Volatile organic compounds (VOCs) are a group of highly-utilized chemicals that have widespread applications, including use as fuel components, as solvents, and as cleaning and liquefying agents in degreasers, polishes, and dry cleaning solutions. VOCs are also used in herbicides and insecticides for agriculture applications.

Laboratory-based methods for analyzing VOCs are well-established; however, analyzing VOCs in the laboratory is time-consuming - obtaining a result may require several hours to several weeks depending on the specific method. Faster, commercially-available methods for analyzing VOCs quickly in the field include use of portable gas chromatographs (GC), mass spectrometers (MS), or gas chromatographs/mass spectrometers (GC/MS), all of which can be used to obtain VOC concentration results within minutes. These instruments can be useful in rapid confirmation of the presence of VOCs in an asset, or for monitoring an asset on a regular basis. In addition, portable VOC analyzers can analyze for a wide range of VOCs, such as toxic industrial chemicals (TICs), chemical warfare agents (CWAs), drugs, explosives, and aromatic compounds. This document summarizes the different types of portable VOC analyzers currently on the market, discusses the various methods they use to quantify VOCs, and highlights the strengths and weaknesses of the different technologies. Once potential users understand the strengths and weaknesses of each technology type, they may be better able to determine the type of system that is appropriate for their needs.

There are several easy-to-use, portable VOC analyzers currently on the market that are effective in evaluating VOC concentrations in the field. These instruments utilize gas chromatography, mass spectroscopy, or a combination of both methods, to provide near laboratory-quality analysis for VOCs. Gas chromatography, mass spectroscopy, and combined gas chromatography/mass spectroscopy are discussed in detail below.

GC uses the differences in the partitioning behavior of different VOCs to separate complex VOC mixtures into individual VOCs. Because GC depends on this partitioning behavior, its use is primarily restricted to contaminants that can be volatilized.

Identifying and quantifying contaminants in a GC requires preparing the samples, and then injecting the samples into the GC. These steps are discussed separately below.

Sample Preparation
Because a GC cannot measure VOCs directly from a water sample, different techniques must be used for preparing water samples prior to injection into a GC. These methods include:

Each of these methods is discussed in detail below.

Direct injection involves injecting the sample directly into the GC. There are two different methods for direct injection - direct aqueous injection and liquid/liquid extraction. In direct aqueous injection, a water sample is injected directly into the port. However, directly injecting an aqueous sample onto the column will eventually degrade the column, and thus this injection method may not be preferable. In addition, water cannot be transferred into an MS. If an MS is used in tandem with a GC, the sample must be transferred into another solvent before it is injected into the GC. In liquid/liquid extraction, the VOCs are first stripped from the aqueous sample using a solvent. The solvent is then separated from the water, and is injected into the GC.

Sampling using SPME-SPDE fibers is an effective but time consuming method that is typically used for air sampling, but can also be used for water sampling. SPME fibers are coated with a liquid (polymer), a solid (sorbent), or a combination of both. The fibers are exposed to the water sample under mixing conditions, for a duration that allows the VOC to adhere to the fiber, either through absorption (in the case of liquid coatings) or adsorption (in the case of solid coatings). The fibers are then exposed to the injection port, where the compounds on the fibers are thermally desorbed into the GC.

Static headspace sampling detects VOCs in the headspace above the water being sampled. Individual samples are heated until the concentration of VOCs in the water equals the concentration of VOCs in the air above the water. One mL of the gas is then injected into the GC. In the dynamic headspace sampling/purge and trap method, helium (or other inert gas) is passed through the sample to strip the VOCs from the liquid into the gas. The VOCs are then deposited on a trap before injection into the GC.


Sample Filtering
While some users working in a laboratory environment filter samples prior to injecting them into a GC, none of the instruments described in this document require filtration before sample analysis.

Sample Identification and Quantification
Once the samples are properly prepared, they are injected into the GC so that the individual contaminants can be separated, identified, and quantified. To accomplish separation in a GC, the sample is passed through a small diameter resin column (referred to as a capillary column or stationary phase) using a carrier gas (such as helium, argon, and nitrogen) or a solvent (the carrier gas or solvent is called the mobile phase). As the mobile phase passes over the column, the contaminant compounds are adsorbed to the column. Next, an eluent is put through the column. This eluent binds the compounds and strips them from the column. Different contaminants have different affinities for the column, and thus different contaminants are stripped off the column at characteristic times. The eluent then flows through a detector, which reads individual contaminants as peaks relative to the baseline established by the eluent. The size and position of each peak relative to the baseline can be used to indicate the specific identity and concentration of each contaminant. After the sample has run through the system, the system is purged, and then the next sample can be injected.

Once the individual VOCs are separated from each other, they are detected and quantified. There are several different types of detection systems that can be used with a GC, including Flame Ionization Detectors (FIDs), Photoionization Detectors (PIDs), Thermal Conductivity Detectors (TCDs), Electrolytic Conductivity Detectors (ELCDs), Surface Acoustic Wave (SAWs) Detectors, Electron Capture Detectors (ECDs), and Mass Spectroscopy (MS). However, the portable field VOC monitors currently available use either SAW, ECD or MS detectors, and therefore only these types of detectors will be discussed in this document.

