Direct measurement of lipid-soluble arsenic species in biological samples with HPLC-ICPMS

Abstract

Lipid-soluble arsenicals (arsenolipids) occur in a wide range of biological samples where they may play a key role in the biosynthesis of organoarsenic compounds from inorganic arsenic. The study of these compounds has been hindered, however, by the lack of a suitable analytical technique able to separate and measure the various lipid species. As a source of arsenolipids, we used 10 crude fish oils from various regions of the world. Total arsenic analyses on the fish oils, performed with ICPMS following acid digestion with microwave-assisted heating, gave concentrations from 4.3 to 10.5 mg As kg⁻¹. All of the arsenic was soluble in non-polar solvents such as hexane. Analysis of the fish oils for arsenolipids was performed by normal phase HPLC-ICPMS with various mixtures of organic solvents as mobile phases. Inherent problems of instability associated with the introduction of organic solvents to the plasma were overcome by the use of reduced column flow, a chilled spray chamber, and the addition of oxygen directly to the plasma. All ten fish oils appeared to contain the same 4–6 major arsenolipids, but in varying amounts depending on the origin of the fish. Further chromatography with both normal phase and reversed-phase conditions on some of the oils indicated the presence of many more minor arsenolipids. Quantification was achieved by external calibration against triphenylarsine oxide or triphenylarsine sulfide, and the sum of species following HPLC of the oils matched well the total arsenic results (92–107%). The method was applied to samples of food supplements (fish oil capsules) and a packaged food product (cod liver) whereby arsenolipids were measured and found to be significant arsenic constituents. This study represents the first attempt to directly measure intact arsenolipids and, with appropriate sample preparation, may be suitable for quantitative measurement of these arsenicals in a range of biological samples, including foodstuffs.