ANALYTICAL CHEMISTRY, VOL. 50, NO. 12, OCTOBER 1978
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Plasma Atomic Emission Spectrometric Determination of Sulfur in the Vacuum Ultraviolet Region of the Spectrum S. R. Ellebracht* Central Laboratory, Dow Chemical, USA, Freeport, Texas 7754 1
C. M. Fairless Analytical Laboratories, Dow Chemical, USA, Midland, Michigan 48640
S. E. Manahan Department of Chemistry, University of Missouri, Columbia, Missouri 6520 I
A dc discharge plasma source has been utilized for the determination of sulfur in solutions using optical atomic emission spectroscopy. The sulfur resonance lines which occur at 180.7, 182.0, and 182.6 nm were used in this work. A simple argon purge system was developed to reduce light absorption by oxygen. This excitatlon source, coupled with the purge system, appropriate optics, and detection equipment, enabled the determlnation of total sulfur in aqueous solutions with the same sensitivity for different sulfur species. A detection limit of 0.5 ppm and a linear dynamic range of 2000 ppm sulfur were attained. Several potential chemical and spectral interferences were investigated. The instrumentation was also used to determine sulfur in organic solvents and to determine Hg, Se, As, I, and P in aqueous solutions.
Electrically generated argon (or helium) plasma sources designed for solution analysis have many advantages in atomic emission elemental determinations. These include an inert gas atmosphere, relatively good stability, a high excitation temperature, and reasonably long sample residence times ( 1 ) . Three major types of plasmas have been developed, including the inductively coupled radio frequency plasma (2-71, the microwave plasma (8-13), and the dc discharge plasma (14-20). Because of its low cost, relative ease of operation, and small size, the dc discharge plasma was chosen for this investigation. The use of this type of plasma as an excitation source for atomic analysis was attempted as early as 1922 by Gerdien and Lots ( 1 4 ) . However, the first useful adaptations for solution analysis came in 1959 when Margoshes and Scribner (15) of the US.and Korolev and Vainshtein (26) of the USSR, independently published applications in this area. Since these initial studies were reported, numerous adaptations, modifications, and improvements have been made on the concept. Recently, an “inverted V” plasma configuration was developed by Elliott (19) which allowed recording the emission occurring just under the plasma, consequently maintaining a high temperature at a reduced background level. This development made this plasma promising as an atomic emission source for elemental determinations (20). Current technology for direct determination of total sulfur, which includes basic X-ray fluorescence (21),colorimetric and turbidimetric procedures (22-24), combustion coulometry or photometry (25, 26), molecular emission cavity analysis (27, 28), and gas chromatography with emission spectrometric detectors (29, 30), suffers either from limitations due to sensitivity, specificity, accuracy, or applicability to various forms of sulfur. These limitations led us to investigate the
Table I. Instrumentation spectrometer system monochromator grating slit widths external lens data acquisition photomultiplier tube PMT power supply/amplifier dc plasma jet power supply
McPherson, 0.5 M Model 216.5 plane, 1200 grooves/mL blazed for 300 nm 26.5 A/nm, 1st order 25-pm entrance 25-pm exit biconvex CaF,, 5-cm focal length EM1 9783B Ortec Model 9511 and 5032 Spectrametrics, Inc. Spectra Jet I1
applications of Elliot’s dc plasma to the determination of sulfur by optical atomic emission spectroscopy. Use of sulfur lines above the vacuum ultraviolet (VUV) region for atomic emission analysis has been discussed by several authors (29,30). However, a potentially advantageous approach to the trace analysis of sulfur is atomic emission spectroscopy using the more sensitive VUV resonance lines (31,32).Preliminary experiments in this region were carried out by Khartsyzov and L’Vov (33) who determined sulfur, phosphorus, and iodine a t sub-200 nm lines. In more recent work, Kirkbright et al. used atomic absorption (nitrogenseparated nitrous oxide-acetylene flame) to observe sulfur a t the 182.7 nm line (34). They have also reported studies using a type of inductively coupled high frequency plasma to observe sub-200 nm emission lines for S, Hg, Se, As, I, and P in aqueous solutions (7, 32). The objective of the work presented here is t o investigate the applicability of the dc discharge plasma to the determination of sulfur using the VUV resonance lines.
