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Anal. Chem. 1989, 6 1 , 790-793
Nonmetal Analyte Introduction Device for Atomic Spectrometry Gregory K. Webster a n d J o n W. C a r n a h a n ”
Department of Chemistry, Northern Illinois University, DeKalb, Illinois 60115 Nonmetal analyte characterization studies with microwave-induced plasma (MIP) and inductively coupled plasma (ICP) systems inevitably involve the application of a specialized sample introduction configuration. Most introduction schemes route gas over pure solid or liquid analyte to establish a steady-state sample infusion or introduce gaseous samples in discrete and/or continuous modes. Ideally, these devices should be simple, introduce reproducible amounts of sample, and leave no detectable trace of the analyte when it is desired to observe the plasma without sample. Nonmetal analyte introduction with gaseous samples for the ICP has been reported by Fry et al. ( I ) . In this study, a gas sampling loop was used to introduce SFs, CF4, CC12F2, and ClF2CCF2Cl. This scheme consisted of a series of airactuated valves for sample routing. The sample was mixed with argon and introduced into the ICP. Though this system works well, it is somewhat complex. Applications to other analytes require that a tank for each gas be available. Nonmetal analyte introduction schemes such as the ones reported in the literature by Tanabe et al. (2), Freeman and Hieftje (3),and Cull and Carnahan (4)have a sampling device in line with the helium plasma gas. These systems typically operate by directing volatile analytes from pure liquids or solids to the plasma. With these sytems, the precise amount of analyte introduced per second is difficult to control, being dictated by the vapor pressure and surface area of the sample. For compounds of high volatility, dry ice baths or other types of cooling are generally utilized. To avoid saturating the system with analyte, compounds with low volatility are usually utilized. These compounds tend to condense in the introduction line and, because of their low vapor pressures, long periods of time are required before residual analyte is removed. This problem makes reproduction of background spectra difficult. Ideally, an adequate sample delivery system will allow room temperature sample introduction and no detectable analyte signal upon cessation of sample introduction. For characterization studies, we have found that adjusting the vapor pressure of a solution consisting of a volatile analyte in a nonvolatile diluent to be very useful for analyte introduction at ambient temperatures. T o estimate the vapor pressure of the solution and as a guide for solution preparation, a Raoult’s law model is used. The simple sample introduction design based on this concept and reported here yields line emission intensities corresponding to those reported by Tanabe et al. (2) for bromine, chlorine, and sulfur with a helium MIP. An inherent advantage of this approach is that it can be utilized for observation of the spectral characteristics of helium plasma with pure helium, during the introduction of small amounts of volatile nonmetal-containing compounds, and again with pure helium within short time periods. EXPERIMENTAL SECTION Sample Introduction Device. The sample introduction design is shown in Figure 1. The sample holder is constructed of a 10.8 mm diameter Teflon rod with a cylindrical hole of 7.8 mm diameter and a depth of 4.2 mm. This hole has a volume of approximately 0.25 cm3. The body of the design was constructed with Pyrex and was connected to the fused silica capillary plasma containment tube with a threaded Teflon seal and nitrile O-rings. The Teflon rod was held in place in the same manner. Figure 1A shows the position of the rod for placing the sample in the cup and during the acquisition of background spectra. In this position, the sample is isolated from the helium purge and
the plasma gases by the gas-tight nitrile O-rings. Figure 1B shows the position of the Teflon rod during the sample purge with helium. In this position, a helium flow separate from the plasma gas line is utilized to allow the plasma gas flow to be independent from the purge gas. Purging with helium removes residual atmospheric gases and envelopes the sample in a helium atmosphere. Figure 1C shows the position of the sampler during analyte introduction. The helium plasma gas flows over the sample producing a steady-state infusion of analyte. A spectrum in the absence of analyte introduction can then be reproduced after moving the sample rod to the position seen in Figure 1A. Materials and Plasma System. The CBr4 (Reagent Grade, Aldrich Chemical), C2H5Br(Reagent Grade, Baker Chemical), CC1,COOH (Reagent Grade, VWR Chemical), CS2 (Reagent Grade, Matheson, Coleman and Bell), squalane (Eastman Organic), and CH2C12(Reagent Grade, Fisher Scientific) were used as received. The helium used in this study was welding grade purchased from Bennett Welding Supply Co. (Franklin Park, IL). The