Anal. Chem. 1997, 69, 3702-3707
Potential of Radio Frequency Glow Discharge Optical Emission Spectrometry for the Analysis of Gaseous Samples Giuseppe Centineo, Matilde Ferna´ndez, Rosario Pereiro, and Alfredo Sanz-Medel*
Department of Physical and Analytical Chemistry, Faculty of Chemistry, University of Oviedo, Julian Claveria, 8, 33006 Oviedo, Spain
The analytical potential of a radiofrequency glow discharge (rf-GD) for the analysis of gaseous samples is evaluated investigating the detection by optical emission spectrometry (OES) of nonmetals in organic vapors using helium as plasma gas. Discrete amounts of low boiling point organic liquids are introduced into the rf-GD through a thermoelectrically heated gas exponential dilutor, and their OES characteristics are measured. The design of the discharge chamber and the effect of operating parameters on the emission signals of typical nonmetallic elements are discussed. Analytical performance is evaluated under the optimum conditions for Cl, C, Br, and S using emission lines in the ultraviolet-visible region. Detection limits in the low picogram-per-second range, good precision (RSD < 5%), and linear ranges of 4-5 decades were observed for the four elements. A comparison of this source is carried out with other, more conventional plasmas for the determination of low levels of nonmetals in organic compounds by OES. The capability of this He-rf-GD for the determination of element ratios in chlorinated hydrocarbons is also investigated, with promising results. In the course of developing element-specific detectors for gas chromatography, many helium plasmas have been proposed for the detection of nonmetals in chromatographic eluents1 by optical emission spectrometry (OES) or mass spectrometry (MS). Although, so far, the atmospheric pressure microwave-induced plasma (MIP)-OES2-4 could be considered the most popular technique for this particular purpose, many other different combinations, using both atmospheric and reduced pressure plasmas, have been evaluated, with different degrees of success.5-12 Atmospheric pressure inductively coupled plasmas (ICPs),5,6 (1) Uden, P. C., Ed. Element-Specific Chromatographic Detection by Atomic Emission Spectrometry; ACS Symposium Series 479; American Chemical Society: Washington, DC, 1992. (2) Estes, S. C.; Uden, P. C.; Barnes, R. M. Anal. Chem. 1991, 53, 1829-1837. (3) Long, G. L.; Ducatte, G. R.; Lancaster, E. D. Spectrochim. Acta 1994, 49B, 75-87. (4) Camun ˜a-Aguilar, J. F.; Pereiro-Garcı´a, R.; Sa´nchez-Urı´a J. E.; Sanz-Medel, A. Spectrochim. Acta 1994, 49B, 545-554. (5) Chan, S. K.; Montaser, A. Spectrochim. Acta 1987, 42B, 591-597. (6) Montaser, A.; Chan, S. K.; Koppenaal, D. W. Anal. Chem. 1987, 59, 12401242. (7) Uchida, H.; Berthod, A.; Winefordner, J. D. Analyst 1990, 115, 933-937. (8) Gross, R.; Platzer, B.; Leitner, E.; Schalk, A.; Sinabell, H.; Zach, H.; Knapp, G. Spectrochim. Acta 1992, 47B, 95-106. (9) Rivie`re, B.; Mermet, J. M.; Deruaz, D. J. Anal. At. Spectrom. 1987, 2, 705709.
