Caffeine and paraxanthine in plasma

[chromeleon]

I found a paper which precipitates the protein using perchloric acid. The author claims it works so well that there's no need for an internal standard. Is this common practice?

I still intend on doing % recovery... run a standard and also process some blank plasma spiked with the same concentration, and compare the two of those.

[tom jupille]

Internal standards are a mixed blessing. They can cancel out correlated errors, but they excerbate independent (uncorrelated) errors. Since you have two peaks, a simple test is to look at the ratio of the areas. If the precision of the ratio is better than that of the areas, then an internal standard will help. If the areas look better than the ratios, then an internal standard will hurt.

[carls]

If you're using MS detection and NOT using a stable labeled version of your analyte as an internal standard then you'll have to run a spike of each sample since matrix effects are highly sample dependent.