Monitoring serum valproic acid by gas chromatography with electron-capture detection

I have developed a procedure to monitor, by gas chromatography with electron-capture detection, the concentration of valproic acid in serum. The valproic acid is derivatized into its phenacyl ester. The assay is very sensitive, measuring as little as 100 pg of valproic acid easily and accurately. Solvent extraction and concentration of the drug are not necessary; instead, one only has to precipitate the serum proteins. n-Octanoic acid is used as the internal standard. The peak-height ratio of phenacyl valproate to phenacyl octanoate is linearly related to concentration up to 200 mg of valproic acid per liter.