This work was supported by the NSF under award MCB-1330685. DWM acknowledges partial support from Cornell University&#39;s Molecular Biophysics Training Grant (NIH T32GM0082567).

CAUTION: Please consult all relevant material safety data sheets (MSDS) before use. Please use all appropriate safety practices when using compressed gases, including appropriate calibrated gas handling regulators and valves, and approved gas tubing. Contact with liquid cryogens can cause&#160; severe frostbite and necrosis. Use appropriate personal protective equipment (face shield, gloves, lab coat, full length pants, closed-toe shoes), all of which must be impermeable to liquid nitrogen. Remain standing and ensure an ...

<ol>
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W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cryo-cooling+in+macromolecular+crystallography:+advantages,+disadvantages+and+optimization.">Cryo-cooling in macromolecular crystallography: advantages, disadvantages and optimization.</a> Q Rev Biophys. 37 (2), 105-119 (2004).</li><li>Alcorn, T., Juers, D. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Progress+in+rational+methods+of+cryoprotection+in+macromolecular+crystallography.">Progress in rational methods of cryoprotection in macromolecular crystallography.</a> Acta Cryst Sect D. 66 (4), 366-373 (2010).</li><li>Warkentin, M., Sethna, J., Thorne, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Critical+Droplet+Theory+Explains+the+Glass+Formability+of+Aqueous+Solutions.">Critical Droplet Theory Explains the Glass Formability of Aqueous Solutions.</a> Phys Rev Lett. 110 (1), 15703 (2013).</li><li>Kriminski, S., Kazmierczak, M., Thorne, R. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Heat+transfer+from+protein+crystals:+implications+for+flash-cooling+and+X-ray+beam+heating.">Heat transfer from protein crystals: implications for flash-cooling and X-ray beam heating.</a> Acta Cryst Sect D. 59 (4), 697-708 (2003).</li><li>Loerting, T., Bauer, M., Kohl, I., Watschinger, K., Winkel, K., Mayer, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cryoflotation:+Densities+of+amorphous+and+crystalline+ices.">Cryoflotation: Densities of amorphous and crystalline ices.</a> J Phys Chem B. 115 (48), 14167-14175 (2011).</li><li>Shen, C., Julius, E. F., Tyree, T. J., Moreau, D. W., Thorne, R. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Thermal+contraction+of+aqueous+glycerol+and+ethylene+glycol+solutions+for+optimized+protein-crystal+cryoprotection+Thermal+contraction+of+aqueous+glycerol+and+ethylene+glycol+solutions+for+optimized+protein-crystal+cryoprotection.">Thermal contraction of aqueous glycerol and ethylene glycol solutions for optimized protein-crystal cryoprotection Thermal contraction of aqueous glycerol and ethylene glycol solutions for optimized protein-crystal cryoprotection.</a> Acta Cryst Sect D. 72 (6), 742-752 (2016).</li><li>Warkentin, M., Berejnov, V., Husseini, N. S., Thorne, R. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hyperquenching+for+protein+cryocrystallography.">Hyperquenching for protein cryocrystallography.</a> J Appl Cryst. 39 (6), 805-811 (2006).</li><li>McFerrin, M. B., Snell, E. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+development+and+application+of+a+method+to+quantify+the+quality+of+cryoprotectant+solutions+using+standard+area-detector+X-ray+images.">The development and application of a method to quantify the quality of cryoprotectant solutions using standard area-detector X-ray images.</a> J Appl Cryst. 35 (5), 538-545 (2002).</li><li>Chinte, U., Shah, B., DeWitt, K., Kirschbaum, K., Pinkerton, A. A., Schall, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Sample+size:+An+important+parameter+in+flash-cooling+macromolecular+crystallization+solutions.">Sample size: An important parameter in flash-cooling macromolecular crystallization solutions.</a> J. Appl. Cryst. 38 (3), 412-419 (2005).</li><li>Bosart, L. W., Snoddy, A. O. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Specific+gravity+of+glycerol.">Specific gravity of glycerol.</a> Ind Eng Chem. 20 (12), 1377-1379 (1928).</li><li>Rodrigues, M., Francesconi, A. Z. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Experimental+study+of+the+excess+molar+volumes+of+binary+and+ternary+mixtures+containing+water+++(1,2-ethanediol,+or+1,2-propanediol,+or+1,3-propanediol,+or+1,2-butanediol)+++(1-n-butyl-3-methylimidazolium+bromide)+at+298.15+K+and+atmospheric+pressure.">Experimental study of the excess molar volumes of binary and ternary mixtures containing water + (1,2-ethanediol, or 1,2-propanediol, or 1,3-propanediol, or 1,2-butanediol) + (1-n-butyl-3-methylimidazolium bromide) at 298.15 K and atmospheric pressure.</a> J Solution Chem. 40 (11), 1863-1873 (2011).</li><li>Berejnov, V., Husseini, N. S., Alsaied, O. A., Thorne, R. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effects+of+cryoprotectant+concentration+and+cooling+rate+on+vitrification+of+aqueous+solutions.">Effects of cryoprotectant concentration and cooling rate on vitrification of aqueous solutions.</a> J Appl Cryst. 39 (2), 244-251 (2006).</li><li>Meisburger, S. P., Warkentin, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Breaking+the+Radiation+Damage+Limit+with+Cryo-SAXS.">Breaking the Radiation Damage Limit with Cryo-SAXS.</a> Biophys J. 104 (1), 227-236 (2013).</li></ol>