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capacitive plasmas,7,8 and reduced pressure discharges such as low-pressure MIP9,10 or low-pressure ICP11 constitute representative examples of plasmas that have been investigated for nonmetal detection of gaseous organic compounds by OES2-5,7-9 and MS.6,10-12 Glow discharges (GDs) have been widely used for the direct analysis of conductive solids. In recent years, the application of radio frequency (rf)-powered GDs for the elemental analysis of solid materials13-15 has increased rapidly because rf-GDs can be employed in the direct analysis of insulating materials, which is not possible with the direct current (dc)-powered GDs. Although the bulk of GD applications presently is addressed to solid sample analysis and the technique has remained relatively unexplored for the analysis of gases, the physical features of a low-pressure GD as a spectrochemical source (low continuum background, high electronic excitation temperatures, etc.), along with its low costs of acquisition and maintenance, make it also a promising detector for nonmetal analysis of gaseous samples. Elemental determinations in gases, organic vapors, and volatilized analytes using dc-GD-OES have been described recently,16-19 while only preliminary studies have addressed the high potential of rf-GD-MS as a gas chromatographic detector for metal speciation.20 In any case, the rf-GD remains unexplored for the excitation of nonmetals in gaseous samples, although previous research has shown that electrons in the rf-GDs have rather high energies and temperatures in comparison with those of the dcpowered devices.21 Therefore, an investigation of the capabilities of rf-GDs for the excitation of high-lying states of nonmetal gaseous analytes and of its features versus the more popular plasma gas chromatographic detectors seemed appropriate and worthwhile. (10) Story, W. C.; Caruso, J. A. J. Anal. At. Spectrom. 1993, 8, 571-576. (11) Castillano, T. M.; Giglio, J. J.; Evans, E. H.; Caruso, J. A. J. Anal. At. Spectrom. 1994, 9, 1335-1340. (12) Mohamad, A. H.; Creed, J. T.; Davidson, T. M.; Caruso, J. A. Appl. Spectrosc. 1989, 43, 1127-1131. (13) Marcus, R. K.; Harville, T. R.; Mei, Y.; Schick, C. R., Jr. Anal. Chem. 1994, 66, 902A-911A. (14) Bordel-Garcı´a, N.; Pereiro-Garcı´a, R.; Ferna´ndez-Garcı´a, M.; Sanz-Medel, A.; Harville, T. R.; Marcus, R. K. J. Anal. At. Spectrom. 1995, 10, 671-676. (15) Marcus, R. K. J. Anal. At. Spectrom. 1996, 11, 821-828. (16) Puig, L.; Sacks, R. Appl. Spectrosc. 1989, 43, 801-810. (17) Ng, K. C.; Ali, A. H.; Winefordner, J. D. Spectrochim. Acta 1991, 46B, 309314. (18) Pereiro, R.; Starn, T. K.; Hieftje, G. M. Appl. Spectrosc. 1995, 49, 616-622. (19) Broekaert, J. A. C.; Pereiro, R.; Starn, T. K.; Hieftje, G. M. Spectrochim. Acta 1993, 48B, 1207-1220. (20) Olson, L. K.; Belkin, M.; Caruso, J. A. J. Anal. At. Spectrom. 1996, 11, 491496. (21) Ye, Y.; Marcus, R. K. Spectrochim. Acta 1996, 51B, 509-531. S0003-2700(97)00412-5 CCC: $14.00
© 1997 American Chemical Society
Figure 1. Experimental setup.
In the present paper, a gas-sampling rf-GD is explored for the determination of nonmetallic elements (carbon, bromine, chlorine, and sulfur) in discrete volumes of volatilized organic compounds using helium as plasma support gas and OES detection. Construction of the discharge chamber and operating parameters that affect the performance of the rf-GD are investigated. The analytical figures of merit of this reduced pressure plasma are evaluated, and the performance of the rf-GD-OES technique is compared with those of other plasmas better known for nonmetal excitation. In the light of this investigation and of the capabilities of the rf-GD-OES for elemental ratio measurements, its analytical potential as a competitive gas chromatographic detector is highlighted. EXPERIMENTAL SECTION Apparatus. A diagram of the setup used is shown schematically in Figure 1. Halogenated hydrocarbons were introduced in a discrete mode into the rf-GD using an exponential dilution chamber.4,18 As depicted in Figure 1, this device consists of a 575 mL glass vessel, containing a magnetically driven stirrer to allow gas homogeneity. The glass vessel is heated at 120 °C with electrical heating tape in order to guarantee the vaporization of the organic compounds (introduced into the vessel, with a Hamilton syringe, through a septum). The interface between the dilution chamber and the rf-GD was also heated to 120 °C to avoid analyte condensations. The exponential dilutor is provided with two three-way stopcocks that allow the He carrier gas to pass directly to the discharge or to go through the vessel, dragging the sample to the plasma. A restriction was made with a throttling clamp on the plastic sampling tube, prior to the rf-GD chamber, to match the rate of aspiration of the vacuum pump with the total flow rate of the carrier. A side arm directed into a 50 mL water-filled Erlenmeyer flask was incorporated into the sampling line as a means of controlling the equilibration of these two flow rates. By maintaining stable this water level in the flask (Figure 1), a sampling efficiency of 100% could be obtained. For safety precautions, the exit of the Erlenmeyer flask was directed to the hood, and the exhaust of the vacuum pump was also directed to the hood. With the above-described system, the sample introduction apparatus before to the restrictor is at atmospheric pressure, and a rotameter (Air Products and Chemicals, Inc., Allentown, PA) was used to measure the carrier gas flow rate. A He carrier flow rate of 750 mL min-1 was kept constant throughout the experiments, while different plasma pressures were achieved by changing the displacement volume of the vacuum pump using a vacuum valve with handled rotary knob (ref. 289021, Leybold AG, Cologne, Germany).
