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(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, 31, 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, 33, 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
ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979
mation of volatile compounds has been postulated by Sambueva and Shipitsyn (6) to volatilize elements in a spark discharge. In this study, in which we have employed a graphite rod electrothermal atomization device for sample introduction into a n ICP, we have avoided the necessity of addition to the sample of matrix modification reagents by using halocarbon/argon gm mixtures in the sheathing gas of the graphite rod and injector gas for the plasma. T h e adoption of this procedure has successfully enabled significant improvement in the determination of several refractory elements by optical emission spectroscopy using this technique.
Table I. Experimental Facilities plasma power supply
spectrometer
optics
EXPERIMENTAL The graphite rod sample introduction device used was identical with that reported previously ( I ) . The graphite rod sheathing gas was taken directly to the plasma injector via 0.5 m of plastic tubing. In initial experiments carbon tetrachloride was introduced into the atomizer manifold simply by passing the argon gas supply stream over a constant surface area of carbon tetrachloride contained in a Dreschel bottle. Samples were applied to the graphite rod using a 10-gL micropipet. The rod was heated by applying a low voltage to the atomizer terminals to facilitate desolvation and the analyte was subsequently vaporized from the rod. Further experiments were carried out using a 0.1% trifluoromethane (Freon 23) in argon mixture (ROC Special Gases Ltd., England) introduced to the graphite rod atomizer manifold via a needle valve and rotameter. Instrumentation. The inductively coupled plasma source system used in this study was the Plasma-Therm Model HFP 2500D (Plasma-Therm, Kresson. N.J.). The generator was of the crystal-controlled type operating at 27.12 MHz with a variable output of 0-2.5 kW. The plasma was sustained using a demountable silica torch constructed of three concentric tubes and was operated exclusively on argon. Radiation emitted from the plasma was focused onto the entrance slit of a 1-m monochromator (Monospek 1000, Rank Hilger Ltd., Margate, Kent) as a 1:l image. The output of the photomultiplier (EM1 9789QB) was taken directly to a fast response potentiometric chart recorder for signal registration. Further details of the experimental system and operating conditions are shown in Table I. Procedures. After allowing the plasma to stabilize, the argon flow rates for the coolant/plasma supply and the injector supply were set a t their optimal values as detailed in Table I; the wavelength of interest was selected at the monochromator. Sample solutions (10 p L ) were deposited on a depression in the graphite rod and were desolvated and Subsequently vaporized into the plasma, carried on a stream of argon or on the argon/ halocarbon mixture. Emission intensities observed in the plasma at the wavelength of interest were recorded at the potentiometric chart recorder.
RESULTS AND DISCUSSION Attempts to determine boron, zirconium, molybdenum, and tungsten using an argon atmosphere, and the graphite rod as a vaporization device for sample introduction into the plasma, were unsuccessful. Even a t the highest vaporization temperatures (ca. 2600 "C) attainable with our system, it was not found possible t o remove memory effects on successive vaporization of aqueous standards of the elements of interest. T h e memory effect caused by the formation of residual carbides of these elements was found t o limit severely the usefulness of this method of sample introduction. Poor detection limits were observed and extremely poor precision was obtained (RSD values typically greater than 0.3). In a n attempt to prevent this type of behavior we have introduced halocarbons into the sheathing gas to form volatile halides of the elements of interest which are easily volatilized from the graphite rod. As the graphite rod heats up t o the vaporization temperature, the particular halocarbons employed decompose to carbon and active halogen radicals which halogenate the species of interest. This type of behavior of
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readout
plasma torch
graphite rod power supply
graphite rod gas flow rates
sample application standard solutions
Plasma Therm Inc., Model HFP 2500D ; operating frequency, 2 7 . 1 2 MHz; power output, 02.5 kW; load coil: 2lI2 turns copper Hilger Monospek 1 0 0 0 : Czerny-Turner scanning monochromator (1200 lines m m - ' grating blazed at 300 nm); reciprocal linear dispersion, 0.8 nm mm-' plasma imaged in 1:l ratio onto entrance slit ( 3 5 ,um) with two 7.5-cm focal length fused silica lenses signal from EM1 97 89QB photomultiplier tube displayed at Oxford 3000 series potentiomet,ric chart recorder demountable fused silica in a brass base; coolant tube, 21 mm 0.~1.;plasma tube, 17 mm 0.~1.;injection tube, 6 mm 0.~1.;injector tip, 1 . 5 mm i.d. Shandon Southern Instruments A33 7 0 electrothermal atomizer; drying or desolvation time, 45 s at 100 'C; atomization time, 1.5 s at temperatures up to 2600 "C rods 70 mm long and 3 mm dia. ; depression at center 8 mm long, 1.5 mm depth. argon or halocarbon/argon flow rate for injector, 1 . 0 L min-'; argon flow rate for coolant and plasma supply, 1 3 . 0 L min-l 10 p L Eppendorf micropipet with disposable polypropylene tips all stock solutions prepared at a concentration of 1000 ppm from analytical grade reagents; working solutions prepared daily from these solutions -
____
halocarbons is well documented and has been used for the synthesis of inorganic halides (7). For example, carbon tetrachloride is photochemically decomposed and also a t elevated temperatures (300-500 "C) readily transfers chloride ion t o various substrates, CCl, radicals often being formed simultaneously. On passing carbon tetrachloride vapor into the vaporization chamber, it was possible t o determine zirconium without evidence of carbide formation. Optimization of vaporization temperature, injector gas flow rate, forward power, and viewing height in the plasma was performed; under the optimum conditions established (temperature, 2600 "C; injector, 1 dm3 min-'; forward power, 1200 W; viewing height, 40 mm) a detection limit (DL) of 0.001 ppm was observed for a 10-pL sample (DL taken as that concentraticin producing an emission intensity at t h e analyte wavelength equal to twice the peak-to-peak noise level of the background emission). A rectilinear calibration curve was constructed over four orders of magnitude with respect to zirconium concentration and is shown in Figure 1. T h e CCl,/argon atmosphere was also employed to determine chromium and boron in aqueous standards. Again complete removal of the elements from the graphite rod was observed during vaporization and again it W I W possible to
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979 104,
I
0m-l
001 Zr
01
10
CONCENTRATlOh
Figure 1. Zirconium calibration graph obtained at 343.8 n m using CCl,/argon
OM
10
as the chamber atmosphere and injector gas
Table 11. Compromise Operating Conditions Employed for the Determination of Zr, B, Mo, Cr, W, and Ag experimental parameter
compromise condition
generator forward power reflected power viewing height above load coil plasma/coolant gas flow rate injector/chamber gas flow rate PMT E.H.T. entrance slit exit slit rod voltage
1.0 kW
