1842
Anal. Chem. 1980, 52, 1842-1844
Gas-Phase Coulometric Detector for Gas Chromatography E. P. Grimsrud" and S. W. Warden Department of Chemistry, Montana State University, Bozeman, Montana 597 17
each pulse and 6 is the fraction of the total excess positive ions left behind which migrate to this same anode. In GPC it is the quantity, I,, which is of interest since a change in f, indicates the number of electrons lost to the sample molecule. The magnitude of 6, which complicates the measurement, will depend on the geometry of the cell. Since eq 1 describes the measured ECD current with or without sample present (7), the measured response to any sample (peak area) is predicted to be similarly reduced by the positive-ion currents by an amount equal to 1 - 6. In previous studies of gas-phase coulometry, 63Niionization cells of cylindrical geometry have been used where an axial pin running the length of the cylinder (concentric coaxial design) was used as the anode. For this design our calculations (7) predict a value for 6 of about 0.25 and, therefore, an error of about 25% for GPC if the complication caused by positive ions is not considered. With a cell of displaced coaxial geometry, however, the value of 6 is predicted to be reduced to about 0.01. This is because the anode then protrudes only to the edge of the ionization volume, and the small contribution of the tip of the pin to the total area of the active volume boundary determines the value of 6 for this design. This configuration, the space-charge model predicts, should provide the better cell for GPC. In this article responses to CCll are described by using a 63Ni cylindrical cell in which the location of its axial anode can be readily varied between repeated analyses. Thus, comparisons of sample responses and the standing current for the displaced and concentric coaxial designs are reliably made by using a single cell and the same chromatographic system, samples, and associated electronics. The results are discussed relative to t h e predictions of the space-charge model.
Recent theoretical considerations of the pulsed electron capture detector (ECD) predict that anode placement should be an important consideration for successful gas-phase coulometry (GPC). This prediction is tested here with a specialized electron capture detector for which the location of its anode can be varied wlthln the cell. It Is demonstrated that the signal obtained from a pulsed ECD is complicated by positive ion effects as described by a space-charge model of the ECD. It is shown that the magnitude of this posltlve ion component of the measured ECD current is much less for the displaced coaxial design than for the concentric coaxial design, and, therefore, the displaced coaxial design is expected to provlde greater accuracy in GPC.
A unique aspect of the electron capture detector (ECD) for gas chromatographic (GC) detection is that it offers the potential for absolute quantitative analyses of certain substances, independent of calibration procedures ( I , 2). This technique, called gas-phase coulometry (GPC), is possible when the stoichiometry of the electron capture reaction is known and the sought for substance reacts sufficiently rapidly with gaseous thermal electrons that a large percentage of the substance is consumed during its transport through the detector. The integrated response to sample (peak area) is taken t o reflect the number of electrons and molecules of sample reacted. Since the initial suggestion of GPC, our understanding of the ECD has increased, and it has become increasingly possible to identify and individually examine potentially complicating factors of GPC. Thus, in recent articles dealing with GPC we find consideration of electron-positive ion recombination ( 3 ) ,electron attachment to carrier gas impurities ( 3 ) ,stoichiometry of the electron-solute reaction ( 4 ) , loss of the sample molecules by reaction with positive ions ( 4 ) , and the inclusion of a second-order rate law for the reaction of electron with sample ( 5 ) . In this paper the separate question of optimum cell design for GPC will be addressed. We have recently described a n alternate model for the pulsed ECD (6, 7) which has direct implications for GPC. In that model (which will be referred to here as the spacecharge model) the thermal electrons created throughout the ECD after each electron-removing pulse are contained within a positive-ion-rich ionized gas. For the sake of GPC this containment of electrons is fortunate since the potential complication of electron loss by neutralization on cell walls during t h e pulse-free periods is thereby eliminated. However, the same force which contains electrons within the ionized gas also tends to drive excess positive ions to the cell boundaries. Thus, some positive ions will migrate to the same electrode which served to collect electrons during the application of each pulse. The time-averaged negative current measured from the pulsed ECD will thereby be lowered and complicated by positive-ion migration currents. In the space-charge model, eq 1 expresses f = (1 - a,f, (1)
