Analysis of Iron and Steel Using XRF for Determination of Inhomogeneous Samples and Product Failure Investigation

The ARL PERFORM’X spectrometer expands the normal capabilities of X-ray fluorescence enabling small spot analysis. These analyses can be either qualified or quantified by simply measuring a chosen spot on the sample surface (Fig. 1, 2). The small spot analysis size enables determination of embedded particles or inhomogeneous specimens and product failure investigations. With a choice of 1.5 or 0.5 mm, the ARL PERFORM’X spectrometer offers a perfect complement to bridge the gap between traditional bulk analysis and standard micro-analysis using microscopic techniques such as scanning electron microscopy (SEM). A camera is included to select the spots of interest.

Figure 1. Example of an inhomogeneous sample.

Figure 2. Cups for small spot analysis with 30 mm and 10 mm apertures.

Instrument

The ARL PERFORM’X spectrometer used in this analysis was a 4200 W system (Fig. 3). This system is configured as a standard system with 6 primary beam filters, 4 collimators, up to nine crystals, two detectors and our 5 GN Rh anode X-ray tube for best performances from ultra light to heavy elements.

The ARL PERFORM’X analyzer also features small spot analysis (1.5 and 0.5 mm areas). All of the results presented in this article used the 0.5 mm spot analysis configuration.

Figure 3. Thermo Scientific ARL PERFORM’X Spectrometer Series.

Iron Base Calibration at 0.5 mm

For evaluating the performance that can be achieved while analyzing a spot of only 0.5 mm, a calibration has been performed using ferrous base standard samples. The standards used in this small spot calibration can have diameters ranging from 11 to 52 mm; however they must be homogeneous in composition. Typical examples of these calibration curves are shown in Figures 4, 5 and 6. Table 1 offers a summary of limits of detection obtained using a set of international steel standards with the 0.5 mm spot. Results were obtained by using 100 seconds per element counting times and the best conditions and parameters in regards to crystal, detector, collimator and power.

Table 1. Limits of detection in ferrous base for 0.5 mm spot size.

ELEMENT LINE LOD (PPM) 100s
Al 840
Si 725
P 208
S 160
Ti 317
V 205
Cr 160
Mn 250
Co 390
Ni 340
Cu 220
Ta 330
Zr 57
Nb 55
Mo 60
Sn 265

Figure 4. V calibration at 0.5 mm.

Figure 5. Si calibration at 0.5 mm.

Figure 6. Cu calibration at 0.5 mm.

Typical Stability and Precision Tests

A stability test, which includes running a typical sample over 10 days was performed. Each element was analyzed using 100 seconds per element. The stability of an instrument reflects the precision that can be obtained. It should be noted that the accuracy of the instrument is dependent upon the accuracy of the standards used to calibrate the instrument. The stability for each element is given in Table 2 along with the certified and calculated concentrations.

Table 2. Accuracy and precision data in ferrous base for 0.5 mm spots.

ELEMENT LINE CERT. CONC. (%) ANALYZED CONC. (%) STD DEV (%)
Al 0.24 0.21 0.047
Cr 1.31 1.32 0.015
Cu 0.1 0.10 0.010
Mn 1.5 1.47 0.026
Mo 0.03 <0.03 0.002
Nb 0.05 0.06 0.003
Ni 0.32 0.32 0.017
P 0.03 0.02 0.008
Si 0.74 0.63 0.041
Sn 0.1 0.10 0.009
Ta N.A. 0.07 0.015
Ti 0.05 0.04 0.012
V 0.31 0.29 0.012
Zr 0.05 0.07 0.003

Standard-less Analysis for Small Spots

The most useful development in the analytical programs in XRF has been the availability of “standard-less” packages. These packages enable quantitative data to be obtained for completely unknown samples. As in a large number of real life situations, obtaining any or enough standards to create a calibration is not always possible. This is certainly the case when analyzing defects or unknown contamination. Thermo Scientific offers one of the most comprehensive standard-less software on the market: Thermo Scientific UniQuant package. It is a factory calibration based on 64 pure element standards that allows for concentration determination of unknown samples in any matrix by using complex mathematical algorithms for up to 79 elements. These algorithms correct for matrix effects as well as inter-elemental effects to provide highly accurate and precise quantitative results.

Conclusion

Small spot analysis can be easily conducted with the ARL PERFORM’X sequential XRF spectrometer. The precision and accuracy are excellent both for routine or R&D analyses. When an approximate analysis of an inclusion or a defect in a sample is enough, a standardless program like UniQuant will provide good results without the use of specific standard samples. Sometimes only a qualitative analysis using a fast scan method may also be sufficient. Furthermore, operation is made easy through the new state-of-the-art OXSAS software which is able to operate with the latest Microsoft Windows 7 system.

About Thermo Fisher Scientific-Elemental Analysis

For over 75 years, Thermo Fisher Scientific has been a worldwide supplier of spectrochemical instrumentation to major industries including steel, transportation, cement, construction, food, pharmaceuticals, chemicals, academic research, petroleum and electronics. They offer unsurpassed capabilities in the areas of optical emission (OE), X-ray fluorescence (XRF), X-ray diffraction (XRD) and automation of spectrometers.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific- Elemental Analysis.

For more information on this source, please visit Thermo Fisher Scientific- Elemental Analysis.

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