Geological Reference Materials by Microwave Digestion with HBF4BF4 (Supplement to Bollen et al; in review)

3. Data description
3.1. Sampling method
Geochemical analyses often require the complete dissolution of geological materials for the determination of elemental concentrations or isotopic ratios. However, achieving complete digestion can be difficult and is made laborious partly due to the necessity of employing reagents which require particularly strict safety measures. Here, we test a high-efficiency method for complete digestion using an automated microwave digestion system, with samples selected to represent a range of compositions commonly encountered in sedimentary paleo-investigations. The method leverages the unique properties of tetrafluoroboric acid (HBF4) as a safer alternative to hydrofluoric acid for the dissolution of geological materials. The method minimises sample preparation time, enables high sample throughput, increases laboratory safety, and decreases infrastructural requirements.

We report the elemental compositions of solutions obtained both immediately post-digestion, and following evaporation to dryness and redissolution in 0.5 M HNO3. Reference materials tested are BHVO-2 (Hawaiian basalt, USGS), NOD-A-1 (Atlantic ferromanganese nodule, USGS), ShBOQ-1 (Boquillas Shale, USGS), SCo-2 (Cody Shale, USGS), SSAR-1 (Animas River sediment, USGS), and MESS-4 (Beaufort Sea marine sediment, NRC Canada).
Elemental concentrations were measured using an Agilent 8700 QQQ-ICP-MS with various gas modes to minimise potential mass interferences. We present data on a wide range of elements, including Be, Na, Mg, Al, Si, P, S, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Rb, Sr, Y, Zr, Nb, Sb, Cs, Ba, REEs, W, Tl, Pb, Th, and U. Elemental recovery is affected by the formation of insoluble precipitates and oxide species, with changes in elemental concentration between the T1 sample (post-digestion aliquot) and T2 sample (dried and redissolved) indicating uptake into or liberation from precipitates.
3.2. Analytical procedure:
Laboratory: Laboratory for Biogeochemical Oceanography Across Time

40 to 60 mg of each reference material was weighed into 8 mL PFA digestion vials, with six replicates for each material and three procedural blanks (39 samples in total). Samples were acidified with 6 mL concentrated HNO3 (Normatom Grade, VWR) and left to react for two hours to dissolve easily soluble phases such as carbonates. Subsequently, 1 mL 38% HBF4 (ultra-pure grade, AnalytiChem Belgium) was added to each vial before loading them into a Milestone ultraCLAVE automated microwave digestion system. A baseload solution in the reactor consisted of 300 mL Milli-Q water (18.2 M, Merck), 10 mL 30% H2O2 (Suprapur Grade, VWR) and 5 mL 18 M H2SO4 (Normatom Grade, VWR), which served to absorb excess microwave energy and ensure uniform heat distribution among reaction vials. The chamber was initially pressurised with N2 gas to 40 bar. A temperature ramp of +4 °C/min was applied for 45 minutes reaching and maintaining 180 °C for 105 minutes, corresponding to a reaction pressure of ~100 bar. The total operational duration, including heating, cooling and depressurisation was approximately three hours. Following digestion, vials were removed from the ultraCLAVE and left cooling in a fume hood for one hour prior to further processing.

Immediately after opening each vial, an aliquot of 70 µL was pipetted from the surface of the digest and diluted into 6.93 mL 0.5 M HNO3 for elemental analysis; these aliquots are hereafter referred to as the T1 samples. The remaining ~7 mL of each digest were transferred into 14 mL PFA vials, and the original digestion vials were rinsed twice with Milli-Q water and these rinses added to the corresponding PFA vial. The combined solution was evaporated to dryness overnight at 120 °C on a hotplate within a fume hood, then redissolved in 10 mL 0.5 M HNO3, referred to as the T2 samples. For elemental analyses, both T1 and T2 samples were further diluted in 0.5 M HNO3 to a total dilution factor of 10,000 relative to the initial average sample mass.

Elemental concentrations were determined on an Agilent 8900 ICP-QQQ at the Faculty of Geosciences and the Environment, University of Lausanne. Measurements were conducted under various modes - He, H2, O2 and no gas – in both single and triple quadrupole configurations to minimise polyatomic and isobaric interferences. An internal standard solution containing Rh and Bi was introduced via on-line addition. Calibration standards were prepared gravimetrically from multi-element reference solution (CMS-1, CMS-3, CMS-4, CMS-5; Inorganic Ventures). Details of gas modes, isotope selection, internal standard correction, and mass-shifts parameters are provided.

Instrumental reproducibility was assessed using the NOD-A-1 reference solution. The 2 relative standard deviation (2RSD) is < 10 % for S, V, Mn, Fe, Co, Ni, Cu, Zn, Sr, Y, Zr, Nb, Mo, Ba, La, Ce, Tb, Tl, Pb, and U and < 20 % for all other measured elements (Na, Mg, Al, Ti, Cr, As, Sb, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, W) except for Hf (~25 %) and Th (~45 %). Potential contamination was evaluated by normalising the average signal intensity of procedural blanks to the mean signal of reference material samples. For most elements, the procedural blank contribution was < 1 % of the total signal. Only Si and Ta exceeded 10%, with Si concentrations falling below the LOQ. Considering the negligible impact of blanks relative to the ± 10 % ICP-QQQ analytical uncertainty, no procedural blank correction was applied to the reported concentrations. Of the 78 measurements, seven outlying samples were removed, with poor data quality attributed to erroneous sample dilution, measurement errors, and handling. We therefore report four and three replicates of the T1 and T2 samples from MESS-4, respectively, and four replicates of the SSAR-1 T2 Sample. All other materials retain six replicates for each of the T1 and T2 samples.
3.3. Data processing
Data reduction was performed in Agilent MassHunter, applying blank subtraction and internal standard correction. Limits of detection (LD) were defined as three times the standard deviation of blank replicates, corresponding to a < 1% probability that the signal arises from instrument noise. Limits of quantification (LOQ) were set to three times the LOD. Concentrations below the LOQ were excluded for further analyses.