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What is distortion for peaks in the chromatogram?

Distortion of peaks in the chromatogram usually is the result of a physical, rather than a chemical problem. When all peaks in the chromatogram show the same type of distortion, the initial problem occurred before the analytes began migrating through the column. The most

Common cause of this type of peak distortion is a partially blocked frit or a void at the head of the column. Columns are not very susceptible to voids unless they are operated outside of their recommended pH range.  Frit blockage remains a common problem.  

The frit at the column inlet typically is 2.0 μm porosity for 5 μm particle columns, and 0.5 μm porosity for columns ≤ 3 μm. If particulate matter from the sample, worn pump seals, or the mobile phase reaches the column, it usually collects on the frit. 

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This material can distort the distribution of the sample at the column inlet, that part of the sample reaches the column via a different flow path and thus later than another portion of the sample. Since no separation has taken place at this point, all analytes are distorted in the same manner and the chromatogram shows distortion for all peaks.

To prevent this problem, filter the mobile phase if it has potential to contain particles (e.g. buffer precipitate or dust); replace the pump seals before they wear enough to shed particles. If the sample contains particulate debris, either filter it or centrifuge it briefly to remove particles. 
To avoid this it is strongly recommend installing a 0.5 μm porosity in-line filter just downstream from the autosampler to catch any particulate matter that inadvertently enters the HPLC system. The system backpressure will rise when this in-line filter frit becomes blocked; replacement of the frit is a simple, fast, and inexpensive task. Use of an in-line filter is one of the least expensive ways to prolong the column lifetime.

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