Software package for XRF analysis XRF-FP

A full-featured package for quantitative elemental XRF analysis.

This package is a software package for quantitative XRF analysis. Fundamental parameter (FP) XRF analysis converts peak intensity into the concentration of the corresponding element or film thickness. It processes the raw X-ray spectra measured with Amptek detectors and pulse processors to obtain (1) elemental peak intensity (the intensity of the peaks corresponding to each element) and (2) elemental concentration or film thickness.

Introduction and working principle

This package is a software package for quantitative XRF analysis. Fundamental Parameter (FP) XRF analysis converts elemental peak intensity into elemental concentration or film thickness. It processes raw X-ray spectra measured with Amptek detectors and pulse processors to obtain (1) elemental peak intensity (intensity of peaks corresponding to each element) and (2) elemental concentration or film thickness.

Fig. 2. The sketch illustrates the data flow into the XRF analyzer.

Once the system has been installed, calibrated and a spectrum has been obtained, the XRF analysis begins:

1. Decoding the detector response to restore the appearance of the photopeak. This step is applied to departure peaks, total peaks, background peaks, etc. The output of this step is a processed spectrum that accurately shows only the photopeaks.
2. Decoding of photopeaks to determine the intensity of interaction of X-ray radiation in the detector. The result of this stage is a table of intensity in each analyzed photopeak.
3. Calculation of attenuation and matrix effects to determine the concentrations of elements in the sample. The result of this stage is a concentration table, which is the final result of the analysis.

Spectrum processing corrects the spectrum for yield peaks, sum peaks, background peaks, Compton, backscatter and other effects. It also corrects for attenuation in the Beryllium window and detector insensitive layers, as well as detector efficiency. Peaks are approximated using theoretical models or by measuring the detector response and can be performed using either linear or nonlinear spectral sweeps. A variety of processing options allows the software to be tailored to a specific detector/spectrometer and application.

Quantitative analysis, the step in which the element concentration or film thickness is calculated from the intensity of the corresponding peak, can be performed either with or without the use of standards for calibration of the analytical parameters. In standard-free analysis, all parameters are specified based on theoretical equations, a fundamental parameter database and accurate modeling of the measurement geometry. Standard-free measurement is possible for simple structures or single-layer thin-film samples when the film thickness is unknown. In standard-based analysis, the fundamental parameters are derived from the measured response for each element. The most accurate analytical results are obtained when using standards with similarity to the material under measurement. The "scattering coefficient" measurement can be used to estimate the fraction of the sample that consists of low-Z materials that cannot be measured by XRF analysis.

The software includes a large number of variables that the user can adjust to suit experimental conditions and to optimize processing.

Fig. 3. The main program window. Shows a table element with various parameters and concentration.

Table 1. Results obtained using the XRF analysis software package for two steel alloys. The data were collected using an Amptek XR100-SDD detector, a PX5 pulse processor, an Amptek mini-X detector with X-ray tube, and an Amptek MP1 mounting plate.

Features of the software package

• Up to 40 elements can be analyzed simultaneously. Analysis can be performed for all elements from H to Fm, using K, L and M line analysis, in the energy range from 0.1 keV to 60 keV.
• Supports measurement of total composition, composition and thickness of one layer or composition of up to 4 layers.
• Includes full correction for losses due to attenuation in air, the detector window, the detector dead layers, the active volume of the detector, and the filter that is inserted between the tube and the sample or between the sample and the detector. They are calculated based on user input of parameters defining the excitation source, the detector, and the measurement geometry.
• Spectrum processing parameters include parameters for background detection, overlapping peaks, and summed peaks.
• Includes absorption corrections for both thick and thin films of secondary fluorescence, i.e. all matrix effects, enhancement and absorption. All possible lines are considered - for both excitation and fluorescence.
• The intensity of a photopeak can be modeled as a Gaussian function, by integrating the area under the peak, or by using the measured response of the photopeak.
• Linear or nonlinear unfolding. In nonlinear unfolding, the center of gravity and peak resolution are selected taking into account the optimal shape.
• Quantitative analysis can be implemented using fundamental parameters, fundamental parameters with scattering coefficient (for samples containing a large amount of low-Z materials), or a simple least squares method.
• Analysis using fundamental parameters can be based on the use of one standard source, several standard samples, or neither. Analysis without standard samples requires a clear definition of the parameters of the X-ray tube, detector, environment, and geometric parameters of the measurement.
• The basic calculations of the parameters are based on the Sherman equation.
• Tube spectra can be supplied by the user or calculated from built-in models (Ebel, Pella et al.). These tube spectra can be convolved with experimental transfer functions to obtain the expected spectrum of tubes passing through an optical such as a polycapillary bundle.
• Includes interactive "expert mode" and "normal mode". In expert mode, the user can see every step of the analysis and parameter setting. In normal mode, after the spectrometer is set up and calibrated, you only need to press one single button to obtain data, process the spectrum, analyze the intensity, and save the results in a report.

