An optimized filling method for capillary DLS

Abstract

Capillary dynamic light scattering (DLS) is a new, simple and enabling technique, that increases the size range of DLS by over an order of magnitude in a cheap, disposable, but high optical quality, glass capillary. Sample loading for other capillary-based modalities, such as blood analysis, is typically achieved by dipping the capillary into the bulk sample, however, DLS is exquisitely sensitive to static scattering such as from a fluid meniscus or sample dried on the outside of the capillary and is sometimes used for extended measurement times where evaporation must be avoided. In this work, we carefully validate capillary dipping and sealing with a clay plug for DLS against reference measurements in a high quality 1 cm cuvette and then introduce a simple capillary loading scheme that reproducibly places a >3 μl sample in the correct location for a DLS measurement. We demonstrate the statistically identical characterisation of the new scheme and dipping against the reference measurements, but in sample volumes reduced by 1 and 3 orders of magnitude, respectively, key for high value applications such as pharmaceutical development where sample costs of $100 k per mg are common and in the environmental & medical sciences where samples may be difficult or unethical to collect in bulk. •Use of the capillary method to characterize high value samples in the lowest, reproducible volume.•Pitfalls and subsequent development of the best reproducible method.

Keywords: Capillary; Dynamic light scattering; Method optimization; Optimized filling strategy for capillary DLS.

Graphical abstract

Fig. 1

Wicking depth at 20 °C for (a) DI Water, (b) Isopropyl-alcohol(IPA) and (c) estimated from the scale in the images (500 mM not shown) meniscus depth for DI water, 500 mM NaCl and IPA.

Fig. 2

Placing the pipette tip into the capillary.

Fig. 3

Pipette onto a slide and into the capillary.

Fig. 4

Mann–Whitney tests for differences in Median between slide and capillary: All numerically identical.

Fig. 5

(a) A 2 μl sample that has failed to wick across the capillary cross section, (b) a 3.0 μl sample.

Fig. 6

Filling the capillary by air pressure. Place the capillary (1) into the fluid (2) and the capillary fills (3). Add the clay slug to the capillary (4) and place the filled capillary (5) into the capillary holder (6) and check for bubbles in the measurement area (7).

Fig. 7

Filling the capillary with a GELoader™ tip, (a) feeding the tip into the capillary, (b) placing the capillary into the ZSU1002 holder, (c) aligning the sample with the cross-hair on the holder.

Fig. 8

Test for normality for 60 nm latex in 150 mM NaCl.

Fig. 9

Student t-tests – for difference in measurement of dilute 60 nm latex in 150 mM NaCl.

Fig. 10

Distribution of (a) Zave and (b) PDI, for 5 μl, 3 μl, 2  μl and a filled 10.0 mm cuvette for the diluted 60 nm latex.