is thus eliminated, a potential throughput advantage of an interferometric or multiplexed instrument for the same measurement may not exist. Finally, the optimum (maximum) S/Nfor the measurement of a modulated atomic line (e.g., Figure 6) occurs if proportional signal carried noise is limiting; however, this can only occur for relatively large signals. For smaller signals, i.e., a t or near the detection limit, the best S/N ratio will result when background shot noise limitation occurs. If the spectrometer disperser (grating or prism) is substituted for one with larger dispersion, the background will be reduced while the signal remains constant. Thus, the resulting S/N ratio will be higher at the same slit setting after such a substitution. Furthermore, the limiting noise may revert back to signal carried shot noise, in which case more improvement could be realized by further opening the slits until background carried shot noise is dominant again. Thus, for atomic fluorescence excited with a line source, the optimum single channel spectrometer should have
the greatest dispersion possible (as allowed by the wavelength of the line being measured, Le., with the use of a grating with groove spacing just greater than half the wavelength of the line) and a slit width wide enough to achieve background shot noise limitation. With a multichannel spectrometer, an increase in dispersion to achieve a S/Ngain will result in a simultaneous decrease in the monitored spectral region, assuming the use of image detector.
LITERATURE CITED ( 1 ) T. L. Chester and J. D. Winefordner, Spectrochim. Acfa, Part 6, 31, 21 (1 976). (2) J. D. Winefordner, R. Avni, T. L. Chester, J. J. Fitzgerald. L. P. Hart, D. J. Johnson, and F. W. Plankey, Specfrochin?.Acfa, Part B, 31, 1 (1976).
RECEIVEDfor review June 30,1976. Accepted October 4,1976. This work supported solely by AF-AFOSR-F44620 76 C 0005. One of the authors (TLC) acknowledges the award of a fellowship sponsored by the Procter & Gamble Company.
Determination of Hydrogen Sulfide, Carbonyl Sulfide, Carbon Disulfide, and Sulfur Dioxide in Gases and Hydrocarbon Streams by Gas Chromatography/Flame Photometric Detection C. David Pearson* and W. J. Hines Phillips Petroleum Company, Research and Development, Bartlesville, Okla. 74004
The application of a gas chromatograph/flame photometric technique to the separation and measurement of four compounds, H2S, COS, CS2, and SO2 in inert gases and hydrocarbon streams, is described. Three columns were necessary to achieve separation of the sulfur compounds from each other and from interferences. These procedures are appllcable up to the 50-ppm level. Higher levels are measured after dilution. The lower detection limits in mol ppm are: COS and CS2,0.2; SO2, 0.4; and H2S, 0.8. The standard deviation for H2Sat 6.70 mol ppm level is 0.23 and for COS at 3.97 mol ppm is 0.06. Although not measured, CS2 is expected to behave like COS and SO2 like H2S. The accuracy of the blends prepared by means of permeation tubes is estimated to be 3 % of the amount present.
The determination of trace sulfur compounds is a difficult problem and the analysis of the four compounds which are the subject of this report is no exception. Chemical methods of analysis are in use for carbon disulfide (CSz), sulfur dioxide (SO?),and hydrogen sulfide (H2S); the first two are limited to liquids. Carbonyl sulfide can be analyzed chemically down to 10 ppm and by gas chromatography down to 0.1%. The development of the flame photometric detector (FPD) ( 1 , 2 ) in conjunction with gas chromatography (GC) has allowed the determination of these four compounds a t levels down to 0.5 mol ppm. Below this level special techniques and apparatus have made possible the detection of selected compounds as low as 10 ppb ( 3 , 4 ) . This report describes the experimental work carried out in developing analytical techniques for the determination of H2S, COS, SO.,, and CS2 in gases and refinery streams. A previous publication described the modifications to the gas chromatograph and the detector (5).Our system will detect down to
0.2 mol ppm of COS and CS2,0.4 mol ppm of SO2 and 0.8 mol ppm of H2S. This is satisfactory for the uses that have been encountered to date. The stainless steel GC columns described in this report are more robust than the glass or Teflon columns used by others for measurement of sulfur compounds at sub-ppm levels ( 3 , 6 ) .Calibration of the GC/FPD is by permeation tube blends which provide a stream of known composition ( 4 , 6, 7). The hydrocarbon portion of a sample frequently extinguishes the flame in the FPD causing a loss of sensitivity and necessitating recalibration. Reversal of the fuel gas inputs, as recently described (8), eliminates this problem and gives satisfactory operation despite a reduction in response by a factor of two or three. In addition, for quantitative analysis it is essential that the sulfur compounds be separated from the hydrocarbons. When hydrocarbon replaces hydrogen in the flame, the excitation energy-and hence the signal-is reduced. Separation has been accomplished of the four sulfur compounds from each other and from hydrocarbons in the list of samples below. Three GC columns are used. Each specific sulfur compound requires a separate calibration. The determination of these four sulfur compounds in natural gas, nitrogen, hydrogen, stack gases, shale oil off-gases, coal liquefaction off-gases, propane, butanes, ethylene, propane/propylene, butenes, Claus unit tail gases, butadiene, light ends of gasoline, and gases given off polymers when heated is described herein.
