Electron Probe X-Ray Analyzer (EPXA) Quantitative Analysis Software

Elements/Components

Can analyze up to 55 elements as individual elements and/or compounds. Unanalyzed elements can be specified stoichiometrically bound with an analyzed element (e.g., oxides or carbonates). Elements can be analyzed in one or more compounds within the same analysis. One compound (or element) can be analyzed by difference. Any number of compounds (or elements) can be “fixed.” For example, solutions, binders and/or hydrated crystals can be analyzed this way.

General Bulk and Thin-Film Analysis

Any bulk or single-layer (unsupported) thin-film sample can be analyzed by either standardless or a calibration-with-standards ZAF approach.

Analysis Method

Complete ZAF analysis is possible, with or without standards, using an internal database of fundamental parameters (FP) such as absorption coefficients, fluorescence yields, transition probabilities, etc.  Analysis can be done without standards if the results can be normalized to 100%. When using standards, thickness can also be determined or the results do not have to be normalized.

When more than one excitation is used, at least one of the elements for each condition must have been calibrated. Calibration factors may be generated using any type of standard (e.g., pure element or analytical “type” standard). A single “type” standard may be used, or the calibration may be done with a different standard for each element, or any combination of standards may be used. If some elements are calibrated and some are not, the latter can use calibration coefficients derived from the former group.

The mass thickness of the sample can either be specified or calculated. If the latter, then the analysis cannot be standardless. Several units are possible for thickness measurement, and the density can be calculated theoretically or specified in the case of linear thickness calculations. Composition units may be ppm or wt%, with the additional output of atomic and mole percent.

Calibration Method

With the FP analysis method, one can choose a “standardless” approach. All of the parameters describing the X-ray tube spectra, filtering, attenuation in air, attenuation in Be windows and dead layers, attenuation and enhancement in the sample, etc. are computed from physical models based on the data the user has entered into the software. It is simple to use standardless analysis but the parameters are only approximate. This is due to approximations inherent in the physical models and in the data the user enters.

With the FP analysis method, one can also choose to calibrate its parameters using either a single standard or multiple standards. Calibration is strongly recommended and will lead to much more accurate analysis results. A single “type” standard may be used, i.e. one can use a single piece of material containing all of the elements which will later be analyzed. For example, one can use a single “standard reference material” of stainless steel and then obtain very accurate analyses of other steel alloys. One can also calibrate with a different standard for each element.

Several types of analysis cannot be standardless, i.e. calibration with standards (reference materials) is required. Least squares analysis cannot be standardless. If the mass thickness of the sample (i.e. the mg/cm2) is calculated, the analysis cannot be standardless.

Detectors

Various detectors (Si-PIN, SDD) and windows (Be, Si3N4) can be fully modeled. The software has provision for the user to input all the required parameters (e.g., thickness, are, dead layer, etc.) associated with these detectors and their windows.

Amptek supplies all the parameters for its XR-100 series of detectors.

Geometry

The complete system geometry can be specified including the sample incidence and take-off angles, the source-to-optic and/or source-to-sample distances, the sample-to-detector distance, as well as the environmental factors.

Figure 4. Geometry angle definitions.

Elements, Lines, and Interelement corrections

Includes full corrections for absorption and both thick and thin-film secondary fluorescence. All possible lines are considered for both excitation and fluorescence. The analysis can be performed for all elements from H through Fm, using K, L or M lines in the energy range from 0.1 keV up to 60 keV.

Spectrum Calibration

Using known peaks in the spectrum, the software calculates the effective gain (eV/channel) and offset (zero shift) for the spectrometer. These factors are applied to subsequent spectra prior to other spectrum processing. The calibration can be specified in the XRS-FP2 software or in the DPPMCA software. XRS-FP2 can automatically import the calibration from DPPMCA.

Background Removal and Blank Subtraction

The background removal module uses iterative filtering to distinguish peaks, leaving behind the smoothly varying spectral background. This background is then removed from the original spectrum, leaving the peaks.

The blank subtraction module is used to remove peaks due to environmental interference or contamination. These peaks are not due to material in the sample but in the spectrometer, for example Ar in the air or Al in the filters or Pb in a user’s shielding. This module subtracts a spectrum acquired from a “blank” reference material, i.e. one without the elements to be analyzed.

Figure 6. Processed spectrum and background.

Escape Peak and Sum Peak Removal

Removes, at the user’s option, both detector escape and sum (pile-up) peaks. The escape peak module uses an internal function to estimate the fraction of x-ray events (above the K edge) that will generate K x-rays that will escape from the front or backside of the detector.

Smoothing

A specified number of 1:2:1 Gaussian smooths can be applied to a spectrum.

Deconvolution: Intensity Extraction

This module operates on the processed spectrum to extract the net peak intensities for the selected elements. It includes several options. First, the peak areas are computed using one of three methods: (1) simple peak integration across a fixed Region-Of-Interest, (2) fitting Gaussians to the peaks, using a known database of line ratios and peak energies, etc., and (3) a reference deconvolution, which uses stored profiles for each element to fit the peaks. Second, the spectrum fitting can be done using either a linear or nonlinear approach. Both utilize a least-squares method. In linear fitting, the peak ratios, energies and widths are fixed. This method is usually very fast. In nonlinear fitting, these parameters are allowed to vary within certain constraints. This method is much more computationally intensive.

All required line energies and resolutions are calculated automatically from the specified analyte line. The Gaussian peak fitting can be done with a linear or nonlinear least-squares approach. The latter allows constrained changes in the peak positions, intra-series line ratios, and peak widths, from their nominal starting points.

In addition to calculating elemental intensities, the software automatically calculates the estimated uncertainty and background values which allows uncertainty and Minimum Detection Limit (MDL) calculations to be performed during the FP analysis.

Figure 9. Plot showing spectra from a stainless steel sample. The black trace is the raw spectrum, the blue trace is the processed spectrum, and the red curves show the result of a Gaussian deconvolution. The quantitative results are in Table 1.

Spectrum Acquisition

There are two methods of spectrum acquisition. The first is to acquire with the Amptek DPPMCA acquisition application which controls the MCA8000D, DP5 Digital Pulse Processor, X-123, or PX5 Digital Processor and Power Supply. Once acquired, the XRS-FP2 can import the file and use the DPPMCA display for spectrum processing. This is called “Expert Mode.” Once the system is set up and calibrated in “Expert Mode”, it can be used in “Routine Mode.” The user places the sample in the spectrometer, then in the screen below clicks “Analyze” and XRS-FP2 acquires the spectrum, saves it, analyzes it, and then saves the report. “Routine Mode” acquires the spectrum directly into the XRS-FP2 software which can then be automatically processed. A repeat measurement capability is provided.

Figure 10. Easy and Expert Modes.

Automatic Peak/Element ID

There are two options:

Using the Amptek DPPMCA application, the user can automatically mark peaks (ROIs) for analysis. If the appropriate element library is loaded into DPPMCA the software will associate the marked peaks with elements. The corresponding elements can then be automatically imported into the XRS-FP2 element table.

Using the XRS-FP2 interface, the software analyzes a spectrum and assigns the most-likely elements and lines to each identified peak, and assembles a complete list of likely elements in the spectrum.

MLSQ

FP calibrations using multiple type standards and various additional regression models used to refine the FP calibration coefficients.

Spectrum Display

In addition to DPPMCA, the XRS-FP2 software can display acquired or processed spectra. Up to 8 spectra can be compared. KLM markers are available for peak identification and various annotation tools are available for adding text and lines to the display. It is not necessary to use DPPMCA program if the XRS-FP2 software is setup to acquire spectra directly from the Amptek DPP hardward (e.g., DP5).

Figure 11. Spectrum display showing element markers.