Bleed is the loss of stationary phase. The column stationary phase is a long polymer chain of repeating groups. These polymers are also bonded to other polymer chains, this is called cross-linking, which helps to stabilize the phase and reduce the amount of bleed.
At the end of the chain, there is typically a more reactive group like an alcohol (PEG) or silanol (siloxane based phase). This more reactive group can react with a previous link in the chain causing a section of the polymer chain to be removed and elute as bleed. As you increase the temperature, the rate of this reaction increases, resulting in increased bleed, which is why the baseline rises as the column temperature increases.
Other things can also cause this reaction in the stationary phase. Very reactive groups like acids, bases, and oxygen can break apart these polymer chains and cause bleed. If reactive chemicals are injected, they will condense onto the phase of the column for long periods of time and have more time to cause damage while they are condensed. This is another reason why trimming the column may help restore chromatographic performance, as this removes the damaged portion of the column in addition to any contamination.
Note that bleed will occur over the entire length of the column and this results in a rise in baseline. If any peaks are observed, even if the ions are consistent with bleed, they are most likely coming from one specific source and being separated by the column. That is, these are likely impurities being “injected” onto the column resulting in peaks, and these are not column bleed.
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