Instrument - High Resolution Time of Flight Chemical Ionization Mass Spectrometer
High Resolution Time of Flight Chemical Ionization Mass Spectrometer
What is being measured:
Data recording software:
Data analysis software:
Raw data time resolution:
Analysis data averaging:
4 pptv (formic acid)
Sensitivity to temperature (and correction method, if applicable):
Sensitivity to relative humidity (and correction method, if applicable):
Sensitivity to RH is compound-specific. Correction involves passing carrier UHP nitrogen through a water bubbler before it joins the sampled gas in the instrument's IMR region. This results in a stream of gas in the IMR region at a stable RH of around 60%.
Sample preparation method:
Sample residence time (chamber to instrument) (seconds):
Length of tubing (cm):
Instrument flow rate:
Tubing inner diameter:
Chemical identification method:
The sampled gas comes into contact with Iodide ions that are produced by a methyl iodide permeation tube within the instrument. In the Ion-Molecule Reaction region, the I- ions form adducts with the sampled gas primarily as I- + C2H6O -> C2H6OI-. The negatively-charged adducts are then passed through a vacuum region where voltages are applied. Based on the relative time of flight of each individual chemical species, we can determine the mass-to-charge ratio of the species. The high mass resolution allows us to do high-resolution identification of compounds within a single unit m/z.
Data analysis method:
An instrument-specific calibration curve is present that allows us to determine m/z of compounds based on their ToF at a 1Hz time resolution. After the initial data processing is done (determining average peak shape, subtracting instrument baseline, etc.), we integrate the data using Igor's Tofware and then we obtain graphs of signal (in ions/s) vs. unit m/z. We can then carry out high-resolution identification of peaks at specific m/z based on our knowledge of the kind of reactants and products that may be found in the experiment.
Currently our instrument is only used qualitatively for most compounds but we are working on a calibration technique. We normalize all signals against the primary ion (I-) to take into account fluctuations in its concentration.
Calibration drift estimate:
Low (on the order of weeks)
Uncertainty estimation method:
There are compounds that interact more strongly with I- than the different VOCs we measure (eg: Cl2, chlorinated compounds). These can cause significant depletion in I- affecting other observed adducts.
Link to supplemental information: