X-Ray Fluorescence Analytical Techniques - CNSTN : Centre ...

One of the inherent advantages in TXRF is that one deals with thin samples so the
simple conversion of fluorescent intensities I into concentration data C is applicable, as there
is a linear correlation between I and C. After establishing a calibration curve either from
known multielement standards or by using the fundamental parameters to calculate the
calibration curve theoretically, the conversion of I into C can be immediately performed. The
addition of one element as internal standard of known concentration into the sample is
recommended to improve the accuracy of the results, because in this case geometric and
volumetric errors will cancel. The simple relation to calculate the concentration of the
unknown is given:
I S
= C ; (III.3)
x std
Cx ⋅ ⋅
Sx
Istd
std
Cx: Concentration of unknown, Cstd: Concentration of Standard,
Ix: Intensity of standard, Istd: Intensity of Standard,
Sx: Sensitivity of unknown, Sst: Sensitivity of Standard.
A sample is “thin” if its thickness does not exceed the critical thickness, which about 4
µm for organic tissue, 0.7 µm for mineral powders, and 0.01 µm for metallic smears. Under
the assumption that the matrix absorption for the analyte differs only slightly from that of the
internal standard element, these values can be generally be higher by a factor of 40 – 400. For
the calculation of these values, the standing-wave field was not taken into account. This effect
and the sample self-absorption can lead to contradictory requirements for the sample
thickness.
VI. Influence on Detection Limits
The advantages of excitation in total reflection geometry are listed:
1. Efficient excitation by both, the primary and the reflected beam- the fluorescent signal is
doubled compared to standard excitation geometries 45° incident - 45° emission angle.
2. The spectral background caused by scattering on the substrate is reduced because the
primary radiation scarcely penetrates into the reflector substrate (high reflectivity, low
transmission into the material). The scatter contribution from the sample itself is a
minimum because of the 90 degree condition between incident and scattered radiation
towards the detector.
3. The detector is mounted closely to the sample of small amounts are required. The samples
must be prepared in aware that thin film approximation is applicable. Therefore no
absorption occurs and a linear correlation between intensity and concentration of the
element is valid.
4. Simultaneous multielement determination is possible due to the use of energy dispersive
detectors.
Due to the argument 1 and 2 automatically the peak to background ratio is increased
compared to standard XRF.
Improvements in the detection limits can be expected if the physical parameters
influencing the minimum detection limits are optimised. The generally accepted definition is: