Real-Time Engine Exhaust Analysis with Ultra-Sensitive PTR-TOFMS

The major source of air pollution has been attributed to automobile exhaust. Combustion engines used in cars, ships, aircraft, power plants, etc release a number of air pollutants that have been shown to have many different negative effects on public health and the natural environment.

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Emission testing of combustion engines has thus been the focus of concentrated studies. in addition to the emissions comprising of nitrogen oxides (NOx), carbon monoxide (CO), particulate matter, and sulfur oxides (SOx), volatile organic compounds (VOCs) and their atmospheric photochemical reaction products have also attracted a lot of attention because of their negative impact on the atmosphere and their hazards to public health.

PTR-MS is an analytical method that enables real-time quantification of hydrocarbons and VOCs with high sensitivity. The capability to monitor emissions in real-time is mainly useful to examine the rapid changing conditions in the combustion process and to assess the impact of control technologies. Particularly, the preliminary cold start phase in cars or idle phase in aircraft engines, which is accountable for a large fraction of discharged compounds, as well as hard acceleration phases, need a real-time (or online) analysis. Looking at the exhaust composition enables examining which parameters and processes are accountable for those emissions.

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Major Reasons Customers Choose PTR-TOFMS to Monitor Exhaust Emissions:

• Emission rates alter rapidly with engine operating conditions, and hence a high time resolution measurement is vital
• High sensitivity and good separation of isobaric compounds
• In contrast to offline methods, real-time analysis enables the measurement of critical compounds. For instance, acrolein is an unstable compound which may easily get lost in an offline sampling procedure

Examples of Real-Time Engines Emission Monitoring with PTR-MS

Measurements of diesel exhaust or compression ignition have been a target of researchers already in the initial days of PTR-MS. In 2005, Jobson et al. used a quadrupole based PTR-MS to evaluate the VOC emissions of a diesel engine. Their main reasons for using PTR-MS was the

• in-situ (real-time) capability of the instrument, and the
• efficient and soft ionization, which provides simpler mass-spectra than Atmospheric Pressure Chemical Ionization (APCI) mass spectrometer.

The PTR-TOF systems, which were later introduced, offered extra benefits for automotive exhaust monitoring:

• The measurement of complete spectra in < 1second
• Separation of isobaric molecules
• Higher sensitivity for large molecules
• Identification of compounds by their chemical composition, and

Setup

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A general setup includes a dilution system, such as a heated sampling line and a constant volume sampler to reduce surface interaction of semi-volatile compounds.

At the California Air Resource Board (CARB), an IONICON PTR-TOF 8000 is employed in studies of automotive vehicle exhaust. As a primary step, Matsunaga et al. [1] validated the PTR-TOF system against a technique utilizing the gold standard, a GC-FID system (SOP MLD 102/103). The research revealed that measurements of BTEX vehicle emissions with the PTR-MS matched with GC-FID to within 15%.

Diesel Exhaust

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In dynamometer vehicle testing, on-board diagnostics (OBD) signals are used to monitor vehicle parameters. Integrating OBD data with PTR-MS measurements of numerous emitted VOCs such as ethanol, acrolein, formaldehyde, acetone, styrene, ethylbenzene, acetaldehyde, benzene, xylene, 1,3-butadiene, toluene, trimethylbenzene, and naphthalene allows for correlation research.

Gasoline/Petrol Exhaust

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Matsunaga et al. ^[1] also researched the emissions from gasoline vehicles and found that the VOC emissions from unburned fuel are very high at the beginning of a cold-start drive cycle but rapidly vanish as the vehicle warms up. The PTR-MS is unique with the ability to quantify a number of VOC species in real-time.

Gasoline/Petrol and Ethanol Blends

Gasoline is frequently mixed with ethanol, such as E10 (10% ethanol in 90% gasoline), which has environmental as well as economic considerations. Impacts of ethanol/gasoline ratio on emissions are keenly studied elsewhere. For such analysis, a real-time instrument like a PTR-MS serves as a robust tool for demonstrating the relationship between vehicle parameters and emissions.

References

[1] Matsunaga et al. (February, 2015): Motor Vehicle Exhaust Analysis with a Proton-Transfer-Reaction Mass Spectrometer (PTR-MS) – Comparison Study with Conventional Methods for BTEX and Other Toxic Air Contaminants. Poster presented at CRC Mobile Source Air Toxics Workshop, Sacramento, CA, U.S.A.

This information has been sourced, reviewed and adapted from materials provided by IONICON Analytik.

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