LITERATURE CITED (1) A. Karmen. MethodEnzymology, 14, 465 (1969). (2) D. C. Hobbs, American Society for Mass Spectrometry, Annual Meeting. San Francisco, Calif., 1970, Abstract 657. (3) C. P. Nulton, J. D. Naworal, I . M. Campbell, and E. W. Grotzinger. Anal. Biochem., 7 5 , 219 (1976). (4) W . H. Braun, E. 0. Madrid. and R . J, Karbowski, Anal. Chem.. 48, 2284 (1976). (5) W. C. Breckenridge and A. Kuksis, Lipids, 5 , 342-352 (1970). (6) D. C. Hobbs. Antimicrob. Ag. Chemother.. 2, 272 (1972). (7) M. Matucha, V. Svoboda, and E. Smolkova, J . Chromatogr., 9 1 , 497 (1974) ( 8 ) J . D. Mahon, K . Egle, and H. Fock, Can. J . Biochem., 5 3 , 609 (1975). (9) J. R. Scaife and G. A. Garton, Biochem SOC.Trans., 3 , 1011 (1975). (10) J . R . B. Slayback, I. M. Campbell, and E. Farish. Anal. Biochem.. 69, 140 (1975). (1 1) A . Kuksis, N. Kovacevic, D. Lau, and M. Vranic, Fed. Proc., 3 4 , 2238 (1975). (12) K. K. Stanley and P. K. Tubbs, Biochem. J . , 150, 77 (1975).
(13) H J G M Derks F A J Muskiet. and N M Drayer. Anal E m h e m , 7_2_ ,391 11976) \ . (14) J. A. Lubkowtz and J. Galobardes, J . Environ. Sci. Heahh, B l l . 49 (1976). (15) H. T h Schneider, B. P. Disboa, and H. Breuer. fresenius' Z. Anal. Chem., 279, 161 (1976). (16) A. Hatanaka, T. Kajiwara, and J. Sekiya, Phytochemistry, 15, 1125 (1976). (17: J. R. B. Slayback, I . M. Campbell, and M. H. Vaughan, Biochim. Biophys. Acta. 431, 217 (1976). (18) J. R. B. Slayback and I. M. Campbell, Biochim Biophys Acta. 450, 33 (1976) (19) C . P. Nulton and I . M. Campbell, Can J , Microbiol., 2 3 , 20 (1977). (20) R D. B. Fraser and E. Suzuki, Anal. Chem., 4 1 , 37 (1969). (21) V. J. Law and R . V. Bailey, Chem. Eng. Sci., 18, 189 (1963).
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RECEILFI) for review February 2 , 1977. Accepted July 11, 1977. T h e authors acknowledge gratefully the financial support of the U.S. Public Health Service (RR-00273) and the University of Pittsburgh Medical Alumni Association (fellowship to S.D.).
Determination of Parts per Billion Levels of Electrodeposited Metals by Energy Dispersive X-ray Fluorescence Spectrometry John A. Boslett, Jr., Robert L. R. Towns,* Robert G. Megargle, and Karl H. Pearson Department of Chemistry, Cleveland State University, Cleveland, Ohio 44 1 15
Thomas C. Furnas, Jr. Molecular Data Corporation, 2869 Scarborough Road, Cleveland Heights, Ohio 44 1 18
Aqueous solutions of nickel(II), copper(II), and zinc(I1) are quantitatively determined to the low part per billion level by energy dispersive x-ray fluorescence (XRF) spectrometry using selective potentiostatic electrodeposition as a preconcentration technique. Novel, cylindrical monochromators between the sample and detector of the XRF system reduce background levels due to scattering from the reflective electrode surface, and yield greatly improved signal-to-noise ratios. Linear callbration curves were obtained. Minimum detection limits are less than 20 ng for the metals studied.
