Proton transfer reaction, PTR

Other MS-fingerprinting techniques that are in commercial development are based on atmospheric pressure ionisation (API), resonance-enhanced multiphoton ionisation (REMPI) TOE and proton-transfer reaction (PTR). They are rapid, sensitive and specific and allow measurements in real time and may play an increasingly important role in the future development of electronic noses and tongues. 

FIGURE 3.1 Quadrupole ion trap mass spectrometers are capable of concatenating multiple, unique ion-processing methods during routine, robust, day-to-day operation for example, a user can combine electron transfer dissociation (ETD), proton transfer reaction (PTR), collision-induced dissociation (CID), and ion attachment (lA) in any order in but a single scan. This ability highlights how trapped-ion instruments are capable both of mass analysis and of functioning as an ion reaction vessel. 

Proton transfer reaction (PTR), whereby an ionic reagent either abstracts or donates a proton to a precursor [34 3]... 

Within mass spectrometry, electron ionization (El) typically results in the extensive fragmentation of the molecule under investigation. Therefore, the interpretation of the mass spectra (i.e., confirmation of molecular weight) can be difficult. A way to avoid fragmentation is via chemical ionization (Cl), which is done in our case using proton transfer reaction (PTR) ionization. Similar to Cl, molecules that are ionized using PTR techniques typically form a protonated molecular ion (M+H), in... 

Not all ionization methods rely on unimolecular conditions as strictly as El does. Chemical ionization (Cl, Chap. 7), for example, makes use of reactive collisions between ions generated from a reactant gas and the neutral analyte to achieve its ionization by some bimolecular process such as proton transfer. The question which reactant ion can protonate a given analyte can be answered from gas phase basicity (GB) or proton affinity (PA) data. Proton transfer, and thus the relative proton affinities of the reactants, also play an important role in many ion-neutral conplex-mediated reactions (Chap. 6.12). In the last decade, proton transfer reaction (PTR) MS has emerged as a tool for analyzing volatile organic compounds (VOCs) in air. Therefore, PTR-MS is interesting for analytical work concerning environmental issues and in occupational health and safety (Chap. 7.3). 

Use of a proton transfer reaction (PTR), time-of-flight mass spectrometer and high sensitivity PTR quadruple mass spectrometer Use of sealed liquid cell for polymer analysis Analysis of ethanol derivative (spec A = 245 nm)... 

Karl, T., Yeretzian, C., Jordan, A., and Lindinger, W. Dynamic measurements of partition coefficients using proton-transfer-reaction mass spectrometry (PTR-MS), Int. J. Mass Spectrom., 223-224 383-395, 2003. 

MDGC, and comprehensive two-dimensional GC, or GCxGC), faster separation techniques (fast GG), fast methods for quality assessment or process control in the flavour area ( electronic noses and fingerprinting MS) and on-line time-resolved methods for analysis of volatile organic compounds (VOGs) such as proton-transfer reaction MS (PTR-MS) and resonance-enhanced multi-photon ionisation coupled with time-of-flight MS (REMPI-TOFMS). The scope of this contribution does not allow for lengthy discussions on all available techniques therefore, only a selection of developments will be described. 

Fig. 15.14 Analytical techniques for time-resolved headspace analysis. An electronic nose can be used as a low-cost process-monitoring device, where chemical information is not mandatory. Electron impact ionisation mass spectrometry (EI-MS) adds sensitivity, speed and some chemical information. Yet, owing to the hard ionisation mode, most chemical information is lost. Proton-transfer-reaction MS (PTR-MS) is a sensitive one-dimensional method, which provides characteristic headspace profiles (detailed fingerprints) and chemical information. Finally, resonance-enhanced multiphoton ionisation (REMPI) TOFMS combines selective ionisation and mass separation and hence represents a two-dimensional method. (Adapted from [190])...

