A ~ I Ctwtn. . lees, 65,3295-3298
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Reductive Pyrolysis for the Determination of Aqueous Sulfur Compounds with a Helium Microwave-Induced Plasma Jorge S. Alvaradot and Jon W. Carnahan' Department of Chemistry, Northern Illinois University, DeKalb, Illinois 60115
A method for the determinationof aqueous sulfur is presented. Sulfur-containing compoundswere converted to HzS by a noncatalytic reductive pyrolysis at 1050 "C in the presence of hydrogen gas. The resultant HzS was trapped by condensation in a liquid nitrogen trap. Following preconcentration, flash vaporization into a flowing helium stream routed the HzS to a 1.6-kW helium microwave-induced plasma for atomic emission detection. Analytical effects of pyrolysis temperature,hydrogen flow rate,and sample introduction rate were studied for aqueous solutions of cysteine, methionine, dimethyl sulfoxide, and ammonium sulfate. Results showed linear responses and detection limits of 30 parts per billion when the 921.3-nm nonresonant sulfur atom emission line was monitored and 400 parts per trillion when the resonant sulfur line at 180.73 nm was observed. INTRODUCTION The determination of low concentrations of sulfur is important for several environmental and technological applications. The development of sensitive procedures for determination of sulfur to levels below 1 part per million allows industrial procedure adjustments to be made to minimize corrosion and pollution, as well as optimize refining processes. As a result of these needs, several methods have been developed. The use of electrochemicaltechniques such as microcoulometry,' coulometry,2and potentiometrys have been implemented with good results. In these methods, the sulfur-containing compounds are converted by oxidation or reductive pyrolysis into an electrochemically determinable sulfur compound such as H2S or S02. Conversion to HsS has been found to be stoichiometricunder suitable conditions for most sulfur compounds.' Langmainier et al.4 reported detection limits of 0.005 pg/ mL using reductive pyrolysis after gas chromatographic separation. Detection was accomplished with a gold-plated porous membrane electrode. Drushel6 used a high-temperature reductive procedure to form H2S, which was monitored photometrically after reaction with lead acetate. This procedure measured the rate of blackening with a H2S analyzer. Detection limits of 0.025 pg/mL for sulfur were reported. Farley and Winklefl proposed a reductive procedure for the analysis of samples with a sulfur concentration
I
I
1 Monochromator
1
1
I .
I
Liquid N2
1 Power 1 Figure 1. Block diagram of the He-MIP and pyrolysis unlt.
range of 0.2-5 pg/mL. Their method utilized a 10% platinum catalyst on a quartz substrate at 1200 OC in a hydrogen stream to form H2S. The product was absorbed by a zinc acetate solution. N,N-Dimethylphenylenediaminewas added to the solution to form methylene blue from the reaction with H2S. Spectrophotometricdetectionwas used. The method suffered from catalyst decomposition at the high temperature. In the determination of sulfur by inductively coupled plasma (ICP) with direct nebulization, Landsberger et al.7 and Lafreniere et al.8 reported detection limits of 0.004 and 0.002 pg/mL at 180.7 nm, respectively. Matousek et al.9 and Alvarado et al.10 reported the determination of sulfur compounds using graphite furnace vaporization into a helium microwave-induced plasma. These procedures showed the possibilityof performing sample analysiswithminimal sample pretreatment. The authors reported sulfur detection limits of 9 pg/mL and arange of 3-5pg/mL when the 527.86-527.89nm unresolved doublet and the 921.3-nm transition line were used, respectively. This article describes a method employing a kilowatt plus microwave-induced plasma (Kip-MIP) of helium for the determination of the totalsulfur content in sulfur-containing compounds using near-infrared (921.3 nm) and vacuumultraviolet (180.73 nm) atomic emission lines. The analyte was converted to hydrogen sulfide by noncatalytic reductive pyrolysis. Hydrogen was used for H2S production without significantlycompromisingthe performance of the discharge. The resultant H2S was condensed and preconcentrated in a liquid nitrogen trap in a manner similarto that used by Holak11 to separate hydrides from bulk gases. Following preconcentration, flash vaporization into a stream of flowing hydrogen routes the H2S to the plasma for atomic emission detection.
