Thursday, October 23, 2008

Liquid Nitrogen for Grownups

Everyone who has ever seen a mad scientist outreach knows that liquid nitrogen is fun. You can freeze people’s balls with it, shrink balloons, cast gasoline candles, and make great clouds of fog. It was even the molecule of the day last week. But there is a little known secret side to this substance. Believe it or not, it is actually a commonly used substance for real labs doing actual work. And while we may occasionally make ice cream with it, that isn’t why our labs buy it by the silo full. So below I’ll go through a couple of the actual scientific uses of LN2.

1. IR detectors. Infra red spectroscopy studies the wavelength of IR radiation given off or absorbed by various materials, depending on their composition, structure, or impurities. However, at room temperature, everything gives off a fair bit of IR radiation. So an IR detector in a microscope will detect the ‘glow’ of the surrounding instrument- as well as the detector components themselves- unless it is cooled down. Liquid nitrogen is the cheapest way to do this.

2. Gas separation. Water and carbon dioxide are two commonly studied fluid phases in geology. Studying them often involves separating them from air or other combustion gasses. Dry ice/ethanol slurries are cold enough to condense water, and liquid nitrogen condenses CO2. so you stick various parts of your gas line in buckets of those substances, and they freeze out for later analysis. These are called cold traps.

3. Gas purification. Sometimes, instead of condensing a gas for analysis, you simply want to remove it. For example, the excimer laser that I used to keep running used a gas mixture of fluorine, argon, and helium. Invariably, water would contaminate the gas reservoir, reacting with the fluorine to form HF and oxygen. The oxygen is no big deal, but the HF prevents the formation of excited dimmers that actually produce the laser light. Since HF is polar, it condenses much more easily than the other gasses, so we use liquid nitrogen to freeze it out. In that case, though, the gas is in a continuous flow, and you can’t let it get too cold or the argon might condense as well. For all you former students reading, this is why I get so pissed off when you guys let that cold trap run dry- if the coil warms up, then the entire 3 months worth of HF gets back into the gas stream and fucks up the electrodes when you activate the trigger.

4. Some solid state charged particle detectors, like the EDS system on the microprobe, also like to run at low temperatures. But I don’t know the exact reason for that.

5. Dry gas. Some of the liquid nitrogen in storage will inevitably boil off in storage as a result of imperfect insulation. This gas is moderately pure, but very dry, and thus is handy for things like venting vacuum systems, flushing furnaces, dusting shit off, and other random tasks that require low pressure, low oxygen, and/or dry gas.

How about y’all?

11 comments:

Sara said...

pre or extra cooling as a jacket around a helium cryostat

Chuck said...

Say, that wasn't your magnet that blew up on the LHC, was it?

ScienceWoman said...

Ha! And I gave you an LN2 container on facebook the other day. I'd tell you what I've used it for, but you've essentially already covered it.

Anonymous said...

RNA preps, RNA preps and many, many, many more RNA preps.

Threadspinner said...

Cooling NMR electromagnets and also in subambient thermal analyses like differential scanning calorimetry and dynamic mechanical analysis.

And also freezing bananas. (When no one else is watching.)

Chuck said...

Why does RNA have to be cold? And are NMR magnets hot superconductors these days, or is there some other reason for cooling them?

Chris Phoenix said...

Re application # 4, cold electronics have less electrical "noise." I don't know if that's the reason, but it sounds plausible.

Anonymous said...

Cooling samples in our differential scanning calorimeter, that mostly we use for measuring glass transition temperatures of polymers at low temperatures.

Threadspinner said...

Hi Chuck. NMR magnets are essentially coils of superconducting wire. They have a nonzero resistivity at temperatures over a few Kelvin, which interferes with the test. Hence, the coils, are usually immersed in liquid helium, and there's a cooling jacket that holds liquid nitrogen to help keep the helium from boiling away. Operating temp is about 4.2K, which does not qualify as a "hot" superconductor.

I don't know if anyone is working on using hot superconductors to make NMR magnets, but the standard instrument still uses conventional materials.

Anonymous said...

Freezing leaves before grinding them up for DNA. It works wonders for really tough, thick leaves. I don't know why you'd need to do that to a banana though.

Don't forget throwing it at your labmate's feet to make him dance!

Threadspinner said...

Why would one need liquid nitrogen for bananas? Frozen bananas are outrageously delicious. Ghirardelli's ice cream shops used to sell them dipped in their noteworthy chocolate and rolled in walnuts. Better than a sundae, for sure. Liquid nitrogen just lets you make them faster.

Make sure you use a new flask for melting the chocolate, though.