Sunday, July 24, 2011

Analysis of ppb Levels of Low MW Organics Compounds Using an 11.7 PID-GC

The ionization potential (IP) of low molecular weight saturated organic compounds are > 10.6 eV but < 11.7 eV. This group of compounds (ethane-butane), chloroalkanes (CH3CL. CHCl3, CCl4, CH2CL2, C2H4Cl2 etc.), oxygenates (methanol & formaldehyde) are difficult to do with any single detector except the PID with an 11.7 lamp. The dynamic range of the PID with an 11.7 eV lamp is about 106 with detection limits in the 50-100 ppb range. These detection limits could be improved using thick film capillary columns. A comparison of the range of the 11.7 eV lamp with other GC detectors is shown below.

The response of low MW alkanes C2-C6 for the PID 11.7 is shown in the chromatograph below:
C2-C6 HC with the PID 11.7
Note that the PID 10.6 eV will only respond to C5 & C6 alkanes.
The chromatogram below via PID 117 detects some chlorinated HC including ethylene dichloride (C2H4Cl4). (IP= 11 eV). Of course, the PID 11.7 also detects low ppm levels of MeOH and chloroform as shown below. Note the high sensitivity of the low MW chlorinated hydrocarbon.

The PID 11.7 also has a high sensitivity to formaldehyde, one of the few detectors that respond to formaldehyde at ppb levels without the need for derivitization. A chromatogram of methanol and formaldehyde is given below.

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