4. Non-target UHPLC/MS Analysis of Emerging Contaminants in Water

Contamination of water resources is one of the major problems to be faced for environment preservation and sustainability. Although anti-pollution strategies taken in the last half-century have consistently reduced in surface water the amount and the presence of many recognised contaminants, other potentially hazardous chemicals are being released into the environment, together with new substances that are continuously synthesized and whose dangerous properties are not well known. Water-pollutant monitoring makes typically use of methods developed for target analysis, focused on priority pollutants. The monitoring of target-compounds based on mass spectrometry (MS) and selected reaction monitoring (SRM) mode is often insufficient to definitely assess the quality of surface water, just because the presence of only a limited number of potential pollutants is considered. Also potentially harmful non-target pollutants simultaneously present must be taken into account.

In the determination of semi-polar and polar pollutants, liquid chromatography coupled with tandem MS is generally the technique of choice. To obtain a complete information on water composition, full-spectrum acquisition techniques, better if with the possibility to obtain information about MS/MS spectra, are required. For this purpose, hybrid mass spectrometers like triple quadrupole/linear ion trap (QqLIT), hybrid quadrupole/time-of-flight (QqTOF) MS and linear ion trap/orbitrap analyzer must be used. The last two instruments are the masters of the non-target approach, because their can offer the advantage of high resolution MS, that combined with a good MS accuracy allows to identify the unknown species. In addition, the hybrid nature of such instruments allows to acquire during a single chromatographic run also information about high resolution MS/MS spectra, that are fundamental in order to attribute to each unknown species a probable chemical structure. An interesting advantage associated with high resolution MS based methodologies consists in the possibility of performing retrospective analysis, since it allows the identification of organic contaminants included in the first screening: this investigation can be done at any time, without the need of new analysis or new sample injection.

In addition, the elevated acquisition speed of TOF makes it compatible with ultra high performance liquid chromatography (UHPLC). UHPLC and high resolution MS provide potent analysis rich in information on sample composition. During a non-target screening analysis, all the compounds eluted from the analytical column can be detected without any kind of selection, a part the obvious limitations derived from chromatography and ionization process in the LC-MS interface. A genuine non-target analysis involves the automated component detection from the total ion current and the use of deconvolution software to detect the presence of multiple species and to produce pure spectra for each individual component. An important limitation to this approach is the lack of availability of large compound libraries similar to those used in gas chromatography-mass spectrometry (GC-MS), making the identification of unknown species very complex and time-consuming. At the moment, spectral libraries for LC-MS are home-made and quite limited. This review presents an overview of published UHPLC-MS methods developed for post-target and non-target screening analysis of water emerging contaminants, such as pesticides and their degradation products, pharmaceuticals and drug side-reaction products, surfactants and illicit drugs. The different aspects of the current MS instrumentations, using tandem, hybrid and high resolution MS systems, are compared and discussed.