Chemistry Reference
In-Depth Information
reagents, with this purpose, photochemical or enzymatic reduction of nitrate by nitrate reductase have been
used instead of cadmium. However, some toxic reagents are still used for the final spectrophotometric
determination of nitrite such as sulfanilamide (SAN) and N-1-naphthyl ethylenediamine dihydrochloride
(NED) or luminol for chemiluminiscent detection. To avoid the use of toxic reagents in nitrate measurement,
direct spectrophotometric analysis at low wavelength, around 200 nm, can be used which reduces considerably
sample preparation. However, the procedure has important drawbacks because other substances represent
serious interferences, such as Cl
−
, NO
2
−
, and Fe
3+
, as well as organic matter. For this reason interferences
elimination by in-line separation of nitrate using an anion-exchange resin allows selective nitrate determination
without employing toxic reagents [27].
An example of green approach for nitrite determination in natural water was proposed by Melchert
et al.
[26] based on flow-injection spectrophotometry. The flow system was designed with solenoid micro-pumps
in order to minimize reagent consumption and waste generation. The approach is based on the Griess
diazocoupling reaction with sulfanilamide and N-(1-naphthyl)ethylenediamine (NED) yielding an azo dye,
followed by photodegradation of the low amount of waste generated based on the photo-Fenton reaction.
The sampling rate was estimated at 108 determinations per hour. The consumption of the most toxic reagent
(NED) in the conventional Griess reaction is reduced 55-fold and 20-fold in comparison to batch method
and flow injection with continuous reagent addition, respectively. A colorless residue was obtained by
in-line photodegradation with reduction of 87
of the total organic carbon content. The results obtained
for natural water samples were in agreement with those achieved by the reference method at the 95
%
%
confidence level.
An alternative procedure on the basis of the reaction of nitrite and iodide to yield iodine has been also
proposed. In this case the sampling rate was estimated at 44 determinations per hour. Despite avoiding the use
of toxic reagents, the nitrite-iodide method presented the worst performance in terms of selectivity and
sensitivity.
5.4 In-field sampling
Several in-field sampling techniques environmentally friendly have been developed until now. In this context,
the use of sorbents is very important since they offer good analytical features while they are solvent-free
techniques or consume little volume of reagents. Solid phase extraction (SPE) and solid phase microextraction
(SPME) allow in situ sample extraction and store samples for analysis. New field samplers have been
developed for SPME [28] and SPME-based approach for sampling and analysis has been used for the
determination of transformation products of highly reactive and toxic unsymmetrical dimethylhydrazine
(UDMH) [29], for propofol exhalation in pigs, humans and sheep during anesthesia [30] and even in vivo
sampling, as applications of SPME for analysis of volatile emissions and drugs in biological fluids [31].
A rolling stir bar sampling procedure has been developed for sampling volatile compounds from agricultural
products, plant materials and human skin for thermal desorption/GC-MS [32].
As mentioned, SPE presents important advantages in relation with the environmental sampling as it
requires less volume of solvent. Furthermore, the cartridges or disks can be used for storage of the species
and SPE provides high enhancement factors if a large volume of water (1-2l) is passed through the sorbent
without breakthrough [33. SPE has been used as 'in field-sampling' technique for the simultaneous
determination of selected endocrine disrupters in water [34] and for extraction and storage of tributyltins
(TBT, DBT and MBT) in sea waters for tin speciation using C
18
cartridges. In this latter example, the sampling
operation is very simple and only requires passing a suitable volume of water through the cartridge with the
help of a syringe, therefore it is not necessary the use of reagents and allows in-field sampling. Stability of
tributyltin in C
18
cartridges was checked for almost two years, although phenyltin species were only stable for
