ANALYTICAL CHEMISTRY, VOL. 51, NO. '12, OCTOBER 1979
1935
Comparison of Several Microwave Cavities for the Simultaneous Determination of Arsenic, Germanium, Antimony, and Tin by Plasma Emission Spectrometry Kevin J. Mulligan, Mark H. Hahn, and Joseph A. Caruso" Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 4522 1
Fred L. Fricke US.
Food and Drug Administration, 114 1 Central Parkway, Cincinnati, Ohio 45202
A Beenakker, 3/4-X Evenson, 'I4-X Evenson, and 3/,-X Broida cavity were successively used to couple microwave power (2450 MHz) to an Ar/He or He plasma at atmospheric pressure. The suitability of each plasma as a spectral excitation source for the simultaneous spectrometric analysis of As, Ge, Sb, and Sn was evaluated. Though detection limits (1 ppb at 3a) and linear dynamic ranges (10') were similar, the pure He plasma sustained in the Beenakker cavity proved to be the easiest to tune and operate plus It provided the most reproducible results. These results are comparable to those obtained by other methods.
The analytical potential of the microwave induced plasma
(MIP) as an excitation source for atomic emission spectroscopy has been outlined in a recent review ( 1 ) . Among the advantages it provides are a high excitation temperature a t a low operating power (less than 200 W) and a typically small molecular background emission. Cooling requirements a t this power level are minimal and the rate of consumption of the support gas is of the order of 1 L/min or less. This stands in marked contrast to the inductively coupled plasma (ICP) which operates a t the kW level and requires large amounts of additional support gas to act as a coolant (2) in spite of a recent improvement in torch design (22). However, the lower electrical field strength within the MIP increases the sensitivity of the impedance of the plasma to the presence of material other than the support gas. As a consequence, sample introduction rates in excess of 1 mg/s can disrupt the impedance match between the microwave power generator and the cavity to the point where the plasma is extinguished. In attempts to minimize this limitation by providing for a more efficient coupling of the microwave power to the plasma, several cavities have been proposed and evaluated (3-5). Of these, the foreshortened 3/4-X and 1/4-X coaxial cavities of Broida and Evenson, respectively, have gained wide acceptance. Skogerboe and Lichte have introduced a modification of the Broida cavity, termed the 3/4-X Evenson cavity, which can sustain a MIP in NZ,Ar, and He a t atmospheric pressure in the presence of aqueous aerosols ( I ) . Recently, Beenakker (7,8) described a cylindrical TMolocavity which also supports a He plasma a t atmospheric pressure. Subsequently, several workers have demonstrated the efficacy of this cavity as part of an element-selective detector for gas chromatography (9,11,24) and in application to the analysis of solutions (10, 12). Workers in this laboratory have coupled a semiautomated hydride generator with the MIP for the spectrometric determination of Ge, As, Se, Sn, and Sb in the single element mode (13). By employing a chromatographic separation to 0003-2700/79/0351-1935$01.OO/O
remove spectral interferents and t o stabilize the plasma, detection limits were obtained a t the sub-ppb level. The demonstrably better fragmentation and excitation characteristics of the He plasma vs. the Ar plasma (9) and the recent improvements in cavity design which permit a He plasma to be sustained at atmospheric pressure have prompted us to undertake a comparative examination of the performance of these four cavities when applied to the simultaneous determination of Ge, As, Sn, and S b as their hydrides.