EXPERIMENTAL Apparatus. A Tracor Model 550 gas chromatograph equipped with dual electrometers and a flame photometric detector, Tracor Analytical Instruments, Austin, Texas, were used in this work. Samples were admitted t o the GC from 1- or 5-cm3 glass sampling loops through a 6-port rotary gas sampling valve, Perkin-Elmer Corp., ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
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Table I. O p e r a t i n g Conditions for the Gas C h r o m a t o g r a p h i c C o l u m n s Approximate retention time, s Column temperature, "C
Helium inlet pressure,psi
PPE on Chrom. G
70
sp. silica gel Q F 1 on Porapak QS PPE on Chrom. G
60 65
30 30 30
90
60
Column 1). 3 m 2 ) . 1.8 m 3). 1.8 m 4). 7.3 m
COS
CSz
SOz
38
38
73 163 143
51 246 143
83 116
68 224 a 188
H2S
a
482
Retention times for CS2 a n d SO:, on column 3 have not been determined, T h e y are very long.
1000
100 .x
.45
I I
L-
l010
100
1000
i
SAMPLE S I Z E (TORR)
Figure 1. Plot of
area response vs. sample size (Torr) for H2S blend
Norwalk, Conn., mounted on the side of the instrument cabinet and connected directly to the column through the oven wall. The sample loop was connected to a vacuum pump and a mercury manometer. Instrument modifications included mounting the detector on the rear wall of the oven with the column connection inside the oven and reversing the hydrogen and oxygen/air feeds to the flame photometric detector. A Tracor Mini-Perm permeation tube apparatus was used for calibration. It was operated a t 30 OC with varying flow rates of nitrogen to produce blends of known composition. Columns a n d Packings. All columns were made of 3-mm 0.d. stainless steel tubing cleaned with a sequence of solvents: benzene, chloroform, acetone, and distilled water. This was followed with a rinse of 3% Siliclad solution (Clay Adams, Parsippany, N.J. 07054) and heated a t 110 "C for 1h. The action of the Siliclad is unknown. It considerably reduces loss of the sulfur compounds, presumably by inhibiting the activity of the surface of the steel by absorption or reaction. Column 1).3.0 m of treated tubing packed with 5% polyphenyl ether (6-ring) and 0.4% phosphoric acid on Chromosorb G AW DMCS 80/100. Column 2). 1.8 m of treated tubing packed with special-treated silica gel (Tracor) Note. Column temperature should never exceed 110 "C. Column 3). 1.8 m of treated tubing packed with 5% silicone QF16500 on 8/100 mesh Porapak QS. Column 4). 7.3 m of treated tubing packed with 10% polyphenyl ether (&ring) and 0.4% phosphoric acid on Chromosorb G AW DMCS 80/100. The operating conditions for these columns are given Table I. Glass Sample Loops. The sample loops were made from 3-mm 0.d. glass tubing wound in 10-cm diameter turns. The inside surface was treated with a 3% solution of Siliclad, rinsing with deionized water and heating a t 110 OC for 1h. Loops up to 5 cm3 in volume were used successfully. Procedure. After a stable baseline has been established, a calibration blend for each sulfur-containing component is admitted to an evacuated sample loop until the required pressure is reached. The 124
* ANALYTICAL CHEMISTRY, VOL. 49, NO. l,JANUARY 1977
I
I
0
1
1 - 1
2
3
4
5
TIME ( M I N I
Figure 2. Separation of sulfur compounds on Tracor special silica gel. Column: 60 OC, 30 psi helium, 5-cm3 glass sample loop
contents of the loop are then injected into the chromatograph. The resulting area response for each component is measured. The procedure is repeated a t different pressures (typically 300, 150, 80 Torr). A suitable quantity of sample is then injected in a similar manner and the area response of each sulfur component is measured. Recalibration is required approximately every 4 h. Calculation. A calibration is obtained for each sulfur compound. A logbog plot of area response as a function of the pressure of the calibration blend is prepared (see Figure 1)and the slope of the plot is calculated. The area response for the unknown should fall within the limits of the linear portion of this plot.
