Standard Test Method for Quantification of Volatile Organic Compounds Using Proton Transfer Reaction Mass Spectrometry

Importancia y uso:

5.1 Vapor intrusion testing has been performed traditionally using multiple canister samples or thermal desorption tube samples. These discontinuous measurements have been shown to be snapshots and provide averages of exposure. In many cases a higher temporal resolution is desirable to identify peaks of emissions due to specific occupancy or environmental changes. For these cases, a continuous real-time monitoring solution is desirable. These continuous monitoring setups can be either short-term or be part of a long-term monitoring plan as described in ASTM guide “Standard Guide for the development of LongTerm Monitoring Plans for Vapor Mitigation Systems” (E2600).

5.2 The PTR-MS provides real-time measurement of multiple VOCs at ultra-trace levels, that is, in the µL/L (ppm) to less than pL/L (ppt) range. Its strengths lie with the ability to measure VOCs in real-time and continuously (that is, ~1 Hz or faster, using time-of-flight analyzers), and with limited sample pre-treatment, compared to a gas chromatograph (GC) system, which is commonly the method of choice to measure VOCs using a variety of detectors. In case of PTR-MS with quadrupole analyzers, the terms would be nearreal-time and semi-continuous. The high temporal resolution of the PTR-MS measurement in the range of second(s) is often desired when studying the atmospheric chemistry or source emissions that result in unpredictable, sudden, and short-term fluctuations. For a detailed description on the design and theory and practical aspects of operation for the different types of PTR-MS, please refer to Yuan et al. (2017)(1).

5.3 For ambient air measurements, such as vapor intrusion (VI) related emission testing, the PTR-MS can be used in three different modes of operation: (1) in scanning mode to identify sources and VI entry points within buildings; (2) in variation identification mode, as a continuous monitoring instrument with seconds to minutes of temporal resolution covering a large number of VOCs; (3) in source tracking mode, as a scanner of indoor and outdoor sources and as a rapid tracking device for external emissions; this requires the instrument to be mounted on a moveable platform, such as on an (autonomous) vehicle or trolley. The same operation can be used to identify various other constituents in air, depending on the application—be it fugitive emissions from toxic materials or illicit materials, or metabolic reactions to infections expressed in different breath emissions.

5.4 Spatial and temporal variability are two common challenges with ambient air measurements and source assessments. Within a given building, the sources for vapors can be few or many and are generally irregularly spaced; they may be obscured from view by floor coverings, furniture or walls, which in itself can be a large source of VOC. The current methods of choice require the use of time-discreet monitoring or time-averaged monitoring of a specific sampling spot. Real-time monitoring provides a method to assess the spatial distribution of vapor concentrations, which may help to rapidly and efficiently identify the location of vapor entry points.

5.5 Real time assessment is valuable as a component of a program of assessment with two or more supporting lines of evidence and can be used to:

5.5.1 Provide support for real-time decisions such as where and when to collect long-term samples for fixed laboratory analysis using canisters or sorbent tubes;

5.5.2 Verify data quality (for example, monitoring the efficacy of soil gas probe purging prior to sampling, providing leak checks; and

5.5.3 Measure changes in VOC vapor concentrations in response to changes in building pressure, temperature, solar irradiation, or other weather conditions and factors affecting vapor fate and transport, including secondary chemistry occurring within the building.

5.5.4 Identify alternative pathways based on prior identified intrusion compounds or based on emissions within such pathways, such as stormwater drains.

5.6 Screening of a property prior to a real estate transaction based on site specific potential sources of concern. The option for voluntary investigative assessments of potential VI in the real estate business is described in ASTM method E2600-15.

Note 1: The quality of the result produced by this standard is dependent on the competence of the personnel performing it, and the suitability of the equipment and facilities used. Agencies that meet the criteria of Practice D3740 are generally considered capable of competent and objective testing/sampling/inspection/etc. Users of this standard are cautioned that compliance with Practice D3740 does not in itself assure reliable results. Reliable results depend on many factors; Practice D3740 provides a means of evaluating some of those factors.

Subcomité:

D18.21

Volúmen:

04.09

Número ICS:

13.040.20 (Ambient atmospheres)

Palabras clave:

air toxics; ambient atmospheric analysis; hazardous vapors; mass spectrometry; proton transfer reaction; real-time environmental monitoring; site characterization; vadose zone gases; vapor intrusion; VOC;