The Evolution of GC Detectors
The first GC detector was invented in 1952 by the originators of the technique, James and Martin [1], and took the form of a titration apparatus situated at the end of the column. One of the original
applications of GC was the separation of a mixture of fatty acids and consequently, the eluent gas was bubbled through a suitable aqueous liquid to absorb the solutes. The solution contained an indicator, the color of which changed as each solute was eluted, and the solution was then manually titrated. Later the titration process was automated by the inventors (probably the first automatic titration apparatus to be made and certainly the only one available at that time) and an integral chromatogram was formed by plotting the volume of base solution added against time. The chromatogram consisted of a series of steps one for each solute. This rather primitive arrangement clearly and plainly demonstrated that gas chromatography would work but, at the same time, it also indicated that a detector with greater sensitivity and a more catholic response was necessary for the effective use of the technique.
The next detector, the first practical detector to be developed, was also invented by James and Martin but, for some reason, was never formally reported in the literature. Its description, however, did appear in a review by A. T. James [2].
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Figure 1
The Martin Gas Density Bridge
A detailed explanation of the function of the detector was given by Munday and Primavesi [3] in the 1956 symposium entitled ''Vapor
Phase Chromatography" (the name originally given to the technique by Martin and later changed to Gas Chromatography). The gas density balance was an extremely complicated and ingenious device and, incidentally, the modern so–called gas density bridge bears little or no resemblance to the original design. In view of its technical ingenuity and because it was the first effective GC detector to be
developed it will be described in some detail. A diagram of the gas density balance is shown in figure 1. The detector consisted of a Wheatstone network of capillary tubes that were drilled out of a high conductivity copper block and was fairly compact. The reference flow of mobile phase and the eluent from the column entered at two opposing junctions of the bridge arms (the center of tube (C)) such that the eluent was contained in one vertical arm (C) and the pure mobile phase in a parallel vertical arms (A) and (B). The increase in pressure at the base of tube (C) due to the presence of solute in (C) applied a pressure to the bottom of tube (A). This caused a flow of gas through the anemometer from tube (A) to tube (B) providing an output that was fed to a recording milliammeter. Subsequently all flows exited from the top and bottom of tube (C).
The anemometer was particularly special. It consisted of a cylindrical chamber about 1.5 cm in diameter and about 4 mm wide. A length of 0.001 in O.D. copper wire, containing 2 mm of 0.001 in Constantan wire arc welded to the copper wire in the center, passed through the conduit connecting the chamber to tubes (A) and (B). (To make one of these dual thermocouples with the equipment available in 1952 was a feat in itself. Beneath the copper Constantan junctions was situated a heater loop that raised the temperature of both junctions by convection currents circulating round the cylindrical chamber as shown in the center of the anemometer diagram. When a flow of gas passed through the anemometer as a result of solute vapor being present in tube (C), the convection currents above the heater loop were displaced so that one junction was cooled and the other heated as shown in the right–hand side of the anemometer diagram. The differential output
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from the two thermocouples was charted on a recording milliammeter.
The detector was robust, but a little difficult to set up initially. It was linear (0.98<r<1.02) over about 3 orders of magnitude of concentration range and had a sensitivity (minimum detectable concentration) of about 5 × 10-7 g/ml (n-heptane). It helped to produce the data for many of the fundamental studies in
GC. Unfortunately, the detector proved to be very difficult to make for the general chromatographer and, even after many attempts, was never produced commercially as an effective GC detector. The lack of an alternative, simple detector provoked a number of chromatographers to develop alternatives. The development of GC detectors was far easier than the development of LC detectors as organic vapors change the physical characteristics of a gas to a far greater extent than they do a liquid. As a
consequence, a number of very effective highly sensitive GC detectors were developed over a relatively short period of time and many of them were manufactured commercially.
The first alternative device to be used as a GC detector was the katherometer introduced by Ray [4] (now more prosaically know as the hot wire detector (HWD)). This detector will be described later as it is still a popular device for the detection of permanent gases and so will only be briefly mentioned here. It consists of two heated filaments, one suspended in the eluent gas from the column and the other in a pure reference stream of gas. The filaments are situated in the arms of a Wheatstone bridge. When a solute is eluted, both the thermal conductivity and the heat capacity of the gas change, which alters the heat loss and consequently the temperature of the filament and its resistance This unbalances the bridge and the out-of-balance signal is passed to a suitable monitoring device. This detector is relatively insensitive but responds to all solutes that differ in heat capacity and thermal conductivity from those of the carrier gas. This detector was used extensively in the early days of GC for the analysis of
hydrocarbon gases. In the early days of detector development there was much discourse and dissent with regards to the exact mechanism of detection involved in the katherometer [5,6] and even today it is
considered to respond to a number of different physical properties of the eluent gas with no one playing a major role.
