instrument LOQ and Method LOQ

Discussions about HPLC, CE, TLC, SFC, and other "liquid phase" separation techniques.

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Dear all
Suppose in a HPLC method, I have to take 1 g sample and during sample preparation it is diluted to 5000 times. my HPLC can quantify 1 ppm of that analyte in pure solvent (instrument LOQ) while 1.5 ppm in sample matrix.

Then during method LOQ determination, whether I have to incorporate all the dilutions or not? If I incorporate all the dilutions steps and inorder to meet my instrument LOQ( 1ppm), that sample should have at least 5000 ppm of that analyte. Is it means my method LOQ 5000 ppm??
or method loq just means incorporation of matrix only in loq determinmatrion ( 1.5 ppm in above example)
Hoping for much responses from the experts.
Thank you
Suraj
Ultimately your limit of quantification applies only to a particular sample type, handled from start to finish in a defined way, so it applies to the entire method including all sample prep and all analytical method details including the instrument and its set-up.
You can use a LOQ determined on a simple liquid standard to estimate the likely LOQ from a longer method with a full extraction and dilution, but it's only an estimate; it should be checked by running standards through the entire method.
The LOQ almost always refers to the amount of analyte needed in the original sample, which in your case is 1g of solid. In your case it is likely to be around 5000ppm rather than 1 or 1.5.
Suraz wrote:
Dear all
Suppose in a HPLC method, I have to take 1 g sample and during sample preparation it is diluted to 5000 times. my HPLC can quantify 1 ppm of that analyte in pure solvent (instrument LOQ) while 1.5 ppm in sample matrix.

Then during method LOQ determination, whether I have to incorporate all the dilutions or not? If I incorporate all the dilutions steps and inorder to meet my instrument LOQ( 1ppm), that sample should have at least 5000 ppm of that analyte. Is it means my method LOQ 5000 ppm??
or method loq just means incorporation of matrix only in loq determinmatrion ( 1.5 ppm in above example)
Hoping for much responses from the experts.
Thank you
Suraj


I have always found the best definition for LOQ to be "the product of the low standard concentration (x ug/L) and the dilution factor of the matrix blank sample (y)". Yes you must account for the dilution factor. I suppose you could say your "instrument" LOQ is the concentration of your lowest standard but the value is really meaningless as far as your method is concerned.
Thank you both of you.
In some cases, there is concentration step rather than dilution step in method. In such cases, method LOQ will be smaller than Instrument LOQ.This is quite odd to hear.

But yes, practically method LOQ has higher significance specially in audit, when accessor spiked at LOQ level in blank matrix and you are asked to analyze it.

If he spiked at instrument LOQ level then you may not find your target due to high dilution.while this problem is not occur in method LOQ. Am I right ?
Easy solution to the dilemma: specify instrument LOQ in mass (or moles). From there, it's easy to get the instrument LOQ in concentration terms (divide mass by volume injected). Multiply that by the dilution factor to get the method LOQ.

As a corollary, "ppm" is *horrible* as a unit of concentration. Far better to use proper SI units (ug/mL, or ug/g, for example). That way there's no ambiguity.
-- Tom Jupille
LC Resources / Separation Science Associates
tjupille@lcresources.com
+ 1 (925) 297-5374
Thank you Tom.
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