SIFT-MS Air Quality Monitoring
Good air quality is an important contributor to quality of life, since pollution affects human health, property and the natural environment. Primarily formed by human activities, volatile organic compounds (VOCs) are significant pollutants that are often directly hazardous to human health and also contribute to secondary effects such as ozone production in photochemical-induced smog.
SIFT-MS is a unique technology that samples and analyzes whole air in real-time, to sub-part-per-trillion-by volume (pptv) level. SIFT-MS provides an ideal solution for monitoring VOCs and certain inorganic gases in ambient air.
This article briefly reviews the most important hazardous airborne volatile organic pollutants, then provides an overview of current air monitoring technologies, before presenting example data obtained from a SIFT-MS instrument used for detection of ambient VOCs.
Hazardous Airborne Volatile Organic Pollutants
VOCs are a diverse group of carbon-based compounds that exist either as gases or volatile liquids at room temperature. As well as causing direct negative health effects, VOCs are significant contributors to photochemical smog. Current methods to detect and quantify VOCs (and the associated health risks) often rely on the measurement of total non- methane organic compound (NMOC) concentration in the atmosphere or from a particular source. The NMOC concentration is then entered into accepted kinetic models for photochemical smog formation.
Unfortunately, measuring total concentrations of VOCs in air samples does not provide a reliable indication of potential health effects from air pollution. This is because VOCs differ tremendously in toxicity, and it is often trace pollutants that pose the greatest health risks.
The United States Environmental Protection Agency (US EPA) lists 189 toxic air pollutants in its revised Clean Air Act. Of the 189 hazardous air pollutants (HAPs) or air toxics, approximately half are VOCs, with the balance comprising primarily metals, other inorganic compounds and semi-volatile organic compounds.
It is, however, very difficult to monitor concentrations of so many HAPs over large areas, and since the majority are traceable to particular sources (e.g., plastics or pesticide manufacture) there is little reason to monitor all 189 in all areas. Therefore the US EPA focused its attention on 33 HAPs that pose “the greatest threat to public health in the largest number of urban centers”. Nineteen of these pollutants are VOCs. Since they pose such a widespread threat, it makes sense to routinely and widely monitor these VOCs in urban centers.
A key hindrance to wide-scale monitoring of even the urban HAPs is that an ideal technology (that is, one that is not only technically capable but also practical and economical) has not been commercially available. Consequently, national environment agencies have targeted only the greatest threats to the wider population. For example:
- The National Air Quality Strategy (NAQS) for the United Kingdom4 “sets objectives for eight air pollutants to protect health” (p7), of which two (benzene and 1,3-butadiene) are VOCs. Note that annual thresholds have been set for about 100 air pollutants (of which around half are VOCs).
- The European Union directive 2008/50/EC seeking cleaner air for Europe 6 lists only one VOC: benzene.
- The New Zealand Ministry for the Environment’s recently revised guidelines for air quality added the “priority organic contaminants”: benzene, 1,3-butadiene, formaldehyde, and acetaldehyde. These VOCs are also indicated as most important by Australian authorities8 and are the four major VOCs targeted in US air monitoring programs.
The remainder of this whitepaper briefly compares the most commonly available ambient monitoring technologies and then focuses on SIFT-MS, which provides a significant advance in real-time monitoring solution.
Technologies for real-time air quality monitoring
There are a number of commercially available technologies for continuous monitoring of ambient VOCs, several of which are compared in Table 1. Most techniques use some form of mass spectrometry, although the infrared spectroscopic technique is also popular.
Table 1 indicates that SIFT-MS offers the best fit for general, real-time VOC analysis in ambient air because it is easy to use, easily integrated, very sensitive and has high stability with respect to calibrations and humidity.
|Characteristic||Long-path FTIR||Fast GC/MS||MIMS||SRI-PTR-MS||SIFT-MS|
|Compounds accessible to technology||Wide||Wide, but limited by column||Wide, but limited by membrane||Wide||Wide|
|Selectivity||Moderate to high||High||Moderate||High||High|
|Limit of detection||10 ppbv||1 ppbv||1 ppbv||<1 pptv||<1 pptv|
|Response time||Seconds||Several minutes||>10 s||100 ms||100 ms|
|Required user skill level||High||Moderate to high||High||High||Caters to wide range of users|
FTIR = Fourier Transform Infrared Spectroscopy; GC/MS = Gas Chromatography Cass Spectrometry; MIMS = Membrane Introduction Mass Spectrometry; SRI-PTR-MS = Switchable Reagent Ion – Proton Transfer Reaction Mass Spectrometry; SIFT-MS = Selected Ion Flow Tube Mass Spectrometry
Hazardous Air Pollutants (HAPs) Monitoring
The high-speed analysis provided by SIFT-MS allows continuous monitoring of VOCs. Figure 1 shows concentration data obtained over a 42-hour period at a school adjacent to an industrial area near Taipei, Taiwan from 3 pm on 19 July to 9 am on 21 July 2011. The data for selected compounds were extracted from full mass scans acquired with five-minute time resolution.
Gusty wind conditions during the trial lead to increased signal variability compared to analysis in static air conditions. Nevertheless, several compounds show interesting trends over the sampling period, including toluene, C3-alkylbenzenes (e.g. mesitylene), methanol, isopropyl alcohol, acetone and N,N- dimethylmethanamide.
The suitability of SIFT-MS for ambient air monitoring is, however, much greater than Figure 1 illustrates. SIFT-MS can analyze air for the vast majority of VOCs and is easily configured for analyzing different compounds. This means SIFT-MS can perform general air monitoring for a wide range of hazardous air pollutants or can be quickly configured to track compounds at a specific source. Table 2 lists some common air pollutants SIFT-MS can monitor in ambient air, together with national authorities that list them.
|Hazardous air pollutant (HAP)||US EPA Hazardous Air Pollutants ||UK DEFRA National Air Quality Strategy ||Australian National Pollutant Inventory ||New Zealand Ministry|
for the Environment
|ethylbenzene + xylene isomers||✓||✓|