How is valve timing optimized on the Lachat QC8500 FIA?


Dokument-ID TE5438


Veröffentlichungsdatum 04.03.2022
How is valve timing optimized on the Lachat QC8500 FIA?
Setting time to valve for Lachat FIA
If valve timing is not correct, it is possible to inject a poor slug of sample, air or wash bath water. Valve timing is not just to fill the sample loop with sample. If the sample only fills the loop and then injects there can be some carrier mixed with the sample and a pure slug of sample will not be injected. This can cause less color development and more variability between injections of the same sample/standard. Correct valve timing will guarantee that the loop is rinsed with sample before actually injecting.

Most valve timing parameters are entered into the Omnion software following the parameters specified in the Lachat Quikchem method.
The exception to this is Time to Valve. Time to Valve will vary from system to system depending on lengths of tubing used between sampler and system as well as whether or not the system includes a dilutor.  The Time to Valve parameter must be measured and then entered into the Omnion software. 

Time to valve is determined by running dye as a sample and timing how long it takes to reach the valve. A stop watch or second hand will need to be referenced.   
  1. Run dye as a sample. 
  2. Start time begins when the probe drops into the sample. 
  3. Stop time is when the dye reaches port six of the valve being timed. 
  4. The time that transpires from start to stop is Time to Valve. 
  5. Enter the time in the Timing tab of the Run Properties Window. Select the channel for the valve being timed on the left hand side of the Timing tab and enter the value in the Time to Valve field.  Once it is entered into the software it can be verified by running the dye again. If it was set correctly, one can hear the valve first turn when the dye reaches the valve (within a second or so).
 Additional concerns that can effect Time to Valve
  • Sample line pump tube - It is helpful to trim the sample line pump tube. As the sample must travel through this pump tube to reach the valve its length affects the Time to Valve. Pump tubes have specifications for inner diameter but do not have a specification for length and this can vary from lot to lot. It is recommended to trim each pump tube used for the sample line so that there is one inch outside of each tab. If the sample line pump tubes are each trimmed to the same length every time there should not be a significant change in Time to Valve when this pump tube is changed.
  • Flow concerns - If time to valve is measured multiple times and it is not consistent, it indicates the sample line has a flow issue of some sort. This could be an old pump tube, a valve in need of service, dirty waste line, clog/crimp in tubing etc. The valve injects the dye onto the manifold when it turns the second time. If the dye does not get injected or struggles to do so, then there is an issue with the flow of the carrier line. 
  • The procedure to determine time to valve in this article should be followed for most one channel methods. An exception to this would be inline digestion/distillation methods and any method which uses a dialysis block prior to the injection valve. 

How can time to valve be checked for a Lachat inline method?
How is valve timing optimized on the Lachat QC8500 FIA when running more than on channel at a time?

The Universal Dye sold by Lachat is obsolete. Please reference the following article: Is there a replacement for the Lachat Dye, Universal 250ml?


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