Selecting the correct ionisation technique can determine whether a mass spectrometry (MS) experiment succeeds or fails. With no universal method, analysts must weigh analyte properties, matrix considerations, and application goals before committing to a strategy. Poor choices risk reduced sensitivity, increased interference, or incomplete data.
To address these challenges, Separation Science invited registrants of the recent “Choosing the Correct Ionisation Technique for Successful Mass Spectrometry” webinar to submit their most pressing ionisation questions. Emeritus Professor John Langley, a recognized authority in MS and hyphenated techniques, shared his insights and practical recommendations.
To learn more about the webinar, read the executive summary of the event. Alternatively, readers can find Langley's responses to attendee questions below.
1. A compound typically analyzed by LC-MS/MS with an ammonium ion adduct produces only a sodium adduct precursor ion in one LC-MS system, despite the absence of sodium in the mobile phase. How can ammonium ionisation be achieved for this compound?
Sodium ions are often present in the ion source due to contamination from earlier samples or the metal capillary. To encourage ammonium adduct formation:
Use ammonium acetate as the chromatography buffer.
Add a post-column supply of ammonium ions using a tee-piece after the column.
These adjustments help minimize sodium interference and improve method reliability.
2. What methods can be used to generate a molecular ion of tris(acetylacetonato)iron(III) [Fe(acac)₃]?
Positive ion electrospray ionisation (ESI) should provide the desired data. For reference spectra, see the NIST Chemistry WebBook.
3. What methods can be used to generate a molecular ion of fullerene C₆₀?
There are several options:
- Electron ionisation with direct insertion probe heating will produce a spectrum where the molecular ion is typically the base peak. Multiply charged species may also appear.
- Atmospheric pressure photoionisation (APPI) of C₆₀ dissolved in toluene yields the molecular ion.
- Matrix-assisted laser desorption ionisation (MALDI) of dried C₆₀ in toluene also works, but laser power must be carefully controlled to avoid forming new fullerene species.
These approaches provide reliable ways to obtain the molecular ion of C₆₀.
4. Can mass spectrometry characterize a molecule that becomes protonated upon the addition of an acid?
Yes. Positive ion ESI, atmospheric pressure ionisation (APCI), or MALDI are all suitable options for analysis.
5. Laboratories often have ESI and APCI sources, but what is meant by a mixed-mode source?
Mixed-mode ionisation provides both ESI and APCI data without changing probes. While APCI data quality may not equal that of a dedicated APCI probe, the mixed-mode source is highly useful for screening complex samples. Analysts can acquire both positive/negative ESI and APCI spectra in a single dataset, with only minor compromises to the chromatographic peak profiles.
6. During analysis of haloacetic acids in water by ESI IC-MS/MS, an increased response for dichloroacetic acid and its internal standard was observed in samples from a specific location. This effect was not seen in calibration, QC, or other samples. What could cause this?
The likely cause is ion suppression or enhancement specific to those samples. Ionisation enhancement is less common but does occur. APCI often exhibits less susceptibility compared to ESI. To mitigate this, consider adjusting the LC column or mobile phase to separate the interfering compound from the target analyte. For further insights, see Rapid Communications in Mass Spectrometry (2010, 24:3103–3108).
Key Takeaways
Selecting the right ionisation method is crucial for sensitivity, accuracy, and reproducibility. The following points highlight key considerations:
- Sample-specific factors, matrix effects, and source design strongly influence results.
- Flexibility in ionisation strategy—such as adopting mixed-mode or altering LC conditions—can overcome analytical challenges.
These factors underscore the importance of carefully evaluating ionisation choices to achieve reliable and reproducible MS data.
Optimizing Ionisation Techniques in Mass Spectrometry
As Professor Langley’s responses show, small changes in source conditions, buffer composition, or analytical strategy can make the difference between ambiguous and reliable results. Analysts who adapt their approach—whether through mixed-mode ionisation, careful matrix control, or alternative source selection—are better equipped to overcome challenges and deliver robust data. By applying these expert insights, laboratories can optimize their workflows and strengthen confidence in their MS results.
