ELECTRON-CAPTURE NEGATIVE IONIZATION MASS-SPECTROMETRIC CHARACTERISTICS OF BIOACCUMULATING METHYL SULFONE-SUBSTITUTED POLYCHLORINATED-BIPHENYLS

Gas chromatography/electron-capture negative ionization mass spectrome try (GC/ECNI-MS) was used to separate and detect a mixture of 24, 3-an d 4-methyl sulfone-substituted polychlorinated biphenyls (MeSO(2)-PCBs ). The dominant fragment ions were [M-CH3](-) and [M-Cl+H](-), and to a lesser extent [M-2Cl](-), [M-2Cl+2H](-), [M-CH3-Cl+H](-) and [M-SO2C H3+H](-)). The structures of the 3-and 4-MeSO(2)-PCBs varied in the de gree (i.e. tetrachloro-to heptachloro-) and position of chlorine subst itution. With the assistance of multivariate statistical analysis, the abundance of several isotopic clusters of fragment ions relative to t hat of the molecular ion (M(-)) was found to vary as a function of MeS O(2)-PCB structure. Compared with other congeners, the relative abunda nce of [M-CH3](-) was significantly lower for MeSO(2)-PCBs containing the 4-MeSO(2)-2,5-dichloro moiety, and the hexachloro-and heptachloro- MeSO(2)-PCBs had higher relative abundances of[M-Cl+H](-). Subtle ECNI mass spectral fragmentation rules therefore exist for 3-and 4-MeSO(2) -PCBs with respect to congener structure; however, both ion source tem perature and pressure must be controlled. Increasing the ion source te mperature from 100 to 260 degrees C resulted in an large increase in t he abundance of [M-CH3](-) relative to M(-) from <20% to >100%, for al l the MeSO(2)-PCBs studied. For the set of ten 3-/4-MeSO(2)-PCB pairs, the [M-CH3](-) relative abundance ratio was independent of ion source temperature, and was similar to 4.5 and similar to 1.5 for congeners possessing 2,5-dichloro and 2,5,6-trichloro substitution on the MeSO(2 )-containing phenyl ring, respectively. The effect of an increase in i on source temperature, over the same temperature range, on the relativ e abundance of hydrogen inclusion-type fragment ions was much less dra matic. [M-CH3-Cl+H](-) and [M-Cl+H](-) abundance often increased, but never exceeded similar to 40%. Decreasing the methane reagent gas pres sure from 0.5 to 0.05 mbar did not significantly alter the ion source temperature effect on the relative abundance of the fragment ions. How ever, the ECNI response sensitivity decreased by an order of magnitude . (C) 1997 by John Whey & Sons, Ltd.