11 How to determine LOD in AAS[1].pdf

8/17/2019 11 How to determine LOD in AAS[1].pdf

1/3

______________________________________________________________________________________

Shimadzu (Asia Pacific) Pte Ltd, Singapore

1

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


2/3

______________________________________________________________________________________


2

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


3/3

______________________________________________________________________________________


3

Reference

(1) www.iupac.org/publications/analytical_compendium/

~ The End ~

11 How to determine LOD in AAS[1].pdf

Documents

Transcript of 11 How to determine LOD in AAS[1].pdf