EXPERIMENTAL SECTION
+ Present address: Argonne National Laboratory, 9700 S.Case Ave.,
Instrumentation. The overall arrangementof the equipment is shown in Figure 1. Hydrogenolyeis waa carried out using an electric multiple-unit furnace(HeviDuty Electric Co.,Milwaukee, WI) with a meximum operating temperature of 1066 OC. Figure 2 shows a furnace diagram. The furnace inner tube was of fused
(4)Langmanier, J.; Opekar, F.; Pacakiva, V. Talanta 1987,34, 463. (6)Druahel, H.V. Anal. Chem. 1978,60,76. (6)Farley, L. L.; Wmkler, R. A. Anal. Chem. 1968,40,962.
(7)Landsberger, S.J. Enuiron. Anal. Chem. 1986,19,219. (8)Lafreniere, B.R.;Houk, R. S.;Fassel, V. A. Anal. Chem. 1987,59, 2276. (9)Matoueek, J. P.; Orr,B. J.; Selby, M. Talanta 1986,39,876. (10)Alvarado, J.; Wu,M.; Carnahan, J. W. J. Anul. Atom. Spectrosc. 1992,7,1253. (11)Holak, W.Anul. Chem. 1969,41,1712.
Author to whom correspondence should be addreeaed.
ER 1203-E141,Argonne,-IL 60439-4843. (1) Wallace, L. D.;Kohlenberger, D. W.; Joyce, R. J.; Moore, R. T.; Riddle, M. E.;McNulty, J. A. Anal. Chem. 1970.42,387. (2)Moore, R. T.;C h t o n , P.;Barger, V. Anal. Chem. 1980,52,760. (3)h i m , R. E.;Ham, D.D.Anal. Chem. 1981,53,1088.
0003-2700/93/0365-3295$04.00/0
0 1993 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 85, NO. 22, NOVEMBER 15, 1993 Pneumatic Nebulizer
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,
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OUTLET
INLET
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1°C I
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Flgurr 2. Furnace dlagram of the trace sulfur apparatus wlth a thermocouple and pneumatic nebulizer. IO
silica with a 210-mm length, 15-mm o.d., and 13-mm i.d. The inlet and outlet tubes were 100 mm long, 7-mm o.d., and 5-mm i.d. To control the temperature, a grounded junction thermocouple with a maximum operating temperature of 1093O C (Omega Engineering, Stamford, CT) with a pyrometer range from 0 to 1100 O C (Omega Engineering, Stamford, CT) was used. The measured temperature in the center of the furnacewas maintained typically at 1050 OC, but temperatures of 750 and lo00 O C were observed at the inlet and outlet, respectively. Samples were introduced into the hydrogenolysis tube with a concentric glass type-C nebulizer (J. E. Meinhard Associates, Santa Ana, CA). Liquid flow to the nebulizer was controlled by a peristaltic pump (Rainin Instrument Co., Woburn, MA). Hydrogen (Airco, Murray Hill, NJ) was used for nebulization and sample transport. The oven outlet was connected with Tygon tubing to an H2O/ ice condenser maintained at 0 "C to condense the solvent vapor. The outlet of the 0 O C condenserwas connect& to aliquid nitrogen trap. This U-shaped trap was made with a Teflon tube 510 mm long, 11-mm o.d., and 9-mm i.d. To preconcentrate the analyte, the trap was placed into a liquid nitrogen containing Dewar. The U-tube was transferred to a 25 O C water bath to vaporize the H2S. The Kip-MIP was used with a forward power of 1.6 kW. In general, the system consist of a 3-kW maximum output, 2450MHz power supply (Raytheon Co., C.A.S. Division, Waltham, MA), waveguides for power transmission (GerlingLaboratories, Modesto, CA), a slotted waveguide tuner (Cober Electronics, Stamford, CT),a waveguide to 1.625-in. coaxial transition, and a 1.625-0.875-in. coaxial reducer (Andrew Corp., Orland Park, IL). The depth of the TMoloresonator cavity was 2.0 cm. Details may be found in the literature.12J3 The plasma torch was described previously by Wu and Carnahan.13 For the analysis using the near-infrared spectralregion, anal* emission was focused on the entrance slit of a 0.35-m focal-length GCA/McPerson Model EU-700 (Acton, MA) scanning monochromator. A 17.0-cmfocal-lengthlens was usedto focus emiasion on the entrance slit. Entrance and exit slits were set at 75 pm. A 600 grooves/" grating with a 1.0-pm blaze wavelength and a Hammamatau 2406 PMT operated at -1100 V were used for dispersion and light detection. In the vacuum-ultraviolet region, a 0.2-m vacuum-ultraviolet monochromator (McPherson, Acton, MA) with a 1200 grooves/" holographic grating coated with