plasma was confined in a capillary fused silica tube (9 cm x 0.6 cm o.d x 0.1 cm i.d.) inserted in a TMoloresonant cavity similar to that described by Beenakker (5). The system impedance was matched by using an adjustable fused silica rod inserted into the cavity and a three-stub tuner ( M a y Microwave Corp., Model HMC-1878B). The microwave generator used was a 125-W, 2450-MHZ Microtherm Model CMDl (Raytheon Manufacturing Company). All spectra were recorded by use of a 0.35 m focal length spectrometer (GCA/McPherson) with a Schoeffel McPherson 8102-0049-11200 grooves/mm holographic grating (useful range, 200-800 nm) and a 1P28 photomultiplier tube (RCA) operated at 900 V. The spectral bandwidth was 0.1 nm. The output of the photomultiplier tube was converted to a voltage with a home-built current-to-voltageconverkr amplifier and scans were monitored with an Omniscribe Model B-5117-5chart recorder. A yellow high pass optical filter (Turner 2B) with a cutoff of approximately 390 nm was placed over the entrance slit of the monochromator to remove second-order radiation. Methods. The sample was placed in the cup and purged with helium for 30 min for the solid samples and 10 min for the liquid samples as shown in Figure 1B. During the purging procedure, the plasma was ignited with a copper wire and allowed to stabilize at 40% power (approximately40 W forward power). The helium flow rate was 50 mL/min. A background spectrum was recorded after the sample was purged and the Teflon rod was moved to the position seen in Figure 1A. These scans were from 400 to 495 nm at 0.2 nm/s for the bromine- and chlorine-containingsamples and from 480 to 587 nm for the sulfur-containing samples. This procedure enabled spectrometer calibration by using helium emission l i e s at 447.15/447.17 nm and 492.19 nm for the halogens and the helium emission lines at 501.57 and 587.56/587.59 nm for sulfur. Upon completion of the background scan, the sample was placed in line as shown in Figure 1C. The sample analysis was performed under the same conditions as the background wan. Because of the low helium flow used for the plasma, the flow was doubled before the Teflon rod was pulled back t o avoid extinguishing the plasma by a “backflow” effect. In other words, if the rate of withdrawal of the Teflon rod volume is greater than the helium flow rate, pulling the Teflon rod from the body of the sampler will stop or reverse the flow of gas in the plasma region, extinguishing the plasma. Once the sample was isolated from the plasma (Figure lA), the flow rate was returned to its original setting. A spectrum was then recorded by scanning the region a final time. RESULTS AND DISCUSSION Solid Samples. The sample introduction device was first evaluated with milligram amounts of solid CBr, for bromine emission and CC1,COOH for chlorine emission. Both compounds have adequate vapor pressure to introduce sufficient
0003-2700/89/0361-0790$01.50/0 C 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 7, APRIL 1, 1989 A
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Figure 1. Schematic for the sample introduction device: (A) position of Teflon rod for sample loading; (B) position of rod during the sample purge with helium; (C) position of rod while obtaining spectra of the sample.
analyte to produce halogen emission, but the use of both compounds results in a residual halogen signal after the sample was again isolated from the plasma. This effect is due to condensation in the sample gas lines. These compounds are slow to vaporize from these surfaces because of their relatively low vapor pressures. Examples of these results are illustrated in Figure 2. A 10-min purge with helium after the CC13COOH analysis was required to remove evidence of residual chlorine in the background spectrum. Results were similar with CBr4. Many solid and liquid sulfur compounds were examined in the same manner as CBr, and CC1,COOH. However, sulfur emission could not be detected. The compounds either were not volatile enough or produced intense C2 emission making absolute verification of sulfur emission lines quite difficult. Raoult’s Law Approximation. The failure to produce sulfur emission and the residual signals seen with low vapor pressure solid and liquid compounds led to the manipulation of the vapor pressure of solutions with compounds too volatile to sample directly. According to Raoult’s law, the vapor pressure of a binary solution may be expressed as
Figure 2. Chlorine emission analysis using CCI,COOH: (A) background spectrum with the sample in the position seen in Figure 1A; (B) spectrum of the sample in the position seen in Figure 1C; (C) background spectrum immediately after moving the sample back to the position of Figure 1A; (D) background spectrum 10 min after moving the sample back to the position of Figure 1A. All scans were recorded at a full scale of 10 mV, chart speed of 12.5 cm/min, scan rate of 0.2 nm/s, amplifier time constant of 0.5 s, and a slit width of 50 km.
