I'm looking for suggestions to know which samples I could use as a standarization and model transfer for NIR portable devices. Right now we are using mixes of urea and glycerol with different molar concentrations. The problem with glycerol is it's fluidity, which is low and we are losing a lot of sample every time we return it to the bottle. Some alternatives that we though about as mixes of water and ethanol were discarded because the evaporation of ethanol. We are looking for samples that could be used for 5 years and be chemically stable over time.

A quick research of the available bibliografy showed that many groups use samples with organic origin, which I don't think will be stable over time. We welcome every suggestion about the samples we could use, especially if they are easy and cheap to prepare.