Selection Criteria for a Discrete Analyser

The automated chemical analyzer industry has seen the traditional Continuous Flow Analyzer (CFA), having an alternative technology available with the Discrete Analyzer.

Discrete analysers were developed in the mid-2000’s for the large clinical laboratories.  The discrete analyzer works by the combination of: special robotic probe/s, accurate stepper motors and pumps to control the fluidic mixing and movement of samples and colour reagents.

Direct or Indirect Read

Discrete Analysers can be differentiated into 2 formats; direct or indirect reading.

The direct reading discrete analyser transfers via the robotic arm, the sample and reagents direct to the cuvette where the optical density will be read.

The indirect reading Discrete Analyser  initially transfers the sample and reagents to an intermediate reaction cuvette where the colour development progress’. Once the sample colour has saturated, optical density plateaus in the reaction cuvette, it is pumped through a common flow-through cell, used for all samples where the optical density is measured.


Benefits of Direct or Indirect reading


Indirect reading Discrete Analysers have the following benefits:

  1. disposable, single use reaction cuvettes,  no clean up of cuvettes
  2. cheaper instrument purchase price


Direct reading Discrete Analysers have the following benefits

  1. reaction occurs in the final measuring cuvette, no intermediate reaction cell cost
  2. greater accuracy with less transferring of solutions
  3. less reactant needed, lower consumable costs
  4. faster sample throughput
  5. no air-bubbles in the measurement cell. Indirect flow through cell can create air bubbles.


Other considerations with different sample types, to consider are:

  1. blank out initial sample colour, ie chlorophyll in plant samples, wine colour, tannins in soil.
  2. over-range samples will need diluting with the extractant rather than DI water. For several different sample extractants there will be a need for matching extractants to be available to the discrete analyzer.
  3. Nitrate analysis will require either a Cd reduction coil or use of the Hydrazene method.
  4. With some in-direct discrete analysers, reactants used in one method may contaminate another analyte, i.e. ammonia and nitrate. Check with existing users.
  5. Electrochem measurements such as pH and conductivity are possible as well as colorimetric assays.