Anal. Chem. 1983, 55, 2193-2194
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CORRESPONDENCE Interferences in Determination of Hydrogen Sulfide in Air by Gas Chromatography with Flame Photometric Detection Sir: Several papers have been published in the past about the determination of sulfur compounds in the atmosphere by gas chromatography using flame photometric detection (1-4, and this method has been recently proposed as suitable for standardization ( 5 ) . In our view, possible interferences by other gaseous air pollutants having the same retention time as hydrogen sulfide must be evaluated to state the overall reliabiity of the method. Of course, selectivity and efficiency of the GC:column play a primary role in the elimination of interferences. These are not due to other sulfur compounds, because the column proposed here for use in this method will separate dl light sulfur compounds that may be found in the atmosphere (6). Thus, the only possible interferences could come frorn light hydrocarbons that are always present in the atmosphere, especially in the urban areas. The specificity of the flame photometric detector (FPD) for sulfur compounds ensures that a negligible response is given to non-sulfurcontaining compounds when these are present within an analogous rangie of cortcentration. However, either a disturbance of the detector signal at low H2S concentrations or a quenching effect when the concentration of the interfering compounds is very high should be expected. I n this paper the results obtained about the influence of different concentrations of interfering compounds on the detector response for €12S are reported. EXPERIMENTAL SECTION GC Column, Packing, and Material. Several column packings that separate H,!S from air, other riulfur compounds, and hydrocarbons are commercially available. From these, we have chosen a 1.6 m X 4 mm i.d. glass column packed with Carbopack B coated with 1.2% XE60 0.8% H3P04,commercially available from Supelco as Carbopack B HT 100. This stationary phase was proposed and used several years ago (6). The reason for this choice is that, from a severe examination of the litmature available, this is apparently the only packing that simultaneously will separate H2S from both SF6 and COS and at the same time ensures the separations posriible with other stationary phases. In fact, quite strangely, most author8 do not indicate I-12S separation from SF,, a constant and widely diffused component of the atmosphere, present at the low parts-per-trillionlevels even in the remote regions of the world (7);thiis compound may exist in much higher concentrations in highly industrialized sites, parts per-billion range, and possibly interfere in the determination of H2Swith FPD detection. As for the use of capillary columns, the high volatility of H2Sand COS, SF6,and SOz from which it should be separated, discourages their use, in spite of the extremely high potential resolving power of such type of oolumns. Furthermore, the injection of large air samples (10 niL) is experimentally difficult with capillary columns and implies cryogenic injection. Mercaptans, are well separated by capillary GC (8), a8 it could be easily predicted, but these compoundri are not of interest in the present work. Pyrex glass has been wed as column material, after it was shown in earlier papers (6,9)thlat treatment of the GC column and gas lines with a stream of dry nitrogen for 2 h at 200 "C makes glass completely inactive toward sulfur compounds. In fact, the layer of water condensed from the atmosphere always adsorbed on glass is responsible for interaction with H2S and other sulfur compounds.
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The column diameter is rather large compared to the usual ones, which range around 2 mm. This is due to the necessity of injecting a large sample volume to obtain an acceptable sensitivity. By use of a column with 4 mm i.d. and a sample loop of 0.8 cm i.d., 20 cm long that yields a total volume injected of 10 mL, the best results are obtained in terms of injection time for a complete sample injection of peak broadening (10). As shown in Figure 1,with such a column, the only interfering compound is ethene, which shows the same retention time as that given by hydrogen sulfide, all other hydrocarbons tested with FID being well separated. Detector and Operating Conditions. Calibration curves for H2S were drawn by means of the experimental apparatus described in ref 2 and 3, using a permeation tube to obtain known concentrations of H2S. Ethene concentrations were obtained by simultaneous syringe injection of ethene diluted in air. In fact, a permeation tube for ethene is not availabile, and on the other side, a perfect knowledge of the concentration of this compound is not necessary because just the interfering effects of ethene are under study, not its determination. For this reason, the automatic apparatus used in ref 2 was used for H2S,while ethene was injected through a normal syringe injector placed in the gas flow at the top of the column. Preliminary experiments showed that with this device simultaneous injections could be very easily obtained for the two compounds, without appreciable discrepancies and that an error of less than 7% is achieved for the ethene concentration. The detector used was a DANI Model FPD 68/7 (DANI, Monza, Italy), which operates with a double flame, according to the original Varian design. This has the advantage of reducing the "quenching" effect of hydrocarbons and prevents flame extinguishing when high concentrations of organic solvents reach the detector. Working conditions were as follows: carrier (Nz) 40 mL/min; hydrogen, 170 mL/min; air 1, 90 mL/min; air 2, 210 mL/min; column temperature, 25 "C; detector temperature, 250 O C .