where A I and A:! are arbitrarily chosen areas (e.g., in Figure 1 they could be 100 and 1000) and P I and P Z are the corresponding Torr values. The concentration of the component in the sample, in mol ppm, is calculated as follows:
where C, = concentration of compound in sample, mol ppm; P, = pressure a t some point on the log plot; C, = concentration of compound in calibration blend, mol ppm; A, = area response of compound in sample; E = slope of calibration log plot; P, = sample pressure in sample loop; and A , = area response a t P,.
DISCUSSION The work reported here is part of a larger analytical scheme for the qualitative and quantitative determination of unknown sulfur compounds i n gases and plant streams. D u r i n g the course of the latter work we developed the separations de-
Table 11. Summary of Columns Required to Obtain Analyses of the Four Title Compounds in Different Matrices Major component Columns required Compounds determined Permanent gases, water vapor Methane, ethane or ethylene Propane, propylene butanes, butenes, butadiene
1
H2S, COS, CS2, SO:!
Column 2. Special silica gel Column 2. Special silica gel Column 3. QF1 Column 2. Special silica gel
soz, cs2 HzS, COS so2
H2S, COS
Column 3. QF1 Column 4.Polyphenyl ether
cs2
Table 111. Precision of the Technique Calculated from Repeated Determinations of Blends Mean No. value, of Compound mol ppm Detmns H2S COS
TIME (MINI
Figure 3. Separation of H2S and COS on 5% QF1 column. Column: 65 OC, 30 psig helium, 5-cm3 glass sample loop
scribed here for four sulfur compounds: H2S, COS, SO2, and
cs2.
The determination of unknown sulfur compounds begins with a survey run. The sample is chromatographed on column 1 a t 30 psi helium inlet pressure and temperature-programmed from 70-200 "C a t 8 OC/min, which elutes all the sulfur compounds up to diamyl disulfide. A column packed with Tracor special silica gel (column 2) is used in most determinations. As shown in Figure 2, this column gives an adequate separation of COS and H2S in less than 2 min with well-separated CS2 and SO2 peaks within 4.5 min. This elution order reverses the normal HzS/COS order and is valuable in measuring traces of COS in the presence of larger quantities of H2S. If the order were reversed, a large H2S peak would swallow up the COS peak so that it would not appear. This column is satisfactory for use with sulfur compounds in air, hydrogen, nitrogen, and other gases which emerge before COS, making possible the analysis of all four sulfur gases in one run. The presence of hydrocarbons complicates matters because of the requirement that the sulfur compounds be separated from them. If not separated, a reduced or no response may be obtained depending on whether the sulfur compound is in high or low concentration relative to the hydrocarbons. This effect is due to the hydrogen supply to the flame being replaced by hydrocarbons with a consequent reduction in excitation energy and, hence, response. The FPD contains two electrodes which facilitates separation studies. The photometric output measures the sulfur-containing components and a flame ionization electrode measures the hydrocarbon components. Both measurements are thus made on the same portion of the sample. The analysis of samples containing hydrocarbons requires that determinations be made on two or more different and
6.9 4.0
11 11
Confidence limits at95% level 0.15 0.04
Std dev 0.23
0.06
Re1 std dev, % 3.3 1.6
separate columns. Methane, ethane, and ethylene overlap the COS and H2S peaks on the silica gel column. When this happens, the QF1 column (column 3) is used to separate and measure COS and H2S. This column gives an excellent separation as shown in Figure 3. Here H2S and COS are separated by 80 s and they emerge between ethane/ethylene and propane/propylene, the latter being a major constituent. H2S and COS are shown on the FPD trace in Figure 3 and the hydrocarbons are shown on the flame detector trace (FID). CS2 and SO2 have retention times on this column that are too long to be useful; they are measured on the silica gel column. We analyzed a number of natural gas samples and one ethylene sample by use of these two columns. When hydrocarbons heavier than methane, ethane, and ethylene are encountered the analysis becomes more complicated because of hydrocarbon interference in measuring HzS, COS, and CS2. Under these conditions only SO2 can be determined on the special silica