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 (C 10 H 16 ) analyses was carried out. The monoterpenes were quantified by SIFT-MS using H 3 O + 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 H 3 O + 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 3carene 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.

The peppermint breath test benchmark for PTR-MS and SIFT-MS

“…The detection of monoterpenes by SIFT-MS has been previously demonstrated [15], but not in relation to the ion signals assignments for the peppermint target compounds. As such, preliminary SIFT-MS studies were undertaken using a gas standard (National Physical Laboratory, Teddington, UK).…”

Hyper-fast gas chromatography and single-photon ionisation time-of-flight mass spectrometry with integrated electrical modulator-based sampling for headspace and online VOC analyses

“…Please do not adjust margins Please do not adjust margins which is set in relation to the standard gas of 1 ppm. From 10 consecutive chromatograms, we obtain 2 ppb for toluene and 0.5 ppb for p-xylene, which is competitive with LODs from previously published fastGC setups with soft ionization mass spectrometry 9,17,18 or fast GC with sensor array detection 6 for analytes of similar volatility. It has been found that the photoionisation cross section at 126 nm of p-xylene differs by less than 25% of the one of toluene 19 , we attribute the lower LOD of p-xylene to the highe r trapping efficiency.…”

Robust Automated SIFT-MS Quantitation of Volatile Compounds in Air Using a Multicomponent Gas Standard

“…Under these conditions SIFT-MS analysis should be conducted with caution, and matrix volatiles should be included in the SIM method, including any observed secondary chemistry, so that potential interference and quantitation impacts can be assessed immediately. 42 Alternatively, a fast-GC separation stage 43 or a thermal desorption unit 44 could be added…”