Chromatography Forum

Hello to everyone as this is my first post in the forum and i apologize beforehand if it is a silly question.

Assume that i have chromatogram like that

time (min)_______area (%)_______component
__3.2_____________53____________ethanol
__6.7_____________28____________butanol

I have hard time assessing this result, which one of the following is true?

a) the ethanol to butanol mass ratio in the sample is 53/28=1.893
b) the ethanol to butanol mol ratio in the sample is 53/28=1.893
c) the ethanol to butanol mol ratio in the sample is (53/2)/(28/4)=3.786

I assume c is right since i think FID detects the carbon molar flow with respect to time (i searched a little about FID's but could not find an answer or i found and did not understand). Therefore given area percentages must be divided by the carbon amounts of the each component respectively before comparing them between each other.

Any help would be much appreciated. Thanks in advance.

NVU_15,

D) None of the above. All you have is the area % of the integrated peaks (total) in the chromatogram. Your problem is compounded by at least two factors. 1) The sum is not 100% so you have something else in your chromatogram and 2) ethanol response factor is not the same as butanol response factor. Butanol is almost a 1 but ethanol is anything but....

Make a solution of these compounds weighed out accurately and then calibrate. At least then can rely on the linearity of the FID.

Best regards,

AICMM

c) is the closest, but relies on the FID being a simple carbon counter, which it is not, especially with small molecules with heteroatoms. AICMM's advice is the best - calibrate by mass (or moles) and then calculate. Failing that you can look up the FID's relative response for ethanol and butanol.

The peak areas being expressed as % is not a problem (as long as they are on the same chromatogram of course).

Peter