Science Buddies: "Ask an Expert"

Liz:

Louise is right on this one. Everyone here are concern about your safety with LN2. Like we've warned you before any detailed discussion took place, LN2 is extremely dangerous to handle. Even college students get hurt, and they are the so-called "matured experimenters". As I indicated, 95% of the seeds will not germinate under 0 degree C. They will go into a dormant state until the temperature goes up to 24 degrees C. To put them into a deep cryogenic preservation state for ages to come, -32 degrees C or lower need to be achieved. This is slightly lower than human cryogenic suspended animation, which tops out at -22 C, and human can only be cryogenic frozen for about a 120 years top at the highest estimated projection, with optimal conditions.

Dry ice is the safest way to go. The experts gave you quite a bit of cryogenic designs that are cheap and efficient. Compare the different type of preservation to see which one is the most effective, vs the control (the seed sitting in soil, in normal conditions).

It seems that safety is a big concern. But, I would add one other consideration.

Using liquid nitrogen has its benefits, despite the safety concerns. For example, the time to reach sub-zero is significantly faster with liquid nitrogen than dry ice, no matter its combination with other compounds. If I'm not mistaken, liquid nitrogen can snap freeze at almost a million degrees/sec. This may be a necessary consideration for freezing and thawing. However, it's not clear what the experimental controls are based on prior posts.

Hope that helps,

James

James wrote:It seems that safety is a big concern. But, I would add one other consideration.

Using liquid nitrogen has its benefits, despite the safety concerns. For example, the time to reach sub-zero is significantly faster with liquid nitrogen than dry ice, no matter its combination with other compounds. If I'm not mistaken, liquid nitrogen can snap freeze at almost a million degrees/sec. This may be a necessary consideration for freezing and thawing. However, it's not clear what the experimental controls are based on prior posts.

Hope that helps,

James

Liquid nitrogen is faster (but not a million degrees per second... there are probably other factors like heat capacity and volume of the sample to consider that define that rate, not the liquid nitrogen itself) But, given that the student has no equipement for handling liquid nitrogen and the significant cost of said equipement, any minor improvement in speed isn't worth it. Plus the student has no source for liquid nitrogen.


Louise

Actually, it is around a million degrees/sec. You are right to point out different variables like heat capacity, volume of sample, etc. However, if you pour liquid nitrogen in an immersive fashion (i.e. pour enough for the sample to soak in the liquid), the temperature drop is significant. This project involves seeds and not metal or any other fancy material. With or without means should not preclude from thinking through different variables when designing an experiment.

James wrote:Actually, it is around a million degrees/sec.

Could I have a citation for this?

Thanks,
Louise

Hi Louise,

I don't know what I was thinking earlier today. It was in the middle of my hospital clerkship and I was trying to juggle a lot. I think I was thinking of something else.

Nonetheless, what I should have said was that liquid nitrogen cooling is fast. There are other cryogens that are faster, but obtaining those cryogens is difficult and more important, difficult to maintain. I suggested bringing liquid nitrogen back into the discussion since the seeds being used are in danger of having crystal formations from too slow a freezing process. During the thawing process, those crystals will leave behind potentially significant damage in the structure of the seed.

I stand corrected.

James

James wrote:Hi Louise,

I don't know what I was thinking earlier today. It was in the middle of my hospital clerkship and I was trying to juggle a lot. I think I was thinking of something else.

Nonetheless, what I should have said was that liquid nitrogen cooling is fast. There are other cryogens that are faster, but obtaining those cryogens is difficult and more important, difficult to maintain. I suggested bringing liquid nitrogen back into the discussion since the seeds being used are in danger of having crystal formations from too slow a freezing process. During the thawing process, those crystals will leave behind potentially significant damage in the structure of the seed.

I stand corrected.

James

I was curious about the calculation. I was really busy too, so my question was probably too terse. I honestly had/have no idea about this issue of speed, other than some experience freezing things both ways, and am interesting in a more quantitative evaluation for the student. Pop over to the experts forum and look at Craig's comments on the problem.

Louise