gel column (column 2). H2S and COS can be determined on the QF1 column (column 3) because they emerge ahead of the CBand heavier hydrocarbons except propylene. CS2 was determined using column 4. This column was also used for the determination of the light mercaptans in some samples. Column 4 has good separating power but it partially absorbs reactive compounds such as SOz. Although column 4 will separate CS2 and SO2 from most light hydrocarbons, it is preferable to use the silica gel column for trace SO2 determinations. The minimum detectable quantity rises from 0.4 to 1.0ppm SO2 when the determination is made on column 4. Table I1 summarizes the above, showing the columns required to obtain analyses of the four title compounds in different matrices. With most of these samples we were able to determine all four components on these three columns in a straightforward manner. A problem arose with the determination of COS in a propane/propylene stream because the C3 peak emerged so early that it covered up the COS peak. When the column 3 temperature was lowered from 65 to 55 "C and the sample size reduced from a 5- to a l-cm'>loop, the C3 retention time increased from 257 to 330 s. The COS retention time determined with a permeation tube blend of COS in nitrogen, was 345 s which apparently puts the COS peak eluting in the middle of the large C3 peak. We found, however, that with a blend of COS in propane the COS retention time decreased to 312 s. On this basis we concluded that the COS peak was eluting before the C3 peak. As none was detected in our sample, we ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
125
were able to calculate a "less than" value. The shift in this retention time is probably due to the hydrocarbon saturating the column and reducing the time that the COS component spends dissolved in the liquid phase as in frontal analysis. (Note: The retention time given for COS in Table I is in nitrogen.) Determination of High Concentrations. The technique described is satisfactory for trace quantities in the 0.5- to 50-ppm range but cannot be used directly with higher concentrations. Both H2S and SO2 in the 0.5 to 8% range have been determined by bleeding a measured pressure of sample into an evacuated container and then admitting nitrogen or air until atmospheric pressure is reached. (Nitrogen should be used when H2S is to be diluted.) After equilibration, the diluted mixture is analyzed by the standard technique or may be diluted again if necessary to bring the response close to that of the calibration standards. Interferences. Other sulfur compounds elute after the four title compounds on columns 1,2, and 3. On column 4, CS2 is separated from all the light sulfides and mercaptans. Interference from hydrocarbons has been eliminated as described above. Accuracy a n d Precision. The accuracy of the determination is considered to be the same as the accuracy of the permeation tube blends. The error in the latter value is due to error in the measurement of the emission rate and the flow rate of the diluent gas. Calculation of error for several per-
meation tube/flow rate combinations shows that the error lies between 2 and 3%of the amount present. The precision of the technique was evaluated by repeated determinations of blends. A full calibration was run for each determination of the blend. The precision for H2S and COS are shown in Table 111. The H2S data have approximately twice the variability of the COS data. This is not unexpected in view of its greater reactiveness. In both cases the precision is satisfactory. Precision data have not been obtained for CS2 and SOz but it is thought that the CS2 data would be similar to COS and the SO2 to HzS because of similar chemical reactivity of these pairs. LITERATURE C I T E D (1) S. S. Brody and J. E. Chaney, J. Gas Chromatogr., 4, 42 (1966). (2) H. W. Grice, M. L. Yates, and D. J. David, J. Chromatogr. Sci., 8, 90 (1970). (3) C. H. Hartmann, Joint Conf. Sensing Environ. Pollutants, Collect. Tech. Pap., November, 1971, AlAA paper 71-1046. (4) R. K. Stevens, A. E. O'Keefe, and G. C. Ortman, Environ. Sci. Techno/.,3, 652 (1969). (5) C. D. Pearson, J. Chromatogr. Sci., 14, 154 (1976). (6) R. K. Stevens, J. D. Mulik, A. E. O'Keeffe,and K. J. Krost, Anal. Chem., 43, 827 (1971). (7) A . E. O'Keeffe and G. C. Ortman, Anal. Chem., 38, 760 (1966). (8) C. A. Burgett and L. E. Green, J. Chromatogr. Sci., 12, 356 (1974).
