Hello!

I've never posted here (or any forum, for that matter) so forgive me if I do something wrong or against "netiquette."

I recently took an exam in my analytical chemistry class and am confused about several of the problems I got wrong. Unfortunately, my professor is not very approachable or helpful. (In fact he said if we come to him for a regrade he'll take more points off than he gives back).

I admit I'm also suspicious of a few of his answers on the key, so I'm hoping you all here can help me either understand it.

I know this is long so I'm hoping to break it up so it's easy to follow. If you can answer even just one part of the my post (one question below) I would greatly appreciate it!!

First question:

1) Shown below is a separation of a mixture of two components. The separation achieved a resolution of 1.5 and the two retention times were 100 and 112, as shown.

(Here he showed an image.. I have a screenshot because it's from the ebook, but I can't figure out how to a add it. It is very simple, two peaks and it says resolution = 1.50. Shows 6sigma between the peaks. Then he added to retention times, 100 and 112 sec.)

How many plates were achieved in this separation?

--I understood that resolution = delta tr / wav so I found that the average width (wav) = 8 sec. I became stuck on the exam because I couldn't remember any equations that relate average width to something useful for number of plates. My professor used w = 4sigma to find sigma then used that in N =tr^2/sigma^2.

I had not seen any problem that does this before, so my question is: can you use an average width in place of width to find number of plates for the separation? His key also lists two answers because he used each retention time and said either is correct.

I still have a final in this course so if this is OK I do need to know for that.

2) Suppose the separation (same as question 1) is an example of gas chromatography. What is the effect on the separation's number of plates of increasing the size of particles on which the stationary phase is coated? (a) N increases (b) N decreases (c) no change. Why? Explain with equation.

--He said to assume gas chromatography questions are referring to open-tubular columns since it is more common than packed. I understood "particles on which the stationary phase is coated" to be referring to SCOT (support-coated open tubular) in which the stationary phase is a liquid film over the support particles, which are NOT stationary phase. Here was the answer I put:

"(c) no change. H = A + B/ux + Cux (Van Deemter) No change because particle size affects the A term for a packed column but this is an open-tubular column. Thickness of film coated on the particles would affect N and resolution, not particle size."

Can anyone explain to me where I went wrong in my reasoning and how particle size would affect resolution in this question? I could not find any reference to affect of particle size for GC unless the particle is the stationary phase, not when a stationary phase is coated on it (as in this question).

3)Suppose the separation (same as question 1) is an example of gas chromatography. What is the effect on the separation's number of plates increasing the carrier gas flow rate? (a) N increases (b) N decreases (c) no change. Why? Explain with equation.

"None of the above. H = A + B/ux + Cux This would have _opposing_ effects, which can be seen in the van Deemter equation. Ux is linear flow rate. If it is increased, H will be increased by Cux which decreases N (N=L/H). However, if Ux is increased it will also decrease the B/ux term which decreases H and increases N."

--Prof actually said no one in the class got full credit on this. His answer was:

"(a) increases _and_ (b) decreases. Depends on where you are on the van Deemter plot (H vs. U). H can go up or down, N can go up or down."

I must say, I think my answer explains why the plot is shaped that way... please let me know if you see something different in my answer vs. his or I overlooked something.

Finally..

4) Using van Deemter equation, explain why longitudinal diffusion is a more serious problem in GC than LC.

My response:

"H = A + B/ux + Cux The A term deals with multiple paths and particle size but in open-tubular GC, this term is irrelevant and the equation becomes H = B/ux + Cux. Longitudinal diffusion is the B terms nd it contributed to plate height, H, more than it does in the packed LC van Deemter equation. Plate height is inversely related to resolution, so when B affects GC more than LC, it negatively affects resolution in GC more than LC."

I was not very confident of this response, because it is just mathematical and less conceptual, so I know I need help on this one!

Again, I don't expect any one person to take the time to answer these. I didn't know if they should be four separate topics or all on one, since they are all about chromatography and van Deemter. I'd be happy to split them if that's what I should do.

Thank you in advance for any advice or explanation you can give me!!!!