The authors have nothing to disclose.

The present apparatus and methods, developed primarily by undergraduates with limited access to instrument-building tools and machinery, nevertheless delivers highly accurate density measurements for individual liquid drops as small as 50 pL. In the concentration range near and above 50% w/w, where small cooling rates are sufficient to obtain vitrified samples, the densities agree with those obtained in previous measurements on bulk samples. Extrapolations of the present densities to 0% concentration &#8211; pure water &...

The physical properties of water and aqueous mixtures in their various phases are of fundamental interest, and are important to in vivo and in vitro understanding of biological systems. In contemporary cryobiology and biological cryopreservation, the vitreous or amorphous phases of aqueous cryoprotectant mixtures are of particular interest1,2. Nucleation and growth of ice crystals can disrupt cells and tissues, and promote protein denaturation and aggregation, so cryopreservation protocols that vitrify the solvent have become increasingly popular. In biomolecular crystallography, the crystallization of solvent in the channels between biomolecules disrupts crystal lattices and degrades diffraction properties. Vitrification is achieved through a combination of rapid cooling, dehydration, and addition of cryoprotective solutes such as glycerol, ethylene glycol, polyethylene glycols (PEGs), alcohols, and salts.
Vitrification limits ice crystallization and growth, but does not eliminate all cooling-related sample damage. For example, crystal mosaicity (a measure of the distribution of crystal plane orientations) routinely increases by a factor of 10 to 100 when protein crystals are cooled into a vitrified state3, and post-thaw survival rates of vitrified sperm cells and oocytes vary widely.
One damage mechanism is differential contraction of solvent and surrounding material during cooling3,4,5. The equilibrium solvent and solute concentrations within a crystal, cell or tissue are temperature dependent, and the solvent plus the solute and surrounding material may contract by different amounts. Rapid cooling may prevent solvent and solute redistribution prior to vitrification, and differential contraction may lead to large, inhomogeneous, nonequilibrium stresses that cause sample damage.
Rational approaches to reducing cooling-induced damage could thus benefit from the knowledge of temperature-dependent densities of liquid and vitrified aqueous mixtures. At solute concentrations above 50% weight of solute to weight of solution (w/w), most aqueous cryoprotectant mixtures can be vitrified with modest cooling rates of 10 K/s or less, allowing production of and density measurements using large vitreous samples6. Density can then be determined using Archimedes&#39; principle, by measuring the apparent weight of the sample when suspended in a liquid cryogen like nitrogen. However, as solute concentration decreases, cooling rates required for vitrification increase rapidly: Cooling rates for aqueous glycerol mixtures increase from &#60;10 K/s at 50% weight of solute in g to volume of solution in mL (w/v) to &#62;1,000 K/s at 25% w/v7. Heat transfer becomes boundary-layer limited, so that achieving larger cooling rates requires smaller and smaller samples8.
Measurements of the density of pure vitreous water and ice have been achieved by depositing micrometer-diameter (femtoliter volume) drops in a vacuum onto a cryogenically-cooled surface so as to build up a macroscopic (gram mass) sample. The density of this sample was determined by cryoflotation in a liquid nitrogen-argon mixture, in which the density of the cryogenic liquid was adjusted until the sample became neutrally buoyant9. However, generating large samples from a large number of small drops in a way that minimizes void volumes &#8211; an important source of error in previous vitreous phase density measurements &#8211; is non-trivial. For aqueous mixtures, differential evaporation of solution components during aerosolization and deposition in a vacuum can lead to substantial uncertainties in deposited concentrations.
We have developed a method, based on cryoflotation, that allows accurate density determination of aqueous mixtures using individual drops as small as 50 pL10. These drops can be rapidly cooled while retaining their original concentrations, and their cryogenic temperature state (vitrified or crystalline) can be assessed using a simple visual assay that correlates with X-ray diffraction measurements. This method is broadly applicable to aqueous and non-aqueous mixtures, and can be extended to a variety of biological samples including cells (e.g., stem and egg), tissue samples, and protein crystals having low-temperature densities between 0.8 and 1.4 g/mL.