Example of definition

Elements / Components

Up to 40 elements can be analyzed as individual elements and/or compounds. Unanalyzed elements can be determined stoichiometrically based on the bond with the analyzed element (e.g. oxides and carbonates). Elements can be analyzed in one or more compounds within a single analysis. One compound (or element) can be analyzed in different ways. Any number of compounds (or elements) can be "fixed". For example, solutions, binders and/or hydrated crystals can be analyzed in this way.

General and thin film analysis

Any volume and single-layer (not supported) thin-film sample can be analyzed using both standard samples and the fundamental parameters method. Additional software is available for simultaneous processing of multilayer samples (up to 6 layers) and thin-film materials using the fundamental parameters method (for more detailed information, please contact Technoanalitpribor specialists).

Quantitative analysis

Method of analysis

The module includes a variation analysis. First, the fundamental parameters method can be selected for quantitative analysis. This module solves a set of nonlinear equations relating the intensity of an X-ray peak to the concentration of elements in a sample. These equations include corrections for attenuation and absorption in the sample, the occurrence of secondary X-rays in the sample, attenuation in windows and air, the scattering spectrum tube, etc. Second, the FP with a spread of indices can be selected. This option is recommended when the sample contains a significant amount of low-Z materials, such as plastic. The estimation is made for the unanalyzed part of the sample by comparing the known elements of the analyte with the measured C/R ratio and assigning an average atomic number to the remaining elements. Third, the simple least squares method can be selected. This is an empirical method that does not use all the information about the spectrum. Instead, it relies on simple calibration coefficients and assumes that the intensity of a particular line depends linearly on the concentration.

When more than one excitation is used, at least one of the elements for each condition must be calibrated. Calibration can be done using any standard sample (e.g. a pure element). One or more standard samples can be used for calibration. If some elements are calibrated and some are not, then for the latter, calibration factors obtained from the first group can be used.

The thickness of a sample can be determined or calculated. In the case of calculation, the analysis cannot be done without a standard sample. Several ways are possible to measure thickness and density. Density can be calculated theoretically or determined in the case of linear thickness calculations. The composition of samples can be up to millionths of a percent.

Calibration method

The fundamental parameters method allows calibration without standard samples. All parameters describing the X-ray tube spectra, filtration, attenuation in air, attenuation in the beryllium window and dead layers of the detector, attenuation and amplification in the sample, etc. are calculated on the basis of physical models built on the basis of the data entered by the user in the program. It is easier to use than analysis with standard samples, but its parameters are approximate. This is due to the approximations inherent in the physical model and the data entered by the user.

With the fundamental parameters method, one can also choose whether to calibrate its parameters using one or more standard samples. Calibration is highly recommended and will lead to much more accurate results. A single common standard sample can be used, i.e. one piece of material containing all the elements that will be analyzed later can be used. For example, a stainless steel "standard reference material" can be used and then very accurate analysis of other alloy steels can be observed. It is possible to calibrate using different standard samples for each element.

Several types of analysis cannot be done without reference standards, i.e. calibration against a reference standard is required. For example, least squares analysis cannot be done without a reference standard. If the density of the sample (i.e. mg/cm2) is calculated, the analysis cannot be done without a reference standard.

Sources of excitement

Either X-ray tubes or isotope sources can be used. For X-ray tubes the software package can model both X-ray reflection and transmission using either the Pell or Ebel models, or using a source-supplied spectrum for a full polychromatic source simulation. Anode, windows and filters can be specified. The tube window can be of any composition (e.g. BeO or glass). Any element can be selected for the anode, as well as the emission angle. Energy can vary from 3 to 60 kV. Support is provided including file transfer efficiency using, for example, polycapillary optics placed between source and sample. Radioactive isotopes can be used using a source file describing the relative line ratios. For secondary excitation of the target, monochromatic excitation is assumed.