In the same symposium the "flame thermocouple detector" was first described by Scott [7] and it was, in fact, the forerunner to the flame ionization detector FID. Either hydrogen or a mixture of hydrogen and nitrogen was used as the carrier gas or the eluent from the column was mixed with hydrogen so that the elution products could be burnt at a small jet. A thermocouple was situated above the jet and was heated by the flame. When a solute was present in the eluent, the heat of combustion of the gas increased, raising the flame temperature and the output from the thermocouple. The electronic circuit was simple, consisting of a backing off circuit to offset the output from the hydrogen flame alone and an attenuating circuit, the output from which passed to a potentiometric recorder. The detector had a linear response over about three orders of magnitude of concentration and a sensitivity of about 1 × 10-6
g/ml (n-heptane). Its response was proportional to the heat of combustion of the solute. This detector was also made commercially but enjoyed a very short life as it was quickly supplanted by the FID.
Figure 2
The Flame Thermocouple Detector
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The β-ray ionization detector was also introduced in the 1956 symposium by Boer [8,9] and as this was the first ionization detector that utilized a radioactive source it will also be described in some detail here. The design of the detector is shown diagramatically in figure 3. It consisted of two cells, a reference cell, through which pure carrier gas passed, and a sensor cell, which carried the eluent from the column. In each cell was placed a 90strontium β emitting source. The decay of 90strontium is in two
stages, each stage emitting a β particle producing the stable atom of 90zirconium.
The ionization currents formed (collected by appropriate electrode potentials) are arranged to oppose one another and consequently any variation in pressure or temperature of the two cells will be balanced out.
Figure 3
Figure 4
Reproduction of Boer's Original Ionization Cell Reproduced from reference 8
The differential signal resulting from the presence of a solute in the column eluent sensor cell is
amplified and recorded. The ionization detector, as originally described by Boer and taken directly from his original publication, is shown in figure 4.
The ionization current increases as the voltage increases until a plateau is reached where the ionization current is constant from about 50 to 200 volts. The ionization current produced depends on the nature of the gas and the ionization cross sectional area of the gas molecule. Curves relating ionization to applied voltage are shown in figure 5. It is seen that the ionization current for nitrogen is much greater than that for hydrogen. It is also seen that in the presence of organic gases such as 10% v/v butane in hydrogen doubles the ionization current relative to that of pure hydrogen. Similarly, 10% v/v of butane in
nitrogen increases the ionization current by about 50%. Providing the ionization voltage is such that the ion current of the eluted material lies on the plateau portion of the response curve, the relationship between ion current and solute concentration is linear.
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Figure 5
Response Curves for the Boer Ionization Detector
Reproduced from reference 8
As the ionization currents are additive the signal(s) from the compensated cell can be described by the following equation.
where (m)
and
is the molar fraction of component (c) in carrier gas (g), is the saturation current of the pure component,
is the saturation current of the carrier gas.
It is seen that it can be shown theoretically that the response of the detector should be linear with concentration.
The sensitivity of the detector was similar to that of the katherometer i.e. about 1 × 10-6 g/ml.
Unfortunately, the practical lifetime of this detector was also relatively short as it was eclipsed by both the FID and the argon ionization family of detectors introduced by Lovelock. Nevertheless, the
development of these early detectors not only helped establish GC as a viable analytical technique but also stimulated the development of other types of vapor sensing devices. The future
detectors would prove to have extremely high sensitivities and wide linear dynamic ranges and to function on widely different principles. However, with the exception of the katherometer, they would also render these early detectors virtually obsolete. Nevertheless, they played an important role in the early days of GC and in their time were exciting devices to operate.
The second generation of GC detectors was introduced at the 1958 Symposium on Gas
Chromatography, probably the most important and technically exciting of all chromatography symposia. At that symposium, many original GC concepts was confirmed but, more importantly, further developments in detector technology were described including two new detectors, the FID and the emissivity detector. In addition, novel column systems were introduced, including the open tubular column by Golay [10] (which was to become the column of the future) together with the first high efficiency packed columns [11]. The contents of this symposium probably represented the climax of GC development and, although a plethora of symposia would appear over the following half century, none would have the excitement and novel technical content of this one.
Detector technology and new detectors were particularly hot topics at the 1958 symposium.
McWilliams and Dewer [12] described the flame ionization detector which was to be the 'work horse' of all future gas chromatographs. Further developments of the flame thermocouple detector were described by Primavesi et al. [13], the design of the katherometer was simplified by Stuve [14] and Grant [15] described the first thermal emissivity detector.
The emissivity detector developed by Grant was an interesting and innovative extension of the flame thermocouple detector. It did not prove particularly popular at the time, but in recent years the concept has been revived and commercial detectors based on the emissivity concept of Grant are now available.