AlMgF2 was used. The entrance and exit slits were set at 50 pm. The scintillator detector was used for vacuum-ultraviolet detection. A cylindrical cone with a 2-mm orifice similar to that used by Houk and co-workers14was used as an interface between the plasma emission source and the detection system. The PMT output currentwas monitored with an operational amplifier based current-to-voltageconvertor/amplifier/filtersystem. Details of the vacuum-ultraviolet spectrometer/KiP-MIP interface may be found in the literature.'6 Reagents. Standard solutions were prepared with analytical grade reagents and distilled, deionized water. Ammoniumsulfate from Fisher Scientific (Fair Lawn, NJ) and methyl sulfoxide (12)Cull, K.B.;Carnahan, J. W. Appl. Spectrosc. 1988,42, 1061. (13)Wu,M.;Carnahan, J. W. Appl. Spectrosc. 1992,46, 163. (14)Houk, R. S.;Fassel, V. A,; LaFreniere, B. R. Appl. Spectrosc. 1986,40, 94. (15)Alvarado, J.; Carnahan, J. W . Appl. Spectrosc., in press.
Figure 3. Effect of temperature on conversion of varlous sulfur compounds to H2S. from Aldrich ChemicalCo. (Milwaukee,WI) wereused. Cysteine and methionine were from Sigma (St. Louis, MO). These compounds were used to prepare solutionscontaining 0.5-lo00 pg of sulfur/mL.
RESULTS AND DISCUSSION PyrolysisConditions. The nebulized sample was passed through a high-temperature region (1050 O C ) in the presence of hydrogen gas, forming HzS. The resultant H2S (mp = -85 O C , bp = -60.7 "C) was condensed in the liquid nitrogen trap (-195.8 "C). After the preconcentration process, the sample undergoes flash vaporization and introduction into the plasma discharge. Four different compounds were used for sulfur determination: cysteine, methionine, methyl sulfoxide, and ammonium sulfate. These compounds were selected to obtain a range of molecular structure and physical properties. The temperature dependance of pyrolysis efficiency was tested from 500 to 1050 "C. Due to instrumental limitations the experiment was not performed a t higher temperatures. The results are shown in Figure 3. Reductive pyrolysis began to take place in the temperatures range of 700-800 "C and was practically complete at 1050 O C . For comparison, Langmainier et ala4reported temperatures of 700 O C when gaseous compounds were analyzedand DrusheF reported 1125 "Cas the optimum pyrolysistemperature when liquid samples of aliphatic sulfides in hexane were studied. Two other parameters were examined the rate of peristaltic pump regulated sample introduction and the flow of hydrogen used to carry and react with the sample. Figure 4 shows a three-dimensional plot with both parameters. For a 1.0-mL sample volume, the signal increased when the rate of sample introduction was decreased. For practical purposes, the rate was maintained not less than 0.2 mL/min. Sampling at rates less than this value will significantly increase the time of the analysis, especiallyif large sample volumes are to be analyzed. The hydrogen flow rate was also an important parameter. Three conditions must be satisfied to obtain the optimum flow. First, the flow must be great enough to create acceptable nebulization conditions. Second, sufficient H2 must be present to react with all of the analyte. Third, the flow rate should not be too great as the maximum residence time of the sample in the reaction tube is desired to facilitate complete formation of H2S. It should be noted that if the flow rate was too small, carbonaceousdeposita tended to form in the reaction tube when the organic samples were examined and a decrease in the signal was seen. This decrease was due, presumably, to adsorption of H2S. Finally, a high flow of hydrogen causes destabilization of the plasma discharge. When flows of hydrogen were greater than 1L/min, hydrogen destabilized or extinguished the Kip-MIP. In all cases, the response
ANALYTICAL CHEMISTRY, VOL. 65, NO. 22, NOVEMBER 15, 1993
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-6.00
log concentrotion (ppm] Flgurr 6. Calibratlon plots for methlonlne and ammonium sulfate at 921.3 nm. Flgurr 4. Effect of hydrogen flow rate and sample lntroductlon rate (pump rate) on the sulfur signal.
