Positive-negative, chemical-ionization, direct exposure mass spectrometry of underivatized prostaglandins

Abstract

Nine underivatized prostaglandins were examined using direct exposure, ammonia, chemical-ionization, pulsed positive-negative ion mass spectrometry. The positive ion spectra were characterized by (M+18) + ion adducts. The negative ion spectra were characterized by ions which dependence upon the functionality present in the cyclopentane ring system (acetal for TXB 2). The E and D series prostaglandins gave (M-18) − as the major negative ion, while the F series and TXB 2 were characterized by negative ions corresponding to (M-1) −, and PGA 2 by the parent (M) − ion. Prostaglandin 6-keto-PGF 1α was anomalous in this respect showing apparent dehydration, interpreted as an overall (M-18+1) + and (M-18-1) − in the positive and negative ion spectra, respectively. All major ion types were shown to give essentially a linear response with respect to concentration in the 10–1000 ng range. Although these initial studies were conducted under ideal conditions, it would appear that direct chemical ionization techniques show promise for providing direct structural information on prostaglandins without the need for prior chemical derivation.