# Simply Applicable Method for Microplastics Determination in Environmental Samples
## 2 Results and Discussion
### Method Applicability in Environmental Samples
#### 2.2.2 Spiked Environmental Matrices  
The identification of MPs in complex environmental matrices can be complicated due to the content of various natural organic materials in the samples of water, wastewater, soil, and air [43]. From a qualitative analysis point of view, samples must be pretreated to eliminate the natural organic material and thus avoid interferences of the matrix during the analytical process, and to preconcentrate the MPs content [43,44]. For matrices with low suspended matter content, such as water and air samples, filtration is a suitable pretreatment, while matrices with higher content of natural organic material, such as wastewater and soil samples, must be subjected to more complex pretreatment techniques, like digestion or density separation [45]. Method applicability of HS-SPME-GC-MS was tested in a polymer blend and a complex organic mixture containing algae biomass and organic compost. A mixture of microalgae, organic compost, and polymer blend to simulate those found in the environment were treated with hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) to eliminate the organic material before analysis (see Section 3.5). Sample digestion with H\({}_{2}\)O\({}_{2}\) is a technique, suitable for the digestion of organic material within the matrix, that has a minimum effect on MP damage or further decomposition [46]. Sample pretreatment with digestion, sieving, and/or density separation of MPs is also needed for reliable identification with thermo analytical techniques such as Py-GC-MS and TED-GC-MS, and with spectroscopy techniques such as FTIR and Raman spectroscopy [39,44]. As matrix interferences were minimized by pretreatment of the sample, ions derived from the matrix were not problematic for the GC emergence of selected fragment ions for polymer identification (Figure 7). Each XIC from the mixture was compared to the XIC of individually analyzed MPs (see Section 2.1) and mass spectra in the determined retention times for selected polymer-specific compounds were compared to confirm the MP polymer type. Synthetic polymers, such as PE, PP, and others analyzed in this study, are derived from petroleum [47]. Therefore, hydrocarbons in the form of alkanes, alkynes, and/or alkenes can be expected thermal decomposition products. The suitability of selected characteristic compounds for PE and PP identification was tested in soil and mineral oil matrix only, and in soil and mineral oil matrix spiked with PP and PE. After the extraction of PP, identification fragment ion \(m/z\) 142, with the peak at \(\mathrm{t_{R}}\) = 7.108 min, was not obtained in the soil and mineral oil mixture while there was an evident peak in the spiked mixture. Based on this finding, the suitability of a selected fragment ion for PP identification in environmental samples also containing petroleum-derived compounds was confirmed (Figure S10). Figure 7: HS-SPME-GC-MS total ion chromatograms of MPs spiked microalgae and organic compost matrix after its pre-treatment and thermal decomposition at 220 \({}^{\circ}\)C for 3 min and 15 min with extracted ion chromatograms of identified polymers (polymer-specific compounds: 3-styrene, 4 and 5-trans (cis)-1,2-diphenylcyclobutane, 6-1-chlorococtane, 7-4-6-dimethyl 2-heptanone, 8-pentadecane, and 1–dimethyl terophthalate). Some problems occurred after the extraction of PE identification fragment ion \(m/z\) 85, as pentadecane (t\({}_{\text{R}}\) = 10.664 min) was also determined in the soil and mineral oil matrix. Overlay of both chromatograms revealed that pentadecane was the last detected higher alkane obtained after analysis of the soil and mineral oil matrix according to the proposed method for MPs identification. However, on the XIC of soil and mineral oil matrix spiked with PP and PE, pentadecane was obtained as previously mentioned, and for PE specific series of higher alkanes, ranging from C\({}_{12}\) (t\({}_{\text{R}}\) = 6.883 min) to C\({}_{20}\) (t\({}_{\text{R}}\) = 16.342 min).