<table border="0" cellpadding="0" cellspacing="0" width="573">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">centrifuge tube</td>
			<td style="width:71px;">Falcon</td>
			<td style="width:119px;">6029236</td>
			<td style="width:167px;">15 mL conical centrifuge tube</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">glycerol, &#62;99.5%</td>
			<td style="width:71px;">Sigma&#160;</td>
			<td style="width:119px;">G9012-100 mL</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">ethylene glycol, &#62;99.8%</td>
			<td style="width:71px;">Sigma</td>
			<td>324558-100 mL</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">analytical microbalance</td>
			<td style="width:71px;">Mettler</td>
			<td style="width:119px;">AE240</td>
			<td>Analytical balance, 0.01 mg resolution, has hook on bottom for weighing below the balance&#160;</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">vortexer</td>
			<td style="width:71px;">Scientific Industries</td>
			<td style="width:119px;">SI-0236</td>
			<td>Vortex-Genie 2</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Apparatus enclosure framing</td>
			<td style="width:71px;">Unistrut&#160;</td>
			<td style="width:119px;">1-5/8&#34; metal framing</td>
			<td>48&#34; wide x 24&#34; deep x 40&#34; tall&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Apparatus enclosure air barrier</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td>any clear plastic sheeting</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">neoprene rubber disk</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td>4&#34; diameter, 1/8&#34; thick</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">dewar flask</td>
			<td style="width:71px;">Scilogix Dilvac</td>
			<td style="width:119px;">SS333</td>
			<td>4.5 liter dewar flask with steel case and clamp lid</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">copper chamber</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td>This fabricated part is comprised of a 1.43&#34; diameter, 0.017&#34; wall thickness copper tube with a solid cylindrical copper base soldered to seal one end.&#160; The copper base is 0.87&#34; tall and the overall chamber height is 7&#34;.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">nitrogen gas</td>
			<td style="width:71px;">Airgas</td>
			<td style="width:119px;">NI HP300</td>
			<td>99.998% pure N2 gas</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">argon gas</td>
			<td style="width:71px;">Airgas</td>
			<td style="width:119px;">AR HP300</td>
			<td>99.998% pure Ar gas</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">rotameter</td>
			<td style="width:71px;">Omega</td>
			<td style="width:119px;">FL3692ST</td>
			<td>2.52 l/min max flow rate</td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:216px;">foam insulating lid</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td>This part is fabricated from 4 lb/ft3 crosslinked polyethylene foam (supplied by Technifab, 1355 Chester Industrial Parkway, Avon, OH), and has an OD of 2.42&#34;, and ID of 1.52&#34;, and a thickness of 0.79&#34;.&#160;&#160;&#160;&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">PTFE test mass</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td>This fabricated part is a 0.246&#34; diameter, 0.580&#34; tall cylinder with a 0.060&#34; diameter hole running perpendicular to and intersecting the cylinder axis ~0.10&#34; from one end.&#160;</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">microbalance platform</td>
			<td style="width:71px;">Unistrut</td>
			<td style="width:119px;">1-5/8&#34; metal framing</td>
			<td>11&#34; wide x 24&#34; long x 24&#34; high rectangular frame with an top aluminum sheet containing a hole for the monofilament and hanging test mass</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">2 mil (50 um) monofilament line</td>
			<td style="width:71px;">Berkley</td>
			<td style="width:119px;">NF1502-CM</td>
			<td>Nanofil fishing line</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Argon precooling coil tubing</td>
			<td style="width:71px;">VWR</td>
			<td style="width:119px;">60985-512</td>
			<td>1/8&#34; ID x 1/4&#34; OD PVC tubing</td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:216px;">perforated copper foil mixer</td>
			<td style="width:71px;"></td>
			<td style="width:119px;"></td>
			<td>1.4&#34; diameter,&#160; 35 micron thick copper disk, cut from 1 ounce/ft2 copper sheet and perforated with holes using an awl or other sharp pointed tool.&#160; Insert 1-2 mm diameter rigid thermally insulating (plastic or wood) rod into the center and fix using epoxy as needed.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">syringe</td>
			<td style="width:71px;">BD</td>
			<td style="width:119px;">309628</td>
			<td>1 ml Luer-Lok tip syringe</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">vacuum generator</td>
			<td style="width:71px;">Gast</td>
			<td style="width:119px;">VG-015-00-00</td>
			<td>compressed air venturi single stage vacuum generator</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">hydrophobic coating spray</td>
			<td style="width:71px;">RainX</td>
			<td style="width:119px;">620036</td>
			<td>plastic water repellent</td>
		</tr>
		<tr height="84">
			<td height="84" style="height:84px;width:216px;">long focal length stereo microscope</td>
			<td style="width:71px;">Bausch and Lomb Stereozoom 6</td>
			<td style="width:119px;"></td>
			<td>0.67-4 x zoom pod with 20x eyepieces, 10 cm working distance&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">LED ring illuminator</td>
			<td style="width:71px;">Amscope</td>
			<td style="width:119px;">LED144S</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">LED spot illuminator</td>
			<td style="width:71px;">Newhouse Lighting</td>
			<td style="width:119px;">NHCLP-LED</td>
			<td>3W LED gooseneck clamp lamp</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">1.8 ml cryo vial</td>
			<td style="width:71px;">Nunc</td>
			<td>V7634-500EA</td>
			<td>Any 1.8 or 2 ml cryovial is adequate</td>
		</tr>
	</tbody>
</table>

