Unexpected dimerization of isoprene in a gas chromatography inlet. A study by gas chromatography/mass spectrometry coupling

Abstract

During analysis of pure isoprene by gas chromatography/mass spectrometry (GC-MS) using a programmed temperature vaporization (PTV) inlet, the presence of several isoprene dimers was detected in the total ion chromatograms (TICs). This study intends to determine the part of the instrument where dimerization occurs and the relative importance of the dimer amounts under different experimental conditions. The reference thermal dimerization of isoprene gives four six-membered cyclic dimers and two eight-membered ones. In all samples containing different amounts of freshly distilled isoprene, only peaks corresponding to the former appeared in TICs. For the same temperature, their amounts increase as the concentration of injected isoprene increases. The main products are diprene (from 80 to 100%) of the total dimers and dipentene (from 1 to 14%). The sum of the two other dimers is never higher than 6%. In conclusion, isomeric dimers are produced through a dimerization in the inlet. No dimerization of isoprene occurs in the mass spectrometer source. Then care is needed when analyzing terpenic compounds in the presence of isoprene by GC-MS because structures, retention times and mass spectra of diprene and dipentene are close.

Dimerization Products of Chloroprene are Background Contaminants Emitted from ALTEF (Polyvinylidene Difhioride) Gas Sampling Bags

“…This suggests that the intact chloroprene spiked into the bag, and then transferred to the Tenax tube was not dimerized during the thermal desorption of the sorbent tube. Estevez and colleagues reported that the same type of dimerization reactions occurred in isoprene, a structural analogue of chloroprene, during the GC analysis where the injection port was maintained at 150 to 300 C. 39 However, none of these isoprene dimers were detected in our breath samples analyzed where isoprene was one of the major VOCs. 20,22 Moreover, the absence of the monomer in the ALTEF bags analyzed strongly suggests that the chloroprene dimers detected from the bags had been formed during the manufacturing process of the bags.…”

Production of highly oxygenated organic molecules (HOMs) from trace contaminants during isoprene oxidation

“…As will be shown below, this route accounts for two-thirds of the total signal detected at m/z = 137.133, which is equivalent to 2 % relative to the isoprene signal. On the other hand, one-third of the total C 10 H + 17 signal or 1 % (relative to isoprene) is attributed to dimerization of isoprene (Estevez et al, 2014, and references therein) inside the gas bottle, which forms [4 + 2] cycloaddition products. The amount of dimerization inside the gas bottle was estimated from a chamber experiment during which the cryotrap was switched off at the beginning and then turned on for the last hours of the measurement.…”

“…Cycloaddition of isoprene has been well documented (Citroni et al, 2007;Compton et al, 1976;Estevez et al, 2014;Groves and Lehrle, 1992;Walling and Peisach, 1958), and thermally induced polymerization in the gas phase has recently been shown for heated gas chromatography inlets. The dimerization leads predominantly to six-membered rings, mainly the [4 + 2] Diels-Alder products sylvestrene and limonene (Estevez et al, 2014, and references therein). This known dimerization is the reason for the addition of p-tert-butylcatechol (TBC) to the liquid isoprene (Sigma Aldrich, 99.8 % purity, with addition of 139 ppm TBC), which was used for creation of the gas standard provided by the supplier.…”

Reply to RC1, RC2, RC3

“…This reaction can explain the observed compound at m/z = 137.133 and m/z = 81.070 (C 6 H + 9 ). Cycloaddition of isoprene has been well documented (Citroni et al, 2007;Compton et al, 1976;Estevez et al, 2014;Groves and Lehrle, 1992;Walling and Peisach, 1958), and thermally induced polymerization in the gas phase has recently been shown for heated gas chromatography inlets. The dimerization leads predominantly to six-membered rings, mainly the [4 + 2] Diels-Alder products sylvestrene and limonene (Estevez et al, 2014, and references therein).…”