The Three Classes of C4D Products: Detectors, Monitors and Meters

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Figure 1. The C4D application matrix

Introduction

This document describes the relationships between various products, developed by eDAQ, based on the use of the Capacitively Coupled Contactless Conductivity Detection method (C4D) in various applications. These applications are divided into three classes described in Figure 1.

Class I: C4D Detectors

These devices are optimised to detect small changes in conductivity caused by analytes in chromatography systems. Output is in mV. Typically the buffer conductivity should be in the range 20µS/cm to 20mS/cm. The inner diameter of the capillary can be adjusted to suit background electrolyte/buffer conductivity, or the buffer conductivity can be modified to suit the capillary used.

Key issues for detectors

  • Sensitivity: More is better.
  • Spatial resolution: Must resolve peaks as narrow as 1 second.
  • Speed: up to 100 data points per second required.
  • Mechanical arrangement: must fit in instrument manufacturer’s enclosure.

Applications of Class I C4D Detectors

  • Capillary Electrophoresis (CE)
  • Microchip Electrophoresis (MCE)
  • High-Performance Liquid Chromatography HPLC
  • Ion Chromatography (IC)
  • Flow Injection Analysis (FIA)

Features specific and common to C4D detectors

  • Best sensitivity achieved with high effective resistance between electrodes
  • Suitable for 365/800/1600 µm OD capillaries and tubing.
  • Sensitivity is improved with smaller capillary ID and larger excitation voltage.
  • Buffer conductivities above 10mS/cm should be avoided
  • Excitation Frequency range: Sinusoidal 50kHz to 1500kHz
  • Excitation voltage: 10Vpp to 100Vpp
  • Configuration: Balanced AC Bridge
  • Electrode separation: small to provide good peak resolution

Advantages

  • NO CONTACT WITH SOLUTION - no cross contamination
  • SYSTEM LONG TERM STABILITY - Electrodes do not degrade
  • LOW VOLUMES - very low volumes can be measured
  • FLOW THROUGH - ideal for capillary based systems
  • FAST RESPONSE - Max 100 samples/sec
  • Can detect colourless analytes
  • CHEMICALLY INERT capillary used

Available products

  • ET120: This headstage was developed specifically for CE applications. It is based on the use of nominal 380 µm OD capillaries.
  • ET130: This headstage was developed for HPLC and IC applications. It is based on the use of nominal 1600 µm tubing. The options for 800 µm and 380 µm OD tubing are available with the ET131 configurable headstage. The ET125 is a general purpose headstage for 1600 µm OD tubing.
  • ET121: This headstage is developed for MCE applications.
  • ET225: This headstage was developed specifically for MCE applications using Micronit chips.

SUPPORT HARDWARE: The headstages above are suppourted by ER225 Data system (with Digital I/O), ER815 and ER825 single and multiple channel systems. Supported by Chart and/or PowerChrom software or by user provided data systems.

Class II: C4D Monitors

These devices are used to measure or monitor conductivity changes in solutions flowing in tubes. In this case, changes in conductivity are measured. Actual determination of conductivity in uS/cm units is not of paramount importance but is available with the correct software. These systems provide flexibility to meet different applications.

Features specific and common to C4D Monitors

  • Suitable for 1600 µm OD capillaries- other OD by special order...
  • Buffer conductivities above 20mS/cm should be avoided.
  • Excitation Frequency range: Sinusoidal 10kHz to 2000kHz
  • Excitation voltage: 1Vpp to 100Vpp
  • Configuration: Single Ended or Balanced AC Bridge
  • Electrode separation: can be adjusted to suit application 4 to 40mm
  • These devices are flexible and can be adjusted to meet customer applications

Applications of Class II C4D Monitors

  • Changes in arctic melt water conductivity
  • Detection of salinity changes in desalination experiments
  • Detection and characterisation of bubbles in a tube
  • Change in conductivity in analytical experiments
  • Titration monitor both single and multi-channel
  • Detecting particles flowing in a capillary

