## 3 Analytical Instrumentation
### High-Resolution Mass Spectrometry (HRMS)  
The analysis of transformation products of CECs has become the subject of intense analysis in the field of environmental chemistry, given that the transformation products may be more toxic or persistent than the parent compounds, thus representing a greater risk for the environment and human health [53]. The LC or GC associated with high-resolution mass spectrometry (HRMS) has been widely applied in the identification of these transformation products [37]. In this context, the amoxicillin's behavior in waste and surface water, which includes identifying its transformation products, was carried out using liquid chromatography that is associated with a QTOF [198]. While QTOF is more suitable for confirmation purposes, as well as for identification of unknown compounds or metabolites [43; 199], because of their ability to provide exact mass measurements, QqLIT is appropriate for both quantitation and confirmation [96]. [PERSON] et al. identified biotic and abiotic transformation products of clarithromycin and carbamazepine [200] and limomycine [201] by liquid chromatography that was coupled to an Orbitrap analyzer. Relative to these instruments, the last decade has witnessed a significant shift from using ion traps, single- and triple-quadrupole mass spectrometers towards employing mass spectrometers that provide an accurate mass of analytes, such as time-of-flight (TOF), Fourier transform ion cyclotron resonance (FT ICR), and Orbitrap detectors [202]. The HRMS instruments, such as TOF and Orbitrap, have revolutionized the research of CECs in the aquatic environment, due to their high sensitivity in full scan mode, their increased mass accuracy, and the capacity to distinguish the isotopic pattern. HRMS instruments can screen for unknowns, due to exact mass measurements. This unique characteristic distinguishes it to other mass spectrometry instruments [203]. In comparison to triple quadrupole mass spectrometers, which operate at unit resolution and generally in the selected reaction monitoring (SRM) or multiple reaction monitoring (MRM) modes for specific target analytes, TOF-mass spectrometers can acquire full-scan mass spectra at high resolution for all analytes without a loss in sensitivity. Because most of the TOF mass spectrometers have a resolution of at least 10,000 at full-width-half-maximum (FWHM) peak height, isotopic patterns are evident and empirical formulas, and chemical structures can be proposed for unknowns or confirmed for target analytes. This also makes it possible to use mass spectral libraries and enable the data file to be reinterrogated months later to find additional unknown contaminants [86]. Hybrid systems, such as HRMS hyphenated to a quadrupole, or linear ion trap (LTQ), such as the LTQ-Orbitrap, combine the tandem mass spectrometric capability that is associated with the LTQ with the high mass resolving power (up to 100,000 FWHM) and mass accuracy capability of the Orbitrap [202; 203]. These hybrid configurations based on HRMS allow for the reliable interpretation of MS/MS spectra. They are valuable when dealing with complex environmental matrices, such as surface water or wastewater, where the co-elution of analytes with matrix interferences can result in ambiguous peaks [203; 204]. This type of hybrid systems has also been used in the monitorization of PhACs in water matrices. [PERSON] et al. [205] illustrated the potential of liquid chromatography coupled to a hybrid linear ion trap Fourier Transform Orbitrap mass spectrometer (LC-LTQ FT Orbitrap MS) for the simultaneous identification and quantification of 24 drugs of abuse and relevant metabolites in influent and effluent sewage water. The acquisition of full-scan accurate-mass data by Orbitrap, together with the simultaneous MS/MS measurements allowed by LTQ, is a powerful combination for confident identification and confirmation. [PERSON] et al.