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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: coolestsoul on March 19, 2008, 06:39:28 PM

Title: basic questions about analysing GC results
Post by: coolestsoul on March 19, 2008, 06:39:28 PM

Hi...

I am a complete beginner as far as the use of GC as aan analytical technique is concerned. I had few questions and answers to them would really help me out in my preliminary work:

1. Is the area of the GC peaks proportional to concentration of the species in the sample in mass terms (e.g. g/mL) or in molar terms (e.g. mol/mL)?

2. Actually, this one is I guess similar to the first question. If I wish to add a known amount of a species as an internal standard, should I add it in a manner such that its final concentration is constant in all samples in molar terms or in mass terms?

3. I am getting peaks of butanol when I run a butanol solution in water through my GC which instead of sharp peaks are triangular in nature with an abrupt and sharp beginning (desired??) and a lot of trailing towards the end? What may be causing this?

Thanks and regards.

Title: Re: basic questions about analysing GC results
Post by: Rivet on March 19, 2008, 07:15:33 PM

1.  Both.  You may calibrate your instrument on either basis.

2.  See above.

3.  There are a numerous causes for tailing such as using a less-than-ideal stationary phase or as a result of a poor injection.  I think we need more information about your protocol to answer this one.

Title: Re: basic questions about analysing GC results
Post by: coolestsoul on March 19, 2008, 07:24:01 PM

I guess I was vague while asking he question. Kindly consider the situation below:

1. If i inject a sample containing two species which give two peaks, say A (mol wt 50) and B (mol wt 100). If the sample contains 1 mmol of A (i.e. 50mg) and 1mmol of B (i.e. 100mg) then should I expect the area of the peaks to be in 1:1 ratio (which will be the case if area is proportional to molar concentration)  or 1:2 ratio (which will be the case if the area is proportional to the mass of the species, and since I have added twice as much weight of B as of A, its peak will have twice the area)?

2. The protocol is that I am injecting 1ul of ~5% v/v BuOH solution in water @ 230 deg C. Heating the column from 50 deg C to 250 deg C over 12 minutes and detecting using an FID at 280 deg C.

Thanks for your help.

Title: Re: basic questions about analysing GC results
Post by: JGK on March 20, 2008, 08:08:17 AM

Quote from: coolestsoul on March 19, 2008, 07:24:01 PM

I guess I was vague while asking he question. Kindly consider the situation below:

1. If i inject a sample containing two species which give two peaks, say A (mol wt 50) and B (mol wt 100). If the sample contains 1 mmol of A (i.e. 50mg) and 1mmol of B (i.e. 100mg) then should I expect the area of the peaks to be in 1:1 ratio (which will be the case if area is proportional to molar concentration)  or 1:2 ratio (which will be the case if the area is proportional to the mass of the species, and since I have added twice as much weight of B as of A, its peak will have twice the area)?

Generally no, each species will have a differing unit response (area/unit concentration), while you may get a relationship with some, but with most it will not work this way.

2. The protocol is that I am injecting 1ul of ~5% v/v BuOH solution in water @ 230 deg C. Heating the column from 50 deg C to 250 deg C over 12 minutes and detecting using an FID at 280 deg C.

Thanks for your help.

Internal STD levels should be identical (same level/concentration) in all samples and standards.

Title: Re: basic questions about analysing GC results
Post by: Rivet on March 20, 2008, 07:02:51 PM

Your oven program sounds reasonable.  Describe your column and your injection mode (split?) and maybe we can solve your tailing problem.