Three methods to verify dynamic light scattering systems
If you are a meticulous researcher you may want to test your new Zetasizer. Is it really providing the correct size? One of the advantages of dynamic light scattering is that there are no tweakable adjustments, it is all physics: laser wave length, scattering angle, refractive index of the dispersant are all known in advance and are given parameters for the optical setup and sample. Nevertheless, it is a good idea to check and verify the performance of the system. One of the most common methods for this verification is the use of latex standards. There are several ways to determine whether your dynamic light scattering system is OK. Here is a selection of the three most often encountered by our customers.
- First the easy convenient method: At Malvern, we have created a transfer standard that contains latex particles at an appropriate concentration, in a pre-loaded 10mL syringe for convenient use as-is. We use this as a quick performance test, and all that is required is for the z-average size to be between 300 to 400nm diameter. This test is part of the automated installation test macro that you (or your service engineer) ran when the system was initially set up in your lab.
- Next, let’s look at the IQ/OQ method: Here we can use a common sizing standard like the 60nm polystyrene latex. We can take 1 drop from the stock bottle ( do not shake the bottle, just gently invert, and possibly sonicate for 10 seconds, it is best to not take the first drop, but instead discard the first couple of drops to waste to avoid any aggregates in the dropper tip) into 5mL of 10mM NaCl saline solution (filtered through 0.2 μm pore size) and the resulting z-average size should be within 2% of the stated range on the bottle. For example, if the bottle states 60nm ± 4nm then we would expect the z-average to fall between 56*0.98=54.9nm and 64*1.02=65.3nm. There may also be a specific designation on the certificate indicating the hydrodynamic size as measured by DLS, for example, Hydrodynamic Diameter: 58 – 68nm (PCS).
- If that is still not good enough, try the Duke method: As stated by Layendecker, Duke, and Brown, the electrolytic activity of the diluent can have a significant influence on the surface chemistry and thus affect the measured hydrodynamic size. They argue that the lack of conductance in ultra-pure 18 MΩ MegaOhm water (<10 μS/cm) can lead to hydrophobic interactions which typically lead to larger apparent sizes and often larger polydispersity index values. To overcome this hydrophobicity, they recommend a solution of 0.16wt% of tetra sodium pyrophosphate (TSPP) for conventional polystyrene (but not carboxylated or other surface-modified) nano particles.
So the next time you are worried that your system is not working, try out a standard according to one of the above recipes. Hopefully it should put all worries to ease.
PS: there is also a technical note listing some results from standards “Measuring Latex Standards by Dynamic Light Scattering”
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