LAWR

STA 2015

Local Authority Waste & Recycling Magazine

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11 Annual Guide 2015 | SOURCE TESTING ASSOCIATION Limit of Detection (LOD) and Limit of Quantification (LOQ) A critical aspect of any measurement is an understanding of the performance of the method used. e usual specifics are; what is being measured, in what matrix, with what range and with what limits of detection/quantification and desired accuracy and pre cision? Such performance is particularly important near the critical level of interest, i.e. near a regulatory limit. ese are essential characteristics that must be known before a measurement campaign can be planned. Unfortunately, in many cases, the procedure used to determine LOD and LOQ is not defined in the methods or standards, and there is not a single inter nationally recognised approach. is can lead to very different values being reported from different laboratories for the same method using the same equipment. LOQ is usually determined at a level higher than the LOD where acceptable method performance is obtained. is might typically be between five and ten times the LOD. e following examples demonstrate the remarkable variety of definitions (where there are any!) of the limits of detection contained within BS EN and BS ISO Standards. Limits of quantification are very rarely mentioned. BS EN 14791:2005 "Stationary source emissions – Determination of mass concentration of sulphur dioxide – Reference method." A definition is given: 3.1.3 "analytical detection limit – concentration value of the measurand below which there is at least 95% level of confidence that the measured value corresponds to a sample free of that measurand". No guidance on how to obtain this value! BS ISO 11338-2:2003 "Stationary source emissions – Determination of gas and particle phase polycyclic aromatic hydrocarbons – Part 2: Sample preparation, clean up and determination". Limit of detection is mentioned, but is not defined anywhere in the text. An expected LOD is given in the text for a certain sample volume taken. BS ISO 15713:2006 "Stationary source emissions – Sampling and determination of gaseous fluoride content" Not defined anywhere in the text, but given the following mention in the characteristics section. "10.1 Detection limits e detection limit of the test method can be estimated to be 0.1 mg/m 3 with a sample volume of 0.1 m 3 (based on a flow rate of 4 l/min for a sample period of 25 min) and a detection limit of 1 mg for the mass of fluoride in solution." BS EN 15549:2008 "Air quality – Standard method for the measurement of the concentration of benzo(a)pyrene in ambient air" is standard contains a quite detailed definition and meth od for determination of the LOD. Even then though it doesn't say whether the ten blanks are run in one batch or paired batches, or what happens if there is no BaP in the blanks. An adaptation of this approach where a low spike would be used when it is known that no peak is detected from the blank materials would be consistent with MCERTs and would also need to be defined as being in ten paired batches. »

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