For the Zetasizer Nano S, S90, Z, ZS or ZS90 instruments, the minimum temperature specification is 0°C . This technical note contains details of the experimental procedures and results obtained from measurements of a NIST traceable polystyrene latex as demonstration of this minimum temperature specification for the Zetasizer Nano range of instruments.
The specification of the temperature range is 0ºC to 90ºC, and the accuracy 0.1ºC at 25ºC, 0.2ºC at 0ºC and 0.5ºC at 90ºC. An extended high temperature option is available that extends the temperature range to 120ºC with an accuracy of 1ºC.
Measuring the absolute temperature of the sample in the cuvette accurately without affecting the measurement requires specialist equipment, so it is not possible for the laboratory scientist to check this parameter.
However, there is a way of checking that the temperature is stable over a range of values including zero degrees. This is by measuring a known size material, in this example a (nominally) 60nm latex standard in 10mM NaCl, at temperatures over which the sample is known to be stable, and observing that the results are repeatable.
The polystyrene latex standard sample was prepared for measurement by adding 4 drops to 20ml of the 10mM NaCl solution.
A temperature trend measurement was used from 20° to 0°C at 5° intervals with a 120 second equilibration time at each temperature. Three repeat measurements were taken at each temperature.
For these tests where the sample is dilute, and the dispersant is water, the Zetasizer calculated the viscosity automatically from the temperature at the outside of the cuvette. This will be within 0.1ºC of the set temperature.
The results shown in figure 1 and summarised in table 1 demonstrate an automated trend plot, with three repeat measurements at 5 degree intervals from 20ºC to 0ºC. The individual temperature points show excellent repeatability, and all the results are within specification for the latex used. The trend line shows a change of 0.4nm over the temperature range. This is on average of ± 0.2nm over the range, which is equivalent to a variation in temperature of ± 0.1ºC.
|
| Temperature (°C) | Measurement | Z-Average Diameter (nm) | Polydispersity Index |
|---|---|---|---|
| 20 | 1 | 63.5 | 0.002 |
| 2 | 63.3 | 0.017 | |
| 3 | 63.2 | 0.020 | |
| Mean (S.D.) | 63.3 (0.15) | 0.013 (0.010) | |
| 15 | 1 | 63.1 | 0.013 |
| 2 | 63.0 | 0.012 | |
| 3 | 62.9 | 0.040 | |
| Mean (S.D.) | 63.0 (0.12) | 0.022 (0.016) | |
| 10 | 1 | 62.4 | 0.023 |
| 2 | 62.5 | 0.010 | |
| 3 | 62.5 | 0.013 | |
| Mean (S.D.) | 62.5 (0.01) | 0.015 (0.007) | |
| 5 | 1 | 62.3 | 0.010 |
| 2 | 63.0 | 0.006 | |
| 3 | 62.5 | 0.020 | |
| Mean (S.D.) | 62.6 (0.38) | 0.012 (0.007) | |
| 0 | 1 | 63.2 | 0.048 |
| 2 | 62.7 | 0.004 | |
| 3 | 63.6 | 0.021 | |
| Mean (S.D.) | 63.2 (0.46) | 0.024 (0.022) |
If the system was not capable of reaching the set temperature, the viscosity calculated by the Zetasizer or input manually for this temperature, would be the wrong value, and an incorrect size would have been calculated. At zero degrees, a temperature error of 1ºC for example, would be equivalent to a size error of 3.2%, or 2nm for this sample.
In order to do measurements below the dew point, which is often at temperatures around 10ºC in temperate climates, a supply of dry air to the inlet at the back of the Zetasizer is required to prevent condensation on the surface of the cuvette. Condensation must be avoided as it will affect the measurement results.
The results reported in this technical note confirm that the minimum temperature specification for the Zetasizer Nano S, S90, Z, ZS or ZS90 instruments is 0°C.
