Chromatography Forum

I have a problem with making a matrix matched calibration curve for detecting endogenously present analytes in human serum, urine and vegetable matrices.

The matrix for spiking standard already contains a tiny amount of analytes. I can only think of measuring a zero calibrator and then subtracting the response of each calibrator from that of the zero calibrator to construct a calibration curve?

Is this valid? Are there any other better ways?

Please help. Thanks.

If the amount of analyte is constant in the matrix you're using for preparing standards, you might be able to use the method of standard addition to initially determine the background concentration, then take that into account when preparing your standards for your calibration curve.

For example I have an ICP method for iron that has 0.3ppm of iron in my matrix, which I determined by standard addition and confirmed with outside analysis. I calibrate using the straight matrix sample and a matrix sample with 50ppm spiked - so my curve is a 0.3ppm point and a 50.3ppm point.

Whether this is acceptable would depend, among other things, on whether you can accurately determine the low levels of analyte in your matrix, what kind of regulatory environment you are in, and any internal SOPs you have to follow.

I really appreciate your comments.

"For example I have an ICP method for iron that has 0.3ppm of iron in my matrix, which I determined by standard addition and confirmed with outside analysis."

How to confirm with outside analysis?

For my analysis, the level of matrix is below the LOQ. Can your approach be used?

Are there any regulatory rules for this situation?

Take your matrix with its little bit of target analyte, and add increasing known amounts of analyte. Run as a calibration. When you plot peak area vs amount added the regression line will pass through the y axis at some none-zero value that corresponds to the amount of analyte present before you added any. You can now re-plot the calibration with the amount of analyte at each point as (amount added + amount already present), and then use that regression equation to calculate the amount in samples from their peak areas.

Peter

Thanks Peter.

I understand now.

Outside analysis: Send it to another laboratory for analysis. Iron is commonly analyzed and I use a contract laboratory to verify my results.

Regulatory rules: I don't know if you're in academia or industry, and what regulations you might have to follow.

Many thanks to your sharing, bluejay.

I work in a hospital clinical laboratory. We usually follow guidelines of US or European authorities.