However, the analyses can mostly be carried out without any changes in the aggregate state of the sample, i.e. the dissolution of solid samples is not required. Therefore, the effort for sample preparation compared to optical methods is relatively low and no or only slight dilution effects reduce the sensitivity of trace detection and therefore avoid analytical errors by contaminations.Nevertheless, it should be noted that even in the case of XRF, sample preparation must be carried out very carefully in order to achieve the desired analytical accuracy.
Besides the analysis of homogeneous volume samples, the characterization of layered systems with XRF is also possible. Under certain conditions, both thickness and composition of layers can be determined. In this case, the mass per unit area can be determined by measurement, which then has to be converted into layer thicknesses and mass fractions by using the material density.The determination of layer thicknesses is a very common analytical problem in industrial process control of mechanical and electronic components or many other electroplated products.
Another very important property of X-ray spectrometry is the possibility of automation, in particular, the automation of the measurement process, including data evaluation. In case there is no change in the sample type even sample preparation can be automated. This results in a fast analysis, but above all it provides for an analysis independent of subjective influences. Sample preparation and measuring operation can then be carried out under equivalent conditions, which reduces the uncertainty range of the measurement.Apart from this effect, the ongoing costs for analyses are reduced by means of automation.
The analytical problem of X-ray spectrometry can be very different and can be classified into various “degrees of difficulty.”Qualitative analysis can be considered as a simple task. In this case, it is only necessary to determine whether certain elements are present in the sample or not.The next stage involves the monitoring of concentration ranges for selected elements. In this instance, it must be determined whether the mass fractions of the elements under consideration in the sample material are below or above a certain limit. Here, often no direct quantitative analyses are required but only a monitoring of the intensity level of the analyte. In this case, the matrix influence can be neglected because samples of similar qualities are investigated and their matrices do not change significantly.Without doubt, the most demanding analytical problem is the quantitative analysis. Here, the elements present in a sample have to be identified at first – for samples of same quality as in the case of quality control in the production process, this is not necessary – and then their mass fractions or their layer parameters have to be determined.
The requirements regarding accuracy and sensitivity of the analysis can be very different, resulting in the selection ofthe sample preparation method (homogenization of the sample, elimination of influences of the surface roughness or of mineralogical effects)the measuring conditions (excitation conditions, measuring times, measuring medium)the evaluation model and, if available, of the reference samples to be used for the calibration to the accuracy requirements.
Further, X-ray spectrometry can also determine element distributions of large sample areas by using specific excitation conditions. For this purpose, the incident beam has to be concentrated on a small sample area. The sample then needs to be moved under the fixed beam into the measuring position. This offers the possibility for the analysis of non-regular sample surfaces and additionally for the characterization of inhomogeneous materials.
By using specific excitation geometries and conditions, it is possible to influence the sensitivity of the method. For example, in the case of a grazing incidence of the primary beam, the spectral background is greatly reduced and hence the sensitivity of the measurement is significantly increased.A similar effect can be obtained by using monoenergetic radiation for the excitation. Here also, the spectral background is reduced and an improvement in sensitivity can be achieved.
A large number of different X-ray spectrometry instruments are available, each of which is designed for specific analytical tasks. A more detailed discussion of the individual instrument types can be found in Section 4.3.
The radiation sources used in laboratory analyses today are X-ray tubes. In the past, isotope sources have been used as well. Another radiation source includes synchrotrons. Their radiation properties, i.e. the high beam brilliance, the polarization of the synchrotron radiation, or the possibility of generating monoenergetic radiation by means of appropriate X-ray optics, allow the use of very dedicated measuring geometries and measurement methods. As a result, new analysis methods are often developed at these radiation sources, which subsequently can be transferred into routine practice. However, these special analyses methods are not discussed within this scope since the very high instrumental effort and the limited availability of measurement time at these sources restrict their routine use.Another type of interesting X-ray source are plasmas in which atoms are ionized by extremely high temperatures. These atoms then emit X-radiation when transferred back to the ground state. It is usually radiation in the lower energy range. Because the plasma is often generated by a laser impact, these sources can be pulsed and consequently be used for time-resolved studies. However, these sources are not yet suitable for real routine use.
2.2 X-ray Radiation and Their Interaction
2.2.1 Parts of an X-ray Spectrum
X-radiation is electromagnetic radiation in the energy range of approximately 0.1–100 keV or with wavelengths in the range of approximately 25 to 0.01 nm. X-radiation is therefore characterized either by its energy E or by its wavelength λ. Both quantities are mutually transferable through the following relationship:
(2.1)
X-ray radiation can be generated by several processes. A continuous broadband spectrum is emitted by the stepwise deceleration of highly energetic charged particles but also by highly ionized plasmas (bremsstrahlung). Line-like spectra (characteristic radiation) are generated when transitioning excited atoms back into the ground state, if the energy difference of the energies involved is within the range described above. For the excitation of X-rays in the laboratory scale, accelerated charged particles, i.e. electrons or protons, as well as high-energy ionizing radiation, i.e. X-rays themselves are used. In the beginning, radioactive elements were also used as radiation sources in laboratory equipment. However, these sources are now used not very often because of the high safety requirements.
The most common way of producing X-ray radiation is the deceleration of accelerated electrons. This is used in X-ray tubes and electron microscopes. The deceleration of the electrons results in both a continuous spectrum and a line-like spectrum.
The continuous spectrum results from the deceleration of the electrons on the tube target by scattering on the atomic nuclei. The intensity of the emitted radiation is described in detail by Kramers' law.
(2.2)
with
| I cont | intensity of the emitted radiation |
| K | proportionality coefficient |
| I | electron current |
| Z |
atomic
|