Detection limit
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In analytical chemistry, the detection limit, or LOD (limit of detection), is the lowest quantity of a substance that can be distinguished from the absence of that substance (a blank value) within a stated confidence limit (generally 1%). There are a number of different "detection limits" that are commonly used. These include the instrument detection limit (IDL), the lower level of detection(LLD), the method detection limit (MDL) and the level of quantitation (LOQ). The IDL is often used to estimate the rest of the detection limits and a relationship can be established between these detection limits as IDL:LLD:MDL:LOQ = 1:2:4:10.
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[edit] Instrument Detection Limit
Most analytical instruments produce a signal even when a blank (matrix without analyte) is analyzed. This signal is referred to as the noise level. The IDL is the analyte concentration that is required to produce a signal greater than three times the standard deviation of the noise level. This is practically measured by analyzing 8 or more standards at the estimated IDL then calculating the standard deviation from the measured concentrations of those standards.
[edit] Lower Level of Detection
This detection limit is the amount of analyte that will produce a signal distinguishable from noise 99% of the time that it is analyzed or better. Again this is practically measured by analyzing standards that are almost zero in concentration then measuring the standard deviation of the measurements. In order to reduce the probability of a false nondetection to 5% the standard deviation is then multiplied by 3.290.
[edit] Method Detection Limit
Many times there is more to the analytical method than just doing a reaction or submitting it to direct analysis. For example it might be necessary to heat a sample that is to be analyzed for a particular metal with the addition of acid first (this is called digestion). Additional steps in an analysis add additional opportunities for error. Since detection limits are defined in terms of error, this will naturally increase the measured detection limit. This detection limit (with all steps of the analysis included) is called the MDL. The practical method for determining the MDL is to analyze 7 samples through the entire analytical process. The standard deviation is then determined. The one-sided t distribution is determined and multiplied versus the determined standard deviation. For seven samples (with six degrees of freedom) the t value is 3.14.
[edit] Limit of Quantification
Just because we can tell something from noise does not mean that we can necessarily know how much of the material there actually is. The LOQ is the limit at which we can reasonably tell the difference between two different values. The LOQ is drastically different between labs so another detection limit is commonly used that is referred to as the Practical Quantitation Limit (PQL). The PQL is defined simply as about 5 times the MDL.
[edit] References
Standard Methods for the Examination of Water and WasteWater, 20th Edition, 1-17
