Headspace time-resolved

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])...

An alternative to chemical ionisation is resonant (and non-resonant) laser ionisation methods [179], i.e. selective and soft laser photoionisation, such as REMPI. A particularly interesting setup is the combination of REMPI with TOFMS for monitoring coffee brew headspace. This chapter deals with technical features and applications of time-resolved analytical methods with particular focus on PTR-MS and resonant and laser ionisation methods (REMPI-TOFMS). 

Yeretzian, C., A. Jordan, H. Brevard, W. Lindinger, Time-resolved headspace analysis by proton-transfer-reaction mass-spectrometry, in Flavor Release, A.J. Taylor, D.D. Roberts, Eds., Amer. Chem Soc., Washington, D.C., 2000, p. 58. 

Capillary columns are used to separate 1,1,1-trichloroethane from the other components in a mixture. Capillary columns provide wider versatility offering superior resolution of components. A comparison of capillary and packed column for analysis of volatile organics by GC is available (Clark and Zalikowski 1990). Narrow-bore capillary columns have high resolving power but may not be suitable for headspace analysis because of easy column saturation (Ohno and Aoyama 1991). Wide-bore capillary columns are suitable in such cases (Ohno and Aoyama 1991). Different detectors can be used ECD, HECD, and MS have been described. The MS is the most selective detector, but the HECD is the most sensitive. Both closed path and open path Fourier transform infrared spectrometry (FTIR) have recently been used for the determination of 1,1,1 -trichloroethane in air (Carter et al. 1992 Trocha and Samimi 1993 Xiao and Levine 1993). Although the FTIR methods have higher detection limits than some of the other conventional methods, they afford the opportunity of remote monitoring of real-time samples (Xiao and Levine 1993). 

The results from the combined analysis of the SMP headspace demonstrate that some ion masses can be attributed to a single compound and therefore quantified reliably. Other ion masses, however, correspond to several compounds and quantification is at best an estimate. This experiment shows that the strength of APIMS lies in its ability to monitor volatile compounds on-line in real time, but its weakness in food flavor systems is its inability to resolve the compounds with the same ion mass. 

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