Validation of the existing analytical method using TXRF for the metals copper, zinc, lead, manganese, chromium, cobalt and nickel

Status:

completed 01/2026

Aims:

Total reflection X-ray fluorescence (TXRF) analysis continues to be of only secondary importance in the analysis of metals in workplace air. In many respects, however, modern TXRF spectrometers are the equal of established analytical methods such as inductively coupled plasma mass spectrometry (ICP-MS) or optical emission spectrometry (ICP-OES). They deliver high sensitivity and selectivity in the ultra-trace range for a wide range of elements, whilst at the same time demonstrating high matrix tolerance. Preparation of particulate dust samples in the form of a suspension is comparatively straightforward and time efficient, and even poorly soluble metals and their compounds can be analysed without the need for complex digestion processes. Furthermore, when compared directly to ICP-MS and ICP-OES, for example, TXRF has a lower environmental impact, besides entailing significantly lower operating and maintenance costs. Owing to the analytical challenges, however, validations are more difficult than with methods involving a digestion process. These challenges include the complex composition of workplace air samples (total dust content, particle size distribution, analytes), critical parameters in TXRF analysis of suspensions (homogeneity, application of the sample, spectral interferences, calibration) and the absence of normative provisions. A key focus of the project was the definition of performance criteria and their evaluation for the suspension-assisted TXRF method.

New methods, employing microwave-assisted pressure digestion and ICP-MS, for the analysis of copper, zinc, lead, manganese and chromium and also cobalt and nickel and their compounds have already been validated and established in the past (including in IFA Folder 7806 and 7808). The aim of the present project was to define validation tests (in the absence of normative requirements) and to validate the existing method of suspension-assisted TXRF for analysis of the above metals and their compounds. The performance characteristics and limits of the method were to be determined and documented. The IFA’s metals interlaboratory test covers the metals stated and their compounds. ICP-MS was to serve in the first instance as the reference method for interlaboratory test samples and routine samples. The results were to be evaluated in accordance with the applicable standards, in particular EN 482 and ISO 21832. The project was intended to make a decisive contribution to suspension-assisted TXRF becoming more established as an analytical method for the analysis of metals and their compounds in the workplace.

Activities/Methods:

In the first stage of this project, the discrete validation steps for the method and the comparative tests performed by means of the validated reference method (IFA Folder, codes 7806 and 7808) were assessed and specified in observance of the EN ISO 21832, DIN 32645 and EN 482 standards. The validation steps (in particular determination of the instrumental detection limit, the method’s limits of detection and quantification, the recovery and the analytical precision) were subsequently performed for the analytical method described, and the results were evaluated against the requirements of the above standards for valid measurement methods. The full scope of testing was carried out on the two legacy TXRF instruments and the newly procured TXRF instrument. Notable characteristics of the new TXRF instrument included its combination of the excitation sources of the two legacy instruments and its generally higher sensitivity. It was not therefore possible to transfer the results obtained with the legacy instruments. To enable the analytical limits of the TXRF analytical method to be evaluated, a comparative analysis was conducted of interlaboratory test samples, and also of suitable real-case workplace air samples from contract analysis in order to take effects of real-case particle distributions into account. Reference methods for the TXRF analyses, in the form of established analytical methods employing digestion and ICP mass spectrometry, were applied to both sample types. A suitable method for transfer of the suspended sample was developed to permit comparative analyses of real-case workplace air samples.

Finally, the results of the validation tests were evaluated with consideration for the relevant assessment criteria and the requirements of the applicable standards. Plans are for the method to be published in the IFA Folder, as a method described in the DGUV Informative publication of the Analysis working group, as a DFG method in the MAK Collection, and as a standard operating procedure in Q.wiki. Following completion of the project, the performance criteria defined are to be incorporated into the international standard.

Results:

In the course of the project, the total reflection X-ray fluorescence analysis of suspensions was validated methodologically and studied with regard to its efficacy for quantitative analysis of the elements of Co, Cr, Cu, Mn, Ni, Pb and Zn. For this purpose, an internal validation scheme for TXRF analysis was developed and applied, and a sequential sample preparation procedure was standardised and established for comparison with the reference method of ICP mass spectrometry. Overall, experiments for determination of the method parameters on all available TXRF systems revealed good values for limits of quantification, precision, accuracy and system comparability within the concentration range under examination. Altogether, the comparative studies conducted on interlaboratory test samples and real-case samples (n = 203) demonstrated close agreement between suspension-assisted TXRF and the ICP-MS reference method. Deviations were studied systematically, and were attributed to influences of the matrix or the method. In the course of this study, methodological constraints were identified and measures described that must be taken into account during contract analysis (such as the extent to which, in the event of certain perturbations, further quantification is to be carried out by means of ICP-MS). In the final evaluation, the performance characteristics determined met the predefined requirements (for example for the recovery limits and the relative standard deviations) and the relevant normative requirements. Consequently, suspension-assisted TXRF can be classified as a suitable method for analysis of Cr, Cu, Mn, Ni, Pb and Zn for selected analytical problems. Considerable potential thus exists, particularly for screening and for use in routine work, in comparison with the acid digestion analysis method established internationally in occupational safety and health. A relevant constraint exists only for cobalt: firstly, the limit of quantification determined by the older generation of TXRF instruments (Picofox) is no longer sufficient to ensure reliable monitoring of 0.2 times the currently valid acceptable concentration; secondly, the mixed body of real-case contract analysis samples examined in comparative studies is not yet sufficient to permit assessment of the method’s robustness across the entire range of analysis. The total data set (n = 203) appears large in statistical terms, but a comparison should also be conducted at the sectoral/activity level. When the data sets – over 200 in number – have been broken down to this level, the resulting number of data sets for comparison for each activity/sector is small. Comparative measurements of real-case samples will therefore be continued in the established work process beyond the project term, thus yielding further observations permitting a comprehensive picture.

Conclusion for the MGU: a validation scheme for TXRF has been developed in consideration of the relevant normative provisions which can also be used to validate other metals. The existing method for quantification of seven metals was developed further and validated in the course of this project. Should perturbing influences be identified in unknown samples in future, the analyses will be supplemented as required by sequential ICP‑MS measurements of the samples.

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