Читать книгу X-Ray Fluorescence in Biological Sciences - Группа авторов - Страница 57

The main reason for the comparatively higher (poorer) detection limits observed in XRF, as stated above, is due to the higher spectral background in XRF. In energy dispersive X‐ray fluorescence (EDXRF), the X‐ray beam is made to fall on the sample at an angle of about 45°. The emitted X‐rays are detected by a detector which is also placed at an angle of 45° from the sample surface. This geometry, having a 90° angle between the detector and sample, is used because the scattered X‐rays produce a minimum background in the sample spectrum when arranged in this geometry [6]. However, in this geometry the X‐rays penetrate deep in to the sample/support up to a few micron level and interact with the atoms present in the sample, resulting in high scattered intensity and thereby high spectral background. In addition to this, the intensity of characteristic X‐rays of the analyte elements emitted from deeper layers are absorbed by the matrix elements when these X‐rays try to escape the sample. Another phenomenon may also occur where the elemental X‐ray lines of matrix elements with higher energies than the absorption edges of the analyte lines may get absorbed by the analyte and thereby can excite the analyte element to emit its characteristic X‐ray lines, thus producing a higher intensity of the analyte X‐ray line compared to that which would have been obtained by exciting the sample by the exciting X‐ray beam only. In the above two situations, the observed intensity of the analyte line shall be less or more, respectively, than the theoretical normal intensity of the analyte X‐ray line which should have been obtained by sole excitation of the exciting beam.

In wavelength dispersive X‐ray fluorescence (WDXRF), unlike in EDXRF, the angle between the exciting X‐ray beam and sample as well as that between detectors and sample changes continuously and hence the background also changes. The background produced in WDXRF is always appreciably high. In addition, the distance between the sample and the detector is also larger in WDXRF (a few centimeters) compared to that in EDXRF (a few millimeters). This reduces the sensitivity of a WDXRF spectrometer, due to the attenuation of X‐ray intensity by air and spectrometer components, the crystal analyzer placed between the sample and the detector, and also due to the smaller solid angle subtended by the detector on the sample. These factors limit the applicability of XRF ‐ EDXRF as well as WDXRF, especially for those samples which contain trace levels of analyte (in a sub‐ppm level ) or where the sample amount available is less, e.g. forensic, precious, biological samples, etc. [7].