EXPERIMENTAL SECTION The ECD is shown in Figure 1. It was fabricated from stainless steel. The "gas in" port is located on the side so that at a later date this source can be attached to a vacuum envelope for mass spectral studies (6). A cylindrical platinum foil embedded with 15 mCi of 63Ni(New England Nuclear, Boston, MA) forms the wall of the ionization cell. The volume of the active region is 1.6 cm3. A ceramic feedthrough is secured to the top of the cell, held in place with a Teflon gasket and clamps. A 1/18-in.stainless pin slides within the feedthrough to the two extremes shown. The entire cell is placed in an oven which was maintained at 184 "C. The main block of the cell can be electrically isolated from ground potential. Glass tubing interfaces the detector to the gas chromatograph. The electronics associated with the ECD are shown in Figure 2. A pulser applies positive or negative 45-V pulses of variable frequency and width to the entire ECD cell block. The ECD current is read from the central pin by use of an operational amplifier (RCA CA3140S) with a capacitor and a precision 108-Q resistor (*I%) as shown. (The currents measured have been found to be independent of the precise values chosen for the feedback resistor and capacitor.) During the course of our studies with several pulsed ECDs, we have noted that currents of various magnitudes are frequently observed in the absence of applied pulses. These have been thought to reflect the presence of contact potentials within the cell. Contact potentials are recognized as an important cause of analysis error when the ECD is operated in the dc mode (8) but are generally found to be overwhelmed in the pulsed mode. As long as the frequency of pulsing is rea-
the net time-averaged current, f, measured with a pulsed ECD, where f, is the time-averaged electron current due t o the collection of all electrons within the ECD a t the anode during 0003-2700/80/0352-1842$0 1.OO/O
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1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 12, OCTOBER 1980
1843
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Figure 3. ECD standing current measured as a function of pulse width for the pin-out (A) and the pin-in (B) placements of the anode. Pulse period is 500 ps Temperature is 184 O S
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Figure 2. Circuit for measuring the pulsed ECD current
sonably rapid ( T 5 500 ps) we have found, also, that the effects of these potentials are largely overwhelmed. Nevertheless, a small potential, Eblas,shown in Figure 1, has been used here in an attempt to compensate for any effects of contact potentials when they are observed. The value of is selected so that the measured current is zero when no pulses are applied to the cell. The value of Eblas necessary to eliminate the pulse-free current for the measurements reported here is about +350 mV. Within the range of pulse periods used here ( T 5 500 ~ s )the , responses to sample are independent of whether or not Eblmis used, while the measured standing current is only slightly affected. (For pulse periods greater than 500 ps, both response and the measured standing current become progressively dependent on the use of Eb,=for a cell showing a pulse-free current. These observations are presently under further investigation.) The gas chromatograph is home built, incorporating a 10 f t by in. stainless column packed with 10% SF-96 on Chromosorb W. The oven is held at 40 "C. Gaseous samples are introduced to the GC by using a 2.0-mL gas sampling loop (Carle 8030). The carrier gas was 10% methane in argon (Matheson) and was first passed through CaS04 and 13X molecular sieve filters. Samples were prepared by successive dilutions into nitrogen of CCl, by use of glass airtight vessels. The final dilution is into a 22-L carboy pressurized to 200 torr above atmospheric pressure. From this reservoir numerous aliquots could be transferred to the GC sample loop with a 100-mL glass syringe. The reproducibility of this method of sample introduction to the GC is excellent (better than & I % ) .
