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?