Analytical Chemiluminescence/Chemiluminescence detection in gas chromatography

D9. Chemiluminescence detection in gas chromatography

Figure D9.1 – Flow chart of a gas chromatograph.

Gas chromatography is a major separation method. It consists of the injection of a gaseous or liquid sample into a gaseous mobile phase which is passed through a column of solid support particles carrying a liquid stationary phase, maintained in an oven at a suitable temperature (which is usually above ambient but need not be above the boiling points of the analytes). Separation is the result of partition between the stationary and mobile phases and the separated constituents of the sample are usually detected by a flame ionization detector. A flow chart of the typical instrumentation for gas chromatography is illustrated in figure D9.1.

Samples of increasing complexity are being analysed by gas chromatography. Universal detectors, such as the flame ionization detector, are not adequate for such a task but selective detectors can provide the additional discrimination that is needed. Nitrogen- and sulfur-containing compounds commonly occur as trace-level analytes in complex samples and highly selective detectors have been developed. Among these, the nitrogen chemiluminescence detector and the sulfur chemiluminescence detector have emerged as powerful tools in gas chromatography, supercritical fluid chromatography and high performance liquid chromatography; stand-alone nitrogen/sulfur analysers can be based on the same chemiluminescence reactions. Detectors of either element are based on the same ozone-induced gas phase chemiluminescence. The chemiluminescence is preceded by high temperature pyrolysis which oxidizes the nitrogen in the sample (RN) to nitric oxide (NO):

(D9.1) Oxidation: RN + O₂ → NO + CO₂ + H₂O

and it is believed that the sulfur in the sample (RS) is converted first into sulfur dioxide (SO₂), which is then reduced in the presence of hydrogen to sulfur monoxide (SO):

(D9.2) Oxidation: RS + O₂ → SO₂ + CO₂ + H₂O

(D9.3) Reduction: SO₂ + H₂ → SO + H₂O

(D9.4) Overall: RS + O₂ + H₂ → SO + CO₂ + H₂O

These reactions produce the species that react with ozone, producing excited nitrogen dioxide and excited sulfur dioxide respectively (eqns. D9.5 and D9.7):

(D9.5) Reaction with ozone: NO + O₃ → NO₂* + O₂

(D9.6) Chemiluminescence: NO₂* → NO₂ + light (~ 1200 nm)

(D9.7) Reaction with ozone: SO + O₃ → SO₂* + O₂

(D9.8) Chemiluminescence: SO₂* → SO₂ + light (~ 360 nm)

The nitrogen chemiluminescence reaction emits in the near infra-red (eqn. D9.6), whereas the sulfur reaction emits in the ultra-violet (eqn. D9.8). This wide spectral separation of the emission bands enables nitrogen and sulfur to be determined selectively. A few small gaseous molecules containing sulfur also enter into the chemiluminescent reaction with ozone without undergoing preliminary pyrolysis.

Figure D9.2 – Flow diagram of a nitrogen-sulfur detector.

The instrumentation for nitrogen-sulfur chemiluminescence detection is depicted in figure D9.2. The pyrolyser converts the analytes in the gas chromatograph column effluent into the corresponding chemiluminescent species, which pass to the reaction chamber where they react with ozone supplied by a generator. The light emitted is detected by a photomultiplier.