Measuring the Densities of Aqueous Glasses at Cryogenic Temperatures

Abstract

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 (N 2 -Ar) mixture. The cryogenic temperature phase of the drop is evaluated using a visual assay that correlates with Xray diffraction measurements. The density of the liquid N 2 -Ar mixture is adjusted by adding N 2 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 <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. Video LinkThe video component of this article can be found at https://www.jove.com/video/55761/ 9 . However, generating large samples from a large number of small drops in a way that minimizes void volumes -an important source