A simple and rapid head space-single drop microextraction-‘spectro-pipette’ (HS-SDME-SP) method for the on-site measurement of arsenic species in natural waters

•Design of a spectro-pipette for microdroplet handling.•Matrix isolation, extraction, redox and chromogenic reactions in a single step.•The spectro-pipette incorporates in-situ absorbance measurement.•A detection limit of 0.1 nM for inorganic arsenic species.•Spectro-pipette sensor (weight 1.1 kg) suitable for field deployment. Arsenic (As) is a ubiquitous element in the environment while its toxicity and chemical properties are highly associated with its chemical form. High quality As speciation data in environmental matrices are thus desirable for understanding its biogeochemistry. Here we present a simple and novel head space-single drop microextraction-spectro-pipette (HS-SDME-SP) method for the on-site measurement of As species in natural waters. Arsine was generated and separated from the sample matrix by borohydride-based hydride generation, which was further extracted into a stable, hanging 5 μL microdroplet on the tip of a spectro-pipette. With the spectro-pipette, the microvolume of extractant is easily handled while gas–liquid absorption, redox and chromogenic reactions and in situ absorbance measurement can be carried out in a single step. The speciation analysis of As(III) and As(V) was achieved by utilizing their different pH requirement for reduction and arsine generation in the hydride generation process. Although the proposed method is a simple sensor based assay, it is sensitive, robust and reliable; the limit of detection (3 × S/N) was 0.1 nM, RSDs for 25 nM As(III) were