I measured the particle size of very diluted chitosan solution in methanol using DLS. The size distribution shows single sharp peaks around 150 nm but with PDI=1. My question is that the PDI value indicates the polydispersibility and the size distribution by both intensity of number represents single sharp peak (not broad) and also the Z-average equals 2513 which is incompatible with the calculated particle size distribution peak (150nm)? How can explain this result. Please see the attached reports.
It should be the distance where particles' distributions are provided.
Usually technic requires a dispersed matrix; wet or dry (if you use methanol it is called as wet dispersion by the company).
A laser focused beam is used on targeted particles and resulting intensity fluctuations are recorded. A statistical distribution over the matrix is then given as particle size distribution.
I am not sure but Z-parameter should be the distance between the well-dispersed particles inside the matrix. You should prove it by asking a technician. Company provides information in its web page.
Hi, Fatma.
For Polydispersity in Zetasizer,the maximum value is arbitrarily limited to 1.0. A PDI value of 1 indicates that the sample has a very broad size distribution and may contain large particles or aggregates that could be slowly sedimenting. If this is the case, the sample may not be suitable for a DLS measurement. However, the results you have shown are good if only single peak has to consider but PDI value should be below 0.7 to give correlating results. Mid-range value of PdI between 0.08 to 0.7is the range over which the distribution algorithms best operate. Thus, I would suggest to go with other concentration ( may be higher or lower) and remove larger particles by centrifugation or filtration (if possible). One more thing check solvent properties (refractive index and solubility) and add that while giving instruction for measurement.
Hope PDI improves. All the best.
http://nano.indiana.edu/Zetasizer_faq.html
Ritu's suggestion to dilute your sample even more is a good idea.
(your report indicates an attenuator position 8; so you can probably dilute your sample by another 10 to 30 times, to reach an attenuator position between 8 and 10)
The fact that you have a single major peak around 150 nm both in intensity- and number-weighted distributions could suggest that you have quite a high concentration of aggregates and/or impurities. It may come from the way you prepare chitosan from raw material.
0.45 or 0.22 µm filtering may help you solve this issue if you have not implemented it yet (make sure that your filter ids compatible with methanol though)
Finally: what concentation is your "very diluted" solution?
Wow, thanks a LOT for your helpful discussion.
The concentration of chitosan in 0.001%. And I'll try to dilute it 20 times and then centrifuge the sample.
But does it mean the sample contains particle size larger then 10000nm which couldn't appear in the scale and this is the cause of high z-average value (2510 nm)?
Also, I'll measure the TEM to check if the particle size is confident with DLS or not?
Another point that I know the polydispersity (PdI) = the square of the standard deviation / mean diameter; and by this way the SD will be equal 2510 which is indescribable.
Ok, so 0.001% is like 10 mg/L, I am pretty sure that diluting a little further can yield better data.
If you have affordable access to TEM (SEM would work too), that will greatly help you interpret these DLS results.
From what I remember, DLS uses one approach to compute sizes that are smaller than the laser wavelength (typically 633 nm), and a different approach for larger sizes. I think that the smaller sizes are derived with better confidence from the scattering photons data, and objects larger than 10 µm are definitely out of range with this method and a red laser. (but it does not mean that their presence will not affect the result, and it may indeed be the cause of your high Z-ave value)
I am not sure about your equation for the PdI, but for sure a PdI of 1.000 is a sign of a highly disperse suspension (probably unfit for measurement by DLS as is) and by no means an accurate measurement.
Quite healthy discussions by Charles Fortier and Murat Sertkol ( however, Z-average is not distance between particles, it is average diameter of particles which had been calculated by a standard process of comparing them with a sphere). Electron microscopes will obviously give better and more confirmative values, before going for TEM, one should check samples in SEM/FESEM .This can give an idea about size and shape. As you may be knowing that DLS treats all particles as an sphere so results usually vary when they are observed under FESEM/TEM. Sample preparation is always important in all the cases. DLS calculates size by comparing number of particles hit by a Laser beam ( usually a large number compared to FESEM/TEM) and under electron microscope we selectively choose an area of our interest.
If particles are not spherical and either very large or too small ,then DLS is not a good option go for other techniques.
your PdI indicates a very broad size distribution >0.7 and light scattering techniques can't be used to accurately measure particle size. Using other measuring techniques could help. The z-average itself indicates presence of very large aggregates in your sample. Tweaking the method of fabrication could help.
how about the PDI is 1 but count rate is 50 kcps. basically we need to increase the concentration to increase the count rate. am i right? so how do i tackle the issues? my product is skincare cleansing.
- Badges
- Science topic