How to Increase the Speed of Moisture Sorption Measurements with DVS Microsample Analysis

During the development of new pharmaceutical products, screening of large numbers of candidate substances for potentially unwanted physical properties such as stability / moisture sorption is very important.

To achieve this objective, the industry is extensively using Dynamic Vapor Sorption (DVS) automated moisture sorption analyzers, reducing the usual analysis times from weeks or months with static sorption methods to hours or days.

Current pharmaceutical development is driven by the need to identify new active substances and introduce them to the market as rapidly as possible. As a result, there is a growing demand for even faster methods of screening the large number of candidate substances generated by the latest pharmaceutical discovery processes.

This article shows how analysis times can be reduced by up to 10 times by utilizing the very high sensitivity of the ultra-balance employed in the DVS instrument.

Moisture Sorption Kinetics

There are two key factors that dominate the moisture sorption kinetics of a solid material (how quickly moisture is taken up):

(1) Mass transport in the gas phase – how quickly moisture (at a specific %RH) is supplied to the sample.
(2) Diffusion into the sample – this can be a combination of diffusion through a powder bed and/or diffusion into individual particles.

The first factor is already optimized in the DVS instrument by having a dynamic flow of 200 sccm of humidified gas over the sample. This ensures that moisture is rapidly delivered to the sample surface. The second factor is primarily a function of the total sample mass and the average particle size of the sample, as shown in Figure 1.

The effect of particle and sample size on moisture diffusion kinetics

Figure 1. The effect of particle and sample size on moisture diffusion kinetics.

In a standard DVS experiment, 10 to 30 mg of particulate sample is used in a flat-bottomed holder or a bowl-shaped sample holder. In the ideal scenario, the sample diffusion kinetics and mass transport to the sample can be optimized by spreading the particulate sample thinly over as large a surface area with as little sample as possible, as shown in Figure 2.

The effect of sample geometry and holder on moisture diffusion kinetics

Figure 2. The effect of sample geometry and holder on moisture diffusion kinetics.

For such small sample sizes the kinetics will then largely be dominated by the relative hygrospicity of the sample being studied. Thus, the sensitivity as well as the long-term stability of the ultra-balance utilized in the sorption analyzer will dictate the ultimate size of the sample.

For samples measuring less than 1 mg in mass, which will be referred to as ‘microsamples’, a sub-µg sensitivity and a base line stability better than 10 µg is preferred for screening of moisture uptakes of higher than 1% by weight.

Method

A DVS automated moisture sorption analyzer is used to perform all the experiments. This system utilizes typical baseline stabilities of 2.5 to 5.0 µg per day and a SMS UltraBalance with a sensitivity of 0.1 µg.

The symmetrical design of the sorption analyzer combined with a single isothermal temperature zone, ensures that optimum balance performance is obtained. Measurements were then made on a Polyvinylpyrrolidone K 25 sample (PVP av. molecular weight 25,000) with sample masses of 5123 mg and 26.5939 mg on two separate instruments over the RH range of 0 to 90% at 25 °C. For the microsample, an additional step of 95% RH was measured to show the effect on kinetics at high humidities.

This microsample was spread uniformly over the base of a shallow convex sample holder, with the larger sample being positioned in the deeper bowl-shaped cup in order to accommodate all the powder.

It must be noted that whenever microsamples are used, care must be taken to ensure that sample holders are fully cleaned because any remaining hygroscopic substances from earlier analyses may have a major impact on the results.

Results

The kinetics of moisture sorption for a microsample (blue) and normal sample (red) of PVP measured by DVS

Figure 3. The kinetics of moisture sorption for a microsample (blue) and normal sample (red) of PVP measured by DVS.

Figure 3 shows the kinetics of moisture sorption for the two samples of PVP. The data clearly shows how the size of the sample significantly affects the overall speed of analysis. In this case, an increased analysis time by up to 10 times is seen for a corresponding decrease in sample mass of about 50 times.

Table 1. Comparison of equilibrium percentage moisture contents during sorption and desorption for the normal and micro PVP samples measured by DVS.

Target RH% Sorption Normal PVP Sorption Micro PVP Desorption Normal PVP Desorption Micro PVP
0.0 0.00 0.00 0.37 -0.64
10.0 3.74 3.71 5.14 4.69
20.0 6.44 6.28 9.23 8.89
30.0 10.21 9.15 13.08 12.88
40.0 14.25 13.63 16.67 16.42
50.0 18.59 18.24 19.41 19.20
60.0 23.62 23.30 23.44 22.82
70.0 30.89 30.56 30.80 29.88
80.0 41.10 41.06 41.39 40.16
90.0 58.84 60.03 58.84 59.22
95.0   79.49   79.49

 

Table 1 illustrates a comparison of the equilibrium moisture contents measured from the moisture sorption profiles in Figure 3.

Based on the data, it is clearly seen that there is a good agreement between the sorption and desorption data for the two samples, particularly in the low % uptake region which is important for screening purposes. It must be noted that at 90% RH, the sorption data for the normal PVP sample had not reached equilibrium within the selected experimental parameters.

Conclusion

This brief study shows how the DVS moisture sorption analyzer can be effectively used for studying the moisture sorption of microsamples (<1.000 mg), resulting in a significant reduction in the analysis time needed for each sample.

This is achieved by using the unprecedented stability and sensitivity of the DVS ultra-balance, together with appropriate preparation of the sample holder and microsampler.

Acknowledgement

SMS thanks Mr. J. Booth, Ms. S. Reutenauer and Mr. C. L. Levoguer for their contributions to this article.

This information has been sourced, reviewed and adapted from materials provided by Surface Measurement Systems Ltd.

For more information on this source, please visit Surface Measurement Systems Ltd.

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