Gas Chromatographic Analysis for Hydrogen Sulfide, Organic Sulfides, Mercaptans, and Carbon Dioxide in Hydrocarbon Matrices Using an Electrolytic Conductivity Detector* R.G. Schiller** and R.B. Bronsky, American Natural Service Company, 5943 Tireman, Detroit, Michigan 48204 Downloaded from https://academic.oup.com/chromsci/article/15/12/541/349030 by guest on 02 October 2021 Abstract Experimental Hydrogen sulfide, mercaptans, and organic sulfides can be A Tracor 550 Chromatograph was used in the experimenta- determined in hydrocarbon matrices using GC and an electro- tion. The chromatograph was equipped with a Flame Photo- lytic conductivity detector. Sensitivity is in the ppb level and interference from the hydrocarbon is minimal. The response metric Detector and a Tracor 310 Hall Electrolytic Conductiv- is linear simplifying calculations when compared to the FPD. ity Detector. A CSI 38 Digital Integrator was used for quanti- Carbon dioxide can also be determined with the same proce- fication. The gaseous sample was injected through a 10 port dure and its detection limit is approximately 0.1 percent. stainless steel valve equipped with a 10 cc sample loop. The columns used were: a 3ft x '/iin stainless steel column packed Introduction with acetone-washed Porapak QS 80/100 mesh (5) carrier gas The Flame Photometric Detector (FPD) has been used in flow 30 cmVmin (this column was used for programmed anal- the past for the determination of gaseous sulfur compounds in yses from 60°C - 160°C), a 36ft x l/8in Teflon column filled hydrocarbon matrices (1). This technique is satisfactory for with 40/60 mesh Teflon powder and flow coated with a poly- natural gas containing (<l%) of the heavier homologues (C3 - phenyl ether, phosphoric acid and acetone mixture (3) carrier C8). Natural gas at the well head quite often has high concen- gas flow 30 cmVmin (this column was used for isothermal trations (>1%) of these heavier homologues. The FPD be- operation below 80°C). The instrument was calibrated using a comes difficult to use in this situation, requiring manipulation Tracor 432 Permeation Device. Permeation tubes of hydrogen of the oven temperature and column preparation to ensure sulfide, carbonyl sulfide, carbon disulfide, methyl mercaptan separation of the sulfur compounds from the hydrocarbon. and dimethyl sulfide were obtained from Metronic Associates, Similarly, the detector is even less useful when analyzing a Inc. Nitrogen was used as both the carrier gas and the permea- liquid hydrocarbon (C3 +) and determining sulfur compounds tion tube diluant. with boiling points higher than methyl mercaptan (1). The FPD is also inconvenient to use because the detector is not linear with response (2,3). The electrolytic conductivity detec- Results and Discussion tor was therefore investigated to determine if it has any advan- Reductive Mode tages over the FPD when analyzing hydrocarbon samples. The Hall Electrolytic Conductivity Detector (4) operates as In this mode the effluent is mixed with hydrogen gas and the follows (see Figure 1): The effluent from the chromatograph is reaction in the furnace converts the sulfur compounds to mixed with a reaction gas (either reducing or oxidizing). This hydrogen sulfide while the hydrocarbons are unchanged (6). stream enters the furnace and the components are converted to The solvent used in the Hall Electrolytic Conductivity Detec- ionizable species or left unchanged depending on the tor is methanol containing a small quantity of dissolved io- conditions in the furnace. The stream is mixed with the liquid dine. The hydrogen sulfide reacts with the iodine forming two solvent and carried to the conductivity cell. The ionizable highly ionizable species. species dissolved in the solvent cause a change in electrical conductivity in the cell. The change in current is detected by H2S 21" the electrometer and converted to a millivolt signal for recording. The solvent is circulated through an ion exchange The hydrocarbons are not ionizable and thus are not de- bed to remove all contaminants enabling it to be reused. The tected. This mode should theoretically provide for maximum two detector modes (reductive and oxidative) both offer sensitivity and minimum interference. This was achieved in means of changing the sulfur compounds to ionizable species actuality. Detection limits in the low ppb level were recorded with minimal conversion of the hydrocarbons. Both will be and no hydrocarbon interference was noted, however, base- discussed in the following investigation. line stability was poor and could not be improved. This resulted because the ion exchange bed could not be used (the ion exchange resin removed the iodine). The solvent rapidly *To obtain a reprint of this article; please circle number 201 on Re- became contaminated and the drift was beyond the zeroing print Request Card. capacity of the electrometer. Because of this problem the re- * Author to whom correspondence should be addressed. ductive mode was dropped as