The key difference between LoD and LoQ is that LoD is the smallest concentration of an analyte in a test sample that we can easily distinguish from zero whereas LoQ is the smallest concentration of an analyte in a test sample that we can determine with acceptable repeatability and accuracy.
The term LoD and LoQ are found in analytical chemistry, mainly under the HPLC technique. The term LoD stands for limit of detection whereas the term LoQ stands for limit of quantitation. LoQ is a derivative of LoD with a slight difference.
CONTENTS
1. Overview and Key Difference
2. What is LoD
3. What is LoQ
4. Side by Side Comparison – LoD vs LoQ in Tabular Form
5. Summary
What is LoD?
The term LoD stands for limit of detection. We can also name it as the detection limit or lower limit of detection. It is the lowest amount of a substance that can be distinguished from the absence of that substance (zero point) with a stated confidence level. Usually, the confidence level for LoD is 99%. We can estimate the detection limit from the mean of the blank, the standard deviation of the blank, and the slope of the calibration plot along with a defined confidence factor. In addition to these, the accuracy of the model used to predict concentration from the raw analytical signal is another factor that affects the LoD of a particular test.
For example, let us consider a calibration plot that follows the model equation f(x) = a + b(x) where “x” is the signal measured, “a” is the point where the equation cuts the ordinates axis, and “b” is the sensitivity of the system. Here we can calculate LoD as the “x” value in which f(x) equals the average value of blank’s “y” plus the “t” times its standard deviation, “s” where “t” is the chosen confidence value. We can get this relationship as a mathematical expression as LoD = (f(x)-a)/b = (y + 3.2s – a)/b. Here, 3.2 has been taken as the most accepted value for this arbitrary value.
There are variations of LoD including IDL (instrument detection limit), MDL (method detection limit), PQL (practical quantification limit), and LoQ (limit of quantification). The following graph shows the relationship between LoD and LoQ.
What is LoQ?
The term LoQ stands for limit of quantitation. It gives the smallest concentration of an analyte in a test sample that we can determine with acceptable repeatability and accuracy. In other words, it is the concentration at which the entire analytical system must give a recognizable signal and acceptable calibration point.
What is the Difference Between LoD and LoQ?
LoD and LoQ are important calculations in HPLC. LoD stands for limit of detection while LoQ stands for limit of quantitation. The key difference between LoD and LoQ is that LoD is the smallest concentration of an analyte in a test sample that we can easily distinguish from zero whereas LoQ is the smallest concentration of an analyte in a test sample that we can determine with acceptable repeatability and accuracy.
Summary – LoD vs LoQ
The term LoD stands for limit of detection while the term LoQ stands for limit of quantitation. The key difference between LoD and LoQ is that LoD is the smallest concentration of an analyte in a test sample that we can easily distinguish from zero whereas LoQ is the smallest concentration of an analyte in a test sample that we can determine with acceptable repeatability and accuracy.
Reference:
1. “Limit of Detection.” an Overview | ScienceDirect Topics, Available here.
Image Courtesy:
1. “LOD” By No machine-readable author provided. Mattj63 assumed (based on copyright claims). – No machine-readable source provided. Own work assumed (based on copyright claims) (Public Domain) via Commons Wikimedia