ANALYTICAL METHODS · PRACTICAL GUIDE
A Peak in the Blank: Understand Background and LC–MS Carryover
Use blank injections to tell a background signal from a signal left by the previous sample.
You run a blank, expecting a flat line, but a peak appears. Where did it come from? A blank can help answer that question, especially if you know what was injected immediately before it.
A blank still passes through the system
A blank normally contains the chosen solvent or sample medium without the target analyte. It still travels through the instrument and can reveal signals from the solvent, containers or system.
Define which blank was used. A solvent blank and a blank carried through sample preparation do not investigate exactly the same sources.
Look at the blank before the sample
If a signal is already present before the target sample is injected, that sample cannot be its only explanation. Look for background or earlier contamination.
The comparison should use the relevant signal and the same method. A peak at a similar time alone is not enough to identify a chemical.
Then look immediately after a strong sample
A signal appearing after a strong sample, then falling in later blanks, suggests that material may have remained in the instrument. This is often called carryover. The sequence is useful evidence, but the cause still needs investigation.
In a teaching example, the sample area is 100,000 and the following blank area is 100. The blank-to-sample ratio is 100 ÷ 100,000 × 100% = 0.1%.
Why a small leftover signal can matter
A small signal relative to a concentrated sample can be large relative to the next low-concentration sample. Think of comparing it with the measurement you are about to make, not just the previous large peak.
Wash steps, system checks and repeated blanks should follow the laboratory’s method. The 0.1% calculation here is not a universal acceptance limit.
What can this blank actually tell you?
Keep the injection order, method and signal data together. Without that context, a blank peak is difficult to interpret.
See the answer
No. The signal was already present before this sample. Investigate background and the earlier sequence as well.
When examining LL-37 or MOTS-c signals
For LL-37 or MOTS-c testing, ask whether relevant blanks were included and where they sit in the injection order. These records help interpret low signals; a product name alone cannot resolve them.
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