Quantitative determination of pharmaceuticals using nano-electrospray ionization mass spectrometry after reversed phase mini-solid phase extraction

The pre-concentration effect of solid phase microextraction (SPE) with nano-electrospray ionization mass spectrometry (nano-ESI MS) for selected pharmaceuticals is presented. An analytical method is developed for the quantitative determination of dicyclomine in serum with cyclopentolate as the internal standard by off-line nano-ESI ion-trap MS with reversed phase mini-SPE. Homemade C18 and C4 mini-SPE cartridges of 0.5 and 1 cm in length and 1.55 mm i.d. have been tested for pre-concentration of samples originally 30 μL in volume. After SPE, the volume of the sample in methanol is about 1-2 μL and 0.5 μL can be injected into the nano-ESI MS instrument. Use of a 1 cm C18 cartridge lowered the detection limit of dicyclomine 100 times to 16.1 fmole. Dicyclomine spiked in serum can be determined by nano-ESI MS after protein precipitation and further clean-up on the 1 cm C18 cartridge. However, the slope of a calibration curve of dicyclomine standards spiked in serum is more than a factor of 10 less than that for a calibration curve of dicyclomine standards prepared in water indicating pre-concentration of pharmaceuticals could be compromised by a complex biological matrix.