Ensuring an Accurate Result in an Analytical Instrumentation System Part 3: Maintaining a Representative Sample

Overview:

The objective of an analytical instrumentation (AI) system is to provide a timely analytical result that is representative of the fluid in the process line at the time the sample was taken. If the AI system alters the sample so the analytical result is changed from what it would have been, then the sample is no longer representative and the outcome is no longer meaningful or useful. Assuming the sample is properly taken at the tap, it may still become unrepresentative under any of the following conditions:
- If deadlegs or dead spaces are introduced at inappropriate locations in the AI system, resulting in a "static leak," a bleeding or leaking of the old sample into the new sample;If the sample is altered through contamination, permeation, or adsorption;
- If the balance of chemicals is upset due to a partial change in phase; or
- If the sample undergoes a chemical reaction.

This article will review the major issues leading to an unrepresentative sample and provide recommendations on how to avoid a compromised sample. It will discuss deadlegs and dead spaces; component design and placement; adsorption and permeation; internal and external leaks; cross contamination in stream selection; and phase preservation.