EP1266396A1 - Method and device for detecting compounds in a gas stream - Google Patents

Abstract

In a method and apparatus for detecting compounds in a gas stream, the gas stream with the compounds to be detected is conducted into an ionization chamber of a mass spectrometer where the gas stream is subjected in the ion chamber in a pulsed manner alternately to UV laser pulses and to vacuum ultraviolet VUV laser pulses and the ions generated thereby are directed into the mass spectrometer for detection therein to determine the compounds in the gas stream.

Description

(74) Lawyer: HORST, Günther; Forschungszentrum KarlPublished: sruhe GmbH, Department of Patents and Licenses, - with international search report PO Box 3640, 76021 Karlsruhe (DE). - before the deadline for changes in claims expires, publication will be repeated if changes

(81) Destination countries (national): CA, JP, US. arrive

To explain the two-letter codes and the others

(84) Destination countries (regional): European patent (AT, abbreviations refer to the declarations ("Guidance Notes on BE, CH, CY, DE, DK, ES, FI, FR, GB, GR, IE, IT, LU, MC "Codes and Abbreviations") at the beginning of every regular edition NL, PT, SE, TR). the PCT Gazette

Method and device for detecting connections in a gas stream

The invention relates to a method and a device for detecting compounds in a gas stream according to the preambles of claims 1 and 8, as is known from publication 1.

State of the art

The resonance-enhanced ultiphoton ionization technology (resonance-enhanced mul tiphoton ionization - REMPI), the UV laser pulses for the selective ionization of e.g. B. uses aromatics, is used as a selective and soft ionization method for mass spectrometry. The selectivity is among others determined by the UV spectroscopic properties and the location of the ionization potentials. A typical application is on-line

Detection of aromatic compounds from combustion gases ^. A disadvantage of the REMPI method is that it is limited to some substance classes and the ionization cross section can also be extremely different for similar compounds.

The single photon ionization (SPI) with VUV laser light allows a partially selective and soft ionization ^. The selectivity is determined by the location of the ionization potentials. A typical application is the detection of compounds that cannot be detected with REMPI. The disadvantage of the SPI method is that some substance classes cannot be detected. Furthermore, the selectivity is lower than with the REMPI method, so that interference can occur with complex samples.

Electron impact ionization (EI) with an electron beam is the standard technique for ionization in mass spectrometry of volatile inorganic and organic compounds. It is very universal (ie not selective) and leads many often cool to a very high level of fragmentation, but is very well suited for the direct measurement of compounds such as 02, N2, C02, S02, CO, C2H2 etc., which cannot be detected as well with VUV or REMPI.

Object of the invention

The object of the invention is to design a method and a device of the generic type such that a large number of compounds in the analysis gas can be characterized almost simultaneously. This object is achieved by the features of claims 1 and 8. The subclaims describe advantageous refinements of the invention.

The combination of (quasi) simultaneous SPI and REMPI ionization in a mass spectrometer brings a number of advantages. Both methods record different subsets of the complex analysis gas with different selectivity. In total, more compounds from the sample can be identified.

If the El ionization technique is also used, other compounds such as C02, H20 or CH4 can be detected, which can neither be sensibly detected with SPI nor with REMPI. The combination of the methods and the device for quasi-parallel use of the same in one device allows the construction of particularly compact analytical MS systems for e.g. B. online analytical field applications (process analysis), which nevertheless have a very high performance. The REMPI and / or VUV and / or EI mass spectrometric data obtained in parallel can also be fed to a chemometric analysis by means of pattern-recognition methods (for example a main component analysis).

embodiments

The invention is explained in more detail below on the basis of exemplary embodiments with the aid of the figures. - -

FIG. 1 shows an example of the ionization region of the mass spectrometer 14 and the gas cell 9.

FIG. 2 schematically shows an optical arrangement for generating a UV laser pulse 10 and a VUV laser pulse 2.

FIG. 3 shows an on-line measurement of NO and naphthalene in the flue gas of a waste incineration plant with alternating SPI ionization (VUV for NO) and REMPI ionization (UV for naphthalene).

