Anal. Chem. 1994,66,1062-1069
Laser Mass Spectrometry for Time-Resolved Multicomponent Analysis of Exhaust Gas C. Weickhardt, U. Boesi,' and E. W. Schlag Institut fur Physikalische und Theoretische Chemie, Technische Universitat Munchen, Lichtenbergstrasse 4, 0-85747 Garching, Germany
A new analytical method for the fast analysis of trace substances in gas mixtures has been developed incorporating a pulsed tunable laser system and a reflectron time-of-fight mass spectrometer. The technique is especially designed for the time-resolved detection of air pollutants in the exhaust gas of motor engines. The purpose of this new analytical method is to achieve high time resolution (
0.6
TEA
Figure 7. Normalized slgnal of NH:, as a function of the two callbratlon gases, representing the laser intensity In the Ionization volume. The standard devlatlon of the single shot is 7.4%.
with nT and nl the density of neutrals. As the ratio of the two calibration gases Rlp represents a function of the laser intensity in the ionization volume, .
.... .... . . . ..: . . . 0 500
700
900
1100
..
.
... .. . .. . . . .. . . . . ... . . . ’..., , . : . . ...
the intensity can be written as
‘I
,
1300
1500
1700
Laser Pulse Energy (arb. u n i t s ) Figure 6. Ion slgnal of NHs as a function of the laser pulse energy. The broaddlstrlbutlon demonstratestheinsufficiencyof a measurement of the pulse energy alone for calbration purpose.
the measured laser pulse energy. The broad distribution of the signal intensity at a fixed pulse energy demonstrates the unfeasibility of calibrating the instrument by simply measuring the energy of each laser pulse. To solve this problem, an “intensity sensor” situated at the point of ionization is necessary. By intensity sensor is meant a process which is sensitive to the spatial and temporal distribution of the laser pulse energy within the laser focus. The local laser intensity can be tested by the ratio of two calibration gases depending differently on laser intensity. In comparison with an external recording of the laser power, the “internal” method of using calibration gases has major advantages: First, the measurement takes place at the point of ionization of the trace molecules without using additional instruments that may introduce additional sources of errors. Second, it takes into account fluctuations of the effective ionization volume and the laser beam shape. Formally, the calibration principle can be described as follows: The ratio of the signal of the trace substances STand the first calibration gas SIis a functionfof the laser intensity 1,
1068 Analytical Chemktry, Vol. 66, No. 7, April 1, 1994
The concentration of the trace substance nT can therefore be calculated as
The function F is specific to each substance and has to be determined by a calibration procedure. The result of such a calibration procedure is shown in Figure 7. Here the normalized signal of NH3 is plotted as a function of the ratio of the two calibration gases propylamine and triethylamine (TEA)representing the laser intensity in the ionization volume. The standard deviation of the single shot is thereby reduced to about 7%. It should be mentioned that the calibration method presented here is only exact for a laser pulse having a Gaussian profile in space and time or being made up of several Gaussian pulses of equal heights. Every deviation from this precondition will lead to an increased error of thequantitative measurement. In practice, however, the assumption of Gaussian pulses emerging from a dye laser can be considered as a good approximation and does not severely restrict the quantitative accuracy. The primary calibration gases used for the analysis of exhaust gas were 1,Cdifluorobenzenein the wavelength region 245-270 nm and perdeuterated acetaldehyde (CD3CDO) in the region 298-3 15 nm and 420-445 nm. Triethylaminecould be used as the secondary internal standard over the whole wavelength range.
