Volatile Organic Compounds (VOC) Analysis

In an environmental analysis context, the VOC designation, or volatile organic contaminants, generally refers to the analysis of compounds in environmental samples with the following chemical properties:+

• Low boiling points (below 200°C)
• Low vapor pressures
• Low-to-medium water solubility
• Organic compounds
• Low molecular weights
• HLC(H)*10-3–10-5 atm*m3/mol'

*Henry's law constants (H) range defined as volatility from liquid to air

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• Halogenated hydrocarbons
• Aromatics

• Ketones
• Nitriles

• Acrylates
• Acetates

• Ethers
• Sulfides

At room temperature the properties of VOC cause these compounds to vaporize, evaporate, or sublimate from the liquid or solid form, allowing them to escape into the atmosphere. Sampling methods and extraction techniques must be designed to limit volatile loss from collections to analysis. Similar to SVOC analysis, the type of sample being analyzed dictates the extraction method required. However, techniques to capture and isolate VOC will differ greatly from semivolatile methods.

There are various techniques and strategies used to prepare and transfer VOC to instrumentation for analysis. Analysis of volatiles is dominated by the use of purge-and-trap followed by gas chromatography (GC) or gas chromatography mass spectrometry (GC/MS), although static headspace is also used frequently. Five of the most common methods for volatile extraction are listed below.

Within a sample vial the unoccupied area above the sample is commonly referred to as the head space. Using static headspace, sealed vials containing sample are gently heated to drive VOC compounds out of the sample matrix into equilibrium with the gas phase. Once stabilized the gas phase within the vial is then collected or directly transferred to the instrument for analysis. This technique is generally suited for compounds with relatively high Henry's law constants including fuel range organics. However, this method may be unsuitable for MTBE or ethylene dibromide (EDB).

Perhaps the most commonly used and most sensitive technique for volatile analysis is purge and trap concentration, also known as dynamic headspace sampling.

• Sample is deposited into the sparging vessel where method analytes are purged from the water using a helium or other inert gas stream to sparge through the sample. As with static headspace heat is sometimes applied to drive VOCs out of liquid phase and into the gas phase.
• Once liberated from the sample VOC’s in the gas phase are transferred to an adsorbent trap where they will transiently bind.
• The trap is then heated and backflushed with carrier gas to drive the analytes into a gas chromatographic.

Note: After purging, the trap may be dry purged for a short period to remove water.

As stated previously the most common technique used to detect, identify and quantitate VOC is gas chromatography with flame ionization (FID), electron capture (ECD) or mass spectrometry (GC-MS) detection. Selection of a detection method requires consideration of various analytical parameters and regulatory requirements. Common stand-alone GC detectors used in VOC analysis with their affinities are listed in the table below.

Mass spectrometry has generally replaced GC stand-alone for detection of vocs due to a higher degree of confidence in compound identification. However, routine monitoring of many volatiles is still accomplished by Gas Chromatography detection today. Using GC- MS methods analytes are identified by comparing the acquired mass spectra and retention times to reference spectra and retention times for calibration standards acquired under identical GC-MS conditions.

Regulatory agencies publish guidance for monitoring volatile contaminants in the environment. Regulations often group VOC based on several criteria including detection methods, intended compound purpose use, compound structural attributes classifications and other justifications.

Although the United States Environmental Protection Agency (EPA) publishes specific methods for analyzing VOCs not all regulatory agencies provide method guidance. This European Water Framework Directive (EUWFD) provides VOC monitoring limits without a designated methodology. The analytical method chosen must meet published detection limits and criteria specified by each governing body. Frequently used VOC methods are listed below.