Comparison of Signal-to-noise, Blank Determination, and - Universiti ...

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using s . From these findings, it seemed that not all the methods used to estimate LOD and y 0 LOQ in this study were equivalent. The differences between the smallest and the largest values estimated by different methods could vary by a factor of five to six for both LOD and LOQ. Reliability of the LOD and LOQ estimates It is difficult to compare the degree of reliability of the LOD and LOQ estimates. In order to check predetermined LOD and LOQ values obtained by statistical or empirical approach, the laboratory can obtain test material with known quantities of analyte at the level of the estimated LOD and LOQ limit, or fortified true blank sample at the level of estimated LOD and LOQ levels, and determine the precision as expressed in the relative standard deviation (RSD). The RSD of 10% is generally acceptable for LOQ (9,10) and 33% for LOD (9). The LOD and LOQ values obtained using different methods can vary significantly based on this experimental study using GC method. In the absence of a uniform definition and guideline for LOD and LOQ determinations, during method validation, the exact procedure for determination of LOD and LOQ must be clearly stated in the documented method so that the estimated LOD and LOQ values can be used for comparison by other analysts or laboratories. Acknowledgements The authors would like to thank Universiti Teknologi Malaysia and the Public Health Laboratory Johor Bahru, Health Department of Johor for technical support, research facilities and studentship to Susie Lu Ling and Zalilah Nasir. References (1) NATA (1998) NATA Technical Note #17, National Association of Testing Authorities, Australia (2) McNaught, A.D. and Wilkinson, A. (1997) IUPAC Compendium of Chemical Terminology 2 nd Ed., Blackwell Science (3) Long, G.L. and Winefordner, J.D. (1983) Anal. Chem. 55, 712-724 (4) Currie, L.A. (1995) Pure & Appl. Chem. 67,1699-1723 (5) Miller, J.N. and Miller, J.C. (2000) Statistics and chemometrics for analytical chemistry, Pearson Education Limited, Edinburgh Gate, England (6) ICH Q2B (1996) Validation of Analytical Procedure: Methodology, International Conference on Harmonisation of Technical Requirements for Registration of Pharmaceuticals for Human Use. (7) EURACHEM (1998) The Fitness for purpose of analytical methods. A Laboratory guide to method validation and related topics, LGC, Queens Rd, Teddington, Middlesex, TW11 0LY, United Kingdom. (8) Vial, J. and Jardy, A. (1997) Anal. Chem. 71, 2672-2677 (9) Thomsen, V., Schatzlein, D. and Mercuro, D. (2003) Spectroscopy 18, 112 – 114 (10) Thompson, M., Ellison, S.L.R and Wood, R. (2002) Pure Appl. Chem., 74, 835-855 6
Table 1. Concentrations of test compounds and number of replicates used for experimental application Method Levels (µg/mL), p Replicates, n Measurements, np Signal-to-noise 0, 0.4 10 20 Blank determination 0, 0.4 10 20 Linear regression 0.2, 0.4, 0.6, 0.8, 1.0, 2.0, 4.0 10 70 Table 2. Data obtained for each test compound based on signal-to-noise method Compound Mean (peak height values), n = 10 Blank % RSD S/N = 3 S/N = 10 Toluene 16 43 49 163 Ethylbenzene 15 44 44 146 iso-Propylbenzene 15 37 45 151 n-Propylbenzene 14 25 41 136 Styrene 13 25 38 126 Table 3. The mean concentration and standard deviation of blank obtained using blank determination method Compound Blank , n = 10 Mean conc. ( i b x ), µg/mL Standard deviation ( sb ) i Toluene 0.054 0.022 Ethylbenzene 0.052 0.021 iso-Propylbenzene 0.056 0.018 n-Propylbenzene 0.051 0.013 Styrene 0.050 0.015 Table 4. Parameters of linear ordinary least-squares regression for the five test compounds at seven different levels of concentration, n = 10 Compound Correlation coefficient, r Slope, b Y-intercept, a Y-intercept standard deviation, s y0 Residual standard deviation, s Toluene 0.992 798 18 16 91 Ethylbenzene 0.990 804 18 18 102 iso-Propylbenzene 0.994 804 15 13 78 n-Propylbenzene 0.992 787 36 15 89 7 res
Page 1 and 2: Comparison of Signal-to-noise, Blan
Page 3 and 4: LOD ≅ x + 3s bi bi bi LOQ ≅ x +
Page 5: Blank determination was carried out
Page 9: Figure 1. GC-FID chromatogram of a

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