Optimisation of a thermal desorption–gas chromatography–mass spectrometry method for the analysis of monoterpenes, sesquiterpenes and diterpenes

Abstract

<jats:p>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 (kO3) of ent-kaurene was experimentally estimated to be 2 orders of magnitude greater than the respective literature kO3 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. </jats:p>

OH reactivity from the emissions of different tree species: investigating the missing reactivity in a boreal forest

“…Volatile organic compounds (VOCs) were measured with two in situ gas chromatographs coupled with mass spectrometers (GC-MSs), which have been previously described in more detail by Hellén et al (2017Hellén et al ( , 2018. One GC-MS measured the concentrations of mono-and sesquiterpenes, isoprene, 2methyl-3-butenol (MBO), and C 5-10 aldehydes in the emissions.…”

“…These compounds were calibrated as well with standards in methanol solutions. The detection limits are in the 1-130 ppt v range, and the uncertainty is 32 %-76 % (Hellén et al, 2017).…”

Sesquiterpenes and oxygenated sesquiterpenes dominate the emissions of downy birches

“…13.5 m length, 3/8" i.d). The target compound losses in these sampling lines and enclosure are expected to be negligible as demonstrated by the acceptable recoveries observed in the laboratory tests (Helin et al, 2020;Hellén et al, 2012), and since high flow rates were used.…”

“…Stainless steel lines inside the online box of the TD unit were changed to FEP tubing and an empty sorbent tube used in the TD inlet line was changed to glass coated stainless steel tube. The optimization of the method is described in Helin et al (2020). These TD unit changes and the use of a GC column with a lower film thickness enabled the measurement of OSQTs.…”

“…(offline sorbent tube analysis, e.g. Helin et al (2020)). These measurements were semi-quantitative and they were only used for qualitative analysis of terpene emission patterns.…”