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Ftgurr 5. Calibration plots for cystelne and dimethyl sulfoxide at 921.3 nm.
decreased with increasing hydrogen flow rate. A hydrogen flow of 165 mL/min (2.75 mL/s) was sufficient to produce aqueous mist without severely perturbing the discharge. Line Selection, Linearity, and Response. Spectral scans were obtained in the near-infrared and vacuum-ultraviolet spectral regions by use of a lo00 pg/mL solution of ammonium sulfate. The lines at 921.3 and 180.73 nm were used. Figures 5-7 show the response with increasingconcentration at 921.3 and 180.73 nm. For this study, preconcentration was performed for 5 min (1.00-mL samples with 0.2 mL/min introduction rates). As shown in Figure 5, the system produced a linear response at 921.3 nm when solution concentrations ranging from 0.7 to 900 pg of sulfur/mL as dimethyl sulfoxide and 1 to 1050 pg of sulfur/mL as cysteine were studied. The log-log slopes were 0.982 and 0.991, respectively. Figure 6 shows calibration plots for methionine and ammonium sulfate. The system exhibited a different behavior for these substances. The log-log slopes using a 1.00-mL sample size with concentrations ranging from 1 to lo00 pglmL were 0.907 and 0.705. For the four elements in study, the log-log calibrationplots had correlationcoefficients (r2)of 0.999. When compounds such as methionine and ammonium sulfate are present in high concentrations, the pyrolysis efficiencies may decrease. An explanation ,of this behavior may be found in the volatilization and decomposition
-' 1
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i
log concentrotion [ppm] Flguro 7. Calibration plot for cystelne at 180.73 nm.
characteristicsof these compounds. When low-concentration analyte-containing solutions are introduced, desolvation of the small sample droplets occurred with the formation of free particles. Because the analyte concentration is low and the particles are small, these free particles are then volatilized and pyrolyzed to form a stoichiometricamount of H2S. With higher concentrations, where particles sizes are greater, these steps required significantly more energy. In the case with larger particles, the extent of pyrolysis will be determined by the vaporization energies (a function of compound boiling point) and molecular decomposition energies (a function of the bond strengths). Table I shows some physical characteristics of the compounds under study. Lower boiling points will contribute to faster atomization by greater molecular exposure to the high-energy plasma environment. On the basis of this idea, one would expect efficient atomizationof dimethyl sulfoxidedue to its relatively low boiling point. This compound exhibits excellent linearity of response. Cysteine, with sulfur in a terminal SH group (RCSH) and a moderate decomposition temperature (240 "C),may be expected to be efficientlydecomposed. The s u m of the bond strengths (C-S and S-HI associated with sulfur is 11.4eV. Again, excellent linearity was seen. Methionine has a bridging sulfur group (C-S-C) with a combinated sulfur bond strength of 15.6 eV (two C-S bonds) and a somewhat high decomposition temperature (281 "C). The slope of the log-log calibration plot is worse, with a value of 0.907. Ammonium sulfate presents an extraordinary high sum of sulfur bond energies. The 03s-0bond strength is not known, but the sum of the other three bonds strengths is 14.7. With
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 22, NOVEMBER 15, 1993
Table I. Physical Characteristics of Compounds in Study compound dimethyl sulfoxide cysteine methionine ammonium sulfate a
CHsSOCH3 HSCH~CH(NHZ)CO~H CH~SCH~CHZCH(NHZ)COZH (NHd2SO4
MW
mp ("C)
bp ("C)
log-log slope
78.13 121.16 149.21 132.14
18.4 240d"
189
0.982 0.991 0.907 0.705
281d 235d
d, decomposes.
