below (p. 212). Needless to say, such a procedure is tedious and, when routine separations are being performed on mixtures whose composition is known to vary only between fairly narrow limits, unnecessary. Other methods of identify­ing components are based on chromatographic behaviour. In routine analysis, provided that the experimental conditions are always the same, the identity of the peaks can be inferred from previous separations. The method depends on the fact that for a given set of experimental conditions, components will always behave in the same way. Another way is to make up synthetic mixtures and compare their behaviour with that of the unknown. An unfamiliar peak can sometimes be identified by adding to the mixture some of a pure component thought to be identical with it. If there is an increase in size of the unknown peak, then there is a good chance that the pure substance added is identical. Before describing the chromatographic methods of identification in more detail it will first be necessary to define the various retention volumes which are used. The treatment applies particularly to gas-liquid chromatography, but provided that more or less symmetrical peaks are obtained, it can also be applied to gas-solid chromatography. Caution is, however, necessary in the latter case because of the inconstancy of adsorbent behaviour that may occur from one batch of material to another. The presentation of retention data is discussed in references 34 and 35.

Retention volumes

Retention volumes are expressed in cm3 and reference is made to Fig. 4.13. When a substance is injected on to a chromatographic column, a small volume of air is usually introduced. It is assumed that air is not retarded by the stationary phase at normal operating temperatures, therefore the volume of carrier-gas required to elute the air is a measure of the volume of the apparatus, between the injection point and the detector, not occupied by column packing. This volume, known as the 'gas hold-up' Vm, is given by:

Vm = F . Ot1

where F is the flow rate (cm3 min-1) of the carrier-gas at the outlet pressure and temperature of the column, and Ot1 is the time taken for the air-peak maximum to appear.

An air peak will not appear with some instruments, such as the flame ionisation detector. Methane, which is detected, is usually retained to a negligible degree by the stationary phase (except in gas-solid chromatography at low temperatures) and may be injected separately to determine the gas hold-up with reasonable accuracy. However, Thombs[36] has re­ported that pulses of noble gases produce responses from the flame ionisation detector and hence may be used for gas hold-up measurements.

The uncorrected retention volume VR of the substance is given by:

VR = F • Ot3

where Ot3 is the time taken for the emergence of the peak maximum of the substance. This includes the gas hold-up of the apparatus and making allowance for this we obtain the adjusted retention volume VR' which is given by:

VR' = F • t1t3

where Ot3 is the time taken for the emergence of the peak maximum measured from the air-peak maximum.

There is a pressure gradient down the column and it is therefore necessary to introduce the so-called 'compressibility factor' (i) of Martin and James[7] which is given by:

.^(^/po2 -^ 1) 7 2(pi^/po3-l)

where pi is the pressure of carrier gas at the column inlet, and po the pressure at the column outlet. Hence is obtained the net retention volume VN given by:

VN = jVR'

Finally, the

specific retention volume Vg, is the net retention volume of the substance per g of stationary phase, at 0°C, thus:

Vg = VN . 273/Tw

where T is the temperature of the column in degrees Kelvin and w is the weight in g of stationary phase in the column.

 Relative retention volumes are obtained by comparing the retention volumes (VR' , VN , or Vg ) of the solute under consideration with some standard solute whose behaviour on the particular column in use is precisely known. Relative retention volumes are conveniently expressed as ratios: for example

VN (Standard) /VN (Test)

Specific, net and relative retention volumes may be used for identification purposes.

Use of specific retention volumes

The specific retention volume of a solute is characteristic and may be used to identify peaks in a chromatogram. It is dependent only on the stationary phase in use in the column, and the temperature provided that the stationary phase is a single, pure compound. If it is a mixture, for example, a silicone, there may be poor reproducibility from batch to batch. In view of the temperature dependence it is obviously important that the value of Vg used is the correct one for the particular column conditions. It is possible to calculate the value of Vg for different temperatures by means of an Antoine equation:

log Vg  = A + B/t+C

where t is the column temperature in degrees centigrade and A,B and C are constants which can be evaluated graphically from gas-liquid Chromatography data. Ambrose and Purnell[37] have determined the constants for a number of organic compounds, and the temperature range over which the. values are valid is quoted. Few Vg values have been determined, probably because of the care needed to obtain accurate results. Adiard, Khan and Whitham[38] have made very careful measurements for benzene and cyclohexane on columns of Celite coated with 20% dinonyl phthalate, in the temperature range 48—110°C. In practice there is a tendency to use relative retention volumes.

Relative retention volumes

Relative retention volumes may be used for identification purposes provided that the experimental conditions are as nearly identical as possible for the test substances and the standard. This may be ensured by adding the standard substance to the original mixture. Alternatively, separate successive runs may be made with the standard and then the mixture. The method is to compare the retention volumes (or times) of the other peaks with those of the standard and see if the ratio corresponds to that of a known compound with the standard. Like Vg values, relative retention volumes are valid for one temperature only. Variations of the relative retention volume method have been described by Kovats[39]