The (quasi) parallel use of ionization with REMPI and SPI thus enables the simultaneous tracking of complex chemical samples. Due to the different selectivity of the two methods, different mass spectra are obtained with the respective methods. FIG. 1 shows the ionization region of the time of flight (TOF) mass spectrometer. The gas stream to be analyzed flows effusively through the inlet needle 12 into the ionization chamber 14 1. Alternatively, supersonic oil jet inlet systems (described, for example, in FIG. 3) can also be used. Analytes from the gas stream are alternately irradiated with UV laser pulses (266 nm) 10 and VUV laser pulses (118 nm) 2 directly below the inlet needle 12. The laser pulse length can be between 1 fs and 100 ns. The ions generated by multi-photon ionization (REMPI, 266 nm) or single-photon ionization (SPI, 118 nm) are drawn off through the opening of the hood 13 into the TOF mass spectrometer, where they are mass analyzed. As an alternative to alternating switching between UV laser pulses (266 nm) and VUV laser pulses (118 nm), several pulses of one wavelength can be irradiated one after the other before switching to the other wavelength. The VUV laser beams (118 nm) 2 are generated in the gas cell 9, which is filled with noble gas (Xe and Ar) 3, by frequency tripling of 355 nm laser pulses 1. The 355 nm laser pulses 1 are focused with a quartz lens 6 and through a quartz window 5 into the gas cell 9. The resulting VUV radiation and the remaining 355 nm - -

Radiation 1 enters the ionization cube 14 of the TOF mass spectrometer through the MgF2 lens 4. The offset irradiation of the 355 nm laser beam 1 relative to the center of the MgF2 lens 4 causes a local separation of the 355 nm laser radiation 1 and 118 nm radiation in the ionization chamber 14. The 355 nm radiation can be intercepted in front of the ionization site by means of an aperture. This leads to less fragmented SPI mass spectra.

The alternating generation of the 266 nm 10 and 118 nm 1 ionization laser pulses takes place with a special optical structure, as shown in FIG. 2. The Nd.YAG laser 15 generates 1064 nm laser radiation 23 which are guided through a frequency doubling crystal 17 via two dichroic mirrors 16. The resulting laser beam 24 consists of 1064 nm 23 and 532 nm 25 laser radiation. A movable dichroic mirror 18 mounted on an arm, which can be swiveled into the beam path quickly and precisely in a computer-controlled manner via a galvanometer, is used to alternately redirect or transmit the laser pulses. When the mirror arm 18 is pivoted out of the beam path, the laser radiation 24 is guided through the sum difference mixed crystal 19 and 355 nm laser light 1 is generated, which is separated by the dichroic mirrors 20 from the colinear 532 nm and 1064 nm radiation and into the gas cell 9 to generate the 118 nm VUV laser radiation 2 is used. If the mirror arm 18 is in the beam path, the 532 nm portion of the radiation 24 is directed via the dichroic mirror through a doubling crystal 17. The resulting 266 nm laser radiation 10 is separated from the 532 nm radiation by the dichroic mirrors 22 and then used for REMPI ionization in the inlet block 14 of the TOF mass spectrometer.

The data acquisition system records the REMPI and VUV-SPI mass spectra separately. If a sufficiently intense YAG laser is used, a partially transparent mirror (dichroic beam splitter) can be used instead of a folding mirror. The blanking of the beam that is not required can be realized via a Pockels cell or a chopper wheel. In addition to the Nd: YAG laser, other pulsed solid-state lasers such as Ti: sapphire lasers can also be used.

The following harmonic frequencies can be generated from the primary wave of the Nd.YAG laser (1064 nm): 523 nm (doubled), 355 nm (tripled), 266 nm (quadrupled), 213 nm (quintuple) and 118 nm (nine times). In addition to the two-beam method described above (266 nm for REMPI and 118 nm for VUV), several wavelengths can be irradiated alternately. With a combination of 266, 213 and 118 nm, for example, in addition to VUV selectivity, two different REMPI selectivities are used simultaneously (i.e. slightly offset). For example, naphthalene and its methylated derivatives

(these compounds are indicators of the efficiency of combustion processes) can be detected particularly efficiently with 213 nm. Depending on the type of solid-state laser, 2, 3 or more wavelengths can be used in parallel for the ionization of compounds from the sample. The different selectivities that are induced by the different REMPI and or VUV wavelengths lead to different mass spectra

(In other words, other compounds are added or disappear from the mass spectrum. If it is not possible to assign the respectively observed compounds to very complex samples or unknown samples, the use of chemometric methods for pattern recognition (e.g. main component analysis) and thus e.g. By using fixed frequency shifting units (eg via optoacoustic coupling, with Raman shifter, with optical parametric oscillator crystal, with dye laser unit), a frequency can also be set to a desired frequency for a selective REMPI Detection of a specific substance can be converted, for example, a wavelength can be tuned to a resonance of monochlorobenzene (for example at approximately 266 nm or at approximately 269.82 nm ⁴ ) Monochlorobenzene is an indicator of the occurrence of toxic polychlorinated Dibenzo-p-dioxme and furane (PCDD / F) and can with REMPI on-line in flue gases from e.g. B. technical combustion processes can be demonstrated. With a wavelength of approx. 269.82 nm, detection of monochlorobenzene (MCB) and a number of other aromatics such as e.g. B. benzene, naphthalene or pyrene possible. Alternatively, MCB can be detected in the case of a resonance that is very close to the quadrupled Nd: YAG wavelength. In certain cases it may be sufficient to do this, the fundamental of the Nd: YAG laser, e.g. B. by manipulation of the laser resonator, easy to detune.

