Is this the correct way to determine total amount?

[Pear]

Hi,

slightly different question than usual here.

I'm using HPLC to detect various aldehydes in an extraction.

- I have 0.5g~ of a solid sample that I extract in 20ml water.
- From this, I correct for actual weight of sample and blank correct.
- This gives me a result in mg/kg for the aldehydes.
- The aldehyde of interest result, is for example 8mg/kg.
- I want to calculate the amount of this aldehyde present in 50g of the original sample type.

Is the below calculation the correct way to go about this?

1. 8 (mg/kg) * 20(ml of extraction water) = 160 mg/kg
2. 160 / 0.5 (g of sample used) = 320 mg/kg
3. (320 /1000) * 50 (g of sample I'm wanting to know) = 16 mg/kg

Thanks for any help.

[James_Ball]

Hi,

slightly different question than usual here.

I'm using HPLC to detect various aldehydes in an extraction.

- I have 0.5g~ of a solid sample that I extract in 20ml water.
- From this, I correct for actual weight of sample and blank correct.
- This gives me a result in mg/kg for the aldehydes.
- The aldehyde of interest result, is for example 8mg/kg.
- I want to calculate the amount of this aldehyde present in 50g of the original sample type.

Is the below calculation the correct way to go about this?

1. 8 (mg/kg) * 20(ml of extraction water) = 160 mg/kg
2. 160 / 0.5 (g of sample used) = 320 mg/kg
3. (320 /1000) * 50 (g of sample I'm wanting to know) = 16 mg/kg

Thanks for any help.

Is your initial result of 8mg/kg per kg of water? I am more used to seeing it as mg/L but essentially the two are interchangeable when using water as the solvent.

If the initial result is 8mg/kg water then you can't multiply 8mg/kg x 20ml or you units would be 8mg/kgml. You would either need to multiply by 0.02kg(20ml/1000ml/kg) which would be the weight of the initial extraction water.

You also need to have the 0.5g in kg or convert the mg/kg to mg/g to end up with the correct units.

8mg/kg * 0.02kg = 0.16mg of aldehyde in the extraction

0.16mg / 0.5g = 0.32mg/g
0.32mg/g * 50g = 16mg total but not per Kg

per Kg would be 0.32mg/g *1000g/kg = 320mg/kg

[Andy F]

I did the calculation a different way, but came to the same answer.
However the final units should be 16 mg/50g shouldn't they?

[James_Ball]

I did the calculation a different way, but came to the same answer.
However the final units should be 16 mg/50g shouldn't they?

Depends how you want the final answer. 16mg/50g or 16mg total will be the same if you always have 50g. If you want to dissolve that into a water solution after, say 1 Liter then you either have (16mg/50g)/1L or 16mg/1L. To get rid of the 50g you have to calculate it as (16mg/50g) / (50g/1L) to get the g to cancel out.

If you check your equations by putting in only the units and no values, then cancel all the units, if you end up with something other than the final units you are looking for then there is definitely a mistake somewhere in the equation.

When I read "total amount" I am looking for a single unit, if it is "concentration" then I expect an amount/amount since concentration is amount solute/amount solvent where "solvent" can be liquid or solid.