1. INTRODUCTION
Ion chromatrography is a common tool for quantifying the concentration
of different species of the same element in natural and experimental
solutions. Examples include, but are not limited to, nitrate and
nitrite, bromide and bromate, chloride and perchlorate, as well as an
array of organic compounds. In recent years, with the discovery of
reduced phosphorus species in natural
settings[1-3], phosphate, phosphite and
hypophosphite have moved into the focus of anion
chromatography[2, 4-6]. Phosphite has even been
detected in ancient sedimentary rocks and holds potential as an
important substrate for prebiotic chemistry and early
life[7, 8]. Reconstructing its biogeochemical
history over geologic time and its distribution in modern environments
therefore promises to yield new insights into the evolution of Earth’s
biosphere. Phosphite is thermodynamically unstable in water (i.e., it
forms in the stability field of H2), but it is
kinetically stable, because oxidation to phosphate is slow and primarily
catalysed by micro-organisms today[7-9]. Phosphite
can be produced biologically[10] or abiotically
under dry, hot conditions[8, 11], by lightning[12-14], or from the dissolution of meteoritic
FeP-minerals[15]. However, as these pathways are
relatively rare, phosphite concentrations can be low in natural fluids
(e.g., 0.15-3 µmol/L in waters from Florida[1] and
up to 0.45 µmol/L in a eutrophic lake in China[3],
but less than 0.06 µmol/L in some geothermal
waters[2] and often undetectable). Concentrations
at the lower end of this range pose analytical challenges, because the
phosphite anion peak is typically dwarfed by those of other anions such
as chloride, sulfate, and phosphate in typical chromatographic setups.
Saturation of the column and/or detector by those other more abundant
anions may impact element separation and detection of
phosphite[6].
To overcome this issue, Han et al.[6] as well as
Ivey & Foster[4] implemented the use of OnGuard
cartridges that simplify the sample matrix, particularly by the removal
of chloride – the most common anionic species in most natural systems.
This allowed injection of larger sample volumes (500-800 µL loop size)
to overcome the detection limit. By coupling the ion chromatograph (IC)
with an inductively-coupled plasma mass spectrometer (ICP-MS), Ivey &
Foster[4] were able to achieve detection limits
around 0.002 µmol/L for phosphite. However, the authors also noted that
OnGuard cartridges can impact the phosphate and phosphite content of the
samples, introducing additional uncertainty into the analytical yield.
Furthermore, OnGuard cartridges add extra costs and labor to the sample
preparation protocol. Alternatively, phosphite can be analysed by UV-VIS
spectrophotometry[16]; however, this may suffer
from interferences with other ions, and the reported detection limit of
0.36 µmol/L[16] is not as good as with the coupled
IC-ICPMS system. Lastly, phosphite measurements can be made by nuclear
magnetic resonance (NMR)[1], but this technique is
generally not optimized for trace quantities. A detection limit has to
our knowledge not yet been published, possibly because it is dependent
on numerous instrument parameters (discussed below).
Here we present a new approach of removing chloride from the sample
matrix on-line within the ion chromatograph by splitting the sample
stream after passage of the chloride fraction through an additional
clean-up column. By coupling this method with an Element 2 ICP-MS in
medium-resolution mode, this allows us to achieve detection limits
better than 0.003 µmol/kg with a small sample volume (37.5 µL loop size
on Valve 1, Fig. 1a). We further present data collected by NMR with a
detection limit of ca. 6.46 µmol/L, which highlights the value of the
IC-ICPMS setup for natural samples.