Analysis of Microsamples by Single-Exposure Spark Source Mass Spectrometry Philippe Chastagner Saoannah River Laboratory, E. I. du Pont de Nemours and Co., Aiken, S. C. 29801
Turbidity, solarization, and adjacency effects produced in llford 42 emulsion by ion exposure were found to be analogous to the same effects produced in conventional emulsions by light exposure. Image broadening caused by the turbidity effect was shown to be a useful measure of ion intensity and, therefore, of impurity concentration over a range >lO*:l. Peak width and transmittance areas were found to be practical measuresof image broadening. Asingle-exposure analysis technique based on the turbidity effect was developed and applied to liquid and solid microsamples and to surface films. This study also indicated that secondary ion fogging of photoplates does not contribute to the density or size of an image produced by primary ions, and suggests that the practice of subtracting background corrections for the fog is incorrect.
THESPARK SOURCE technique of mass spectrometry for the analysis of trace impurities in solids is now well established. As the result of a need for analyses of microsamples a t Savannah River Laboratory, a single-exposure analysis technique based on the turbidity effect in Ilford 4 2 emulsion was developed and is now used routinely. The single-exposure technique is especially useful for the analysis of surface films and highly radioactive materials, where sample size is limited. In contrast, in conventional spark source mass spectrometry where sample size is not a limiting factor, a series of graded exposures is made. Every element of interest in the sample is represented on the photoplate by one or more images having optical densities within the straight line portion of a conventional densitometric calibration curve. The conventional techniques have been described by Brown, Craig, and Elliot ( I ) and by Ahearn (2). Excellent reviews of the precise measurement and interpretation of conventional spectra have been published by Owens (3) and Schuy and Franzen ( 4 ) . A description of the characteristics of some ion sensitive emulsions has been given by McCrea (5). In this investigation, the characteristics of images produced in Ilford 4 2 emulsion by ion exposure were studied and the basic fundamentals and techniques of the single-exposure analysis method were established. The image effects studied are analogous to those occurring in conventional emulsions on exposure to light, and are: turbidity, which is the property of a n emulsion that causes the image width to increase with exposure; solarization, or image reversal, which is a function of exposure and is thought to result from bromine depletion; and adjacency effects-Le., the Eberhard effect and edge effects, which result from chemical changes produced in the (1) R. Brown, R. D. Craig, and R. M. Elliot, “Spark Source Mass
Spectrometry as an Analytical Technique,” in “Advances in Mass Spectrometry,” Vol. 2, R. M. Elliot, Ed., Macmillan, New York, 1963, pp 141-56. (2) A. J. Ahearn, “Mass Spectrometric Analysis of Solids,” Elsevier, New York, 1966. (3) E. B. Owens, in “Mass Spectrometric Analysis of Solids,” Chap. 111, A. J. Ahearn, Ed., Elsevier, New York, 1966. (4) K. D. Schuy and J. Franzen, 2.Anal. Chem., 225,260 (1967). ( 5 ) J. M. McCrea, Appl. Spectrosc., 21,305 (1967). 796
0
ANALYTICAL CHEMISTRY
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8
1
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Emulsion Fog Level
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Figure 1. Image width and transmittance area measurements developer by the development process. A complete discussion of these effects as produced by light in conventional ernulsions is given by Mees (6),and his nomenclature is used throughout this paper. No published work was found about the production of these effects by ion exposure of gelatinpoor emulsions such as Ilford 4 2 . The solarization and adjacency effects observed in the present work with ions were identical to those described by Mees and could be controlled adequately by vigorous agitation during development ; therefore, they are not discussed here. EXPERIMENTAL
Apparatus. Ilford 4 2 photoplates were exposed to ions in an A.E.I. MS7 double focusing mass spectrometer of Mattauch-Herzog geometry. The plates were processed in a nitrogen-burst developing apparatus, developed for 3 minutes in Kodak D-19, (Eastman Kodak Co.) and fixed for 1 minute in Kodak Rapid Fix. A Jarrell-Ash Model JA-203 recording microphotometer equipped with a Bristol strip chart recorder linear in transmittance was used for densitometric measurements. Procedure. Conventional series of graded exposures were made to obtain stepwise mass spectral data for the study of turbidity, solarization, and edge effects. Short exposures -
(6) C. E. K. Mees, “The Theory of the Photographic Process,” Revised ed., Chaps, 7 and 24, Macmillan, New York, 1954.
