Commodore 128 PC with Preparative Gas Chromatograph

[amiman99]

Not mine but interesting find. I think it comes with everything.

 

Commodore 128 PC with Preparative Gas Chromatograph

 

http://austin.craigslist.org/ele/5605746717.html

 

[Keatah]

$400 worth of spare parts and ICs! Easy..

[CGQuarterly]

Speaking both as a classic computer enthusiast and as an analytical chemist, that shit looks awesome.

[amiman99]

Speaking both as a classic computer enthusiast and as an analytical chemist, that shit looks awesome.

Please explain, because after reading about "Preparative Gas Chromatography", I still dont know what it does.

[CGQuarterly]

Please explain, because after reading about "Preparative Gas Chromatography", I still dont know what it does.

 

 

OK. This is going to be a long post. I almost never talk about my job so this will be interesting to try to type out.

 

So this is how normal gas chromatography works. Generally speaking, you have a small vial with a volatile solvent in it. In that solvent are going to be your chemicals-of-interest. You probably got them in there by extracting whatever environmental media you were interested in into the solvent. It's kind of hard to explain in theory, so here's an example. This is going to be overly-simplified because you probably don't want to hear the tedious details.

 

So let's say that you have some cabbage, and somebody wants to know what kinds of pesticides are present on/in it and in what quantities. You would take the cabbage and shred it (like just with a knife), weigh out a sub-sample of say 100g, and add an appropriate amount of some extraction solvent like ethyl acetate (maybe 50 ml).

 

You'd then homogenize the sample in a laboratory blender. Organic compounds like pesticides would rather go into the solvent than stay with the cabbage, because they are relatively non-polar and are therefore more soluble in a non-polar organic solvent than in water. Let it sit for a while so the solvent can do its thing (maybe but it on a shaker or something), put it in a centrifuge to separate the solids from the liquid, pull of the supernatant liquid and concentrate it all the way down to 1 ml (the solvent evaporates while your target compounds stay in the liquid), then shoot it on the GC.

 

The way the GC works is it draws 1 or 2 microliters (1/1000th of a milliliter) of the solvent and shoots it into a hot glass tube called an inlet liner. A fairly normal inlet temperature is 250C, which is about 480 degrees Fahrenheit. This instantly vaporizes both the solvent and the chemicals that you extracted out of the cabbage. If you're looking for chemicals that have such a high boiling point that they don't vaporize under say 300C, then you have to use a different technique (called liquid chromatography). There's a "carrier gas" (almost always helium) blowing through the inlet that carries your now-vaporized compounds onto what's called a "column". A gas chromatography column is usually (but not always) 30 meters long (so about 100 ft), is a very thin (0.3 mm I.D) tube made out of glass that's coiled up on a cage so that the whole thing is only about 6 inches in diameter. Even though it's made of glass, it's so thin that it's very flexible. This column runs out of the inlet, and the cage hangs in a programmable oven. Generally you would start the oven off at a low temperature, say 50C. This is so that as soon as the compounds are on the head of the column, but have cleared the inlet, they re-condense and stick to the inner walls. The inner walls of the column are lined with a VERY thin layer of material called the "stationary phase" (the helium carrier gas is the "mobile phase"). The oven is programmed so that it slowly heats up according to whatever temperature program you set. As the oven heats up the column, your target compounds begin to partition between the mobile and stationary phases and in doing so they make their way through the column. How long it takes each compound to reach the end of the column depends on it's boiling point, polarity, and size (all three of those are inter-related anyway). So while you injected a mixture into one end of the column, you will (ideally) get each compound coming out the other end separately.

 

Normally, you'd have the business end of the column feeding into some kind of detector. An example of a simple and common detector would be a flame ionization detector, in which the column effluent exits the column at the base of a flame. Helium of course doesn't burn, but organic compounds all will. So as each compound comes out of the end of the column, it burns in the flame. This combustion creates ions, and you can measure the conductivity across the flame. When there are ions, the conductivity of the flame increases, allowing more current to pass through it. The system can plot the conductivity of the flame as a function of time, and you will see a peak every time something comes out. Looks like this:

 

There are all kinds of detectors for looking at particular classes of compounds. You can also have the column go into a mass spectrometer, which is what I mostly deal with at my job. How that works is a whole different kettle of fish.

