ner. A calibration curve was made by aspirating the diluted CDC serum standards into the nitrous oxide-acetylene flame in the conventional manner. The calcium 422.7-nm line was determined using the vidicon flame spectrometer with a slit width of 20 Fm. No filters were employed. The results of these determinations are listed in Table V as referee values for calcium. The average deviation between the simultaneous multielement vidicon results and the singleelement flame emission calcium determinations is 1.81%.A standard t-test for paired variance indicates that even for a confidence level of only lo%, no significant difference exists between these sets of data. Considering the good agreement obtained in the analysis of control serum shown in Table IV, and the good agreement obtained with the referee method, the vidicon results are considered to be more reliable than the calcium determinations made using the cresolphthalein complexone method.
CONCLUSIONS The success of the vidicon flame spectrometer for the simultaneous determination of sodium, potassium, and calcium in serum results from a combination of instrumental components and processes. They act in a complementary fashion, such that elimination of any one obviates the goals achieved. The nitrous oxide-acetylene flame is necessary to eliminate the phosphate interference on calcium and to provide the necessary excitation for the detection of the chosen potassium lines with the vidicon. The sample introduction system allows the direct injection of small samples into the flame. The vidicon detector allows simultaneous measurement of intensities over a range of wavelengths and also permits integration of the transient signals produced by the sample introduction system. The wide spectral response of the detector permits the detection of the long wavelength sodium lines. The analytical lines chosen permit the direct determination of sodium, potassium, and calcium at the levels encountered in serum without dilution, and the chosen window allows the monitoring of these lines in an apparent 40-nm window which avoids detector
saturation from flame bands and other intense analyte emission lines. The advantages of the vidicon flame spectrometer are speed, simplicity, ease of operation, accuracy, precision, and versatility. Compared with most flame systems used in clinical analyses, which generally determine only two elements simultaneously, the vidicon flame spectrometer permits the rapid simultaneous determination of the three most frequently clinically determined cation electrolytes on a single sample. At present, the system is operated in a manual mode. The system, however, is easily amenable to automation, and future efforts in this direction will substantially reduce the analysis time still further. The system is not only simple to operate, but possesses no mechanical moving parts, and should therefore be reasonably maintenance-free. Since the analysis is direct and requires no sample manipulation, potential sources of error are eliminated. Samples can be analyzed with an accuracy of 2.2% or better with a precision of 2% or better. The system is versatile and may be used for the determination of other cation electrolytes such as magnesium and lithium as well as other trace elements present in serum. The system may also be applied to the analysis of other body fluids, such as urine, by using appropriate standards.
ACKNOWLEDGMENT The authors would like to thank J. H. Boutwell and D. D. Bayse of the Center for Disease Control, Atlanta, Ga., for supplying the samples of analyzed bovine serum. Thanks are also extended to the National Bureau of Standards for supplying the results of the isotope dilution mass spectrometric analyses of the CDC samples for potassium. Finally, thanks are extended to Robert McGovern of the Tompkins County Hospital for providing the human serum samples and analyses. Received for review March 4, 1974. Accepted April 15, 1974. This work was supported by the National Institutes of Health, Grant No. GM-19905-02.
Hollow Cathode Ionization for the Mass Spectrometric Analysis of Conducting Solids B. N. Colby'
and C. A. Evans, Jr.
Materials Research Laboratory, University of Illinois, Urbana, 111. 6 180 1
A simple hollow cathode ion source for the mass spectrornetry of conducting solids is described. Using clean argon for the discharge support gas, broad elemental coverage was obtained with detection limits in the sub-ppma region possible. The presence of molecular gases in the discharge del Present address, E. I. du Pont de Nemours and Co., Inc., Instrument Products Division, 1500 S. Shamrock Ave., Monrovia, Calif. 91016.
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ANALYTICAL CHEMISTRY, VOL. 46,
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creased sensitivity by as much as three orders of magnitude. Ions representative of the hollow cathode sample appear primarily as singly charged, monoatornic species although some polyatornic ions were observed. Ionization of the elements is such that standardless analysis to within a factor of three is possible. Highest sensitivity is achieved using a high current, high voltage discharge, although this may result i some nonuniform ionization for volatile elements.
