That makes sense, but as long as the propane is kept cold by LN2, vapor pressure isn't a problem. The problem comes when the fluid used for freezing is the same fluid used for cooling (i.e. the LN2. The fluid used for cooling will necessarily be at or very near it's boiling point, so vapor pressure is one atmosphere. It cannot be heated (barring superheating), so every nanojoule of heat it absorbs goes into gas production- hence the insulating layer. Using two fluids, the one with the lower boiling point for cooling and the one with higher boiling point for heat transfer from the sample (cooling), the fluid absorbing the heat is well below its boiling point and so no gas will be produced unless a lot of heat is absorbed and not dissipated.
The way I was taught the Yale method, the propane was cooled until it started to solidify, making a slush of propane crystals in liquid propane at the freezing (not boiling) point. Melting of the slush keeps it there if the vial needs to be removed from nitrogen before the plunge.
eab

On 08/11/2015 09:59 PM, Tristan Croll wrote:
> Thanks for that. I think the vapour issue is why isopentane is preferred over propane: it's (barely) liquid at room temperature, so you never encounter its boiling point - which means no chance of even transient bubble formation. Vapour pressure above the pool is also minimised.
>
> Cheers,
>
> Tristan
>
> Tristan Croll
> Lecturer
> Faculty of Health
> School of Biomedical Sciences
> Institute of Health and Biomedical Engineering
> Queensland University of Technology
> 60 Musk Ave
> Kelvin Grove QLD 4059 Australia
> +61 7 3138 6443
> This email and its attachments (if any) contain confidential information intended for use by the addressee and may be privileged.  We do not waive any confidentiality, privilege or copyright associated with the email or the attachments.  If you are not the intended addressee, you must not use, transmit, disclose or copy the email or any attachments.  If you receive this email by mistake, please notify the sender immediately and delete the original email.
>
> On 12 Aug 2015, at 11:37 am, Patrick Loll <[log in to unmask] <mailto:[log in to unmask]>> wrote:
>
>> Here’s the answer:
>>
>> J Appl Crystallogr. 2006 Dec 1; 39(6): 805–811.
>> doi: 10.1107/S0021889806037484 <http://dx.doi.org/10.1107%2FS0021889806037484>
>>
>>> On 11 Aug 2015, at 5:51 PM, Tristan Croll <[log in to unmask] <mailto:[log in to unmask]>> wrote:
>>>
>>> I have a more general question: is plunging into liquid nitrogen the standard approach for freezing crystals? This actually gives relatively slow cooling, due to the Leidenfrost effect - the boiling nitrogen creates an insulating gaseous layer. In cryo-histology circles, the gold standard is to cool a small bowl of isopentane (melting point ~112K) in liquid nitrogen until it's almost all solidified, then dip the sample into the liquid pool in the middle. This yields an order of magnitude faster cooling rate, and drastically improves the quality of the final sections.
>>>
>>> Cheers,
>>>
>>> Tristan
>>> Tristan Croll
>>> Lecturer
>>> Faculty of Health
>>> School of Biomedical Sciences
>>> Institute of Health and Biomedical Engineering
>>> Queensland University of Technology
>>> 60 Musk Ave
>>> Kelvin Grove QLD 4059 Australia
>>> +61 7 3138 6443
>>> This email and its attachments (if any) contain confidential information intended for use by the addressee and may be privileged.  We do not waive any confidentiality, privilege or copyright associated with the email or the attachments.  If you are not the intended addressee, you must not use, transmit, disclose or copy the email or any attachments.  If you receive this email by mistake, please notify the sender immediately and delete the original email.
>>>
>>> On 12 Aug 2015, at 12:21 am, Roger Rowlett <[log in to unmask] <mailto:[log in to unmask]>> wrote:
>>>
>>>> Frost can readily accumulate on loops under high humidity conditions. We have even seen ice crystals suddenly appear in real time in the cryojet. We usually remove such surface ice by gently and quickly swiping with a 10-cent fine paintbrush. Once the nucleating ice crystals are removed further frost accumulation is inhibited and the drying effect of the cryostream usually keeps it frost free. Frost often accumulates in the cryojet shadow of the loop and can be minimized at kappa angles that have the cryojet nearly parallel to the pin. We have found that the backside of the mitegen loops will accumulate frost under very humid conditions. Nylon loops are less problematic under these conditions. It's a summer nuisance even with powerful dehumidifiers in our home source room.
