Holger Nagel

Application of Laser Ionization Mass Spectrometry for On‐line Monitoring of Volatiles in the Headspace of Food Products: Roasting and Brewing of Coffee

Resonance-enhanced multi-photon ionization time-of-flight mass spectrometry (REMPI/TOFMS) 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 EMPI@266nm 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 instant-coffee manufacturing could be promising industrial applications.

On-line emission control of combustion processes by laser-induced resonance-enhanced multi-photon ionization/mass spectrometry

Laser-induced resonance-enhanced multi-photon ionization (REMPI) represents an efficient and highly compound selective (even isomer discrimination can be achieved) ionization method and therefore is ideally adapted as an ion source for analytical time-of-flight mass spectrometry (TOFMS). The combination of mass and optical selectivity (high-resolution UV spectroscopy is involved in the REMPI-ionization process), its high sensitivity, and the high measurement speed makes the REMPI/TOFMS approach a powerful tool for on-line emission control of combustion processes. A REMPI/TOFMS-based monitor for highly time resolved analysis (sampling rate 50 Hz) of organic and inorganic compounds in the exhaust gas of internal combustion engines is already in operation; an exhaust-gas measurement is presented in the paper. However, recent investigations focus on the question of whether the REMPI/TOFMS technique combined with a supersonic Jet-inlet system (Jet-REMPI/TOFMS) is also applicable for continuous realtime monitoring of highly toxic combustion trace products. Relevant examples are polycyclic aromatic hydrocarbons (PAH, e.g., benzo[a]pyrene) and chlorinated aromatics (e.g., chlorinated dibenzo-p-dioxins/furans, PCDD/F). Such an on-line monitor is currently needed for emission control of industrial combustion processes as, for example, waste incineration (WI) facilities. In addition to some technical improvements (e.g., increase in on-line sensitivity or coupling with gas chromatography), one major prerequisite for practical applications is a thorough knowledge of the spectroscopic properties of the relevant target compounds. This contribution focuses on these spectroscopic preconditions. In conclusion, concepts for an on-line emission monitoring of the PCDD/F-content in flue gases of WI plants by means of the Jet-REMPI/TOFMS technique are presented. They are based on either the indicator parameter relationship between, for example, chlorinated benzenes and PCDD/F in the WI-exhaust gases (real-time, on-line technique) or a fast, repetitive discontinuous trapping and enrichment before a fast gas chromatography-Jet-REMPI/TOFMS detection.

FAST MONITORING OF MOTOR EXHAUST COMPONENTS BY RESONANT MULTI-PHOTON IONISATION AND TIME-OF-FLIGHT MASS SPECTROMETRY

Abstract A new analytical procedure is provided by the combination of two types of spectroscopy. Resonant ionization of selected compounds by multiphoton ionization is based on results of absorption spectroscopy for the compound molecules of interest and time-of-flight mass spectrometry serves for the unambigious detection of these compounds. An interesting application of this method is the fast exhaust gas analysis. In the development of future combustion engines, the management of dynamic motor processes becomes predominant because by more than 90 % of all the dangerous exhaust pollutions are produced in instationary motor phases such as fast speed or load changes. The investigation of dynamic processes however, requires fast analytical procedures with millisecond time resolution together with the capability to measure individual components in a very complex gas mixture The objectives for a development project of such an instrument were set by the Research Association for Combustion Engines (Forschungsvereinigung Verbrennungskraftmaschinen, FVV, Germany): Up to ten substances should be monitored synchroneously with a time resolution of about 10 milliseconds, with concentration limits of 1 part per million and with a precision better than 10 % relative standard deviation. Such a laser mass spectrometer for fast multi-component automotive exhaust analyses has been developed in a joint research project by Bruker-Franzen Analytik GmbH, Dornier GmbH and the Technical University of Munich. The system has been applied at a motor test facility to investigate the emissions of the aromatic hydrocarbons benzene, toluene and xylene, of nitric oxide and acetaldehyde in stationary and dynamic engine operation. These measurements demonstrate that strong emission of these pollutants takes place at instationary engine operation and in particular that these compounds are emitted at different times, giving new information about the processes in the combustion chamber and in the exhaust pipe.

