EP1266396B1 - 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

The invention relates to a method and an apparatus for detecting compounds in a gas stream according to the preambles of claims 1 and 8, as known from the publication 3.

State of the art

The resonance-enhanced multiphoton ionization technique (REMPI), the UV laser pulses for the selective ionization of z. B. aromatics is used as a selective and soft ionization method for mass spectrometry. The selectivity is determined, inter alia, by the UV spectroscopic properties and the position of the ionization potentials. A typical application is the on-line detection of aromatic compounds from combustion exhaust gases ¹ . A disadvantage of the REMPI method is that it is limited to some classes of substances and the ionization cross-section can also be extremely different for similar compounds.

Single photon ionization ( SPI) with VUV laser light allows for a partially selective and soft ionization ² . The selectivity is determined by the position of the ionization potentials. A typical application is the detection of compounds that can not be detected by REMPI. A disadvantage of the SPI method is that some substance classes can not be detected. Furthermore, the selectivity is smaller than in the REMPI method, so that interference can occur more intensively with complex samples.

Electron impact ionization (EI) with an electron beam is the standard technique for ionization in the mass spectrometry of volatile inorganic and organic compounds. It is very universal (ie non-selective) and results in many molecules often leads to a very strong fragmentation, but is very well suited for a direct measurement of compounds such as O 2, N 2, CO 2, SO 2, CO, C 2 H 2, etc., which can not be detected so 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 multiplicity 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 described advantageous embodiments of the invention.

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

If the EI ionization technique is still used, further compounds such as CO2, H2O or CH4 can be detected, which can not be sensibly detected with either SPI or 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 z. B. on-line analytical field applications (process analysis), which nevertheless have a very high performance. The parallel obtained REMPI and / or VUV and / or EI mass spectropetric data can also be supplied to a chemometric analysis by pattern recognizing methods (eg a main component analysis).

embodiments

The invention will be explained in more detail below with reference to exemplary embodiments with the aid of the figures.

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

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

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

The (quasi-) parallel use of ionization with REMPI and SPI thus allows 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. The 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 molecular beam inlet systems (described, for example, in ^{Figure 3} ) may also be employed. Analytes from the gas stream are irradiated directly below the inlet needle 12 alternately with UV laser pulses (266 nm) 10 and VUV laser pulses (118 nm) 2. The laser pulse length can be between 1 fs and 100 ns. The ions produced by multiphoton ionization (REMPI, 266 nm) or one-photon ionization (SPI, 118 nm) are drawn off through the opening of the discharge aperture 13 into the TOF mass spectrometer and mass analyzed there. As an alternative to alternating switching between UV laser pulses (266 nm) and VUV laser pulses (118 nm), it is also possible to irradiate several pulses of one wavelength in succession before switching to the other wavelength. The VUV laser beams (118 nm) 2 are generated in the gas cell 9, which is filled with inert gas (Xe and Ar) 3, by frequency tripling 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 MgF 2 lens 4. The offset irradiation of the 355 nm laser beam 1 relative to the center of the MgF 2 lens 4 causes a local separation of the 355 nm laser radiation 1 and 118 nm radiation in the ionization chamber 14. By a diaphragm, the 355 nm radiation can be intercepted before the ionization. This leads to fragmented SPI mass spectra.

The alternating generation of the 266 nm 10 and 118 nm 1 ionization laser pulses is carried out with a special optical structure, as it FIG. 2 is shown. The Nd: YAG laser 15 generates 1064 nm laser radiation 23, which are guided via two dichroic mirrors 16 through a frequency doubling crystal 17. The resulting laser beam 24 consists of 1064 nm 23 and 532 nm 25 laser radiation. A movably mounted on an arm dichroid mirror 18, which can be computer controlled via a galvanometer quickly and precisely swung in the beam path is used to redirect the laser pulses alternately or pass. When the mirror arm 18 is pivoted out of the beam path, the laser radiation 24 is passed through the sum difference mixed crystal 19 and generates 355 nm laser light 1 separated by the dichroic mirror 20 from the collinear 532nm and 1064nm radiation and into the gas cell 9 to produce 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 through the dichroic mirror by 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, then a partially transparent mirror (dichroid beam splitter) can be used instead of a folding mirror. The suppression of the respectively unneeded beam can be realized via a Pockels cell or a chopper wheel. In addition to the Nd: YAG laser, other pulsed third-party lasers such as Ti: sapphire lasers can be used.

