Analysis for underivatized theophylline by gas-chromatography on a silicone stationary phase, SP-2510-DA

I describe a flame-ionization gas-chromatographic method for the quantitative analysis of serum theophylline without the need for derivatization. The analysis is performed isothermally on a new silicone stationary phase, 2% SP 2510-DA (Supelco). With this liquid phase, gas-chromatographic properties and sensitivities are comparable to methods requiring derivatization. A salt-solvent pair of ammonium sulfate and methylene chloride/hexane/acetic acid (80:20:0.1 by vol) is used for extraction. Ammonium sulfate greatly reduces co-extractable interferences from icteric, lipemic, and hemolyzed serum. The density of methylene chloride is adjusted with hexane so that the solvent is less dense than the salt-saturated serum and can be removed by decanting. Acetic acid, used to adjust the pH of the serum samples, is combined with the extraction solvent rather than being added individually to the serum samples. Theophylline values obtained by this method agree closely with those determined by enzyme immunoassay (correlation coefficient, 0.988).