Analytical Methods for Plastic (Microplastic) Determination in Environmental Samples

Abstract

Beside several studies about the occurrence of microplastic (MP) there is still a huge gap of knowledge regarding the dynamic processes of MP distribution and fate. Consequently, there is a need for reliable, fast, and robust analytical methods for MP monitoring. However, due to the physicochemical attributes of plastic, new analytical approaches fundamentally different from those for most other environmental contaminants are required. Promising strategies include spectroscopic and thermo-analytical methods. The two vibrational spectroscopic methods, Fourier-transform infrared spectroscopy (FT-IR) and Raman spectroscopy, have been implemented for MP detection. Especially in combination with particle finding software or a focal plane array (FPA) detector, they enable reliable determination of MP particle numbers in environmental samples. In recent years, different thermo-analytical techniques, such as pyrolysis (Py), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) have been adapted for MP detection. All thermo-analytical methods are based upon measurement of physical or chemical changes of the polymer under thermal treatment. While DSC measures differences in heat flux caused by phase transitions of the polymer, TGA-MS is based upon detection of specific thermal degradation products. By means of a gas chromatographic separation step, an enhanced detection of the marker compounds is possible, enabling a more sensitive MP detection even in complex matrices. The extent of analytical information obtained as well as the complexity and effort of the methods increase by TGA-DSC < TGA-MS < Py-GC-MS/TED-GC-MS. The results are comparable to those of spectroscopic methods (FT-IR, Raman), but both techniques have different benefits and limitations. While thermo-analytical methods require minor sample pretreatment and reveal mass concentrations, spectroscopic methods are non-destructive and yield particle numbers and size distribution by imaging techniques. Whichever is the most suitable method depends on the scientific question and what kind of information is required.