RESULTS AND DISCUSSION I t is known that ethene concentrations in the open atmosphere usually range between 2 and 25 ppb and that GC/FPD methods not using preconcentration techniques (2) have a detection limit of about 20 ppb H2S. Much lower detection limits are obtained by feeding the FPD directly with the air, but in this case the specificity is completely lost. When ethene is present a t low concentration levels, it can be stated that it has in practice no effect on the FPD response. However, because of the possibility of monitoring H2Sin particular sites, e.g., oil refinery spills, it would be interesting to explore the detector response to H2S in the presence of very high concentrations of ethene. In Table I the results obtained by injecting H2S and CzH4simultaeously as described are reported as percent variation of the peak area obtained when injecting pure H2S. The influence of the presence of high concentrations of ethene is positive at low HzS concentrations and negative at high concentrations. In fact, when the 8ensitivity of the detector is high, a positive peak is registered from FPD if high concentrations of ethene are injected, and this signal gives its additive contribution to the peak area at low concentrations of H2S. This occurs when the H2S concentration is below 0.3 ppm and at least 10 ppm of ethene is
0003-2700/83/0355-2193$01.50/0 0 1983 American Chemical Soclety
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 13, NOVEMBER 1983
Table I. Percent Variation of the Square Root of the Peak Area at Various Ethene Concentrations for Different H,S Concentrations H,S,
PPm
1
0
2
min.
Flgure 1. FID and FPD responses to light hydrocarbons and HzS; column, Carbopack B coated with 0.8% H,PO, and 1.2% XE60.
G a.u
10
0.02 +3.5 0.05 t1.8 0.08 t 0.8 0.14 0 (3.17 0 0.19 0 0.23 0 0.31 0 0.47 0 0.60 0 0.70 0 0.80 0 1.09 0 1.20 0 1.35 0 1.50 0
50
ethene, ppm 100 200
+22 +130 t13 t 25 t 5.9 t 20 t3.3 +7.7 1-2.3 t3.9 t1.4 t3.6 t0.5 +1.4 t0.2 t1.5 0 t0.6 0 t 0.2 0 0 0 -0.8 0 -2.3 0 -2.0 0 -1.4 0 -1.4
400
t353 t500 t75 t137 + 50 t 91 t29 + 59 t21 + 39 t20 t 38 t8.6 +20 t4.5 t11 t1 t1.7 +1.4 +1.7 0 0 -1.85 -2.1 -3.9 -8 - 4.2 -6 -3.9 -8.3 -3.8 -8
in air in the entire concentration range tested if the ethene concentration is less than 10 ppm. The method can still be used at higher concentrations (Table I), if the interference is below, say, 2 or 3 % . The numerical data reported here are of course a characteristic of the particuliir detector and separating column used, but the type and extent of interference of ethene in the detector response to H2S can be extended to other FPD detectors, which should be tested in a similar way. Registry No. H2S,7783-06-4; CzH4,74-85-1.
20
15
LITERATURE CITED
10
05
10
15
ppm H2S
Figure 2. Calibration curves for H,S with different ethene concentrations: (a) HzSonly; (b) HzS 100 ppm ethene; (c) H,S 400 ppm
ethene.
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present. At higher concentrations of H2Sthe additive effect of ethene becomes negligible and a “quenching” effect takes place if the ethene concentration is much higher. It is interesting to note that a very good linearity of the calibration curves is observed even a t high ethene concentrations, as shown in Figure 2. The slope of the calibration curve decreases as the concentration of ethene increases. In conclusion, the gas chromatographic method described here can be used for the determination of hydrogen sulfide
(1) Stevens, R. K.; Mulik, J. D.; O’Keeffe, A. E.: Krost, K. J. Anal. Chem. 1971, 43, 827. (2) Bruner, F.; Llbertl, A,; Possanzini, M.; Allegrini, I.Anal. Chem. 1972, 4 4 . 2007-2014. (3) BrUGr, F.; Canulll, C.; Possanzini, M. Anal. Chem. 1973, 4 5 , 1970-1971. (4) Bruner, F.; Ciccioli, P.; Di Nardo, F. Anal. Chem. 1975, 4 7 , 141-144. (5) Document from Internatlonal Standard Organlzation 1982 (ISO/TC 146ISL 3/WG N 7, I 1 Revision) available from SIS Tegnergatan, Stockholm, Sweden. (6) Bruner, F.; Clccioli, P.; Bertoni, G. J. Chromatogr. 1978, 120, 200-202. (7) Slngh, H. B.; Salas, L. J.; Shigeishi, H.; Scribner, E. Science 1979, 203, 899-903. (8) Cox, R. D.; Earp, R. F. Anal. Chem. 1982, 5 4 , 2265-2270. (9) Bruner, F.; Clccloli, P.; Di Nardo, F. J. Chromatogr. 1974, 99, 661-672. (IO) Bruner, F.; Ciccloli, P. Proceedings of TECOMAP, Helsinki 30 July-4 August 1973 WMO Geneva, Switzerland, 1974; pp 324-333.
Filippo Mangani Fabrizio B r u n e r “ Istituto di Scienze Chimiche Universith di Urbino P z z a Rinascimento, 6-61029 Urbino, Italy Nunzio P e n n a Cattedra di Microchimica Universith di Urbino Via Saffi, 2-61029Urbino, Italy RECEIVED for review December 28,1982. Accepted June 27, 1983.
