Particle size by tem found to be 350 nm while dls give size of 200 nm, what could be possible reason?
@ Rajat, probably during preparation of your samples in the dry state for TEM analysis some aggregation may occur or larger particles may come during sampling for TEM as sample size is very less for TEM (only a few hundred particles ) . When you have a monodisperse size distribution, ie all particles of the same size then a size measurement by TEM may give a size similar to that measured by DLS. TEM is a number based particle size measurement whereas DLS is an intensity based one. Therefore, DLS is more accurate than TEM in terms of size measurement.
@J.C. tarafdar sir can you please provide any reference regarding this explanation.
@ Rajat, please consult any nanoparticle characterization book. You may consult also our book Tarafdar, J. C. and Raliya R. (2011). The Nanotechnology. Scientific Publisher (India). pp. 215; or my book Tarafdar, J.C. (2021). Nanofertilizers: Challenges and Prospects. Scientific publisher (India). pp.363.
Rajat Garg Thank you for your question. It’s the usual scenario that number-based single particle electron microscopy results are smaller than the ensemble intensity based DLS results, so your results are unusual in this respect. It would be useful if you could post any TEM image where you have deduced 350 nm and your DLS result (with any error report) where we can see the width of the distribution. OK, possible reasons for this discrepancy:
• What is your system and the particle density? Do you observe settling over a period of time in the DLS system? This could be removing the larger material from the measurement zone. This is usually obvious to see in a series of measurements
• There is a major difference between ‘particle size’ and ‘particle size distribution’ and distribution is the bane of all size analysis techniques. In the whole history of electron microscopy, no more than a few tens of grams in total has been imaged. How many particles did you image to draw a conclusion of 350 nm? You need to measure 10000 random particles to get a standard error of 1% on the mean. It may be that you just haven’t got a representative sample in your microscopy result
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