MEETINGS
WORKSHOP ON MASS SPECTROMETRIC ANALYSIS OF SOLIDS at the National Bureau of Standards in Gaithersburg, Md.,
Nov. 18 and 19,1968 A. J . Ahearn and P. J . Paulsen, National Bureau of Standards, Washington, D. C. 20234
WORKSHOP on Mass Spectrometric
A Analysis of Solids sponsored by the
National Bureau of Standards was held at the Bureau's Gaithersburg, Md., lab oratories on November 18 and 19, 1968. Ninety-five people from 62 institu tions (12 U.S. government laboratories, 6 universities, 2 research institutes, 7 instrument makers, 27 industrial lab oratories, and 8 institutions from coun tries other than the TJ.S.A.) participated in the discussions of problems. The purpose of this workshop was to promote discussions of basic problems and the exchange of ideas, experiences, techniques, etc., bearing on the factors that influence the accuracy and pre cision of analysis of solids by mass spectrometric techniques. Most of the registrants use electrical discharge type ion sources and many of the problems were thus oriented. Nevertheless, the capabilities of ion probe mass analyzing techniques were also explored. The agenda consisted of five topics selected by questionnaire. Discussion leaders were used. Short informal so licited talks were given to initiate the discussion. Five committees of four people each met during the first evening or at the end of the workshop to sum marize the discussion of the five topics. That which follows is an abstract of these summaries. The first topic led by R. K. Skogerboe (Cornell University) dealt with the
accuracy and precision problem. I n spark source mass spectrometry there are two main error categories: those associated with the measurement pro cess and those attributable to the va porization ionization process. The role of statistics in data interpretation was discussed at the basic level. A number of useful suggestions were made, one of which involved the pooling of data on a few replicates from each of a number of different samples to estimate the overall precision of the analysis pro cedure. An in-depth comparative analysis be tween a limited number of laboratories on NBS platinum SRM 680 and 681 and on boron-doped silicon is being made to obtain a fundamental under standing of any observed differences. Electrical detection may eliminate some of the problems of the photoplate in that it offers the opportunity to ob tain higher speed, precision, and accu racy. Tests indicate that good preci sion results when the ratio of the mass resolved ion current to the unresolved (monitor) current is measured. A sali ent characteristic of the electrical detection technique is the ability to obtain immediate feedback on the effects of experimental changes. Negative ion spectra, adequate for analysis, are obtained with some ele ments at a lower sample consumption than needed for positive ion spectra.
Examples are the halogens and, to a lesser extent, the group VI elements. The second topic led by W. M. Hickam (Westinghouse Research Lab oratories) explored the capabilities of ion probe techniques particularly for surface analysis. Mass analysis of the sample sputtered as ions is employed to characterize the sample. Two advan tages—increased secondary beam inten sity and stability—of chemically active primary ion beams vs. frequently used rare gas ion beams were discussed. The degree of control of the bombarded area varied from several square millimeters to a spot about one μ,πι in diameter. Penetration rates from a few to sev eral thousand angstroms per hour have been achieved. Instrumental capabilities include: (1) mass analysis of both positive and negative secon dary ion beams but not concurrently, (2) scanning with spatial resolution as small as a μ,ηι, (3) determinations of concentrations to as low as a fractional part per million, (4) detection of monolayer surface coverage, (5) com positional displays by means of elec trical, photographic, or visual read out. Among the applications presented were composition and size of segregates and studies of isotopic, elemental, and compound gradients of both bulk and thin film samples. The third topic led by E. R. Blosser (Battelle Memorial Institute) con-
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Meetings sidered a variety of sample problems. With geological samples, steps must always be taken to ensure that the test sample truly represents the specimen collected in the field. Homogenizing techniques involving sample fusion were evaluated. In the analysis of an ion sample from its recorded mass spectrum, frequently only the number of singly charged ions of trace and matrix are measured. The ratio FT of singly charged to total number of trace ions may not equal FM the corresponding ratio for the matrix. Consequently, the ratio FT/FM must be included in the determination of the composition of the ion sample from its mass spectrum. As an alternative to polyethylene or rigid steel dies, the use of a naphthalene mold in an isostatic press for the preparation of pelletized powder samples was described. After pressing, the naphthalene is removed by sublimation, thereby minimizing interferences. Special techniques are required for microgram samples or highly radioactive powder samples. The sample may be pressed onto the end of a high purity gold wire, or as a solution or a liquid sludge, it may be deposited on a wire after which the solvent is evaporated. The fourth topic led by F. D. Leipziger (Kennecott Copper Co.) was concerned with electrical discharge type ion sources. A repetitive time-gated low voltage discharge type ion source was described. Among the advantages presented are stable ion current and consumption of cathode material only, except during the initial part of the pulse which is excluded in the time gating. With it, analysis precision is stated to be better than 5%. This ion source cannot be used with materials that are poor electrical conductors. Electrical detection was employed in a study of the influence of source parameters on the composition of the mass resolved ion beams using a radio frequency spark source. Tests indicate that the source parameters, in descending order of their importance, are accelerating voltage, spark displacement off instrument axis, spark gap, and duty cycle. With mass spectra recorded on ion sensitive plates, a correlation of ion transmission and mass line profiles was noted. Deviations from symmetrical profiles were most noticeable at low mass. Consequently, an adequate measure of the integrated ion intensity profile of the mass line is not obtained from the product of the peak intensity and half width. A questionnaire was distributed in an attempt to pool and correlate Relative Sensitivity Factors for different ma80 A
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trices, instruments, operating condi tions, and data handling techniques. This survey is sponsored by a Task Force of Subcommittee VII in the ASTM E-14 Conference on Mass Spec trometry. The fifth topic led by J. R. Woolston (RCA Laboratories) was a panel-type discussion of ion-sensitive plate prob lems. The effect of grain statistics on the precision of mass spectral photog raphy was reviewed. This led to the conclusion that for Q2 plates the opti mum precision is obtained in the range 30 to 70% optical absorption, and that for rectangular mass line profiles, the precision improves with increasing line width. "Trumpets" and "halo effects," the highly detrimental emulsion darkenings in the vicinity of strong mass lines, were discussed. In this context, the relative merits of developing the sur face image versus the internal image were presented. The reduction of this detrimental blackening by an electric ally conducting film above or below the emulsion was also described. Tests made on commercial ion-sensi tive plates and commercial developers tentatively indicated that the optimum detection sensitivity is obtained with Q2 plates presoaked and developed in ID 13 at 24 °C for three minutes. The properties and performance of gelatine-free ion-sensitive plates were described. These consist of a polycrystalline layer of contiguous domains 0.2 μτη thick. At low mass, they exhibit lower sensitivity, much shorter dynamic range and lower background than emul sions commonly used. Above m/e = 400, the gelatine-free plates have greater sensitivity and higher resolving power, indicating their usefulness for molecular spectroscopy. The last of the four half-day sessions was devoted to clinics by instrument makers for their own clients, general tours of NBS, and personal visits to individual laboratories. The open discussion of common prob lems and the free exchange of ideas that characterized this workshop was achieved in part by two conventions, first used at the corresponding 1967 NBS workshop—no photographing of slides would be done, and no one would be quoted directly. Hence, the anonym ity that permeates this account. A half-day workshop will be held during the "ASTM Committee E-14 Conference on Mass Spectrometry and Allied Topics meeting in Dallas on May 18-23, 1969. This will be similar in character to that held during the 1968 E-14 conference and again will be spon sored and organized by Subcommittee VII. VOL. 4 1 , NO. 2, FEBRUARY 1969
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