Direct, Isomer-Specific Quantitation of Polycyclic
Aromatic Hydrocarbons in Soils Using Membrane Introduction Mass Spectrometry
and Chemical Ionization
posted on 2020-10-20, 04:44authored byGregory
W. Vandergrift, Erik T. Krogh, Chris G. Gill
Polycyclic aromatic hydrocarbons
(PAHs) are routinely screened
for in soils, where quantitation of structural isomers is critical
due to varying toxicity within PAH isomer classes. While chromatographic
methods provide isomer resolution, such strategies are cost and time
intensive. To address these challenges, we present condensed phase
membrane introduction mass spectrometry using liquid electron ionization/chemical
ionization (CP-MIMS-LEI/CI) as a direct mass spectrometry technique
that provides rapid, quantitative results for PAH isomer measurements
in soil samples. A methanol acceptor phase is flowed through a probe-mounted
polydimethylsiloxane hollow fiber membrane directly immersed into
a dichloromethane/soil slurry. PAHs and dichloromethane co-permeate
the membrane into the acceptor solvent, whereas particulates and charged
matrix components remain in the sample. A nanoflow of the membrane
permeate is then directly infused into a LEI/CI interfaced triple
quadrupole mass spectrometer. Diagnostic PAH adduct ions were formed
at either M + 45 ([M + CH2Cl + CH3OH–HCl]+) or M + 47 ([M + CHCl2–HCl]+). This allowed the development of specific MS/MS transitions for
individual PAH isomers. These transitions were subsequently used for
the direct analyses of PAHs in real soils where CP-MIMS-LEI/CI was
shown to be rapid (15 soil samples/h) and sensitive (ng/g level detection
limits). CP-MIMS-LEI/CI results compared well to those obtained using
GC–MS (average percent difference of −9% across 9 PAHs
in 8 soil samples), presenting a compelling argument for direct, quantitative
screening of PAHs in soils by CP-MIMS-LEI/CI, particularly given the
simple workflow and short analytical duty cycle.