I have prepared salt solutions with salinity value in the range 0 to 1 ppt with step incremental value of 0.1 ppt by dissolving NaCl salt to distilled water . I have prepared the calibration curve and it shows good linearity in my salinity sensor. Now I want to find the limit of detection. Can anyone please tell me which method I should follow between ICH and IUPAC? Can I take the distilled water as blank solution?
Since your technique is non colorimetric, so distilled water can be taken as blank solution.
LOD 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 . The detection limit is estimated from the mean of the blank, another consideration that affects the detection limit is the accuracy of the model used to predict concentration from the raw analytical signal.
Usually, I use the following equation:
LOD=3*SD/m
where SD=magnitude of error bar at blank, m=slope of the calibration curve at blank
Sensitivity= slope of the calibration curve at a certain target concentration
Exactly, use distilled water free from all types of contaminants and follow.
Detection limits LOD, LOQ can be calculated using following equation (as per ICH Q2 guide lines) DL = 3.3 * Sigma/ Slope. , and LOQ = 10* Sigma/ slope, Where Sigma is Residual of standard deviation of regression or STD deviation of y intercept. Sigma can be calculated by following Microsoft office Excel functions 1.Regression 2.Formula “”STYEX”” - 3.Array Function “” LINEST”” - For tutorial of the use of above functions are watch the youtube video.. https://youtu.be/Ii2Z3hqwRGM/
( 1 ) For a particular volume of the analyte / target / biomarker containing solution, create a titration / calibration curve of different concentrations. These will give you the signal responses. Do each concentration at least 12 times to get a statistically significant calibration curve. Most often, for publication purpose, a minimum of 3 replicates are considered.
( 2 ) Also determine the blank response using a solution of the same volume but not containing the biomarker you want to detect. This will provide the background response. You can call this as noise. Repeat this for "n" number of times.
(3) Next, determine the lowest concentration of the biomarker containing solution which has ratio of signal to noise >= 3. This will give you the limit of detection(LOD). The number 3 is derived from statistical calculation (3 sigma, 99.7 % chance that it is the correct response).
(4) Sensitivity is defined as the ratio of change in signal to change in concentration. Low value means it is less sensitive to change in concentration of target. If change in signal is dY and change in concentration is dX, then sensitivity is dY/dX ~ slope of the calibration curve.
(5) In some literature signal-noise ratio is defined as ( (mean of signal response - background response) ) / standard deviation of background response). But from my experience, it is rare to use this definition while writing biosensor papers. Hope this will help you.
You may check my biosensor publication detailing the above procedure :
Article Label Free Thrombin Detection in Presence of High Concentrat...
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