Hi!
A colleague of mine was doing some laser diffraction analysis for a waste water. We observed some strange effect which we couldn't explain: an analysis with and without ultrasound showed similar results (it said, that there are too large particles, although there are NO visible ones; the range was 0-350 micron). This cannot be true. We cleaned and checked the device (with a standard) and remeasured the sample which we had before: the one already treated with US. The result seems ok. A repition of an original sample with longer US lead to "too large particles" again.
Now my question is:
Are there any molecules/chemical componentes which influence the analysis and pretend to be large particles? Acids, colour, solvents, ....
Hi Johanna,
Laser diffraction measures particle size distributions by measuring the angular variation in intensity of light scattered as a laser beam passes through a dispersed particulate sample. Large particles scatter light at small angles relative to the laser beam and small particles scatter light at large angles, as illustrated below. The angular scattering intensity data is then analyzed to calculate the size of the particles responsible for creating the scattering pattern, using the Mie theory of light scattering. The particle size is reported as a volume equivalent sphere diameter.
Please check whether the samples contains any liquids that are not missible. If small quantity immissible liquid present in solvent and both are transparent, this type of error message can popup, even though the visual you cannot detect.
Also make sure that your sample is free from air bubble. Sometime it is also associate with air bubble also.
Have you got particles in your waste water (if so what is the approximate concentration) or are you simply measuring noise-on-noise? A laser diffraction instrument is not a particle counter and needs sufficient concentration of particles for a measurement. Are you generating thermal effects with your sonication (that cause convection and thus RI variation in the liquid causing the laser to move into the inner rings - "beam steering" and causing a ghost peak: this is easily observed with volatile liquids and suspensions). If you're using a Malvern help is near you with Malvern GmbH. Just take a look at http://www.malvern.com/en/support/contact-support/
Thanks for you valuable answers! I can imagine, that some immiscible liquids (oils, BTEX) are present in the water sample (although nothing is visible at the water surface...). It could be a possible explanation! Air bubbles are not present, I already checked.
Alan, I don't think we genereate thermal effects, as the type of analysis works well with similar waters...
Hello Johanna I still have no idea what the level of concentration of particles is in you sample. Can you post data on this site (if you're using a Malvern) or send to me? [email protected]
How do you measure the background for your sample? Normally one has the dispersant liquid in the accessory and then adds drops of suspension, paste or emulsion to the accessory until the appropriate optical concentration for measurement is reached. If you're trying to measure the background first in DI water, then emptying, then adding your sample then you'll have a major problem in at least 3 cases - dissolved salt(s) altering the RI and thus the alignment of the system, possible thermal effects if the temperatures are different and thermal equilibration isn't achieved, the inability to remove dissolved air (easy if DI water is the background as sonication can be used here.
Sorry, I forgot that. The optical concentration is indeed quite low, only 1,6-3,2%. It should be 10-20%, right? Well, I just accepted that.....
We defined the background with DI water indeed. Do you mean "dissolving salts" by mixing some of the remaining DI with the actual sample? What is actually RI? Do you think we should try to measure the background with a filtered sample of the water?
I am aware, that my sample is not well suited for a laser diffraction method. However, I can accept uncertainties to some extend.
A attached the diagrams of the original sample and the sample remeasured after ultrasound.
The optical concentration should be fine at 1 - 3%. How many units of scattering do you have over background? Can you see the raw intensity-detector (angle) plot? If you have dissolved material in the waste water you want to measure then the refractive index will be different to DI water and the alignment will be changed. Even some slight heating via the ultrasound could then 'tip you over the edge'. Your lens on the Sympatec is restricting your top end measurement - large particles will be few in number but will scatter strongly. The refractive index of the material (and background liquid) is very important - it governs how much light is scattered, as well as the size, and concentration.
Hi Alan!
I talked to my colleague and we tried to figure out some answers to your questions. However, we couldn't. The device is already quite old: maybe it does not have the functions or we are just too unexperienced. It's always a problem, if no one is an expert in complex analytical devices.....
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