Amptek supplies all parameters for its mini-X X-ray tube and filter kit. If a tube from another supplier is used, the customer must find its parameters independently.

Radioactive isotopes can be used by using a source file describing the relative line ratio. For secondary excitation of the target, monochromatic excitation is assumed.

Detectors

Various detectors (Si-PIN, SDD, CdTe, Si (Li) and Ge) and windows can be fully simulated. The program has provision for users to enter all necessary parameters (e.g. thickness, are, dead layer, etc.) associated with these detectors and their windows. The spectral processing of CdTe includes some significant changes in the processing procedure. For more information on CdTe detector analysis see this page.

Amptek supplies all the parameters of its XR100 series of detectors, including the XR100-SDD, XR100CR for SiPIN and XR100-CdTe. If a detector from another supplier is used, the customer must find the critical parameters.

Geometry

Complete system geometry can be specified including sample rate and take-off angles, source-to-optical and/or source-to-sample distances, sample-to-detector distances, and environmental factors. Amptek supplies all parameters in its MP1 mounting plate, for use with the mini-X and XR100/X-123.

Figure 4. Definitions of angle geometry.

Figure 5. When using the MP1 plate mounting, refer to this diagram and the table below. The distance from the edge of the sample plate should be 0.375 inches (1 cm) in order to be at the intersection point of the detector and the x-ray tube. All dimensions are in centimeters (cm).

Table 2. When using the MP1 mounting plate with the XRF-FP software, enter these values in the Geometry Setup dialog.

Elements, lines and inter-element corrections

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

Spectrum processing

Calibration spectrum

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. Calibration can be specified in the XRF-FP software or in the ADMCA software. XRF-FP can automatically import ADMCA calibrations.

Background Removal and Empty Subtraction

The background removal module uses iterative filtering to isolate peaks, leaving a smoohtly different spectral background. This background is removed from the original spectrum, leaving the peaks.

The empty subtraction module is used to remove peaks due to environmental interference or contamination. These peaks are not due to material in the sample but to the spectrometer, such as Ar in the air or in Al filters or Pb in the user's protection. This module subtracts the spectrum acquired from "white" reference material, i.e. one without elements to analyze.

Figure 6. Original background spectrum before removal.

Figure 7. Processed spectrum and background. Blue curve is remove background.

Escape Peak and Peak Sum Removal

Removes, at user choice, both detector escape and sum (cluster) peaks. Moudle escape peak uses internal functions to estimate the fraction of x-ray events (above the K edge) that will generate K x-rays that can escape from the front or back side of the detector. Includes options for Si and CdTe.

Figure 8. The plot shows a tungsten (W) x-ray tube output spectrum taken from a CdTe detector after processing to remove escape events. The grey trace shows the original spectrum. The green trace illustrates the escape events in the original spectrum. They are subtracted from this original spectrum, then the correct energy is calculated (by adding in the energies that escaped). The red line shows the corrected escape events, which are then summed with the grey trace. The dark black trace shows the final result of the processing with the events in their correct channels.

Smoothing

The specified amount of 1:02:01 Gaussian smoothing can be applied to the spectrum.

Deconvolutions: mining intensity

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 calculated using one of three methods: (1) a simple peak integration over a fixed region of interest, (2) a Gaussian fit to the peaks using a known database of line ratios and peak energies, etc. and (3) link 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 use the least squares method. In a linear fit, the peak ratios, energies and widths are fixed. This method is generally very fast. In a nonlinear fit, these parameters can vary within certain constraints. This method is much more computationally intensive.

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

In addition to calculating elemental intensities, the program automatically calculates uncertainty estimates and background values, which allows uncertainty and minimum detection limit (MDL) calculations to be performed during FP analysis.

Automatic Peak / Element ID

There are two options:

With the Amptek ADMCA application, the user can automatically mark peaks (transformations) for analysis. If a corresponding library element is loaded into the ADMCA software, the marked peaks will be associated with the elements. The corresponding elements can be automatically imported into the XRF-FP element table.

Using the RFA-FP interface with Spectra-X, the program analyzes the spectrum and assigns the most probable elements and lines to each of which a peak is defined, and collects a complete list of possible elements in the spectrum.

MLSQ

Calibration using fundamental parameter methods is carried out using several standard paths and various additional regression models to refine the FP calibration coefficients.

