When I measure DLS for nanparticles size, I see that there are no rule in the samples concentration and the Average Count rate index in mine. I wonder that if the higher concentration of samples, the lager average count rate value.
Could everybody give me advice? Many thankfull.
One key point in DLS is to collect enough photons to obtain a statistically significant result. While you could in principle measure at very low average count rates, you might then have to collect data for very long run times. At high concentration the count rates could get very high, to the point that the detector in the system could be damaged. In order to avoid this, many instruments have an 'automatic attenutation mode' in addition to a 'manual mode'. In a typical auto-attenuation mode, the system would aim for an average count rate of about 100 to 500 kcps (= kilo counts per second). Built-in attenuators have certain "step sizes" to cover a large dynamic range, as an example, in the Malvern Zetasizer the transmission steps are 100%, 30%, 10%, 3%, 1%,...
If you perform measurements in automatic mode, it can then happen that the actual measured countrate is smaller, even though the concentration is increased, and this could then lead to a perceived lower countrate. For example, let's say your sample scatters 300kcps at 100% transmission. If you double the concentration, the countrate should be near 600kcps at 100% transmission, however the automatic mode then wants to optimize and selects 30% transmission which would lead to a count rate of 200kcps. This is easily noticed if one looks at the derived count rate (a parameter that can be calculated and displayed in the software), which would be the theoretical countrate assuming 100% transmission.
In general in the dilute regine, the larger the concentration, the larger the derived countrate. However, if the particles are interacting, the countrate might be higher (for attractive interaction) or lower (for repulsive interaction) than expected (due to second Virial effects). For very high concentration, the scattering behavior changes from single scattering to multiple scattering, which could make the apparent count rate smaller again (depends on optical setup, the level of multiple scattering and other parameters).
As a rule of thumb for anyone beginning with dynamic light scattering I would recommmend an average countrate between 100 - 500 kcps, and a concentration such that the sample is optically clear and dilute enough that it is not optically turbid.
Be careful which DLS instrument you are using.
Some instruments, if you do not set up the measurement method to prevent it, will use auto-attenuation ( a neutral density filter wheel) to incrementally vary the input intensity so as to keep the intensity on the photocounter at a more constant level. If you work up a count rate ladder of particle concentrations and see little count variability, or occasional sharp reversals in that count rate vs particle concentration, then you may have an auto-attenuating instrument.
Best regards,
Mike
The relationship should be linear, at low concentrations. At elevated concentrations, the inverse osmotic compressibility is more noticeable in its actions.. However, if you have a complicated "automatic" instrument, all sorts of issues may arise.
One key point in DLS is to collect enough photons to obtain a statistically significant result. While you could in principle measure at very low average count rates, you might then have to collect data for very long run times. At high concentration the count rates could get very high, to the point that the detector in the system could be damaged. In order to avoid this, many instruments have an 'automatic attenutation mode' in addition to a 'manual mode'. In a typical auto-attenuation mode, the system would aim for an average count rate of about 100 to 500 kcps (= kilo counts per second). Built-in attenuators have certain "step sizes" to cover a large dynamic range, as an example, in the Malvern Zetasizer the transmission steps are 100%, 30%, 10%, 3%, 1%,...
If you perform measurements in automatic mode, it can then happen that the actual measured countrate is smaller, even though the concentration is increased, and this could then lead to a perceived lower countrate. For example, let's say your sample scatters 300kcps at 100% transmission. If you double the concentration, the countrate should be near 600kcps at 100% transmission, however the automatic mode then wants to optimize and selects 30% transmission which would lead to a count rate of 200kcps. This is easily noticed if one looks at the derived count rate (a parameter that can be calculated and displayed in the software), which would be the theoretical countrate assuming 100% transmission.
In general in the dilute regine, the larger the concentration, the larger the derived countrate. However, if the particles are interacting, the countrate might be higher (for attractive interaction) or lower (for repulsive interaction) than expected (due to second Virial effects). For very high concentration, the scattering behavior changes from single scattering to multiple scattering, which could make the apparent count rate smaller again (depends on optical setup, the level of multiple scattering and other parameters).
As a rule of thumb for anyone beginning with dynamic light scattering I would recommmend an average countrate between 100 - 500 kcps, and a concentration such that the sample is optically clear and dilute enough that it is not optically turbid.
To see the trend you need to set up every thing manually: the attenuation and the position (Malvern instruments). in principle, the count rate increases when the concentration increases but I am not sure if it is linear in general. I think Malvern was aware about the linearity problem and this is why they do not include this kind of application. I read one article to use the count rate to measure the concentration and the author stated that this application was not described by the manufacturer.
Check the real concentration of your compound in the sample you used for DLS using an indepent method (UV, refractive index, titration, elemental analysis:
Filtering highly concentrated "charged" colloids may actually reduce your concentration
Check the linear range of your detection system
Every detection system has a finite response time and you may loose counts due to "detector paralysis"
As above; check "automated setting changes"
I join colleagues and I add the little.
With what nanoparticles you work and on what angle you measure light scattering? For example, if it is the absorbing particles (nanoparticles of gold or silver and measurement are conducted on an angle of 90 degrees), they can absorb (real adsorption) very strongly laser radiation at big concentration.
Look on a cell sideways. If the beam of the laser meets in a cone, absorption is very great and intensity of light scattering falls as a result of strong absorption.
Simple answer is to use the "Derived Count Rate". This will normalize for any attenuation filters, etc...That way you don't need to mess with other parameters. Of course this only corresponds with the Malvern instrument. For other instruments you need to contact the vendor.
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