calculating weight/weight conc. from weight/volume

[MikeT]

Hey guys,
I have been spending so much time thinking about this but I'm still confused. I made a calibration curve for caffeine by injecting 5 ul of solution into the HPLC. The conc.'s I used were from 0.4 to 5 mg/ml. Then for my sample, I diluted .1517 g of a supplement powder (mixture of a lot of things) in 100 ml solvent, giving a conc. of 1.517 mg/ml. The amount injected was 5E-3, so I turned every concentration to mg on column. I need to get the caffeine amount in this 0.1517 g of powder by mg/g basis. How do I do that? Thanks so much.

[mattmullaney]

Hi Mike,

So then, you have standard curves rendered in amount (mg caffeine) and concentration (mg/mL caffeine). You know the volume of caffiene-containing sample you injected on to the same system (same volume as standards, 5 uL?) as well as the concentration of the sample in mg/mL...151.7 mg/mL as you wrote below.

The mass of Sample Injected on Column: Inj. Vol. (0.005 mL) x 151.7 mg/mL = 0.7485 mg

Mass of Caffeine Determined in 0.7485 mg Sample...well, this is what you determine from one of your standard curves.

(Mass of Caffeine Determined in sample, mg) / (0.7585 mg Sample Injected on Column x / 1000 mg / g) = mg caffeine / g sample.

Good Luck.

[MikeT]

Thank you so much !

[mattmullaney]

Hi MikeT,

Glad to help when/if I can...probably every member here has helped/been helped at one time or another. I belong in both categories, at least. Sometimes I get stuck on things, too...don't we all?

Best Wishes!