Analysis by ICP spectroscopy (with the exception of laser ablation systems), requires samples to be completely dissolved (digested) into a solution. Sediment digestions are most commonly achieved either by lithium metaborate (LiBO2) flux fusion or by a combined acid attack, using one or more of hydroflouric (HF), nitric (HNO3), and hydrochloric (HCl) acids. Current shipboard procedures for the ICP-AES, following Murray et al. (2000) and Quintin et al. (2002), recommend using flux fusion. The flux fusion procedure was originally recommended for both practical and analytical purposes. In the practical sense, the use of flux fusions for the ICP-AES allowed for significant financial savings because much of the required apparatus was already acquired and in use to prepare samples for the wavelength dispersive X-ray fluorescence instrument that was on board the JOIDES Resolution at the time. For analytical purposes, flux fusions were recommended over acid digestions for shipboard analyses because acid digestions (1) did not allow for the analysis of Si, due to the volatilization of Si in the presence of HF, (2) did not yield usable results for refractory elements (e.g., Ti, Cr, and Zr) hosted in minerals that are difficult to dissolve (respectively, rutile, chromite, and zircon), and (3) presented safety concerns with using HF in a shipboard environment.
The geochemical laboratory on board the ship during Leg 199 (Paleogene Equatorial Transect; Lyle, Wilson, Janecek et al., 2002; Quintin et al., 2002) and during many other legs has successfully demonstrated that flux fusion sample preparation produces reliable geochemical data with an acceptable degree of precision and highlighted the utility of using such data for first-order paleoceanographic interpretations. However, whereas the flux fusion scheme results in complete dissolution of all phases (Potts, 1987, and references therein), the use of the lithium metaborate potentially contaminates the laboratory with Li and B, elements of interest to both pore water chemists and igneous geochemists. Although cross contamination has been minimized by maintaining separate inventories of instrument glassware for the ICP-AES, if such a flux fusion-based solution is passed through an ICP-MS, the instrument will be thoroughly, and perhaps permanently, contaminated with Li. Because of the addition of matrix, flux fusion also potentially compromises the analysis of some key trace metals (e.g., Ni, V, Cr, Zn, and rare earth elements) by raising the procedural detection limit.
Thus, in order to extend the element menu and provide a more analytically palatable laboratory environment, the ability to prepare samples using an acid digestion scheme would be desirable. Whereas acid digestions have their limitations, as listed above, a newly advanced microwave-assisted protocol has been developed at Boston University to overcome most of those limitations. The microwave-assisted approach minimizes digestion times, and by using boric acid as part of the reagent cocktail, the formation of insoluble fluorides can be inhibited and Si preserved and thus measured. Overall, the microwave-assisted acid technique with the use of boric acid is a quick and safe method for digesting sediment and perhaps other lithologies that could be used in the shipboard environment.
In this paper, we compare shipboard and shore-based flux fusion results. We also compare shore-based flux fusion results to those derived from microwave-assisted acid digestions. We show that (1) the current shipboard methods are adequate for both shipboard and shore-based sedimentary chemical uses, (2) the microwave-assisted acid technique yields a complete (and HF-safe) digestion, and (3) the microwave-assisted acid digestion holds great potential to be the primary means of digesting sediments on future IODP expeditions. We discuss preliminary results from ODP Leg 206, Site 1256, as it provides an excellent sample suite to compare flux fusion with the newly developed microwave-assisted acid digestion technique and contrast shore-based measurements with the shipboard initial results on Leg 206.