Chromatography Forum

Hi!
A couple of weeks ago I and two friends analyzed perfumes using GC-MS technology. We acquired graphs that says RT in the y-axis and time in the x-axis. And over each peak is a number, 13.26, for example.
How can i explain this so people without knowledge of this will get what it is all about. Anyone?

If RT means retention time that doesn't make sense for the the y-axis? Please verify or explain.

I have developed what I call the "shopping mall" theory of retention in chromatography which goes as follows. Outline a theoretical shopping mall consisting of a long, single hallway (column) with doors at each end (inlet and outlet). The mall is filled with only shoe stores (or sporting goods or whatever, depending on your audience). This is the stationary phase. Three shoppers (analytes) enter the mall at one end (inlet) at the same time. The time it takes for each shopper to traverse to the end door (outlet) will be monitored. One shopper is only slightly interested in shoes so only stops at a couple of stores and exits the mall in 15 minutes. The second shopper has a greater interest in shoes and stops at several stores and exits the mall in 30 minutes. The third shopper really likes shoes and stops at almost all of the stores and exits at 60 minutes. Retention is based on how much attraction there is to the shoe stores (stationary phase) and is measured as the time it takes to make it from one end of the mall to another (retention time). Use your imagination to expand or modify this theory as necessary. My apologies in advance to all you theoretical chromatographers who may cringe at this explanation.

I personally prefer the river analogy. You have the river which represents the mobile phase and all obstacles along can be considered the stationary phase. You introduce balls of different size and weight upstream and you count balls downstream. The balls will be separated according their physichemical characteristics (in this case size, weight etc). Each peak in the chromatogram indicates nature of ball and amount of the same type of balls introduced. Different rivers can be different mobile and stationary phase (or chromatographic conditions in general).

Now about your chromatogram, x-axis is in general retention time (RT) and y-axis is intensity. The numbers on top of the peaks is either retention time in the Apex of the peak (you can verify this by it's correlation with the x-axis) or sometimes I have also seen people putting the masses of the compounds eluting at that peak (not very likely in your case).

Nice and pedagogic explainations!
Oh and the y-axis consists of values 1-100.
That's the relative abundance. But haw can I explain how the GC measures this?

I like the following analogy:

There is a long, long street with many pubs, kind of like Bourbon Street in New Orleans.

You have three groups of people that enter the street together: a group of Saudis, a group of Germans, and a group of Irish. As they walk down the street, they enter the pubs and check them out. The Saudis don’t drink, so they leave the pubs immediately again. The Germans hang out in each pub, have a beer, and leave again. Since it takes some time to drink a beer, it takes the Germans much longer to get to the end of the street than it took the Saudis, which migrated through the pubs without being retained. Since the Germans do not drink completely in unison, they spread out a bit after a while, but they arrive at the end of the street not to much spread out from each other. The Irish on the other hand drink more than one beer at each pub. Then they need to go to the bathroom etc. This makes their travel much longer than the travel by the Germans. Also, since they drink different quantities, they spread out much more than the Germans.

So the Saudis got through Bourbon Street unretained, the Germans had a retention factor of one beer, and the Irish had a retention factor of 2 beers or more. The spreading of the bands is in proportion to the retention (or beer consumption).

We also can expand the story with a group of Americans, which drink whisky and get stuck in the first pub…

I would use the river analogy but slightly different than Kostas (he sounds like he does a lot of size exclusion chromatography).

Nobody wants to be washed down the river so when they are dropped in the first thing they do is latch on to the bank and hold on. All your various compounds, like people, have different degrees of strength so the weak ones (polar compounds) can’t hold on very long and are washed downstream quickly. Your strong individuals (non-polar) can sometimes hang on forever. This is where solvent strength comes into play. Seeing as the rivers flow is constant, an increase in solvent is not an increase in flow but an additive to the river that makes everything slippery and more difficult to hang on to. Eventually, with enough solvent introduced into the river nobody can hold on and they are all washed away.

This also explains why if you are only dropping one individual in the river you don’t need to run a gradient. You get to know this individual’s strength and can introduce just the right amount of your slippery additive such that he holds on as long as you want him to. When he exits he is easy to detect because he is all alone.

However, when you drop a whole crowd in the river at the same time (a dirty sample) you need to run a gradient. You add very little solvent such that everybody gets a good grip immediately. Then as you start ramping your slippery additive the individuals start loosing their grip and washing away one at a time such that when the guy you are looking for lets go he doesn’t do it at the same time as anyone else and also exits alone and easy to detect.

This also explains the use of different rivers (columns) when analyzing various people. Your C18 has a whole bunch of handholds for people to grab on to so if you are analyzing really strong people you might want to use one that doesn’t have so many.

To a non-chromatographer I often explain the separation in chromatography by asking if they have ever spilled iced tea on the Sunday comics. Most people have seen the ink spread and I point out how the colors separate one will flow with the tea faster than another and the colors separate. We just do the same thing in a chromatographic column - just in a more controlled manner.

Generally I don't have to get into a discussion of chromatographic mechanisims or how we can detect compunds that have no color. The've actually seen the separation. And our graph is just how much ink there is going out from the original spot in the Sunday comics page.

(Oh the terror of the paperless home - no Sunday paper! I'd just have to go on line looking for message boards to haunt.)

[quote="Uwe Neue"]I like the following analogy:

There is a long, long street with many pubs, kind of like Bourbon Street in New Orleans...."

I love your explanation! Funny and clear!