Optimisation of a thermal desorption–gas chromatography–mass spectrometry method for the analysis of monoterpenes, sesquiterpenes and diterpenes
Abstract. In this study, a thermal desorption–gas
chromatography–mass spectrometry (TD–GC–MS) method following sorbent tube
sampling was developed for the determination of monoterpenes (MTs),
sesquiterpenes (SQTs) and diterpenes (DTs) in gas-phase samples. The
analytical figures of merit were determined, and the method performance was
tested by conducting experiments related to, for example, sampling recovery,
storage stability and ozone reactivity. The limit-of-quantification values
were 13–518 pg (0.5–9.3 pptv), intermediate precision was in the range of
3 %–10 % and the expanded measurement uncertainty was in the range of
16 %–55 % for terpenes. The sampling recoveries of terpenes were
approximately within 100±20  % with different inlet lines (15 m
long Teflon and 1 m long heated stainless steel) and branch enclosure
cuvette (6 L Teflon bag) tested. Ozone is an important factor causing losses
of the studied compounds during sampling. Therefore, losses of terpenes upon
ozone exposure were studied and the reaction rate coefficients were
estimated. The ozone reaction rate coefficient ( k O 3 ) of ent-kaurene was
experimentally estimated to be 2 orders of magnitude greater than the
respective literature k O 3 value, demonstrating the potential
underestimation of DT contribution to atmospheric reactivity. The
preliminary comparison between offline- and online-mode TD–GC–MS sampling and
analysis revealed that diterpenes and oxygenated sesquiterpenes are lost in
excessive amounts in online-mode sampling, hindering the online-mode
applicability for the quantitative analysis of these compounds. A few
applications to real samples were tested to identify DTs potentially emitted
by boreal forest tree species. In dynamic headspace samples of pine needles
and spruce twigs heated to 60  ∘ C, five DTs and 13 DTs could be
detected in emissions, respectively. The semi-quantitatively estimated
emission rates of DTs were roughly 1 to 3 orders of magnitude lower than
those of MTs and SQTs. Similarly, in spruce branch enclosure emissions from
a living tree, six DTs were detected once the enclosure was heated to ca. 60  ∘ C. In summary, the developed analytical procedure was
demonstrated to be applicable for the analysis of MTs, SQTs and DTs. In
addition, DTs could be detected in needles, twigs and branch enclosure
emissions; however, high temperatures were required to promote the emissions
and for obtaining detectable concentrations.