Emerald Group Publishing Limited
Copyright © 2008, Emerald Group Publishing Limited
Precise analysis with rotary cone sample divider
Article Type: Methods From: Anti-Corrosion Methods and Materials, Volume 55, Issue 2.
Today modern analytical instruments for particle characterisation can achieve previously unheard-of levels of precision. Verification limits of trace analysis in the parts per million or parts per billion range have now become standard. Unfortunately, it is frequently overlooked that the overall error in analysis is comprised of the measurement error of the analytical instrument plus the sampling error. In other words: an error of 15 per cent made in taking of the sample will never disappear irrespective of the precision of the measuring instrument.
The error due to sampling becomes larger when, as is often the case, an analysis quantity of less than 200mg is required and the original sample quantity available to the lab is, for example 2,000g. For this reason, it is necessary to divide the total quantity into a representative sub-quantity. The aliquot should be representative and have the same physical and chemical properties as the whole quantity. The ideal way to obtain a representative sample is to use the Laborette 27 rotary cone sample divider from Fritsch. The design is based on the combination of various dividing processes in one instrument. The sample is fed through a funnel to a dividing cone with a shape that allows for quartering and coning of the material. Against the surface of the cone, the sample material is accelerated outward through rotation of the entire system and divided by guide channels into eight, ten, or 30 individual material flows depending on the dividing head used.
Owing to the rotation of the dividing head, the number of dividing steps is increased up to 2,600per min, so that the final reduced sample is made up of a very large number of individual samples. The result of the sample division is more accurate the slower and more uniformly the sample is introduced. For this reason, Fritsch recommends use of the Laborette 24 vibratory feeder. With rotary sample dividers of other manufacturers, the sample introduction takes place eccentrically into the individual glass sample bottles. Unfortunately, this can result in systematic errors. When using a feeder channel, segregation of the sample can occur in the channel. This results in a reduced dividing accuracy.
The introduction of the sample over the central dividing cone in the Laborette 27 reduces this error. Through rotation of the cone, the flow of material is spread out radially and evenly over 3608 and thereby also the random or systematically embedded properties of the sample. Naturally, the handling and cleaning of the sample divider is equally important. The dividing head of the Laborette 27 can be easily removed from the drive shaft in order to either clean it with a brush or blast it with compressed air. It is also possible to rinse it with water, place in an ultrasonic bath, or wash it in a dishwasher. The dividing head is available in various versions. These allow for division ratios of 1:8 (in 500 or 250ml bottles, i.e. a maximum of 4l sample), 1:10 (in 250ml bottles, 2.5l sample) or 1:30 (15, 20 or 30ml bottles, maximum 300ml sample). In addition, the individual dividing heads are available in aluminium and polyoxymethylane plastic. Fritsch also offers the dividing head 1:30 with PTFE coating.
For further information, please visit the web site: www.fritsch.de