Ultra-sensitive speciation analysis of tellurium by manganese and iron assisted photochemical vapor generation coupled to ICP-MS/MS.

Photochemical vapor generation (PVG) of Te4+ was undertaken with a simple reactor consisting of a polytetrafluoroethylene reaction coil wrapped around a low-pressure mercury tube lamp and using a flow-injection for sample delivery. The composition of a reaction medium, the influence of irradiation time and the effect of added sensitizers and interferents were investigated using high-resolution continuum source atomic absorption spectrometry and a miniature diffusion flame atomizer. A mixture of 5 M acetic acid and 3.5 M formic acid and sample flow rate of 4 mL min-1 permitting a 36 s irradiation time were found optimal for PVG of Te4+. The addition of 250 mg L-1 Mn2+ and 15 mg L-1 Fe2+ ions as sensitizers enhanced the overall PVG efficiency 2.75-fold to 50 ± 2%. In order to achieve higher sensitivity necessary for determination of Te in real environmental samples, PVG was coupled to inductively coupled plasma triple quadrupole mass spectrometer and detection was performed with O2 in the reaction cell utilizing a mass shift mode of measurement (m/z 128 → m/z 144) to ensure interference free ion detection. A limit of detection 1.3 ng L-1 and repeatability (RSD) 0.9% at 250 ng L-1 were achieved. This ultrasensitive methodology was validated for speciation analysis of Te in water samples of various matrix complexities (fresh water, well water, seawater and contaminated water). Since no response was observed from Te6+ under optimal PVG conditions, Te4+ was selectively determined by direct PVG. The sum of Te4+ and Te6+ was determined after pre-reduction of Te6+ in 6 M HCl (95 °C), evaporation to dryness and reconstitution in the reaction medium containing sensitizers. Very good accuracy was demonstrated by spiked recoveries for both Te4+ and total Te in water samples and also by total Te determination in fresh water Standard Reference Material NIST 1643f.