Identification Of Halocarbons Present In Samples
ECD Detectors
- Contain radioactive 63Ni and requires a license to operate
- Extremely sensitive to compounds with high affinity to thermal electrons such as organic compounds with Cl, Br, I, S, N, and O in their structure
- Very sensitive to O2, but also sensitive to H2S, N2O, and H2O
- Use ultra-pure N2 or Ar-CH4 mixture as the carrier gas, which is more difficult to use with capillary columns that work best with helium as the carrier gas
- Easily contaminated, especially with capillary column bleed
- Very sensitive to pressure changes associated with temperature ramping
- Calibration in non-linear and many standards are required for calibration
The GC, column, pre-column, and cryogenic trap are optimized for the quantitative analysis of CFC-11, CFC-12, and CFC-113.
Other compounds that can be measured qualitatively during CFC analysis:
- Nitrous oxide
- Oxygen
- HCFC-22
- CFC-114
- Halon-1211
- Hydrogen sulfide
- Vinyl chloride
- Methyl chloride
- Methyl chloroform
- Methyl bromide
- Methylene chloride
- Chloroform
- Trichloroethylene
- Carbon tetrachloride
- Tetrachloroethylene
Identification of specific halocarbons should be confirmed by GC-mass spectroscopy since more than one compound can have the same or similar retention time. The retention time can vary as a result of changes in the oven temperature, the length of the analytical column, the carrier gas composition (N2, or Ar-CH4), and the carrier gas flow rate.
GC-Mass Spectroscopy
- Universal detector as compared to selective detectors (ECD, PID, FID, TCD, etc.)
- Used to quantify and identify compounds
- Has total ion mode and selective ion mode
- Total ion mode measures the molecular ion and all fragments produced within a specified mass range (30 to 250 AU)- the larger the mass range, the fewer the scans and the higher the detection limit
- Selective ion mode does not scan the entire mass range but measures only one (or more) ions at any given time interval (example—mass 51, or 49 and 51 AU between 3.40 and 3.60 minutes)
- Spectrum is unique for each compound
- From the spectrum, the compound can be identified using library searches
- Isotopic ratios can be used to determine the number of carbons (~99% 12 AU and 1% 13AU), chlorine (~76% 35AU and 24% 37AU), bromine (~50% 79AU and 50% 81AU), etc. present