The rf discharge chamber was basically constructed after the Marcus design.13 However, such a GD chamber was designed for solid analyses; therefore, some modifications addressed to improve the analytical performance for the analysis of gases were introduced in our system. Figure 2a shows a general view of the modified source used, where the cathode was refrigerated by circulating water through an stainless steel hollow flat disk. As can be seen, the only entrance of gases (carrier He gas with the vaporized analyte) in this chamber is allowed through the part called “limiting disk”, made of stainless steel, which is grounded. The design and dimensions of the limiting disk proved to be a key part of this system; therefore, several modifications were evaluated. A detailed scheme with the dimensions of the limiting disk finally selected after such experiments is shown in Figure 2b. The cathode was electrically isolated from the limiting disk by a Viton O-ring. It is well-known that the sputtering rates in rf-GDs are lower than those in dc-GDs;21 in any case, in order to avoid potential problems related to lack of performance or short-circuiting due to cathode erosion and redepositions, the cathode surface was polished and cleaned on a daily basis. Details of the rf-GD instrumental components and the spectrometric equipment are given in Table 1. Shielding should prevent rf radiation outside the generator, the impedance matching network, and the transmission line. A Faraday cage, made with a fine mesh screen, covering the GD source is recommended. Peak heights of the transient analyte emission signals were recorded, and the signal integration time was 0.1 s. For optimization experiments, three replicate injections were made. In our studies, the time elapsed between injections was 10 min. Considering the equation c(t) ) c0 exp(-Ft/V), where c0 is the initial concentration, F the carrier gas flow rate, t the time spent after the stopcock was turned, and V the volume of the vessel, the time for decreasing the analyte concentration in the vessel to 0.001% of the initial concentration was 9 min. Reagents. Three reference materials from MBH Analytical Ltd. (Herts, U.K.) were evaluated as cathode: iron ref. M BS 50 D, zinc ref. 41 XZ4, and austenitic stainless steel ref. 13X12533. Helium (99.999%) was used as plasma gas. Chemicals (with purity higher than 99.0%) were obtained from Merck (Darmstadt, Germany). To avoid problems associated with condensations in the dilutor or in the interface, simple organic compounds with low boiling points were selected. Compounds investigated as models as well as the optical emission wavelengths chosen for this study are compiled in Table 2. Studies on the capability of the rf-GD-OES system for element ratio determinations were carried out for carbon tetrachloride, chloroform, 1,2Analytical Chemistry, Vol. 69, No. 18, September 15, 1997
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Figure 2. Diagram of the rf discharge chamber. (a) General scheme of the chamber. (b) Design of the limiting disk for gas analysis. Dimensions are given in millimeters. Table 1. Instrumental Equipment components power supply
rotary vane pump pressure measurement optical arrangement monochromator photomultiplier read-out system a
description and specifications Glow Discharge 13.56 MHz radiofrequency generatora maximum power output, 500 W output impedance, 50 ( 5 Ω control mode, forward power control Model RF-5S, RF Power Products Vacuubrand, Model RZ8 MSK Baratron capacitance pressure transducer, Model 122B; response, 0.1-100 Torr Detection System plasma viewed axially and imaged 1:1 on the entrance slit using a fused-silica lens 1 m Jobin-Yvon HR-1000M Czerny-Turner mount with a grating of 2400 grooves mm-1; slit width, 0.1 mm Hamamatsu R-212 Jobin-Yvon Spectralink system controlled by a computer
Automatic matching network, Model AM-5, RF Power Products.
