Iron oxide nanoparticles have been measured with DLS before and should work if prepared in a stable suspension. It is not clear how different your results from your expected size are, a few nanometers could be due to hydration and different preparation conditions. More significant differences could be due to aggregation, and the stronger scattering from the larger species 'overshadowing' the signal from the smaller components.

Also keep in mind that sizes stated by suppliers are often based on number-based techniques such as TEM, and thus the reported values would often be different (usually smaller) than the z-average in DLS.

http://www.materials-talks.com/blog/2014/08/05/faq-how-important-are-refractive-index-absorption-for-nanoparticles/

http://www.materials-talks.com/blog/2014/01/23/intensity-volume-number-which-size-is-correct/

http://www.materials-talks.com/blog/2014/07/10/faq-peak-size-or-z-average-size-which-one-to-pick-in-dls/

Iron oxide nanoparticles have been measured with DLS before and should work if prepared in a stable suspension. It is not clear how different your results from your expected size are, a few nanometers could be due to hydration and different preparation conditions. More significant differences could be due to aggregation, and the stronger scattering from the larger species 'overshadowing' the signal from the smaller components.

Also keep in mind that sizes stated by suppliers are often based on number-based techniques such as TEM, and thus the reported values would often be different (usually smaller) than the z-average in DLS.

http://www.materials-talks.com/blog/2014/08/05/faq-how-important-are-refractive-index-absorption-for-nanoparticles/

http://www.materials-talks.com/blog/2014/01/23/intensity-volume-number-which-size-is-correct/

http://www.materials-talks.com/blog/2014/07/10/faq-peak-size-or-z-average-size-which-one-to-pick-in-dls/

Quasielastic light scattering is not a technique that you can easily use out of the box.

there are a considerable number of points at which life can become interesting.

I am going to start at the very beginning, so hopefully your reaction is "of course I did that".  Do you have a standard that can be used to test the instrument under exactly the same conditions? I strongly recommend carboxylate-modified polystyrene latex spheres; they are usually well behaved. With the standard: Is your spectrum close to a single exponential? What was the measured signal to noise ratio: The RMS spread of the measurements around the fitted smooth curve (single exponential or cumulant series).  The eye test is to prepare a semilog plot of the measurement (log of spectrum - baseline) against linear in time.   How far down can you follow the spectrum?  A few parts per thousand of the original amplitude is good.  If the spectrum fades into the noise when you are above 1% of the original amplitude, something is wrong.  Many things could be wrong.

What is the count rate (photons per second)?  If you are close to the limiting acceptable count rate (scattering too bright) you will have issues.  If you are down at, say, 10,000 counts per second, something is not good.  I can go on.

With respect to your sample: Is the sample close to water-white (i.e., transparent)? If you cannot see through it, you are getting multiple scattering.  How did you filter the sample?   I can go on. 

I should emphasize that the above is not a complete list of places where things can go wrong, just a starting point.

Dear Lael Badakhshaneian

As George Phillies presented in his answer, there is a number of issues that have to be considered when using DLS. If you are more of a beginner, I would recomend to visit the Malvern website (manufacturer of Zetasizer instrument) and have a look at the webinars there. They discuss many of the limitations and problems that might occur.

Regards

Josef Chmelar