Ind. Eng. Chem. Res. 1999, 38, 2203-2209
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Evaluation of a Commercial Mass Spectrometer for Its Potential To Measure Auto Exhaust Constituents in Real Time Mark A. Dearth Ford Motor Company, P.O. Box 2053, Dearborn, Michigan 48121-2053
A newly available commercial instrument, the V&F Instruments (Austria) AirSense 500 mass spectrometer, was evaluated for its ability to measure various hydrocarbons in auto exhaust, in real time. The measurements obtained were compared to off-line high-resolution gas chromatography and other measurement systems during an evaluation period that extended for more than 7 months and involved various Ford vehicle testing facilities. The device was found to provide real-time (1 Hz or faster) concentration data for 10 hydrocarbon exhaust constituents with a precision and accuracy comparable to current off-line gas chromatographic measurement methods. The AirSense 500 was carefully evaluated for its measurement of ethene and other olefins and benzene and other aromatic compounds. The mass spectral measurements agreed, on average within (17% of the reported values for the same vehicle tests as measured by offline GC/FID, when comparing the slope of the linear regression line resulting from direct comparison of both techniques. Linear correlations between the two techniques averaged an r2 of 0.85 for measurements mostly in the 0.01-0.2 ppm range. The instrument was also shown to be useful for the measurement of H2S in dilute vehicle exhaust, at levels down to 20 ppb. In comparison to current methods of H2S measurement, the AirSense 500 was found to be more robust to potentially interfering compounds. Introduction Vehicle exhaust emissions continue to be of concern to regulators and environmentalists. Even as exhaust emission limits reach new mandated low levels in the U.S., the push is on to reduce emissions in Europe and elsewhere. It is likely that regulators will mandate further emission reductions of CO2 and possibly CO, hydrocarbons, and the oxides of nitrogen. The test and measurement systems that were adequate a few years ago are no longer satisfactory for quantifying some emissions at the levels emitted by the next generation of vehicles. This potential problem was recognized, and efforts were begun to address these measurement challenges by the U.S. automakers starting in 1989. One of the results of these efforts was the development of instrumentation that could quantify emissions on a species-specific basis in a real-time manner. Real-time analysis allows one to connect cause and effect and to modify experiments as they progress. The push to develop faster, real-time measurements within the automotive emissions community has paralleled a similar trend within the general analytical chemistry community. There has been a concerted move away from time-consuming, laboratory-bound analytical techniques (such as traditional gas and liquid chromatography) to portable, direct sampling GCs, in situ spectroscopicbased or biosensors, and on-line mass spectrometry. There are many benefits of on-line, real-time analysis including the elimination of sample collection, transportation, preparation, and the reduction of errors associated with these processes. Additional benefits include reduced development time, more interactive testing, and the ability to observe transient chemical phenomena. These benefits result in significant cost and time savings, as well as more robust products. The synergy between the development of instruments capable of such measurements and applications and
processes designed or requiring such measurement capability is still ongoing, but the applications of such devices in the automotive emissions arena continue to grow. One aspect of real-time monitoring of automotive emissions has focused on the quantification of hydrocarbon species. Hydrocarbons, especially compounds such as olefins, aromatics, and aldehydes, have significant ozone-forming potential in the lower troposphere. The state of California requires that these compounds be quantified in auto exhaust so that the ozone-forming potential of the vehicle exhaust can be predicted. The methods adopted by the automakers and regulators use high-performance gas chromatography (GC) coupled with flame ionization detection (FID). Because FID is a nonselective detector (it responds to methylene, i.e., CH2), each compound in the complex milieu of auto exhaust must be resolved to be unambiguously identified. Though standard mass spectrometry cannot quantify in real time a broad range of hydrocarbon species, previous works1,2 have shown that atmospheric pressure ionization and glow discharge ionization mass spectrometry can. Until recently, there has been a gap between what was possible in the laboratory using prototype equipment and a robust commercial reality. We have found that the AirSense 500, made by V&F Instruments can, in real time, quantify individual hydrocarbons and other species. The focus of this work is not to carefully compare dynamic ranges, limits of detection, and linearity (work already done elsewhere3) but to complete the evaluation of the AirSense 500 in a real-world setting. This device was designed to perform in harsh environments and provide real-time data on dynamic processes in the hands of relatively unskilled operators. Such design constraints dictate many compromises. The question is, have these compromises diminished the utility of the device? In the world of
10.1021/ie980116s CCC: $18.00 © 1999 American Chemical Society Published on Web 02/25/1999
2204 Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999 Scheme 1a
Table 1. Potential Compounds of Interest in Vehicle Exhausta major ions
compd
a
A is the primary gas inlet. B is the sample gas inlet.