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Figure 2. Turbidity effect-6aCu image width -4
were made before long ones to assure that primary ion images would be formedin regions essentially free of secondary ion fog. Single-exposure spectra of microsamples were made by the technique reported previously (7-9), in which the samplebearing surface is swept past a fixed, wedge-shaped counter electrode that is carefully aligned with the ion optical axis of the source. The technique usually produces an exposure of about three nanocoulombs. The zero density reference for photometric measurements was the glass plate scraped clear of emulsion. Emulsion fog density was measured in an unexposed area of the emulsion at the high mass end of each plate where no secondary fogging occurred. Image widths and peak areas were measured on microphotometer tracings of image profiles extrapolated to the emulsion fog level without regard to secondary fogging or solarization. Image width is the distance in millimeters between the extrapolated sides of the image profile at the emulsion fog level. Transmittance area is the area in cm2 bounded by the extrapolated lines, the emulsion fog level, and the top of the peak. The measurements are illustrated in Figure 1 for both normal ( A ) and overexposed ( B ) images. These measurements are in contrast to the area measurements of Skogerboe, Harrington, and Morrison, (10) in which only the area above the adjacent fog was determined. To be consistent with the nomenclature of Mees, all measures of blackness are expressed as optical density, even though this requires nonlinear scales in illustrations of transmittance profiles. RESULTS AND DISCUSSION
Turbidity Effect. Image width, which is a function of the turbidity effect, was found to increase linearly with the logarithm of the ion exposure, just as image width increases on light sensitive plates (11, 12) with exposure to light. As shown in Figure 2, this relationship is linear over five decades of exposure. The transmittance area is also a linear function of the logarithm of the ion exposure over five decades of exposure as shown in Figure 3. The reproducibility of images in the usual exposure range for single-exposure spectra is given in Table I. Either the width or area of extrapolated image profiles can be used as a measure of ion intensity.
(7) P. Chastagner, Twelfth Annual Conf. on Mass Spectr. and Allied Topics, Montreal, 1964, Paper 55. (8) A. J. Ahearn,J. Appl. Phys., 32,1197 (1961). (9) Chem. Eng. News, 42, 79 (July 13, 1964). (10) R. K. Skogerboe, W. L. Harrington, and G. H. Morrison, ANAL.CHEM., 38,1408 (1966). (11) C. E. K. Mees, “The Theory of the Photographic Process,” Revised ed., Macmillan, New York, 1954, pp 1003-7. (12) R. Wildt, 2.fur wissenschaftliche Photographie, Photophysik andPhotochemie, 25,153 (1925).
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Figure 4. Photoplate fogging Role of Secondary Ion Fogging. Images formed in regions of the photoplate fogged by secondary ions appear to be superimposed on the fog as shown in Figure 4. When subtractive corrections are made for the fog as described by Owens (3), and Schuy and Franzen (4), the image sizes of the isotopes of an element are often disproportionate, such as the abnormally small 57Fe image illustrated. However, if the image profiles are extrapolated to the emulsion fog level, their widths and areas after a square root of the mass correction are proportional to the logarithms of the relative isotopic abundances and, therefore, to the logarithms of their relative exposures as shown in Figure 5. This relationship indicates that the secondary ion fog does not add to the density of the photographic image of a primary ion beam and suggests that the emulsion response may be different for primary and secondary ions. Current practice for background corrections assumes that the response is the same, and thus may be incorrect. These observations are consistent with those of VOL. 41, NO. 6, MAY 1969
797
Table I. Reproducibility of Copper Spectra
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6aCu
Exposures, nanocoulombs
No. of Exposures
1 10
13 13
Average image width, mm 36.0 57.0
Re1 std dev,
Average transmittance area, an2
2.3 8.9
60.0 90.5
z
Average image width, mm 30.4 43.6
Re1 std dev, 1.2 5.2
z
Average transmittance area, cm2 42.6 71.7
Re1 std dev, 1.5 7.4
z
Re1 std dev, 1.2
z
5.2
Pulse rate, 100 pps. Pulse length, 25 ~ s e c .
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