KJ3055-Chapter 8 (X Ray Spectrometry)

Sample penetration depth: effect on excitation and fluorescence

Fig. 1. Effect of sample penetration depth on excitation an emission. In XRF, excitation and emission conditions vary with the sample penetration depth (d). Due to absorption, the excitation beam intensity decreases with d (according to Lambert –Beer law) and renders the excitation less efficient at a higher d (Fig. 1, Curve 1). Fluorescence radiation is partially re-adsorbed in the sample before leaving it. That is why the amount of radiation arising from a longer d is smaller (Fig. 1, Curve 2). The effect of the lower excitation intensity at a longer d adds to the re-adsorption effect and contributes to the decrease in fluorescence efficiency with the increase in d. Such processes are dependent on matrix properties and should be corrected for. Graph adapted from

Fig. 2. The fate of the emitted radiation is illustrated in Fig. 2 for the simple case of sample containing elements A and B. Fluorescence radiation emitted by excited A atoms can escape to the detector (4) but it is partially lost by various processes such as: · scattering by B (1) and A (6) atoms; · re-absorption by A atoms, followed by emission to a different direction and with a different wavelength (5); · absorption by B atoms, followed by emission of B specific radiation (2) provided that the A photon energy is higher than the ionisation energy of B (secondary fluorescence). This effect adds to B primary fluorescence(3) and enhances the B signal. Although this description is far from being complete, it gives an insight in the complexity of the phenomena that affect the signal by inter-element effects. Suitable algorithms and software allow correcting for inter-element effects.

F.G. Banica, 09-03-20