EXPERIMENTAL Although the basic configuration of the system has been described elsewhere ( I 3 ) , it is reproduced here as Figure 1 for convenience. Hydride Generation. A 20.0-mL aliquot of an aqueous acid solution (10% v/v HC1 and 2.5% v / v HzS04Baker Analyzed Reagents) was placed in the reaction tube of a semiautomated hydride generator, modeled after that of Fiorino et al. (14). Then the mixture was spiked with an appropriate amount of a stock solution which contained 10 ppm of each of the analytes. This solution was prepared daily from AAS standards (Alfa-Ventron Co. and Fisher Scientific Co.). Upon initiation of the generation cycle, 8 mL of a 4% w/v solution of NaBH4 (Matheson, Coleman and Bell, 98%) which contained 5% w/v of NaOH (Matheson, Coleman and Bell ACS Reagent Grade) was delivered uniformly over a 15-s interval. After a 5-s delay, the delivery tube was flushed with 10 mL of doubly deionized HzO for 5 s. These conditions provided the optimal compromise for the simultaneous generation of the hydrides and were essentially the same as those that have been used t o generate the hydrides individually (23). SeH, is effectively generated from more acidic solution and so was not included in this study. The NaOH used to stabilize the NaBH, solution was drawn from a 50% w/v aqueous solution to minimize the amount of Na2C03in the mixture and, consequently, the amount of COz produced during the generation reaction. Also, to ensure the quantitative production of the hydrides, fresh NaBH, solution was prepared daily. System Operation. The effluent from the reaction tube, which consisted of the hydrides, Hz, COP,HC1, and H20, was passed through a drying tube, D1, which was packed with CaC1, (Baker Reagent Grade 8 Mesh Granular) to remove the bulk of the HzO. Then, valve, V1, routed the flow into a thick-walled silanized glass condensation tube (1.5-cm i.d. and filled to the extent of 40 cm with 1 mm silanized glass helices) which was immersed in liquid N1. While the bulk of the reaction products were collected, V3 permitted the hydrogen to be vented to the atmosphere through D2. At this point, V1 was turned to admit the He carrier gas to the system and V2 was opened to provide a total He flow rate of 400 mL/min. This served to rapidly eliminate H2 from the condensation tube. After 15 s, V2 was closed and the flow rate fell to 100 mL/min. These flow rates were controlled by adjustment of flow controllers, FC1 and FC2. 1979 American Chemical Society
1936
ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979 FC2
VliRChIVE GE\ERATOR
721.50 r-z
I
.I
I1
!.
a3
Figure 2.
Scale representation of the 3/4-X Evenson and the Beenakker
cavities Figure 1.
Schematic diagram of the apparatus
Then, V3 was turned so that the carrier flow entered the remainder of the system and the condensation tube was transferred to a hot water bath at 75-80 "C. The condensed materials were volatilized and carried through an additional drying tube, D3, to remove the remaining HzO. Subsequently, they passed through a chromatographic column (4.7 mm i.d. X 2.5 in.) at ambient temperature which was packed with Chromosorb 102. The column served to reduce the rate of sample throughput to a point compatible with the maintenance of a stable plasma. In the case of all cavities except the Beenakker, it was necessary to vent the early portion of the flow from the column in order to sustain the plasma. The required vent times were as follows: 3/4-X Evenson and ' l 4 - X Broida cavities. 15 s; and 'I4-h Evenson cavity, 20 s. All times were measured from the point at which the condensation tube was transferred to the hot water bath. After this vent interval had elapsed, V4 was turned to allow the carrier gas to enter a mixing chamber where it mingled with additional support gas of a suitable flow rate. For the Beenakker and 3/4-h Evenson cavities, this gas was He at 400 mL/min whereas for the 3/4-X Broida it was Ar at 400 mL/min. The 1/4-h Evenson cavity required Ar at 600 mL/min. Finally, the mixture passed into a quartz containment tube (Amersil-HeraeusInc. 