What is the purpose of a back pressure loop added after the flow cell in some Lachat methods?

Document ID

Document ID TE7714

Version

Version 6.0

Status

Status Published

Published Date

Published Date 04/30/2019
Question
What is the purpose of a back pressure loop added after the flow cell in some Lachat methods?
Summary
The purpose of back pressure loops in Lachat methods.
Answer
In general, back pressure loops are added to help minimize air spikes. In some heated chemistries (like ammonia), as the temperature increases, gas solubility decreases and so air spikes are more likely to form. Back pressure loops are added to help force gas back into solution (increases in pressure result in increased gas solubility). For chemistries like phosphorous where the temperature only reaches 37 °C, back pressure loops may not be necessary depending on how many air spikes are observed.
 
If one decides to add a back pressure loop, additions of only 50 cm at a time are recommended (use the shortest length of coiled tubing that helps with air spikes). If too large of a back pressure loop is added, it could restrict and slow down fluid flow which will increase reaction time and result in increased peak heights over time. 

For other suggestions on reducing air spikes, see the articles: Do Lachat reagents have to be degassed with Helium? and Are there any tips for getting rid of bubbles in my Lachat FIA flow cell?

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