Major levels, classical analytical methods

The concentration range of the analyte may well limit the number of feasible methods. If, for example, we wish to determine an element present at the parts-per-billion or parts-per-million level, gravimetric or volumetric methods can generally be eliminated, and spectrometric, potentiometric, and other more sensitive methods become likely candidates. For components in the parts-per-billion and parts-per-million range, even small losses resulting from coprecipitation or volatility and contamination from reagents and apparatus become major concerns. In contrast, if the analyte is a major component of the sample, these considerations are less important, and a classical analytical method may well be preferable. 

The essential aspects have been discussed in the introduction on the use of RMs and CRMs. It should be noted that inorganic CRMs, in particular pure metals, are available on the market from several reliable suppliers. They show usually purity values with associated uncertainties that are negligible compared to the uncertainty of the majority of spectrometric methods in which they serve as calibrants. It is usual to find materials of stated (not by definition certified) purity of 99.999% (five nines in analytical jargon) or better. This would mean that any impurity is below 0.001% as a mass fraction. No relative analytical method has precision performances that go down to such levels. Suppliers of ultra pure metals are numerous. NIST sells such metals as certified RMs (SRMs). The certification of the purity is discussed briefly in Chapter 5. It can be mentioned that the measurements are often based on absolute methods. The ultimate detection of impurities can be made with spark source MS. For pure metals the uncertainty linked to the calculated purity is small. Therefore, compared to the intended use and the uncertainty of classical methods applied by the analyst for the determination of elements, it is totally negligible. 

XRF nowadays provides accurate concentration data at major and low trace levels for nearly all the elements in a wide variety of materials. Hardware and software advances enable on-line application of the fundamental approach in either classical or influence coefficient algorithms for the correction of absorption and enhancement effects. Vendors software packages, such as QuantAS (ARL), SSQ (Siemens), X40, IQ+ and SuperQ (Philips), are precalibrated analytical programs, allowing semiquantitative to quantitative analysis for elements in any type of (unknown) material measured on a specific X-ray spectrometer without standards or specific calibrations. The basis is the fundamental parameter method for calculation of correction coefficients for matrix elements (inter-element influences) from fundamental physical values such as absorption and secondary fluorescence. UniQuant (ODS) calibrates instrumental sensitivity factors (k values) for 79 elements with a set of standards of the pure element. In this approach to inter-element effects, it is not necessary to determine a calibration curve for each element in a matrix. Calibration of k values with pure standards may still lead to systematic errors for unknown polymer samples. UniQuant provides semiquantitative XRF analysis [242]. 

The production of SWIFT-WFD RMs had a valuable impact in the evaluation of European laboratories performances using both classical and screening methods for four classes of analytes trace elements, major components, PAHs, pesticides. The produced RMs allowed the evaluation of analytical performances at EU level at different levels of concentration, in different matrices (river water, spring water) with different composition. 

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