We have a "cryogen freezing bath" device from Balzers, which consists of a small container on a tube which fits into a Dewar of LN2. The small container is filled with e.g., a freezing slush of Freon or propane, into which (cryoprotected) samples are plunged. Someone else on campus would like to have one of these devices, and the machine shops are really backed up. Do any of you have an idea where this kind of thing can be purchased? If you can't picture this, a schematic can be found in Bozzola and Russell's Electron Microscopy on p. 312. 

I presume you're talking about plunge freezing. If you do this directly into liquid N2, the N boils, producing a layer of gaseous N around the tissue. That's a good insulator, so you get slow freezing. So you use an intermediate coolant that will stay liquid up to ~-80, for rapid heat transfer. When I was still in the lab (I'm retired) we used Freon 22, which is no longer permitted, or propane, which explodes if you're careless. 

... the intended purpose is to freeze mouse heart/aorta and liver for cryosections ... This group was just plunging the tissues into straight LN2 and wondering why they had big holes...! 

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There is no real need to purchase the quench cooling device you describe. All you need is a copper cylinder which will sit in a Dewer of LN2. Have a little plastic lid on the cylinder, When the cylinder is cool, slowly condense some propane into the cold cylinder (do this in a spark proof hood). Once the propane is cooled put the lid on. The purpose of this little lid is to stop LN2 bubbling into the cylinder with the propane. When you are ready to quench your sample, get a warm rid and make a little pool in the solid propane, watch the propane pool and as the surface just begins to freeze over, plunge in little bits of your sample. Let then stay for 30 secs then quickly bring them out and put them into a dewar of LN2. 

Variations of this are endless. You can use ethane instead of propane (bit dangerous) and you can have the whole dewer-cylinder sitting on a magnetic stirrer with a magnetic flea in the secondary cryogen. If you are really serious, read my book "Low temperature Microscopy and Analysis" 

Isopentane had been used as a freezing compound a long time ago, but I sure was glad when propane was advocated as a better alternative. I started using propane about 15 years ago. Isopentane behaves like a "gone off" rubber solution near its freezing point and freezes well before propane. 

ETHANE melting point 90K gives a mean cooling rate of 13-15 kKsec-1, PROPANE melting point 84K gives a mean cooling rate of 10-12 kKsec-1. Your stuff, ISOPENTANE melting point 113K does not cool as fast as the other two although I do not have the figures to hand. If you want to read more see Chapter 3 in "Low Temperature Microscopy and Analysis" by Patrick Echlin, Plenum Press, New York 1992. When choosing a cryogen there are many factors to consider including specific heat, thermal conductivity, relative cooling efficiency, viscosity at the cryogen melting point and thermal inertia. 

(1) Being old fashioned, I still call 2-methyl butane ISOPENTANE, which might help if you're doing a literature search; 

The 2-methyl butane is actually the cryogen (freezing agent) and the LN2 is what is used to bring it down the the proper temperature. The reason that agents like 2-methyl butane, propane, ethane and freons etc. are used is that they have a higher boiling point than LN2 (-196 oC). LN2's low boiling point creats a vapor barrier (Leidenfrost phenomena) to form around tissues that are immersed directly into it which prevents the rapid conduction of heat from the tissue to the cryogen. The best cryogens have high boiling points and and low freezing points. 

... I had overestimated iso-pentane, partly as a result of getting my Celsius and Kelvin mixed up! But for less critical work, like rapid quenching of polymer melts, it is sometimes helpful where working with a bottle of gas might not be allowed. 

Which is the better cryoagent ... Propane gas liquefied by cooling is a much, much better cryo-agent than is isopentane. Its easy to store in a lab a small gas cylinder with a blunt needle on a bit of tubing as the outlet. With little gas flow rub the needle over the small metal cup that is cooled by liq N2. Soon you will have a couple of ml of liquid propane. Do this in a fumehood, which is a good idea when using solvents too. 

Isopentane becomes very rubbery near liquid nitrogen temperature. The temptation is to warm it a little with a metal rod, reducing the temperature differential some more. Propane has greater freezing speeds making vitrification possible. These things where published 20 and more years ago when cryo fixation was developed. I would be surprised if propane penetrates the specimen to any extent because snap freezing is so rapid. Later the specimen is sublimed and the propane would be the first phase to go. Also propane has been used to widely that somebody would have noticed an effect on lipids - if that was greater than isopentane's. 

I notice that propane is actually somewhat soluble in (liquid) water, and I wonder if it diffuses into the sample to cause artifacts, perhaps altering membranes or extracting lipid droplets, which are my main interest. I would assume that IF this happens, isopentane being less soluble in water would have less of this kind of effect. Has anyone noticed? 

... for biological applications we have empirically determined that 10-12% ethane in propane is a good cryogen. I forget your application but I hope this is helpful. 

(2) If you have things that don't like being dunked in a hydrocarbon (rubbers and some plastics) it is possible to get quite powerful cooling by using a roughly equal mixture of acetone and methanol, which goes down to at least -110^C and has a higher boiling point than the isopentane. The b.p. of isopentane is twenty-something, which would make it hard to store in Arizona and similar places warmer than the UK. 

Mixtures naturally have lower freezing points than the pure solvents, which is the principle of Dowtherm, an industrial heat transfer fluid which is mixture of Naphthalene and Diphenyl Ether.