## 1 Introduction  
Arsenic is relatively scarce in the natural environment (20 th position in elemental abundances on the continental crust), but it is widely distributed. It can be found in detectable amounts in nearly all rocks and soils. As in soils relates to the geological substratum and consequently a rather wide range of As levels have been reported in soils around the world with an average of 5-10 mg/kg in uncontaminated soils [1]. However, As may accumulate in soils due to human activities such as waste discharges of metal processing plants, burning of fossil fuels, mining of As containing ores and agricultural use of arsenical pesticides. In this respect it is important to know that As has different degrees of toxicity depending on the speciation of As, dose of exposure, individual and local tissue susceptibilities [2]. The oxidation state of As is important since it affects movement, persistence and toxicity in soils and sediments. In oxygen-rich environments and well-drained soils, arsenic (AsO\({}_{4}\)\({}^{3-}\)) species dominate, whereas under reducing conditions, such as regularly flooded soils, arsenic (AsO\({}_{3}\)\({}^{-}\)) is the stable oxidation state, but elemental As and arsine (AsH\({}_{3}\)) can also be present in strongly reducing environments. As may also exist in organometallic forms such as monomethylarsonic acid (MMAA, CH\({}_{5}\)AsO\({}_{3}\)), dimethylarsinic acid (DMAA, C\({}_{2}\)H\({}_{7}\)AsO\({}_{2}\)) and the more volatile methylarsines. Iron, aluminum and calcium arsenates are, like the corresponding phosphates, virtually insoluble in water and arsenate exhibits a phosphate-like specific adsorption (inner sphere complexation) on oxide and clay surfaces. Arsenites, however, are 10 times more soluble and mobile and are, furthermore, more toxic than arsenates [3]. Trace elements occur in soils in certain 'pools' or'sinks' of different solubility and mobility, of which in general six categories can be differentiated in the solid phase: (i) occupying exchangeable sites as diffuse ion or as outer-sphere complexes; (ii) specific adsorption as inner-sphere complexes; (iii) associated with insoluble organic matter; (iv) (co)precipitated as pure or mixed solids; and finally present in the structure of (v) secondary or (vi) primary minerals [4]. Trace element speciation analysis in soils must almost always be performed by operational fractionation methods like chemical extractions since direct determination of specific chemical compounds is mostly impossible. Chemical extraction is, however, complicated by the fact that no chemical solution uniquely extracts trace elements out of one of these pools. Sequential extractions, in which a soil sample is reacted with a series of carefully selected chemical solutions of increasing strengths, were developed to increase extraction selectivity of the distinct geochemical fractions [5]. Today, sequential extractions are the most frequently applied analytical method for studying the solid phase speciation of environmentally relevant heavy metals [6]. However, sequential extractions determine only an operational defined speciation (i.e., a fractionation), which depends on different factors like choice and order of extraction agents, the extraction duration, solid/liquid ratio, and sample preparation and conservation [7; 8]. Sample preparation and conservation are very important, especially for samples taken from reducing environments, since (labile) phases can actually be transformed into other phases during sample preparation. Some significant differences in operational speciation are even encountered when the same scheme is applied by different analysts [9]. However, lack of precision (especially at concentrations close to the detection limit) and selectivity are the main problems that occur among sequential extractions [10; 11; 12].