>>>>
>>>> Roger Rowlett
>>>>
>>>> On Aug 11, 2015 9:54 AM, "Katherine Sippel" <[log in to unmask] <mailto:[log in to unmask]>> wrote:
>>>>
>>>>     Hi Sergei,
>>>>
>>>>     Another possible cause could be ambient humidity. Living in the subtropics, I noticed that I definitely had less icing problems on lower humidity days and tried to plan accordingly (crystals and weather permitting). If you plan on freezing for a trip, it might be worth checking out the weather forecast and freezing on a low humidity day, even if it a bit earlier than you had planned. Obviously you can't plan your beam time around humidity, but this will at least limit one potential source of ice.
>>>>
>>>>     Cheers,
>>>>     Katherine
>>>>
>>>>     On Tue, Aug 11, 2015 at 7:21 AM, Sergei Strelkov <[log in to unmask] <mailto:[log in to unmask]>> wrote:
>>>>
>>>>         Dear All,
>>>>
>>>>         What I wanted to discuss specifically is the ice crystals that we often find
>>>>         on the outside of the loops rather than bulk ice formation which is due
>>>>         to insufficient cryoprotection (although it is sometimes difficult to distinguish
>>>>         between the two cases). Sometimes these ice crystals on the surface can be removed
>>>>         by pouring LN2 onto the loop, but more often this has little effect.
>>>>         'Annealing' (thawing and freezing again) always resolves the problem
>>>>         but this is of course not an option most of the time (as the protein crystal gets damaged).
>>>>
>>>>         We measure at synchrotrons on a regular basis and the ice formation problem
>>>>         seems to vary quite a bit from trip to trip. Thus far we could not quite figure
>>>>         out what causes it.
>>>>
>>>>         We usually mount our crystals (with cryoprotectant...) by plunging them
>>>>         into LN2 on a bench. Thereafter they are stored either on canes or unipacks
>>>>         in a dry shipper. As we travel by car or train, we always keep the dry shipper filled
>>>>         with LN2 anyway. At the beamline they are usually mounted by a robot.
>>>>
>>>>         We mostly use the fiber loops (Hampton) but sometimes also the 'plastic' loops
>>>>         from Mitegen. I could not see much difference between them in terms of ice formation,
>>>>         but I wondered what the others' experiences are.
>>>>
>>>>         The choice of cryoprotectant matters obviously but annoyingly we can sometimes
>>>>         happily use some cryoprotecting conditions initially, but suddenly have problems
>>>>         with exactly the same crystals and cryoprotectant on the next trip...
>>>>
>>>>         The ice accumulation in any one of the LN2 vessels used at different stages
>>>>         (LN2 tank, the open reservoir we use for plunge-freezing, or the crystal storage/shipping dewar)
>>>>         /might/ be the cause. Specifically,  there will /always/ be /some/ ice accumulation in the open reservoir.
>>>>         Thus far we were not too paranoic about refreshing LN2 every five minutes -- but maybe we should?...
>>>>
>>>>         Or could the (variable) presence of ice in the 100l LN2 tank that our internal services fill for us
>>>>         be a problem?...
>>>>
>>>>         Thanks and best wishes,
>>>>         Sergei
>>>>
>>>>         --
>>>>         Prof. Sergei V. Strelkov
>>>>         Laboratory for Biocrystallography
>>>>         Dept of Pharmaceutical and Pharmacological Sciences, KU Leuven
>>>>         Herestraat 49 bus 822, 3000 Leuven, Belgium
>>>>         Work phone:+32 16 330845  <tel:%2B32%2016%20330845>    Mobile:+32 486 294132  <tel:%2B32%20486%20294132>
>>>>         Lab pages:http://pharm.kuleuven.be/Biocrystallography
>>>>
>>>>
>>>>
>>>>
>>>>     --
>>>>     "Nil illegitimo carborundum"/- /Didactylos
>>>>
>>
>>