Ion-Pair Chromatography Coupled to Inductively Coupled Plasma–Mass Spectrometry (IPC-ICP-MS) as a Method for Thiomolybdate Speciation in Natural Waters

Abstract

Molybdenum precipitates preferentially under reducing conditions; therefore, its occurrence in sediment records is used as an indicator of paleoredox conditions. Although thiomolybdates (MoO4-xSx(2-) with x = 1-4) supposedly are necessary intermediates in the process of molybdenum precipitation under anoxic conditions, there is no information about their abundance in natural environments, because of a lack of element-specific methods with sufficiently low detection limits. Here, we optimized ion-pair chromatographic separation for coupling to an inductively coupled plasma-mass spectrometry detector (IPC-ICP-MS). 2-Propanol (10%-25% gradient) replaced the previously used acetonitrile (25%-75%) as the solvent, to reduce the carbon load into the plasma. In synthetic solutions, formation of thiomolybdates was found to occur spontaneously in the presence of excess sulfide and the degree of thiolation was highest at pH 7. Excess hydroxyl led to a transformation of thiomolybdates to molybdate. Under acidic to neutral conditions, precipitation of molybdenum and hydrolysis of tetrathiomolybdate were observed. Flash-freezing was found to be suitable to stabilize tetrathiomolybdate, with <4% transformation over more than two months. High ionic strengths matrices (>2 mM) negatively affected the detection of molybdate, which eluted mainly in the dead volume, but had no negative effect on higher thiolated molybdates. Detection limits were ∼10 nM. With the newly developed IPC-ICP-MS method, thiomolybdates were found to form spontaneously in euxinic marine waters after adding a molybdate spike and occur naturally in sulfidic geothermal waters.

Experimental Confirmation of Isotope Fractionation in Thiomolybdates Using Ion Chromatographic Separation and Detection by Multicollector ICPMS

“…The MoS4 2stock solution was filtered (0.2 µm, celluloseacetate filter, Membrex or cellulose-nitrate, Whatmann) to remove insoluble particles, flash-frozen, and stored in liquid nitrogen immediately after preparation to ensure stability. 38 Aliquots were thawed directly before starting the experiments to minimize alteration. All pipette tips (Sarstedt) and screw cap vials (1.5 mL glass (VWR), 2 mL PP (Hartenstein), and 10 mL PP (Sarstedt)) were acidcleaned with 3 M HCl (Grüssing) for 24 h and rinsed with ultrapure water.…”

“…The chromatographic method for Mo speciation analysis has been described in detail elsewhere. 38 Briefly, it uses high-performance liquid chromatography (Merck Hitachi HPLC) coupled to UV/Vis detection. The chromatographic separation is based on ion-pairing using tetrabutylammonium hydroxide as ion pair reagent, a sodium carbonate buffer, and a gradient of 25-50% acetonitrile as eluent on a reversed-phase polymeric column (Dionex, IonPac NS1, 10 μm, 250 × 4 mm).…”

“…For the formation experiment, the goal was to get a mixture of different thiomolybdates in equilibrium where each thiomolybdate had high enough concentrations to be measurable by MC-ICP-MS. Since we knew from a previous experiment that a 10-fold excess of sulfide would dominantly yield MoOS3 but little MoS4 2-, 38 here we used 250 µM (NH4)2MoO4 with a 20-and 50-fold excess of Na2S. The starting pH (WTW multimeter (Multi 340i) with WTW SenTix 41-3 pH electrode) was adjusted to 7.3 with HCl (Grüssing) because we previously observed the fastest reaction kinetics at that pH.…”

“…The starting pH (WTW multimeter (Multi 340i) with WTW SenTix 41-3 pH electrode) was adjusted to 7.3 with HCl (Grüssing) because we previously observed the fastest reaction kinetics at that pH. 38 We used screw-capped 50 mL PFA beakers (Savillex) and allowed solutions to equilibrate during preparation and sampling with the glovebox atmosphere, which resulted in some sulfide loss and a corresponding pH increase up to pH of approximately 9. At that pH, we had previously observed thiomolybdate formation and transformation to be almost negligible.…”

Atomic Spectrometry Update: review of advances in elemental speciation

“…A method has been developed to determine the concentration of thiomolybdates in natural waters using an ion pair chromatographic (Dionex, IonPac NS1, 250  4 mm  10 mm) column on line coupled to ICP-MS. 161 The use of 2-propanol (10-25% gradient) in the mobile phase (2 mmol L À1 tetrabutylammonium hydroxide + 1 mmol L À1 Na 2 CO 3 ) was found to decrease the formation of carbon, thus eliminating the need of oxygen addition to the instrument plasma gases. A crucial step in the precipitation of Mo in anoxic environments is the formation of thiomolybdates (MoO 4Àx S x 2À with x values of 1-4) as intermediates.…”