An atmospheric pressure chemical ionization–ion‐trap mass spectrometer for the on‐line analysis of volatile compounds in foods: a tool for linking aroma release to aroma perception

Abstract

An atmospheric pressure chemical ionization ion-trap mass spectrometer was set up for the on-line analysis of aroma compounds. This instrument, which has been successfully employed for some years in several in vitro and in vivo flavour release studies, is described for the first time in detail. The ion source was fashioned from polyether ether ketone and operated at ambient pressure and temperature making use of a discharge corona pin facing coaxially the capillary ion entrance of the ion-trap mass spectrometer. Linear dynamic ranges (LDR), limits of detection (LOD) and other analytical characteristics have been re-evaluated. LDRs and LODs have been found fully compatible with the concentrations of aroma compounds commonly found in foods. Thus, detection limits have been found in the low ppt range for common flavouring aroma compounds (for example 5.3 ppt (0.82 ppbV) for ethyl hexanoate and 4.8 ppt (1.0 ppbV) for 2,5-dimethylpyrazine). This makes the instrument applicable for in vitro and in vivo aroma release investigations. The use of dynamic sensory techniques such as the temporal dominance of sensations (TDS) method conducted simultaneously with in vivo aroma release measurements allowed to get some new insights in the link between flavour release and flavour perception.

Modified proton transfer reaction mass spectrometry (PTR‐MS) operating conditions for in vitro and in vivo analysis of wine aroma

“…Moreover, some impurities present in ethanol increased the background noise of the determination ion that also affected the LOD calculation. Nevertheless, these LODs were in the range commonly found in APCI experiments . They were also in the range of the LODs determined for aroma compounds using quadrupole PTR‐MS or PTR‐ToF‐MS, and are fully compatible with the concentrations generally found for aroma compounds in foodstuffs and beverages.…”

“…Therefore, the formation of this 3‐methylbutanol dimer seemed more related to the concentration used in this study for the molecule rather than depending on the presence of ethanol. Supporting this hypothesis, the dimer‐related ions were also present when aqueous 3‐methylbutan‐1‐ol solutions at the same concentration were subjected to conventional PTR‐MS conditions (data not shown), and, moreover, it was recently demonstrated in APCI that the formation of proton bound dimer ions of oxygenated compounds was highly dependent on the concentration used …”

“…Moreover, contrarily to published data ethanol adducts (M + C 2 H 5 OH)H + resulting from association of M with C 2 H 5 OH 2 + or from ligand switching from ethanol cluster ions (C 2 H 5 OH) 2 H + were found insignificant as the (M + C 2 H 5 OH)H + ions and their dehydrated products never surpassed 1% of the main molecular ions (Table ). For these linear esters, the main pathway for ion production was clearly proton transfer from the reagent ions (C 2 H 5 OH)H + , their respective proton affinity (a value of 833 kJ/mol has been calculated for the PA of ethyl hexanoate and a measured value of 848–852 kJ/mol reported for ethyl butanoate) being favorable to this only pathway . These new findings concerning esters fragmentation could be rationalized in terms of lower used E/N values and for ethanol‐related cluster formation, in terms of temperature used in the drift‐tube, set to higher values than in SIFT instruments that generally determined product ion distributions of these reactions and to higher values than previous PTR‐MS studies conducted at a drift‐tube temperature of 323 K together with higher E/N values .…”

“…The linear dynamic curves for ethyl hexanoate in the hydro‐alcoholic solution and in water may be found in Supplementary Materials ‐ Figure S1. LODs were expressed as 3 times the standard deviation of the background noise, ie, calculated from the intensities of the determination ion m/z 145.122 measured in each background signal (6 values × 3 replicates), using the slopes determined for the linearity curves. Solution LODs of ethyl hexanoate were found at 0.314 ppb (μg/L) in the ethanolic solution (10% ethanol v/v) and at 0.047 ppb in water.…”

Proton-Transfer-Reaction Time-of-Flight Mass Spectrometry (PTR-TOFMS) for Aroma Compound Detection in Real-Time: Technology, Developments, and Applications

“…First and foremost, the rapid detection ability of a complete mass spectrum enables the entire complex mixture of aroma compounds to be monitored almost simultaneously, which is not achievable with quadrupole mass filter based instruments such as PTR-QMS and APCI-MS. It should be noted here that APCI-MS employing ion-trap systems also exist: these similarly allow full mass spectral scans to be performed within a fraction of a second and additionally offer the advantage of MS n capabilities for compound structure elucidation, but such systems are not widespread and the latter feature is mainly reserved for specific scientific study rather than routine use for flavour release applications (7). Secondly, the high mass resolving power afforded by the PTR-TOFMS, which is in the region of m/m 5000-6000, offers the possibility for seperating isobaric compounds and improving compound identification certainties in the analysis of complex gas matrices, as often encountered with food.…”