Table 111. Measurement of Atmospheric H2S Concentration, ppt Location Date NCAR 27 September 1971 33 66 28 September 1971 4 October 1971 55 11 October 1971 48 19 October 1971 430 Lake Eldora 20 October 1971 West wind (a.m.) 20 33 East wind ( p m . ) Precision, Accuracy, and Sensitivity. The reproducibility of parallel samples taken in the 0.05- to 2-ppb H2S range was
+lOZ. The accuracy of the method was in part demonstrated by putting aliquots of solutions, containing a dissolved, weighed amount of Na2S.9H20, onto impregnated filters. The recovery of the S2-from the filter was demonstrated to be better than 95 Z by analysis. Since precisely known concentrations of H2S in the concentration range 0.05 to 2 ppb could not be prepared, the absolute accuracy of the method could not be assessed. [The gas dilution device used (11) could not be used as a precise source of H2Sat these levels.] For a 1000-1. air sample at 600 mm Hg pressure with 5 ppt H2S, the sample captured would equal the blank levels for S2-. However, it is conceivable that the method could be used for H2S concentrations below 1 ppt if large enough air samples were taken to overcome the filter blank. Field Sampling. The analytical method reported here was used to measure atmospheric H2S levels at NCAR and at a remote mountain area (Lake Eldora Ski Area). The air at both sites should be relatively clean and free from local pollution when there is a west wind. Table I11 shows the results of these measurements. At NCAR, the H2Slevel varied from 30-60 ppt. However, on a day of high local pollution, the H2S concentration was measured at 430 ppt. The result is hardly surprising in view of the very poor ventilation during the sampling period. The H S levels at Lake Eldora were measured for only a single day and showed 20 ppt for a morning west wind and 33 ppt for an afternoon east wind. All samples were collected for several hours at 6.6 l./min. Long sampling times were used because the levels of H2S
were quite unknown. For higher H2S concentrations, obviously much shorter sampling times or slower flow rates could be selected, or the washings of the filter could be diluted to a greater extent. Alternative Methods for Analysis. The analysis described herein relies on a fluorimetric determination of the amount of S2- retained as Ag,S on a filter impregnated with AgN03. This procedure has proved extremely satisfactory. However, quantitative analysis could also be achieved by determining the amount of Ag+ combined with S2- as Ag& If this procedure is used, it is necessary first to remove all soluble unreacted AgN03 by washing. The remaining Ag+ could then be estimated by: 1). Dissolving the Ag2Swith NaCNNaOH solution and determining the Ag(CN),@-l)- complexes by atomic absorption spectroscopy; 2). Determining the Ag+ present as Ag,S by neutron activation analysis; 3). Using radioactive llornAgso that the remaining Ag2S can be estimated by or y counting; 4). With H2S concentrations above 10 ppb, densitometric methods remain the simplest to use. This latter procedure is described in detail elsewhere (8,101. The first three methods have been investigated and shown to possess very high sensitivity. However, the limiting factor is the efficiency of removal of unreacted AgN03 from the filters without disturbing the Ag2S. This can be accomplished using the sulfur-containing ligands S20a2-,SCN-, or (NH& CS in the washing solution. However, although good quanti. tative results can be obtained, the background levels of Ag+ on washed papers are such that the sensitivity of H2S determinations is limited to about 50 ppt for a 1000-1. sample. Thus, the high sensitivities of detection by the above three methods are not realized. Work is currently in progress to lower the amount of Ag+ remaining in washed filters without disturbing collected Ag&. ACKNOWLEDGMENT
The authors wish to thank Tell Ertl for the use of his facilities during sampling at the Lake Eldora Ski Area.
RECEIVED for review February 2, 1972. Accepted May 25, 1972. The National Center for Atmospheric Research is sponsored by the National Science Foundation. One of us (D.F.S.N.) wishes to acknowledge the support of a Senior Traveling Fellowship awarded under the Fulbright-Hays program for International Exchange of Persons.