X-ray fluorescence analysis of aqueous solutions is hampered by difficulties encountered in preparing suitable samples. In addition, elements of interest are frequently found a t levels which are below the minimum detection limits of conventional x-ray fluorescence spectrometry ( X R F ) . As a result, several investigators have employed enrichment techniques to extend the range of x-ray fluorescence analysis of ions present in solution. Methods of preconcentrating metal ions have included the use of ion-exchange resins ( I , 2 ) , ion-exchange resin impregnated paper ( 3 ) , and chelating functional groups immobilized on suitable substrates ( 4 ) . Elder, Perry, and Brady ( 5 ) have used ammonium-1-pyrrolidine dithiocarbamate as a precipitating agent to remove trace elements from environmental water samples. T h e precipitate was removed by filtering for subsequent energy dispersive X R F analysis. Vassos et al. (6) have used constant current electrodeposition of reducible metal ions upon a pyrolytic graphite rod to prepare samples for wavelength dispersive x-ray fluorescence analysis. Each of the methods has inherent problems, but, with certain limitations and with strict control over experi1734
ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
mental conditions, preconcentration techniques have proved useful in analyzing solutions with metal concentrations in the part per million (ppm) range. This research describes a technique by which trace amounts of the aqueous metal ions nickel(II), copper(II), and zinc(I1) are preconcentrated on the end face of a n ordinary spectrographic graphite rod by potentiostatic electrodeposition. T h e thin metal film t h a t results from the electrodeposition is analyzed by energy dispersive x-ray fluorescence spectrometry. Controlled potential electrodeposition has t h e capability to selectively separate trace concentration metal ions from a solution t h a t may contain interfering metal ions. Background due to scattering of the incident radiation from the reflective graphite substrate is minimized by the use of specially designed cylindrical monochromators designed, built and supplied by Molecular Data Corporation, 2869 Scarborough Road, Cleveland Heights, Ohio 44118. T h e estimated 80- to 100-fold reduction in background radiation attributable to use of these monochromators significantly improves the signal-to-noise ratio a t all energies and dramatically lowers the minimum detection limit ( 7 ) . T h e quantitative analysis of nickel(II), copper(II), and zinc(I1) a t the 2-100 part per billion (ppb) level from 120 mL of solution is reported.
EXPERIMENTAL Solutions. Stock solutions of zinc(I1) acetate. copper(I1) perchlorate, and nickel(I1) chloride were prepared by dissolving reagent grade salts in distilled-deionized water. The stock solutions were standardized against dried primary standard disodium dihydrogen ethylenediaminetetraacetate dihydrate (Na2H2EDTA.2H20) and determined to be 0.0996 F Zn(C,H,O&, 0.1039 F Cu(ClO,), and 0.0991 F NiC12. Solutions containing trace levels of the metal ions were prepared by diluting microliter amounts of the stock solution in distilled-deionized water. Supporting electrolyte was prepared in concentrated form by dissolving reagent grade sodium acetate in distilled-deionized
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Figure 1. Schematic representation of the electrodeposition cell and potentiostatic controller
water and adjusting to p H 6.0 with glacial acetic acid. The total analytical concentration of acetate was 2.60 F. This solution was cleaned of metal contaminants by electrolysis for 18 h over a cathodic mercury pool at -1.700 V vs. saturated calomel electrode (SCE). Apparatus. The electrodeposition cell and potentiostatic controller are shown schematically in Figure 1. This apparatus was fabricated from basic components and is an updated version of other designs (8,9). It includes a section to scan cell potentials that consists of amplifier A l , operated as an integrator of a constant voltage, and various controls to select the scan rate and determine the operating mode. A fraction of the scan output can be used for the x-axis of a recorder for voltammetric experiments and for charging-current compensation in the cell current measuring circuit. Switch S1 allows the entire scan circuit to be disconnected; the instrument was used in the nonscan mode for this study. Amplifier A2 adds the scan, if used, to a constant offset and delivers a control signal to amplifier A3 that regulates the cell. The transistor network within the feedback loop is a current booster with a current capability of about 30-40 mA. A voltage is applied to the auxiliary electrode (AUX) by A3 to keep the potential of the reference electrode (REF) equal to the control signal. Since the working electrode (WE) is held a t ground or virtual ground, the effect is to maintain the potential difference between the WE and R E F at the negative of the control signal. The principal cell is a Nalgene low form, polypropylene beaker whose curved rim with pour spout was removed to accommodate a tight fitting Teflon cap. The cap was bored to accept and support an auxiliary electrode cell, a saturated calomel electrode, a Teflon stirring rod, a fritted glass gas purge tube and vent, and a working electrode. Working electrodes were prepared from ultrapure spectrographic graphite rods, diameter 0.615 cm, supplied by the Carbon Products Division, Union Carbide Corporation. Prior to use, each lot of graphite electrodes was
