To enhance dopamine detection limits in LC-MS/MS, solid-phase extraction (SPE) can effectively concentrate the analyte by selectively binding it to a sorbent. Alternative methods include liquid-liquid extraction, micro-solid-phase extraction, dispersive solid-phase extraction, and molecularly imprinted polymers, each offering unique advantages for improving sensitivity.

Yes, solid phase extraction (SPE) is a viable method for concentrating dopamine or other target substances from a sample before LC-MS/MS analysis. SPE improves sensitivity by removing interferences and pre-concentrating the analyte.

How to Perform Solid Phase Extraction (SPE) for Dopamine:

To improve the lowest detection limit (LOD) of dopamine in LC-MS/MS, you need to enhance signal intensity, reduce background noise, and increase concentration. Here are some key strategies:

1. Sample Preparation Improvements

a. Solid Phase Extraction (SPE) Optimization

Use a strong cation-exchange (SCX) SPE column to selectively bind dopamine and remove interferences.

Optimize the elution solvent (e.g., methanol with 1-2% formic acid) to maximize dopamine recovery.

Reduce elution volume to concentrate the analyte before injection.

b. Liquid-Liquid Extraction (LLE)

Use pH adjustment to increase dopamine partitioning into an organic solvent (e.g., ethyl acetate or chloroform).

Evaporate and reconstitute in a smaller volume for higher concentration.

c. Pre-column Derivatization

Dopamine lacks strong ionization properties in LC-MS, so derivatizing agents (e.g., dansyl chloride, o-phthalaldehyde) can improve detection sensitivity.

This enhances ionization efficiency and reduces LOD.

2. LC-MS/MS Optimization

a. Column Selection

Use a high-efficiency C18 or HILIC column with smaller particle sizes (≤2 µm) for better peak shape and resolution.

A polar end-capped column can also improve retention and signal.

b. Mobile Phase Modifications

Optimize pH (3–5) with formic acid or acetic acid to improve dopamine ionization.

Use higher organic content (acetonitrile or methanol) in the mobile phase to reduce matrix effects.

c. Injection Volume

Increase the injection volume (e.g., from 10 µL to 50 µL) if your LC system allows, to load more dopamine into the column.

3. MS/MS Settings Optimization

a. Improve Ionization Efficiency

Use Electrospray Ionization (ESI, positive mode) for dopamine.

Optimize ion source parameters (e.g., nebulizer gas flow, capillary voltage).

Add dopamine-specific ion-pairing reagents (e.g., heptafluorobutyric acid) to enhance ionization.

b. Optimize Mass Transitions

Select the best precursor-to-product ion transitions (MRM mode) to maximize sensitivity.

Example:

Dopamine [M+H]+ = 154.1 → 137.1 m/z

Another transition: 154.1 → 119.1 m/z

Adjust collision energy for better fragmentation.

4. Minimize Matrix Effects

Use matrix-matched calibration curves (spiking dopamine in real samples) to correct suppression effects.

Apply post-column infusion to detect and compensate for suppression zones.

5. Instrument Enhancements

Use higher-resolution mass spectrometers (e.g., QqQ or Orbitrap).

Apply nano-LC-MS to reduce solvent use and enhance ionization.

Use dual ionization sources (ESI & APCI) to compare signal strength.

It may be worth challenging the widespread notion that LC-MS is the only technique worth bench space in a modern bioanalytical lab. It's true that a single instrument can obviate the need to invest in several dedicated instruments and the associated know-how. However, this instrumentation remains fairly useless for many purposes, particularly when there may be unknown metabolites ("metabolomics"), or when everything of interest is masked by trailing peaks of lipids and other major metabolites and drug excipients that interfere with ionisation.

For a long-running project it might be simpler and more economical to use, in this case, LC with electrochemical detection. Serotonin and the catecholamines have conveniently low oxidation potentials and, probably, you can still buy a ready-to-use kit from an instrument supplier. Old methods are not necessarily obsolete.

Returning to LC-MS, it may be surprising that tandem LC is still so little used for combining sample enrichment with improved chromatographic selectivity. The approach is as old as column chromatography and no mass spectrometer can compensate for inadequate separation.