With the VUV laser wavelength, compounds such as NH3, NO, many aldehydes and ketones etc. can then be detected in parallel, which cannot be detected with REMPI at the MCB resonance.

An analytical laser mass spectrometer can furthermore advantageously be designed for certain applications with an inlet system for generating a supersonic molecular beam (jet). The adiabatic cooling achievable thereby increases the

Selectivity of REMPI-TOFMS Method ⁶ and reduces the fragmentation in SPI and egg ionization.

El ionization only achieves much smaller cross sections than laser ionization (with conventional pulse energies), however, the repetition rate of the laser ionization processes, which are pulsed, is limited to 10-20 Hz in many compact laser systems. Since the acquisition of a mass spectrum after the ionization pulse only takes a few 10 μs, the mass spectrometer is not used for most of the time. The El ionization uses an electron gun that accelerates electrons with kinetic energies of 2-200 eV to the sample molecules. The normally continuous EI method can also be used with the time-of-flight mass spectrometer using pulsed electron guns and pulsed trigger fields. This is also possible in parallel with the use of laser ionization methods (REMPI, SPI). The information about the laser ionization methods is typically recorded via a transient recorder. records true information from the El-ionization on payment cards. The integration of electron impact ionization allows the direct on-line measurement of the compounds which are present in higher concentrations and which cannot be detected with REMPI or SPI.

applications

The method and the device described above can in principle be used for a large number of applications. Four application examples are given below:

Application example 1: Monitoring of combustion processes

REMPI has proven to be a very powerful analytical method for online analysis of aromatic hydrocarbons, dioxin indicators (MCB) and other compounds. At the same time, information such as nitrogen compounds such as NO, NH3 or aldehydes was important. These connections can be verified with VUV. Thus, the VUV, SPI and REMPI ionization methods complement each other and can be used together advantageously for a good characterization of the combustion process. If parallel egg ionization is implemented, a very comprehensive characterization is finally achieved, since some main chemical parameters, such as concentrations of C02, 02 and smaller organic molecules such as acetyls (important for the formation of polycyclic aromatics and soot aerosols) ) cannot be detected with the usual SPI VUV wavelengths or with 2 photon REMPI processes. The method with a device of the generic type is suitable for characterizing and analyzing combustion and pyrolysis processes of all types. FIG. 3 shows the concentration profiles of naphthalene and NO in the flue gas of a domestic waste incineration plant (raw gas at 700 ° C.) recorded with parallel VUV-SPI and REMPI ionization. - 8th -

Example of use 2: On-line analysis of process gases in food technology

Mass spectrometric methods can be used on-line to monitor food technology processes (drying processes, rusting or cooking processes, etc.) and for quality control of raw materials (mold growth, quality) or evaluation of sensory quality. First experiences have already been made with the REMPI method in the field of coffee roasting ⁷ . With REMPI (266 nm) the degree of roasting can be determined via the composition of differently substituted substances. By contrast, many flavor-relevant compounds (aliphatic aldehydes and ketones, furan derivatives, nitrogen heterocycles etc.) can be detected very well with VUV ionization. Electron impact ionization allows the tracking of the primary coffee roast products C02 and H20. In principle, a large number of such processes can be comprehensively checked and validated using the method and a device of the generic type.

Example of use 3: On-line analysis of headspace samples of complex mixtures

The method can be used with a device of the generic type for the analysis of complex substance mixtures (solid, solution / liquid, gas phase). Suitable auxiliary devices (headspace sampling, thermal desorber etc.) can be used to obtain a representative gas sample. For example, process solutions from the chemical industry, mineral oil products but also end products such as perfume or deodorant can be analyzed and monitored.

Example 4: On-line analysis of medically relevant samples:

The method can be used with a device of the generic type for analyzing the breathing air (exhaled) from patients and control persons. Certain volatile substances such as acetone / indicate illnesses or general health condition. The headspace can also be analyzed using medical samples (blood, urine, etc.).