 

So to go back to the cabbage example, you would have a standard mixture of target pesticides that you would run on the GC, and then compare it to what came out of the cabbage sample. If you were using a modern mass spec (which these days you totally would) then you could identify compounds without needing a standard, and the standard would be used only to quantify them.

 

So preparatory GC. Look at the picture in the craigslist ad. See how there are 6 little glass thingies at the bottom of that GC? What those are doing are collecting fractions of the column effluent, rather than sending the effluent into a detector. So the point of that system is to take a sample, separate out the compounds in that sample according to their boiling points, and collect them in up to 6 different fractions in glass vials. You can then take those samples and analyze them in some other way. There are a number of reason that you might want to do that, and they're kind of too complicated to try to explain here. Preparatory GC is actually really rare because it's generally unnecessary, especially with modern technology. I've done preparatory LC work, but have just kluged together a system like that on a benchtop (because it's simpler than a GC).

 

That system in the CL ad is really old (as if you couldn't tell by the fact that it's run by a Commodore.) That thing almost certainly doesn't even use the kind of GC column that I described because they weren't in common use yet. It uses a much shorter, wider-bore "packed" column that does a much crappier job of separating things. That may be why you'd need a preparatory GC back then. Separate your sample into 6 fractions based on their relative volatilities, then send them back through a GC column with a less aggressive temperature program in order to get better separation. It was kind of the wild west back then. Now technology is so advanced that you don't have to do stuff like this, and a lot of the "art" of chromatography has been lost. Kids today are spoiled.

 

Anyway, that's about it. Maybe I should start an "ask a chemist" thread in OT. I'm sure no one would read it.

[labrat]

To be fair to the old instrument in the ad, preparatory chromatography is used for larger volumes and generally simpler mixtures than analytical chromatography, like isolating the product of a synthesis from the reactants.

 

I suspect this is probably still useful for cases where two molecules have similar boiling points but different polarities.

 

-"There are dozens of us!"

[amiman99]

Thanks for the explanation @Jibbajaba

 

So I guess it works like a spectrometer.

[CGQuarterly]

A spectrometer generally plots intensity as a function of wavelength, while a GC plots intensity as a function of "retention time" (how long it takes each compound to traverse the column).

[TPA5]

 

 

 

OK. This is going to be a long post. I almost never talk about my job so this will be interesting to try to type out.

 

So this is how normal gas chromatography works. Generally speaking, you have a small vial with a volatile solvent in it. In that solvent are going to be your chemicals-of-interest. You probably got them in there by extracting whatever environmental media you were interested in into the solvent. It's kind of hard to explain in theory, so here's an example. This is going to be overly-simplified because you probably don't want to hear the tedious details.

 

So let's say that you have some cabbage, and somebody wants to know what kinds of pesticides are present on/in it and in what quantities. You would take the cabbage and shred it (like just with a knife), weigh out a sub-sample of say 100g, and add an appropriate amount of some extraction solvent like ethyl acetate (maybe 50 ml).

 

You'd then homogenize the sample in a laboratory blender. Organic compounds like pesticides would rather go into the solvent than stay with the cabbage, because they are relatively non-polar and are therefore more soluble in a non-polar organic solvent than in water. Let it sit for a while so the solvent can do its thing (maybe but it on a shaker or something), put it in a centrifuge to separate the solids from the liquid, pull of the supernatant liquid and concentrate it all the way down to 1 ml (the solvent evaporates while your target compounds stay in the liquid), then shoot it on the GC.