checked visually for defects and by x-ray fluorescence for contamination. The lateral surface of each rod was spray coated with acrylic lacquer (Borden 13021, and one end was bored to receive a standard banana pin. The other end was polished on a carborundum disk rotating at approximately 2500 rpm to prepare the working electrode surface. Stirring in the cell is accomplished by a Teflon propeller and shaft coupled to a synchronous motor operating a t 1600 rpm. The x-ray fluorescence instrumentation used for analysis is unique in that it incorporates cylindrical monochromators in both the incident and analyzing beams to reduce background levels and improve signal-to-noise ratios. The XRF instrument is shown schematically in Figure 2. The x-ray generator and high-voltage power supply is a Picker Ultra-stable Model No. 6238 and uses a Dunlee molybdenum target x-ray diffraction tube for excitation of the sample. A cylindrical monochromator was constructed by Molecular Data Corporation, Cleveland, Ohio, using curved, high reflectivity-low resolution?compression-annealed LCAR Class A graphite crystals (Union Carbide Corporation) inlaid around the inside of a cylinder. Curved crystals are used in the incident beam to increase reflectivity of the monochromator and to provide a more highly resolved beam, i.e. decreased levels of white radiation. It is positioned in the incident beam between the tube and the sample. A beam-stop is placed in the center of the cylinder to prevent passage of the direct beam. The sample is positioned at the focal point of the hollow cone of diffracted K n and KB radiation and is oriented a t an angle 45' to the axis of the cone. The analyzing beam of fluoresced radiation from the sample is collected at angles of 4 5 O to the sample and 90' to the axis of the incident beam. A second cylindrical monochromator using flat, compression-annealed graphite crystals of the type described earlier is placed in the analyzing beam. The focusing nature of the cylindrical monochromators permits selection of a bandpass of energies to the detector, thus preventing the scattering incident ANALYTICAL CHEMISTRY, VOL. 49, NO. 1 2 , OCTOBER 1977
1735
Figure 3. Plot of Cu K a net integrated intensity vs. electrodeposition time for 120-mL solutions containing 13.2 Fg Cu" (110 ppb)
Figure 2. Schematic representation of the x-ray fluorescence analysis system
radiation from reaching the detector and contributing both to the total spectrum and to the background at energies lower than the incident radiation. A particular bandpass of energies may be selected by positioning both the analyzing monochromator and the detector at the proper focal length to satisfy the Bragg relationship for the monochromator crystals. The bandpass of the analyzing monochromator when used with our particular detector is approximately 2 keV, and the accessible energy range for this first prototype configuration is from about 5 keV (Cr Koc) to 11 keV (GA Koc). The focusing nature of the cylindrical monochromator leads to a translational rather than angular geometry for positioning of both the detector and monochromator. The translational geometry of this monochromator/detector system is considerably simpler and less sensitive to position settings than angular geometries are t o angular settings. Furthermore, the cylindrical nature of the monochromator permits focusing of a considerably larger solid angle of radiation than do other crystal monochromators, yielding greater flux at the sample and a t the detector. The detection and data acquisition portion of the system consists of an ORTEC Si(Li) detector (resolution 180 eV FWHM at Mn Kcu), an ORTEC 459A amplifier, and a Nuclear Data 4410 computer-based x-ray data system. Procedure. A solution of distilled-deionized water and sufficient amounts of stock supporting electrolyte to make the solution 0.10 F in total acetate a t pH 6.0 was prepared and degassed by purging with nitrogen for 10 min. The appropriate microliter amount of previously standarized stock metal solution was added. The final volume in all cases was 120.0 mL. Electrodeposition proceeded at -1.300 V vs. SCE, with vigorous stirring. All electrodepositions were run at ambient temperature. On completion of the deposition, the graphite rod was removed from the cell, with the voltage applied, and rinsed quickly in distilled-deionized water. After air drying, the working surface was sprayed lightly with the acrylic lacquer to fix the deposit, and a 1 / 4 - to 1/2-inch length of the carbon rod was cut off for analysis. Counting times were generally 400 s (live time), except for the analysis of solutions of very low concentrations (
