Depth of knowledge

[dekoyl]

I encountered the following [simplified] question.
and have same molecular formula, same molar mass but different structural formula. A sample containing both compounds are analysed by GLC using flame ionisation detector.
If the sample has the same concentration of both chemicals, will the peaks produced have the same area?

The Answer: "No. The peak area depends on the sensitivity to the flame ionisation detector. This depends on the structure of the molecule and the ions that are formed in the FID cell. Consequently, if the concentration of more than one constituent of a sample is to be determined, separate calibration curves must be used for each compound."

I'm wondering - is this just general knowledge required for this chromatography area of study?

[dekoyl]

And somewhat related:
Should HPLC and GLC have different "sharpness" of peaks on their chromatograms? Ie. The HPLC chromatogram has sharper peaks whilst the GLC one has blunt peaks.

Thanks

[Chemkid]

lol, similar stuff was on this thread http://vcenotes.com/forum/index.php/topic,11591.0.html

I encountered that same question in heniemann (about the flame ionisation detector) but its highly unlikely that we need to know that according to my teacher. However, it is better to know it, as it "leads in to the idea of spectroscopy" (in the words of my teacher)

i have no idea about the second one    

[TrueTears]

In reply to your first question, that kind of questions is probably unlikely to be tested but it's good to know about it.
Basically what happens in the flame ionisation detector is that organic compounds are burnt in a hydrogen-oxygen flame and it basically 'breaks' up the organic compound into their ions. These ions are then attracted to electrodes and cause a current to flow, the amount of current determines the type of compound it is.

Even though the 2 organic compounds have the same molecular formula, they would not 'break' up into the same ions because they have different structural formulas. Therefore different ions are produced. The area under the peaks refer to the AMOUNT of compound present, even though they have the same concentration it does not mean they come in exactly the same quantity.

Your second question

The sharpness of the peaks refer to the recorder response(ie absorbance/concentration) and so it wouldn't really matter whether GLC or HPLC is sharper. The important thing is the height of the peaks and the place on the chromatogram which they appear. The higher the peaks are means there is more of this substance in the whole sample. The place where the peaks occur determine its attraction to the stationary phrase (ie the retention time). If a substance has a high retention time it means it is more attracted to the stationary phrase and hence would take longer to be eluted. So if you compare the heights of the peaks with a reference sample and the heights match and they are also in the same place on the chromatogram(same retention time), it would mean they are the same compound.

Hope that helps

[dekoyl]

^ Epic help TrueTears. Thanks

[jackinthepatch]

I have an additional question, based on the same sort of thing.

What are some factors that can affect the retention time of a component? I know that molecular mass has something to do with it, but what exactly? Like will a higher molecular mass result in a longer or shorter retention time? Help me lol.

[TrueTears]

Basically retention time is the time for the compound to be eluted. So, this means its attractions with the stationary phrase. Something with a high molecular mass would probably take longer to be eluted say, in HLPC. This is because it is much harder for the LARGE compound to 'squeeze' through the stationary phrase and get eluted. However if you have a smaller molecular mass, normally it means it can 'squeeze' through the stationary phrase easier and be eluted faster. But ofcourse there are many more factors to be considered, for example, say if the stationary phrase was a very polar substance and the compound being tested is also very polar, then dipole-dipole bonds will form between the stationary phrase and the compound resulting in high attraction so it would take longer for the compound to be eluted hence higher retention time. If the compound wasn't polar then it would not be attracted to the stationary phrase and be eluted faster, ie lower retention time. Sometimes the compound might form hydrogen bonds with the stationary phrase, resulting in even stronger attraction hence higher retention time.

[jackinthepatch]

I love you TrueTears.

Thanks so much for that, it's much clearer now

Bit more karma heading your way

[shinny]

Oh wow there actually was a reason behind that question from Heinemann (well I think that's where it was from). The general consensus from last year was that it was an error (no one could find any solutions to it, and even teachers across a few schools thought it was wrong too). Long story cut short; I doubt you'll be expected to know that, but like others have said, now you do, and it's not the kind of thing you'll forget and it's nice to know I guess for better understanding.

[dekoyl]

^ Thanks again TrueTears

Will we come across knowledge of bonding again in unit 3/4 or is it meant to be retained from unit 1/2? Intramolecular bonds, hydrogen bonds, dipole-dipole etc.

[jackinthepatch]

From what I've heard so far we just have to remember it all. It's assumed knowledge, along with the rules of solubility, the valency table and sooo many other things.

It sucks doesn't it lol. Bloody chemistry.

[TrueTears]

From what I've heard so far we just have to remember it all. It's assumed knowledge, along with the rules of solubility, the valency table and sooo many other things.

It sucks doesn't it lol. Bloody chemistry.

yeap exactly what jack said, not too hard to remember those anyway but solubility table kills me -_-

[shinny]

The above concepts aren't something that will be DIRECTLY tested, but knowing it will definitely help a lot with the conceptual understanding of...well...pretty much everything in units 3 and 4. This is the key reason why I stressed throughout the summer for people to revise unit 1 and 2 rather than studying ahead to 3 and 4 and I reckon my understanding of these retained from actually trying in units 1 and 2 is the reason why I picked up chemistry concepts so quickly in year 12.

[jackinthepatch]

Yeah same here! I don't know why, but the solubility rules just will not stick in my head. They're a bitch to learn.

Ah well I guess I'll get it eventually, with practice. I hope lol.