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The persistent 'memory effect' of triethylamine in atmospheric pressure ionization sources can be effectively controlled by decontamination with a iPrOH/AcOH mixture.

RATIONALE
Triethylamine (TEA)-containing mobile phases are commonly used in negative ion LC/MS separations of oligonucleotides and other compounds. However, TEA tends to cause persistent contamination of atmospheric pressure ionization (API) sources and LC system components resulting in intense background peaks in positive mode ESI mass spectra.


METHODS
LC/MS separations were carried out on a high resolution / accurate mass LC/MS system consisting of Agilent 1260 Infinity LC and Agilent 6530 QTOF mass spectrometer. Mobile phases containing 4-mM TEA and 100-mM hexafluoroisopropanol (HFIP) were used for negative ion LC/MS, and mobile phases containing 0.1% formic acid were used for positive ion LC/MS.


RESULTS
In accordance with previous reports, we observed that the use of TEA-containing mobile phases in negative ion LC/MS results in profound and persistent contamination of mass spectrometer's API source with TEA that greatly impedes the subsequent use of that instrument in positive ion mode. After unsuccessfully trying several different solvents, we found that the mixture containing 50% isopropanol (iPrOH), 10% acetic acid (AcOH), and 40% H2 O is very effective in removing such persistent contamination of API source and LC fluid path.


CONCLUSIONS
The simple, inexpensive, and broadly applicable decontamination procedure we described here can reduce TEA contamination of various API sources and LC systems to manageable levels and allow for uninterrupted operation of multi-user LC/MS instruments.