Difference between revisions of "Measuring the Conductivity of a Liquid in a Pipette Tip"

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''The conductivity of a solution inside a pipette tip can be measured using the [https://www.edaq.com/ER818 ER818] Octal Contactless Conductivity System.''
 
''The conductivity of a solution inside a pipette tip can be measured using the [https://www.edaq.com/ER818 ER818] Octal Contactless Conductivity System.''
[[File:Setup C4D ER818 ET128.jpg|450px|thumb|right|Figure 1. Equipment setup with ER815 and ET128]]
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[[File:Setup C4D ER818 ET128.jpg|300px|thumb|right|Figure 1. Equipment setup with ER815 and ET128]]
  
 
=== Introduction ===
 
=== Introduction ===
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[[File:Sample tube types C4D ER818 ET128.jpg|300px|thumb|right|Figure 2. Typical sample mediums]]
 
[[File:Sample tube types C4D ER818 ET128.jpg|300px|thumb|right|Figure 2. Typical sample mediums]]
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Typical sample mediums are shown in Figure 2. All are based on individual samples, except for the 1600 µm outer diameter (OD) capillary, which allows for either a continuous or sampled liquid to be monitored.
 
Typical sample mediums are shown in Figure 2. All are based on individual samples, except for the 1600 µm outer diameter (OD) capillary, which allows for either a continuous or sampled liquid to be monitored.
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[[File:Hamilton pipette tip inside C4D ER818 ET128.jpg|300px|thumb|right|Figure 3. Hamilton pipette tip and the C4D electrodes positioning]]
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[[File:Hamilton pipette tip inside C4D ER818 ET128.jpg|100px|thumb|left|Figure 3. Hamilton pipette tip and the C4D electrodes positioning]]
 
The system uses a [https://www.edaq.com/c4d-contactless-conductivity-introduction capacitively-coupled contactless conductivity] measurement (C4D). This applies a high frequency signal to an excitation electrode which couples the signal to the sample. A detection electrode located some distance from the sample measures the resulting current, which is proportional to the sample conductivity. The measured quantity will depend on the physical dimensions of the test volume and its conductivity.
 
The system uses a [https://www.edaq.com/c4d-contactless-conductivity-introduction capacitively-coupled contactless conductivity] measurement (C4D). This applies a high frequency signal to an excitation electrode which couples the signal to the sample. A detection electrode located some distance from the sample measures the resulting current, which is proportional to the sample conductivity. The measured quantity will depend on the physical dimensions of the test volume and its conductivity.
  

Revision as of 18:32, 2 January 2017

The conductivity of a solution inside a pipette tip can be measured using the ER818 Octal Contactless Conductivity System.

Figure 1. Equipment setup with ER815 and ET128

Introduction

The ER818A system was developed to enable the measurement of conductivity of samples with the following general characteristics:

  • Low sample volumes
  • Low sample wastage
  • Zero cross-contamination
  • 100 µS/cm to 20,000 µS/cm conductivity range
  • Ready operation in an automated system
Figure 2. Typical sample mediums


Typical sample mediums are shown in Figure 2. All are based on individual samples, except for the 1600 µm outer diameter (OD) capillary, which allows for either a continuous or sampled liquid to be monitored.

1 Straight tube tip

2 Tapered tube tip

3 Capillary tubing with 1600 µm OD

4 Melting point disposable glass tube with 1600 µm OD

5 NMR tube with 5mm OD


Figure 3. Hamilton pipette tip and the C4D electrodes positioning

The system uses a capacitively-coupled contactless conductivity measurement (C4D). This applies a high frequency signal to an excitation electrode which couples the signal to the sample. A detection electrode located some distance from the sample measures the resulting current, which is proportional to the sample conductivity. The measured quantity will depend on the physical dimensions of the test volume and its conductivity.

Equipment Required

  • ER818 Octal Contactless Conductivity System, which includes:
    • ER815 Contactless Conductivity Detector
    • ET128 Octal headstage
  • A computer with your own software to record the data (for example, Tera Term, LabVIEW, C#, WinWedge or HyperACCESS)
  • Your pipette tips with samples
  • A standard solution with conductivity in the middle of the measuring range, say 5 mS/cm

Experiment Procedure

A Quick Start guide describes how to connect the system, how to use Tera Term software to calibrate the system using a standard solution, adjust the gain and measure the conductivity of an unknown sample. The Quick Start guide can be downloaded from here.


Notes

Important Note about Temperature

The volumes of liquid involved in these measurements are very small and subject to temperature variation. Typically there will be a 4 to 5 °C temperature difference between the ambient laboratory air and the internal temperature of the headstage. This corresponds to a potential error of approximately 5-6%.

Since the internal temperature of the headstage is available – some temperature compensation can be performed.

Allowing some time for the temperature to equalise can also help in reducing the temperature error. Another approach could be to measure the conductivity as quickly as possible before the sample temperature increases.

Use of a Reference Channel

In some applications where small changes in conductivity are to be measured, a reference channel is provided which operates as follows.

A reference solution is selected, say 3 mS/cm. The unknown solutions to be tested have a conductivity of say 3 mS/cm +/- 1 mS/cm. By selecting the reference solution, it effectively subtracts this value from the unknown solutions so that only the changes are displayed. An advantage of this technique is that any temperature drift in the reference will be removed from the unknown solutions, thereby providing temperature compensation.