Once the sample is dried, there should be no noticeable degradation of the sample. In a previous job, we used to dry samples at 80C for 48 h (or longer) in brown paper bags (shiny white paper often has finishing agents which included zinc which was a target element for us). Don't over fill the bags or put too many in the oven at one time. Insufficient airflow just ends up with hot wet samples that are messy and useless, and at worst, a burnt-out oven. If you don't have enough room in the oven, rotate the samples every 24h so the first batch is "dry enough" not to go mouldy, then do the next batch.

We also stored dry samples in paper "coin" envelopes (about 5x10 cm, but whatever suits your samples) after grinding. As long as the paper is not being abraded by the sample, there should be minimal contamination occurring.

Grinding of samples often adds as much or more contamination than storage, as the metal parts of grinders do wear away on the sample. It may pay to grind an abrasive material like polished white rice for 5-10 replications and see whether there are any random spikes of metals (Fe, Cr, etc), indicative of grinder contamination, or a gradual decrease/increase of other elements suggesting insufficient cleaning of the grinder between the seaweed samples. I would imagine the salt levels might be quite significant in the first couple of rice "blanks".

Speak to the staff at the analysis lab and see what they recommend. I assure you they would rather talk to you about potential contamination issues BEFORE the samples are processed, rather than call you about an issue AFTER the samples have been analysed.

Matthew Wheal Thanks so much for your super thorough answer. I have seen coin envelopes used and they seem ideal, and much easier to get than small plastic vials. The paper bag method sounds good as well! I initially had just spread the seaweed on a plastic tray in the drying oven, however as it became more dry, small fragments fell off while transferring it to the scale to ensure the dried seaweed reached a constant weight. I think using a paper bag could definitely solve this. Is the bag left open or is it folded shut?

I think we left the drying bags open or folded once loosely 2 cm or so to let moisture escape but not let so much dust etc in. Once dry though, the coin envelopes are fine if sealed or folded over.

It might pay to send a few empty envelopes with the samples so the lab can check for potential contamination (rinse for a few seconds with their blank solution) just to see if anything unexpected is present in the paper bags themselves.

Matthew Wheal regarding the shiny white paper you mentioned, if zinc was not one of my target elements I assume it would be ok to utilize it? The use of metals trays (e.g. disposable aluminum baking trays) I also assume would be fine if I only intend to test for %C, %N and %P?

If you are only considering macro elements in the samples, then paper will not contribute significant amounts of contamination.

Offering my bias here towards micro nutrients: Why not consider some other elements, as C is pretty much related to biomass, N will probably also be related but subject to volatility, P might be a good indicator of rapid consumption by microbial breakdown, but what about S, Mg and Fe, which are all important for photosynthesis and protein metabolism, and relatively rare in the marine environment. If you extended your suite of analyses, you might get a wider range of responses for your study.

@Matther Wheal I didn't include other elements more due to finances and I really more wanted to focus on N and P in respect to their contribution in eutrophication of coastal marine systems. Thanks again for your assistance and recommendations. Additionally, how do you store your ground plant (e.g in a dry cupboard or refrigerated?) and for how long is it safe to be stored?

As long as samples are fully dried (which might be difficult with some algae as they tend to go quite gelatinous), you should be fine with room temperature storage in the paper bags. For the macro elements this will be years unless there is some sort of cupboard moth/silverfish/weevil that takes a liking to salty powdered seaweed. We used to have naphthalene traps hung in our store cupboard, but only held samples for a month after results were sent out, otherwise we would have run out of room very quickly.

@MatthewWheal thanks so much for your assistance! You've been a great help!