Quantitative Analysis

The main function of an electron microprobe is to perform quantitative analysis using wavelength dispersive spectrometry (WDS). This technique provides accurate, non-destructive, quantitative chemical analyses of areas a few microns in size in solid materials. Most solid materials can be analyzed if properly prepared. Samples ranging in width from a few 100s of microns to up to ~ 50 mm (2.5") can be accommodated in our microprobe.

High-quality WDS analysis requires a stable well maintained instrument as well as a suite of reliable, well characterized standards. The CAMECA SX50 in our lab has been maintained under a full service contract with the manufacturer throughout its 19 year lifetime and has proven to be exceptionally stable over that time. The lab maintains a diverse and ever growing collection of almost 300 natural and synthetic standards to provide for the analysis of a wide range of different materials.

The minimum detection limits (MDL) and analytical accuracy of a WDS analysis depends in large part on the electron beam conditions and counting times chosen for the analysis (although other factors can also be significant in certain situations). A typical geochemical analysis on our instrument for 11 elements under conditions of 15 keV accelerating voltage, 20 nA beam current, 2 μm beam size, and 30 second peak count time would require ~4 min per analysis and would yield MDLs of ~100 - 300 ppm (3σ) for all elements and a 1σ accuracy of 0.5 - 1.5% relative for major elements (those present at concentrations > 1 wt%). MDLs and 1σ errors could both be improved (if desired) by increasing counting times, beam current, or both. Light elements (Z < 9) are more difficult to analyze on the electron microprobe and analyses for these elements have larger MDLs and 1σ errors than do analyses for heavier elements made under the same conditions.

Our microprobe is often used for trace element analysis (elements present at concentrations < 0.1 wt%) using higher accelerating voltages, beam currents, and counting times. Accelerating voltages of up 25 keV and regulated beam currents up to 300 nA can be employed on suitable samples (unregulated beam currents of 300 - 800 nA are also possible) and can produce MDLs as low as 5 ppm in favorable cases.

Computer automated analysis allows users to set up points, lines, or a combination of both to be analyzed unattended at a later time (usually overnight). Results are given in one or more custom, tab-delimited reports which are compatible with any spreadsheet program.

Our CAMECA SX50 is equipped with four WDS spectrometers containing 12 diffracting crystals which allow analysis of all elements with Z ≥ 4 (beryllium). The spectrometer configuration of our SX50 is shown below.

Our CAMECA SX100 is equipped with five WDS spectrometers containing 14 diffracting crystals which allow analysis of all elements with Z ≥ 5 (boron). Three of the spectrometers are equiped with large LPET and LLIF crystals which provide more counts and better peak-background ratios than do normal sized crystals of the same type. The spectrometer configuration of our SX100 is as follows.