an estimate of the fourth bond of one-third of this value, the s u m of the sulfur bond strengths approaches 20 eV. Further, ammonium sulfate is not particularly volatile. From this discussion,it seems logicalthat pyrolysis efficiencydecreases for higher concentrations and the log-log slope for sulfur as ammonium sulfate is less than 1 (0.705). Figure 7 shows a linear sulfur response from 0.02 to 100 pg/mL from cysteine at 180.73 nm. The log-log slope from 0.02 to 100 pg of sulfur/mL was 0.984 with a correlation of 0.999. A "rollover" in the plot was seen at concentrations greater than 100 pg of sulfur/mL. The log-log slope from 0.02 to lo00 pg of sulfur/mL was 0.850 with a correlation of 0.994. By comparison of this results with those observed for cysteine a t 921.3 nm, the rollover of the signal is due probably to self-absorption in the plasma discharge when the atomic resonant line for sulfur a t 180.73 nm was used. Detection Limits. Limits of detection using the pyrolysis system are listed in Table 11. The detection limits are defined as the concentration which yields a signal-to-noise ratio of 3. The noise is defined as the standard deviation of the baseline signal. For all compounds, a t 921.3 nm, detection limits were 0.03 pg/mL when 5.00-mL solutions of each were introduced. At 180.7 nm, using the same volume, detection limits were improved to 0.0004 pg/mL for cysteine and methionine and 0.0006 pg/mL for dimethyl sulfoxide. These results can be improved by increasing sample size with a subsequent increase in preconcentration time. Table I1 lists detection limits obtained using different sample introduction devices for sulfur-containingcompound. Detection limits are improved by greater than or equal to 2 orders of magnitude when pyrolysis is compared to electrothermal vaporization. Comparing results using the pyrolysis/ preconcentration procedure with the near-infrared and the vacuum-ultraviolet emission lines, detection limits are 2 orders of magnitude better. To our knowledge, these are the best detection limits reported using atomic emission spectrometry for sulfur analysis. The discussed approach compares favorably to other techniques as well.
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(16) Wu, M. Ph.D.Dissertation, Northern Illinois University,DeKalb, IL, 1990. (17) Blades, M. W.; Hauser, P. Anal. Chim. Acta 1984,157,163. (18) Stubley, E. A,; Horlick, G . Appl. Spectrosc. 1984, 38, 162. (19) Chan, S.; Tan, H.; Montaser, A. Appl. Spectrosc. 1989, 43, 92. (20) Wallace, G. F. At. Spectrosc. 1980, I , 38. (21) Nygaard, D. D.; Leighty, D. A. Appl. Spectrosc. 1985, 39, 968. (22) Hayakawa, T.; Kiku, F.; Ikeda, S. Spectrochim. Acta 1982,374 1069.
Table 11. Comparison Detection Limits for Sulfur USNL ETVu PNU pyrolysis compound h (nm) (pg/mL) (rg/mL) (ag/mL) (rcg/mL) thiourea
527.86/ 527.89 (NH.&SOd 545.4 250'J (NH&SO, 921.3 30'J cysteine 921.3 methionine 921.3 S (xylene soln) 921.3 DMSO(aq) 921.7 5fa (NH&Sa(aq) 921.3 cysteine 180.7 methionine 180.7 DMSO 180.7 0.002gsn S(aq) 180.7 S(xy1ene) 180.7 0.05hS~q S(aq) 180.7 O.O2'J9p4
9mJ 0.03' 0.03' 0.03'
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Normalize to S/N = 3. From ref 14. From ref 10. From ref 15. e From ref 16. f From ref 17.8 From ref 8. From ref 18. From ref 19. j From ref 20. From ref 7. This work. From ref 9. P Sulfur compound not reported. q Inductively coupled plasma (Ar-ICP). Helium microwaveinduced plasma (HaMIP). * Helium inductively coupled plasma (He-ICP). Ultrasonicnebulization. Electrothermal vaporization. u Pneumatic nebulization.
CONCLUSIONS The combination of the 1.6-kW plasma with a pyrolysis preconcentration system is well-suited for the determination of sulfur. Noncatalytic hydrogenolyeis presents a procedure easy to implement without reducing the effectiveness of the He-MIP or dealing with a catalyst which may become fouled. The analysis of aqueous samples for organic and inorganic sulfur-containing compounds yield the best detection limits ever reported by atomic spectrometric methods. This value is 400 parta per trillion. Possibilities of further sample preconcentration imply that even lower levels of sulfur may be detectable. It may be possible to utilize this approach for other elements in direct analysis approaches as well as for liquid chromatography detection.
RECEIVED for review May 14, 1993. Accepted August 20, 1993.'
* Abstract published in Advance ACS Abstracts, October 1, 1993.
