INSTRUMENTATION
Total-Reflection X-Ray Fluorescence Spectroscopy Reinhold Klockenkàmper Institut fur Spektrochemie und angewandte Spektroskopie Bunsen-Kirchhoff-StraBe 11 D-4600 Dortmund 1 Germany
Joachim Knoth, Andreas Prange, and Heinrich Schwenke GKSS-Forschungszentrum Geesthacht GmbH Institut fur Physik Max-Planck-StraBe D-2054 Geesthacht Germany
Total-reflection X-ray fluorescence (TXRF) is a relatively new technique for micro- and trace analysis and for surface analysis. It differs fundamentally from classical X-ray fluorescence (XRF) but has many similarities with atomic spectroscopic methods such as atomic absorption spectroscopy (AAS) a n d t h e inductively coupled p l a s m a t e c h n i q u e s (ICP-AES and ICPMS) for trace element analysis, a n d with X-ray photoelectron spectroscopy (XPS), Rutherford backscattering spectroscopy (RBS), and secondary ion mass s p e c t r o m e t r y (SIMS) for surface analysis. The unique role of TXRF is based on the formation of standing waves above surfaces and within the near-surface layers, resulting in a surface sensitivity of a few nanometers. In this INSTRUMENTATION we describe the fundamentals of TXRF, provide details on the instrumental 0003-2700/92/0364-1115A/$03.00/0 © 1992 American Chemical Society
r e q u i r e m e n t s of t h e method, a n d present a few selected applications.
Development of TXRF The foundation of spectrochemical analysis is ascribed to Bunsen and Kirchhoff who, in 1859, vaporized a salt in a flame and determined some alkaline and alkaline earth metals by using the first optical spectroscope. Today, optical atomic spectroscopy includes a variety of analytical techniques such as flame and graphite furnace AAS, ICP-AES, and ICPMS. In 1895 Rôntgen discovered a new kind of r a d i a t i o n t h a t h e called X-rays, and in 1913 Moseley established t h e basis of X-ray spectral analysis by relating the wavelengths of characteristic lines to the atomic n u m b e r s of t h e e l e m e n t s . I n t h e years since, XRF has developed into a powerful analytical method. Because it does not require timeconsuming sample preparation a n d is nondestructive, XRF is especially suitable for analysis of solid samples for industrial production, geological prospecting, and environmental control. XRF is not suitable, however, for trace analysis and is notorious for systematic errors caused by matrix effects. Extensive efforts have been made to overcome the drawbacks of XRF by forming thin films, layers, or surface residues. The most significant progress in performance a n d accuracy w a s achieved when t h e phenomenon of total reflection, discovered in 1930 by Compton, was used for XRF analysis
in 1971 by Yoneda and Horiuchi (1). They directed t h e p r i m a r y X-ray beam onto a polished quartz carrier with glancing incidence a t angles smaller than the critical angle of total reflection. In this mode of operation, quantities < 10~ 9 g became detectable for the first time using a n energy-dispersive X-ray detector (2). Although this method, known a s TXRF, p a r t i a l l y compromises t h e nondestructiveness of XRF, it lowers the detection limits from 10~ 7 g to < 10~ 12 g, offering new possibilities in trace analysis. Today TXRF is used primarily for micro- and trace analyses, in which small quantities of the sample are placed on optical flats, and for surface analysis, in which the nearsurface layers are characterized.
Fundamentals In contrast to XRF, which uses a n gles of incidence of - 45°, TXRF uses a primary beam with a grazing incidence angle of < 0.1°. Basic components common to both techniques include a n X-ray source, a n energydispersive detector, and pulseprocessing electronics. Nevertheless, TXRF differs fundamentally from classical XRF, especially with respect to sample type and preparation, calibration, data analysis, and detection performance; it is more similar to AAS, XPS, RBS, and SIMS than to any other X-ray spectrometric technique.
R e f l e c t i o n and r e f r a c t i o n . X-rays are both reflected and re-
ANALYTICAL CHEMISTRY, VOL. 64, NO. 23, DECEMBER 1, 1992 · 1115 A
INSTRUMENTATION fracted at the interface between two different homogeneous media, just as light is. Reflection and refraction are both determined by the index of re fraction in its complex notation η = 1 - δ - ίβ
(1)
where the real term δ represents the dispersion and the imaginary term β stands for the attenuation in matter. In this equation, δ is dependent pri marily on the density and β on the mass absorption coefficient of the medium; both quantities are depen dent on the wavelength. For X-rays, both δ and β are very small; δ is on t h e order of 10~ 6 a n d β is even smaller. Consequently, the real part of η is slightly smaller than 1, and the X-rays penetrating from vacuum (or air) into matter are slightly re fracted toward the surface plane. Total reflection occurs if X-rays strike a medium with grazing inci dence. The glancing angle must be smaller t h a n a critical value (pc, which can be calculated according to Snell's law
%=ΛΜ
(2)
For photon energies of -10 keV the critical angle is only about 0.1°. X-rays totally reflected at a plane surface penetrate into the medium very little. Reflectivity and penetra tion depth can be calculated using the Fresnel formulas based on classi-
(a) 1.0 |
cal electrodynamics. As shown in Figure 1, both are dependent on the angle of incidence φ. At the critical angle