Table 2. Selected Wavelengths and “Model” Compounds element
wavelength, nm
organic compound
Cl C S Br
II, 479.45 I, 247.86 II, 545.38 II, 470.49
1,2-dichloroethane 1,2-dichloroethane carbon disulfide 1,2-dibromoethane
dichloroethane, 1,2-dichloropropane, and dichloromethane. For sample handling, eyes and skin should be protected with safety goggles and poly(vinyl alcohol) gloves, respectively. Volumes of 0.5 µL of analyte/hexane mixtures were introduced into the exponential dilutor chamber to give the required analyte concentration in the 575 mL glass vessel. RESULTS AND DISCUSSION The evaluation of modifications in the design of the discharge chamber as well as the selection of the operating conditions such as pressure and forward power were carried out using the ratio 3704 Analytical Chemistry, Vol. 69, No. 18, September 15, 1997
IN/IB (where IN is the net analyte emission intensity and IB the background emission intensity) as the criterion to be maximized. Selection of the Limiting Disk and Evaluation of the Effect of the Cathode Material. In the chamber described by Marcus et al.13 for the analysis of solids by rf-GD-OES, the plasma gas is introduced through the main body of the discharge chamber.13-15 However, the experiments for the analysis of previously vaporized analytes showed that much better analytical sensitivities were obtained when the analyte was introduced along with the plasma gas through the limiting disk (Figure 2a) instead of through the main body of the chamber. Experiments on the effect of the design of the limiting disk have been plotted in Figure 3. It shows that the effect of the inner diameter of the central orifice (i.d. in Figure 2b) for Cl, C, S, and Br is an important experimental parameter. Limiting disks with inner diameters of 3, 4, and 6 mm were evaluated in our setup, and similar behavior was observed for the four elements under study (note the different scales in Figure 3a-c). While slight increases in background emission intensities were observed with increasing orificie diameters, the
Figure 4. Plots of IN/IB versus rf forward power for the nonmetals under study. (9) Chlorine, (2) bromine, ([) sulfur, and (b) carbon.
Figure 3. Effect of the inner diameter (i.d.) of the central orifice in the limiting disk on the IN/IB values at different pressures. (9) Chlorine, (2) bromine, ([) sulfur, and (b) carbon. (a) 3 mm i.d., (b) 4 mm i.d., and (c) 6 mm i.d.
net analyte emission was always higher for the 4 mm i.d. disk, giving rise to better IN/IB signals. This disk, of 4 mm, was thus used throughout for further studies. The introduction of a secondary He gas flow through the main body of the chamber was also tested. No effect on the analytical signal was observed, compared with that observed using a single entry from the limiting disk. Therefore, this second gas flow through the main body of the chamber was discarded. Zinc, iron, and stainless steel were evaluated as cathode materials for the determination of chlorine. Neither differences in plasma stability nor noticeable variations in the IN/IB signal were observed for any of the three materials studied as the cathode of the glow discharge. The use of stainless steel,