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Figure 6 The Emissivity Detector
The original detector was very simple in design and the original diagram of the device is shown in figure 6. In principle the column eluent was mixed with a combustible gas and burnt at a small jet in the same manner as the flame thermocouple detector. In fact Grant used coal gas (largely a mixture of hydrogen, methane and carbon monoxide) with which the column eluent was mixed prior to
combustion. Opposite the flame was situated a lens that focused the light emitted onto a photo cell. The output from the coal gas flame light was balanced out by a simple potentiometer network and the signal passed to a potentiometric recorder. The detector gave a partially selective response to aromatic
hydrocarbons or any solute that either increased the luminosity or imparted color to the flame. The sensitivity varied widely with the solute being detected. Aromatics that impart strong luminosity to the flame giving a strong response and consequently a high sensitivity (ca 1 × 10-6 g/ml). Conversely,
saturated hydrocarbons impart little luminosity to the flame and thus have a weak response and a very low sensitivity–the very features for which the detector was designed. The linear range is difficult to determine from the original publication but appears to be in excess of two orders of magnitude. The performance of the detector is illustrated in figure 7.
Figure 7
Chromatograms Demonstrating the Selectivity of the Emissivity Detector
Reproduced from reference 8
The separation shown in figure 7 of a mixture of aliphatic and aromatic hydrocarbons was monitored using both a katherometer detector that responded to all solutes and the emissivity detector that selectively responded to the aromatics. It is seen that the emissivity
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detector clearly discriminates the solutes benzene and toluene from the aliphatic hydrocarbons, which give very little response.
In retrospect the triad of symposia held in 1956, 1958 and 1960 not only established GC as a unique and reliable analytical technique but also contained the first disclosure of 90% of the devices used in
modern GC. Between the 1958 and 1960 symposia. Lovelock developed a family of ionization
detectors starting with the Argon Detector [16,17] in 1958 of which three different models were created and demonstrated to function well, and ending in 1959 with the electron capture detector [18], which, second to the FID, is probably the most commonly used detector today.
At the 1960 symposium, research into the mechanism of detection of a number of the more important GC detectors was reported. The FID was carefully examined by Ongkiehong [19] and Desty et al. [20], the different argon detectors were described in detail by Lovelock [21] and the performance of the FID and Argon Detectors were compared by Condon et al. [22]. The integral and differential ionization detectors were also discussed by Matousek [23]. At the end of the 1960 symposium, the majority of the GC detectors that are used today had been described and their function explained and understood. Since that time few, if any, new GC detectors have been introduced and innovations that have been described have been largely extensions and improvements of older concepts.
Perhaps, before discussing the general properties of detectors one other GC detector might be
mentioned as it possibly represents the only GC detector that functions by absolute measurement. This detector, developed by Bevan and Thorburn [24] in the early 1960s, was the mass detector.
Unfortunately, this detector never progressed beyond the research prototype. It consisted of an efficient charcoal vapor adsorbent suspended from the arm of a recording microbalance. The column eluent was arranged to pass through the adsorbent without disturbing the balance. As solutes were eluted from the column they were quantitatively adsorbed on the charcoal and the weight recorded
by the balance. As a consequence, the balance record produced an integral chromatogram, each step being an absolute measure of the mass of solute eluted. Although it had an absolute response, this detector was never developed seriously as its sensitivity was relatively poor, which placed severe restrictions on the type of column with which it could be used.
The General Properties of GC Detectors
The basic function of the GC detector is to respond to the presence of very small quantities of vapor in a permanent gas. This is tantamount to the detection of relatively high boiling compounds contained at very small concentrations in very low boiling substances. Because the physical and chemical properties of permanent gases differ widely from those of a vapor, a very wide range of detection methods can be employed. Such methods range from the measurement of standard physical properties such as thermal conductivity and light adsorption to ionization potentials and heats of combustion.
The response of a GC detector can be general or specific. A detector with a catholic response such as the FID is used widely in routine analysis. The specific detector, such as the nitrogen-phosphorus detector (NPD), is extremely useful for measuring particular types of compounds such as herbicides and pesticides, where the compounds of interest are not eluted discretely but mixed with a number of other contaminating compounds. Examples of this type of application will be given when the NPD is
discussed in detail.
In general GC detectors should be insensitive to changes in flow rate but, as already discussed, few detectors have this attribute although some, for example the FID, are virtually insensitive to changes in column flow. This is advantageous as it permits the use of flow programming development. Flow programming attempts to achieve the same result as temperature programming, which is to accelerate the strongly retained peaks through the column. It is less effective than temperature programming as the effect of temperature change on
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solute retention is exponential whereas the effect of flow programming on solute retention is linear with change in flow rate. Nevertheless, it is useful where, due to the solutes being thermally labile,
temperature programming is not possible. Consequently, detectors that are insensitive to column flow rate change can be advantageous.
Some detectors require no other gas than that used as the carrier gas, other require specific gases to be added to the columns eluent for them to function. In some cases the detector prescribes a certain gas to be used as the carrier gas. It will be seen later that the sensitivity of the katherometer is greater when helium is used as the carrier gas. In addition, if the gas chromatograph is being used for permanent-gas analysis, then helium must be used to differentiate the carrier gas from the other gases being analyzed. The use of extra gases increases the operating costs, particularly if they are relatively expensive such as helium or argon. In Europe helium can be very costly relative to other gases as it is mostly obtained from the United States.
All gas chromatographs are designed to operate over a wide range of temperatures, the extremes being