Multiple reaction monitoring (MRM), also called selected reaction monitoring (SRM) when one transition is emphasized, is a targeted mass spectrometry method that quantifies predefined precursor-to-product ion transitions on a triple quadrupole. Quadrupole 1 (Q1) isolates a precursor m/z, the collision cell fragments that ion population, and quadrupole 3 (Q3) passes selected product ions for detection. The quantitative signal is the chromatographic peak area of those transitions for a closed peptide list. MRM is not a discovery acquisition mode like data-dependent acquisition (DDA) or data-independent acquisition (DIA). Relative same-peptide comparison is the default claim; absolute amount requires a matched stable isotope-labeled standard (SIS) and a stated calibration model (Lange et al. 2008).
Researchers who already have a closed peptide panel or a specific validation objective can also review the MtoZ Biolabs SRM-MRM Quantitative Proteomics Services for project-specific feasibility and assay planning.
What MRM and SRM Mean in Practice
In proteomics, MRM and SRM usually describe the same triple-quadrupole transition logic. SRM historically names monitoring of a selected precursor-product pair. MRM names monitoring of multiple such pairs in one method, which is the common laboratory practice for multi-peptide panels. Neither name by itself specifies liquid chromatography conditions, dwell time, or whether an SIS was used.
The assay begins after targets are closed. Proteotypic peptides are chosen from discovery data, spectral libraries, or literature. For each peptide, one or more transitions are optimized for collision energy, selectivity, and retention-time schedule. Only those transitions are recorded during the run. Proteins outside the list remain invisible even if they are abundant.
Figure 1. MRM/SRM isolates a precursor in Q1, fragments it, selects product ions in Q3, and quantifies from transition peak areas for a closed peptide list.
An MRM signal is the peak area of predefined transitions, not a survey of the full proteome.
How an MRM Assay Is Operated
A workable MRM project follows a fixed order. Skipping transition validation cannot be repaired by higher sample load alone.
1. Close the peptide and transition list
Select peptides that are unique where possible, ionize well, and produce stable fragments. Define quantitative and qualifier transitions before the cohort is queued. Shared peptides report gene-family ambiguity rather than a single protein.
2. Decide relative versus absolute readout
Same-peptide peak areas support relative comparison when digestion and loading stay comparable. Absolute concentration needs matched SIS peptides, with spike timing recorded. An SIS added after failed digestion cannot reconstruct the lost protein.
3. Prepare matched digests and chromatography
Protein extraction, digestion, and cleanup must keep samples comparable. Liquid chromatography separates peptides so that scheduled retention-time windows can catch each transition. Electrospray ionization is the usual interface for peptide MRM. Naming MALDI as a generic first step confuses common proteomics practice with other analyte classes.
4. Acquire and integrate
The triple quadrupole cycles through the transition list with declared dwell times. Peak area, retention time, and qualifier ratios are reviewed in software such as Skyline. Interference at unit-resolution Q3 can still admit background ions that share the same nominal masses.
Figure 2. MRM operation locks transitions before acquisition; absolute claims require SIS, while relative claims still need matched processing and clean chromatograms.
Judgment Criteria: When MRM Fits
Study need |
MRM / SRM often fits |
Reassess before scheduling |
|---|---|---|
Target status |
Closed, mature peptide and transition list |
Open protein inventory still required |
Throughput |
Large panel must run repeatedly on a triple quadrupole |
Early fragment review is more important than multiplex |
Matrix |
Transitions already shown to be clean in the matrix |
Severe unit-resolution interference needs high-resolution fragment review |
Claim type |
Relative confirmation; absolute with SIS |
Discovery fold changes across an open proteome |
Adjacent option |
Production-scale targeted panel |
Parallel reaction monitoring (PRM) for post-run fragment choice |
Prefer MRM when transitions are locked and multiplex efficiency on a triple quadrupole is the limiting need. Prefer PRM when the panel is still being refined and accurate-mass fragment inspection matters in a complex digest. Researchers comparing those routes can refer to PRM vs. MRM: Comparative Analysis in Quantitative Proteomics.
Figure 3. MRM fits a closed, mature transition panel on a triple quadrupole; keep DDA or DIA while discovery is open, and consider PRM when fragment review is the scarce resource.
Limits and Application Boundaries
MRM cannot recover proteins that were never scheduled. Unit-resolution Q3 can accept interfering ions that share precursor and product nominal masses. Dwell time and peak width limit how many transitions fit in one method. Calling MRM highly sensitive without stating peptide amount, matrix, and transition quality overstates what the instrument class alone decides.
Typical applications are biomarker candidate validation, pathway-protein panels, pharmacodynamic monitoring, and selected modified-peptide assays when site-discriminating transitions can be defined. A common research sequence is discovery by DDA or DIA, optional PRM confirmation, then MRM for a locked panel that must run at scale. Diagnostic cutoffs and clinical decision rules sit outside ordinary research MRM reporting unless a separately validated assay framework is in place.
Once the peptide list, transition set, and relative-versus-absolute design are defined, the project can be evaluated as an MRM/SRM study. Researchers can review the MtoZ Biolabs SRM-MRM Quantitative Proteomics Services for sample evaluation, method feasibility, and workflow planning. For PRM-centered follow-up after discovery, see the Parallel Reaction Monitoring (PRM) Service.
Frequently Asked Questions
1. Are MRM and SRM different methods?
In proteomics practice they usually describe the same triple-quadrupole transition principle. SRM emphasizes a selected pair; MRM emphasizes monitoring many pairs in one method.
2. Does MRM provide absolute quantification by itself?
No. Absolute amount requires matched SIS peptides and a calibration model. Without SIS, the default claim is relative.
3. Can MRM replace DDA or DIA discovery?
No. MRM measures only scheduled transitions. Keep discovery acquisition while new protein names are still needed.
4. Why must transitions be chosen before the run?
Q3 records only the product ions that were programmed. Unlike PRM, MRM does not store a full high-resolution fragment spectrum for later ion selection.
5. When is PRM the better next experiment than MRM?
PRM is often better when the panel is still under fragment review in a complex matrix. MRM is often better when a mature transition list must run at very high multiplex on a triple quadrupole.
Reference
- V. Lange, P. Picotti, B. Domon, R. Aebersold (2008). Selected reaction monitoring for quantitative proteomics: a tutorial. Mol. Syst. Biol., 4, 222. https://doi.org/10.1038/msb.2008.61
- A.C. Peterson, J.D. Russell, D.J. Bailey, M.S. Westphall, J.J. Coon (2012). Parallel reaction monitoring for high resolution and high mass accuracy quantitative, targeted proteomics. Mol. Cell. Proteomics, 11, 1475-1488. https://doi.org/10.1074/mcp.O112.020131