Tablo 2. Components d Exhaust Gases Investbated for Analyrls by Laser Mass Spectrometrya
component
M (amu)
IP (eV)
IP (Amu, nm)
(n+ m)-MPI
X range (nm)
hydrogen ammonia water acetylene prussic acid ethene carbon monoxide nitrogen monoxide formaldehyde oxygen methanol hydrogen sulfide acetaldehyde dinitrogenoxide nitrogen dioxide butadiene
2 17 18 26 27 28 28 30 30 32 32 34 44 44 46 54
15.4 10.2 12.6 11.4 13.8 10.5 14.0 9.2 10.8 12.0 10.8 10.4 10.2 12.9 9.8 9.1
acrolein propionaldehyde acetone carbonyl sulfide
56 58 58 60
10.1 10.0 9.7 11.2
sulfur dioxide benzene toluene xylene trimethylbenzene
64 78 92 106 120
12.3 9.2 8.5 8.5 8.3
322 486 393 327 270 472 354 268 342 308 457 477 364 384 507 410 547 368 372 511 332 443 301 268 281 293 300
3+1 2+1+1 3+1 2+1 2+1 3+1 3+1 2 1+1+1 2+1 3+1 2+1+1 1+1+1 3+1 1+3 2+1 3+1 1+1+1 1+1+1 3+1 2+1 3+1 2+1 1+1 1+1 1+1 1+1
211-273 430-454 355 260-265 266275 430-454 430-454 25&270 298-315 286295 430-454 430-454 298-315 355-360 500-520 385-410 430-455 298-315 298-315 480-530 270-282 405-423 298-308 245-270 246270 245-270 245-270
Main Wavelength Ranges X (nm)
compd
245-270 298-315 430-454
acetylene, prussic acid, NO, aromatics NO, SOz, aldehydes, ketones ammonia, CO, HzS, methanol, butadiene, ethylene
0 The list contains name, formula, maas (M), and ionization threshold (IP) of the molecules, order of resonant multiphoton process ((n+ m)-PI), and wavelength range within which spectra have been recorded.
In summary, the calibration method presented above has the following characteristics: (i) The calibration is carried out for every single laser shot and therefore is very rapid. (ii) Trace component and calibration gas are subject to identical conditions of ionization and detection. (iii) No additional external measuring instrument is used. (iv) To examine the 25 typical exhaust gas components listed in Table 3, only three calibration gases are needed. (v) The required amount of calibration gas is very small. Nevertheless,for each exhaust gas component a precalibration must be performed; this is necessary, as the ratio of trace signal and calibration gas for one fixed laser intensity is individual for each component.
APPLICATION TO EXHAUST EMISSIONS OF COMBUSTION ENGINES The goal of this new analysis technique is, in addition to speed, the applicability to as many different trace components as possible. Therefore, a preliminary list of 25 substances has been established in cooperation with chemists and engineers from the automotive industrye8 For each of these substances a suitable ionization scheme had to be devised, pertaining to as far as appropriate laser wavelength and order of multiphoton absorption. Therefore, UV spectroscopy at predetermined conditions (e.g., gas temperature, order of optimum multiphoton absorption) had to be performed and the fragmentation behavior during the ionization examined. Great emphasis has been placed on few and narrow wavelength regions in order to save material (i.e., laser dyes) and time
(Le., for changing laser dyes). The emerging generation of tunable lasers (e.g., OPOZ3and titan sapphire lasers) will allow new possibilities of wavelength ranges and tunability. The results are listed in Table 2. The first two columns give molecular names and formulas, the third and fourth columns present molecular masses and ionization threshold energies. The sixth column lists the order of the optimum multiphoton ionization process; n is the number of photons absorbed to access the resonant intermediate state and m the number of photons absorbed from there to reach the ionization continuum. For some molecules even two resonant intermediate states are involved. The fifth column gives the longest wavelength to reach the ionization threshold (column 4) with (n m)photons. Finally, in the last column the wavelength range is listed where spectra of the individual molecules have been measured. These spectra are due to n-photon absorption to the intermediate states of the (n + m) ionization. Three important wavelength ranges have crystallized. In the first (245-270 nm), aromatic hydrocarbons are ionized by (1 + 1)-MPI using low-lying ( T * , T ) transitions (Figure 8). In the second (298-315 nm), aldehydes and ketones are ionized by (1 1 1)-MPI via ( T * , n) transitions (Figure 9). Small inorganic molecules with high-lying excited states (e.g., HzS,OCS, NHs) are detectable in the third wavelength region (420-445 nm) by (3 1)- (2 1 1)-MPI via (a* n) and (a*, a) transitions (Figure 10). All spectra were measured by injecting the substance under investigation into the vacuum system at a temperature of 200 O C .
+
+ +
+
+ +
AnaiyticaiChemistry, Vol. 66, No. 7, April 1, 1994
1067
t
Wavelength [nml
I
A
~
Flgure 8. UVREWI spectra of exhaustgas componentsto bedetected in the wavelength region 245-275 nm. Gas temperature was 200 O C .
