# Applications of Nanomaterial Coatings in Solid-Phase Microextraction (SPME)
## 1 Introduction  
Microextraction is a sampling technique characterized by the use of a significantly smaller volume of extractant (liquid or solid) relative to the sample volume. Consequently, only a portion of the analyte is extracted, resulting in a non-exhaustive process. SPME is a specific microextraction method that relies on analyte partitioning between a sample matrix and a stationary phase immobilized on a fiber. This equilibrium-based process is governed by Nernst's partition coefficient, with higher values indicating greater analyte extraction efficiency. The selection of the fiber coating is crucial, as it determines the selectivity and capacity for different analytes [1,2]. Invented in the late 1980s, SPME revolutionized sample preparation by offering a solvent-free, single-step approach. SPME eliminates the need for bulky extraction processes, allowing for rapid analysis both in the lab and at the sampling site. This innovative technique utilizes a tiny fiber coated with a specific material to directly extract and concentrateanalytes from the sample. This translates to faster processing times, reduced waste disposal costs, and potentially improved detection limits [3; 4; 5]. SPME is a solventless sample preparation method that relies on achieving equilibrium between the sample and a coated fiber, and involves these steps:  
1. Extraction: A fused silica fiber with a specific coating (polymer or sorbent) is exposed to the sample matrix. Target analytes partition from the sample into the coating based on their affinities for each. This selectively concentrates the analytes on the fiber. Extraction can be achieved through direct immersion (DI) or headspace (HS) sampling [6]. 2. Desorption: The fiber, loaded with concentrated analytes, is then transferred to an instrument like a gas chromatograph (GC) or high-performance liquid chromatograph (HPLC). Inside the instrument's injector, the analytes are desorbed from the coating and introduced into the analytical system for separation and quantification [7]. Desorption is primarily accomplished by exposing fibers to high temperatures in the GC injector, although solvent immersion can also be used [6]. With two opposing effects, the extraction temperature in SPME has a complicated role. On the one hand, analytes' diffusion coefficient increases with increasing temperature, encouraging their migration towards the extraction phase. In contrast, high temperatures decrease the analytes' distribution constant, which lessens their retention on the fiber because adsorption is an exothermic process. [PERSON] developed a unique mechanism that cools the fiber and heats the sample at the same time in order to address this difficulty. This dual-action configuration optimizes both the diffusion and adsorption processes, increasing the mass transfer of analytes from the sample to the fiber coating and increasing extraction efficiency [8]. The type of coating applied to the SPME fiber is critical to its extraction and desorption effectiveness because it influences the distribution coefficient of analytes between the stationary and bulk phases. It serves to distinguish analytes from other substances. Various coatings have been created to improve the extraction of a wide spectrum of analytes. For SPME procedures to be effective under different conditions, a robust sorbent must be properly selected. Surface active groups, molecule size, and polarity all affect the selectivity of coating materials [9; 10]. In the traditional in-fiber mode of SPME, a sorptive phase immobilized on an inert support is used to extract analytes directly from the sample or its headspace. Polymeric coatings' exceptional chemical and thermal durability makes them popular sorptive phases. For a wide variety of target analytes, their polarity can be adjusted by mixing various materials, resulting in the best extraction performance.