## 1 Introduction  
Formaldehyde occurs naturally in the environment. Most organisms produce it in small amounts during their metabolism and it is also a by-product of combustion and other natural processes. Formaldehyde is also a product of anthropogenic activities such as burning fuel, residential wood burning, and smoking tobacco [1]. However, for many years, formaldehyde has been also applied in industries mainly with antimicrobial action. Therefore, it is commonly used as a fungicide, germicide, disinfectant, and preservative in medical laboratories [1, 2]. Formaldehyde is also widely used as a chemical solvent, raw material for the production of resins, wood fixatives as well as paper and pulp production, cosmetics, food, and the textile industry. Such a variety of applications of formaldehyde affects its continuous presence in the environment [1, 3]. Formaldehyde is toxic for people, causes irritation of the eyes and respiratory tract, headaches, nausea, drowsiness, and allergic skin reactions [2, 3, 4]. It is considered by the International Agency for Research on Cancer (IARC) as a carcinogen as well as classified by the Environmental Protection Agency (EPA) as a probable human carcinogen (the EPA's BI classification). Most of the research on the carcinogenicity of formaldehyde focuses on chronic respiratory exposure because this is the main way of industrial and household exposure [4, 5]. Over the years, analytical methods based mainly on high-performance liquid chromatography [6; 7; 8; 9; 10; 11; 12], gas chromatography [13; 14; 15; 16], spectrophotometry [17; 18; 19], and spectrofluorimetry [20] have been developed to determine formaldehyde in different kinds of samples. However, the direct determination of formaldehyde is complicated owing to its volatility, high polarity and chemical reactivity, as well as lack of a chromophore (or a fluorophore). Derivatization successfully solves these problems. Different reagents such as 2,4-dinitrophenylhydrazine (DNPH) [6; 7; 8; 11; 12; 13], O-2,3,4,5,6-(pentafluorobenzyl)hydroxylamine hydrochloride (PFBHA) [15; 16; 21], and Nash reagent (prepared by mixing acetylacetone, acetic acid and ammonium acetate) [9; 14; 17; 18; 19; 22] have been most often used for the derivatization of formaldehyde. Sample preparation often includes preconcentration of analytes and is considered an essential part of analytical methods. Therefore, for the isolation and preconcentration of trace amounts of formaldehyde different techniques are used such as liquid-phase microextraction (LPME) [6; 10; 11], ultrasound-assisted emulsification microextraction (USAEME) [17], dispersive liquid-liquid microextraction (DLLME) [7; 8; 9; 12; 18; 19; 22], singe drop microextraction (SDME) [13; 20] and solid-phase microextraction (SPME) [14; 15; 16; 21; 23]. SPME is an established analytical technique and relies on analyte isolation with a sorbent-coated fiber [24]. The fiber is fixed in the needle that is exposed to a sample to adsorb analytes that are subsequently desorbed thermally directly into a gas chromatograph or using solvent when high-performance liquid chromatography is to be used [25; 26]. Extraction in SPME is based on the partition equilibrium of the analytes between a sample and a fiber combining extraction and preconcentration into a single step [25; 27]. Because this technique can be easily used for sample preparation in the determination of trace amounts of analytes, many studies have been performed aiming at an increase of the extraction efficiency by using different sorbents and different geometries of supports [26; 27; 28; 29]. Thin-film SPME was introduced as a new geometry of SPME in 2003 [30]. It takes the advantage of a greater sorbent volume with a more developed surface area allowing to gain bigger sensitivity [31]. In the present study, a stainless steel mesh support is proposed that enables the deposition of a large sorbent mass.