Lab Science News

In the early 2000s, the EPA began to investigate the synthetic compound Perfluorooctanoic Acid (PFOA or C8) and its salts, primarily Ammonium Perfluorooctanate (APFO) and other fluoropolymers that may metabolize or degrade into PFOA. These compounds are of interest because of their similarity to another compound known as Perfluorooctyl Sulfonate (PFOS). PFOS was designated a persistent organic pollutant and the primary worldwide manufacturer ceased making it in 2001.

There is still controversy over PFOA’s toxicity, though the compound is persistent (doesn’t biodegrade, hydrolyse or photolyse), bioaccumulates in human and animal tissue (binds to proteins in the blood and liver), and biomagnifies up the food chain. In 2007, the Center for Disease Control and Prevention published the results of two studies on the levels of 11 different polyfluorochemicals in humans. In those studies PFOS, PFOA and Perfluorohexane Sulfonic Acid (PFHS) were found in 98% of those tested, confirming widespread exposure to these compounds. Exposure may occur through consumption of contaminated food or water or through the use of products containing these compounds, but not all sources are known or understood.

PFOA is a polymerization aid used in the manufacturing of fluoropolymers. The carbon fluorine part of the molecule is water resistant, which makes them valuable in producing fluoropolymer products that can repel water, grease and oil. These compounds are used in making non-stick surfaces for cookware, stain resistant clothing, carpets and other fabrics and in fire fighting foams. It is because of its unique polar anionic chemical properties that traditional models used to predict chemical behavior of non-polar organic chemicals, like PCBs or dioxins, in wildlife and humans, cannot be extrapolated from standard experimental data on mice and rats. In rodents PFOA has been shown to be carcinogen and immunotoxic, but whether this can be translated into information about its effect on humans is not clear. Studies continue. It should be noted, in February 2006, the EPA’s Science Advisory Board voted to approve a recommendation that PFOA should be considered a likely carcinogen.

The principal fluoropolymer producers committed to a minimum 50-percent reduction in total global emissions by 2006 (using 2000 as the baseline year), 95% reduction in emissions and product content by 2010 and elimination of its use altogether by 2015. However, because of the persistence of these compounds in the environment and the bioaccumulation and biomagnification in the food chain these compounds will continue to be in the environment long after manufacturing ceases.

Perfluorinated compounds are large molecules and are not amenable to common analytical techniques such as Gas Chromatography/Mass Spectroscopy (GC/MS).

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If you’ve had your laboratory run low-level polyaromatic hydrocarbons (PAHs) or other low level analyses, chances are you have heard of Gas Chromatography/Mass Spectroscopy- Selective Ion Monitoring (GC/MS-SIM). Over the years clients have asked us “What’s the difference between GC/MS-Full Scan and GC/ MS-SIM?” To address this question we must start with the basics. (For our example we will be talking about a standard quadrupole mass spectrometer using electron ionization.)

GC/MS is an instrumental analytical technique comprised of a gas chromatograph and a mass spectrometer. In general, the GC is used to separate complex chemical mixtures into individual components. Once separated, the chemicals can be identified and quantified by the mass spectrometer.

Before analysis can occur a sample must be prepared, usually by extracting the analytes of interest into a liquid solvent phase. This extract is then injected into the GC where it is swept onto a separation column by an inert carrier gas such as hydrogen or helium. The analytes in the mixture are carried through the column by the carrier gas where they are separated from one another by their interaction between the coating (stationary phase) on the inside wall of the column and the carrier gas. Each analyte interacts with the stationary phase at different rates. Those that react very little move through the column quickly and will exit into the mass spectrometer before those analytes having longer interaction and retention times.

When the individual analytes exit the GC column they enter the ionization area (ion source) of the MS. Here they are bombarded with electrons which form ionized fragments of the analyte. These ionized fragments are then accelerated into the quadrapole via a series of lenses and separated based on their mass to charge ratio. This separation is accomplished by applying alternating RF frequency and DC voltage to diagonally opposite ends of the quadrapole, which in turn allows a specific mass fragment to pass through the quadrapole filter. From here the fragments enter the mass detector (electron multiplier) and are recorded. The MS computer graphs a mass spectrum scan showing the abundance of each ionized mass fragment.

A GC/MS system in Full Scan mode will monitor a range of masses know as mass to charge ratio (abbreviated m/z). A typical mass scan range will cover from 35-500 m/z four times per second and will detect compound fragments within that range over a set time period. Laboratories have extensive computer libraries containing mass-spectra of many different compounds to compare to the unknown analyte spectrum. The Full Scan mode is quite useful when identifying unknown compounds in a sample and providing confirmation of results from GC using other types of detectors.

Operation of a GC/MS in SIM mode allows for detection of specific analytes with increased sensitivity relative to full scan mode. In SIM mode the MS gathers data for masses of interest rather than looking for all masses over a wide range. Because the instrument is set to look for only masses of interest it can be specific for a particular analyte of interest. Typically two to four ions are monitored per compound and the ratios of those ions will be unique to the analyte of interest. In order to increase sensitivity, the mass scan rate and dwell times (the time spent looking at each mass) are adjusted.

When properly setup and calibrated, GC/MS-SIM can increase sensitivity by a factor of 10 to 100 times that of GC/MS-Full Scan. Because unwanted ions are being filtered, the selectivity is greatly enhanced providing an additional tool to eliminate difficult matrix interferences.

The ability of the mass spectrometer to identify unknowns in the full scan mode and quantitiate know target analytes in the SIM mode, makes it one of the most powerful tools available for trace level quantitative analysis in the lab today.