ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978
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Comparison of Quenching Effects in Single- and Dual-Flame Photometric Detectors Paul L. Patterson Varian Instrument Division, 2700 Mitchell Drive, Walnut Creek, California 94598
A recently developed dual-flame photometric detector can be easily converted to a single-flame mode. Using this burner, single- vs. dual-flame operation was compared with respect to problems of quenching of light emissions. I n the case where both sample decompositlon and light emission occur in a single flame, light emissions from sulfur and phosphorus compounds are severely quenched by the presence of a hydrocarbon background in the flame. I n addition, single-flame operation exhibits quenched sample responses dependent on the molecular structure of the sample. By separating the region of sample decomposition from the region of optical emission in a dual-flame mode of operation, these quenching effects are minlmlred. A good application of the dual-flame Operation Is shown to be the analysis of sulfur compounds in gasoline.
In gas chromatography, the flame photometric detector (FPD) is commonly used for the specific detection of sulfuror phosphorus-bearing compounds. This detector senses light emissions from S2and H P O molecules that are formed by the combustion of samples in hydrogen-rich, hydrogen-air flames. In the past, the most popular version of the F P D ( I ) has utilized a single flame for the dual purpose of decomposing incoming samples as well as generating the desired optical emissions, Two well-known problems in these single-flame detectors have been sample responses which are strongly compound dependent ( 2 ) ,and sample responses which are severely quenched by the presence of a hydrocarbon background in the flame (3-7). In order to overcome the hydrocarbon quenching problem, it has been necessary to ensure t h a t the detected sulfur or phosphorus compounds are well-resolved from major organic compounds which also elute from the gas chromatograph (GC) column. For complex samples, this requirement has often dictated the use of very high-resolution GC columns such as capillary columns (8). In a preceding paper by Patterson, Howe, and Abu-Shumays (9), the design and performance of a new dual-flame F P D is described. One of the features of the burner employed in this dual-flame F P D is that a simple on-off control of an air source allows a n easy conversion t o a single-flame mode of operation. Consequently, the same burner can be operated in either a single- or a dual-flame mode so that the two methods of operation can be readily compared. The purpose of the present paper is to describe such a comparison of singleand dual-flame operation, especially with respect to the problem of quenching of emissions in the flames. I t will be shown that one of the most significant advantages of dualflame operation is the minimization of quenching effects on the sulfur and phosphorus emissions.
EXPERIMENTAL The work described in this paper employed the flame photometric detector manufactured by Varian (Palo Alto, Calif.) and a Varian Model 3700 gas chromatograph. Chromatographic columns and operating conditions are indicated where used. Solvents used to prepare test samples were nanograde 2,2,4trimethylpentane obtained from Mallinckrodt (St. Louis, Mo.) 0003-2700/78/0350-0345$01 .OO/O
and chromatoquality grade butyl alcohol obtained from Matheson, Coleman, and Bell (Norwood, Ohio). Thiophosphate pesticides were obtained from Polyscience (Niles, Ill.) as 1%solutions by weight in benzene. These thiophosphate samples were further adjusted to desired concentrations by dilutions in 2,2,4-trimethylpentane. All other sample mixtures were prepared by dilutions in either 2,2,4-trimethylpentaneor butyl alcohol using pure chemicals obtained from Chem Service (West Chester, Pa.). Gasoline samples were obtained from service stations representing two major oil companies and two independents. In the Varian dual-flame FPD, hydrogen and air flow rates normally supplied to the burner are as follows: H2, 140 mL/min; air 1, 80 mL/min; and air 2, 170 mL/min. The combination of H2and air 1 forms a first flame which is used to achieve sample decomposition, The unburnt H2 and combustion products from the first flame are then combined with air 2 to form a second flame for the purpose of generating the desired optical emissions. For operation in a single-flame mode, the air 1 source is turned off, and the single-flame formed from the combination of H2 and air 2 then is used for both sample decomposition and generation of the optical emission. From signal-to-noise considerations, the H2 and air 2 flow rates listed above are reasonable optimal choices for single-flame as well as dual-flame operation. For sulfur detection, the Varian FPD employs a broad-bandpass filter with peak optical transmission at 365 nm. For phosphorus detection, the filter is a narrow-bandpass filter peaked at 530 nm. In the dual-flame mode of operation, the sulfur response varies according to a square-law dependence on sulfur amount, while the phosphorus response is a linear dependence on phosphorus amount.
