2.1. Reagents and equipment
All sample preparation and IC-ICPMS analyses were carried out in the St
Andrews Isotope Geochemistry laboratory (StAIG). NMR analyses were
carried out in the School of Chemistry at the University of St Andrews.
Solutions containing chloride, nitrite, nitrate, sulfate, phosphite,
phosphate and in some cases hypophosphite and pyrophosphate were
prepared in LDPE bottles from pure reagents (NaCl, Sigma-Aldrich p/n
1.06404.0500; KNO2, Fisher Scientific p/n 11328016;
NaNO3, Fisher Scientific p/n 10696842;
MgSO4, Fisher Scientific p/n 11377658;
Na2HPO2, Fisher Scientific p/n
222791000; Na2HPO3.5H2O,
Fisher Scientific p/n 11994281; Na2HPO4,
Acros Organics p/n 204855000;
K4P2O7, Fisher
Scientific p/n 10378860) dissolved in 18.2 MΩ∙cm-1deionized water, which was generated with a Smart2Pure system. During
the analysis, a 1 mmol/L NaOH solution was used, which was prepared each
day by dilution of a concentrated stock solution (500 g/L,
carbonate-free NaOH, VWR p/n 87938.290). This stock bottle was stored
and handled with minimal agitation to avoid mixing with atmospheric
CO2 gas that may lead to elevated carbonate
concentrations. An aliquot of 0.16 ml was pipetted into 2 L of DI-water,
and the pipette was dipped as deeply into the stock bottle as possible
to avoid carbonate-enriched solution from the upper layer closest to the
lid. The bottle with the diluted 1 mmol/L NaOH solution was only shaken
up after purging with N2 to remove air. The headspace
was then pressurized with N2 to further avoid ingrowth
of atmospheric CO2 during the analysis. For the NMR
analysis, 10-20 % heavy water (D2O) was used
(Sigma-Aldrich p/n 151882-100G) to prepare a 0.6 ml sample solution.
The ion chromatograph used in this study was a Dionex ICS-6000 from
Thermo Fisher, equipped with an AS-AP autosampler, a 37.5 µL sample
loop, a gradient pump, an eluent generator with an RFIC degasser, a
CR-ATC 600, an EGC 500 KOH cartridge, an AG17-C guard column, an AS17-C
analytical column, an ADRS 600 2mm suppressor, and a conductivity
detector (Thermo Scientific p/n 061830). It was run with a constant flow
rate of 0.5 ml/min. KOH was used as an eluent, and its concentration was
ramped from 1 mmol/L to 40 mmol/L over the course of each run. Analyses
with pyrophosphate were set to last for 55 minutes, and here the KOH was
ramped up between 5.5-23 minutes. After 45 minutes of total run time,
the KOH concentration was decreased back to 1 mmol/L over a duration of
5 minutes. Analyses of solutions without pyrophosphate were set to last
for 27 minutes, the KOH was ramped up between 5.5-23 minutes and ramped
back down between 23-26 minutes. Containers used for samples and
standards were soaked in hot 2M HCl overnight and rinsed several times
with deionized water prior to use.
The ICP-MS was an Element 2 from Thermo Fisher. It was equipped with a
Scott quartz spray chamber and a quartz nebulizer rated for a solution
flow rate of 1 ml/min. Argon gas flow rates were 16 L/min for the cool
gas flow, 0.8 L/min for the auxiliary flow, and 1 L/min for the sample
carrier flow. The RF power was set to 1250 W. Prior to the start of the
run, the ICP-MS was tuned with a multi-element solution at a
concentration of 1 ppb in 5% HNO3 (Thermo Scientific
p/n 1099601). The instrument was operated in medium resolution mode
(measured resolution was ca. 4200 Δm/m) to avoid HNO interferences with
phosphorus at m/z 31. Oxide % measured determined from UO/O was ca.
3-5%.
The NMR used in this study was a Bruker AVIII 500 MHz NMR instrument
equipped with nitrogen cooled broadband cryoprobe. It was operated in
proton-decoupled mode with 3000-7000 scans per analysis.