Type of Procedure
This is not a detailed set of instructions for using the NanoStar. It is a set of notes on various topics which may be of interest to users. For operating instructions, please contact Mike.
1. Dust and other contamination:
Wyatt makes a point of noting that dust in solutions can be a problem with their instrument. If you are working with material straight of a SEC column, it’s unlikely to be a concern, but otherwise you should be aware of the issue. Wyatt’s recommendations for removal of dust are either filtration through a 0.2micron filter or spinning the sample for 10-15 minutes at 3000 rpm (sic). Since they don’t specify the rotor, a reasonable guess would be an RCF of 1000-1500 x g.
2. Temperature control (or Why is there a N2 tank on the benchtop?):
The NanoStar is equipped for temperature controlled measurements. Data can be collected between 4°C and 100°C with a claimed accuracy of 0.01°C. Operating temperature is set in the Dynamics software by opening the Instrument panel in the Parameters node and entering a value for Set Temp (°C).
If the set point is 20°C or lower, the nitrogen purge gas must be used to carry away condensation from the Peltier cooling unit. Open the main stem valve and set the line pressure to 20psi with the needle valve.
In practice, it takes only a few minutes to reach and stabilize target temperatures. The NanoStar ramps at approximately 15 degrees per minute, so it should only take about 4 minutes to get from +4°C to 60°C (assuming an ambient temperature of ~25°C). If you notice longer stabilization time periods, please contact Mike.
3. How many readings should I take?
See page 2-14 of the Dynamics Users Guide
Sigma 0.3 micron latex bead suspension (LB3-1ML) diluted 1:2000 gives a signal in the 3.0e6 counts/second range with a mean population diameter of about 500nM and a %polydispersity < 10.
Sigma 5nm gold colloid suspension (G1402-25ML) diluted 1:10 also gives a strong signal (2.5e6 cnt/s) but has a bimodal population. SoS’s are in the 500 range and while there is a small peak at a diameter of about 5nm, there is a larger peak around 50nm
A 1:10 dilution of 2mg/ml BSA was also used. Counts were lower (3e5) and had much larger variation in SoS’s. Compiling SoS’s under 500 or so gave a reasonable estimate of the population.
5. Is this data any good?
There are some limitations to what the DLS unit can do. A mixture of particles with similar radii of hydration (less than a factor of 5) cannot be resolved. Th polydispersity index will increase, but only a single size will be reported.
The Wyatt handbook states that for a homogenous, monodisperse particle population, the SoS value reported on the List view screen should be below 20. While this is a good number to track, there are a few other things to pay attention to as well.