Comparing Dynamic Vapor Sorption for the COST 90 Procedure for Moisture Sorption Determination

Microcrystalline cellulose (RM 302) is a standard EC reference material used for validating the measurement of water sorption isotherms of food materials after the COST 90 procedure [1].

This procedure involves the periodic weighing of samples stored over saturated salt solutions until equilibrium is established (normally after 7 days). The moisture content RM 302 at 10 specified relative humidities (from Stokes and Robinson) [2] has been certified in a study between 10 independent laboratories within the EC [1].

When compared to the COST 90 procedure, the DVS instrument offers many potential benefits for isotherm measurement including independently validated humidity generation, much faster equilibration times and fully automated weighing of samples, thus removing some sources of error during the measurement.

This article provides data from two independent labs using DVS instruments to determine water sorption isotherms on RM 302 for comparison with the COST 90 procedure.

Method

The Laboratory of the Government Chemist, UK, supplied samples of microcrystalline cellulose RM 302 in sealed sachets, which were stored at 4 °C until they were ready for use. Sample sizes ranging from 10 to 15 mg were used to ensure an optimum balance between homogeneity, sensitivity and equilibration time.

Ten humidity steps were selected to correspond to the saturated salt solutions used in the EC study, and equilibration times of between 2-6 hours for each step were chosen. All samples were analyzed at 25 °C, with data collection intervals of 20 seconds and a total flow rate of 200 sccm. The total acquisition time for a complete sorption isotherm including drying curve was 33.5 hours.

Results

Figure 1 shows typical kinetic data for a sorption isotherm on RM 302 using the method described above, indicating that equilibrium is established quickly for all the sorption steps.

Kinetic data for moisture sorption of RM 302 on a DVS instrument

Figure 1. Kinetic data for moisture sorption of RM 302 on a DVS instrument.

Table 1 shows a summary of the certified equilibrium moisture content of RM 302 as determined by the COST 90 procedure and using a DVS gravimetric vapor sorption instrument.

The DVS data illustrated is the mean of four data sets, three obtained from one instrument in the same lab and one from a different instrument in another lab. Also shown ae the standard deviations of the four data sets to provide some indication of the repeatability of the DVS measurement.

Table 1. Moisture sorption isotherm data for RM 302.

Relative Humidity (%) Mean % H2O Content COST 90 Mean % H2O Content DVS Standard Deviation DVS
11.05 2.13 ± 0.11 2.45 0.026
22.45 3.24 ± 0.12 3.59 0.041
33.00 4.15 ± 0.09 4.40 0.029
42.76 5.16 ± 0.09 5.02 0.027
52.86 5.97 ± 0.14 5.89 0.026
57.70 6.48 ± 0.15 6.43 0.029
70.83 8.25 ± 0.17 8.29 0.060
75.28 8.90 ± 0.24 8.84 0.081
84.26 11.00 ± 0.33 10.98 0.071
90.19 13.27 ± 0.43 12.82 0.160

 

Conclusion

The DVS data illustrated above matches with the COST 90 data for humidities over 33%, where all the data points fall within the 95% confidence limits of the EC study. In the case of humidities of 33% and lower, the DVS data is shown to be consistently over this confidence limit.

This indicates that systematic differences may exist between the two procedures, because over this range, the standard deviation of the DVS data set is extremely small. This data, in combination with saturated salt solutions, provides a rapid way to validate humidity generation in the DVS instrument.

Acknowledgement

SMS acknowledge the contributions of Mr. C. L. Levoguer and Mr. J. Booth, for this article.

References

1 Jowitt R. and Wagstaffe P.J., Community Bureau of Reference, EUR 12429EN.

2 Stokes R.H. and Robinson R.A., Ind. Eng. Chem. , 41 (1949) 2013.

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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