AUGUST 1974
Of the techniques used for the mass spectrometric analysis of solids, spark source mass spectrometry (SSMS) is considered by many to be the mainstay (1).Since the original works of Dempster (2, 3 ) , SSMS has been continually improved and its capabilities have been expanded ( 4 , 5 ) . Recently the use of electrical detection (6) in place of photographic detection has allowed precisions and accuracies on the order of a few percent, if samples are sufficiently homogeneous and sparking parameters are properly controlled (7). Throughout the development of SSMS, three features played a major part in its wide acceptance. First, it is nonspecific so essentially all elements may be analyzed; second, the majority of elements have ion yields within a factor of three ( X or +) regardless of the sample matrix; and third, the rf spark provides a sensitive analysis with detection limits approaching 1 ppb. Because of these three features, the many shortcomings of the rf spark source have been accepted. One of the major problems of the rf spark is the wide initial kinetic energy spread (approximately 1000 V) given to the ions. Large double-focusing mass spectrometers become essential and high accelerating voltages (20 kV) are necessary to gain both sensitivity and spectral resolution. Combined with the source power supply, this results in a considerable investment in instrumentation. Other shortcomings include localized sampling of the electrodes by the spark, general instability of the ion beam, and thermal effects for certain elements. Because of these shortcomings of SSMS, the investigation of alternate ion sources for the mass spectrometry of solids is in order. Several other techniques of solid sampling have been used in the past. The most successful has been secondary ion mass spectrometry (SIMS) or ion probe mass spectrometry (8-10). The elemental coverage and sensitivity of SIMS is similar to SSMS but the uniformity of excitation ranges over about lo4 for different elements, and there are strong matrix effects. The sampling of a material by the sputtering process, however, has shown itself to be dependable even on a microscale. Utilization of sputter sampling, which produces primarily neutral species (11, 121, coupled with a nonspecific form of ionization should provide an excellent ion source for the mass spectrometric analysis of solids provided the range of initial kinetic energies is small and the intensity is high. An ion source which might provide these things is found in glow discharge sources. Aston (13) described the use of glow discharges for the mass spectrometric determination of isotope ratios, and Bainbridge ( 1 4 ) used a glow discharge system in which zinc was sputtered from the cathode to correctly determine its isotope ratios. More recently Coburn and Kay (15-1 7) have described a planar diode sput(1) A. J. Ahearn, Ed., "Trace Analysis by Mass Spectrometry," Academic Press, New York, N.Y., 1972. (2) A. J. Dempster, Nature(London), 135, 542 (1935). (3) A. J. Dempster, Rev. Sci. Instrum., 7, 46 (1935). (4) C. A. Evans, Jr., and G. H. Morrison, Anal. Chem., 40, 869 (1968). (5) S. R. Taylor, Geochim. Cosmochim. Acta, 29, 1243 (1965). (6) C. A. Evans, Jr., R. J. Guidoboni, and F. D. Leipziger. Appl. Spectrosc., 24, 85 (1970). (7) G. H. Morrison and B. N. Colby, Anal. Chem., 44, 1263 (1972). (8)R. E. Honig. J . Appl. Phys., 27, 549 (1958). (9) Ya. M. Fogel', lnt. J. Mass Spectrom. /on. Phys., 9, 109 (1972). (10) C. A. Evans, Jr., Anal. Chem., 44 (13), 67A (1972). (11) R. G. Wilson and G. R. Brewer, "Ion Beams," Wiley-lnterscience, New York, N.Y., 1973, p 35. (12) /bid., p 71. (13) F. W. Aston, "Mass Spectra and Isotopes," Edward Arnold & Co., London, England, 1933. (14) K. T. Bainbridge, Phys. Rev., 39, 847 (1932). (15) J. W. Coburn, Rev. Sci. lnstrum., 41, 1219 (1970). (16) J. W. Coburn and E. Kay, Appl. Phys. Lett., 18, 435 (1971). (17) J. W. Coburn and E. Kay, Appl. Phys. Lett., 19, 350 (1971).
Beam center n g
II
I
A-ode
l l
L a v i t e irsulatoHollc&
Gas
cathode
IE +
Figure 1. Hollow cathode ion source
tering system where ionization takes place in a glow discharge and have analyzed several elements. in thin film samples. Harrison and Magee (18) have sampled the glow discharge in a hollow cathode mass spectrometrically and found peaks in the spectrum which arise from the cathode material with observed sensitivities to sub-ppm levels. Sampling of hollow cathode discharges has been done by several other workers (19, 20) primarily for the purpose of studying the gas phase chemistry and kinetics of the glow discharge rather than for the analysis of the cathode material. The purpose of this paper is to describe a hollooow cathod ionization source (HCIS) for the elemental analysis of solid conducting materials. Some of its operating parameters and both qualitative and quantitative aspects of the spectra are discussed.
EXPERIMENTAL The hollow cathode ion source used for this study is depicted in Figure 1. The cathode sample consists of a 0.25-in. diam rod with a 0.133-in. diam X 0.375-in. cavity to contain the discharge. The cathode is surrounded by a Lavite insulating shield which serves as a mount for the anode and also helps contain the argon gas used to support the discharge. Argon was chosen as the support gas for this study because of its high ionization potential and simple isotopic pattern. Argon from the inlet system passes between the cathode and insulating shield into the discharge region. Ions in the discharge are extracted through a 0.010-in. diam aperture in the anode center. They then pass through an Einzel lens which is followed by beam centering electrodes. The ion beam is focused on the source slit of an AEI MS-902 double focusing mass spectrometer (Associated Electrical Industries, Manchester, England). The only necessary modification to the mass spectrometer was the addition of a 4-in. i.d. X 6-in. extension to the existing source housing. Details of this modification may be obtained from the authors. Argon is taken from a standard gas cylinder a t a pressure of 1.5 1b/im2 gauge and passed through a tube containing titanium-zirconium alloy a t 900 "C (Figure 2). This removes any Hz,0 2 , N2, HzO, and other molecular gases from the argon. The clean gas then passes through an 18-in. section of %-in. 0.d. Teflon (du Pont) tube which serves as electrical insulation between the ion source and ground. The needle valve used to meter the gas into the source is floated a t the ion accelerating voltage and was adjusted with a 6in. long nylon knob. The source is mounted on a water-coo!ed vacuum flange which is sealed with an O-ring to a Teflon (du Pont) insert for electrical isolation. Water cooling is essential to prevent thermal decomposition of the O-ring seal when the discharge is operated for extended periods a t high power. (18) W. W. Harrison and C. W. Magee. 21st Annual Conference on Mass Spectrometry and Allied Topics, San Francisco, Calif., 1973. (19) R. 6 . Tombers and L. M. Chanin, J. Appl. Phys., 42, 5204 (1971). (20) F. Howorka, W. Lindinger, and M. Pahl, lnt. J. Mass Spectrom. ion Phys., 12, 67 (1973).
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