Sample Preparation Laser Particle Sizer "analysette 22"
Sample Preparation Laser Particle Sizer "analysette 22"
Sample Preparation Laser Particle Sizer "analysette 22"
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<strong>Sample</strong> <strong>Preparation</strong><br />
for the<br />
<strong>Laser</strong> <strong>Particle</strong> <strong>Sizer</strong><br />
"<strong>analysette</strong> 22"<br />
Proper Sampling<br />
<strong>Sample</strong> Dividing<br />
<strong>Sample</strong> <strong>Preparation</strong><br />
Dispersion<br />
Tips and Tricks<br />
Materials and Suitable Measurement Liquids<br />
U.Gerber<br />
August 2004
Proper Sampling<br />
It is always surprising how the market for high-precision, fully automatic analysis<br />
equipment is constantly growing, but so little importance is placed on the sampling<br />
techniques and preparation, which are at least as important as the actual equipment.<br />
One of the most frequent errors encountered in an analysis occurs already when the<br />
sample is obtained and is introduced into the analysis process from the very beginning.<br />
Process technicians are consistently amazed at how frivolously samples are taken and<br />
how easily the analyses results obtained are accepted without question.<br />
Deviations in repeated measurements often give rise to doubts about the measurement<br />
equipment, but the cause of the error lies deeper: inhomogenous sampling.<br />
The results are only reproducible if the analysed sample is highly representative of the<br />
material being tested, in other words, if the sample taken is equivalent to the entire<br />
material batch.<br />
1. <strong>Sample</strong> Dividing<br />
The sample for the laser particle sizer (approx. 200 mg - 1g) should correspond to<br />
the entire quantity of source material with regard to particle type and distribution.<br />
The rotary cone sample divider "laborette 27" is very well suited for dividing of dry<br />
laboratory samples or suspensions because various dividing heads with different<br />
division ratios can be selected (see special printing of Aufbereitungs-Technik,<br />
Volume 9 / 1996). Depending on the dividing head used, up to 3000 divisions per<br />
minute can be achieved.<br />
Possible divisions: 1:8, 1:10 and 1:30<br />
Example: An existing laboratory sample of 50 ml (g) or 100 ml (g) could first be<br />
divided with the installed dividing head 1:10 and then with the dividing head 1:30 so<br />
that the quantity of 200 - 400 mg required for the laser particle sizer is available.<br />
This representative sample must be deagglomerated.<br />
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2. <strong>Sample</strong> <strong>Preparation</strong><br />
Preliminary tests must be performed to determine the manner in which the sample<br />
material can be wetted and dispersed.<br />
The liquid should wet the solid as spontaneously and completely as possible!<br />
The additional ultrasound assistance with the highest possible output generally<br />
reduces the time required for dispersion significantly.<br />
• The solid particles should be present in the suspension individually - free of<br />
agglomerates.<br />
• The dispersed condition must be stable throughout the entire measurement,<br />
coagulation / flocculent precipitation may not occur.<br />
The coagulation of a suspension can be recognised based on a light, greasy<br />
sediment that floats in cloud-like form as a 2nd phase or layer in the sample glass<br />
when moved lightly.<br />
• Floating to the surface of the liquid indicates sample material that is not properly<br />
wetted, meaning that the amount of any dispersing agent added (tensides or wetting<br />
/ interlacing agents or salts) is too low.<br />
• The sample may not be destroyed or broken during the dispersion. For thin, plateletshaped<br />
materials such as mica, terra alba, clay and inorg. salts, this is particularly<br />
important.<br />
In such cases, it is recommended that a small quanitity of the sample be mixed into<br />
a paste on a slide with some liquid and wetting agent. The grain spectrum and the<br />
maximum grain size can be identified under a microscope.<br />
After completing the dispersion process, a micropscope is also used to check<br />
whether the coarse grains are still present or were destroyed.<br />
• The suspension / measurement liquid must always have a lower or the same<br />
specific gravity (density) than the solid to be measured.<br />
• The sample may not solvate, dissolve or swell!<br />
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2.1 Dispersion<br />
Easy samples that submerse directly in water without significant effort and do not<br />