4.7 mm o.d./2.9 mm id. x 110 mm) located axially within the cavity of interest to provide maximal power transfer to the plasma (6). Cavity Description and Operating Conditions. The 3/4-X Broida and 1/4-h Evenson cavities were purchased from Opthos Instrument Co. The 3/4-X Evemon cavity was constructed from brass based on details provided by R. K. Skogerboe ( 1 5 ) . It consists of a tube 44.5-mni i.d. and 86 mm in depth to which is attached a suitable RF connector and a tuning screw assembly. The quartz containment tube passes up the middle of the cavity and rests lightly on a brass probe through which the microwave power is introduced. The Beenakker cavity was machined from a single piece of copper stock in keeping with the guidelines suggested by Beenakker (7). This cavity has an internal diameter of 91.5 mm and a depth of 10 mm. ti 1-mm diameter coupling loop joins the cavity perpendicularly at a distance of 10 mm from the wall. These latter two cavities are represented in Figure 2. Microwave power at 2450 MHz was provided by a microwave power generator (Opthos Instrument Co.) and conducted through RG 214/U cable (Beldon) to a three-stub tuner (Maury Microwave Inc. Model 1823) which was affixed to the cavity. All cavities were operated at 100 51: forward power and 0 to 0.5 W reflected power and the plasma was maintained throughout the entire course of the experiment. Because of the power level employed, each cavity was cooled by passing a stream of air through it. To this end, the tuning screws with which the Beenakker cavity had been provided during construction were removed so that the interior of the cavity was accessible. Spectrometric Detection. The emitted radiation was viewed axially. This arrangement offered an improved S / N ratio and a larger total signal per unit analyte (16) as well as avoided
235.0 m
101.9 nm
259.8
~1
12nd order1
i !
Ge
Sb -
Chromatograms obtained using the 3/,-X Evenson cavity. The upper tracings represent a 0.5-pg sample of the analyte (As at 0.1 pg). The lower tracings reflect the blank response on each channel Figure 3.
difficulties associated with the degradation of the optical transparency of the containment tube during the operation of the plasma. This signal was focused on the 25-pm entrance slit of a 1.5-m direct reading polychromator (Jarrell-Ash Model 66-000) by a quartz lens (f = 189 mm) located a distance of 176 mm from the entrance slit. The polychromator is configured in a modified Paschen-Runge mounting with a concave grating ruled at 1180 grooves/mm and blazed at 360 nm. With a fixed angle of incidence of approximately 34O, the instrument provides a reciprocal linear dispersion on the order of 0.56 nm/mm. The 75-pm exit slits sit in front of a series of RCA 1P28 PM tubes. Exit slits were positioned to allow examination of the following lines: As, 234.984 nm; Sb, 259.806 (2nd order); Ge, 303.906 nm and Sn, 317.502 nm (2nd order). Corex filters were employed to prevent spectral overlap of visible radiation with the 2nd order lines. In preliminary experiments to determine the proper integration interval, the output of individual PM tubes was recorded as a function of time on a strip chart recorder (Hewlett-Packard Model 701B). Chromatograms obtained using the 3/4-X Evenson cavity are presented in Figure 3. The tracings initiate at the point where V4 was turned to admit the flow of analytes to the plasma. As a result of the examination of such information, a pre-integration delay of 30 s was employed from the time a t which the con-
ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBEiR 1979
Table 11. Relative Standard Deviation,
Table I. Detection Limitsa, ngb
114-h
3i4+
element
analytical line, nm
As
234.984 259.806 (2nd order) 303.906 317.502 (2nd order)
Sb Ge Sn
Evenson element cavity
EvenEven- Broi- son Beecav- nakker son da cavity cavity ity cavity 23 35 44 40 34 44 52 38 1i4+
140 350
3i4-h
26 75
54 50
29 100
a Determined at three times the standard deviation of the blanks. For concentration values in ppb, divide by the sample volume of 20 mL.
densation tube was transferred to the hot water bath. Integration occurred for 60 s. During the integration period, capacitors were charged with the currents generated by the PM tubes. Subsequently, the capacitors were successively discharged through a current-tofrequency converter and the results were displayed on a 4 1 / 2 decade frequency counter. Our investigations of linear dynamic range were confined to approximately two orders of magnitude because of limitations in the measurement system, the size of the blank signals, and the fact that the analytes plated out on the walls of the containment tube when quantities in excess of a few micrograms were introduced into the plasma.
