Report
Workshop on Mass Spectrometric Analysis of Solids Summary Report by Committee VII—Study of Solids American Society for Mass Spectrometry The American Society for Mass Spectrometry sponsored a workshop on Mass Spectrometric Analysis of Solids on May 29,1975, at the Shamrock Hilton Hotel, Houston, Tex., as part of the 23rd Annual Conference on Mass Spectrometry and Allied Topics. Two topics were selected for particular attention at the workshop. Topic I. Multielement Isotope Dilution. Moderator: J. C. Franklin (Union Carbide, Oak Ridge, Tenn.) The topic centered about a panel discussion of the method, its utility, and the present status of this type of analysis. Each panel member gave a brief description of multielement isotope dilution as used in his individual laboratory to provide a starting point for the group discussion. The group first differentiated between the isotope dilution method involving equilibration of the sample solution with the spike isotope, and the process of adding solid enriched isotopes to powder samples as an isotopically enriched internal standard. The discussion concerned five major areas of interest: • Sample types. Multielement methods had been used by members of the group for analysis of pure acids, pure metals, biological materials, fuels (gasoline, oil, etc.), and nuclear materials. • A large part of the discussion was directed toward equilibration of the spike and sample, preconcentrations,
extractions, and other chemical preparation required for the method. In general, the method requires much more preparation time than the usual spark-source methods. • Concentration ranges and sensitivity of the method require careful designation of terms. A practical working consideration is perhaps that quantity on an electrode that can be detected and analyzed, estimated presently at about 1 0 - 1 0 gram of an element. With a few nanograms on the electrode, a precision of from 5 to 10% is feasible. • Most of the work being done is with photographic detection, although some laboratories are now working with electrical detection methods. The traditional photoplate problems of emulsion calibration, linear range, and background are still encountered. The isotope dilution technique removes the errors associated with the ion source, but the precision and accuracy are still limited by the photoplate at 3-5%. The limited experience with electrical detection by peak switching indicates somewhat better precision— perhaps 1%. Several laboratories at the workshop have plans to do work with electrical detection. • A broad group of general problems and requirements were discussed. The choice and availability of enriched stable isotopes is not a trivial decision. In general, the isotope of choice should be available in a highly enriched state; if possible, the spike should be of low abundance in the
natural element; and it is very convenient if the spike isotope has an odd mass so that the multiply charged ions may be used if needed. Enriched isotopes also lend themselves as internal standards for determination of reliable relative sensitivity factors. This information is available from every spiked sample and provides a growing list of sensitivity factors for survey analysis. In conclusion, the method of multielement isotope dilution retains the elemental survey capability of sparksource mass spectrometry, retains the high sensitivity of the spark, and adds precision and accuracy capabilities of 5-10% to the analysis. Topic II. Empirical Quantitation Procedures in Secondary Ion Mass Spectrometry. Moderator: J. A. McHugh (General Electric, KAPL, Schnectady, N.Y.) This portion of the workshop focused directly on empirical methods for quantitation of SIMS data rather than considering all quantitation schemes (empirical, semiempirical, and theoretical). The various subjects touched upon were standards, precision and accuracy, important instrumental parameters, matrix effects, and a comparison of relative elemental sensitivity factors from a number of SIMS instruments. Eight laboratories contributed relative sensitivity factor data for workshop discussion and data exchange.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975 ·
1059A
The relative elemental sensitivity factor for element "A" is defined as
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SA
hei'C ref
+
where C and i are, respectively, the atomic concentration and total ele ment ion current for element " A " and the reference element " r e f . The eight laboratories analyzed a variety of NBS Fe and Ni base alloys, glass fiber stan dards, minerals, and semiconductors. Sixty-four standards representing these different matrices were ana lyzed; these analyses produced ap proximately 1200 relative sensitivity factors for a wide range of elements. Most of the data contributed to this study were produced by Applied Re search Laboratories ion microprobes. Comparisons of SA values produced by similar instruments and from simi lar matrices showed a good degree of consistency in absolute value. Com parisons between different instrument types showed similar general trends as regards relative element ionization ef ficiency; however, the absolute value of SA exhibited a greater spread. In strument operating conditions (e.g., the primary beam parameters) are major factors affecting data compari sons. Other factors discussed were: the influence of the ambient atmosphere on sample behavior; the value of mon itoring total secondary ion current at a particular aperture in the secondary analyzer; and methods for controlling or eliminating mass bias in the secon dary ion detector. Sample inhomogeneities and sur face contamination caused the SA values for a number of elements and standards to show considerable varia tion. In general, the accuracy of em pirical methods is quite satisfactory for homogeneous materials where there exist reference standards. Most of the laboratories employ empirical methods for quantitation; theoretical methods are employed to a much less er extent. This session of the work shop gave the participating laborato ries the opportunity to share informa tion and to expose workshop at tendees to empirical quantitation methods. A final discussion suggested a round robin experiment among will ing laboratories using a single set of samples containing a suitable glass matrix standard and an alloy stan dard. Such a set will be prepared and distributed within the next 13 months. Committee VII W. W. Harrison, Chairman J. C. Franklin J. A. McHugh 23rd Annual Conference on Mass Spectrometry and Allied Topics, May 29, 1975, Houston, Tex.