From the primary wave of the Nd: YAG laser (1064 nm), the following harmonic frequencies can be generated: 523 nm (doubled), 355 nm (tripled), 266 nm (quadrupled), 213 nm (quintupled), and 118 nm (nine times). As an extension to the two-beam method described above (266 nm for REMPI and 118 nm for VUV), several wavelengths can also be irradiated alternately. In a combination of 266, 213 and 118 nm, for example, in addition to the VUV selectivity simultaneously (ie slightly offset) still exploited two different REMPI selectivities. For example, naphthalene and its methylated derivatives (these compounds are indicators of the efficiency of combustion processes) can be detected particularly efficiently at 213 nm. Depending on the solid-state laser type, it is thus possible to use 2, 3 or more wavelengths in parallel with the ionization of compounds from the sample. The different selectivities induced by the different REMPI and / or VUV wavelengths lead to different mass spectra (ie different compounds are added or disappear from the mass spectrum.) If very complex samples or unknown samples can not be assigned to the observed compounds, then use of chemometric methods for pattern recognition (eg principal component analysis) and thus, for example, for the phenomenological characterization, by using fixed frequency-shifting units (eg via optoacoustic coupling, with Raman shifters, with optical parametric Oscillator crystal, with dye laser unit), a frequency can also be converted to a desired frequency for a selective REMPI detection of a particular substance, for example, one wavelength can be tuned to a resonance of monochlorobenzene (eg at about 266 nm or at about 269.82 nm ⁴ ). Monochlorobenzene is an indicator of the presence of toxic polychlorinated dibenzo-p-dioxins and -furans (PCDD / F) and may be associated with REMPI on-line in flue gases from z. B. technical combustion processes are detected ⁵ . With a wavelength of about 269.82 nm is a detection of monochlorobenzene (MCB) and a number of other aromatics such. As benzene, naphthalene or pyrene possible. Alternatively, MCB can be detected at a resonance that is close to four times the Nd: YAG wavelength ⁴ . For this purpose, it may be sufficient in certain cases, the fundamental wave of the Nd: YAG laser, z. B. by manipulation of the laser resonator, easy to detune.

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

An analytical laser mass spectrometer can also be advantageously designed for certain applications with an inlet system for generating a supersonic molecular beam (jet). The resulting adiabatic cooling increases the selectivity of the REMPI-TOFMS method 6 and reduces the fragmentation in SPI and EI ionization.

The EI ionization achieves only much smaller cross sections than the laser ionization (with common pulse energies), however, the repetition rate of the laser ionization processes, which are indeed pulsed, limited in many compact laser systems to 10-20 Hz. Since the absorption of a mass spectrum after the ionization pulse takes only a few 10 μs, the mass spectrometer is not used for most of the time. The EI ionization uses an electron gun that accelerates electrons with kinetic energies of 2-200 eV to the sample molecules. Using pulsed electron guns and pulsed extraction fields, the normally continuous EI method can also be used with time-of-flight mass spectrometry. This is also possible in parallel with the use of laser ionization methods (REMPI, SPI). Typically, the information of the laser ionization methods is recorded via a transient recorder while the information from the EI ionization is done via counting cards. The incorporation of electron impact ionization allows direct on-line measurement of the higher concentration compounds that can not be detected by REMPI or SPI.

applications

The method and apparatus described above can be used in principle for a variety of applications. In the following four application examples are given:

Application Example 1 Monitoring of Combustion Processes

REMPI has proven to be a very powerful analytical method for on-line analysis of aromatic hydrocarbons, dioxin indicators (MCB) and other compounds. 1. In parallel, information such as nitrogen compounds such as NO, NH3 or aldehydes would be important. These compounds can be detected with VUV. Thus, the VUV, SPI and REMPI ionization methods complement each other and together can be used advantageously for a good characterization of the combustion process. Finally, implementing parallel EI ionization results in a very comprehensive characterization, as some key chemical parameters, such as concentrations of CO2, O2 and smaller organic molecules such as acetylene (important for the construction of polycyclic aromatics and carbon black aerosols) are not can 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 kinds. The FIG. 3 shows the concentration curves of naphthalene and NO in the flue gas of a household waste incineration plant (raw gas at 700 ° C) recorded with parallel VUV-SPI and REMPI ionization.

Application example 2: On-line analysis of process gases in food technology

In order to monitor food technology processes (drying processes, roasting or cooking processes, etc.) as well as quality control of raw materials (mold, quality) or evaluation of sensory quality, on-line mass spectrometric methods can be used. 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 by the composition of different substituted ones. Many aroma-relevant compounds (aliphatic aldehydes and ketones, furan derivatives, nitrogen heterocycles, etc.), however, can be detected very well with VUV ionization. Electron impact ionization allows tracking of the primary coffee roast products CO2 and H2O. In principle, a large number of such processes must be comprehensively controlled and validated with the method and a device of the generic type.

Application Example 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 mixture (solid, solution / liquid, gas phase). Suitable auxiliary devices (headspace samples, thermal desorbers, etc.) can be used to obtain a representative gas sample. By way of 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 breath) of patients and control persons. Certain volatile substances, such as acetone, indicate illness or general health out. Furthermore, the headspace can be analyzed via medical samples (blood, urine, etc.).