RESULTS AND DISCUSSION Experimentally, all cavities with the exception of the 1/4-h Evenson proved to be relatively easy to tune and operate. The use of a three-stub tuner in place of the tuners with which the 3/4-X Evenson and Beenakker cavities had been provided greatly facilitated their tuning by virtue of the fact that the three-stub tuner offers a finer control of the impedance transformation responsible for matching the load with the impedance of the microwave power generator. Subsequent experience has shown the three-stub to be superior to a two-stub tuner (Maury Microwave Inc. Model 1723) in this respect. Recent modifications to the tuning assembly of the Beenakker cavity by van Dalen et al. (17 ) may diminish this advantage, however. Pure He plasmas could be sustained only in the 3/4-X Evenson and the Beenakker cavities which illustrates that these cavities provide a more efficient coupling of the microwave power to the plasma than the other two. Moreover, the fact that no vent time (introduction of considerably more reaction products) was necessary after the transfer of the condensation tube to the hot water bath when the Beenakker cavity was employed suggests that it has the best power transfer characteristics. The chromatograms of Figure 3 are typical of those obtained with each cavity. An examination of the blank signals revealed the presence of significant spectral interferents on the As, Sn, and Ge channels. While we can only speculate on the nature of the species involved, the collection of peaks a t 235.0 nm is probably derived from C02. possibly as an emission from CO+ (B28-X2Z a t 235.25 nm). Also, the negative character of the signals a t 303.9 nm and, more markedly, a t 317.5 nm could result from absorption by 03. Moreover the positive portion of these signals appears to be due to HC1+ ( 2 P n 3 , , R-head at 317.73 rim with a weaker continuum which has an intensity maximum a t 300.0 nm) (18). With the Ar/He plasmas, the blank value constituted 15 to 45% of the integrated signals obtained for a 0.5-pg sample, For the pure He plasmas, this value ranged from 60 to 75%. This demonstrates the more powerful nature of the He plasma vs. the Ar/He plasma and suggests that improved performance
Asb Sb Ge Sn a
3.22 5.27 7.24 4.14
Determined at 0.5 p g .
3 i 4 3
1937
%a
3i4+
Broida cavity
Evenson cavity
2.73 5.43 5.05 2.83
1.09 1.18
1.70 1.61
Beenakker cavity 1.26 2.18 1.65 0.91
Determined at 0.1 p g .
could be obtained by using narrower exit slits. Calibration curves were linear from 0.05 to 2.5 pg. Detection limits determined at three times the standard deviation of the blank values are presented in Table I. That these detection limits are essentially the same in spite of the differences in the magnitude of the relative blank values can be explained by referring to the precision data contained in Table 11. These values were obtained from 8 successive determinations a t the 0.5-pg level with the exception of As, which wias determined a t the 0.1-pg level because of the magnitude of the signal involved. It would seem that the apparent advantage of the reduced spectral background of the Ar/He plasma is offset by the decreased stability of this plasma in the presence of foreign material. The decidedly poorer results given by the L/4-X Evenson cavity for Sn and Ge can be ascribed in some rneasure to the considerable difficulties encountered in maintaining a steady-state plasma with this cavity. In terms of the ease of operation and the sensitivity and precision of results, the pure He plasma swtained in the Beenakker cavity proved to be the inost suitable excitation source of those investigated for the simultaneous analysis of As, Ge, Sn, and S b as their hydrides using th.e MIP. Thompson et al. (19,20) have reported the simultaneous determination of As, Sb, Bi, Se, and T e following the generation of their hydrides by a continuous mixing of the analyte solution with a solution of NaBH4. The evolved gases are fed to an inductively coupled plasma and the emitted radiation is analyzed by a polychromator similar to the one employed above. With a 20-s integration interval, they observed linear ranges of 2.5 to 3 orders of magnitude and detection limits of 0.8 to 1.0 ppb. In another approach, Skogerboe and Bejmuk (21) generated the hydrides of As, Sb, and Ge by reaction of 100 mL of the sample solution with NaBH4 pellets and subsequently chromatographed them on silica gel. Using a thermal conductivity detector, they obtained linear ranges of 3 to 4 orders of magnitude and detection limits (at two times the standard deviation of the base line) as follows: As, 1 ppb; Ge, 0.3 ppb; and Sb, 10 ppb. The total time required for this analysis was about 25 min. With the Beenakker cavity, we have obtained detection limits of 1.5 to 5 ppb based on a 20-mL sample volume with a linear dynamic range of two orders of magnitude. The analysis time was 2 min with a 1.5-min delay between runs while the condensation tube cooled to liquid N:! temperature. As may be seen, these results are comparable with those indicated above (19-21).
