Real-time PTR-MS vehicle exhaust analysis
Monitoring exhaust gas from combustion engines, emitting a large number of different compounds and with rapidly changing concentrations due to variations in operating conditions, necessitates an instrument that measures in real-time and with high sensitivity: an IONICON PTR-TOFMS.
Introduction – Real-time exhaust emission monitoring
Air pollution has been attributed to automobile exhaust as the major source. Combustion engines used in cars, aircraft, ships, power plants, etc. emit many air pollutants that have been shown to have a variety of negative effects on public health and the natural environment.
Emission testing of combustion engines has therefore been the focus of intense studies. Apart from the emissions comprising carbon monoxide (CO), nitrogen oxides (NOx), particulate matter, and sulfur oxides (SOx), also volatile organic compounds (VOCs) and their atmospheric photochemical reaction products have lately drawn much attention due to their negative impact on the atmosphere and their dangers to public health.
PTR-MS is an analytical technique to quantify hydrocarbons and VOCs with high sensitivity and in real-time. The capability to monitor emissions in real-time is particularly useful to investigate the fast changing conditions in the combustion process and to evaluate the impact of control technologies. Especially the initial cold start phase in cars or idle phase in aircraft engines, which is responsible for a large fraction of emitted compounds, as well as hard acceleration phases, require a real-time (or online) analysis. Looking at the exhaust composition allows investigating which processes and parameters are responsible for those emissions.
Major reasons our customers choose PTR-TOFMS to monitor exhaust emissions:
- Emission rates change rapidly with engine operating conditions, thus a high time resolution measurement is important.
- In contrast to offline methods, real-time analysis allows the measurement of critical compounds. For example, acrolein is an unstable compound which may easily get lost in offline sampling procedure.
- High sensitivity and good separation of isobaric compounds.
Examples of real-time engines emission monitoring with PTR-MS
Diesel exhaust (compression ignition) measurements have been a target of researchers already in the early days of PTR-MS. In 2005 Jobson et al. employed a quadrupole based PTR-MS to assess the VOC emissions of a diesel engine. Their key reasons for using PTR-MS was the
- in-situ (real-time) capability of the instrument, and the
- efficient and soft ionization,
- which delivers simpler mass-spectra than Atmospheric Pressure Chemical Ionization (APCI) mass spectrometer.
The later introduced PTR-TOF systems offered additional advantages for automotive exhaust monitoring:
- the measurement of complete spectra in < 1s,
- separation of isobaric molecules,
- identification of compounds by their chemical composition, and
- higher sensitivity for large molecules.
A typical setup includes a dilution system, such as a constant volume sampler and a heated sampling line to minimize surface interaction of semi volatile compounds.
At the California Air Resources Board (CARB), an IONICON PTR-TOF 8000 is used in studies of automotive vehicle exhaust. As a first step, Matsunaga et al.  validated the PTR-TOF setup against a method utilizing the gold standard, a GC-FID system (SOP MLD 102/103). The study showed that measurements of BTEX vehicle emissions with the PTR-MS agreed with GC-FID to within 15%.
In dynamometer vehicle testing, on-board diagnostics (OBD) signals are used to monitor vehicle parameters. Combining OBD data with PTR-MS measurements of a variety of emitted VOCs such as acrolein, ethanol, styrene, formaldehyde, acetone, ethylbenzene, benzene, acetaldehyde, xylene, trimethylbenzene, 1,3-butadiene, toluene, and naphthalene makes correlation studies possible.
Matsunaga et al.  also studied the emissions from gasoline vehicles and found that the VOC emissions from unburned fuel are extremely high at the beginning of a cold-start drive cycle but quickly vanish as the vehicle warms up. The PTR-MS is unique with the ability to quantify multiple VOC species in real-time.
Gasoline/Petrol and ethanol blends
Gasoline is often blended with ethanol, such as E10 (10% ethanol in 90% gasoline), which has economic as well as environmental considerations. Impacts of ethanol/gasoline ratio on emissions are actively studied elsewhere. For such analysis, a real-time instrument like a PTR-MS is a powerful tool for revealing the relationship between emissions and vehicle parameters.
 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.