A pyrolysis cell as simulator for an automobile catalytic converter

Abstract We studied the behaviour of some aromatic hydrocarbon molecules (benzene, toluene and xylene) and their reaction with water as a function of temperature and water content in a pyrolysis cell. We also investigated these substances as parts of two types of unleaded gasolines of two brands available in the Portuguese market.

Fast Exhaust Gas Probe for Multicomponent Analysis: Scientific/Technical Principle

A new analytical method for the automobile industry has been developed using a pulsed tunable laser system and a Reflectron time-of-flight mass spectrometer. The goal was to achieve the following conditions: High time resolution (less than 100 ms), high sensitivity (down to 1 ppm), high accuracy (10%) and applicability to most exhaust emission components. The main problem is the large number of components with very different and fast varying concentrations. For a preliminary list of 25 exhaust emission components, all necessary parameters have been determined. First results obtained from a real exhaust gas sample will be presented. (A) For the covering abstract see IRRD 858138.

A quantitative calibration method for resonant multiphoton ionization

A technique for the fast quantitative analysis of gas mixtures by means of laser mass spectrometry is presented. The signal ratio of two calibration gases with different orders of multiphoton ionization processes, which are added to the sample gas, serves as a sensor for the laser intensity within the ionization volume. While thereby strong shot‐to‐shot signal fluctuations due to higher order MPI processes can be normalized, relative measurement of the sample signal in respect to one of the calibration gases eliminates long term drifts of the apparatus. Quantitative accuracy better than 10% could be achieved using a single lasershot.

Resonance ionization and time‐of‐flight mass spectrometry for the analysis of trace substances in complex gas mixtures

The analysis of mixtures of technical gases still comprises a lot of problems: the large number of components with very different and often rapidly varying concentrations makes great demands on analytical methods. By use of conventional analytical methods, signals of trace substances may interfere with signals of main components, whereas small signals representing low concentrations are covered by signals of main substances.The resonant‐enhanced multiphoton ionization (REMPI) makes use of excited intermediate states of molecules. As these states are characteristic of each substance, one or more components of interest can be ionized with high efficiency without interference of other molecules by using a special laser‐wavelength. The combination of the above mentioned ionization method with a reflectron time‐of‐flight mass spectrometer permits a very fast and sensitive detection of preselected trace substances.As ionization processes of higher order strongly depend on the laser intensity, there is no direct...

Resonante Laser-Massenspektrometrie: Neue Möglichkeiten für die schnelle chemische Analytik

Modernen Analytiklabors stehen heute Methoden des chemischen Spuren-und Ultraspurennachweises zur Verfugung, die in Bezug auf Empfindlichkeit und Selektivitat ausgesprochen weit entwickelt sind. Dies ist vor allem durch den Einsatz gekoppelter mehrdimensionaler Techniken gelungen. Der Preis fur diesen hohen Standard ist jedoch eine sehr zeit- und arbeitsaufwendige Probenvorbereitung. So werden fur speziesselektive Spurenanalytik organischer Schadstoffe, wie z.B. Dioxin- und Pestizidruckstande, einige Tage oder sogar Wochen benotigt [1].

Resonance ionization mass spectrometry and its application to trace analysis of emissions from combustion engines

The principle of resonance-ionization mass spectrometry with lasers is the combination of two analytical tools, UV- spectroscopy in the gas phase and time-of-flight mass selection. The special features of this combination are: very high selectivity, high speed, multicomponent ability, and adaptability to many different problems. Examples for the latter are on-line trace analysis of emissions from combustion processes, such as from combustion engines as well as from municipal incinerators. But also monitoring of industrial procedures, e.g. food processing, are interesting applications of REMPI-MS. In this paper the principles will be shortly explained and results for the analysis of exhaust emissions from motorized vehicles presented.