Improved Gas Chromatographic Method for the Determination of Sulfur Compounds at the PPB Level in Air Fabrizio Bruner, Arnaldo Liberti, Massimiliano Possanzini, and Ivo Allegrini Laboratorio Inquinamento Atmosferico del C N R , c/o Istituto di Chimica Analitica, Citth Universitaria, 00185 Roma, Italy
A gas chromatographic apparatus for monitoring sulfur air pollutants (SOZ, HzS,CHaSH, CH3SCHa) i s described. The column is made of Teflon tubing 3-mm i.d., 1.25 m long packed with Graphitized Carbon Black treated with 0.5% of phosphoric acid and 0.3% Dexsil. An exponential dilution flask constructed of Teflon is used to obtain calibration Curves (Area Vs. concentration) and its feasibility The gas chromatographic procedure is compared with integral methods using bromine coulometry and colorimetry. GASCHROMATOGRAPHY is in general the most suitable technique for trace analysis in gaseous mixtures. In recent years, 2070
the development of ultrasensitive and selective detectors together with the classical-i.e. flame ionization and electron capture-considerably simplified the problem, and the main difficulty is, at present, the selection of the proper COhmn to obtain a specific performance. This is particularly true when strongly reactive and polar compounds have to be measured at very low concentrations. Adsorption by the support plays a very important role and may be prohibitive when parts per billion have to be measured. On the other side, the monitoring of sulfur air pollutants by gas chromatography has a number of advantages over other methods:
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
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AIR MONITORING 8 -W A LV E~
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Figure 1. Scheme of the automatic gas chromatographic apparatus for monitoring sulfur air pollutants
Selectivity: interference from other pollutants may be eliminated and discrimination among the sulfur compounds may be achieved. Absence of intermediate reactions; the detector response is directly related to the presence of a certain compound in the gaseous mixture without the need of reactive solutions. A large range of concentrations is detectable. A very frequent sampling can be realized and consequently a more detailed picture of the pollutants concentrations as a function of time can be obtained. In a recent paper, Stevens et af. (I) succeeded in analyzing H2S, SOs, and mercaptans at the ppb level by using a 36-feet Teflon FEP column packed with TFE Teflon coated with 1 polyphenyl ether and orthophosphoric acid and a Flame Photometric Detector (FPD). Such a column was selected to prevent the strong interactions which otherwise might occur between the reactive compounds and the column packing. The moderate selectivity of the stationary phase was compensated by a considerable column length. Recently, some new selective and extremely inert supports have been developed in this laboratory to obtain the elution of very reactive compounds (2). This paper reports the results obtained with the use of columns prepared with these supports in the analysis of sulfur air pollutants. The use of the exponential dilution method of calibration for SOz, HzS,and CHsSH is also discussed. (1) R. K. Stevens, J. D. Mulik, A. E. O'Keeffe, and K. J. Krost, ANAL.CHEM., 43, 827 (1971). (2) A. Di Corcia, P. Ciccioli, and F. Bruner, J. Chromatogr., 62, 128 (1971).
EXPERIMENTAL Gas Chromatographic Apparatus. A flame photometric detector, 750-volt power supply and electrometer (Tracor Inc. Austin, Texas) coupled with a gas chromatograph, equipped with a n auto-injection system (Carlo Erba Milan, Italy, Model G I 450) is used. Sample loop and connecting lines, before and after the column, were made entirely of Teflon (Du Pont). The scheme of the apparatus is shown in Figure 1. Column. The column consists of Teflon tubing 3 m m i.d., 1.25 m long, packed with Graphon, a type of graphitized Carbon Black (Cabot Corp., Billerica, Mass.) with a surface area of about 90 mz/gram, previously treated with hydrogen at 1000 "C, according to a procedure elsewhere described (3). This material was found to yield symmetrical peaks for SO:! and H2S, but even at room temperature H2S was scarcely retained, while mercaptans showed too high retention times. To eliminate this limitation, Graphon was added with small amounts of orthophosphoric acid and of a nonpolar nonbleeding liquid phase; the former increases the retention times of HsS and SOz, which scarcely interact with the pure Graphon surface, while the latter reduces the surface area of the adsorbent. In this way the retention of the first members of the mercaptans series is strongly reduced and the range of retention times of the compounds under examination is compressed. Such a column operates under the experimental conditions of gas-liquid-solid chromatography as has been shown elsewhere ( 4 ) . The amount of stationary phase exerts a critical influence on the retention times in gas-liquid-solid chromatography; many attempts have been made to reach the desired column performance, and at the same time to avoid badly skewed peaks. (3) A. Di Corcia and F. Bruner, ANAL. CHEM., 43, 1634 (1971). (4) A. Di Corcia and F. Bruner, J . Chromatogr., 62, 462, (1971).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
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--5
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Figure 2. ( a ) Exponential dilution diagram for calibration of sulfur dioxide. Column, as in the text; T, 40 "C; Column flow rate, 100 ml/min; inlet pressure, 0.6 kg/cmZ; injection frequency, 2 minutes; EDP flow rate, 60 ml/min ( b ) Sulfur dioxide monitoring in air, Rome, CNR Building, November 17, 1971. Exponential condition in a ; SOz concentration, 0.1-0.15 ppm; arrows indicate SO2 peaks
