## 2 Materials and Methods  
### Chemicals, Reagents and Standard Solutions  
Methanol (MeOH) and acetonitrile (ACN) were of high purity, suitable for LC-MS analysis, while dichloromethane (DCM) was over 99.5% pure, all supplied by Fisher Scientific (Leicestershire, UK). Acetone with purity greater than 99.5% was obtained from Honeywell (Morris Plains, NJ, USA). Ultra-pure water with conductivity \(<\) 0.055 \(\mu\)S/cm, provided by an Evoqua Ultrapure Water System Ultra Clear 20 TWF EDI (Evoqua Water Technologies, Pittsburgh, PA, USA), was used in all extraction steps, except for the preparation of the chromatographic eluent system, where water of LC-MS purity purchased from Fisher Scientific (Leicestershire, UK) was used. Formic acid (FA) and acetic acid (HAc) of 98-100% purity from Merck (Darmstadt, Germany) were used for the adjustment of pH value. Ammonium formate of 98-100% purity, 25% ammonia solution, and sodium hydroxide (NaOH) of 98% purity were also purchased from Merck.  
The standard compounds were of high purity (>97%) and were supplied in solid form by Sigma Aldrich (Darmstadt, Germany). The selected pesticides, including herbicides, insecticides, and fungicides, demonstrated mostly medium to high polarity and were thiamethoxam, dimethoate, acetamipid, thiacloprid, fluoreturon, chlorantraniliprole, metalaxyl, diuron, bosalid, myclobutanil, linuron, S-metolachlor, prometryn, tebupirifos, chlorpyrifos, fluazifop-p-butyl, imazalil, and fenpyroximate. A KERN ACS 80-4N (Balingen, Germany) four-decimal precision analytical balance was used to weigh standard compounds. Eppendorf (Vienna, Austria) automatic adjustable volume pipettes (10-100 \(\mu\)L, 100-1000 \(\mu\)L) and glass microsyringes (5 \(\mu\)L, 10 \(\mu\)L, 50 \(\mu\)L, 100 \(\mu\)L, 250 \(\mu\)L, and 500 \(\mu\)L) from Innovative Labor Systeme (Stutterbach, Germany) were used to collect and transfer different sample volumes.  
Stock solutions of each compound were prepared separately at a concentration of 2000 mg L\({}^{-1}\) in methanol. The solutions were checked for quality by mass spectrometry, transferred to 8 mL glass-sealed vials, and stored at \(-\)20 \({}^{\circ}\)C. Diluted standard working solutions of the selected compounds and their mixtures were prepared prior to analysis at various concentrations, with appropriate dilutions in methanol and water/methanol (90:10 \(v/v\)).  
A Millipore filtration device (Darmstadt, Germany) and Daihan Labtech (Wilmington, NC, USA) magnetic stirrer were used for water filtration and stirring, respectively. A Thermo Fisher Scientific (Rockwood, TN, USA) HyperSep Glass Block 60104-243 extraction device connected to a vacuum pump was used for solid phase extraction (SPE). For the optimization and application of the solid-phase extraction method, Oasis HLB (divinylbenzene/N-vinylpyrrolidone co-polymer cartridges, 200 mg, 6 mL) extraction cartridges from Waters Corporation (Milford, MA, USA) were used. For preconcentration of the extracts, a Techne Dri-Block DB-3D (Staffordshire, UK) heating evaporator under a gentle stream of nitrogen was used. In all cases, prior to injection into the LC-MS system, the samples were filtered through 0.22 \(\mu\)m pore diameter polytetrafluoroethylene (PTFE)-filled syringe filters (Millipore, Cork, Ireland).  
A Niskin-type sampler was used to collect water samples, and glass fiber filters GF/B, 0.7 mm from Whatman (Maidstone, UK) were used before the extraction. A Consort C932 pH meter (Turnhout, Belgium) and WTW LF 325-B conductometer (Weilheim, Germany) were used to measure the physicochemical characteristics of the water samples.