SAW condensing detectors are often used to detect and quantify chemical warfare agents and hydrocarbons. These detectors contain a crystal-based resonator that resonates at a known frequency. As different sample fractions come off the stationary phase, they pass into the resonator and onto the crystal. By measuring changes in the resonance of the crystal in response to the sample, the specific identity and concentration of the contaminant can be determined.

An ECD is a radioactive source that produces high-energy electrons. As samples enter the chamber, they are bombarded with these electrons, creating negatively charged ions. Voltage is then pulsed through the cell electrodes, and the output through the cell is measured. Changes in the in the pulse rate relative to the applied pulse indicate the extent to which the sample has captured electrons. By comparing the actual voltage measured in the cell to reference voltages, the specific sample contaminant can be determined. Because of its high sensitivity and selectivity, the ECD is most widely used for halogenated compounds.

MS detectors are discussed in their own section below.

Compounds Measured
A GC compares individual results from the sample against a set of standard reference measurements to identify and quantify concentrations of compounds in a sample. This requires either that the GC be manually standardized using a set of known compounds, or that it use a standard reference library that users can upload into the GC. In general, a GC can measure 20 percent of organic compounds known to exist, and perhaps 60-80 percent of those known to be hazardous to human health. In the event an unknown compound is identified, the GC can be preprogrammed with an alarm to alert the operator to conduct additional evaluation of the unknown.

Advantages and Disadvantages
The major advantage relative of a GC relative to other VOC detection technologies is its resolving (i.e., separating) power. Its major limitation is that the samples to be analyzed must be sufficiently volatile for the instrument to function properly (see discussion of sample preparation above for requirements for volatilizing samples for analysis).

In MS, the sample is bombarded with an electron beam having sufficient energy to fragment the molecular structure of the sample. The positively-charged fragments which are produced from this fragmentation are accelerated in a vacuum through a magnetic field and are sorted on the basis of mass-to-charge ratio. The MS then interprets this information and back-calculates information about the original molecule. MS is therefore useful for quantitation of atoms or molecules and also for determining chemical and structural information about molecules.

There are currently three types of MS available on the market: quadrupole MS; ion trap MS; and magnetic sector MS. However, only the quadrupole MS is available for portable VOC detection devices. Therefore, only the quadrupole MS will be discussed in this document.

The quadrupole MS is used primarily for soil and wastewater analysis. It consists of four parallel metal rods arranged in the shape of a diamond. The sample is passed through the center of the rods, and then an electric current is applied to the system. The current affects the trajectory of ions traveling between rods, and at any given voltage, only ions of a certain mass-to-charge ratio pass through the quadrupole filter; all other ions are thrown out of their original path. By monitoring the ions passing through the quadrupole filter as the voltages on the rods are varied, a mass spectrum of the sample can be obtained.

A GC to an MS is easily accomplished because the GC's mobile phase is gaseous. A GC/MS system incorporates the separation power of a GC with the identification power of an MS. As discussed above, most of the detectors used with a GC can only identify compounds for which they are preprogrammed and calibrated; however, MS can be used to positively identify and quantify an unknown compound. Therefore, a combined GC/MS can provide legally defensible data and a positive identification of known and unknown compounds. While the GC/MS can positively identify unknowns, its detection capabilities increase if the unit is pre-calibrated to identify specific contaminants.

Seven commercially available portable VOC analyzers were identified and reviewed as part of EPA's Environmental Technology Verification (ETV) program. EPA analyzed a range of VOCs from contaminated groundwater sources with each instrument and evaluated that instrument for accuracy, precision, linearity, method detection limit, matrix interference effects, inter-unit reproducibility, rate of false positives/false negatives, and other factors. A summary of results of these evaluations are presented below. For a full discussion of the tests and results, see http://www.epa.gov/etv/verifications/vcenter1-4.html and http://www.epa.gov/ETV/verifications/vcenter1-5.html.

Table 1 summarizes the features of the portable GC, MS, and GC/MS systems reviewed under the ETV program, while Table 2 summarizes the sampling methods and several of the advantages/disadvantages of using each.

As mentioned above, each of the instruments evaluated by the ETV program was assessed for accuracy and precision. A summary of the accuracy and precision of each of the instruments is provided below.

Accuracy is a measure of how close a measurement is to the true value of the parameter being measured. The measured difference between the sample reading and the true value is referred to as "bias." Bias is reported as a percentage of the measured value relative to the true value, and can be positive (sample reading is above the true value) or negative (sample reading is below the true value). Bias can vary in magnitude between different analytical techniques or procedures. It should also be noted that small errors can result in a large bias when the actual concentration in the sample is low.

Precision measures the repeatability of a measurement (e.g., the bias in measuring the same sample a number of times). EPA's ETV program expresses precision as the Relative Standard Deviation (RSD) of replicate analyses.

Detection limit is dependent on the VOC that is being measured. The detection limits reported are typical limits for most VOCs. Table 3 summarizes the detection limit, accuracy, and precision results of the EPA ETV.

Table 4 summarizes the setup and analysis times of the VOC detection devices, as well as the types of reference libraries used.

Cost for the various portable VOC monitoring devices are summarized in Table 5.