measuring the densities of aqueous glasses at cryogenic temperatures

We demonstrate a method for determining the vitreous phase cryogenic temperature densities of aqueous mixtures, and other samples that require rapid cooling, to prepare the desired cryogenic temperature phase. Microliter to picoliter size drops are cooled by projection into a liquid nitrogen-argon (N2-Ar) mixture. The cryogenic temperature phase of the drop is evaluated using a visual assay that correlates with X-ray diffraction measurements. The density of the liquid N2-Ar mixture is adjusted by adding N2 or Ar until the drop becomes neutrally buoyant. The density of this mixture and thus of the drop is determined using a test mass and Archimedes principle. With appropriate care in drop preparation, management of gas above the liquid cryogen mixture to minimize icing, and regular mixing of the cryogenic mixture to prevent density stratification and phase separation, densities accurate to &#60;0.5% of drops as small as 50 pL can readily be determined. Measurements on aqueous cryoprotectant mixtures provide insight into cryoprotectant action, and provide quantitative data to facilitate thermal contraction matching in biological cryopreservation.

Density measurements at T = 77 K for vitrified drops of aqueous glycerol and ethylene glycol versus cryoprotectant concentration are shown in Figure 1A and Figure 1B respectively, and the corresponding change in specific volume between T = 298 K and 77 K, calculated using previously determined T = 298 K densities, is shown in Figure 2. At h...

Watch this Scientific Journal Video about Measuring the Densities of Aqueous Glasses at Cryogenic Temperatures at JoVE.com

Measuring the Densities of Aqueous Glasses at Cryogenic Temperatures

A protocol for determining the vitreous phase densities of micro- to pico-liter size drops of aqueous mixtures at cryogenic temperatures is described.

We demonstrate a method for determining the vitreous phase cryogenic temperature densities of aqueous mixtures, and other samples that require rapid ...

