Detection Limits and Accuracy of the Proton ... - Science Direct

Micron Vol. 28, No. 3, pp. 231 239, 1997 i'; 1997ElsevierScienceLtd All rights reserved.Printedin Great Britain 09684328/97 $17.0(1+0.0(t

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Detection Limits and Accuracy of the Electron and Proton Microprobe D. R. COUSENS, *+ R. RASCH* and C. G. R Y A N t *Centre for Microscopy and Microanalysis, The University of Queensland, St Lucia 4072, Australia t C S I R O Division of Exploration and Mining, P.O. Box 136, North Ryde 2113, Australia (Received 27 February 1996; accepted 5 December 1996)

Abstract--The accuracy of and detection limits for trace element analysis using the electron and proton microprobes have been compared using glass samples prepared from whole rock standard powders BE-N basalt, AC-E granite and W-2 diabase. The capabilities and advantages and disadvantages of both techniques for trace element analysis have been examined. The effect of varying the kV on the detection limits in electron microprobe analysis is examinedfor a number of standard referencesamples for both high and low energy lines. © 1997 ElsevierScience Ltd Key words: electron microprobe, proton microprobe, detection limits accuracy, USGS whole rock standards.

INTRODUCTION A variety of microbeam analytical methods has been developed for the trace element analysis of materials in recent times. Many of these have overlapping and complementary capabilities in terms of analytical sensitivity, element range, and information content. The range of application and the relative advantages and disadvantages of these methods are rarely addressed in the literature. This paper considers two charged particle beam techniques for X-ray microanalysis, electron and proton, which differ primarily in the charge and mass of the ionising particles. Trace element sensitivities and accuracies have been examined for glasses prepared from powdered whole rock samples of the USGS standard materials W-2 diabase, the AC-E granite and the BE-N basalt (Gladney and Roelandts, 1988; Govindaraju, 1989; Govindaraju and Roelandts, 1993) and a number of silicate standards.

PIXE MICROANALYSIS

The CSIRO proton microprobe is based on a Russian-quadruplet electrostatic quadrupole lens system (Sic and Ryan, 1986), focusing a 3 MeV proton beam from a Tandetron tandem electrostatic accelerator. The typical size of the beam spot on the sample for this work is ~ 20-30/~m, with X-rays excited at depths extending to beyond ~30¢tm. Beam currents of 10-20nA are typical, and counts are accumulated on grains to a uniform live charge of 3.0 ktC in an automated sequence controlled by computer and using stepper motor sample positioning. X-rays are detected using a 30 mm 2 Si(Li) spectrometer. A 200¢tm A1 absorber is normally interposed between the target and detector to attenuate the intense X-rays excited from major elements such as Fe, Si, A1, Mg, Ca and K. Data are reduced using the GeoPIXE standardless quantitative method in parallel with data collection, triggered automatically by the data acquisition computer at the attainment of the preset charge. The method described in detail elsewhere (Ryan et al., SAMPLES AND PREPARATION 1990a, b, 1995) has been successfully applied to the analysis of 100,000 PIXE spectra. The spectra are deThe BE-N, AC-E and W-2 samples were prepared at composed into their elemental components using an the ion probe laboratory of the C.N.R.S. Centre for adaptive least-squares fitting procedure and converted Crystal Chemistry and Crystallography in Pavia by to elemental concentrations using calculated PIXE fusing the powdered rock standards on an Ir strip heater X-ray yields (Cousens et al., 1987). The continuum and quenching to a glass (Bottazzi et al., 1991). Stanbackground is approximated using the SNIP algorithm dard samples for electron microprobe analysis were optimised for the reliable determination of low statistics mounted in epoxy resin, polished to l/4/tm diamond, trace element peaks (Ryan et al., 1988) and corrected and coated with ~ 20 nm of carbon prior to both proton for absorption (Ryan et al., 1990b). The PIXE yield induced X-ray emission (PIXE) and electron microprobe calculation uses the major element mineral composition (EMP) analyses. obtained by EMP analysis, includes absorption and secondary-fluorescence contributions, and can treat :~Corresponding author. layered structures. The result is a standardless 231

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Table 1. Trace element analyses by electron microprobe--the estimated precision of the analyses is shown in parentheses* Point V Cr Mn Ni Sr Y Zr Ba Cu La Zn Ce Nd Yh

BE-N basalt glass (ppm)

AC-E granite glass (ppm)

W-2 diabase glass (ppm)

MDL (ppm)

256 (1.9%) 345 (0.5%) 1568 (0.2%) 283 (0.7%) 0 0 0 1099 (0.4%) 24 (13%) 0 139 (1.8%) 382 (1.2%) 0 55 (50%)

0 0 386 (0.7%) 13 (14%) 0 0 0 120 (3.1%) 0 0 211 (1.1%) 175 (2.4%) 0 47 (53%)

272 (1.7%) 70 (2.3%) 1179 (0.3%) 64 (2.9%) 0 0 0 176 (2.2%) 22 (14%) 0 77 (3.0%) 73 (5.8%) 0 98 (27"/0)

6 2 3 2 --5 4 3 6 24

* 0 indicates