RESULTS AND DISCUSSION In determining the appropriate operating parameters of a pulsed ECD, it is necessary to determine first the width of the pulse necessary to collect all of the electrons a t the anode during each pulse. A plot of the standing current vs. pulse width for the pin-in and pin-out configurations of the ECD a t a pulse period, T , of 500 ps is shown in Figure 3. This experiment will also provide the first test of our predictions discussed previously concerning eq 1. With the pin centered within the cell, the measured current reaches a plateau level with the application of very short pulses. Our pulse generator is incapable of creating square wave forms of pulse widths less than about 0.3 f i s . With this pulse width, a standing current plateau a t about 5.4 nA is already achieved for the concentric
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Figure 4. Chromatograms showing ECD responses to a CCI, sample under the following detector conditions: (A) displaced pin, (B) concentric pin, (C) disphced pin, reverse polarity pulsing, (D) concentric pin, reverse polarity pulsing. Pulse period IS 500 ps Temperature is 184 OC
geometry. With the central pin pulled back, a standing current plateau is also obtained, but a greater pulse width of about 6 ps is required to achieve it. This difference for the displaced pin geometry is expected due to the 'longer average distance the electrons must travel to reach this anode and because the applied electrostatic field experienced in remote regions of the cell during each pulse will be somewhat weaker. I t is important t o note in Figure 3 the relative magnitudes the standing currents achieved with the two configurations. With the pin-out geometry, the current reaches its plateau a t about 7.0 nA. At the same pulse width, the pin-in current is about 5.4 nA. According to the space-charge model of the pulsed ECD (3, the measured negative current will be lessened by a positive ion migration component, the magnitude of which will be dependent on cell design. In terms of eq 1, previously discussed, it is reasonable tcl expect that the pin-out current will be larger than the pin-in current by the ratio 1.32 (where 6 = 0.25 in eq 1 for the concentric and 6 = 0.01 for the displaced geometries). The observed ratio of standing currents, 1.30, is in good agreement with this expectation. In Figure 4 are shown the portions of four repeated chromatograms recorded during the elution of CC1, from the column. A pulse period of T = 500 LLS is used in each case. In chromatogram A the usual negative ECD current has been monitored (by applying pulses of negative polarity to the cell wall) while the cell is in the pin-out configuration. Conditions are identical for chromatogram B, except t h a t the cell is in the pin-in configuration. As predicted, the magnitude of the response to sample is significantly reduced for the concentric
1844
ANALYTICAL CHEMISTRY, VOL. 52, NO. 12, OCTOBER 1980
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Figure 5. Relatrve peak area responses to CCI, under the same detector conditions as described in Figure 4, but for a range of pulse periods. The points indicated X are the sums of the corresponding points on curves €3 and D
anode. Furthermore, the reduction of response for the concentric anode relative to the response measured with the displaced anode is very close to the predicted value of 25%. T h a t this is due primarily to the effect of space-charge-driven positive ions can be seen directly by performing another simple experiment. In chromatograms C and D, the polarity of the pulser is reversed to provide positive pulses of 45 V to the cell wall. I n these cases, electrons are drawn to the wall during each pulse and are not measured by the electrometer. The remaining positive-ion component of the ECD signal is then measured a t the pin. In the case of ECD current with reverse polarity pulsing, the positive-ion response to the CC14 sample is shown in chromatogram C with a displaced pin and in chromatogram r)with the concentric pin placement. It is seen here, perhaps more directly than in any other experiment, that the contribution of positive ions to the ECD signal is, indeed, significantly larger with the concentric than with the displaced anode. T h e experiments shown in Figure 4 were repeated by using progressively shorter pulse periods typical of those used in other studies of GPC (1-3). The relative peak areas thus obtained are shown in Figure 5 where again the points on curves A and B are for normal polarity pulsing while those of curves C and D are for reverse polarity pulsing. Curves A and C are for the pin-out geometry and curves B and D are for the pin-in geometry. In all cases the response to sample is approaching a limiting value as the period approaches 500 ps. This effect is