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

Figure VI.1: Schematic view of net peak and background definition.
Assuming a confidence level of 95 % the total deviation is given by 2σ. The standard
deviation from peak and background is assumed to be equal:
3⋅ NB 3⋅ IB⋅t 3 I
LLD = ⋅ m = ⋅ m = ⋅ B
NN IN ⋅t
S t
2 2 2
σ T = σ P +σ B = 2σ
B
2 2
LLD = 2σ T = 2 2σB ≈3 σ B = 3 NB
with S = INm, (VI.2)
generally normalized to 1000 s measuring time.
The detection limit is a method to compare the power of different analytical methods of
trace element determination. Generally detection limits are derived from single element
samples, so no line interference is considered. They are idealized and extrapolated values and
one has to multiply these values by about a factor of 3 to come to the minimum measurable
amount in real sample, but these values are good to compare different analytical methods for
trace element determination.
Table VI.1 shows a comparison of detection limits in µg/cm 2 for WD-XRF of
Nuclepore filters and for secondary target EDXRF of Teflon filters.
Table VI.1: Comparison of Detection Limits for WDXRF and EDXRF of filter
materials.
Detection Limit Detection Limit
Element
Element
WDXRF EDXRF
WDXRF EDXRF
Na 310 Ge 3
Mg 60 As 4
Al 6.7 130 Se 2
Si 16 45 Br 16 2
P 8.2 Rb 3
S 5.6 15 Sr 20 3
Cl 2.2 13 Zr 8
K 3.1 6 Mo 5
Ca 1.2 5 Ag 5
Ti 4.3 30 Cd 7.5 (Lα) 6
V 1.6 20 In 6