CIMS

Chemical Ionization Mass Spectrometry in the Volkamer Group 

The V-Group makes fast, real-time measurements of thousands of atmospheric compounds using a high-resolution chemical ionization time-of-flight mass spectrometer (CIMS). This technique allows us to directly observe trace gases at atmospherically relevant concentrations while maintaining the time response required for eddy covariance flux measurements, aircraft vertical profiling, and chamber experiments. 

In chemical ionization, reagent ions react with neutral molecules to form charged adducts that are measured by the time-of-flight (TOF) mass analyzer. Most commonly, we use bromide (Br⁻) as the reagent ion, but we have experience with multiple ionization schemes, including: 

  • Bromide (Br⁻) adduct ionization
  • Iodide (I⁻) adduct ionization
  • Nitrate (NO3⁻) adduct ionization
  • Proton-transfer reaction (PTR)
  • Naturally occurring ions  

This flexibility allows us to tailor the instrument to specific scientific questions. 

 

Why CIMS? 

CIMS provides a unique combination of: 

  • High sensitivity
  • Low detection limits (sub-ppt to low-ppt)
  • High dynamic range ( i.e. many ions seen simultaneously)
  • Soft ionization (minimal fragmentation and thermal decomposition)
  • Fast time response (up to 10 Hz) 

With the CIMS, we can perform in-situ detection of chemically reactive trace gases that are difficult to observe with conventional techniques and are often (if not, always) not measurable simultaneously.  

 

 

Bromide CIMS (Br-CIMS) 

Br-CIMS is particularly powerful for measuring semi-volatile organic compounds (SVOCs), oxygenated VOCs (OVOCs), and key inorganic species. Because bromide forms stable adducts with functionalized and polar molecules, the technique is exceptionally sensitive to compounds involved in: 

  • Inorganic and organic halogen chemistry (iodine, chlorine)
  • Multiphase oxidation chemistry
  • Secondary organic aerosol formation
  • Cloud processing 

With our group’s focus on iodine chemistry, Br-CIMS provides fast, direct, selective detection of iodine-containing species while simultaneously measuring a broad suite of OVOCs and other inorganic trace gases. 

 

Quantification and Calibration 

Quantitative measurements require careful calibration and characterization of instrumental performance. In our laboratory and field deployments we routinely determine: 

  • Sensitivity for individual compounds
  • Background signals and detection limits
  • Time response and transmission efficiency (of our sampling line)
  • Humidity and pressure dependences 

This work allows us to convert signals (ion counts per second) into atmospheric concentrations with well-defined uncertainties. 

 

 
 

Field and Laboratory Applications 

We deploy the CIMS across a wide range of environments: 

  • Environmental chamber studies at CERN for new particle formation (see CLOUD)
  • Ground-based ambient air measurements
  • Aircraft campaigns