The Use of Liquid Chromatography with Tandem Mass Spectrometry for the Analysis of Emerging Environmental Contaminants

Jul 01, 2008

Liquid chromatography with tandem mass spectrometry (LC–MS-MS) led to a revolution in environmental testing. The coupling of liquid chromatography with tandem mass spectrometry created a powerful analytical tool for the analysis of emerging environmental contaminants. Pharmaceuticals and personal care products, perfluorinated compounds, brominated flame retardants, and disinfection byproducts were chosen as examples to illustrate the use of this new technique in environmental analysis.

Traditionally, environmental scientists focused on the analysis of more "classical" environmental pollutants such as dioxins, polychlorinated biphenyls, and chlorinated pesticides (such as DDT). Gas chromatography with mass spectrometry (GC–MS) typically is used to identify and quantify these persistent organic compounds.

However, many organic chemicals are difficult to analyze using traditional GC–MS. Time-consuming and extensive sample preparation is required to reach required sensitivity levels. Liquid chromatography with tandem mass spectrometry (LC–MS-MS) led to a revolution in environmental research over the last decade. LC–MS-MS allows the identification and quantitation of highly polar organic compounds down to nanogram-per-liter levels without time-consuming derivatization and with minimal sample cleanup.

Many emerging environmental contaminants enter the environment after domestic use and are not removed completely by wastewater treatment. Environmental pollutants studied by LC–MS-MS in water and other environmental samples include pharmaceuticals and personal care products (PPCP) such as painkillers, antibiotics, antidiabetics, antidepressants, contraceptives, and lipid regulators. Hormones and other compounds with estrogenic effects and antibiotics are especially a major concern due to a possible development of bacterial resistance. LC–MS-MS is also the analytical technique of choice for the analysis of perfluorinated compounds (PFC), brominated flame retardants (BFR), and drinking water disinfection byproducts (DBP), including haloacetic acids and bromate (1–4).

LC–MS-MS Technology

An LC system with a pump, an autosampler, a column, and a column oven is used in front of the mass spectrometer. Mostly reversed-phase columns with gradients of water, acetonitrile, and methanol with volatile buffers of ammonium formate and formic acid are used for LC–MS-MS to separate analytes of interest and to separate analytes from sample matrix components. After separation, the eluent enters the ion source of the mass spectrometer.

Electrospray ionization (ESI) is the most popular ionization technique. Other LC–MS-MS ion sources are atmospheric pressure chemical ionization (APCI) and atmospheric pressure photo ionization (APPI), which can be used to detect less polar chemicals. In ESI, the eluent is sprayed through a capillary while applying a high voltage to the tip of the spray needle. This procedure generates micrometer-sized droplets containing positively and negatively charged analyte ions. Now gentle heating is used to evaporate solvents and release ions from the droplets. Finally, ions carrying positive or negative charge — depending upon the polarity of the mass spectrometer — are guided by electrical fields into the mass analyzer region. In comparison to traditional GC–MS ionization techniques, such as electron impact (EI), ESI is a very soft technique and produces mostly protonated or deprotonated molecular ions.

Figure 1: Schematics of LC–MS-MS (LC system, ion source, and triple quadrupole).
Different mass analyzers are used in LC–MS-MS laboratories, with triple-quadrupole systems being used widely. New hybrid technologies, such as triple quadrupole coupled with linear ion trap (QTRAP system, Applied Biosystems, Foster City, California) have been developed recently and are gaining popularity. Triple-quadrupole mass analyzers can be used in many different scan modes with multiple reaction monitoring (MRM) being most popular for quantitation. In MRM mode, the molecular ion is filtered in the first quadrupole (ions of different mass-to-charge ratio [m/z] are not allowed to pass the quadrupole). The filtered molecular ion is then fragmented into product ions by collision with a neutral collision gas (such as nitrogen) in the second quadrupole and the product ion of interest is filtered in the third quadrupole (Figure 1). This double filtering reduces the background noise greatly, allowing quantitation with optimal selectivity and sensitivity, excellent linearity, and reproducibility.

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