Chromatography Forum

"Limit of Detection and Limit of Quantitation: The parameters LOD and
LOQ were determined on the basis of response and slope of the regression
equation"

Can somebody explain this to me. I do have my standard curve and regression analyis. what was the formula used to estimate LOD and LOQ

Regression equation y=0.2657x-0.0285
Correlation coefficient (r) 0.9999
Stand error on estimations (Sc) -0.02271
Standard deviation on slope (Sb) 0.00079
Standard deviation on intercept (Sa) 0.01689


Thanks

hi mk12...
well, firstly you should consider this: the standard curve should be constructed at low concentration levels because you want to calculate a concentration which is called the lower limit of quantitation (LLOQ or LOQ, it is the same). So i suppose that the standard curve that you present here,is that curve, and not your daily standard curve.Let me give you an example. Suppose that you have a method with linear range of concentrations 10ppb-3ppm.If you want to find LOD-LOQ you should construct a calibration curve with 5-6 points at very low levels of concentrations eg, 5ppb,10ppb,20ppb,30ppb,40ppb,50ppb.From this calibration curve you will be able to determine accurately your LOD-LOQ. At the other hand, your daily calibration curve could be with 3 points such as, 50ppb,500ppb,2ppm.
So from your standard curve we will have the expression: LOD=3,3Sa/b
or if you want to be more precise you should use this expression: LOD=3,3Sc/b. If your calibration curve (at the low concentration levels) is made correctly, then Sa and Sc (Sy/x) will be very similar. Then LOQ will be 3XLOD or LOQ=10Sa/b or LOQ=10Sc/b.

A nice reference would be Grant T. Wernimont, use of statistics to develop and evaluate analytical methods, AOAC, ISBN 0-935584-31-5. In the third printing of 1990, page 76 and further.

Regards Bert

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