ACKNOWLEDGMENT The authors thank E. J. Younginger for several useful technical discussions.
LITERATURE CITED (1) Skogerboe, R. K.; Coleman, G. N. Anal. Chem. 1976, 48, 611A-622A. (2) Barnes. Raymond M. Crlt. Rev. Anal. Chem. 1978, 7(3), 203-296. (3) Fehsenfeld, F. C.; Evenson, K. M.; Broida, H. P. Rev. Scl. Instrum. 1965, 26, 294-298. (4) McCarrol, Bruce Rev. Sci. Instrum. 1970, 4 1 , 279-280.
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979
(5) Broida, H. P.; Chapman, M. W. Anal. Chem. 1958, 30, 2049-2055. (6) Lichte, F. E.; Skogerboe, R. K. Anal. Chem. 1973, 45, 399-401. (7) Beenakker, C. I.M. Spectrochim. Acta, Part 6 1976, 3 1 , 483-486. (8) Beenakker, C. I. M.; Boumans, P. W. J. M. SDectrochim. Acta, Part6 1978, 33, 53-54. (9) Beenakker, C. I.M. Spectrochim. Acta, Part B 1977, 32, 173-187. (10) Beenakker, C. I. M.; Bosman, 6.; Boumans, P. W. J. M. Spectrochim. Acta, Part B 1978, 3 3 , 373-381. (11) Quimby, Bruce D.; Uden, Peter C.; Barnes, Raymond M. Anal. Chem. 1978. 50. 2112-2118. (12) Zander, Andrew T.; Hieftje, Gary M. Anal. Chem. 1978, 50, 1257-1260. (13) Fricke, Fred L.; Robbins, Wayne B.; Caruso, Joseph A. J . Assoc. Off. Anal. Chem. 1978, 60(5), 1118-1123. (14) Fiorino, John A.; Jones, John W.; Capar, Stephen W. Anal. Chem. 1976, 48, 120-125. (15) Skogerboe, R. K., private communication. (16) Coleman, Geoffrey N. "Automated Background Correction in Multieiement Emission Spectrometry", W.D. Thesis, Colorado State University, 1975. (17) van Dalen, J. P. J.; de Lezenne Coubnder, P. A,; de Gabn, L. Spectrochim. Acta, Part B 1978, 33, 545-549.
(18) Pearse, R. W. B.; Gaydon, A. G. "The Identification of Molecuhr Spectra", 2nd ed.; John Wiley & Sons: New York, 1950. (19) Thompson, M.; Pahlavanpour, B.; Walton. S. J.; Kirkbright, G. F. Analyst (London) 1978, 103, 568-579. (20) Thompson, M.; Pahlavanpour, B.; Walton, S. J.; Kirkbright, G. F. Analyst (London) 1978, 103, 7 0 5 . (21) Skogerboe, R. K.; Bejmuk, A. P. Anal. Chim. Acta 1977, 94, 297-305. (22) Savage, R. N.; Hieftje, G. M. Anal. Chem. 1979, 51, 408-413. (23) Robbins, Wayne B.; Caruso, Joseph A,; Fricke, Fred L. Analyst(London) 1979, 104, 35-40. (24) Quimby, Bruce D.; Debney, Michael F.; Uden, Peter C.; Barnes, Raymond M. Anal. Chem. 1979, 5 1 , 875-880.
