Unexpected drop in sensitivity or low sensitivity for particular analytes - WKB230976

SYMPTOMS

• Unexpected drop in sensitivity
• Low sensitivity for particular analytes
• Slight shift in retention times and change in resolution
• Sensitivity on MS seems to be fine when checked with setup solution
• No issue with desolvation heater
• The same UPLC system and LC column are used for different assays and mobile phases

ENVIRONMENT

• Quattro Premier XE
• ACQUITY TQD
• Xevo TQD
• Xevo TQ MS
• Xevo TQ-S
• Xevo TQ-S micro
• Xevo TQ-XS
• Xevo G2
• Xevo G2-S
• Xevo G2-XS

CAUSE

1. Ion suppression caused by LC-MS formic acid in plastic bottle. Use of LC-MS formic acid from a glass bottle is recommended..
2. Rapid degradation of formic acid, especially in methanol at ambient temperature, is a documented issue.
3. Usage of premixed 0.1% formic acid in water or premixed 0.1% formic acid in methanol. Acid degradation plus ion suppression for analytes of interest (caused by the PEG) may occur.
4. Because the analytes are giving different responses depending on:
   a) the mobile phases used (methanol or acetonitrile) and
   b) the mobile phase pH,
   use of the same LC column for different assays (with different mobile phases) it is not recommended owing to the memory effect on column.
   See bibliography.

Bibliography:

1. Dwight R. Stoll - "Contaminants Everywhere! Tips and Tricks for Reducing Background Signals When Using LC–MS." LCGC NORTH AMERICA VOLUME 36 NUMBER 8 AUGUST 2018, p. 498-504.
2. Dwight R. Stoll - "What’s Trending in LC Troubleshooting?" LCGC NORTH AMERICA VOLUME 37 NUMBER 1 JANUARY 2019, p. 18-23.
3. Neal M. Fox, Anthony R. Kemperman, Sandra M. Lorenz, Karel A.J. Snoble, J.T. Przybytek - "Stability of Formic Acid in Methanol Solutions and the Implications for Use in LC–MS Gradient Elution Analysis" LCGC NORTH AMERICA  VOLUME 26, ISSUE 9, p. 946–950.
   https://www.chromatographyonline.com/view/stability-formic-acid-methanol-solutions-and-implications-use-lc-ms-gradient-elution-analysis
4. Geneen Baynham, Joanne Mather, Warren Potts and Paul Rainville - "Use of pH to Increase the Sensitivity of a Bioanalytical Assay", THE PEAK MARCH 2010, p. 14-21.

FIX or WORKAROUND

1. Make up mobile phases in small amounts (not on a large scale!) daily, especially methanol with formic acid, in order to avoid issues of additive degradation or bacteria growth in aqueous mobile phase.
2. It is not recommended to use formic acid from plastic containers, even when marked by vendor as LCMS grade.
3. If positive/negative switching is applied in the MS method, decrease the concentration of formic acid to 0.05% in order to improve ionization in negative mode, or use acetic acid.
4. Use a dedicated column for each assay.

ADDITIONAL INFORMATION