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Monitoring Environmental Pollutants with Gas Chromatography Techniques

Monitoring Environmental Pollutants with Gas Chromatography Techniques

Sampling technique is the foundation for reliable monitoring of environmental pollutants via GC

Brandoch Cook, PhD

Before the Industrial Revolution, life was often squalid, brutal, and short. Subsequently, people found new ways to persist and even flourish, but did so amidst a noxious brew of factory effluent and airborne vesicants. Exposure to manmade environmental calamity increased correspondingly with prosperity, perhaps reaching its peak in the American consciousness with images of the flaming Cuyahoga River flowing from Cleveland into a moribund Lake Erie. This event was sandwiched between the publication of Silent Spring detailing accumulation of the pesticide dichlorodiphenyltrichloroethane (DDT) in fatty tissues, and the horror of Love Canal, a makeshift dumping ground for the Hooker Chemical Company’s toxic waste that eventually inaugurated the Superfund program.

Fortuitously, increased environmental concern evident in legislation such as the Clean Water Act, and the establishment of the Environmental Protection Agency (EPA), coincided with the development and availability of gas chromatography (GC) techniques and instrumentation pioneered by James and Martin. The high resolution of GC, combined with its accuracy and dynamic concentration range, make it supremely adaptable to separation of environmental pollutants, and coupled to mass spectroscopy, their identification and quantification. Moreover, the regulatory mandate requires identification of as many compounds as possible under constraints of time and expenditure, which is optimal for single capillary GC injections that can be replicated and judged against standards to make pragmatic decisions about what we can drive, eat, breathe, or throw away.

Three broad categories of environmental pollutants are analyzed by GC under EPA guidelines, and the accuracy and fidelity of their measurement is dependent on proper sampling and extraction. 1) Volatile organic compounds (VOCs) include aromatic hydrocarbons such as BTEX (benzene, toluene, ethylbenzene, and xylene) largely found in petroleum derivatives; halocarbons like trichloroethane from industrial solvents; freons used in refrigeration; and alcohols often used as oxygenating agents in gasoline. 2) Semi-volatile organic compounds (SVOCs) include acid-extractable phenols and base-extractable analines and amines (including nitrosamines); and polycyclic aromatic hydrocarbons, putative carcinogens in petroleum emissions. 3) Pesticides and polychlorinated biphenyls (PCBs) are typically toxic at lower levels than other pollutants, with structural stabilities that portend acute or chronic problems linked to exposure. PCBs and organochloride pesticides such as DDT are highly persistent and resistant to breakdown, while phosphorous- and nitrogen-based pesticides are more acutely toxic but move through ecosystems more quickly.

Principles and methods of sampling and extraction

Raoult’s and Henry’s laws determine the efficiency with which VOCs can be extracted after sampling. The vapor pressure of a volatile compound in solution is proportional to its molarity, and by extension the amount of a gas dissolved in liquid is proportional to its partial pressure above that liquid. Headspace sampling is the preferred method for extraction of VOCs for GC injection, either in a static format or, more commonly, via purge and trap (P&T). In static headspace measurements, VOCs in a closed sample vial migrate into the upper, vapor phase, over time to reach an equilibrium. One then injects a portion of this “headspace” into the GC apparatus. Headspace constituents can also be extracted dynamically, greatly increasing efficiency, using P&T devices. In P&T, an inert gas (often helium) purges the sample to extract VOCs, which are retained in an adsorbent trap. Heating the trap releases VOCs in a plug, which is injected into the GC port by back-flushing the trap with carrier gas—helium, hydrogen, or nitrogen. Traps are commonly composed of layers of different adsorbent materials to optimize extraction in a gradient of relative volatility that can be desorbed in reverse order to avoid trapping and desorbing water, which can confound GC analysis.

SVOC extraction is intrinsically more complicated because their higher boiling points preclude gas-phase extraction techniques. Therefore, investigators use liquid-liquid and solid phase extractions, in addition to Soxhlet extraction, all of which commonly use methylene chloride and acetone as solvents. Soxhlet extraction applies a drying agent to samples, which are placed in a thimble between opposing layers of glass wool, with a collection flask from which solvent cycles in a loop, purifying analytes to be subsequently applied to GC. Soxhlet extraction can be fully automated using the Foss Analytics Soxtec 8000, but is also straightforward enough to be assembled from standard laboratory glassware and supplies.

Pesticides and PCBs can be extracted from various matrices in analogous manners. However, because of their greater propensities to be ingested directly as residues on food, lower thresholds are usually allowed based on a combination of 1) adverse effect testing in animals; and 2) natural rates of decay. This condition therefore requires GC detectors with greater sensitivities. An electron capture device detects organochlorines in the range of parts per billion, and a nitrogen-phosphorous detector functions similarly for organophosphorous and nitrogenous pesticides. Although PCBs have been banned in the United States since 1979, they are highly stable and cycle between water and soil; at least 30 percent of Superfund sites on the National Priorities List still have confirmed PCB contamination.

Sampling and measurement in the field

Field-based investigation of pollutants can be informally categorized into two parallel streams: 1) quality-control inspections of industrial facilities to measure evaporative or effluent by-products under EPA regulatory purview; and 2) mapping and sampling of uncontrolled environments to identify potential stressors on regulated resources or impact sites. An example of the former is testing for residual solvents in pharmaceutical manufacturing, which can release several toxic VOCs. For these tests, standard GC detectors can be supplemented with static headspace samplers such as the Gerstel MPS, or P&T sample concentrators like the OI Analytical Eclipse 4660. The addition of emerging technology such as vacuum ultraviolet spectroscopy can increase efficiency via its focus on a narrow band of wavelengths relevant to VOC absorbance. However, because of the need to standardize and replicate measurements, EPA-conforming systems must often follow legacy procedures. In environmental monitoring applications farther afield, Agilent supplies the 490-Mobile Micro GC unit sized as carry-on luggage so that users can run analyses on-site using a variety of sampling methods. Regardless of the field—indoors or out, near or far—GC remains a powerful and versatile method of environmental monitoring and measurement, and proper sampling is its foundation.

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