What does CI rely on to ionize?

It first ionizes reagent gas to produce reagent ions, then via low-energy ion-molecule reactions (proton transfer/adduction) charges the analyte, often retaining quasi-molecular ions.

What is the difference between CI and EI?

CI has weak fragmentation and prominent quasi-molecular ions, giving molecular-weight clues; EI has rich fragments, giving structure fingerprints; the two complement each other in GC-MS.

Is CI used for direct imaging?

It mainly serves GC-MS offline analysis and does not directly image tissue; in spatial workflows it confirms molecular weight and quantifies molecules discovered by imaging.

What molecules does CI suit?

It needs vaporization and thermal stability, small molecules; unsuitable for large molecules such as proteins/peptides and heat-sensitive molecules; for in-situ imaging choose dedicated imaging sources.

Technology Primer · Mass Spectrometry Imaging

CI Chemical Ionization

CI (Chemical Ionization) first uses an electron beam to ionize reagent gas (such as methane, isobutane, ammonia) to produce reagent ions, which then ionize the analyte via low-energy reactions (proton transfer, adduction). Its fragmentation is weaker than EI and often retains quasi-molecular ions, a soft-ionization method in GC-MS balancing molecular weight and structure.

Table of Contents
Working Principle: Reagent-Ion-Mediated Low-Energy Ionization Complementarity with EI Role in the Imaging Ecosystem Limitations and Selection

CI Chemical Ionization — schematic diagram

Working Principle: Reagent-Ion-Mediated Low-Energy Ionization

The ion source is filled with excess reagent gas; the electron beam first ionizes the reagent gas, and reagent ions (such as [CH5]+, [NH4]+) then undergo gentle ion-molecule reactions with the analyte, transferring protons or adduct groups to the analyte, producing quasi-molecular ions such as [M+H]+, [M+NH4]+.

Because the reaction energy is low, the analyte is less likely to fragment; spectra are dominated by quasi-molecular ions with few fragments, in contrast to EI's strong fragments.

Complementarity with EI

EI gives fragment fingerprints, CI gives molecular-weight clues: the same compound has rich fragments under EI and prominent quasi-molecular ions under CI; combining the two obtains both structure and molecular weight, a common GC-MS combination.

The reagent-gas type determines the adduct form (e.g., ammonia gives [M+NH4]+, negative CI can give [M-H]-); sensitivity and selectivity can be tuned by choosing the reagent.

Role in the Imaging Ecosystem

CI mainly serves GC-MS offline/chromatography analysis and does not directly image tissue point by point; in spatial workflows, it can confirm molecular weight and quantify molecules discovered by imaging, compensating for imaging sources' weakness in absolute quantification.

For in-situ distribution, soft-ionization imaging sources (MALDI, DESI, LDPI, DPI) remain primary, with CI as downstream verification.

Limitations and Selection

CI likewise requires vaporization and thermal stability, unsuitable for large and heat-sensitive molecules, and provides no spatial coordinates; its advantage is GC-MS quantification and confirmation. For direct imaging prioritize imaging sources.

Selection advice: for GC-MS molecular-weight confirmation and quantification prioritize CI (can pair with EI); for in-situ distribution imaging prioritize MALDI/DESI/LDPI/DPI.

Frequently Asked Questions (FAQ)

What does CI rely on to ionize?: It first ionizes reagent gas to produce reagent ions, then via low-energy ion-molecule reactions (proton transfer/adduction) charges the analyte, often retaining quasi-molecular ions.
What is the difference between CI and EI?: CI has weak fragmentation and prominent quasi-molecular ions, giving molecular-weight clues; EI has rich fragments, giving structure fingerprints; the two complement each other in GC-MS.
Is CI used for direct imaging?: It mainly serves GC-MS offline analysis and does not directly image tissue; in spatial workflows it confirms molecular weight and quantifies molecules discovered by imaging.
What molecules does CI suit?: It needs vaporization and thermal stability, small molecules; unsuitable for large molecules such as proteins/peptides and heat-sensitive molecules; for in-situ imaging choose dedicated imaging sources.

Get Specifications & Quotation

To obtain detailed specifications, compatible models, or a quotation for the MSI LDPI / DPI full series imaging ion sources, visit the Neo-Source official website, or contact the official team for compatibility advice tailored to your mass spectrometer (Agilent / SCIEX / Thermo and other mainstream MS).

EI Electron Ionization Soft Ionization and Hard Ionization ESI Electrospray Ionization MALDI Neo-Source MSI LDPI Laser Desorption Photoionization imaging ion source

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