however, was rejected because it showed a perceptible background signal from carbon emission. Iron, exhibiting poorer sputtering characteristics than zinc, was eventually selected as the cathode material. This factor, added to the relatively inefficient sputtering of rf-powered GDs21 and combined with the poor sputtering characteristics of He as a plasma gas, resulted in very slow cathode erosion. Optimization of the Operating Conditions. The effect of discharge pressure on the analytical signals of the different elements under study can be observed in Figure 3 for the interval 15-40 Torr. As can be seen, a quite similar trend was obtained for the three limiting disks under study. Previous work using other plasma discharges better known for gas analysis, such as the MIP, has shown similar patterns for the effect of pressure on the analytical signals,22-24 and in the MIP this behavior has been attributed to a balance between atomization of the molecule, which is promoted by high gas temperatures and subsequently high pressures and excitation, which could be promoted by the high electron temperatures that occur at low pressures. In the case of the selected limiting disk (4 mm i.d.), the best discharge pressure for all the analytes turned out to be around 30 Torr (28 Torr was optimum for chlorine, 33 Torr for sulfur, and 32 Torr for bromine and carbon). The effect of rf forward power on the emission line-tobackground ratios for Cl, Br, C, and S was studied in the range 30-80 W. Although increases in both background and analyte emissions were observed when augmenting the power, an IN/IB increase with delivered power is obtained for the four nonmetals evaluated, as can be observed in Figure 4. The observed IN/IB trend can be attributed to a better efficacy of the plasma for the atomization/excitation processes as the power increases in the interval under study. Rf powers higher than 80 W were not assayed since instabilities due to lack of effective refrigeration of the cathode were noted several minutes after ignition of the discharge. Analytical Performance Characteristics. Table 3 collects the analytical figures of merit for Cl, C, Br, and S using the rfGD-OES at 80 W of rf-delivered power under the optimum pressure selected for each element. Detection limits (DLs) (22) Rivie`re, B.; Mermet, J.-M.; Deruaz, D. J. Anal. At. Spectrom. 1987, 2, 705709. (23) Brassem, P.; Maessen, F. J. M. J. Spectrochim. Acta 1974, 29B, 203-213 (24) Costa-Ferna´ndez, J. M.; Lunzer, F.; Pereiro-Garcı´a, R.; Sanz-Medel, A.; BordelGarcı´a, N. J. Anal. At. Spectrom. 1995, 10, 1019-1025.
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Table 3. Analytical Performance of rf-GD-OES for the Analysis of Nonmetals in Volatilized Organic Compoundsa this study element
wavelength (nm)
DL (pg/s)
RSDb (%)
upper value in linear range (ng/s)
MIP3 DL (pg/s)
479.45 837.59 193.09 247.86 833.51 470.49 827.24 190.03 545.38
0.7
3.9 (2.2)
1000
8.1
Cl C Br S
ICP5 DL (pg/s)
dc-GD18 DL (pg/s)
CMP7 DL (pg/s)
5000
7000
400
100
800 13 0.3
2.0 (0.7)
360
4.6 (3.0)
900
2200 11
9.5
10 000 1000 1000
6
3.2 (4.4)
900
58
a For comparison, detection limits (DL) with other helium plasmas and OES are collected. DL calculations are based on 3 times the standard deviation of 10 independent background readings. b RSD ) relative standard deviation. The analyte concentration in ng/s at which the RSD was evaluated is cited in parentheses.
Table 4. Elemental Ratio Determinations (Normalized to Carbon)a concn (M)
CCl4
CHCl3
C2H4Cl2
C3H6Cl2
7.0 × 4.5 × 10-10 3.0 × 10-10 1.2 × 10-10
3.95 (4) ( 0.13 3.89 (4) ( 0.16 4.12 (4) ( 0.17 3.89 (4) ( 0.13
3.08 (3) ( 0.11 3.14 (3) ( 0.15 3.11 (3) ( 0.15 3.16 (3) ( 0.17
0.97 (1) ( 0.10 1.01 (1) ( 0.04 1.02 (1) ( 0.07 1.05 (1) ( 0.05
0.66 (0.67) ( 0.04 0.68 (0.67) ( 0.05 0.69 (0.67) ( 0.07 0.72 (0.67) ( 0.07
10-10
a
CH2Cl2 was chosen as reference compound. Theoretical values are given in parentheses.