RESULTS AND DISCUSSION In Figure 1 there is shown a simple analysis of a 10-ppm concentration of carbon disulfide (CS,) in butyl alcohol. In this analysis, the CS2 comes off the GC column first, followed by the butyl alcohol solvent peak. In the chromatograms shown in Figure 1, the sample was injected twice with the second injection timed so that the second CS, peak came off the column a t the same time as the butyl alcohol peak from the first injection. Figure 1 shows that the dual-flame burner produces a peak for the second CS2 sample which is approximately 80% of the peak height of the first CS2, even though the second peak is present in the detector at the same time as a large concentration of butyl alcohol. When the FPD burner is operated in a single-flame mode, Figure 1 shows that the first CS, peak is larger than that obtained with dual flames, but there is absolutely no response for the second CS2 peak. In other words, in the single-flame mode the butyl alcohol solvent so alters the flame environment that light emission is drastically quenched, including light emission from the butyl alcohol as well as the CS2. The observed difference in sensitivities in the case of the first CS2 peak may be associated with different concentrations of H atoms in the emission flames. Sulfur emission depends strongly on H-atom concentration (9), and the additional air 1 supply in the dual-flame mode could result in a lowering of the H-atom concentration. T h e chromatograms shown in Figure 1 provide a clear illustration of the hydrocarbon quenching effect in the single-flame FPD. The chromatograms shown in Figure 2 provide a more subtle example of this hydrocarbon quenching. In C 1978 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978 FLAMES 1 I 10-OA I
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Figure 1. Dual- and single-flame anaiyses of 10 ppm CS2in butyl alcohol with the second injection timed to cause CS2 to pass through FPD at the same time as butyl alcohol. GC conditions: column, 200 cm X 6 mm 0.d. X 2 mm i.d., Pyrex, 5 % OV-101 on 100/120 Chrom W; carrier flow, 25 mL/min He: column temperature, 60 OC; injector temperature, 130 OC; detector temperature, 230 OC SINGLE F U M E
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Figure 2. Single-flame chromatograms of sample containing 5 ppm each of five thiophosphate pesticides. GC conditions: column, same as Figure 1; carrier flow, 30 mL/min He; column, injector, and detector temperatures all 200 O C
Figure 2, there is illustrated an analysis of a sample containing approximately 5 ppm each of five different thiophosphate pesticides. The analysis was performed using the single-flame mode and using the S filter. The two chromatograms shown correspond to injections of 1.0 and 10.0 ILLof the sample with the current range for the lO.O-~Linjection being 100 times greater than t h a t of the 1.0-pL injection. If these sulfur
Figure 3. Dual-fhme chromatograms of same sample and Gc conditions as Figure 2
emissions obeyed a pure square-law dependence on sulfur content, then the heights of the sample peaks should be the same in the two chromatograms. That clearly is not the case. In the 10-pL chromatogram, the three peaks at early retention times are greatly diminished in amplitude. This effect is caused by the underlying tail of the isooctane solvent peak which is much greater in the case of the 10.0-pL injection than in the case of the 1.0-pL injection. Although this solvent tail does not show up on the chromatogram because it does not give off any detectable light, that solvent tail is nevertheless present in the FPD flame, and it quenches the light emission from the sample peaks a t early retention times. These two chromatograms also provide a good illustration of the confusion that can develop concerning the sulfur response of single-flame detectors. For example, from the data shown in Figure 2, it could be erroneously concluded that the disyston response is approximately linear with sample amount, the methyl trithion and ethion responses exhibit a pure square-law dependence on sample amount, and the methyl parathion and malathion responses are somewhere between a linear and a pure square-law dependence on sample amount. However, if the injected sample volume is held fixed at 1.0 pL, and sulfur response is checked instead by varying sample concentrations, then it is found that all five sulfur compounds exhibit a square-law response. Consequently, variations in both sample volume and sample concentration revealed in this case that solvent quenching is occurring in the data shown in Figure 2. Figure 3 shows the use of the dual-flame burner for the same sample analysis described in Figure 2. With dual flames, both the 1.0-pL and 10.0-pL chromatograms exhibit the same relative peak height distribution. Also, all five sample peaks exhibit a square-law sulfur response within the accuracy of the GC analysis. Hence, the dual-flame burner eliminates the distortion of the chromatogram caused by the quenching of sample response by an underlying solvent tail. Figure 3 also illustrates the advantage of injecting larger sample volumes. By increasing sample volume by a factor of 10, the signalto-noise ratio has been increased by a factor of 100. Also, the selectivity of sulfur response can be expected to improve with larger sample volumes because sulfur peaks increase as the square of sample amount while interfering hydrocarbon peaks
ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978
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0 - R I C H FLAME 430 nm (CHI
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Figure 4. Dual- and single-flame analyses of 20 ng each of tri-
butylphosphate and methyl parathion in isooctane solvent. 1.0 pL sample volume. GC conditions: column, 50 cm X 3 mm, 5% OV-101 on 100/120 Chrom G; carrier flow, 30 mL/min He; column temperature, 180 OC; injector and detector temperatures, 220 O C increase only linearly with sample amount. Figure 4 illustrates the analyses of 20 ng each of tributylphosphate and methyl parathion using the P filter and both the dual- and single-flame modes of operation. With dual flames, the F P D responds to tributylphosphate and methyl parathion with exactly the same sensitivity of 20 nA/(ng P/s). However, in the single-flame mode, the response to tributylphosphate is negligible. In this case, it seems unlikely t h a t the absence of response to tributylphosphate is due entirely to quenching by the solvent tail. Rather, it appears that this single-flame chromatogram is exhibiting a quenching effect associated with the different molecular structures of tributylphosphate and methyl parathion. Similar observations of compound-dependent sample responses have been reported in previous work with single-flame detectors (2). Another area where quenching of sample response has been a problem in the past is in the analysis of samples so complex that the GC column does not completely separate the sulfur and phosphorus compounds from overlapping hydrocarbon compounds. An example of this situation is the analysis of gasoline using a packed column. Figure 5 shows two chromatograms representing the analysis of a gasoline sample on a packed column using column oven programming from 35 to 200 "C. The top chromatogram in Figure 5 was obtained with the dual-flame F P D operated with an oxygen-rich emission flame and a special optical filter peaked a t the most intense hydrocarbon emission. This chromatogram of hydrocarbon emissions shows that there are a great many unresolved hydrocarbon peaks being eluted from the GC column. T h e bottom chromatogram in Figure 5 shows the same analysis performed using the dual-flame FPD with a normal hydrogen-rich emission flame and using the sulfur filter. Despite the simultaneous presence of many overlapping hydrocarbon compounds, the S-mode chromatogram reveals a relatively small number of large, well-resolved peaks. The identity of these S-mode peaks was not established. However, if the sample volume was increased by a factor of two, most of the peak heights increased by a factor of four. Hence, most
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Figure 5. Analysis of 2 pL gasoline sample using dual-flame FPD in hydrocarbon-responsive mode in top chromatogram and sulfur mode in bottom chromatogram. GC conditions: column and carrier flow, same as Figure 1; injector and detector temperatures, 200 O C ; column temperature program, hold at 35 O C for 3 min, program at 4 'C/min from 35 to 120 O C and 84 OC/min from 120 to 200 O C , hold at 200 O C for 3 min DUAL F L A M E S
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Figure 6. Anavsis of same gasoline sample as Figure 5 using duakflame and single-flame FPD operation. Column temperature program, hold at 35 O C for 3 min, program at 4 OC/min from 35 to 100 O C and 84 OC/min from 100 to 200 O C , hold at 200 O C for 2 min. Other GC conditions same as Figure 5
of the prominent peaks in the bottom chromatogram exhibited the square-law response dependence expected for S-containing compounds.
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978
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GASOLINE S MODE BRAND 4 - REGULAR
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Figure 8. Dual-flame, sulfur-mode analyses of 2-wL samples of different grades of same gasoline brand. GC conditions same as Figure 6 20
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Dual-flame, sulfur-mode analyses of 2-pL samples of three different gasoline brands. GC conditions same as Figure 6 Figure 7.
