Arno,

Water is s unique situation because 1liter of water weighs 1 kilogram at common laboratory/room temperatures. Therefore, for aqueous solutions 1 ppm is interchangeable with 1 mg/L or 1 mg/kg, or the weight per weight and weight per volume measures about both ppm for water. .

Acetonitrile is less dense that water and acetonitrile/water mixtures will not have same density as water. If one mixes 0.5 L acetonitrile with 0.5 L water, the resulting volume will be less that 1 L due to interaction of water and acetonitrile molecules.

A work-around to the volume.density changes of acetonitrile/water mixtures is to prepare and report mass per volume (e.g., mg/L) concentrations for your solutions and NOT ppm.

You first need to do a volumetric study of your solvent mixture. There should be available literature on this topic so start there first. After you know how your mixture behaves you add 10 ne millionth parts of your metabolites in it.

The preparation is carried out by weighing each component, solute and solvent. Then 50/50 solvent density is used to go from mg / Kg to mg / L. This article can help you with densities.

https://doi.org/10.1002/jccs.200700162.

Arno,

the practical question is, if you really need to get precisely 10,0 ppm concentration?

It is for quantitative method? like external standard/calibration curve? If so, the question is in which solvent you will inject your real sample. If the goal is to analyse something in water - use water as solvent for calibration mixtures. If sample preparation is included, and finally you dissolve the "extract" with analytes in AcCN/Water 50/50 prepare calibration in this mixture (however in this case, if few preparation steps are used think about internal standard method).

In both cases you can express the concentration as mg/L of solvent and you can forget about problem with "ppm".

regards,

GB