The best compromise has been found when Graphon (40-60 mesh) is coated with 0.3z Dexsil 300 (Analabs, Inc., North Haven, Conn.) and 0.5 orthophosphoric acid. Phosphoric acid is used also to deactivate some possible residual alkaline sites, while Dexsil serves to decrease the surface area to the optimum value for the relative retention of the compounds of interest. No stripper column was found necessary, because retention times in adsorption gas chromatography are sharply increased with the number of carbon atoms, and possible peaks corresponding to higher terms are so broadened that they are practically undetected. N o detectable increase of the base line was observed after two months of continuous monitoring of sulfur air pollutants. Water is quite rapidly 2072
eluted from this column and anyway is not detected by the FPD. Calibration System. An exponential dilution flask (EDF) of the type described by Williams and Winefordner (9, but constructed entirely in Teflon was used; calibration was made by injecting 2 ml of the sample, taken from the original cylinder with a Teflon syringe, into a 2-liter Teflon flask, equipped with an air stirring system and previously evacuated. Air purified by means of an active charcoal filter was then allowed into the flask, equipped with a silicone rubber septum of about 4 mm* area in order to minimize the contact of the rubber with the sample. With another Teflon syringe, the sample is then injected into the dilution flask (275 ml). An initial concentration of 7.300 ppb is thus obtained in the exponential dilution flask. A flow of 60 mlimin is used for the dilution gas (pure nitrogen). Automatic injections of the effluent from the dilution flask are made every 2 minutes. Reproducibility of the injections is ensured if the injection time is 15 sec. Lower times can cause the incomplete elution of the sample, while longer injections are useless. A 10-ml sample loop was always used. In this way a complete scanning over the entire range of concentrations is obtained in about 30 minutes. A typical chromatogram taken during one of these experiments is shown in Figure 2a. Preliminary experiments made by substituting the column with a short Teflon capillary showed that practically no absorption of SOz,H& and mercaptans occurs in the sample lines, valves, and connection between the gas chromatograph and the FPD. Operating Conditions. The detector temperature was kept at 115 "C, to prevent some sample loss, which might occur at higher temperature as reported by Stevens et a/. ( I ) . Column temperature was 40 "C for monitoring sulfur dioxide; this choice was the result of a compromise between the analysis time and the necessity of separating the SO? peak from that of air and of H& if present. Flow rate of the air to be sampled was 800 ml/min. The column flow rate was about 100 ml/min with a carrier inlet pressure of 0.6 kg/cm2. Such reduced inlet pressure avoided any gas losses from the various parts of the apparatus, even though all lines were made in Teflon. Detector gases, of research grade, were previously purified by means of 4A molecular sieves traps. Optimum flow rates were found 150 mlimin for hydrogen, 10 mlimin for oxygen, and 30 mlimin for air. These values are slightly different from those advised by the manufacturer, but a better signal to noise ratio was obtained. A typical chromatogram relative to continuous monitoring of sulfur dioxide in air is shown in Figure 2b. RESULTS AND DISCUSSION
Two main novelties characterize this work: the use of a short adsorption column, that allows a working pressure less than 1 kg cm2, and the use of the exponential dilution flask for calibration. According to the experimental. conditions described, only the peaks of H2S, SO?, and CH3SCHa are very sharp and well defined in all the range of concentration; the peak due to methyl mercaptan shows, however, a remarkable tail at concentrations below 100 ppm. If the chromatogram is carried out at higher temperature (80 "C), CH3SH yields a quite symmetrical peak; SOyis still partially separated from air, whereas H2S is practically unretained and cannot be separated from air. The column can thus be used to analyze mercaptans in air at 80 "C, down to less than 30 ppb as can be seen from the calibration curve (Figure 3) ( 5 ) H. P. Williams and J. D. Winefordner, J . Gas Chromarogr., 4, 271 (1966).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
Peak area ( A ) , arbitrary units; C,p.p.b. Different dots refer to different runs. Column temperature: SO2 and H2S, 40 "C; CH3SH, 80 "C
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Figure 4. Chromatogram of sulfur compounds, experimental conditions as in Figure 2a 300
(natural logarithm of peak area us. natural logarithm of concentration) for CHISH : a linear range of concentrations between 30 and 7000 ppb has been used for experiments. The calibration curves for HrS and SO?, obtained at 40 "C are reported also in Figure 3; a linear relationship has been observed between 20 and 7000 ppb for the former and a still better situation has been obtained for SO2, which can be measured down to 10 ppb. The limitation observed for H2S and CH3SH is due to the incomplete elimination of some specific active sites from the Graphon surface, which strongly interact with the -SH group. The dimethylsulfide peak is quite symmetrical at about 80 ppb. This value has been obtained with rough approximation by comparing the peak area of CH3SH and CH,S-CH3 and by accounting for the relative response for these two compounds as given by Stevens et at. (I). The different dots reported in Figure 3, refer to different runs. The slopes of the curves are 1.86, 1.82, 1.74, and their standard deviations 0.10, 0.08, and 0.18 for H2S, SO2, and CH,SH, respectively. These slopes are slightly below the theoretical value of 2 predicted by the fact that the radiation detected by the F P D is due to the Sr group. Similar results were reported by Stevens et a/., who found somewhat higher
