Ultra-low Current Electrospray Ionization of Chloroform Solution for the
Analysis of Perfluorinated Sulfonic Acids
Abstract
Rationale. Femtoamp and picoamp electrospray ionization characteristics
of a non-polar solvent were explored. The direct ESI-MS analysis of
chloroform extract solution enabled rapid analysis of perfluorinated
sulfonic acid (PFS) analytes in drinking water. Methods. Neat chloroform
solvent and extracts were directly used in a typical wire-in ESI setup
using micrometer emitter tips. Ionization currents were measured with
femtoamp sensitivity while ramping the spray voltage from 0 to -5000 V.
Methanol was used to illustrate the characteristics of spraying
chloroform. The effect of spray voltage and inlet temperature was
studied. A liquid-liquid extraction workflow was developed to analyze
PFOS in drinking water using an ion trap mass spectrometer. Results. The
ionization onset of chloroform solution was 41 ± 17 fA at 300 V. The
ionization current gradually increased with voltage while remaining
below 100 pA when using voltage up to -5000 V. PFOS ion signal was
significantly enhanced to improve the detection limit to 25 ppt in
chloroform. Coupled with a liquid-liquid extraction workflow, detection
limits of 0.38-5.1 ppt, and a quantitation range of 5-400 ppt were
achieved for perfluorinated sulfonic compounds in 1 mL drinking water
samples. Conclusions. Femtoamp and picoamp modes expand the solvent
compatibility range of electrospray ionization and can enable
quantitative analysis in ppt concentrations.