Total digestion of marine particles
Place filter into PFA vial, ideally with sample side facing downwards or folded inwards.
Add 1.5 mL of sulfuric acid to all samples and blanks. 
Place lid atop loosely and leave filter to soak for 20 min at room temperature. 
Lift lid slightly to add 0.5mL of hydrogen peroxide.
Gently, swirl vial to mix.
React for 60 min in a hot plate at 110˚C with lid placed on loosely. 
Add another 0.5 mL peroxide, replace caps loosely and increase hotplate temperature to 200˚C. 
If undissolved filter pieces or semi-digested viscous material persist, let it cool down and add 100-200µL aliquots of peroxide to digest this material. 
Dry vial contents at 235-250˚C. 
Note: If a small droplet of sulfuric acid remains at this point, let vial cool and suspend contents in a small aliquot of Milli-Q water or 8N nitric acid and re-dry at 235˚C until the droplet is removed.
Resuspend dried samples in 2 mL of a freshly prepared mixture of HNO3, HCl, and HF acids (4M each) in milli-Q water.
Heat for 4 hours at 100-110˚C.
Let vials cool to room temperature, uncap and dry at 100-110˚C (overnight).
Add 2mL of freshly prepared 50% HNO3/15% H2O2 (v/v). 
Take vials to dryness on a hotplate at 100-110˚C. 
Resuspend sample in 200-500 µL of 50% HNO3/15% H2O2 (v/v).
Cap loosely and heat at 110˚C.
After vigorous bubbling cease, uncap vials and dry at 135˚C. 
Add 100 µL concentrated HNO3 and heat at 110˚C to dryness redissolved in 1mL of 0.1 M HNO3. 
Dilute sample to 5% of original concentration with 0.1 M HNO3 for analysis. 
After concentration analysis, follow Conway et al. (2013) to prepare samples for isotopic analysis.