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Figure 5. Comparison of different monitoring methods
- gasRome, chromatographic, - Coulometric, - - - West and Gaeke sampling frequency, CNR Building, Nov. 16, 1971. (a)
GC
15 minutes (b) Rome, CNR Building, Nov. 30, 1971. GC sampling frequency. 3 minutes Other conditions, see text
values. The standard deviations look slightly higher than those observed with the permeation tubes. However, for monitoring purposes, the E D F method of calibration gives quite satisfactory results, showing good reproducibility within the limits indicated. Figure 4 shows a chromatogram of the elution of the compounds above.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
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These results show that the apparatus reported is feasible for monitoring SO*,H& and CH3SHwithin the limits shown. Sample injections can be made every 15 min; however, if only SOzis monitored, as n o other sulfur pollutants are usually present in the air, injections can be made every 2 minutes. The reduction of the sampling times between one injection and the next makes the total SO2 monitoring with the gas chromatographic method, which is by definition a discontinuous one, approaching the value of the integral method obtained with a coulometric analyzer. Contemporaneous monitoring of the same ambient air has been carried out with the gas chromatographic apparatus described here, with the automatic coulometer available in our laboratory (6), and with the classical semimanual colorimetric system (7). Figure 5a shows a comparison of the results obtained with these three systems. The sampling rate of the chromatograph was 15 minutes and the same time elapsed between one titration and the next. The results of the West and Gaeke method are also reported and the integration is made in this case over a three-hour period. Figure 5b reports the same comparison, the only difference being the sampling rate of the FPD which was in this case 3 minutes. From a comparison of Figure 5a and 56, it can be seen that with the sampling rate of 3 minutes, the two curves, coulometric and gas chromatographic, are closer than in the 15-minute sampling, as it could be easily predicted by the fact that the latter is discontinuous while the former gives a n integral response. (6) A. Liberti, M. Possanzini, and R. Ricci, I/zqui/zamento, 9, 28 (1971). (7) P. W. West and G. C. Gaeke, ANAL. CHEM., 28, 1916 (1956).
The data of Figure 56 show, however, that the two procedures, gas chromatographic and coulometric, are comparable, and a discrepancy of measurements of about 7 % only is observed. As the analytical techniques vary and the detecting principles are completely different, it can be said that the agreement is very good. Concerning the West and Gaeke method, it can be said that, even though the total under the curve corresponds to that of the other two, a very rough picture of the variations of the SO2 concentration is observed. In addition, the standardization of the coulometric apparatus has been carried out with a sulfur dioxide permeation tube; the agreement of the graphs in Figure 5 confirms also that consistent results are obtained with the two calibration systems. Concluding, the EDF calibration system appears to be simpler than that of permeation tubes, which requires a very long procedure in order to know the permeation rate. Moreover, the permeation tubes system needs a very accurate mixing device which must be frequently operated t o obtain the calibration curve. With the EDF, one has only to inject the sample into the flask and wait about half an hour to get complete data for the calibration curve. ACKNOWLEDGMENT
The authors wish to thank Antonio D i Corcia for discussions, Roberto Rastelli, Giorgio Sirilli, and Claudio Canulli for technical assistance. RECEIVEDfor review March 6, 1972. Accepted June 12, 1972.
NOTES
Use of the Microwave-Excited Emissive Detector for Gas Chromatography for Quantitative Measurement of Inter-Element Ratios R. M. Dagnall, T. S. West, and Paul Whitehead Department of Chemistry, Imperial College of Science and Technology, London, S W7 2AY. U.K. A WIDE RANGE of selective detectors for gas chromatography has been described in recent years. In general, these detectors have been used to provide better detection for particular types of compounds, to enable the determination of such compounds in the presence of a high background, and to obtain qualitative information on heteroatoms present in the chromatographic eluates. Except for the use of mass spectrometers, only very limited attempts have been made to determine the quantitative ratios between elements present in an eluate. Many detectors highly suited to sensitive selective detection are unsuitable for this purpose. Some have an unpredictable response or enhance the response of more than one element (e.g., electron-capture detectors) and others, such as coulometric detectors, would require a second de2074
tector to determine the mass of compound present. In principle, emissive detectors are best suited to determine several elements ; ideally, the response should be structure independent and the emission at each selected wavelength characteristic of the element in question. The use of metal sensitized flames utilizing the emission from metal salts has been reported. Na2S04-sensitizedflame detectors for chlorine, bromine, and iodine compounds have been described by Nowak and Malmstadt (I), and Bowman, Beroza, and Hill (2), but the methods d o not distinguish (1) A. V. Nowak and H. V. Malmstadt, ANAL. CHEM., 40, 1108
(1968). (2) M. C. Bowman, M. Beroza, and K. M. Hill, J . Chromatogr. Sci., 9, 162 (1971).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
