Need help calculating / converting GC-FID result

Basic questions from students; resources for projects and reports.

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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
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
-- Tom Jupille
LC Resources / Separation Science Associates
+ 1 (925) 297-5374
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.
The label claim is 10mg/20g
That's 0.05%
-- Tom Jupille
LC Resources / Separation Science Associates
+ 1 (925) 297-5374
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