A newly developed, mobile laser mass spectrometer (resonance-enhanced multiphoton ionization - time-of-flight mass spectrometer, REMPI-TOFMS) was applied to on-line measurements at a waste incineration pilot plant. REMPI-TOFMS combines the optical selectivity of resonance-enhanced multiphoton ionization with a time-of-flight mass analysis to give a two-dimensional analytical method. Special care was taken to build up a sampling and inlet system suitable for on-line measurements of large, semivolatile polycyclic aromatic hydrocarbons (PAHs). An effusive molecular beam inlet in combination with a fixed frequency UV laser (Nd:YAG at 266 nm or KrF excimer at 248 nm) was used. Under these conditions, many different PAHs can be ionized selectively from the complex flue gas matrix. For example, the achieved detection limit for naphthalene is in the 10 parts-per-trillion by volume (pptv) concentration range. Calibration was performed by using external concentration standards supplied in low ppbv concentrations. The instrumentation is sufficiently robust to be operated under industrial conditions at incineration plants, for instance. The REMPI mass spectra can be acquired at 5-50 Hz. Time profiles of the concentrations of different PAHs in the flue gas were monitored with a time resolution of 200 ms. Significant variations in the concentration profile of several PAHs up to mass 276 amu (e.g., benzo[ghi]perylene) and methylated PAHs have been observed while combustion parameters were changing. In summary, it was demonstrated that laser mass spectrometry (REMPI-TOFMS) enables a real-time on-line trace analysis of combustion flue gases or industrial process gases.

No abstract

Resonance-enhanced multi-photon ionization time-of-flight mass spectrometry (REMPIITOFMS) has been applied to the detection of volatiles in the headspace of brewed coffee and in the coffee roasting process-gas. A frequency quadrupled Nd : YAG laser (266 nm) was used for REMPI ionization (REMPI@266nm) of the volatiles in an effusive molecular beam inside the ion source of a linear TOF mass spectrometer. A special sampling system provided a time correlated sampling. Under these circumstances REMPI@266nm is highly selective for ionization of phenolic compounds. Several phenolic compounds, such as the flavour-active 4-vinylguaiacol, can be detected in the headspace of coffee brew as well as in the roast off-gas with the application of this approach.Moreover, the nitrogen heterocyclic compounds, indole and caffeine, were detected in both cases.During the roasting process the relative changes in concentration of some volatile components of coffee have been recorded by EMPIm266nm with a time resolution of 1 Hz. The different volatiles exhibit characteristic concentration profiles as a function of the roast time. These results demonstrate the applicability of REMPI-TOFMS for on-line monitoring of coffee processing technologies. Such an on-line monitoring technique is of particular interest for process-control purposes, e.g. quality-protection or feedback process control. For example, monitoring of off-gases from the coffee roast process or monitoring of certain unit operations during the instantcoffee manufacturing could be promising industrial applications.Next to wheat, coffee represents the second most important food in world trade.' Taking the high value of coffee and its world production of 5 . 4~ lo6 tons into account, the large commercial importance of coffee-processing is obvious. This is particularly true for the roast process, as the typical coffee flavour mostly arises during the roasting of the green coffee beans. Standard roast processes last 8-12 min at 180-240 "C. Compared to this, more progressive, faster processes take 2-3 min at 300 OC.' During the roast process, thousands of different chemical compounds are are produced by pyrolysis of the organic material of the green coffee beans. About 800'. volatile compounds have been identified and many of them contribute to the coffee flavour.The analysis of coffee volatiles is still a challenge for modem trace analysis. Usually, extensive and time-consuming enrichment, sample preparation and clean-up procedures (adsorptive trapping, steam distillation, fractionation. liquid chromatographic separations etc.) are necessary prior to an instrumental analysis by methods such as gas chromatography/mass spectrometry (GUMS) or highperformance liquid chromatography (HPLC).' A lot of work has been done, using these highly sophisticated analytical techniques. to identify compounds that are decisive for the unique taste and flavour of coffee and, therefore, probably most of the major flavour-active substance classes are known.Due to the fundamental importance of the roast ...

No abstract