RECEIVEDfor review September 10,1976. Accepted October 8, 1976.
Determination of Total Sulfur in Gasoline by Gas Chromatography with a Flame Photometric Detector Dwight A. Clay,' Crystal H. Rogers, and Robert H. Jungers
US.Environmental Protection Agency, - Environmental Monitoring and Support LaboratoryIRTP, Analytical Chemistry Branch,
Research
Triangle Park, N.C. 2771 1
Total sulfur in gasoline is determined by gas chromatography with a flame photometric detector. The analysis time is less than 5 min wlth a detection limit of 0.002% S (by weight) and a precision of f10% for duplicate results by the same operator. The main source of error Is the impreclsion of injection of very small sample volumes (0.1-0.4 PI). Gasoline samples wlth a sulfur concentration of up to 0.12% sulfur can be analyzed without dilution or sample preparation.
Analyses for sulfur in gasoline is a routine determination in refineries and petroleum oriented laboratories. The standard methods used for this analysis are American Society for Testing and Materials (ASTM) methods D1266 (Lamp Method) ( I ) and D-2622 (X-Ray Spectrographic Method) ( 2 ) . The primary disadvantage of the Lamp Method is the time required for analysis; an analyst may be able to analyze, a t most, 11samples plus a blank in an 8-h period. Although the x-ray spectrographic method is fast, it also has the disadvantages of being expensive and of requiring a highly trained operator. The specificity and sensitivity of the flame photometric detector (FPD) for sulfur compounds has been well documented (3-5).It therefore constitutes the basis for the method described below (Figure 1). 126
ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
EXPERIMENTAL Apparatus. A Perkin-Elmer Model 900 Gas Chromatograph (GC) was equipped with an F P D and control unit from Perkin-Elmer. In addition, a gas mixing device (Figure 2) made of glass with a volume of 60 ml was installed in the oven instead of a column. This gas mixing device is not commercially available and was fabricated from glassware on hand in the laboratory. Any gas mixing device that is close to the same volume (50-70 ml) should work as well. A Hamilton Co. No. 7001 syringe was used for sample injection. The data readout was via a Perkin-Elmer Model 56 Recorder and a Perkin-Elmer M-2 Calculating Integrator. A Varian Aerograph Model 9652 Hydrogen Generator was the source of hydrogen. All gas supplies to the GC were filtered with Perkin-Elmer Filter Drier Assemblies. Reagents. Prepurified grade nitrogen from Union Carbide was used as the carrier gas, and house air was used for the FPD. T h e calibration standards used were 0.012, 0.030, 0.080% sulfur by weight made by diluting a 0.120% sulfur standard with sulfur-free reference fuel. The blank was sulfur-free reference fuel. Procedure. The operating parameters for the GC/FPD were as follows: carrier gas, 60 ml/min; air, 100 ml/min; and hydrogen, 150 ml/min.; injection port temperature, 100 "C; oven temperature, 170 "C; detector temperature, 130 "C; manifold temperature, 160 "C. The manifold is not a mixing manifold but is a plumbing area between the column outlet and the detector that is used to direct the column effluent to either or both of the dual detectors in the Perkin-Elmer Model 900 GC. The sample was injected by syringe and in 3-5 min the peak area was printed out on the integrator. A calibration curve was drawn by plotting the logarithm of the concentration as a function of the logarithm of the peak area. The sample concentration was then
