Why are check standards increasing as the run progresses on my Lachat QC8500?

Document ID

Document ID TE7975

Version

Version 3.0

Status

Status Published

Published Date

Published Date 05/21/2019
Question
Why are check standards increasing as the run progresses on my Lachat QC8500?
Summary
Check standards increasing over time
Answer
There are a number of things to consider:
  • Change in temperature. The software will give an error if the temperature gets more than 2 degrees from the set point so it would not typically be a heater issue. However, concerns can arise if reagents and/or samples/standards are refrigerated and not at room temperature when the run is started. It can be a problem if the room temperature is not stable, which would include the instrument or solutions being located near a vent or window. Even if the chemistry is a heated chemistry, further color can develop if the temperatures of solutions change during the run.
  • Flow issues. This could be from a clog or crimp anywhere in the manifold, but the waste line is a common source and easy to isolate. The waste line (the line leaving the flow cell and going to waste) may be restricted, or the line may be submerged in a waste container. Remove or replace the waste line to prevent any back pressure occurring because of a partial clog. If the line goes into a waste container, make sure the line is not submerged in the waste water. If the waste line can be detached and the waste can be collected in a beaker the waste line can be ruled out (or perhaps confirmed) as the issue.
  • Make certain PVC pump tubes are being used if the Lachat method specifies them. PVC tubing will be specified for high back pressure methods as duraprene/santoprene will not handle back pressure as well.
  • A pump speed test can be performed to verify the pump speed is steady.
  • Airborne contamination.
  • Evaporation out of the sample tube, such as if a draft blows over the sampler or the lab is hot.
  • Check for staining on the manifold or in the flow cell. If this occurs the baseline is most likely drifting upwards as well.

Was this answer helpful?

Submitting...
Thank you for your feedback!
There was an error with your submission. Please try again.