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**Hollow** » Mon Mar 25, 2024 11:22 pm

but 5 mg/kg is a mass fraction, not a concentration...

general remark that I try to teach my students as well:

the more one uses the units and terms correctly, the less confusion is introduced...

**concentration **= something **per volume**

**amount; mass fraction **= something **per mass**

If your calibration standard are really in the unit of mg analyte/kg solute, then your result is also in this unit. So you must check, if this is the case or if there are additional calculations being made.

If the calibration is in mg/kg, then you would need to know the density of your solute, which in this case (water) can be assumed as 1 kg/L (or use the exact value from reference sources).

Then you have your matrix-concentration of 15 g sample in 1 kg water = 15 g/kg

from that, you measured an analyte-concentration of 5 mg/kg.

So 5 mg/kg / 15 g/kg = 5 mg analyte /15 g sample (= mass fraction = 0.33 mg/g sample)

But double check if this is correct and that the 15 g are not already contained in the calculation for the 5 mg/kg

(e.g. because you always use 15 g / 1L, so this factor is already implemented).

Just do the calculation by hand from the peak areas of your sample and calibration runs.