Ghost Peaks

Ghost Peaks and Carryover

Welcome back to this 7^th course unit which is looking at ghost peaks and carryover. Unwanted peaks, typically known as ghost peaks, and carryover can both interfere with accurate data interpretation, especially if they coelute with target analytes or persist between injections. Consequently, avoiding these occurring peaks and those arising from contaminants is a critical aspect of the analysis.

1. Ghost Peaks (Unexpected Peaks in Blanks)

Ghost peaks can significantly interfere with data evaluation and potentially produce false results. They are rarely an issue for targeted MRM type analysis but can be a greater issue for scan analysis.

Issue	Solution
Contaminated Eluents	Replace solvent containers regularly, and do not top-up bottles. Maintain and monitor waste purification systems. Use a gradient to wash off contaminants which have accumulated at the head of the column.
Incorrect Solvent Quality	Some LCMS grade solvents may contain UV active contaminants, affecting UV detection. However, they may not ionise in the ion source, therefore it is important to use the right quality solvent Install a ghost trap which can be replaced if contaminated     Switch solvent batches or manufacturers if ghost peaks persist Fig. 1 The QArray Skimmer from an LCMS instrument which was using LC grade solvents
Inadequate Column Equilibration / Carryover from Previous Run	Include post-run rinsing and ensure full equilibration before next injection This is essential in complex sample matrices to ensure all polarity compounds are eluted from the column
Contaminated Sample Solvent / System Flow Path	Degas solvents (remove dissolved gases) Clean flow lines; trapped impurities can elute during gradients

2. LCMS Carryover (Residual Signal Between Injections)

Issue	Solution
Column Contamination	Flush column or replace guard column / analytical column if fouling persists.
Injector Issues	Verify programmed injection volumes  Use a system suitability test to detect injection volumes     Check the batch is programmed for the correct injection volume Purge the injector metering pump
Inadequate Wash Settings	Check wash solvent compatibility with analyte Increase needle and loop flushing cycles Fig. 2 Minimal carryover when appropriate rinse solvents are used
High Sample Concentration / Overload	Inject smaller sample volumes or dilute samples
Insufficient LC gradient	Extend the high-strength solvent hold at the end of the gradient to elute strongly hydrophobic compounds

3. Extra Peaks

Daily Checklist to Prevent Ghost Peaks & Carryover

Unwanted peaks in the chromatogram are disturbing during evaluation and can deliver false results if they coelute with target analytes. Therefore, attempts are always made to avoid ghost peaks as well as regular peaks from contaminants. 

Fig. 3 Common sources of ghost peaks and contamination.

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