How can time to valve be checked for a Lachat inline method?

Document ID

Document ID TE8476

Version

Version 2.0

Status

Status Published

Published Date

Published Date 06/07/2018
Question
How can time to valve be checked for a Lachat inline method?
Summary
Inline time to valve
Answer
Different considerations sometimes need to be made for valve timing for inline Lachat methods.  The image below shows an example of the various timing settings on an inline method.



Things to consider:
  1. Load plus inject in this example is less than the method cycle period in the capture. However, the time the valve spends in the load load state plus the time spent in the inject state will always equal cycle period, even if the times specified for load and inject do not..
  2. Load periods are chosen to allow the sample loop to fill completely plus have excess on either side of the valve when it turns to inject.
  3. Probe in sample should be longer than load --The additional time is allowed to flush the loop, and there should be sample on both sides of the loop when the valve turns to inject. This extra time gives some leeway for pump tube wear, and for faster and slower flows when pump tunes are changed, without always having to make changes in the time to valve.
  4. Unless the user changes the sample loop size, load time will not change. It can be made shorter at times, to improve throughput and minimize the amount of sample needed, but this is very unusual and not supported.
  5. Inject period will always be the difference between the total cycle period, and the load period. (In the instance above, load is 80s so inject will be 120s).
  6. When doing the dye test (not possible with all in-line methods as this is not volatile and will only pass through the membrane when it is wetted!), verify that there are roughly equal portions of sample on either side of the valve when the loop is loaded and the valve turns. It is crucial that the center of the sample slug is the portion that is injected.
  7. We would not generally expect to see a difference outside of +/-20% of the time to valve provided in the method unless the user has a very different set up. So, if the method states 200s time to valve, it is likely that it will actually fall between 180 and 220s. For a “regular method” if the time to valve is 25 in the method, it is likely to fall within the 20-30s range.
  8. On an in-line method, for example with a published time to valve of 200s, if you see 400 or 500 or 600 seconds- something is wrong (Between the sampler, and port 6 on the valve where the sample enters).
Of all of these, TIME TO VALVE is the parameter that may vary between sites and instruments. This is true even for non-in-line methods.



For in-line methods that have a gas diffusion step:


Timing can be difficult and there is unfortunately not a simple way to determine this.
  1. A first step is to put the probe directly into a larger volume of sample. Allow the instrument to go through 5 or so injection cycles. This takes timing out of the equation as sample is continuously pumping to the valve.
  2. Monitor the peak heights and areas: later, this will help you to determine when the timing is correct as it is the maximum signal you can get for that standard.
  3. Set up a series of runs using super scheduler, or work your way through until you get peaks similar to those acheived in the first two steps. The time to valve in the method can be the mid-point, with longer and shorter times also chosen (180, 190, 200, 210, 220, etc,.). Usually, what you see are smaller peaks, which get larger until they are about equal to those with the probe in sample, followed by smaller peaks again.
  4. Do additional fine tuning if needed.

Examples of peaks using the 'probe in standard' technique:



Examples of peaks using super scheduler:




 

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