XYLENE (0.5 ppm)
L
MASS
Flgure 11. Signal of 0.5 ppm pxylene in nitrogen. The Intense signal refers to toluene, which was introduced to adjust the apparatus.
Wavelength [nml Flgure 9. As in Figure 8, but X
= 295-320 nm.
Butadiene
can be studied relative to one another as a function of time. The information supplied this way is essential for understanding the chemical processes during and after the combustion. There are two ways to perform the method of analysis presented here: (i) optimization for the determination of one selected trace substance with highest sensitivity and accuracy or (ii) synchronous detection of several different exhaust gas components with somewhat lower sensitivity. The maximum sensitivity depends essentially on the order of the multiphoton ionization process used for detection of a certain substance. For (1 1) processes (used, for example, for aromatics) and a single laser shot, we determined a limit of detection well below 1 ppm. Figure 1 1 shows the signal of 0.5 ppmp-xylene in nitrogen. Acetaldehyde with a concentration of 3 ppm was detectable by (1 + 1 + 1)-MPI. (3 + 1) photoionization of HIS resulted in a limit of detection of 30 ppm. All values were determined at a quantitative single-shotaccuracy of 10%. Because these values are obtained for single laser shots, they are independent of repetition rate (no averaging) and therefore valid for the maximum achievable time resolution. The sensitivityfor the higher-order processes is not yet satisfactory, but the use of an OPO system providing higher laser pulse energies should easily lower the detection limits below 1 ppm. In Figure 12, first qualitative laser mass spectra measured with real exhaust gas are presented. The conventional mass spectrum is obtained by using electron impact ionization (70 eV), representing undiscriminated concentrations within a motor exhaust gas. It has been measured by the identical apparatus as the mass spectra above; the only difference was a wire tip positioned near the gas inlet cannula within the ion optics. The laser wavelength has been varied in order toprevent any absorption within the molecular gas, and the laser focus has been shifted to hit the wire tip. The intensiveshort electron pulse produced in this way was completely sufficient for ionization. This electron impact mass spectrum emphasizes the signal of the most abundant, but uninteresting exhaust gas components: nitrogen, carbon dioxide, oxygen, and argon. For the mass spectra marked UV1 up to UV4, wavelengths have been chosen so that selective ionization of special trace components was achieved. In spectrum UV1, the wavelength was 307.6 nm for selective ionization of nitric oxide. In mass spectrum UV2, 301.5 nm has been chosen for acetaldehyde, which has the same mass as carbon dioxide and therefore cannot be detected in the conventional mass spectrum (at the
+
m 4jO
425
4j5’
440
445
i
450
455
Wavelength [nml Flgure 10. As in Figure 8, but A = 425-455 nm.
+
One calibration gas is required for (1 1) ionization processes, for higher-order ionization two calibration gases are required. One calibration gas, triethylamine, can be used in each of the three wavelength ranges. The other calibration gases are perdeuterated acetone and fluorotoluene. Thus, only three gases are necessary for the calibration gas mixture. Certain molecules, such as 02, N20,and NO*, absorb in none of the three wavelength ranges and therefore need special wavelengths for ionization. In a few special cases, other calibration gases may be necessary because of interfering calibration gas and trace components. In principle, the list of components can be easily extended. Thereby attention must be given to mass interferences arising from substances or fragments with the same mass as an exhaust gas component of interest. In most cases this problem can be solved by using a different laser wavelength for ionization. A few organic exhaust gas components, namely, small alkanes, have only very unfavorable intermediate states and ionization thresholds if considered from the viewpoint of energy. Tunable lasers of the next generation, which will supply shorter wavelengths and higher pulse energies, may overcome this problem.22 The restriction to a few wavelength ranges has the additional advantages that laser wavelengths are easily found at which several components can be registered synchronously for each single laser shot. Thus, the concentrations of these compounds 1000
Ana&ticalChetnlstry, Vol. 66,No. 7, April 1, 1994
-nitrogen
monoxide
100
r-
-acetaldehyde
,
-------
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2 EO C
2
-
2
60-
d
--toluene
a
v
0
,
.
40-
e,
g
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I
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I /-xylene
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I1 / I .
