help with a question

[deep]

can someone here please help me with this question on Single point calibration without internal standard that im having problems with:

234.2mg paracetamol analytical standard was made up to 100ml with mobile phase. 5ml of this solution was diluted to 100ml. HPLC analysis of the diluted solution gave an AUC for paracetamol of 18528.
1.2464g of paracetamol oral solution (stated content 120mg/5ml) was made up to 200ml. HPLC analysis of the sample solution gave an AUC for paracetamol of 20834.
Determine the actual paracetamol content of the oral solution and compare it to the stated content. Does it comply with the BP limits of +/- 5% of the stated content? (Density of the oral solution = 1.085g/ml)


thanks

[DR]

1st try - let's see if we get same thing...

21.128mg APAP (paracetamol) per g sample or 81.139% of claim. If that's what you got, you're no where near ±5%.

How's your APAP peak shape? Is it on scale, but with a flat top? If so, dilute further. Few detectors are linear beyond a couple of micrograms passing through the cell despite the relatively low output voltage. APAP is notorious for doing that (having a flat topped peak well below maximum detector output voltage).

[deep]

thanks for the answer, but how did you reach it as its the method that im really interested in.
Im not given the graph, just the values of AUC and supposed to use
AUC (unknown) X Known concentration
AUC (known)

so I would appreciate it if you showed me your method

thanks

[DR]

=(234.2/100*5/100)/18528*20834*200/1.2464
yields mg APAP/g sample

=120/5*1.085
is claim in mg APAP/g sample

100* first formula / second formula = % claim

[tom jupille]

In addition, if you want a tutorial, check here:

http://www.lcresources.com/resources/ge ... ction4.htm

[DR]

...Im not given the graph, just the values of AUC and supposed to use...

You should insist on seeing the chromatograms. If the peaks have flat tops instead of points (you may well have to zoom in a bit to inspect this), you have a problem with the method used to get said peaks (samples too concentrated or injection volume too high - either way, too much APAP for the detector).