Mastersizer2000 Application: Verification Method for Laser Diffraction Measurement-2

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Robustness

– Sample Dispersion

  As part of method development, the process of sample dispersion should be explored and understood.

  In the case of dry measurement, users should understand how the measured particle size changes with the selected pneumatic pressure (air pressure) for dispersion[2]. The suitable pressure allows for dispersion without grinding the particles. For pharmaceutical materials, the tested samples should be easily breakable, and if the dispersion pressure is set too high, the particles may be ground to a finer size. Dispersion and grinding often occur simultaneously (causing broadening of the distribution)[10]. The best way to demonstrate that attrition is not occurring is to obtain nearly identical results for wet and dry dispersion. All details concerning the development of the dry method are presented elsewhere in this document[8].

  For measurements using wet dispersion, it is necessary to understand the role of ultrasound in promoting dispersion[2]. Using high-energy ultrasound can cause fracture in some crystalline materials (a very rare phenomenon). As part of method development, the effect of varying ultrasound application times and intensities on particle size should be investigated. When examining the impact of ultrasound disintegration on the robustness of the measurement, it is advisable to conduct measurements both during and before/after ultrasound application. Additionally, microscopic images should be acquired to ensure that particle dispersion does not occur during mixing.

  Additionally, it should be noted that in some cases, the application of ultrasound can induce agglomeration. If agglomeration occurs, it is necessary to explore the use of various dispersing agents to secure stability. Details regarding wet method development are presented elsewhere in this document[9].

– Pump and Stirring Rate

  The pump and stirring speed used in wet measurements should be investigated as part of method development. The chosen conditions should be able to maintain all materials in a suspended state without forming air bubbles (a specific issue when surfactants are used).

  Figure 4 illustrates how the results obtained for a sample of lactic acid vary with the stirrer settings. As seen, the results stabilize at 2,000 rpm. At this point, all materials are accurately in suspension and dispersed. The result values are 2,000 rpm depending on sample sedimentation.

Figure 4] Effects of Stirring Speed on Results for Generic Lactic Acid 

– Confirmation of Refractive Index

  As part of method development, the selection of refractive index should be reviewed. The values for fluids can be used to provide experimental evidence for the real refractive index. If particles smaller than 1/40th of the wavelength of light used for measurement (25 microns for He-Ne red laser light source) are present in distribution[1], the Fraunhofer assumption cannot be used as it may report incorrect information regarding the presence of fine material. ISO 13320-1 provides guidelines regarding the importance of optical properties when setting up laser diffraction analysis[2].

Linearity and Obscuration

Since sizing methods rarely respond linearly as a function of particle size, the evaluation of the linearity of sizing methods is not considered as part of method development. However, it’s crucial to consider how sample obscuration influences measurements. Obscuration, which measures the amount of light scattered by the sample, correlates with the density of material present in the measurement region. For most particle size distributions, the reported particle size should be independent of measurement obscuration across a wide range of obscuration values. Low obscuration levels can result in high coefficient of variation (COV) values due to low signal-to-noise ratios, while very high concentrations may result in measured results being artificially lower due to multiple scattering effects. It is recommended to study obscuration at levels of 10, 15, 20, 25% in the same way that measurements are executed during testing, and to define an acceptable COV similarly.

Figure 5 shows an example of how multiple scattering affects obtained results for pharmaceutical powders. At low levels of obscuration, results remain consistent. However, when obscuration exceeds 10%, multiple scattering causes the reported size to be reduced. In this case, an obscuration level of about 7.5% enables a robust methodology that ensures small changes in sample obscuration do not significantly affect results. 

Figure 5] Change in Dv50 Shielding Function for Pharmaceutical Powder 

Reproducibility 

  Bell et al. defined ‘reproducibility’ as an indicator of precision between laboratories[5]. While it indeed demonstrates this, Malvern Instruments’ experience indicates that reproducibility largely measures the effectiveness of the chosen sample collection method. Additionally, reproducibility can be utilized to mitigate differences among various instruments (whether identical or different models). Small temperature differences, particularly when saturated solutions are used as dispersants, can lead to changes in results due to particle dissolution or recrystallization; hence, the environmental conditions experienced in different labs should also be considered.

  To assess reproducibility, multiple samples (at least 5) should be collected from the same batch according to the investigated method and tested. For each sample, measurements should be repeated at least 5 times to obtain individual and average results. The COV across samples should be determined and must fall within specified allowable ranges stated in ISO13320[2], and in any case within the allowable ranges of USP<429>[4].

  Table 2 shows an example of results obtained using a scoop sampling method for lactic acid excipients. In this case, the obtained COV was within the limits anticipated based on the sample collection statistics presented in Figure 1.

 Table 2] Variations in Results for 7 Scooped Lactic Acid Samples

Intermediate Precision

In assessing intermediate precision using secondary analysts or secondary equipment (or both), users should recognize the variability of the method. This is essentially a repeat of the reproducibility test, with similar COV limits applying. Thereafter, both results should be combined to obtain a joint mean and joint RSD (aiming for <3%).

Table 3 presents reproducibility data obtained for a secondary analyst analyzing the aforementioned lactic acid samples. These joint mean and COV can be determined by both operators and are presented in Table 4. As evident, the deviation in Dv50 falls within the allowable ranges of ISO13320. This relates to the sampling methods used in this study. If the powder was sampled using a rotary riffler, it would enhance overall precision. The wider allowable limits used in USP<429> could be applied if scoop sampling is the only effective method for obtaining samples.