334
Anal. Chem. 1986, 58,334-337
Linearity Assessment of Detectors in Gas Chromatographic Systems by Sample-Loop Supercharge Ewan R. Colson Gas and Fuel Corporation of Victoria, Scientific Services Department, P.O. Box 83, Highett, 3190, Victoria, Australia
For nonilnear detector/system combinations, the exponent, I , of a commonly assumed response vs. sample quantity relation deviates from unity. If the ratio of two sample amounts is known with good precision, then a mean I value over the range between these sample levels can be assessed with slmllar precision, even without such precise knowledge of these levels. A gas sampling valve loop, charged to two accurately measured pressures, Is a convenient device for injecting known analyte quantity ratlos. Methane/ntIrogen and propane/nitrogen samples were separated, and linearity aberrations ( I < 0.998) were observed in two commercial flame Ionization detectors at peak currents exceeding 9 X lo-' A, or 4 X lo-' g/s of carbon. Thermal conductivity detector systems were also evaluated. Deviatlons from iinearlty were noted at peak concentrations of 1.7% nitrogen ( I = 0.993) and 18% methane ( I = 0.965) In helium carrier gas.
This work was commenced in order to investigate the belief, tacitly assumed and expressed only infrequently (e.g., by Krugers (I)),that the flame ionization detector (FID) of McWilliam and Dewar (2) becomes progressively more linear as the sample feed rate is reduced. The linearity of the FID has been measured (3) by the exponent n in the relation
S = A(drn/dt)"
(1)
where S is the detector signal, dm/dt is the sample mass flow rate, and A is a sensitivity constant. The conclusion about progressive onset of linearity requires 1 as dm/dt 0, and hence the reliable deproof that n tection of sensitivity variations of the order of 0.1%. Such proof is prerequisite to the method applied to the evaluation of FID high-end linearity in ref 4. In practical gas chromatography, the detector is only part of a complete system, and it responds to a varying sample mass flow or concentration during the elution of a peak from the column. Thus a weighted average of detector linearity deviations over the peak profile combines with other sources of nonlinearity to influence the peak area/sample amount calibration curve. With low concentrations of light, nonpolar sample molecules, it is possible to avoid significant peak distortion attributable to column factors so that the study of peak area ratios vs. sample amount ratios can provide detector linearity information, limited in accuracy only by the precision of the sample amount ratios and by the system stability. The same technique, with larger sample sizes, can provide in situ linearity measurements more pertinent to actual laboratory practice. Shatkay and Flavian (5) and Bromly and Roga (6) fitted FID peak area/sample size data to relations of type 1 with different variables, e.g.,
-
-
R = BQ1
(2)
where R is the peak area response, Q is the injected molar
quantity of sample species, B is another sensitivity constant, and 1 is another exponent. These and other workers (7, 8 ) observed FID nonlinearities by comparison of peak area ratios of two species over a range of sample loadings, injected as liquid dilutions with a microsyringe. Bromly and Roga (6) tested two FID's and a thermal conductivity detector (TCD) against eq 2 using measured gas dilutions injected with a gas sampling valve (GSV). In the work now presented, detector linearity was assessed by the use of a GSV with two sample loops with a volume ratio of about 5. During each experiment, peaks from supercharged small loop samples were interspersed with peaks from ambient-pressurized loops and the areas were stored. The exponent 1 for two commercial FID's under several operating conditions was calculated from the mean of a specific peak area ratio, at several concentration levels of each of the test species, methane and propane. The trend of 1 was critically examined as sample concentration levels were reduced. The technique was also applied to a TCD in systems with helium and argon carrier gases.
EXPERIMENTAL SECTION Equipment and Gas Flow System. Figure 1 shows the interconnection scheme. Air and hydrogen were supplied to the FID's, D1 and D2, and helium or argon was supplied to the TCD, D3, at rates according to the detector manufacturers' recommendations. In the case of the Hewlett-Packard FID, the hydrogen and air flows were nominally 31 mL/min and 420 mL/min, respectively. The remaining gas flow to the FID's consisted of carrier gas and makeup gas supplied from inlet I. Interconnecting tube was 1.6 mm o.d., 0.8 mm i.d., and was made of stainless steel or nickel. The jumper, J, was a short link within the air oven, TCZ, to the Packard-Becker FID D1 or a 1200-mm unheated line feeding the Hewlett-Packard detectors, D2 and D3, in the adjacent 5880A oven. In the TCD work, the column C1 was replaced with a column C2 in this separate oven together with a reference column. The flow path from the sample source, S, following the continuous lines through valves V4A,VlA,VlB,V6,and V4B,enabled charging of the small loop, SL, at ambient pressure, prior to sample injection into the column by V6 operation. V6 was then in readiness for the charging of the alternative large loop. V1B was used to isolate V6,the sample loops, and pressure transducer, P, immediately prior to sample injection by V6 operation. V1A was operated to allow venting (via L1) of the upstream end of a sample loop when atmospheric pressure sampling was required. It operated in parallel with VlB,but because of its different design, it vented before the slow V1B started to move. This venting operation was enabled by a logic gate only when V4A and V4B were in the ambient pressure position, as illustrated. Superchargingof the small loop was arranged by switching V4A and V4B to the alternative positions shown in Figure 1. Then the sample, at the pressure set by its controller, was fed without significant restriction to the loop, the flow now being regulated by restrictor R3 and vented via H. Table I shows details of the experimental program as applied to evaluate Hewlett-Packard FID and TCD systems. The Packard-Becker FID was also studied with the same analyte compositions. The suffix letters on the file numbers indicate a range of similar experiments (e.g., there were four experiments
0003-2700/88/0358-0334$01.50/00 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 2, FEBRUARY 1986
335
L I S T CLOCk T I n L 12:31 JUN 2 8 , 1982 READ BFIROMCTER
'1 R1 &A
2 '
ax.
6 ,
li. 3 .
e.*. ?.5.
1
Figure 1. Supercharge system: (C 1, C2) akernative column locations; ( D l , D2, D3) detectors; (FC) flow controllers; (H, L1, L2) hlgh- and low-pressure vents; (I) carrier and makeup gas inlet; (J) jumper link; (LL, SL) large and small sample loops; (P) pressure transducer; (PC) pressure controllers; (Rl, R2, R3) restrictors; (S) sample source; (TCZ) temperature controlled zone-air oven; ( V lA, V4A, V4B) valves, type EVO-3 (Clippard Instrument Laboratory, Inc., Cincinnatl, OH); (VlB) six-port valve, type AH 3 CV-6-HPa (Valco Instruments Co., Houston, TX); (V6) eight-port valve, type AH3 CV-8-HPa (Valco). Valve numbers correspond to 5880A software addresses.
Table I. Selected FID and TCD Experiments analyte compd 70
carrier flow rate, gas mL/min
data file group
detector
87A-T 88A-D 89A 93A-B 94A-B 101A-C
FI FI FI FI FI FI
CHI CHI CHI C3Ha C3Hs CHI
0.032 0.165 0.256 0.010 0.157
Nz Nz He
29.6 29.5 30.1 29.1 29.2 29.5
96A 97B-N 98A-C 99A-B lOOA
TC TC TC TC TC
He CHI
4.3 0.47
Ar He He He He
20.2 24.3 23.9 24.3 24.2
N2
C3Hs CH,
0.857
5.5
0.42 100
Nz Nz
Nz
of type 88). The analyte, at the percentage noted, was blended in a pressure vessel with the carrier gas used for each type of experiment. The last column lists the carrier gas flow rates, including makeup gas for the FID. Where a reported flow is a mean over several similar experiments, the coefficient of variation was