expected and is well understood for the case of CC14 (1-3), since with the longer pulse periods and a t the flow rates used here the fraction of CC14 which undergoes electron capture is approaching a limiting value close to unity. At the shorter pulse periods the average electron population in the cell is significantly less causing the reaction efficiency (and measured response) to be lowered. T h e data of Figure 5 allow a quantitative assessment of the responses a t all pulse periods to be made. First, eq 1and our previous calculations for the concentric geometry (7) predict that t h e ratio of signals in curve D to curve B at each value of T should be 6 / ( 1 - 6) = 0.25/(1 - 0.25) = 0.33. T h e measured ratios, ranging from 0.25 to 0.28, are in reasonable agreement with the expectation. For the pin-out configuration, the ratio of peak areas C to A is predicted to be 6 / ( 1 - 6) = 0.01/(1- 0.01) = 0.01. In each case the measured ratio is about 0.034.04. Although this measured ratio is small, it is clearly larger than the prediction. Possibly, with reverse polarity pulsing, the unavoidable transport of some positive ions to
the pin during the application of the 6-ps pulse causes 6 to appear greater than it is with normal polarity pulsing. In support of this suspicion is the observation that in curve C the measured response a t T = 100 p s is actually somewhat larger than that measured at T = 500 ps. In any case, the value of 6 for the displaced anode is clearly quite small relative to that of the concentric design. If the efficiency of electron attachment to CC1, in a given cell is independent of the position of the anode, the sum of the responses at each pulse period on curves B and D of Figure 5 would be expected to equal the sum of curves A and C. At each 100-ps increment the sum of the points on curves B and D is indicated with an x. Since the values on curve C are small and contribute relatively little to the sum A C, the value of x can be compared to the corresponding point on curve A. I t is seen that a t 100 p s the sum X is only about 65% as great as the value on curve A. As the pulse periods are made longer, the sums of the responses for the two configurations approach each other in magnitude as was initially expected. The clearly unequal sums a t short pulse periods may be reflecting the importance of another prediction which follows from the space-charge model. T h a t is that the electrons are held within a specific region of space within the active volume of the ECD (7). With fast pulsing (100 p s ) the size of this region will be small relative to the total ionization volume (7). Furthermore, the shape of the region containing electrons will be very different for the two configurations. With the concentric pin, electron growth will be initiated a t the points in space half of the distance from t h e center of the central pin to the cell wall (see Figure 3 of ref 7 ) . For this configuration the electron cloud takes the shape of the walls of a cylinder. With the pin-out configuration and fast pulsing, the electrons will be contained by a different positive-ion spacecharge field to the very center of the ionization cell. The electron cloud here assumes the shape of a solid cylinder centered within the cell. It seems quite possible that the two shapes of the electron clouds thus envisioned are traversed in different ways by the flow pattern within the ECD. Thus, design-dependent efficiencies of ionization may be observed. Our data would indicate that the second electron cloud described is traversed more effectively by the flow patterns within our cell, if this is, in fact, the cause of our observations. At longer values of pulse period, the sizes of the electron clouds are large in both cases and differences in the effects of their shapes become minimized. In summary, the experiments described here provide additional insight into the operation of the pulsed ECD and demonstrate the superiority of the displaced coaxial design for applications such as GPC where a direct measurement of the electron population within the cell is desired.
+
LITERATURE CITED (1) Lovelock. J. E.; Maggs, R. J.; Adlard, E. R. Anal. Chem. 1971, 43,
1962-1 965. (2) Lillian, D.; Singh, H. B. Anal. Chem. 1974, 46, 1060-1063. (3) Lovelock, J. E.; Watson, A. J. J . Cbromatogr. 1978, 158, 123-138. (4) Grirnsrud, E. P.; Kim, S. H. Anal. Chem. 1979, 57,537-541. (5) Lee, J. D.: Hirsch, R. G. Atmos. Environ. 1979, 73, 1305-1309. (6) Grimsrud, E. P.; Kim, S. H.: Gobby, P. L. Anal. Chem. 1979, 57, 223-229. (7) Gobby, P. L.; Grimsrud, E. P.; Warden, S. W. Anal. Chem. 1960, 52, 473-482.
(8) Lovelock, J. E. Anal. Chem. 1983, 35,474-481.
RECEIVED for review March 10,1980. Accepted June 25,1980. This material is based upon work supported by the National Science Foundation under Grant No. CHE-7824515.
