What can causes negative peaks in a Lachat run?


Dokument-ID TE12214


Veröffentlichungsdatum 30.12.2019
What can causes negative peaks in a Lachat run?
Descriptions of some of the causes for negative peaks in a Lachat run and steps that can be taken to avoid them.
Here are some things that could cause negative peaks on a Lachat Flow Injection Analyzer (FIA):
  • Inverse or bipolar methods are intended to have negative peaks. To verify if the method is inverse or bipolar check the Lachat Quikchem™ method document. If the method support data on the method document shows negative peaks and the chemistry is defined as inverse or bipolar, then it is expected that the peaks would be negative. An example of a Bipolar method is Fluoride by Ion Selective Electrode (ISE). Some examples of inverse methods are Alkalinity and Hardness.
  • Carrier contamination can be a source of negative peaks. This is most common for Phosphorous and Nitrogen chemistries. If some of the analyte is present in the carrier solution, a negative peak will be generated if a solution is injected that is at a lower concentration than the concentration in the contaminated carrier solution. These peaks will look like normal peaks but upside down. The best way to resolve this is to remake the carrier solution.
  • A difference in refractive index between the sample and carrier solution can lead to negative peaks. This is most common with saltwater samples or soil extractant being injected into a deionized carrier water. In these cases the difference in density between the sample and carrier solution causes a difference in refractive index. The light which passes through the flow cell will be effected differently by different refractive indexes caused by differences in density. This will cause a fluctuation in the baseline at the start and/or end of the peak.
  • A valve turning artifact can also cause negative peaks. This can be from a problem with a valve, or too much back pressure on the manifold. When a valve turns there is a kick in the flow that manifests as a fluctuation in the baseline. This will typically occur at the start and/or end of the peak.
Because a fluctuation in the baseline leading to a negative peak at the start and/or end of the peak is a symptom of both refractive index as well as a valve turning artifact, here are a couple of ways to clarify which one is the cause of this symptom.
  • If the issue only occurs for a specific sample or specific samples, then it is likely caused by the refractive index. If the samples are rerun and the problem continues for the same samples but not for others, this makes it more likely to be sample specific issue.
  • If it is not clear if the concern is sample specific, or the issue has been replicated on all injections and the method is for digested or distilled samples, then it would be recommended to run carrier as a sample to see if the issue is replicated. Assuming valve timing is set correctly, the carrier will be injected into carrier. Since it is known that the solution is the same, the presence of baseline fluctuations would confirm the issue is caused by valve turning artifacts. If these fluctuations are absent when running carrier as sample, this would confirm the issue is caused by refractive index.
See also:
What causes refractive indexes and how can they be eliminated?
How can refractive indexes be dealt with for specific sample types?
What can cause a dip before the samples/standards that is not a refractive index?

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