AOPs are widely studied and applied for degradation of organic
pollutants (e.g., dyes). It has become increasingly important to build
a library of different intermediates and products formed during dye
degradation, as degradation products are potentially more hazardous
than the pollutant itself. LC-MS has gained popularity as a method
of choice for the analysis of degraded dye samples for several
reasons (e.g., relatively low operating cost, minimal sample preparation,
ease of separation, and compatibility of the LC-MS system with
aqueous samples). Moreover, the soft ionization techniques used in
the MS can provide information about the mass of the parent compound
and intermediates under analysis. Selected compounds can
be further analyzed by MS/MS fragmentation for structural clues.
Although the LC-MS technique is powerful, it is limited in its use
for distinguishing isomers unless standards are available. For example,
the exact position of an OH on an aryl group cannot be known
using LC-MS alone. Many of compounds reported in the literature
are mere speculation as the exact structures have not been investigated.
In future, LC-MS should be coupledwith 1D and 2DNMR techniques
to provide structural informationaboutunknowncompounds.
As far as mechanisms are concerned, most have been formulated
based on existing knowledge, whereas it is only possible to establish
a detailed mechanism through experimental evidence. It has also
been noticed that some of the intermediates that are proposed in
the literature are unstable. This field of research is still in its infancy
and will require more contributions from the research community