Plastics are hydrophobic in nature and they can be extracted in a purely aqueous medium. Crichton et al., in their study, exploited the oleophilic property of fragmented plastics to separate them from aqueous medium by partitioning them into water-immiscible oil . This method only separated microplastics from their inorganic matter and is compatible with FTIR. This process may cause damage to low melting point polymers.
Optical Vibrational Spectroscopy
This technique is based on irradiating the sample to generate molecular vibration, producing an optical spectrum. Based on the particular peaks of the spectrum, identification of the material is done. This method provides reproducible results and requires only a very small quantity of samples. Also, the process is non-intrusive and nondestructive in nature. Detection of microplastics is usually done by ultraviolet-induced photodegradation. FTIR (Fourier-transform infrared) spectroscopy and Raman spectroscopy are also used. Raman spectroscopy is a laser-based technique which provides better resolution than IR spectroscopy. They are capable of identifying microplastics of about a 1 pm size [48,84]. This technique is widely used for detection of particulate matter in combination with FTIR for qualitative analysis, detection and characterization.
Thermal analysis is usually used in combination with gas chromatography mass spectrometric technique (GC-MS). This method is destructive in nature and provides no, or limited, data about the size and shape of the particulate matter.
Fluorescent Tagging with Nile Red
Nile red has been exploited to fluorescently label microplastics upon irradiation with blue light. This method can quantify microplastics of 20 pm-1 mm [85-87]. The dye is relatively cheap and readily available. Erni-Cassola et al., Maes et al. and Shim et al. have reported tagging of the microplastics with Nile red and subsequently applying density separation and filtration of particles on black polycarbonate paper [85-87].
Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS)
This technique provides ambient information about the chemical composition of particulate solid samples upon bombarding the sample surface with a focused pulse ion beam. After that, the material is analyzed under a time-of flight mass spectrometer, which provides information about the elemental and molecular composition of the sample. Microplastics can also be analyzed by this technique . However, it is an expensive technique with complex instrumentation.