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Flguro 13. Toluene concentration in the exhaust gas of a four-stroke engine runningat 2160 rpm (36 s-’). A measurementis taken for each individualemission stroke. The peaks conespondto manually induced ignition failures.
conventional mass s p e c t r u m argon
c a r b o n dioxide -oxygen nitrogen
-MASSFlguo 12. Messspectraofexhaustgsfrwnanactuaiengh.le.(Bottom) Nonselective electron impact ionizatkm. (UV1-UV4) selectbe laser ionization. For further information (wavelengths UV1-UW), see text.
bottom of Figure 12). In mass spectrum UV3, at 260.9 nm, toluene was selectively ionized and in mass spectrum UV4, at 264.1 nm, synchronous detection of toluene, xylene, and trimethylbenzene has been achieved. These are particularly important for the formation of ozone in the areas of smog.3 As a further development of this project, a system will be devised to help chemists and engineers from the automotive industry find the appropriate laser wavelength for measuring a certain combination of trace components synchronously; of course, this is possible only when the MPI spectra of the substances to be measured simultaneouslyshow overlapping regions. Time Resolution. To test the time resolution achievable with this new technique, the toluene emission of a four-cylinder, four-stroke motor was monitored for 5 s. Exhaust gas was sampled immediatelydownstream from the output valve. The rate was 2160 rpm for this measurement. Analysis was therefore performed at a repetition rate of 18 Hz. The result is shown in Figure 13. The ignition of this motor could be manually turned off for single motor cycles leading to a sudden rise of the toluene concentration in the exhaust gas during one stroke. As can be seen in Figure 13, the concentration could be measured for every single emission of the motor. No smearing out of the signal over several motor cycles was observed. Our system allows time resolution down to 0.01 s (24) Lubman,D. M.;TrembcuU,R.;Sin,C.H. Anal. Chem. 1985,57,1084,1186. Letokhov, V.S.h e r Photoionization Spectroscopy;AcademicPress: Orlando, FL, 1981. (25) Wcinkaur, R.; Walter, K.; Weickhardt, C.; B m l , U.; Schlag, E. W. Z . Naturforsch. 1989, IIA, 1219. (26) KUhlewind, H.; Newer, H. J.; Schlag, E.W. J. Phys. Chem. 1985.89.5993, 5600. (27) Boesl, U.;Grotemeyer, J.; Walter, K.; Schlag, E. W.And. Instrum. 1987,16, 151.
(Le., a repetition rate of 100 Hz) limited by the speed of the sampling and inlet system. By varying the delay between motor emission and sample analysis in the laser mass spectrometer, it is even possible to scan the temporal profile of the exhaust gas pulse emitted by the motor.
CONCLUSION A new technique for time-resolved analysis of trace substances in complex gas mixtures has been presented. It is particularly adapted to the detection of air pollutants emitted by combustion engines. At present, this is the first and only method which fulfills the following conditions simultaneously: (i) time resolution ( < l o ms), (ii) sensitivity (1 ppm), (iii) synchronousdetection ofvariouscomponents, (iv) applicability to most exhaust gas components, and (v) applicability to gas mixtures with highly varying concentrations. A specific calibration method has been developed in order to obtain high accuracy even for a single laser shot. For aromatics, an accuracy better than 10% has been measured. The possibilities of resonant laser mass spectrometry as a chemical analysis technique go far beyond detection of traces in exhaust gases. As an example, isomeric substances can be identified or selectivelydetected by using either spectroscopy,24 laser tandem mass ~pectrometry?~ or multiphoton dissociation.Z6 A combination of resonant laser mass spectrometry and laser des0rption2~may even facilitate the analysis of solid probes, such as soot particles, without a complicatedchemical purification process. ACKNOWLEDGMENT This project has been financed by the German Federal Ministry for Research and Technology (BMFT) and by the Research Union for Combustion Engines (FVV, Frankfurt). The authors thank Mr. Holger Nagel and Mr. Stefan Schmidt for their important contributions, Dr. V. Schiifer (BMW, Munich) and the members of the FVV study group ”Laser Mass Spectrometry” for helpful discussion and support, Dr. Franzen and Dr. R. Frey (Bruker-Franzen, Bremen) for successfulcooperation, and Dr. R. Whetten for critical reading of the manuscript. Received for review September 10, 1993. Accepted January 11, 1994.’ a
Abstract published in Aduunce ACS Abstructs, February 15, 1994.
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