where P is the vapor pressure of the solution, X , is the mole fraction of component a, Pa is the standard vapor pressure of component a, Xb is the mole fraction of component b, and Pb is the standard vapor pressure of component b. Squdane was chosen as the solvent since its vapor pressure is negligible and because it readily dissolves a host of compounds. Letting squalane be component b and assuming the vapor pressure of squalane to be negligible, eq 1 is reduced to
P = XaPa
(2) This approach clearly has an advantage for using very volatile
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 7, APRIL 1, 1989 1 I
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Figure 3. Sulfur emission analysis using 0.03 mL of a 3% (v/v) solution of CS2 in squalane: (A) background spectrum: (B) sample spectrum. The background spectrum 3 rnin after moving the sample to the position of Figure 1A is essentially identical with that of part A. All scans were recorded with the same conditions listed in Figure 2, except the slit width was 38 pm.
analytes. Solution vapor pressures may be easily manipulated by adjusting the mole fraction of the volatile solute. Squalane Solvent. Spectra were obtained with and without 0.03-mL samples of squalane to verify that it had a negligible effect on the background. It was observed that no C2 emission was present for this hydrocarbon, illustrating that the squalane solvent will have a negligible effect on analyte and background emission spectra. Sulfur Emission. A spectrum of the helium plasma in the region of intense sulfur emission is shown on Figure 3A. Figure 3B illustrates the spectrum obtained with a 0.03-mL sample of 3% (v/v) CS2 in squalane. The calculated vapor pressure of the solution is 81 Torr. Upon isolation of the sample from the plasma gas (Figure l B ) , the spectrum duplicates the original background within 3 min. Although not shown, this spectrum is essentially identical with that of Figure 3A. Thus, residual problems previously seen with the solid samples were not present in this analysis. When left in the position of Figure lC, a 0.03-mL sample of the CS2 solution generated a detectable emission signal for 40 min. After that period of time, the sulfur emission signal was indistinguishable from background noise. Chlorine Emission. Many compounds readily available in most laboratories, such as CCl,, CHC13, and CH2C12,contain chlorine and can be utilized for emission studies. CCl, extinguished the plasma in a 25% (v/v) solution in squalane. The calculated vapor pressure of this solution is 81.6 Torr, so it appears that this compound deviates from our Raoult's law assumption. Therefore, a 2.8% (v/v) solution of CH2C12 in squalane was studied. This solution has a calculated vapor pressure of 84 Torr. Figure 4 shows the analysis of a 0.03-mL sample of this solution. The analysis shows chlorine emission lines a t 479.45, 481.01, and 481.95 nm with a return to the original spectral characteristics within 3 min of isolating the
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Figure 4. Chlorine emission analysis using 0.03 mL of a 2.8% (v/v) solution of CH2C12in squalane: (A) background analysis; (B) sample spectrum. The background spectrum 3 min after moving the sample to the position of Figure 1A is essentially identical with that of part A. All scans were recorded with the same conditions as listed in Figure 2 and the text.
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Figure 5. Bromine emission analysis using 0.03 mL of a 3.5% (vlv) solution of C2H,Br in squalane: (A) background spectrum: (B) sample spectrum. The background spectrum 3 rnin after moving the sample to the position of Figure 1A is essentially identlcal with that of part A. All scans were recorded with the same conditions as listed in Figure 2 and the text.
sample in the position of Figure 1B. Bromine Emission. The bromine analogues of the chlorine compounds mentioned above do not have the desired vapor
Anal. Chem. 1989, 61, 793-796
pressure for emission studies in the manner described here. The choice of compounds that have adequate vapor pressure and contain bromine is quite limited. Figure 5 illustrates the analysis of a 0.03-mL sample of a 3.5% (v/v) solution of C,H& in squalane. The calculated vapor pressure is 82 Torr. The analysis shows bromine emissions at 470.49,478.55, and 481.67 nm, again with a reproduction of the original background within 3 min from the time the sample was isolated.
CONCLUSION With the 40-W MIP system discussed, we have found that solutions with calculated vapor pressures near 80 T are useful for producing nonmetal analyte emission with a Raoult’s law approximation and our sample introduction device. The emission lines and intensities for sulfur, chlorine, and bromine are in agreement with those reported by Tanabe et al. (2). With the sampling scheme reported, nonmetal analyte characterization studies in atomic spectrometry no longer dictate consideration of complex sampling schemes, low temperatures for analyte introduction, or residual effects in the background. Modification of this concept for other systems
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with differing sample mass introduction requirements should be straightforward.