The overall goal of this experiment is to determine the glass or vitreous state densities of aqueous solutions at cryogenic temperatures. This method can help answer key questions in the fields of Cryobiology and Cryocrystallography, such as, how much do aqueous cryo protected mixtures contract or expand when cooled into a vitreous state at cryogenic temperatures. The main advantage of this technique is that both cryogenic temperature sample density and phase, can be determined using small samples that can be cooled rapidly.Though this method was developed for cryoprotecting solutions, it can also be used to determine cryogenic temperature densities of tissues, cell aggregates and other biological samples. I first had the idea for this method when I learned from Thomas Lutin, how to make a cryogenic liquid with a tunable density and the range between 0.9 and 1.3 grams per milliliter. To begin this procedure, place a disk of neoprene rubber on the bottom of a 4.5 liter glass dewar flask, to protect the dewar flask from damage.Carefully insert a high thermal conductivity copper chamber into the flask, until it rests on the rubber disk. Adjust struts projecting outward from the chamber to the dewar flask walls, so that the chamber centered and has no tendency to rock. Insert the outlet of a gas tube with dry nitrogen gas flowing at approximately two liters per minute, down to the bottom of the copper chamber and purge the chamber of moist air.Now, slowly pour liquid nitrogen into the dewar flask, outside the copper chamber, allowing time for nitrogen boil off. Remove the dry nitrogen gas tube from the copper chamber and insert it into a matching opening in a foam insulating lid. Then cover the outer portion of the dewar flask with the lid.Secure the lid in place with elastic straps. Slowly pour liquid nitrogen into the copper chamber, so that the final fill level after boiling has ceased, is within approximately five centimeters of the top of the copper chamber. Following this, place a thin optically transparent plastic sheet over the central opening in the lid.Reduce the nitrogen gas flow rate to approximately 0.2 liters per minute, leaving a slight overpressure of nitrogen gas within the gas spaces above the cryogenic liquids. Determine the apparent mass of a one gram 0.4 milliliter PTFE test mass, in the air, at 298 Kelvin, by placing it on the pan of a calibrated analytical micro balance. Next, suspend the test mass using a 50 micron monofilament line strung from the hook on the underside of the micro balance and through a hole in the test mass.Then, determine the apparent mass in air and compare to the mass measurement, correcting as needed for the mass of the line. To determine the volume of the test mass at 77 Kelvin, lower the test mass into the copper chamber containing liquid nitrogen until it is fully submerged. When boiling has ceased, measure the apparent mass.Flow Argon gas at a flow rate of approximately two liters per minute through a coiled tube to its outlet. Place the coiled tube on top of the upper struts that stabilize the position of the copper chamber, just above the liquid nitrogen level and below top surface in the dewar flask. After allowing the coiled tube to cool for five minutes, place the outlet and the tube in the copper chamber at least 10 centimeters below the surface and the liquid nitrogen.Then, cover the dewar flask with the annular lid and transparent sheet. Following this, adjust the Argon flow rate until bubbles rise from the tube outlet to the top surface of the liquid nitrogen. Then, reduce the flow rate until the bubbles form at the outlet but dissolve or liquefy just before breaking the liquid nitrogen surface.Periodically, mix the liquid by inserting a thin circular sheet of copper foil attached to a thin insulating rod into the copper chamber and slowly moving it up and down like a piston. After measuring the apparent mass of the test mass in air, remove the insulating dewar cover and cool the mass to 77 Kelvin by lowering it into the liquid nitrogen outside the copper chamber. Raise the cool test mass into the cold layer of nitrogen gas above the