How can the presence of a Water Dip in a Lachat Chromatogram be used for troubleshooting?

Document ID

Document ID TE12788

Published Date

Published Date 12/09/2020
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Question
How can the presence of a Water Dip in a Lachat Chromatogram be used for troubleshooting?
Summary
Lachat Omnion IC chromatogram
Answer
A water dip will occur in a Lachat Ion Chromatograph (IC) for anions when the ionic strength of the sample is different than the ionic strength of the eluent.
 
The water and cations from the sample are not retained by the column (the anions are retained). These will displace a volume of eluent (carbonate and bicarbonate) when the sample is injected. If the ionic strength of the cations in the sample is less than that of the eluent, this creates the negative "water dip" in the baseline.
 
If the ionic strength of the cations of the sample is greater than that of the eluent, then there is a positive "water dip". If the ionic strength of the cations of the sample is the same as the eluent, there is no "water dip" and the baseline should remain stable. 
 
The presence of a "water dip" is tied to the injection of the sample onto the manifold for IC (in most IC methods, there will a negative water dip). If no peaks are appearing in a chromatogram, and the signal corresponding to the suppresser cartridge going on and offline is visible, than the presence or absence of a water dip can indicate whether or not the 6 port IC valve is injecting sample (and by extension if the valve requires service).
 
Here are a few pictures of a "water dip" taken from the support data of methods 10-510-00-1-A (for the negative water dip) and 10-510-00-1-F (for the positive water dip):
 

 
 

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