4.8: Photo-induced chemiluminescence

Photo-induced chemiluminescence (PICL) involves irradiating an analyte with ultra-violet light in order to convert it to a photoproduct of different chemiluminescence behaviour, usually substantially increased emission. Such reactions form the basis of highly sensitive and selective analytical techniques. Irradiating a molecule can break it into fragments of smaller molecular weight (photolysis) or can induce reactions such as oxidation, reduction, cyclization or isomerization. Direct photolysis involves absorption of photons by the target molecule; in indirect photolysis, the target molecule absorbs energy from another molecule that has previously absorbed photons^[1].

Photochemistry is concerned with excited electronic states induced by the absorption of photons. Photoexcitation is more selective than thermal excitation and leads to a different energy distribution within the molecule. The excited molecule can undergo photochemical processes, the products of which are sometimes involved in side processes. In analytically useful photochemical reactions the light is strongly absorbed by the analyte but not by the photoproducts; the photochemical yield is high; the photoproducts are stable for as long as is needed to complete the analysis and are structurally rigid enough for the emission to have an adequate quantum yield. Successful analytical application also depends on appropriately designed photoreactors. When those conditions are fulfilled, using light has several advantages over the use of chemical derivatization. Lamps are inexpensive and their stable light output allows reproducible results. They differ in their spectral characteristics, which gives scope for increasing selectivity. The use of light has minimal environmental impact and can be effected in ambient conditions. Analysis times are shorter because photochemical reactions are fast and can be shortened further by optimizing reactor configuration or increasing lamp power. PICL has a linear relationship with analyte concentration over a wide concentration range and extends the range of analytes that can be detected by chemiluminescence. It is not necessary to identify or separate the photoproducts.

In PICL-based methods, the sample is irradiated on-line and subsequently merged with the chemiluminescence reagents prior to reaching the flow cell in front of the detector. Flow methods allow the irradiation time to be easily controlled and provide better reproducibility than stationary methods, coping better with the very fast rate of chemiluminescence reactions. Sample throughput, ease of automation and reagent consumption are also improved using flow methods.

PICL has the same instrumentation as other chemiluminescence but, in addition, a photoreactor is required and this has two essential elements – a light source and a container for the sample. Lamps are selected on the basis of power and spectrum (continuous or discrete). Continuous spectra span a wide zone, whereas discrete spectra are series of individual lines in a narrow wavelength range. The mercury-xenon lamp provides a continuous spectrum and is used when its high power is necessary, though it needs cooling. The low-pressure mercury lamp generates little heat and is a typical discrete-spectrum lamp, emitting over the range 200-320 nm, maximally at 254 nm; most substances absorb in this zone. The absorption zone of the selected lamp must be the most useful for excitation and bond-breaking. In flow systems, the sample is usually contained in PTFE tubing, which admits little light but maximises its effect by repeated reflections from the inner tube surfaces; the tubing can be coiled around a low-power lamp. Batch methods instead use quartz cells, which are transparent to ultra-violet. Quartz is more inert than PTFE, but also more fragile.