the succe of PTR_有没有beofsuccess
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the succe of PTR-MS triggered interest in further improving its performance.indeed,PRT-MS is a one-dimensional technique,and ions from a complex headspace,e.g.coffee,can often only be tentatively aigned.ions from different compounds(parent and fragment ions)can overlap in PTR-MS and prevent an unambiguous identification of VOCs in a complex mixture(198).Therefore,the attempt has been made to addre this problem and propose an extension of PTR-MS which allows for an unambiguous identification of headspace compounds.this is achieved by coupling GC with simultaneous PTR-MS and EI-MS detection.in this chapter,we can only introduce the basic features of the new setup:a technical and analytical extension of PTR-MS which removes this shortcoming,while preserving its salient and unique features.combining separation of VOCs by GC with simultaneous and parallel detection of the GC effluent by PTR-MS and EI-MS,an unambiguous inerpretation of complex PTR-MS spectra becomes feasible.A more detailed description of characteristic performance parameters,such as resolution,linear range and detection limit,has been published in a recent paper(199).as an example,the novel setup was applied to the characterisation of coffee headspace as a complex food system basically,an aliquot of the headspace is rapped in defined time periods on several tenax adsorbents for characterisation by GC-MS。figure 15.17 shows the simultaneously recorded total ion counts of the EI—MS（top frame）and PRT-MS（bottom frame）for VOCs trapped on the first tenax cartridge.The GC-separated pure compounds are identified by
thcomparison of their EI-MS fragmentation patterns with the Wiley database(Wiley 7edition)as
well as their retention indices obtained with reference compounds.The PTR-MS spectrum allows the PTR-MS fragmentation pattern of the GC-separated purecompounds to be identified.GC-traces over the entire 40 min of the GC run are shown in Figs.15.18 and 15.19,for the compounds desorbed from Tenax cartridge no.1(trapping time window between 1 and 3 min).The data reveal that the PRT-MS ion signal at m/z 111 is a superposition of ions originating from two different compounds,i.e.2-acetylfuran and 5-methylfurfural, contributing with 29 and 71%,respectively,to the total ion peak intensity at m/z 111.Similarly,the PRT-MS ion signal at m/z 87 is a superposition of 57% 2-methy1-propanal and 39% 2-butanone,with traces from 4-methyl-2-pentanone and 2-methyl tetrahydrofuran-3-one(2% each).While the single PRT-MS traces shown in Fig.15.18 represent a superposition of several compounds,a series of PTR-MS ion traces are shown in Fig.15.19 that are nearly pure(more than 89%),indicating that eentially only one single compound contributes to the ion signal(with only traces from other VOCs).Hence,in an on-line PTR-MS measurement of coffee headspace ,the ion maes at m/z68,75,80 and 95 can be aigned to pyrrole,acetol,pyridine and 2-methylpyrazine, respectively.The coupling of PTR-MS with GC-MS,as introduced here,allows identification and quantification of the VOCs that contribute to a single PRT-MS ion signal.Resonance-Enhanced Multiphoton lonisation
Time-of-Flight Ma Spectrometry
Selective and time-resolved monitoring can be achieved by REMPI at 266 nm coupled to a direct-inlet TOFMS device.Selectivity was introduced into the ionisation step by resonant ionisation at a fixed UV laser wavelength.The photoexcitation energy scheme for REMPI is illustrated in Fig.15.20.Depending on molecular resonances,VOCs with an optical(electronic)absorption at 266 nm absorb a laser photon,while those transparent at 266 nm remain in the ground state.The width of
optical absorptions is given by the ground-state population ,and broadens with the molecule's temperature,which itself depends on the expansion conditions at the inlet system.Since an effusive molecular beam was used(no cooling),a range of rotational and vibrational states was populated,resulting in broad absorption bands.Consequently,a range of compounds may be ionised simultaneously,owing to overlapping absorption bands[200].Technical reviews on REMPI can be found in the literature[200-202]
In a typical REMPI scheme, molecules absorb a first photon and are excited into a UV electronic state.There excited molecules are subsequently ionised by absorbing a second photon.For effective and selective REMPI detection,the following conditions have to be fulfilled:
1.Resonance condition:the molecule has a UV-active excited state,whose energy corresponds to the energy of the laser photon.2.Lifetime condition:the excited state has a lifetime which is long enough for it to absorb a second photon for ionisation.3.Ionisation condition :the energy of two photons is equal to or higher than the ionisation energy of the molecule.The on-line VOC sampling depicted in Fig.15.21 gives a schematic overview of the experimental setup,to illustrate the sampling of the roaster gas and the introduction of the volatiles into the TOP ma spectrometer[203].A quartztube with a paivated inner surface of 10-mm inner diameter was used to sample gas from the roaster.The tube reached about 2 cm into the tube to prevent solid contamination such as dust or silver skins reaching the capillary inlet system.All sampling lines were heated to 250℃，to minimise condensation of low-volatile compounds.A typical REMPI at 266 nm ma spectrum is shown Fig.15.22,obtained by roasting 80g of Arabica coffee at 225℃.The laser power density was adjusted to 106-107w/cm2 in order to avoid non-resonant ionisation procees.The spectrum contains predominantly molecular ions.Chemical aignment of the ion peaks was based on three distinct pieces of information：the literature on coffee flavour compounds[204],the ma as observed in TOFMS and optical absorption properties.With this information,many volatiles observed in Fig.15.22 were unambiguously identified.A full three-dimensional representation-ma,time,intensity-of a typical roasting proce at 200℃,recorded at 10 Hz by REMPI at 248 nm is shown in Fig.15.23,panel a [179].Characteristic cro-sections through the three-dimensional surface are in Fig.15.23,panels b and c.Figure 15.23,panel b gives a cro-section of the roast gas composition at a fixed time(approximately 12min).In Fig.15.23,panel c two cro-sections at fixed maes m/z 94 and m/z 150 are shown, corresponding to t-I profiles of phenol and 4-vinylguaiacol.
