Feb 3 2008
Researchers from St. Petersburg have designed and produced a device that enables to promptly and accurately identify the sizes of micro- and nano-particles. The laser ray illuminates the object in the device so that to judge about the particle sizes by intensity of light diffused by the particles.
Specialists of the St. Petersburg Research-and-Production Firm for Analytical Instrument-Making (LUMEX) demonstrated a device that had been long awaited by representatives of various areas of science and engineering. This is the so-called laser analyzer of particle sizes – a device that enables to promptly and accurately identify the sizes of microscopic particles and to distribute them by sizes. In other words, the analyzer helps to measure not only the average diameter of particles, but also to determine the quantity of particles of a certain size in the mixture.
The device operates as follows. The sample is poured into a transparent cuvette. This can be a suspended matter of smallest particles or emulsion – that does not matter. As solid particles (more than a micron in size) usually accumulate rather quickly at the bottom, the sample in the cuvette is being constantly agitated by a special stirrer, at that, the rate of stirring may be, if needed, very high – more than a thousand revolutions per minute. The stirrer is designed in such a way that no gas holes occur in the analyzable medium, which could impede the analysis. However, the stirrer may not be switched on – then there is an opportunity to track for example the speed of precipitation of particles of different sizes.
The cuvette is illuminated by a laser ray. Microparticles diffuse its light, with the angle of deflection being determined by the size of each particle. The multielement detector records this scattered radiation, thus allowing to measure intensity of beaming at different angles of deflection. Certainly, it is impossible to “pull out” the information on the particle sizes directly from this data, but the light diffusion theory has already been developed for this purpose – “three-story” equations comprehensible only for specialists, which are not needed to others for the most part. It is sufficient for the users to know that the particle size is promptly and accurately calculated in such a way by the computer, certainly with the help of the software also developed by the LUMEX specialists. The only assumption to be kept in mind is that the particle size is determined by the so-called “hydrodynamic radius” – a “fur coat” consisting of water molecules, ions or some molecules adjacent to the particle and moving together with the particle. So, the size determined for some particles via this method is bigger than it would be seen under the microscope – however, in this case it is needed to compare once the particle analysis results obtained through independent methods and to apply the necessary correction later on. This is always the case with determining the sizes by the laser light diffusion method, though.
It should be noted that till recently the device (called “Laska” by the designers) was intended for analysis of rather big particles, their diameter being no less than a micron. This niche at the analytical instrument-making market was not practically filled – as a rule, the task was solved either with the help of a microscope, which is rather labor-intensive and not always possible, or with the help of highly specialized devices, for example, blood cell counters. As for the nano-range sizes, i.e. the particles smaller than a micron, there existed only one device for them – the so-called nanosizer, the device being good and operating based on the same principle of laser light diffusion, but it is so expensive that there are only few of them available in Moscow .
“Our device already enables to perform analysis of particles of up to 500 nanometers in size, i.e., up to half a micron, says Vladimir Krivoshlyk, one of the developers, head of the group, dispersion department, LUMEX. However, we are working now on the analyzer modification, which would allow to measure sizes of smaller particles. We know already how to do this.”