Need help calculating / converting GC-FID result

[ntward]

Hello,

I'm having trouble figuring out how to calculate a gc result from my calculation.

this is my usual process to calculate the percentage of the substance.

Sample prep for example is 165.9mg of sample
20ml of meoh
50ul of sample / 950ul MeOH (1:20 dilution) into gc autosampler vial.


chemstation result is 100.0 ng/uL

100.0ng/uL = 100ug/mL x 20 x20ml/1000 = 40mg
40mg/165.9(sample weight) = 24.1%

What im having trouble with is that im being asked to determine the amount of a substance per 20g

my sample prep is the same as above
Sample prep for example is 325mg of sample into 20ml of meoh
50ul of above sample + 950ul MeOH (1:20 dilution) into gc autosampler vial.

my result is 18.37 ng/ul
Id like to see step by step how to convert the 18.37ng/ul to mg/20g

im sure its a simple conversion, but I must be missing something because my calculations arent making much sense.

thank you

[tom jupille]

It's basically the same as you did before. Once you know the percentage, you simply take that percentage of 20 g (why would someone want to know the amount per 20 g ?).

So, in your second example, you have determined that your analyte concentration is 18.4 ng/uL, which is the same as 18.4 ug/mL, which is the same as 0.0184 mg/mL [I really doubt that you can justify four significant figures here; doing a 1:20 dilution introduces at least 1% uncertainty and even 18.4 would be pushing things].

Your total sample was effectively 325 mg in 400 mL (accounting for that 20:1 dilution), which is 0.812 mg/mL. The percentage of your analyte is then 0.0184/0.812 = 0.0227 or 2.27% .

0.0227 X 20 = 0.45 g

[ntward]

Thank you. The request for mg/20g was based off the serving size of the product.
So than based off the calculation the analyte concentration is 450mg/20g?

The label claim is 10mg/20g. I'll try the extraction again tomorrow and forgo the 1:20 dilution and see how things go.

[tom jupille]

The label claim is 10mg/20g

That's 0.05%