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Reference symbol list:

1 55 nm laser beam

2 18 nm laser beam

3 as filling (e.g. 0.001 bar Xe)

4 converging lens made of MgF ₂

5 entrance windows for 355 nm made of quartz

6 quartz lens

7 sealing ring

8 nozzle for filling / evacuation of the gas cell 9

9 gas cell

10 266 nm laser

11 entrance windows for 266 nm made of quartz

12 gas inlet (needle)

13 Trigger aperture of the TOF mass spectrometer

14 ionization cubes of the TOF mass spectrometer

15 Nd: YAG laser

16 dichroic mirrors for 1064 nm

17 crystal for frequency doubling

18 Computer controlled folding arm with dichroic mirror for 532 nm

19 crystal for sum frequency mixing

20 dichroic mirrors for 355 nm

21 dichroic mirror for 532 nm

22 dichroic mirror for 266 nm

23 1064 nm laser beam

24 Colineare 1064 nm and 532 nm laser beams

25 532 nm laser beam

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credentials

(1) Heger, H. J.; Zimmermann, R.; Dorfner, R.; Beckmann, M.; Griebel, H. ; Kettrup, A.; Boesl, U. Anal. Chem. 1999, 71, 46-57.

(2) Butcher, D.J .; Goeringer, D.E. ; Hurst, G.B. Anal. Chem. 1999, 71, 489-496.

(3) Rohlfing, E. A. In 22nd Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1988, pp 1843-1850.

(4) Heger, H. J.; Boesl, U.; Zimmermann, R.; Dorfner, R.; Kettrup, A. Eur. Mass Spectrom. 1999, 5, 51-57.

(5) Zimmermann, R.; Heger, H. J.; Blumenstock, M.; Dorfner, R.; Schramm, K.-.; Boesl, U.; Kettrup, A. Rapid Comm. Measure Spectrom. 1999, 13, 307-314.

(6) Tembreull, R.; Lubman, D. M. Anal. Chem. 1984, 56, 1962-1967.

(7) Zimmermann, R.; Heger, H. J.; Yeretzian, C. ; Nagel, H .; Boesl, U. Rapid Comm. Measure Spectrom. 1996, 10, 1975-1979.

Claims

- -Patent claims 1. A method for the detection of compounds in a gas stream, in which a) the gas stream with the compounds is passed into the ionization chamber (14) of a mass spectrometer, b) the gas stream in the ionization chamber (14) is irradiated with a UV laser pulse (10) and c) the ions formed are detected in the mass spectrometer, characterized in that, d) alternating with the irradiation with UV laser pulses (10), the gas stream in the ionization space is irradiated with a vacuum ultraviolet (VUV) laser pulse (2) at regular or irregular intervals and the resulting ions are detected in the mass spectrometer. 2. The method according to claim 1, characterized in that the UV laser pulse (10) and the VUV laser pulse (2) are generated with the aid of a solid-state laser (15). 3. The method according to claim 1 or 2, characterized in that the UV laser pulse is generated from the solid-state laser pulse by frequency mixing and / or multiplication. 4. The method according to claim 3, characterized in that the UV laser pulse (10) is tuned by using a dye laser or an optical parametric oscillator. 5. The method according to any one of claims 1 to 4, characterized in that the VUV laser pulse (2) from the solid-state laser pulse (23, 24) is generated by frequency mixing and / or doubling with a subsequent frequency tripling in a gas cell (9). 6. The method according to any one of claims 1 to 5, characterized in that the wavelength of the VUV laser pulse (2) by using a dye laser or an optical para- Metric oscillator can be tuned in the gas cell (9) before the frequency is tripled. 7. The method according to any one of claims 1 to 6, characterized in that in the period between the VUV laser pulses (2) or UV laser pulses (10) the gas stream in the ionization chamber (14) is irradiated with an electron beam for electron impact ionization and the resulting ions can be detected in the mass spectrometer. 8. Device for detecting compounds in a gas stream consisting of a) a mass spectrometer with gas inlet which mouths in the ionization chamber (14) of the ion source of the mass spectrometer, b) a solid-state laser (15) with optical elements for mixing and / or multiplying a basic frequency of the solid-state laser (23), a UV laser pulse (10) being generated from the fundamental frequency (23) and in the area in front of the gas inlet (12) is irradiated in the ionization chamber (14), c) a data acquisition and processing system for the mass spectrometer, characterized by, d) an optical component (18) for dividing the laser pulse into two partial beams (24, 25), one of which Partial beam (25) of the UV laser pulse (10) is generated with the aid of further optical elements (21, 17, 22) and is irradiated into the area in front of the gas inlet (12) in the ionization chamber (14) and e) further optical components (19 , 20) and a gas cell (9) with a suitable filling (3), a VUV laser pulse (2) being generated from the frequency-doubled and guided partial beam (24) in the gas cell (9) by tripling the frequency and in the area in front of the gas inlet ( 12) is irradiated in the ionization space (14). 9. The device according to claim 8, characterized by a dye laser or an optical parametric oscillator in one or more of the beam paths (23), (24) or (25). - - 10. The device according to claim 8 or 9, characterized by a pulsable electron gun for electron impact ionization of the compounds in the gas jet in front of the gas inlet (12) in the ionization chamber (14) of the mass spectrometer.