Determination of oxytocin in a dilute IV solution by LC–MSn

The most common drug prescribed to induce labor in the United States is oxytocin, a peptide hormone composed of nine amino acids. Oxytocin is often reconstituted in intravenous (IV) saline solutions at less than 0.05 units ml−1 (125 ng ml−1) to be delivered at 1–4 drops per minute. Existing LC–UV methods for oxytocin do not have sufficient detection limits to quantitate and/or confirm oxytocin in IV solutions without sample concentration. A determinative and confirmatory method for oxytocin was developed using an LC–MSn ion trap instrument with an electrospray ionization (ESI) interface in positive ion mode. Separation was achieved on a C-18 column using an isocratic elution of water with 50% acetonitrile (v/v) and water with 0.05% formic acid (v/v) at a flow rate of 250 μl min−1. Data was acquired from the selected ion monitoring (SIM) of the precursor ion (m/z 1007.3) and MS2 scans from the collision induced dissociation of m/z 1007.3 at 30% collision energy. In this method, MS2 full scans were utilized to obtain three structurally significant ions for the unambiguous identification of oxytocin. Calibration standards, prepared in de-ionized water from 0.006 to 0.046 units ml−1, were linear with an R2 value of 0.9983. The methods LOD and LOQ were 0.00084 and 0.0029 units ml−1 (2 and 7 ng ml−1), respectively. This LC–MSn method was used to determine the amount of oxytocin in a 0.04 units ml−1 clinical sample that was prepared in 0.9% sodium chloride IV solution.