Figure 6 shows the same gasoline sample analyzed with both the dual-flame method and a single-flame method. Changes in the attenuation factors and a slight change in the column oven program account for a difference in appearance of the dual-flame chromatogram in Figure 6 from that shown in Figure 5 . There are three significant differences between the single-flame chromatogram and the dual-flame chromatogram. First, the peaks exhibited in the dual-flame chromatogram are a factor of 10 to 100 times larger than the peaks in the single-flame chromatogram. Second, at early retention times the single-flame chromatogram exhibits some of the same unresolved hydrocarbon peaks seen in the oxygen-rich chromatogram of Figure 5 . Finally, the single-flame chromatogram is completely missing a few of the very largest peaks present in the dual-flame chromatogram. Hence, it is clear that the dual-flame method is much superior t o the singleflame method in the analysis of complex mixtures such as the gasoline samples described here. Figures 7 and 8 illustrate the potential application of the dual-flame F P D t o the analysis of sulfur compounds in different brands and grades of gasoline. In Figure 7, there are shown chromatograms of three different brands of gasoline. All three chromatograms were obtained under exactly the same attenuations. Hence, it can be seen that there are significant differences in peak amplitudes as well as peak distributions between the three gas samples. Figure 8 shows the analysis of a fourth brand of gasoline, and illustrates the difference between the regular and premium grades of that brand. Again, there are significant differences in the distribution of peaks present in the two chromatograms.
CONCLUSIONS Unlike the single-flame FPD, the dual-flame method of operation allows excellent sample response to be obtained even for analyses where there exists a hydrocarbon background eluting from the GC column at the same time as the sulfur
or phosphorus compounds. This characteristic of the dualflame FPD means that low resolution GC columns can often suffice to analyze complex mixtures. It also suggests that less elaborate sample preparations may be employed before selectively analyzing samples for S or P compounds. However, some reduction of sample response can still occur at very high levels of hydrocarbon background. Consequently, for accurate quantitative results, the dual-flame FPD should be calibrated with standards prepared in a sample matrix that resembles as much as possible the matrix of the unknown samples. For some sulfur compounds which are well resolved from hydrocarbon interferences, the single-flame method of operation can give greater sample response than dual-flame operation. However, single-flame operation is more likely to provide sample responses that are strongly dependent on the molecular structure of the sample compound. Also, singleflame detectors are known (10) to often exhibit sulfur responses that deviate from the expected pure square-law dependence, with the amount of deviation again being compound-dependent. Dual-flame F P D operation may not be as sensitive as single-flame operation in some cases. However, dual-flame operation will generally provide more uniform response to samples, irrespective of sample composition or concentration.
LITERATURE CITED (1) (2) (3) (4)
S. S. Brody and J. E. Chaney, J . Gas Chromatogr., 4, 42 (1966). S. 0. Farwell and R. A. Rasmussen, J . Chromatogr. Sci., 14, 224 (1976). W. E. Rupprecht and T. R. Phillips, Anal. Chim. Acta, 47, 439 (1969). S. G. Perry and F. W. G. Carter, The Chromatography of Sulfur Compounds", in R. Stock, Ed., "Gas Chromatography 1970", Proceedings
of the 8th International Symposium on Gas Chromatography, Dublin, 1970, Institute of Petroleum, London, 1971, p 381. (5) T. Sugiyama, Y. Suzuki, and T. Takeuchi, J. Chromatogr., 80, 61 (1973). (6) C. D. Pearson and W. J. Hines, Anal. Chem., 49, 123 (1977). (7) D. A. Clay, C. H. Rogers, and R. H. Jungers, AM/. Chern.,49, 126 (1977). (8) L. Blomberg, J . Chromatogr., 125, 389 (1976). (9) P. L. Patterson, R. L. Howe, and A. Abu-Shumays, Anal. Chem., 50, preceding paper in this issue. (10) C. H. Burnett, D. F. Adams. and S. 0. Farwell, J . Chromatogr. Sci., 15, 230 (1977).
RECEIVED for review August 22, 1977. Accepted Pu'ovember 4, 1977.
