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 .