 

The way the GC works is it draws 1 or 2 microliters (1/1000th of a milliliter) of the solvent and shoots it into a hot glass tube called an inlet liner. A fairly normal inlet temperature is 250C, which is about 480 degrees Fahrenheit. This instantly vaporizes both the solvent and the chemicals that you extracted out of the cabbage. If you're looking for chemicals that have such a high boiling point that they don't vaporize under say 300C, then you have to use a different technique (called liquid chromatography). There's a "carrier gas" (almost always helium) blowing through the inlet that carries your now-vaporized compounds onto what's called a "column". A gas chromatography column is usually (but not always) 30 meters long (so about 100 ft), is a very thin (0.3 mm I.D) tube made out of glass that's coiled up on a cage so that the whole thing is only about 6 inches in diameter. Even though it's made of glass, it's so thin that it's very flexible. This column runs out of the inlet, and the cage hangs in a programmable oven. Generally you would start the oven off at a low temperature, say 50C. This is so that as soon as the compounds are on the head of the column, but have cleared the inlet, they re-condense and stick to the inner walls. The inner walls of the column are lined with a VERY thin layer of material called the "stationary phase" (the helium carrier gas is the "mobile phase"). The oven is programmed so that it slowly heats up according to whatever temperature program you set. As the oven heats up the column, your target compounds begin to partition between the mobile and stationary phases and in doing so they make their way through the column. How long it takes each compound to reach the end of the column depends on it's boiling point, polarity, and size (all three of those are inter-related anyway). So while you injected a mixture into one end of the column, you will (ideally) get each compound coming out the other end separately.

 

Normally, you'd have the business end of the column feeding into some kind of detector. An example of a simple and common detector would be a flame ionization detector, in which the column effluent exits the column at the base of a flame. Helium of course doesn't burn, but organic compounds all will. So as each compound comes out of the end of the column, it burns in the flame. This combustion creates ions, and you can measure the conductivity across the flame. When there are ions, the conductivity of the flame increases, allowing more current to pass through it. The system can plot the conductivity of the flame as a function of time, and you will see a peak every time something comes out. Looks like this:

 

There are all kinds of detectors for looking at particular classes of compounds. You can also have the column go into a mass spectrometer, which is what I mostly deal with at my job. How that works is a whole different kettle of fish.

 

So to go back to the cabbage example, you would have a standard mixture of target pesticides that you would run on the GC, and then compare it to what came out of the cabbage sample. If you were using a modern mass spec (which these days you totally would) then you could identify compounds without needing a standard, and the standard would be used only to quantify them.

 

So preparatory GC. Look at the picture in the craigslist ad. See how there are 6 little glass thingies at the bottom of that GC? What those are doing are collecting fractions of the column effluent, rather than sending the effluent into a detector. So the point of that system is to take a sample, separate out the compounds in that sample according to their boiling points, and collect them in up to 6 different fractions in glass vials. You can then take those samples and analyze them in some other way. There are a number of reason that you might want to do that, and they're kind of too complicated to try to explain here. Preparatory GC is actually really rare because it's generally unnecessary, especially with modern technology. I've done preparatory LC work, but have just kluged together a system like that on a benchtop (because it's simpler than a GC).

 

That system in the CL ad is really old (as if you couldn't tell by the fact that it's run by a Commodore.) That thing almost certainly doesn't even use the kind of GC column that I described because they weren't in common use yet. It uses a much shorter, wider-bore "packed" column that does a much crappier job of separating things. That may be why you'd need a preparatory GC back then. Separate your sample into 6 fractions based on their relative volatilities, then send them back through a GC column with a less aggressive temperature program in order to get better separation. It was kind of the wild west back then. Now technology is so advanced that you don't have to do stuff like this, and a lot of the "art" of chromatography has been lost. Kids today are spoiled.

 

Anyway, that's about it. Maybe I should start an "ask a chemist" thread in OT. I'm sure no one would read it.

 

That was fascinating, even though I didn't understand half of it.

 

 

Sent from my iPhone using Tapatalk

Edited June 3, 2016 by TPA5

[Gandor]

When I was studying chemistry they used a BBC computer to do this

[CGQuarterly]

When I was an undergrad, the GC we used in the rudimentary class didn't even have a computer. It had a plotter with a little mini felt-tip pen hooked directly up to the GC.

[+slx]

I find it great that the machine needs the C64 mode of the 128 to run.....about like running across a setup like this for the Atari requiring a classic 800 but no XL...