scale and sub-micron polystyrene latex spheres has been described. Since these products are characterized by a narrow distribution, only a limited number of ...
Quantitative particle characterization using SEM B.H. Lich*, L. DesRosiers*, J. Elands*, A.P. Tinke** *FEI Company, Eindhoven, The Netherlands **Johnson and Johnson Pharmaceutical Research and Development, Beerse, Belgium
In the pharmaceutical industry, scanning electron microscopy (SEM) is often used as a qualitative tool for the analysis of drug substances and drug products in order to obtain information about the shape and surface structure of the material. Another application for SEM is studying structural changes induced by variations in ambient conditions. SEM also plays an important role in the characterization of nanoscale and sub-micron particles. More than just qualitative analysis; here we study if SEM can play a role in the quantitative analysis for characterization of size of sub-micron particulate systems. Furthermore, the technique can serve as an absolute reference standard to support the development of QC particle size distribution techniques. In literature, the particle size distribution analysis of nanoscale and sub-micron polystyrene latex spheres has been described. Since these products are characterized by a narrow distribution, only a limited number of particles need to be analyzed, which allowed the use of elaborate manual procedures in the SEM analysis of these products. However, once the size distribution of the product gets broader and more particles need to be counted, the automation offered by the use of SEM technology becomes particularly beneficial. For this poster, a study has been done to verify SEM particle characterization results versus the calibration standards provided by the standards manufacturers, as well as to characterize products with broad particle size distribution.
As a minimum limit, the signal level, which has a 10 % deviation from the base level, is visible. This implies that detection based on backscattered electrons has a lower limit of around 100 nm and particles could appear smaller than they really are due to the limited contrast at the edge of the particle. Hence for submicron particles the secondary electron signal is the preferred signal. Considerations of operation in low vacuum imaging mode Imaging in low vacuum mode is desired because it allows using non-conductive samples in the microscope without any sample preparation. The drawback of using a low vacuum regime is that the poor vacuum causes the primary electron beam to spread. The beam spread is a function of the pressure in the sample chamber of the SEM and the distance that the beam travels through this vacuum, the beam gas path length. In order to minimize the beamspread we used a proprietary high vacuum extension cone.
Statistics, required number of particles based on geometrical standard deviation according ISO 13322-1 Annex A
Aspirin on bisphenol
Thickness (nm)
Schematic of SEM low vacuum system
Evaluation of pharmaceutical sample with broad distribution profile
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Beam-sample interaction model
SEM analysis of a 300 nm latex sphere calibration sample SE image of broad profile pharmaceutical product
Electron optical theoretical consideration of signal discrimination To study the signal levels using aspirin as a target sample on bisphenol (typical filter material), the thickness of aspirin particles was assumed as one parameter and the resulting stack from sample + substrate was used as being representative for the signal level (using a 5 kV electron beam). The y-axis actually gives the backscatter(BS) coefficient of the combination of materials, hence the signal strength. This can be compared with the signal strength of the bulk material (the base or the filter material where the sample is attached, i.e. thickness = 0). Each individual point has been simulated using an electron flight simulator and has a relative accuracy of around 1 – 2 % The BS coefficient is at a level of around 0.051 and with the full sample thickness, the value increases to around 0.061 maximum. Normally BS detectors are optimal for discrimination levels around Z = 29 (e.g. Cu) and can give good contrast with delta Z = 0.1. For the aspirin on bisphenol, the signal level is a factor of 6 lower, but the difference in signal between base and sample is larger. Hence imaging the sample is no problem (from a signal to noise point of view).
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Results for 300 nm particle standard, size = 301 nm +/- 10 nm (centrifugal sedimentation) (DC20000, CPS instruments) Results SEM analysis SE D[4,3] = 307 nm, GSD = 1.035, n=1291 (nmin_95% = 68) BSE D[4,3] = 250 nm, GSD = 1.180, n= 1170
Comparison results of different signals (SE, BSE)
References
Discussion
• Internal standard method for size calibration of sub-micrometer spherical particles by electron microscope, Technical Note 010B, June 15 (2003) Duke Scientific corporation. • Particle size analysis – image analysis method; Part 1: static image analysis method, ISO 13322-1:2004
• SEM combined with image analysis is a promising technique for generating particle distribution profiles with the possibility to visually re-evaluate the data by re-assessing the particle. • The technique holds promise for characterization of the size and shape of unknown products with relatively wide distribution profiles from the nanometer to the micron range.
