[faith]

how do determine the following if u carry the experiment using HPLC
1) the average retention times for caffeine
2) area under a calibrated curve for caffiene
3) Construct a standard curve by plotting the average peak area vs. concentration of
standard.
4) Calculate the concentration of caffeine in given solution,
5) the mass of caffeine in the analgesic pill.

[Arkcon]

You appear to be new, so welcome to the forums.  We like to help, but we do so by giving hints, or trying to help you understand, or tell you, after you show us your work, where you've gone wrong. 

That is quicker and easier for us, than explaining everything, and it's less like cheating, and you end up better off during an examination, because you've found it out yourself. 

Your six questions are basically asking for a few classroom lectures worth of definitions, that you could just as easily look up in your text, yourself. 

What can you tell us about what you know about these subjects, and what is the first thing that confuses you the most?

[faith]

after the experiment l obtained a table that showed the time (secs) and voltage (V). I took the range between the time the calibrated curved was obtained, and traced these ranges to the its voltage. I asummed that the voltage is my absorbance so l plotted the absorbance against the concentration that was given. I know this method is wrong so that why l need HELP.

[faith]

method l used to solve the problem which l think l am wrong
1) retension time was the point of injection to the highest peak for each concentration. I then took the average of the retention time to obtained average retension time.

2) the concetration of caffiene was the slope of the graph
3) to get the mass l calculated the mole of caffiene using: mole = volume of caffiene  * concentration
mass = molar mass of caffiene * mole of caffiene
l will like if some one could show me an example using a table with its  graph

[Arkcon]

Humph.  Most modern chromatographic systems draw a tracing for you.  Oh well.  Can you plot time vs. voltage, and answer a few of the questions yourself?  Can you plot the cal curve data, and your experimental data, and see some similarities?

[Arkcon]

method l used to solve the problem which l think l am wrong
1) retension time was the point of injection to the highest peak for each concentration. I then took the average of the retention time to obtained average retension time.

That is not a bad method.  Try to check the standard deviation, if it is outside what you or an instructor would accept, try to realize -- your separation has been affected, and this makes all results suspect.

2) the concetration of caffiene was the slope of the graph

Ummm...you're probably close, just leaving out words, I'm guessing.  Try to think about it more, so you can write something better on the write-up.

3) to get the mass l calculated the mole of caffiene using: mole = volume of caffiene  * concentration
mass = molar mass of caffiene * mole of caffiene

Nothing here is absolutely wrong, if you can justify it with units canceling, then trust your instincts.

l will like if some one could show me an example using a table with its  graph

Nobody does chromatography that way anymore.  I'm serious, your assignment, while probably meant to teach you some of the more fundamental concepts of analytical chemistry as it relates to separation science -- is crazy.  Not the bit where you generate a standard curve from the results, that can be done by hand, or the integrator, or the chromatography software.  That's true.  But you getting a table of time vs. voltage, instead of a tracing, that is what borders on the insane.  I've never been subjected to that, and I started HPLC as a college intern, in 1990, that's close to two decades ago.

Occasionally, my instructors would mention, way before the 1990's, of analog integrators failing.  And how they'd determine relative areas under the peak by cutting out the trace and weighing the paper on an analytical balance.  But I never did that.  I'm just warning you, some of the problems you're facing are going to be outside the experience of most people you'll meet.

[JGK]

l will like if some one could show me an example using a table with its  graph

Nobody does chromatography that way anymore.  I'm serious, your assignment, while probably meant to teach you some of the more fundamental concepts of analytical chemistry as it relates to separation science -- is crazy.  Not the bit where you generate a standard curve from the results, that can be done by hand, or the integrator, or the chromatography software.  That's true.  But you getting a table of time vs. voltage, instead of a tracing, that is what borders on the insane.  I've never been subjected to that, and I started HPLC as a college intern, in 1990, that's close to two decades ago.

Occasionally, my instructors would mention, way before the 1990's, of analog integrators failing.  And how they'd determine relative areas under the peak by cutting out the trace and weighing the paper on an analytical balance.  But I never did that.  I'm just warning you, some of the problems you're facing are going to be outside the experience of most people you'll meet.

I started working with HPLC in the early 1980s and even then only heard of cutting peaks out of traces and weighing  as an "amusing aside" in lectures or as an alternative to calculus methods in computing integrator validation. Even in the 80's computing integrators were common and I never had to perform voltage/time plots.