HNO3 acid is required to preserve the water samples to minimize metal cation precipitation and adsorption onto the sample. Adding Concentrated acid is required mostly for microbial deactivated and acting as perservative and also minimize metal precipition but hydrochloric acid is the preferred solvent for this purpose.

The maximum holding time for most metals before analysis is 6 months, and they should be stored in a fridge

Muhammad Amjad, Thank you

During cold storage you must be sure that evaporation is minimised or accounted for. The surest approach is to store control solutions with the samples, and to weigh the containers before and after storage.

Paul Milham Thank you,

You are welcome

It all depends on how do you keep your sample in a cold/frozen storage condition. The storage temperature should be super stable. The principle behind this is to stop any microbial activity (example: Acid loving bacteria) during the storage. We dilute the sample after acid digestion, so the pH would not be super low.

In general, +4 deg C if you analyze within 2-4 weeks; -20 deg C for extended storage; if storage over many years, you need quick-frozen in liquid nitrogen at –196°C and then stored at or below –70°C.

I think the recommendation of @Paramsothy is superb. I also go along with his suggestions. Analysis within a week or two may not pose any issue after acid digestion but for later time, you need further processes as recommended.

You can contact with Dr Kenan Zengin ([email protected]).

All the best, Ergin