emissions sampling, the life of an instrument is often harsh and unpleasant. Many instruments that have performed well in the lab fail when asked to reliably measure 24 h a day on the test-cell floor, adjacent to industrial machinery, in an uncontrolled environment, and in the hands of relatively unskilled operators. We were looking for an instrument that was robust, easy to use, and provided real-time data and meaningful analytical results. We report here the real-world evaluation of this device for the dynamic measurement of hydrocarbons and other species of interest in dilute vehicle exhaust, in a rugged, industrial environment. Experimental Section V&F Mass Spectrometer. Principles of Operation. The AirSense 500 is a linear quadrupole instrument that was developed specifically to monitor gaseous organic and inorganic molecules.3,4 A drawing of the device is shown in Scheme 1. The instrument uses electron ionization to create a primary ion beam of mercury (Hg), xenon (Xe), or krypton (Kr). This primary ion beam is transferred by a radio-frequency octapole to a collision region where sample gas is tangentially introduced. Collisions between the primary ion beam and the neutral sample gases result in charge exchange if the sample molecule has a lower ionization potential. One can tailor the ionization process by selecting different ionization gases. For instance, by using Hg+ ions as the primary gas, one will avoid ionizing gases whose ionization potentials are above 10.44 eV. Since the small permanent gases such as N2, O2, H2, H2O, CO2, CO, etc., require more energy to ionize, they will not be observed in the resulting mass spectra. Species such as C2H4, NO, NO2, H2S, and most of the olefins and aromatic molecules (see Table 1) are ionized by Hg+ ions. This selective ionization provides for ultratrace (parts per billion) detection for some compounds in the presence of bulk amounts of the normal atmospheric gases. Physical Description. The device (see Figure 1) is a compact system consisting of three stacked modules. The top module contains a computer and various control electronics, the middle module the mass spectrometer and sampling system hardware, and the bottom module the various pumps. The entire system weighs about 100 kg and is about 100 cm high by 50 cm square. The device is mobile, is self-contained (including all pumps), and operates on standard 110 V in the U.S. The system uses a Windows 95-based data system and PC. Analytical Methods. GC and MS. Quantification of the hydrocarbon species followed procedures previously developed for the Auto/Oil Air Quality Improvement Research Program and described elsewhere.5
ionization Kr: molec potential, 13.9 formula MW eV eV
methane acetylene ethylene ethane propadiene propene propane 1,3-butadiene butenes butane 1,3-pentadiene pentene pentane benzene hexene hexane toluene heptene heptane xylenes trimethylpentane trimethylbenzenes
CH4 C2H2 C2H4 C2H6 C3H4 C3H6 C3H8 C4H6 C4H8 C4H10 C5H6 C5H10 C5H12 C6H6 C6H12 C6H14 C7H8 C7H14 C7H16 C8H10 C8H18 C9H12
nitrogen carbon monoxide carbon dioxide hydrogen cyanide nitrous oxide sulfur dioxide oxygen hydrogen sulfide nitric oxide nitrogen dioxide
Inorganic Gases N2 28 15.6 CO 28 13.9 CO2 44 13.7 HCN 27 13.6 N2O 44 12.6 SO2 64 12.3 O2 32 12.1 H2S 34 10.5 NO 30 9.3 NO2 46 9.8
16 26 28 30 40 42 44 54 56 58 66 70 72 78 84 86 92 98 100 106 114 120
12.7 11.4 10.4 11.5 10.1 9.7 11.1 9.1 9.7 10.6 8.9 9.5 10.4 9.3 9.5 10.2 8.8 9.9 9.9 8.5