obtained by OES with other He plasma sources (introducing the analytes as volatilized organic compounds in the discharge) are also included in Table 3 for comparison. As can be seen, DLs in the order of low picograms per second are obtained with the rf-GD-OES technique for the four nonmetals under study. The figures obtained for Cl and C detection are particularly good. These DLs compare favorably with those obtained with the traditional He-MIP.3 It is worth noting the gain in sensitivity using rf-GD (this work) versus dc-GD.18 In order to explain the important differences observed between the analytical performance of dc-GD and rf-GD, it should be noted that the source design was not the same in both cases (e.g., a more diffuse plasma was generated in the dc-chamber18). Although this last factor could give rise to a higher analyte dilution, there is no doubt that the rf-GD source seems more convenient than dc-GDs for nonmetal excitation. The reproducibility of the rf-GD-OES technique was evaluated in each case for five replicate sample injections. Results are collected in Table 3, showing that the RSDs were between 2.0 and 3.9% for C, S, and Cl. The RSD for Br was about 4.6% (this higher value could be attributed to the high background observed at the 470.49 nm emission line). To determine linearity limits, increasing concentrations of analytes were introduced in the discharge, and the first concentration discarded was that giving a net emission signal 10% lower than the signal expected from the slope of the linear graph. It should be noted that the linear range of the calibration curves (Table 3) extended over more than 4 orders of magnitude for any of the four nonmetals evaluated. Moreover, it is important to note that the rf-GD was found to have an excellent day-to-day stability. Element Ratio Determinations. The use of relative emission intensities for the determination of element ratios and empirical formula has been proposed for different excitation sources.4,18,25 On the other hand, to use the rf-GD for elemental GC detection, its ability to break-up any compound into its constituent atoms 3706 Analytical Chemistry, Vol. 69, No. 18, September 15, 1997
with the same efficiency should be demonstrated. In the present study, the potential of the He rf-GD-OES technique for element ratio measurements and compound atomization/excitation has been investigated for carbon and chlorine in organochlorinated compounds containing two, three, and four halogen atoms per molecule. Low boiling point compounds were selected in order to use the previously described analyte introduction system. Table 4 shows the results obtained for different concentrations of the four organic compounds tested. Dichloromethane was chosen to define the reference compound ICl/IC ratio, and all other molecules were referenced to this standard. As can be seen, a good correlation between theoretical values and experimental measurements were observed for all the concentrations under study, demonstrating the ability of this spectrochemical source both to atomize/excite chlorinated hydrocarbons and to determine empirical formulas of such compounds. CONCLUSIONS The preliminary analytical evaluation shown in Table 3 for the detection of previously volatilized nonmetal analytes by rf-GDOES indicates that this spectrochemical source offers a more sensitive alternative to the more familiar excitation sources employed up to date. Thus, the high analytical potential of this technique for the determination of low levels of nonmetallic elements introduced into the discharge in the gas phase is demonstrated. Moreover, the experiments described here for chloroalkanes show the capability of this source for elemental ratio measurements of these compounds. Although different hydrocarbons containing sulfur, oxygen, and nitrogen should be also tested to perform a critical evaluation of a GD detector for element ratios,1 the experiments carried out in this study, showing high sensitivity and the ability to atomize/excite chlorinated hydrocar(25) Uden, P. C.; Slatkavitz, K. J.; Barnes, R. M.; Deming, R. L. Anal. Chim. Acta 1986, 180, 401-416.
bons of different formulas with high efficiency (thus allowing empirical formula determinations), point to the potential advantageous use of this reduced pressure spectrochemical source for GC element-specific detection. The proposed source is easy to build, stable over periods of at least several hours, and inexpensive to operate. All these practical features, along with the analytical capabilities of the rf-GD-OES, make this new technique highly attractive for its investigation as a gas chromatographic detector. Studies addressed to investigate the proposed rf-GD-OES technique for the analysis of volatilized nonmetals, including the determination of the content of chloride in aqueous samples by on-line chemical generation of the gaseous chlorine26 and of (26) Camun ˜a, F.; Sanchez-Urı´a, J. E.; Sanz-Medel, A. Spectrochim. Acta 1993, 48B, 1115-1125.
organometallics for speciation,24 as well as the interfacing of the rf-GD to a gas chromatograph for element-specific detection by optical emission spectrometry and its comparison with the wellestablished GC-MIP-OES technique,3 are currently in progress in our laboratory. ACKNOWLEDGMENT Financial support from DGICYT (Spain) through Project No. PB94-1331 is gratefully acknowledged. Thanks are due to Talleres Jesus Alvarez (Asturias, Spain) for the thoroughness in the reliable construction of the glow discharge chamber. Received for review April 21, 1997. 1997.X
Accepted July 5,
AC970412Z X
Abstract published in Advance ACS Abstracts, August 15, 1997.
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