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8/17/2019 11 How to determine LOD in AAS[1].pdf
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Shimadzu (Asia Pacific) Pte Ltd, Singapore
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HOW TO DETERMINE LIMIT OF DETECTION IN
ATOMIC ABSORPTION SPECTROSCOPY
Introduction
According to IUPAC(1), limit of detection (LOD), expressed as the concentration, cL is
derived from the smallest measure, xL, that can be detected with reasonable certainty for a
given analytical procedure. The value for xL is given by the equation:
(1) xL = x bl + ks bl where
x bl = mean of blank measurements
s bl = standard deviation of the blank measurements
The values for x bl and s bl are found experimentally by making a sufficiently large number ofmeasurements, say 20. Also, a value of 3 for k in equation (1) is strongly recommended.
Hence, equation (1) becomes:
(2) xL = x bl + 3s bl
In atomic absorption spectroscopy (AAS), a calibration curve is used to determine the
unknown concentration in a sample. The calibration curve can be represented by the
equation:
(3) Absorbance = (Slope x concentration) + intercept
For determination of LOD:
Absorbance = xL
Concentration = cL = LOD
Intercept = x bl
Hence, from equations (2) and (3),
xL = (Slope x cL ) + x bl
x bl + 3s bl = (Slope x LOD) + x bl 3s bl = Slope x LOD
(4) LOD = 3s bl / Slope
The Slope in equation (4), which is also known as sensitivity, is determined using:
(5) Slope = (x1 - x bl) / (c1 - c bl) where
x1 = Mean absorbance of first standard solution
x bl = Mean absorbance of blank solution
c1 = Concentration of first standard solution
c bl = Concentration of blank solution = 0
SAP/CSC/CAM/AAS-031
Oct 2004
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In equation (5), the slope is determined using a blank and standard solution. Preferably, the
concentration of the standard solution should be close to the concentration of LOD. As an
example, a rough estimate can be obtained from characteristic concentration (1% absorption)
– the concentration of c1 is about 10 times higher than concentration of characteristicconcentration. Alternatively, the slope can also be determined by using a series of standard
solutions to generate the calibration curve - in most AAS instruments, the slope is calculated by the software and displayed in the results.
By substituting equation (5) into equation (4), this becomes:
(6) LOD = 3s bl .c1 / (x1 - x bl)
Method
(a) Generate a calibration curve using a blank solution and a standard solution with
concentration about 10 times higher than concentration of characteristic concentration toobtain c1, x1 and x bl .
(b) Analyse a blank solution about 20 times to obtain s bl .(c) Substitute the values obtained in steps (a) and (b) in equation (6) to obtain LOD.
The calculations are shown below as an example.
Conc (ppm) Abs
0.0 0.0036
0.2 0.0533
0.5 0.1222
1.0 0.2241
SD of blank solution measured 20
times = 0.0001295
Based on the table above, s bl = 0.0001295; c1 = 0.2 ppm; x1 = 0.0533; x bl = 0.0036Hence, LOD = (3 x 0.001295 x 0.2) / (0.0533 – 0.0036)
= 0.016 ppm
Alternatively, the calculation using the slope value obtained by the AAS software is also
shown below.
Abs = 0.219383Conc + 0.00756212
From equation (4), LOD = 3s bl / Slope= 3 x 0.001295 / 0.219383
= 0.018 ppm
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Reference
(1) www.iupac.org/publications/analytical_compendium/
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