Chemistry Reference
In-Depth Information
1) Two successive injections of sample are performed, using two different appropriate carrier-eluting
solutions, one for each of them. The sensing surface responds each time to only one analyte in the sample
or, in some cases, to the sum of both analytes in one of the injections. High selectivity is achieved just by
the interaction of the analyte with the solid phase in the different surface microenvironment appropriately
provided by the respective carrier. They really are very single environment-friendly systems (and, hence,
simple GAC methods) which have proved their utility [39].
2)
Two successive injections of sample are performed, combined with the use of two sensing supports exhibiting
each of them different chemical nature. Such as system has been described using as analytical property the
intrinsic UV absorbance of the analytes. The supports were placed, respectively, in the measurement and the
reference cells in a double beam spectrophotometer, so obtaining, respectively, positive (measurement cell)
and negative peaks (reference cell) depending on the cell where the analyte was measured [40].
12.4.1.2
Solid phase molecular spectroscopy: multicommutated procedures
Multicommutation (MC) allows the design of dynamic manifolds based on the use of discrete commutation
devices (three way solenoid valves) so that each analytical step of the flow system procedure can be
implemented independently, and the systems can easily be reconfigured by means of software [41]. Only the
reagent necessary for sample analysis is introduced in the system, thus being in compliance with the GAC
principles more closely than conventional FIA approaches.
The combination of MC with SPMS (SPMS-MC) has become a very fruitful implementation in the field
of optosensors. Both, single- and multi-analyte SPMS-MC systems based on analyte native properties
(using UV and fluorescent detection techniques) have been described [42, 43], using the same strategies
applied for SPMS-FIA multi-optosensors. SPMS-MC implementation exhibits interesting properties in line
with the main principles of GAC concept [43]: (1) minimization of both sample and reagents consumption
(inherent to multicommutation principle) is achieved by re-cycling solutions to their respective vessels
while they are not being used in the flow system; (2) no derivatizing reagent is used; and, (3) one single
injection allows multicomponent determination together with enhanced sensitivity and selectivity provided
by the solid phase.
12.4.2
Monitoring derivative species
Flow-SPMS procedures based on the use of derivative reactions involves a less ideal GAC approach than
those earlier described. Nevertheless, it is worthwhile mentioning them because they still keep very interesting
features related with the GAC principles. The number of described flow-through optosensors monitoring
derivative product is lower than those measuring a native analytical signal from the analyte(s). It is due,
probably, to the difficulty of regenerating the solid sensing zone when retaining derivatized species.
12.4.2.1
FlA-solid phase molecular spectroscopy with derivatizing reactions
There are two different designs for sensors which monitor a reaction product: (1) on-line derivatization,
before the sample plug reaches the solid phase area and retention of the derivative species on the sensing
zone; or (2) to immobilize the reagent on the sensing support and to generate the reaction product just when
the analyte in the sample plug reaches the sensing microbeads. In this second option, successive reutilization
of the immobilized reagent onto the solid phase is performed. Sensors based on this option integrate the
reaction, the retention (separation/preconcentration) and the detection processes on the solid phase, so these
three processes occur at the same time and in the same place of the flow system, just on the solid phase, while
the former design integrates only the retention and detection processes.
