**Abstract**

Selenium is an important element in environmental and living organisms that is being essential in very narrow concentration range, while deficiency or toxicity occurs outside this range. However, its toxicity depends not only on its dose but also on its chemical form. In environmental samples, selenium can exist in inorganic forms (as elemental selenium, metal selenides, selenite, or selenate anions) and as organic species with direct C-Se bonds (methylated compounds, selenoaminoacids, and selenoproteins). Thus, the development of reliable techniques to study the speciation of selenium in environmental samples is necessary. The main purpose of this chapter is to provide an update on the recent literature concerning the strategies for selenium speciation in environmental samples. Liquid chromatography coupled with sensitive detector is a commonly used technique for selenium separation. Gas chromatography can also be applied for such purpose; however derivatization step is usually required before analysis. Direct determination of selenium species at the concentration levels present in natural samples is very often difficult or even impossible. For this, several preconcentration/separation procedures for selenium have been proposed, including coprecipitation, extraction into an organic solvent, or application of solid sorbents.

**Keywords:** selenium, speciation, chromatography, gas chromatography, liquid chromatography

### **1. Introduction**

Selenium and its several species have been demonstrated to be essential for living organisms [1, 2]. It plays a key role in many important metabolic pathways such as thyroid hormone metabolism and antioxidant defense systems [3]. For these reasons, it should be present in human diet. The range between the deficiency and toxicity of selenium is very narrow. The nutritionally required daily uptake of selenium is 55 μg; however some studies suggest that it should be 100 μg [4]. It is estimated that the diets of 1 billion people might lack sufficiency for their wellbeing [5, 6]. The toxic dose of selenium is very much dependent on its chemical form, with different toxicity for organic and inorganic forms [7]. Selenoaminoacids are principal dietary forms of selenium, and selenomethionine (SeMet) is derived from plants, while selenocysteine (SeCys) from animals [8, 9].

Selenium naturally exists in many different inorganic (elemental selenium, selenide, selenite, and selenate ions) and organic forms (methylated compounds, selenoaminoacids, and selenoproteins). Inorganic selenium, present in water and soil, can be easily transformed into volatile compounds by plants and fungi.


**Table 1.**

*Principal selenium species present in environmental samples.*

#### *Chromatographic Analysis of Selenium Species DOI: http://dx.doi.org/10.5772/intechopen.87053*

Organic species of selenium form covalent C-Se bonds. SeCys is included into selenoproteins and participates in redox reactions. The metabolic pathway of selenium in the human body is complicated [10]. In general it can be divided into three groups including reduction of inorganic species by glutathione (GHS) to selenide, cleavage reaction of organic species by β-lyase; utilization according to the UGA codon leading the synthesis of selenoproteins; and finally excretion after being metabolized to methylated species.

Apart from the importance of the selenium in living organisms, this element is also spread throughout the environment. Sulfur-containing minerals are natural sources of selenium, but it is also produced by combustion of fossil fuels. It should be noticed that selenium is used in electronic industries and agriculture as a component of fertilizers.

Several analytical procedures for determination of selenium at low concentration levels in environmental samples have been proposed and recently reviewed [11, 12]. For selenium speciation analysis, the coupling of chromatographic techniques such as gas chromatography (GC) or high-performance liquid chromatography (HPLC) with a highly sensitive and selective detector is very useful [13]. Even though GC exhibits high efficiency and simplicity, HPLC has the ability of dealing with nonvolatile compounds, extending the range of application and avoiding a derivatization step. This chapter will focus on the recent progress in the application of HPLC in different modes for selenium speciation analysis in water, soil, and plants. Sample pretreatment procedures will be also considered. The principal selenium species present in environmental samples are summarized in **Table 1**.
