Instrument - Filter Inlet for Gases and AEROsols coupled to High Resolution Time-of-Flight Iodide Chemical Ionization Mass Spectrometer
FIGAERO-HR-ToF-CIMS (far, gas, mass spec)
Filter Inlet for Gases and AEROsols coupled to High Resolution Time-of-Flight Iodide Chemical Ionization Mass Spectrometer
What is being measured:
Aerodyne Research Inc.
Data recording software:
Data analysis software:
Raw data time resolution:
Analysis data averaging:
Sensitivity to temperature (and correction method, if applicable):
Sensitivity to relative humidity (and correction method, if applicable):
Sensitivity varies depending on water vapor pressure in IMR. We add excess water vapors directly to IMR so H2O·I- does not varies substantially.
Direct sampling for gas-phase measurement. Filter collection followed by thermal desorption for particle-phase measurement.
Sample preparation method:
Sample residence time (chamber to instrument) (seconds):
Length of tubing (cm):
Instrument flow rate:
Tubing inner diameter:
FEP teflon / stainless steel
Chemical identification method:
Mass spectrometry technique with selective ionization toward polar species (e.g., organic acids, polyols, multi-functional, nitrates, some inorganic species etc) by iodide ions. Though the resolving power (4000-5500 depending on tuning) is limited, mass assignment is done carefully based on knowledge on iodide selectivity and precursor VOC chemistry.
Data analysis method:
Raw data (1 Hz) are averaged to 10-s data using Tofware 2.5.1x in Igor 6. Take a close look at diagnostic data (e.g., pressures, temperatures, flow readings) and remove data points if abnormal. Instrument functions Baseline is determined using either smooth or median average routine built in Tofware using an integration width of 15 to account for a large negative mass defect of iodide ion. Peak width as a function of m/z is determined using a built-in algorithm in Tofware using a tolerance of 5 to 10. Peak shape is determined using a built-in algorithm in Tofware and manually removing ions with bad shapes. Time-series mass calibration built in Tofware, in general, is done at 1-min interval, using I-, H2O·I-, CHOOH·I-, HNO3·I-, CF3COOH·I-, I2-, and I3- as mass calibrants. Exact choice changes depending on types of experiments. Peak search range of 10, fit type of custom shape, and 2-param fit equation are generally used though they vary depending on application. High resolution peak fitting routine is performed following Stark et al. (2015, IJMS) and based on known precursor VOC chemistry as well as knowledge on iodide selectivity. Batch HR calculation is done by using fully constrained fitting, applying ToF duty cycle at m/z 127 (I-), subtracting baseline before fitting, and constraining isotopes and reallocating signals. Calculated signals are then normalized by ([I-]+[H2O·I-]/106) and dilution factor if used.
All signals are normalized by ([I-]+[H2O·I-]/106) to account for changes in abundance of reagent ions over time. When dilution flow is desired and employed (not to deplete reagent ions and not to have secondary ion chemistry), signals are further corrected accordingly. Signals from particle-phase measurements are converted to gas-phase measurement equivalent signals based on the collection volume, gas-phase sampling flow rate,
We do 2 types of calibrations: sensitivity and volatility calibration. For sensitivity calibration, we either flow a known concentration of analyte (e.g., permeation tube) to gas-phase inlet and measure signal at multiple levels of concentration, or we deposit a known mass of analyte on filter (FIGAERO), desorb it and measure signal at multiple levels of mass. For volatility calibration, we deposit a mixture of species with known subcooled vapor pressures (e.g., organic acids, polyols) and relate them to the desorption temperature (Tmax) obtained by a Gaussian fit.
Calibration drift estimate:
Uncertainty estimation method:
We use an error provided by Tofware.
Changes in water vapor pressure in IMR changes sensitivity greatly, especially at a lower end of vapor pressure.
Link to supplemental information: