Can Breath Isoprene Be Measured by Ozone Chemiluminescence?

Isoprene, involved in the biosynthetic pathway to cholesterol, is the prevalent hydrocarbon in breath. Breath
isoprene measurement is of great interest as a measure
of basal cholesterol production rate. We investigated the
merits and pitfalls of isoprene measurement via its chemiluminescence (CL) reaction with ozone. For many subjects, apparent concentrations measured are higher than
those obtained by a gas chromatography (GC) reference
method that can be traced to ozone-induced CL with
simultaneously present lower olefins and sulfur compounds. A warm column preconcentration method eliminates the lower olefins and greatly improves sensitivity
while a silver-form, ion-exchange resin can remove the
sulfur gases. The breath sample is captured on a miniature synthetic carbon sorbent column maintained at 55
°C, under which conditions ethylene, propylene, and water
vapor are not significantly captured while the preconcentration process greatly improves the limit of detection for
isoprene to 0.6 ppbv (S/N = 3). The captured isoprene
is released by heating the column to 150 °C. Breath
samples from different subjects were collected both before
and after meals and analyzed in a double-blind fashion
in two laboratories, with the second laboratory performing
quantitation by cryofocusing GC-flame ionization detection
with parallel measurement by mass spectrometry to
provide identity confirmation. For all individuals studied,
the CL and the GC results agreed when both warm column
preconcentration and passage through Ag+-form cation-exchange resin, which removes divalent sulfur gases, were
implemented prior to CL measurement. The intensity of
CL from the reaction with ozone can be much higher for
some sulfur gases than for isoprene. Even though present
at lower concentrations than isoprene, unless removed
prior to CL measurement, for some individuals sulfur
gases can cause unacceptably large (up to 500%) errors,
making the sulfur gas removal step critical.