ACKNOWLEDGMENT The authors thank Larry Gregersen and Ed Hyland of Northern Illinois University for their help with the construction of the sample introduction device and Karen D’Arcy of Governors State University for the CC1,COOH. G.K.W. would also like to thank Sigma Xi for a Grant-in-Aid to support this work. LITERATURE CITED (1) Fry, R. C.; Northway, F. J.; Brown, R. M.; Hughes, S.K. Anal. Chem. 1980, 5 2 , 1716-1722. (2) Tanabe, K.; Haraguchi, H.; Fuwa, K. Spectrochim. Acta, Part 8 1981, 368, 119-127. ( 3 ) Freeman, J. E.; Hieftje, G. M. Spectrochim. Acta, Part 8 1985, 408, 653-664. (4) Cull, K. B.; Carnahan, J. W. Appl. Spectrosc. 1988, 42, 1061-1065. ( 5 ) Beenakker, C. I . M. Spectrochim. Acta, Part 8 1978, 378, 483-486.
RECENEDfor review September 27,1988. Accepted January 3, 1989.
Simultaneous Determination of Major and Trace Elements by Inductively Coupled Plasma Mass Spectrometry/OptlcaI Emission Spectrometry John R. Garbarino* and Howard E. Taylor
US.Geological Survey, Box 25046, M S 408, Denver Federal Center, Denver, Colorado 80225 William C. Batie Analyte Corporation, 1651 East Edinger Avenue, Santa Ana, California 92705 Inductively coupled plasma mass spectrometry (ICP-MS) is capable of determining a wide range of different elements a t sub-bg/L concentration levels ( 1 ) . The linear dynamic measurement range extends 4-5 orders of magnitude. However, because of its high sensitivity, the determination of analyte concentrations above 1 mg/L requires special measures. The ion flux from analyte concentrations greater than 1mg/L saturates an electron multiplier operating in the pulse counting mode. Many elements, for example Na, Mg, Ca, and Si, often occur a t concentrations greater than 1 mg/L in natural water ( 2 ) . In late 1988, instrument manufacturers introduced modifications that extended the analytical range of the technique. Instrumentation employed in this work uses computer-controlled quadrupole rod offset potential to reduce the sensitivity of selected mass-to-charge ratios. While solving the basic problem, this approach requires extreme caution when the application involves spectral corrections based on isotope abundance values. An alternative to using a different rod offset potential to reduce the sensitivity would be the selection of a less abundant isotope for quantitation. However, oftentimes the isotopic abundance does not offer an appropriate combination of decreased sensitivity and dynamic range. In addition, some analytes are monoisotopic, making selection of another isotope impossible. Also, some elements have significant background interferences that are associated with isobaric, multiply charged, or polyatomic ions that limit their analytical usefulness in some applications ( 3 , 4 ) . Therefore, the combination of high sensitivity and spectral interferences can affect the determination of selected analytes by ICP-MS. Inductively coupled plasma optical emission spectroscopy (ICP-OES) exhibits a linear dynamic range similar to that of
ICP-MS, although sensitivities for selected elements generally are less by at least a factor of 10 (5). When alternate analytical wavelengths are selected, spectral interferences can be minimized, and variable sensitivity can be obtained. Recognizing that each of these techniques has both advantages and disadvantages, combination of the two would provide substantially greater analytical flexibility. This technical note describes a technique that couples mass spectrometric and optical emission spectrometric detection by using a single inductively coupled argon plasma for ion production and as a light source. The technique provides simultaneous determination of major and trace elements by effectively combining and extending the analytical concentration range of the individual techniques.
EXPERIMENTAL SECTION The ICP-MS instrumentation used in this study has been described previously by Garbarino and Taylor (6). The analytical isotopes used were 51V,52Cr,and g3Cu. The pneumatic nebulizer employed was a modified Babington type similar to the one described by Wolcott and Sobel (7). The plasma on the ICP-MS instrument was used as the emission source for the optical spectrometer. The optical emission spectrometer was composed of two separate 0.75-m polychromators manufactured by Spectro Analytical Instruments, Inc., Fitchburg, MA. Each polychromator had optimum reciprocal linear dispersion and blaze wavelength for the wavelength range of interest. Analytical wavelengths used were as follows (nm): V 309.3, Cr 205.6, Fe 239.2, Cu 324.8, Ca 393.4, Mg 279.6, Na 589.0, K 766.5, and Si 251.6. Light emitted by the plasma was transmitted to each polychromator by using two quartz optical fibers, each 2 m in length and having a diameter of about 1.5 mm, that could be physically positioned independently to view the optimum emission region in the plasma for elements
0003-2700/89/0361-0793$01.50/00 1989 American Chemical Society
