Addition of fast gas chromatography to selected ion flow tube mass spectrometry for analysis of individual monoterpenes in mixtures
Abstract. Soft chemical ionization mass spectrometry (SCI-MS)
techniques can be used to accurately quantify volatile organic compounds
(VOCs) in air in real time; however, differentiation of isomers still
represents a challenge. A suitable pre-separation technique is thus needed,
ideally capable of analyses over a few tens of seconds. To this end, a bespoke
fast gas chromatography (GC) instrument with an electrically heated 5 m long metallic capillary column was
coupled to selected ion flow tube mass spectrometry (SIFT-MS) measurements. To assess the
performance of this combination, a case study of monoterpene isomer
( C10H16 ) analyses was carried out. The monoterpenes were
quantified by SIFT-MS using H3O+ reagent ions (analyte ions
 C 10 H 17 + , m∕z  137, and C 6 H 9 + , m∕z  81) and NO+ 
reagent ions (analyte ions C 10 H 16 + , m∕z  136, and
 C 7 H 9 + , m∕z  93). The combinations of the fragment ion relative
intensities obtained using H3O+ and NO+ were shown to be
characteristic of the individual monoterpenes. Two non-polar GC columns
(Restek Inc.) were tested: the advantage of MXT-1 was shorter retention,
whilst the advantage of MXT-Volatiles was better separation. Thus, it is
possible to identify components of a monoterpene mixture in less than 45 s
using the MXT-1 column and to separate them in less than 180 s using the
MXT-Volatiles column. Quality of the separation and the sensitivity of present
technique (limit of detection, LOD, ∼16  ppbv) was found to be inferior compared to
commercially available fast GC solutions coupled with proton transfer
reaction mass spectrometry (PTR-MS, LOD ∼1  ppbv) due to the limited
sample flow through the column. However, using combinations of two reagent
ions improved identification of monoterpenes not well resolved by the
chromatograms. As an illustrative example, the headspace of needle samples
of three conifer species was analysed by both reagent ions and with both
columns showing that mainly α -pinene, β -pinene and 3-carene
were present. The system can thus be used for direct rapid monitoring of
monoterpenes above 20 ppbv, such as applications in laboratory studies of
monoterpene standards and leaf headspace analysis. Limitation of the
sensitivity due to the total sample flow can be improved using a multi-column
pre-separation.