LIST OF REFERENCE NUMBERS

1

55 nm laser beam

2

18 nm laser beam

3

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

4

Lens of MgF ₂

5

Entry window for 355 nm quartz

6

Condensing lens made of quartz

7

sealing ring

8th

Connecting piece for filling / evacuation of the gas cell 9

9

gas cell

10

266 nm laser

11

Entry window for 266 nm of quartz

12

Gas inlet (needle)

13

Exhaust orifice of the TOF mass spectrometer

14

Ionization cube of the TOF mass spectrometer

15

Nd: YAG laser

16

Dichroic mirror for 1064 nm

17

Crystal for frequency doubling

18

Computer-controlled folding arm with dichroic mirror for 532 nm

19

Crystal to sum frequency mixing

20

Dichroic mirror for 355 nm

21

Dichroic mirror for 532 nm

22

Dichroic mirror for 266 nm

23

1064 nm laser beam

24

Colinear 1064 nm and 532 nm laser beams

25

532 nm laser beam

references

1. (1) Heger, HJ; Zimmermann, R .; Dorfner, R .; Beckmann, M .; Griebel, H .; Kettrup, A .; Boesl, U. Anal. Chem. 1999, 71, 46-57 ,
2. (2) Butcher, DJ; Goeringer, DE; Hurst, GB Anal. Chem. 1999, 71, 489-496 ,
3. (3) Rohlfing, EA In 22nd Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1988, pp 1843-1850 ,
4. (4) Heger, HJ; Boesl, U .; Zimmermann, R .; Dorfner, R .; Kettrup, A. Eur. Mass Spectrom. 1999, 5, 51-57 ,
5. (5) Zimmermann, R .; Heger, HJ; Blumeristock, M .; Dorfner, R .; Schramm, K.-W .; Boesl, U .; Kettrup, A. Rapid Comm. Mass. Spectrom. 1999, 13, 307-314 ,
6. (6) Tembreull, R .; Lubman, DM Anal. Chem. 1984, 56, 1962-1967 ,
7. (7) Zimmermann, R .; Heger, HJ; Yeretzian, C .; Nagel, H .; Boesl, U. Rapid Comm. Mass. Spectrom. 1996, 10, 1975-1979 ,

Claims (10)

1. Process for the detection of compounds in a gas flow, in which

   a) the gas flow with the compounds is passed into the ionization chamber (14) of a mass spectrometer,

   b) the gas flow is irradiated by an UV laser pulse (10) in the ionization chamber (14), and

   c) the resulting ions are detected in the mass spectrometer, characterized by

   d) the gas flow in the ionization chamber being irradiated by a vacuum ultraviolet (VUV) laser pulse (2) at regular or irregular intervals alternately with irradiation by UV laser pulses (10) and the thus generated ions being detected in the mass spectrometer.
2. Process according to Claim 1, characterized by the UV laser pulse (10) and the VUV laser pulse (2) being generated with the help of a solid-state laser (15).
3. Process according to Claim 1 or 2, characterized by the UV laser pulse being generated from the solid-state laser pulse by frequency mixing and/or frequency multiplication.
4. Process according to Claim 3, characterized by the UV laser pulse (10) being adjusted by using a dye laser or an optical parametric oscillator.
5. Process according to one of Claims 1 through 4, characterized by the VUV laser pulse (2) being generated from the solid-state laser pulse (23, 24) by frequency mixing and/or frequency doubling with subsequent frequency tripling in a gas cell (9).
6. Process according to one of Claims 1 through 5, characterized by the wavelength of the VUV laser pulse (2) being adjusted by using a dye laser or an optical parametric oscillator prior to frequency tripling in the gas cell (9).
7. Process according to one of Claims 1 though 6, characterized by the gas flow in the ionization chamber (14) being irradiated by an electron beam for electron impact ionization in-between the VUV laser pulses (2) or UV laser pulses (10) and the resulting ions being detected in the mass spectrometer.
8. System for the detection of compounds in a gas flow, consisting of

   a) a mass spectrometer with a gas inlet that opens out into 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), with an UV laser pulse (10) being generated from the basic frequency (23) and irradiated into the area in front of the gas inlet (12) of the ionization chamber (14),

   c) a data acquisition and processing system for the mass spectrometer, and

   d) an optical component (18) to split the laser pulse into two partial beams (24, 25), with the UV laser pulse (10) being generated from one partial beam (25) with the help of other optical elements (21, 17, 22) and irradiated into the area in front of the gas inlet (12) of the ionization chamber (14), characterized by

   e) other optical components (19, 20) and a gas cell (9) with an appropriate filling (3), in which the frequency-doubled and guided partial beam (24) in the gas cell (9) generates a VUV laser pulse (2) by frequency tripling and irradiates it into (14) the area in front of the gas inlet (12) of the ionization chamber.
9. System according to Claim 8, characterized by a dye laser or an optical parametric oscillator being arranged in one or several of the beam paths (23), (24) or (25).
10. System according to Claim 8 or 9, characterized by a pulsing electron gun for electron impact ionization of the compounds being installed in the gas flow in front of the gas inlet (12) of the ionization chamber (14) of the mass spectrometer.

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