Diagnostics; Quenching Effects; Pmt Saturation; Detector Temperature Considerations - Agilent Technologies 6890 Series Service Manual

Gas chromatograph

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Flame Photometric Detector (FPD)

Diagnostics

Hydrocarbon quenching occurs when a high concentration of carbon dioxide

from a hydrocarbon peak is in the flame at the same time as the sulfur species.

Part of the light emitted by the sulfur species is absorbed by some CO

Self-quenching occurs at high concentrations of the heteroatom species. Some

other ground state (inactivated) species reabsorbs the emitted photon,

preventing it from reaching the PMT.

These effects are reduced by good chromatographic practices. The column

should provide good separation of the compounds, those that contain sulfur

or phosphorus as well as those that do not but may absorb light. A careful,

multilevel calibration is well worth the investment!

Detector and gas cleanliness must be maintained to have consistent

responses. Since most sulfur and phosphorus compounds contain chemically

active sites, the injection and column systems must be kept very clean.

The photomultiplier tube may saturate if light intensity is too high. When this

happens, increasing concentration produces little or no increase in signal and

peak tops are rounded or flattened. The sample must be diluted to prevent

The FPD flame produces considerable water vapor. The detector must be

operated above 120°C to prevent condensation.

Unnecessarily high temperatures can cause thermal decomposition of many

thermally labile phosphorus and sulfur compounds.

Detector temperature can have a significant effect on sulfur sensitivity. If

analyzing compounds with high boiling points, the detector temperature

should be set to 25°C above the final oven temperature—if allowed by the

temperature limit of 250°C.

Agilent 6890 Gas Chromatograph Service Manual

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