Hence, most of the relevant proton-transfer reactions involving H3O+ are slightly exoergic, and H3O+ will perform proton-transfer reactions with nearly any kind of VOC in the headspace of food products. However, H3O+ does not react with the natural components of air such as O2, N2, CO2, CO or others (see Table 15.4). The exoergicity of the proton-transfer reaction with most VOCs, however, is low enough that breakup seldom occurs. On the basis of this ionisation principle, a PTR-MS setup was developed applicable to trace-gas analysis, and aimed at speed, sensitivity, versatility and simple handling. 

Lindinger, W., Fall, R., Karl, T.G. (2001) Environmental, food and medical applications of proton-transfer-reaction mass spectrometry (PTR-MS). Adv. Gas Phase Ion Chem. 4 1-48. 

A proton transfer reaction-mass spectrometer (PTR-MS) system has been developed which allows on-line measurement of VOCs with concentrations as low as a few pptv (parts per trillion by volume) (Hansel et al., 1998). The acute measurement sensitivity to VOCs and real-time characteristics make it a very powerful tool in both indoor and outdoor environmental research. 

These exothermic proton-transfer reactions occur on every collision with well known rate constants, having typical values 1.5 x 10"9cm3s 1 < k < 3 x 10 9 cm s 1. An additional advantage of using primary H30+ ions is that many of their proton-transfer processes are nondissociative, so that only one product ion species occurs for each neutral reactant In order to allow for an accurate quantification of the neutral reactants from measured primary and product ion signals, the reactions of H30+ with the neutrals must occur under well-defined conditions. This is assured in the PTR-MS system by allowing the H30+ reactions to proceed within a DT (Hansel et al., 1998). 

In general, any analytical equipment or procedure used in the field of natural products chemistry and environmental engineering is also helpful in aroma analysis 64,65 The history and principles of such art are described in detail elsewhere and will not be featured here. Gas chromatography (GC), GC-mass spectrometry (MS), and nuclear magnetic resonance (NMR) are the most frequently used techniques along with rather specialized setups such as proton transfer reaction-mass spectrometry66 (PTR-MS) used in retronasal aroma analysis (see Chapters 9.02, 9.06, 9.10-9.11). 

However, the desire to reduce or even eliminate the required sample preparation has led to the development of newer techniques, such as proton-transfer-reaction mass spectrometry (PTR-MS) and selected ion flow tube mass spectrometry coupled with ion mobility spectrometry (MS-SIFT IMS). These techniques allow real-time measurements within a single second. This enables analysis of gas composition during technological processes, monitoring of indoor air, and investigation of VOCs emitted by living organisms. 

Zhan, X., Duan, J., Duan, Y. Recent developments of proton-transfer reaction mass spectrometry (PTR-MS) and its applications in medical research. Mass Spectrom. Rev. 32, 143-165 (2013)... 

PTR-FTICR proton-transfer reaction Fourier transform ion cyclotron resonance... 

The traditional equilibrium method of flavor release study mentioned above is extremely time consuming, and several weeks are commonly needed to obtain full release profiles of flavors from powders. Recently, thanks to the pioneering work of Dronen and Reineccius (2003), proton transfer reaction mass spectrometry (PTR-MS) has been used as a rapid analysis to measure the release time-courses of flavors from spray-dried powders. The PTR-MS method has been applied extensively to analyze the release kinetics of volatile organic compounds from roasted and ground coffee beans. The release profiles could then be mathematically analyzed by means of Equation 1.1 to obtain the release kinetic parameters, A and n (Mateus et al., 2007). 

ENVIRONMENTAL, EOOD AND MEDICAL APPLICATIONS OF PROTON-TRANSFER-REACTION MASS SPECTROMETRY (PTR-MS)... 

The development of proton-transfer-reaction mass spectrometry (PTR-MS) as a tool for the analysis of volatile organic compounds (VOCs) is described. PTR-MS is based on the rapid, non-dissociative transfer of protons from H30 to most common VOCs, but not to the principal gases in the air sample. Recent developments in the design of PTR-MS instruments allow detection of some VOCs in the parts per trillion by volume range. This sensitivity and the capability of PTR-MS instruments to be operated for extended periods in both laboratory and field settings has allowed exploration of many aspects of VOC analysis in environmental, food and medical applications. 

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