Determination of the β‐blocker betaxolol and labelled analogues by gas chromatography/mass spectrometry with selected ion monitoring of the α‐cleavage fragment ( m / z 72)

Low concentrations of betaxolol in blood plasma and physiological buffers were determined by selected ion monitoring of the intense m / z 72 fragment [CH 2 NHCH(CH 3 ) 2 ] + formed by electron impact ionization of the O ‐trimethylsilyl derivative. At a mass spectrometric resolution of 3000, fewer potentially interfering peaks were seen than at low resolution. There remained a chemical interference, corresponding to 100 pg/sample, which arose during treatment of the samples. This method is more sensitive than previous ones, but it is restricted to situations where the specificity can be controlled. When the m / z 72 fragment was mass‐shifted by using betaxolol appropriately labelled with deuterium or 13 C, both the interference and the baseline noise were greatly reduced; concentrations of labelled betaxolol as low as 10–20 pg/sample can be determined with little difficulty.