Analytical Chemistry Concentration Calc Question

[TrueStar]

A lab mate and I are discussing how to properly interpret LCMS results and coming to different conclusions.

We are taking a solid food sample and extracting it with 50:50 acetonitrile/water to determine the concentration of melamine in the food sample. Standard solutions were made that range between 0 and 5ppm of melamine.

The question is whether we read the LCMS results at face value. For example, we could take 100mg of food sample and extracted it with 10mL of solution. If the LCMS reads that the sample has 0.2ppm of melamine in the sample, I don't think that represents what was in the solid. I think we would have to calculate it as the mass of melamine in the LCMS sample to be 0.002mg and because we used 100mg of food sample, then that means the food sample actually contains 20ppm of melamine.

So the question is do we read the LCMS results at face value or do a calculation based on the amount of food sample used?

 

[Borek]

If you have calibrated your detector with the standard solutions, the ppm you have measured is the concentration in the solution, (in your case the extract), not in the sample itself.

 

[mjc123]

Yuo will have to be more specific about what you mean by your terms - especially "sample". What output does the LCMS actually give? It doesn't know how you have treated your sample, so it can't give a reading of 0.2 ppm in the [food] sample. What it measures is amount of melamine injected, which depends on solution concentration and injection volume. If you have done a calibration for constant injection volume of solutions of different concentration, you will get a value for the concentration of your solution, so 0.2 ppm in the [solution] sample would imply a mass of melamine in the solution of about 2 µg. (Note, not exactly 2 µg, if it's ppm by weight, as the density of 50:50 AN/water is not 1 g/mL. Are you working with ppm w/v? You seem to be working with ppm w/w as regards the food sample.)
You don't seem to really understand the procedure you're using. Go back and check how the LCMS was calibrated. What does "0.2ppm of melamine in the sample" actually mean?

 

[TrueStar]

If you have calibrated your detector with the standard solutions, the ppm you have measured is the concentration in the solution, (in your case the extract), not in the sample itself.

Yes, we created a curve with our standard solutions. I looked around a bit more and it sounds very similar to a situation like this -- http://www.chemicalforums.com/index.php?topic=49489.0

I think we should be doing the same thing.