Limit of Quantitation

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Quantitation of analytes at or near the Limit of Detection (LOD) is usually poor and gives low precision. We define the limit of quantitation (LOQ) as the lowest concentration of analyte that can be determined using the method with a given amount of precision and accuracy. The LOQ > LOD. One rule-of-thumb is that the LOQ should be 10× the standard deviation (s) of the noise (the mean + 10s).

          lower limit of quantitation[1]

Results that fall between LOD < x <LOQ should be reported as ‘detected but not quantifiable’.[2]

Instrument Limits of Quantitation

For the lab analyses of water samples on this project, we are able to calculate the limit of quantitation for the ion chromatrograph (IC) (salts analysis) and the optical emission spectrometer (OES) (metals analysis) for each species. Each salt or metal has a different level of quantitation limit because of the instrument's selectivity towards that species.

Current Limits of Quantitation for Salts


Species Limit of Quantitation (ppm)
fluoride (F-) 0.025
chloride (Cl-) 0.057
bromide (Br-) 0.009
nitrate (NO3-) 0.28
phosphate (PO43-) 0.19
sulfate (SO42-) 0.12

For project lab data tables, these are the current ion chromatographic analyses limits of quantitation. Any instrument values fall between the value above and the limit of detection value are denoted by '<LQ'. That is, we can say that we have detected the species in question, but we cannot say for certain if the amount if correct or not.

Current Limits of Quantitation for Metals


Species Limit of Quantitation (ppm)
calcium (Ca) 0.46
magnesium (Mg) 1.38
iron (Fe) 0.21
manganese (Mn) 0.039
aluminum (Al) 0.20
strontium (Sr) 0.006
lead (Pb) 0.023
arsenic (As) 0.023
mercury (Hg) 0.025
chromium (Cr) 0.0014
barium (Ba) 0.005
cadmium (Cd) 0.0013

For project lab data tables, these are the current optical emission spectroscopic analyses limits of quantitation. Any instrument values fall between the value above and the limit of detection value are denoted by '<LQ'. That is, we can say that we have detected the species in question, but we cannot say for certain if the amount if correct or not.

See also: Limit of Detection

References

  1. Harris, D. C. Quantitative Chemical Analysis, 8th ed.; W.H. Freeman and Co: New York, 2010; p. 105
  2. In the case of this occurring in lab analyses of Beaver Run data, results are marked as "DNQ".



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