Why are check standards increasing as the run progresses on my Lachat QC8500?
Check standards increasing over time
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.