liquid nitrogen and wait for residual liquid nitrogen to evaporate off the test mass.Then, lower the cool, dried test mass into the liquid nitrogen Argon mixture until it is fully submerged and within two centimeters of the liquid surface. After any boiling and surface waves have disappeared, measure the apparent mass of the test mass. Immediately prior to drop dispensing in cooling, mix the nitrogen argon cryogenic liquid, as previously described.After removing the sample tubes cap, extract up to one milliliter solution using a clean one milliliter syringe. Attach a 27 to 33 gauge needle onto the syringe and then push a small amount of sample through the needle to expel air and any residues from previous dispensing. For samples that require faster cooling for vitrification, place the outlet of a gas tube connected to a vacuum generator, in the gas space above the liquid nitrogen argon mixture and gently suction away the cold gas layer that forms.For samples with large non-aqueous component concentrations that can be vitrified with modest cooling rates, lightly press the syringe to displace a small diameter, 10 nanoliter to one microliter drop that hangs from the needle tip by surface tension. Gently tap the needle to detach and project the drop towards the liquid nitrogen argon mixture. Using a long working distance binocular microscope, and bright, cool illumination from an LED illuminator, carefully examine the drop while keeping it immersed in the liquid nitrogen argon mixture.Vitrified drops should appear clear. Reject drops are cloudy or show an optical imperfections including cracks. The drop floats, decrease the density of the liquid nitrogen argon mixture by adding liquid nitrogen using a 1.8 milliliter cryo vial.Gently mix the liquid nitrogen argon mixture, using a thin perforated copper foil sheet and moving it up and down in the copper chamber. After each liquid nitrogen addition, move a small prequel rod beneath the floating drop to gently displace it downward into the liquid and observe its speed as it returns to the surface. Density measurements at 77 Kelvin for vitrified drops of aqueous glycerol and ethylene glycol, versus cryo protecting concentration, are shown here.The corresponding change in specific body in between 298 and 77 Kelvin, calculated using previously determined 298 Kelvin densities is displayed here. Near 20%to 25%solutions of both cryo protectants are predicted to show no net expansion or contraction. The slope of the volume change versus concentration has the largest magnitude below 40%where the effects of additional cryo protectant on waters, tetrahedral low temperature structure are most pronounced.Once mastered, the initial high density liquid nitrogen argon cryogenic solution can be prepared in two to two and a half hours and the densities of individual drops can be determined in 10 to 15 minutes. While attempting this procedure, it's important to remember to watch for icing and remove as much of the cold gas layer that forms on the liquid nitrogen argon mixture surface as possible, using a vacuum generator. Following this procedure, drops can be collected, stored in liquid nitrogen and later measured using cryocrystallography to verify the phase of a drop.Don't forget, that working with liquid cryogens can be extremely hazardous and precautions such as, wearing appropriate gloves, face shields, clothing and footwear, should always be taken while performing this procedure.

measuring-the-densities-of-aqueous-glasses-at-cryogenic-temperatures

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JoVE Journal

Engineering

8.6K Görüntüleme.  Cornell University. The overall goal of this experiment is to determine the glass or vitreous state densities of aqueous solutions at cryogenic temperatures. This method can help answer key questions in the fields of Cryobiology and Cryocrystallography, such as, how much do aqueous cryo protected mixtures contract or expand when cooled into a vitreous state at cryogenic temperatures. The main advantage of this technique is that both cryogenic temperature sample density and phase, can be determined using small sa...