have a large fine portion are added to the dispersing unit as a solid portion by<br />
portion using a spatula and can be measured reproducibly after a brief ultrasound<br />
exposure / dispersion (30 - 60 s).<br />
If the double measurement performed exhibits excessively large differences in the<br />
grain size distribution, this may have several reasons:<br />
• pump speed is too low - coarse material is settling out<br />
• stirrer speed is too high - air bubbles form<br />
• dispersion time is too short<br />
• increasing fine portion - longer ultrasound exposure required<br />
• - add dispersion agent<br />
• - adhesion of fine particles to the cell glass<br />
• - blockage by coarse grain > 1.5 mm<br />
• - coarse material settled out: spec. gravity too high<br />
• curve appears too coarse - sample is swelling or flaking out<br />
• - sample is agglomerated<br />
• - sample is magnetic<br />
• decreasing coarse and fine range - sample dissolves<br />
• - beam absorption decreases<br />
In addition, inaccurate measurements can result from pinched or kinked tube<br />
connections (due to movement of the measurement cell for multi-range<br />
measurements).<br />
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Difficult to disperse samples may exhibit the following properties: static charge (e.g.<br />
for polymers), adhesion or cohesion forces – samples tend to stick/clump (e.g.<br />
clays, soil samples, terra alba), magnetism, hydrophobic properties – waterrepelling<br />
molecular components (e.g. pharmaceuticals, medications, toner, graphite,<br />
titanium dioxide), coagulation (e.g. clays, terra alba, chalk, gypsum).<br />
a) Static charge or hydrophobic properties:<br />
A spatula portion of the material should be placed in a small 50 ml Erlenmeyer flask<br />
and then first wet with 1 (to 2) drops of a wetting agent (tenside) and mixed into a<br />
paste until the sample is completely wetted! Then add water drop by drop and stir<br />
some more. The approx. 20 - 30 ml suspension is dispersed in the ultrasound bath.<br />
If the sample is already located in the dispersion unit and floats to the surface, the<br />
sample can be wetted as follows:<br />
with a glass rod or the tip of a spatula, a small drop of a wetting agent (e.g. Imbentin<br />
(SG/45/AG), Teepol or Pril) is placed on / touched against the liquid surface and<br />
distributed. It can immediately be seen that the skin formed on the surface breaks<br />
open and the fine portion enters the suspension.<br />
b) Adhesion forces:<br />
The surface adhesion of the particles. These surface forces can be reduced by<br />
creating adsorption layers of tensides or macromolecules at the solid/liquid phase<br />
borders. One refers here to the coating, shielding or masking of the solid or to<br />
wetting.<br />
For this reason, sodium diphosphate (sodium pyrophosphate: Na 4 P 2 O 7 ) or polysalts<br />
in concentrations of approx. 0.5-1% are generally used.<br />
c) Cohesion forces and magnetism:<br />
The effects of attractive forces between atoms or molecules of a body are referred to<br />
as cohesion force or polarity or as magnetic properties. The magnetic properties are<br />
difficult to eliminate: either one uses a highly viscous liquid such as ethylene glycol<br />
or glycerin/water mixtures for low magnetism or one heats the sample to over 100°C.<br />
However this is hardly possible in the laboratory and rarely performed. For this<br />
reason, such materials with strong magnetism are not suited for particle size<br />
analysis.<br />
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d) Coagulation:<br />
This is the flocculent precipitation of a sample resulting from colloidal particles<br />
balling together in a suspension. This can occur when the quantity of solids is too<br />
large or if the pH range is unfavourable. By adding a few drops of dilute acid (e.g.<br />
hydrochloric acid) before adding the sample (!!) for the acidic range or diluted bases<br />
(e.g. caustic soda hydrated, ammonia or soda solution) in the alkaline range, the pH<br />
value can be decreased or increased in order to counteract the reaction of the<br />
sample (e.g. for chalks, terra alba, slaked lime and clay).<br />
2.2 Tips and Tricks<br />
The larger the fine portion of a sample is, the greater the effort required for<br />
dispersion. If required, ultrasound exposure should take place for several minutes in<br />
the external ultrasound bath "laborette 17".<br />
We recommend placing a spatula tip's worth of sample (approx. 0.5 - 1 g) in a 50 ml<br />
Erlenmeyer flask and, for easily wetted substances, adding approx. 20 - 30 ml of<br />