EXPERIMENTAL Apparatus. The plasma source was a model 53000 dc discharge spectra jet 11,manufactured by Spectrametrics, Inc. This source has three argon gas streams, one from each electrode (plasma stream) and one from the sample aspirator. These create a region of maximum sample concentration just below the sharp bend in the arc (20). The best signal-to-noise ratio was found in an observation zone from 0.5 mm to 2 mm below the center of the arc and 1mm to either side of the center. This observation zone was used for all the work presented here. Other instrumentation is listed in Table I. Argon (99.995% pure) was used to purge the system from the source to the detector (Figure 1). The area around the source was purged by forcing a stream of argon at a rate of 2 L/min
0003-2700/78/0350-1649$01.00/0 C 1978 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 12, OCTOBER 1978
Monochromator and P M l
Glass frit
Glass Cylinders
Figure 1. Schematic diagram of argon purge system
24
Table 111. Sulfur Detection Limit with and without the Argon Purge with purge without purge wavelength, detection relative detection relative nm limit sensitivity limit sensitivity 180.7 182.0 182.6
I
0.5 1.0 1.5
100 50 20
250 50 15
0.2 1.0 3.3
Table IV. Aspirator Argon Flo+ flow rate, intensity intensity L/min 50 ppm Sb backgroundb
(a)
0 c
cc.
OD
-N O DN
. .
1.75 2.00 2.25 2.50 2.75 3.00
20 24 26 32 27 25
2 3 3 4
3 1
0 0 )
Wavelength ( n m )
Figure 2. Scan over the wavelength region of interest while aspirating a 50-ppm aqueous sulfur standard. (a) Dark current, (b) background, (c) analytical signal
Table 11. Slope and Intercept Values for Aqueous Sulfur Solutions (concentration range 2 ppm to 2000 ppm S) Using the 180.7 nm Line compound slope intercept RZ ammonium sulfate 2.55 -0.22 9997 thiourea 2.48 0.30 9999 sulfamic acid 2.61 0.21 9987 ammonium pyrrolidine2.59 -0.10 9993 carbodithioate through a 30-mm i.d. medium pore glass frit located behind and directed toward the source. The region from the source to the lens and from the lens to the monochromator was enclosed by use of two glass cylinders 30-mm i.d. and 75 and 95 mm in length. These, as well as the monochromator, were purged with an argon flow rate of 2 L/min. Reagents. Sulfur compounds for aqueous work were chosen to encompass a variety of sulfur oxidation states and chemical forms. These were ammonium sulfate, thiourea, sulfamic acid, and ammonium pyrrolidine-carbodithioate. For the analysis of organics, several organic solvents and several different sources of sulfur, including different oxidation states and chemical forms, were used. The organic solvents were: benzene, butanol, ethyl acetate, dimethylformamide, and hexane. The sulfur compounds were: thiophene, dimethyl sulfoxide, dimethyl sulfone, octyl mercaptan and dithiooxamide. From these, 18 standard solutions listed in Table IX (see below) were made. Reagent grade sulfur compounds and “distilled in glass” solvents were used throughout the investigation.
RESULTS AND DISCUSSION T h e locations of the sulfur resonance lines were established by scanning the 180-184 nm wavelength region while aspirating a solution of 50 ppm S in water (Figure 2 ) . Background and dark current levels are also shown. A practical resolution (line width at 1/2 peak height) of 0.09 nm was attained for the instrument. Aqueous solutions of the four sulfur compounds gave the same sensitivities at a given line. T h e slopes and intercepts of analytical curves (from 0.5 to 2000 ppm S)drawn from the four sulfur compounds at the 180.7 nm line are given in Table 11. Similar agreement was found a t the other lines. Reproducibility in ten duplicate runs of a 100 p p m S solution gave a 2u value of 2.0 ppm S. Relative sensitivities and detection limits for the VUV lines are listed in Table 111. The detection limit is defined as that
Measured a t the 180.7 nm line. trary intensity units. Table V. Plasma Argon Flowa flow rate, intensity, L/min 50 ppm Sb 0.50 0.75 1.00 1.25 1.50 1.75
30 33 32 29 25 20
Measured a t the 180.7 nm line. trary intensity units.
Expressed in arbi-
intensity backgroundb 2.0 2.0 2.0 2.0 1.5 1.0
Expressed in arbi-
Table VI. Interferences Incurred when Aspirating a 50 ppm S Solution Spiked with 5000 ppm Potential InterferenF wavelength potential interferent 180.7 nm 182.0 nm 182.6 nm t 5% NH4C2 H 3 O2 t 2% MgC1,.6H,O 0 0 KH,P04 -2% -2% KI - 2% -6% NaC, H,O, -6% -18% NaNO, -2% -12% a Expressed as a relative % deviation from of S in a 50-ppm aqueous solution.