Available Products

  • ET125/ET130/ET131 – Supported by ER225 Data system (with Digital I/O), ER815 and ER825 single and multiple channel data systems supported by Chart and/or PowerChrom software or by user provided data systems. Available with 1600µm (1/16 inch) capillaries Other OD by special order.
  • ET127 – experimental system developed to measure relatively slow conductivity changes. Supported directly by customer supplied data system. Subject to special Quote.
  • EPU359 – experimental system developed to measure conductivity in 1600u capillaries. Supported by customer data system or by direct serial output. Subject to special Quote until official release in 2018. Units supplied to customers and currently in use.
  • EPU360 – experimental system based on isoPod configuration supported by Pod-Vu and serial protocol. Subject to special Quote until official release in 2018. Pre-production units available soon.
  • ER818 – Octal C4D system developed to measure the conductivity of a CONTINOUS flowing liquid in a tube or capillary, or SAMPLED system where the sample can be presented in a suitable pipette, melting point tube or NMR tube. Supported by ER815 system in the range of 20 µS to 20 mS/cm. Samples to be presented in a suitable glass tube up to 5000µm OD.

Class III: C4D Meters

Single channel C4D meter

  • EPU359 – experimental system developed to measure conductivity in 1600 um capillaries. Supported by customer data system or by direct serial output. Read out in mS/cm available. Subject to special Quote until official release 4th qtr 2018. Units supplied to customers and currently in use. Available on special order.
  • EPU360 – experimental system based on isoPod configuration supported by Pod-Vu and serial protocol. Subject to special Quote until official release in 2018. Read out in mS/cm available. Subject to special Quote until official release 4th qtr 2018. Units supplied to customers and currently in use. Available on special order.

Multi-Channel C4D Meter

  • ER818 Octal C4D system developed to measure the conductivity of a CONTINOUS flowing liquid in a tube, capillary or SAMPLED system where the sample can be presented in a suitable pipette, melting point tube or NMR tube. Supported by ER815 system in the range of 20 µS to 20 mS/cm. Read out in mS/cm available. Samples to be presented in a suitable glass tube up to 5000 µm OD. A number of systems delivered to customers – available on special order.

Laboratory C4D Meter:

These devices, currently under development, will function as normal “stand alone” laboratory contactless conductivity meter– output is in mS/cm - with three ranges planned 20, 2 and 0.2mS/cm. This system will allow conductivity of a solution in a suitable tube to be measured. Currently the likely choice of the sampling tube is a 5mm OD NMR tube. This device will have fixed specifications and be easy to use. This product is a long term development.

Advantages of C4D Meter

  1. NO CONTACT WITH SOLUTION – no cross contamination
  2. SYSTEM LONG TERM STABILITY - Electrodes do not degrade
  3. LOW VOLUMES – very low volumes can be measured
  4. CONTINUOUS OR SAMPLED based systems
  5. Fast response
  6. Chemically inert tubing can be used
  7. Direct output in mS/cm

APPENDIX

C4D Measurement System Parameters and Variables.

Conductivity of liquids covers 7 orders of magnitude!

  • 1 uS/cm
  • 10 uS/cm
  • 100 uS/cm
  • 1 mS/cm
  • 10 mS/cm
  • 100 mS/cm
  • 1000 mS/cm NO!

A single instrument cannot cover the complete range from 1 uS/cm to 1000 mS/cm. A practical goal is to be able to cover three orders of magnitude within the above range.

At the low end, conductivities of less than 10 µS/cm can prove difficult to measure. For example, deionized water is <2 µS/cm and normal drinking water is in the range 50-1000 µS/cm. At the high end, measuring above 100 mS/cm is also difficult to measure with conventional systems. For example, seawater is 53,000 µS/cm or 53mS/cm. eDAQ C4D measurement systems have a number of variables or parameters that can be selected or adjusted in order to achieve the required application measurement range. The C4D measurement system variables are described in Figure 2.

Figure 2. C4D measurement system variables

The following parameters are adjustable or selectable in C4D systems:

  • Sine Wave Excitation Voltage = E can vary from 1 to 100 Vpp - Readily adjustable.
  • Square Wave Excitation Voltage = E can vary from 0 to 4 Vpp - Readily adjustable.
  • Electrode Gap = g can vary from 2mm to 100mm.
  • Electrode Length = e can vary from 1mm to 10mm.
  • Tube/Capillary OD = D can be chosen from 380, 800 1600 and 3200µm.
  • Tube/Capillary ID = d can be chosen from many sizes in 5 µm increments.
  • Buffer resistance Rb = (g+e) / ρA where A = π (d/2)2
  • Buffer Conductivity = ρ in S/m

Coupling Capacitance Cc = 2π εr ε0 e / (ln⁡ (D/d))

Note for Silica εr=4

For best results the product of Rb and Cc should be greater than 10-8