Spectrum window

In addition to ADMCA, the Spectra-X module displays acquired or processed spectra. Up to 8 spectra can be compared. KLM markers for peak identification and various other tools are available for adding text and lines to the screen.

Fig. 11. Spectra-X display showing element markers.

Extended description

There are only two steps in the XRF non-primary parameter (FP) method. The first step is to calibrate the response function for each element from one or more standards (the so-called "Calibration" step). The second step is to prepare an analysis sample from a given material using previously stored calibration factors, and the FP algorithms given the sample definition (i.e., the number of layers, and which elements are in which layers).

The program will support single layer or compound mass and thickness analysis of up to 40 elements, calculated either as elements and/or compounds. Up to 4 or more excitation "conditions" are allowed in an analysis. Each condition describes a separate analysis, and can be freely defined with any combination of experimental conditions, such as kV, anode tube, filter, detector filter, environment (air, vacuum, He) and acquisition time. This allows the analyst to evaluate some elements with one condition and others completely differently, such that each analysis can be optimized for a specific element or group of elements. In addition, spectra processing steps can be freely defined, and all parts of the setup code state.

The FP analysis software will support single or multiple standard calibration schemes, or completely standard-less analysis if the tube, detector, environmental and geometric parameters are known. Calibration standards must be transferred one at a time and the merging of standard calibration information is handled internally. After each calibration step, a set of calibration factors and associated information for each of the defined elements is returned, which can be used immediately if only one standard is applied. When using multiple calibration standards, all factors will be combined into a single set, and this final set is then available for subsequent quantitative analysis.

Layer thickness must be established for standardless analysis. Results may be normalized to any value, and must be normalized for standardless analysis or when layer thickness is calculated. Elements (or compounds) may be calculated, fixed, or determined by difference. Elements may be determined by stoichiometry from compound formula. Composition results may be calculated in units of W%, or per minute, and for thin films, units such as ug/cm2 and mg/cm2 are used for mass thickness. The latter may be converted to thickness (microns, microinches, nm, etc.) if the density is known. Densities may be entered or additionally calculated theoretically.

All relevant FP calculations are performed both in the calibration process and quantitatively using calculations based on the Sherman equation. The tube spectra required for direct fluorescence calculations can be provided by the user or calculated from built-in models (Ebel, Pella et al.). These tube spectra can be convolved with experimental transfer functions to obtain the expected spectrum of tubes passing through an optical such as a polycapillary bundle. The presence of air paths will also be calculated from the input geometry parameters for the source and detector paths. Single-element filters can also be inserted between the tube and sample or between the sample and detector, and the software can accommodate both.

The detector parameters (window, thickness, area, etc.) will also be used to calculate the various absorption effects and efficiencies when the x-rays pass through the window and get deposited in the detector material. This is only strictly necessary when performing standard-free analysis, but the calculation is always done this way for consistency, and to facilitate comparison of calibration factors between elements. If the theory were perfect all the calibration factors would have the same value. In practice the differences should be relatively small, especially when compared with factors that do not fully compensate for detector efficiency. Usually when calibrating elements that all use the same line series (e.g. K), the coefficient of variation is small (<30%), but is often larger when calibrating from mixed lines (e.g. K and L) because it is difficult to make absolute calculations that incorporate line series information (e.g. fluorescence yield).

It is not necessary to collect pure element for FP spectra analysis, as no direct ratioing is needed for elemental intensity. The calculation is done this way to make it easier to do standard-less analysis. Of course, one can use pure element standards if desired, and a full FP calibration can be done this way, without any "type" standards used at all. This is useful if the analyst does not have type standards available.

Both direct and secondary fluorescence effects are taken into account in FP calculations. Included in the FP database are all the necessary parameters to calculate or recall absorption coefficients, fluorescence yields, jump factors, Coster-Kronig transitions, energy lines, line ratios, transition probabilities, etc.

The program consists of a main program window that provides the user with an interface. It runs on standard PCs (Windows XP and above) with at least 256 MB of RAM memory. The RFA-FP software is fully compatible and integrated with the Amptek ADMCA display and acquisition software. It can also directly control all Amptek electronics to provide auto/repeat/continuous operation.

Complete system includes XRF

• X-123 Spectrometer

• Mini-X Detector with USB Control

• Software for XRF analysis

• Mounting plate MP1