-5%
+ 5%
-2.5% -20% -30%
-40%
the intensity
concentration of analyte in aqueous solution which produces a signal equal to twice the standard deviation obtained in the background noise observed for solutions of concentrations near the detection limit. T h e oxygen absorption at the 180.7 n m line decreases by a factor of 500 the detection limit for that line. T o optimize the signal-to-noise ratio, we sequentially varied both aspirator and argon flows. T h e optimum aspirator flow was 2.5 L/min (Table IV). Optimum plasma flow rate was 0.75 to 1.0 L/min (Table V). These optimum argon flow rates were the same for all three lines and were used throughout our work. Some chemical interferences were investigated by aspirating 50 ppm sulfur (as ammonium sulfate) in water spiked with 5000 ppm potential interferent and observing the emission signal at each of the three lines relative to a 50 ppm aqueous sulfur solution. Results of these investigations are shown in Table VI. T h e most sensitive sulfur line (180.7 nm) shows the least relative interference in all solutions.
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Table VII. Interferences Incurred when Aspirating a 50 ppm S Solution Spiked with 5000 ppm Potential Interferenta wavelength element 180.7 nm 182.0 nm 182.6 nm Fe 0 0 t 12% Mn 0 + 2% 0 Ni 0 + 2.9% 0 cu 0 + 2.9% t 3% Mo 0 t 3% t 28% a Expressed as a relative % deviation from the intensity of S in a 50-ppm aqueous solution.
Table IX. Detection Limits for Different Sulfur Compounds in Different Solventsa N-octyl mer- thio- dithio- dimethyl dimethyl solvent captan phene oxamide sulfoxide sulfone -_ hexane 15 10 -15 -butanol 4 1 -4 ethyl 5 2 -7 5 acetate dimethyl3 1 3 5 2 formamide -benzene 15 10 -15 a Expressed in ppm.
Table VIII. Detection Limits for Various Elements Using Our Instrumentation
where both the sulfur compound and the solvent are known. These investigations indicate t h a t a dc plasma atomic emission system can be easily and economically built or modified, with appropriate optics and purging systems, to have the added capacity of being able to determine sub-200 nm elemental resonance lines. The added usefulness of these instruments over conventional instruments is obvious.
element Hg Hg Se Se As
As As As I I P P
ox state
I1 I1 IV IV I11 V I11 V -1 -1 V
V
wavelength, nm 184.96 253.65 196.09 206.28 193.69 193.69 228.81 228.81 183.04 206.16 185.92 213.62
detection limit, ppm 0.1 0.2 1.0 4.0 0.4 0.4 0.2 0.2 4.0 10.0
0.4 0.1
No appreciable effect on signal intensity was observed from standards made up in 1 M HC1 and 1 M NH3. Standards spiked with various amounts of NaCl indicated signal (above background) differences were observed a t NaCl concentrations above 0.5%. However, if the concentration of NaCl in the standard is closely matched to the NaCl concentrations in the samples, solutions containing up to 5% NaCl can be accurately analyzed for sulfur. Some spectral interferences were investigated by spiking 50 ppm sulfur (from ammonium sulfate) solutions with 5000 ppm of the potential interfering metal species. These metals were chosen from those with emission lines near the VUV sulfur lines (Table VII). An appreciable positive interference occurred only from copper, molybdenum, and iron at the 182.6 nm line and nickel and copper a t the 182.0 nm line. No interferences were observed a t the 180.7 nm line. We also determined Hg, Se, As, I, and P in aqueous solutions a t their VUV resonance lines. Detection limits for these elements are given in Table VIII. Preliminary experiments were performed to investigate the utility of the dc plasma for the determination of sulfur in organic liquids. Solvents and sulfur compounds are listed in Table IX. We found that different sulfur compounds in one solvent yielded substantially different observed sulfur sensitivities. Also, the same sulfur compound in different solvents yielded different sensitivities. The optimum conditions found in the aqueous solution analysis were also used in the organic work. No effort was made to correct for different nebulization efficiencies, aerosol droplet sizes, or plasma configuration. Under these conditions, the system is useful in the determination of sulfur in organic solvents only in those instances
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RECEIVED for review August 31, 1977. Accepted July 24,1978.