a useful technique. Reproduction (photocopying) of editorial content of this journal is prohibited without publisher's permission. JOURNAL OF CHROMATOGRAPHIC SCIENCE»VOL. 15 DECEMBER 1977*541 REACTION GAS ION EXCHANGE BED Downloaded from https://academic.oup.com/chromsci/article/15/12/541/349030 by guest on 02 October 2021 SOLVENT RESERVOIR RECORDER Figure 1. Diagram of the Hall Electrolytic Conductivity Detector. of conversion of carbonyl sulfide or dimethyl disulfide. It was The Oxidative Mode thought that the column was retaining these two compounds In this mode, the furnace temperature, oxidant flow (air or but when the column was switched to the FPD, excellent res- oxygen), and furnace contact time are used as a means of ponse for these two compounds was achieved. Methane is in- selectively converting as much of the sulfur compounds to cluded in Figure 2 showing the conversion of 100% methane sulfur oxides with as little conversion of the hydrocarbons to to carbon dioxide at various temperatures. carbon dioxide as possible. This mode therefore requires the Hydrocarbons other than methane (C2 - C6) including un- manipulation of many variables to achieve maximum sensitiv- saturates and aromatics were tested and gave a response ity and selectivity. similar to methane. Selectivity between mercaptan and sulfide Hall (4) recommended the use of a non-aqueous solvent to can be achieved at the lower furnace temperature (600° C). promote selectivity between sulfur dioxide and carbon dioxide Sulfides should be determined, for best sensitivity, between and the use of a 1 mm i.d. quartz tube packed with quartz 8OO-9OO°C. wool to increase contact time of the chromatograph effluent in The quartz tube (6 cm long with 1 mm i.d.) is extremely dif- the furnace (7). Since hydrocarbons are more stable than the ficult to pack with quartz wool. Tests were run to determine sulfur compounds, a low furnace temperature and minimal whether this packing is necessary. Two 1 mm quartz tubes amounts of oxidant would seem to be necessary to improve were used; one with and the other without the quartz wool. All selectivity. With this in mind, the instrument conditions were other parameters were held constant. adjusted using methanol as a solvent, and air as the oxidant at Three (3) ppm methyl mercaptan was used as the sulfur a flow of 4-5 cc/minute. The furnace was equipped with a 1 compound in a nitrogen matrix. The packed tube, in this in- mm i.d. quartz tube with 2 mm in length of quartz wool stance, improved the sensitivity but increased the tailing. The packing. The furnace temperature was varied to check the effect of simultaneous eluting hydrocarbon and sulfur com- conversion of specific sulfur compounds to the sulfur oxides. pounds was checked. A short 6 in Porapak QS column and A three ppm standard of each compound: hydrogen sulfide, a 130°C oven temperature was used. A 3 ppm methyl mercap- ethyl mercaptan, dimethyl sulfide, carbonyl sulfide, and di- tan standard was made up in a methane matrix and also in a methyl disulfide in nitrogen was introduced into the chro- nitrogen matrix. The unpacked tube had a 20% reduction in matograph through the 10 ml gas sample loop. Figure 2 shows sensitivity when the methane eluted with the methyl mercap- the conversion relationships. There was no indication tan. The packed tube showed little change in sensitivity when 542OECEMBER1977 JOURNAL OF CHROMATOGRAPHIC SCIENCE«VOL. 15 Downloaded from https://academic.oup.com/chromsci/article/15/12/541/349030 by guest on 02 October 2021 DEGREES C/100 Figure 2. Conversion and response relationship at various furnace temperatures A: Three (3) ppm ethyl mercaptan, sensitivity (3x1) B: Three (3) ppm hydrogen sulfide, sensitivity (3 x 4) C: Three (3) ppm dimethyl sulfide, sensitivity (3x1) D: 100% methane, sensitivity (3x1) the methane eluted with the methyl mercaptan (Figure 3). It showing the approximate detection limit with the present con- appears that the packing is very important in reducing hydro- figuration. Detection limits for the lower boiling sulfur com- carbon interference. pounds (e.g., hydrogen sulfide, methyl mercaptan) are better Methanol proved to be the best solvent for the Hall Electro- than with high boiling sulfur compounds because of less peak lytic Conductivity Detector. Other solvents such as ethanol spreading. The determination of carbon dioxide at mole per- and propanol were tried, but the tailing was excessive with cent levels is possible using this detector. Figure 5 shows that these solvents. Up to 8% v/v water was added to the methanol carbon dioxide is not linear and standards closely matching to increase sensitivity but the signal-to-noise ratio was best samples would be necessary. This technique compares with with pure methanol. A satisfactory oxidant flow was approxi- that of a thermal conductivity detector. mately 5 cc/min. Faster flow increased the tailing while slower Figure 6 is a chromatogram of a typical odorized natural flow showed little change.