Vitamin B1 (VB1), also known as thiamine, has a molecular weight of 337.3 Da and is easily soluble in water. It is the first vitamin to have been found with the capability of maintaining the human body's sugar metabolism and nerve conduction.
Due to the quite small molecular volume and low molecular weight, the scattering effect of VB1 is very weak. To measure the particle size of such small molecules, the excellent sensitivity of a dynamic light scattering optical system is required.
Here the Nanoptic 90 Plus (nanoparticle size analyzer) from Bettersize Instruments Ltd. will be used to analyze the particle size and size distribution of the Vitamin B1 molecule. As shown in Table 1, VB1 solutions at various concentrations were prepared.
The measurement temperature is set as 25 ± 0.1 °C by the built-in temperature control system. Water was used as the dispersant with a viscosity of 0.88 cp at 25°C and a refractive index of 1.33. Since the particles are quite small and the scattering effect is extremely weak, the presence of impurities (e.g. dust) can have a significant effect on the test results. Therefore, the sample has to be pre-purified and a 220 nm PES filter was used for sample filtration. In addition, the amount of sample tested was around 1.5 mL. The temperature equilibrium duration was set as 120s for each sample, and each sample was measured at least three times so that we could ascertain the repeatability and hence the relative standard deviation of the results.
Figure 1. Correlation functions of VB1 solution at various concentrations
The correlation functions of VB1 samples at various concentrations were calculated and obtained from the original scattered signals by the Nanoptic 90 Plus.
Due to the rapid Brownian motion of the small molecules, the decay rates of the correlation functions were very fast, and the correlation functions reached the baseline within 10 μs, as shown in Figure 1.
For all prepared samples, the intercepts of the correlation functions were distributed between 0.55-0.65 which illustrated good signal-noise ratios. The good repeatability of the correlation functions suggested very high sensitivity and stability of the Nanoptic 90 Plus optical system.
Figure 2. The particle size distributions of VB1 at various concentrations
The results of multiple measurements are shown in Table 2. The Nanoptic 90 Plus provided very good repeatability results of VB1 at various concentrations in terms of the Z-average particle size and the PD.I. The measured particle sizes were less than 1.0 nm for all concentrations, and the particle size slightly decreased with the concentration. What is the reason for the decrease of the particle size with the VB1 solution concentration? The answer is that the viscosity of the VB1 solution increases when there is an increase in concentration. The particle size measured decreases as the VB1 molecules with higher viscosity diffuse slower and perform slower Brownian motion.
Table 2. Particle sizes and distribution coefficients of VB1 at different concentrations
To conclude, as can be seen from the experimental results, the Nanoptic 90 Plus system provides highly reliable and repeatable particle size results even for sub-nano particles down to 0.5 nm with very weak scattering effect as it equips a 50mW solid-state laser, a high-sensitivity APD detector, a single-mode fiber detection system and an ultra-high-speed multi-mode correlator system. The Nanoptic 90 Plus with an unprecedented sensitivity and lower limit measurement range for very small particles has thus proven to be a powerful particle size analyzer using the dynamic light scattering technique.