The construction,
critical evaluation, and performance assessment
of a medium-pressure (2–13 mbar), high-temperature chemical
ionization (CI) source for application in GC-MS is described. The
ion source is coupled to a commercial time-of-flight (TOF) mass analyzer.
Reagent ions are generated in a two staged process. The first stage
uses a filament free, helical resonator plasma (HRP) driven ion source
for H3
+ generation. Reagent gases, for example,
nitrogen, isobutane, and methane are added in a second stage to the
H3
+ stream, which leads to the formation of
final protonation reagents. The GC effluent is added subsequently
to the reagent ion gas stream. Designed for the hyphenation with gas
chromatography, this GC-CI-TOFMS combination produces GC limited Gaussian
peak shapes even for high boiling point compounds. Limits of detection
for the compounds investigated are determined as 0.4–1.2 pg
on column with nitrogen, 0.6–12.6 pg with isobutane, and 2
pg to >25 pg with methane as reagent gas, respectively. An EPA
8270
LCS mix containing 78 main EPA pollutants is used to evaluate the
selectivity of the different reagent ions. Using nitrogen as reagent
gas, 74 of 78 compounds are detected. In comparison, 41 of 78 compounds
and 62 of 78 compounds are detected with isobutane or methane as CI
reagent gas, respectively.