Gas Chromatography Detectors - The Emissivity or Photometric Detector > Page 51

The system is very similar to that originally devised by Grant. The end of the capillary column is led into the flame jet where the column eluent mixes with the hydrogen flow and is burnt. The jet and the actual flame is shielded to prevent light from the flame itself falling directly on to the photo-multiplier. The base of the jet is heated to prevent vapor condensation. The light emitted above the flame, first passes through two heat filters and then through the wavelength selector filter and finally on to the photo-multiplier. The response of the detector to sulfur is fairly insensitive to changes in hydrogen flow rate. However, the response to phosphorus compounds shows a maximum at a particular hydrogen flow rate, the magnitude of which varies with the air flow.

A serious problem that can occur in the FPD is the quenching or re-absorption of the light emitted by the selected species. Hydrocarbon quenching can occur when the peak containing sulfur is co-eluted with another hydrocarbon in relatively high concentration. The high concentration of carbon dioxide appears to suppress the characteristic light emission from a sulfur compound. Quenching can also occur from excess of the selected species itself. It has been suggested that this can be due to collisional energy absorption, competing chemical reactions or the re-absorption of the photon by inactivated species.

The flame conditions can be critical and the gas flows and jet diameter must be adjusted to ensure optimum emission in the detection zone. Hydrogen flow rates, air flow rates and jet temperature must also be optimized to ensure maximum sensitivity and selectivity and to avoid sample condensation in the burner conduits. An example of the use of the FPD in the analysis of thiophene and substituted thiophenes in a hydrocarbon mixture is shown in figure 27.

It is seen that excellent selectivity is afforded by the FPD. The filters allow the sulfur compounds to be clearly and unambiguously selected from the multitude of hydrocarbons present in the mixture thus, greatly simplifying the analysis and improving its accuracy and precision.