Three methods to verify dynamic light scattering systems with Latex Standards
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. DLS is all physics: laser wave length, scattering angle, refractive index of the dispersant are all known parameters in advance. The optical setup and sample determine the selection of these. 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.
How can I test a latex standard in my Zetasizer?
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. To pass, the requirement is for the z-average size to be between 300nm to 400nm diameter. This test is part of the automated installation test macro that you (or your service engineer) ran at the initial system 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 standard. [ 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]. We can take 1 drop from the stock bottle into 5mL of 10mM NaCl saline solution. [Filter the saline through 0.2 μm pore size first.) 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 latex 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”
PPS: here is a selection of Polystyrene Latex Nanosphere Size standards suitable for the Zetasizer, available from the malvernstore website: 20nm, 30nm, 40nm , 50nm, 60nm, 70nm, 80nm, 90nm, 100nm, 125nm, 200nm, 220nm, 240nm, 300nm. All are highly uniform, calibrated by NIST traceable standards, 1% solids in 15mL, optimized for easy dispersal and colloidal stability.
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