Accurate Viscosity Measurements of Small Samples

Most viscometers available in the market require in excess of 1ml of sample volume for viscosity measurements. When measuring viscosity with micro-volume sample levels, accuracy cannot be guaranteed. This makes the device unsuitable to measure high shear rate, and for that reason micro-volume measurements are restricted to shear rates of up to 5,000 s-1.

The m-VROC viscometer from RheoSense precisely and reliably performs viscosity measurements with only 50µl of sample. Moreover, it can reliably and accurately measure viscosities of samples volumes down to 20µl, making it suitable for use in areas requiring extremely small sample volumes, such as high-cost and limited early stage drugs. This article discusses a test method of measuring viscosity for these sample volumes.

Instrumentation

Along with m-VROC viscometer, RheoSense recommends the following accessories for micro-volume measurements: Pipette (PN: H-100-2); Compressed Gas Coupler (PN: VROC-800-4); and 100 µl Zero Dead Volume Syringe (PN: VROC-300-100-0 ).

VROC Cell Cleaning and Preparation

Proper cleaning and preparation of the VROC cell is essential for reliable and accurate micro-volume measurements. At first, the VROC cell is continuously flushed with an appropriate cleaning agent and the cleaning agent’s viscosity is continuously measured at the same time. The flushing continues until the measured viscosity equals the standard value estimated for the cleaning agent. This indicates that all previous materials have been removed.

Figure 1. Compressed Gas Coupler Connection

The Compressed Gas Coupler’s inflow port is then connected to any air or inert gas line, as shown in Figure 1. It is necessary to use a very clean dry air or nitrogen gas with an output pressure set to 1psi. The coupler’s outflow port is then connected to the VROC cell valve inlet and the chip is dried for five minutes by the passing nitrogen gas. After five minutes, the nitrogen supply is withdrawn and the coupler is removed from the VROC cell.

Sample Preparation

After cleaning and drying the chip, the next step is the sample preparation. RheoSense suggests the use of the zero dead volume syringe to obtain the 20µl volume level. However, the standard 100µl syringe can also be utilized by sacrificing the sample remained in the syringe at full compression. When using the 100µl syringe, care must be taken to prevent air bubble entrapment.

Experimental Procedure

The 100µl syringe is carefully connected to the VROC cell without introducing any air bubbles. The m-VROC pusher block is then positioned so that the pusher block of the pump makes contact with the plunger of the syringe, thereby ensuring that the actual dispensed volume will match the reported dispensed volume. The sample temperature is then allowed to become equal to the syringe jacket temperature.

About 12µl of sample is then injected into the VROC cell to prime the VROC cell channel. This is the volume of the VROC channel and tube/valve fittings. The m-VROC Control software is used for this purpose. The following settings are entered into the -VROC Control software to dispense 12µl into the VROC channel:

Flow rate (μl/min) Meas. Time (s) Pause time (s) Volume (μl)
100 6 1 12

The following steps followed for viscosity measurement:

  1. The viscosity is measured by entering the following settings into the m-VROC software:
    Flow rate (μl/min) Meas. Time (s) Pause time (s) Volume (μl)
    100 1.1 1 3.5
  2. The reliability of the viscosity measurement is confirmed by observing whether the value of the R2 number displayed in the m-VROC software is ≥0.99.

If the R2 value is below 0.99, then the VROC cell channel may not have been completely primed. So again, the aforementioned steps are followed to obtain a good measurement. If the R2 value is ≥0.99 on the first run, then a second viscosity measurement is performed by repeating the aforementioned steps with the remaining sample volume.

Measurement Results

The analysis of a 100 mg/ml BSA solution was performed by following the aforementioned procedure for demonstration purposes. The results obtained are shown in the following table:

Flow rate (μl/min) Meas. Time (s) Pause time (s) Volume (μl) Viscosity, mPa-s
100 6 1 12 -
100 1.1 1 3.5 ~1.49
100 1.1 1 3.5 1.50
100 1.1 1 3.5 1.50
200 1.1 1 3.5 1.50

Figure 2. Expected Viscosity Values of BSA Solution with Time

Figure 2 illustrates the data of the third row in the table, showing the expected viscosity values measured with time after filling the flow channel before viscosity measurement.

Conclusion

The aforementioned test procedure shows the possibility of measuring viscosity for sample volume down to 17µl. Nevertheless, the recommended volume is 20µl, owing to variations that happen between individual measurements. Practice and familiarity with the measurement device is also important for successful and error-free measurements.

About RheoSense

RheoSense Inc., is a global viscosity measurement equipment manufacturer. Its VROC (Viscometer-Rheometer-On-a-Chip) technology is the only MEMSmicrofluidic chip-based viscometry technology in the world.

This innovation provides accurate, repeatable viscosity measurement using today’s technology to reduce device footprint and sample size and is being readily adopted in BioPharmaceutical, Blood, Ink and other applications.

This information has been sourced, reviewed and adapted from materials provided by RheoSense.

For more information on this source, please visit RheoSense.

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