the measurement fluid + the dissolved / mixed dispersion aid/tensides. When using<br />
tensides, we recommend low-foam tensides, such as Imbentin SG/45/AG (from the<br />
company Dr. W. Kolb, CH-Hedingen or STS Tensid Service GmbH<br />
D-68649 Gross-Rohrheim).<br />
After shaking briefly, hang the Erlenmeyer flask on a laboratory clamp in the<br />
ultrasound bath in such a way that the interior liquid level lies below the liquid<br />
surface of the ultrasound bath.<br />
After the deagglomeration time determined in preliminary tests, the suspension<br />
quantity required for the measurement is added to the dispersion unit of the laser<br />
with a pipette (while agitating).<br />
If the required quantity of solid is precisely known, the complete contents can be<br />
transferred with a wash bottle by weighing into the Erlenmeyer flask and<br />
subsequent dispersion; this means that no demixing can take place when taking the<br />
sample with a pipette.<br />
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Difficult samples, such as fly ash, sulphur, coal, polymers or pigments can be<br />
quickly dispersed even in water with a few small tricks.<br />
The laid out sample is combined with one drop of tenside and mixed to a paste with<br />
a glass rod or spatula after adding one to two drops of water. Due to the relatively<br />
high tenside quantity compared to the water, the cavitation is reduced and the<br />
analysis sample is very quickly wetted.<br />
After adding a few drops of water while stirring, one can check whether particles<br />
that are not yet wetted are floating on the surface of the liquid.<br />
Then dilute the suspension to approx. 20 - 30 ml and deagglomerate it in the<br />
ultrasound bath.<br />
When using simple tensides like flushing agent (e.g Pril), it often happens that<br />
"foam" arises while mixing to a paste and mixing into the sample; this foam floats on<br />
the surface after dispersing and is transferred to the dispersing unit.<br />
If the stirrer intensity is too great, the foam is pulled into the measurement circuit<br />
and "coarse grains" are measured that are not present.<br />
To avoid this "mis-measurement", it is possible to destroy the "foam" in the<br />
Erlenmeyer flask: with a glass rod the tip of which is dipped in n-butanol such that<br />
the glass rod is only moistened but no drop can be seen.<br />
A light contact with the foam surface causes it to collapse away and the suspension<br />
can be analysed for particle size without difficulty.<br />
Another option for substances that are difficult to wet is adding 2 to 3 drops of (e.g.<br />
alcohol (ethanol)) to the dry laboratory sample, which immediately soaks up the<br />
alcohol like a sponge. Water + wetting agent can now be added and the sample<br />
dispersed accordingly.<br />
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The user who works with the "liquid dispersing unit for small quantities“ naturally<br />
also has the option of using alcohols, alkanes, high-boiling-point benzines or other<br />
organic liquids.<br />
It should be mentioned here that the liquid dispersing unit for small quantities is not<br />
explosion-safe -select appropriate liquids - and should only be operated with good<br />
ventilation.<br />
Changing the measurement fluid from alcohol, for example, to other liquids is<br />
relatively simple because many organic liquids mix well together.<br />
The compatibility of the solvent with the the connection tubes should also be<br />
considered. The sealing rings in the measurement cell and the connection tubes are<br />
made of Viton.<br />
Some liquids that cannot be used are: acetone (ketones) and paint thinner.<br />
The resistance list available from the hose manufacturers contains a rating of the<br />
chemical reistsnace to various operating media (liquids) for the most common<br />
elastomers.<br />
After completing a measurement with the laser particle sizer "<strong>analysette</strong> 22", one<br />
should absolutely practice the habit of rinsing out the measurement system<br />
immediately, to prevent unnecessary "depositing" or "sticking" of particles in the<br />
measurement circuit and, above all, on the measurement cell.<br />
An "intermediate rinsing" with a tenside solution is very helpful for this.<br />
Tap water or water with a high lime content can lead to deposits on the<br />
measurement cell. One should either switch to distilled water or it is necessary to<br />
clean the measurement cell from time to time.<br />
Rinsing with an approximately 10% hydrochloric acid will remove the lime coating<br />
after just a few minutes. Then rinse twice with normal water.<br />
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