RECEIVED for review April 16, 1979. Accepted July 12, 1979. K.J.M., M.H.H., and J.A.C. gratefully acknowledge the National Institute of Occupational Safety and Health for partial support of this work through grant p OH 00739.
Volatilization of Refractory Compound Forming Elements from a Graphite Electrothermal Atomization Device for Sample Introduction into an Inductively Coupled Argon Plasma G. F. Kirkbright and R. D. Snook* Chemistry Department, Imperial College, London S W7 2A Y, England
The use of a halocarbon/argon atmosphere in the sampling manlfold of a graphlte rod electrothermal vaporization device employed for the introduction of samples into a higkfrequency, inductively coupled argon plasma source for optical emlssion spectroscopy is shown to permit sensitive determination of elements such as boron, molybdenum, zirconium, chromium, and tungsten which form refractory oxides or carbides which limit attainable sensitivity when an argon atmosphere alone is employed. Typical improvements in the detection limits obtained with thls technique are between one and two orders of magnitude, so that subnanogram amounts of these elements may be detected; linear dynamic concentration ranges of four orders of magnitude have been obtained wlth the procedure employed.
A previous publication from this laboratory (1) has described the application of graphite rod electrothermal vaporization as a technique for sample introduction into a high-frequency, inductively coupled argon plasma source (ICP) for optical emission spectroscopy. In this technique a resistively heated graphite rod was employed to desolvate aqueous samples and subsequently to vaporize the analyte into the ICP source. This was facilitated by enclosing the graphite rod in a glass manifold connected to the plasma injector tube via a short length (0.5 m) of plastic tubing. The argon sheathing gas for the graphite rod was used as the injector supply for the plasma, and therefore vaporized species were carried to the plasma for analysis on a stream of argon. Using this apparatus the authors were able to determine 16 elements a t the subnanogram level with adequate precision and excellent powers of detection in small sample volumes (10 pL). None of the elements determined in this early work, however, was of the type which forms refractory compounds
on, or with, the graphite rod during the analytical heating cycle. Difficulty is often encountered in trying to remove completely such refractory compounds from the graphite rod with consequent poor precision and the appearance of memory effects in subsequent determinations. This type of behavior has been noted by several authors who have employed electrothermal atomization in atomic absorption spectrometry (AAS). Thus the formation of refractory compounds such as carbides has made difficult the sensitive, routine determination of such elements as zirconium, boron, molybdenum, tungsten, and, to some degree, chromium, although well-designed atomization devices which are capable of reaching high atomization temperatures (ca. 2800 "C) have enabled the satisfactory determination of this latter element. Several authors have described attempts made to minimize the possibility of carbide formation with the carbon tube or rod electrothermal atomizer. Thus Renshaw ( 2 ) introduced samples into a graphite tube in a tantalum boat to avoid contact between the sample and the tube material and was able to report increased sensitivity for the determination of barium. Dagnall (3) reported that by using tubes saturated with tungsten the AAS sensitivity obtained for molybdenum was 70% higher and better precision was obtainable. In this paper we report an in situ method for preventing carbide formation by preferential formation of volatile halides of the elements of interest. A similar approach has been reported by Ediger ( 4 ) who added ammonium to sea water in order to volatilize sodium chloride, a source of major spectral background absorption in AAS, from an electrothermal atomizer tube as ammonium chloride and sodium nitrate at relatively low temperatures (ca. 500 "C). Similarly, Alder and da Cuhna (5)employed ammonium fluoride solution to volatilize uranium from a graphite rod device used for sample introduction into a low-power microwave induced plasma source. The use of halocarbons to promote the for-
0003-2700/79/0351-1938$01,00/0 C 1979 American Chemical Society
