Selenium speciation in feed ingredients and in salmon feed

Selenium (Se) is an essential element for fish, including Atlantic salmon (Salmo salar). The element exists in different chemical forms, or Se species. The chemical properties of the Se species, combined with the concentrations of Se, affect the uptake and metabolism of the element. For Atlantic salmon, the major source of Se is the diet.

The average Se concentration of selected fish feed produced in Norway in 2018 contained a concentration of 0.7 mg/kg and was ranging from 0.24 to 2.3 mg/kg (n= 76) [1]. The level of Se in complete feed depends on the feed composition and the natural levels of Se in the feed ingredients used. Traditionally, fish feed mainly composed of marine feed ingredients, where as today`s feed contains a higher inclusion of plant-based proteins.

Currently, there is also a focus on new feed materials, such as insects. Insect meal was recently approved as a feed ingredient for aqua culture production in the European Union (EU) [2]. Se is regulated as a feed additive in EU. The supplementation of Se is regulated with an established maximum level of 0.5 mg/kg feed [3], and the addition of organic selenium (Se-yeast) is limited to a maximum level of 0.2 mg/kg feed [4,5].

To control whether feed comply with the legislation there is a need for an analytical method that can discriminate between inorganic and organic selenium species in feed. Furthermore, few data exist on the Se concentrations and Se species present in feed ingredients, both marine, plant-based and insects.

Speciation data on Se in feed ingredients can provide information on the natural composition of Se originating from the feed ingredients. In the present study, Se species were determined using high performance liquid chromatography coupled to inductively coupled plasma mass spectrometry (HPLC-ICPMS).

The Se species were separated using ion-exchange chromatography, and different extraction procedures were applied to evaluate the extraction efficiency of inorganic and organic Se species in the feed ingredients and in the complete feed. Results from analysis of feed ingredients and fish feed will be presented and discussed with regards to analytical challenges.