Chemistry Reference
In-Depth Information
Multi-analyte procedures
Multi-analyte flow-through optosensors are able to determine more than one
analyte (typically two or three analytes) per sample. Figure 12.4 summarizes the described strategies for these
sensors. As can be seen, there are two main groups:
1)
Systems with simultaneous determination of two or three analytes from only one injection.
2)
Systems performing sequential determination of two analytes (requiring two successive injection of sample).
Simultaneous determinations
There are two main options for performing simultaneous determinations:
1)
Without performing separation previous to detection. These are systems which combine SPMS-FIA with
multicalibration chemometric approaches. Up to three analytes can easily be simultaneously determined
with only a single injection in a carrier stream using a diode array detector to acquire the solid phase UV
spectral data [35] or a spectrofluorimeter when the analytes pass through the sensing zone.
2)
On-line separation previous to detection. Several options have been described:
a) A minicolumn placed on-line retains one (or two) of the analytes, while another one passes to the
sensing zone generating its transitory signal. The further elution of the second (and, if it is the case,
the third one) analyte from the minicolumn by appropriately chosen eluting solution(s) allows the
simultaneous multicomponent analysis with only one sample injection in a manifold which provides
the temporary sequentiation of the analyte arrival to the sensing microbeads, by means of the
separation performed in the minicolumn [36].
b) The use of photo induced fluorescence (PIF) has also been implemented in the development of
sensors for simultaneous determination of two analytes. Similar strategy to that just above described
was used. Although a photoreactor on line is included before the detector [37], no chemical reagent
is used, so these multisensors can be also considered as belonging to the same class that those ones
included in this section.
The multidetermination in these three described cases is performed on the sensing support for all
the analytes. This design imposes an additional selectivity condition: the retention of the analytes in
the minicolumn, so making the multisensor more versatile and selective. In this easy way,
multideterminations are performed with a single GAC SPMS system.
c) The described set up in point (a) can still be simplified without using the minicolumn and substituting
it by a little additional amount of sensing microbeads inside the flow cell above the detection area.
This strategy works in a similar way to that from on-line minicolumn separation, but it saves solid
phase, and simplifies the manifold, with favorable subsequent advantages [19]. In this system, the
analytes have to show a very different affinity behavior towards the sensing phase in order to get the
separation inside the flow cell.
d) Another interesting strategy in multi-determination sensors is to perform a dual functioning of the
detection zone containing the sensing support: (i) detection in the interstitial (homogeneous) solution
between solid microbeads, which offers a very low effective path length (first analyte) and (ii) the solid
phase transitory measurement of the second analyte as it passes through the flow cell, after eluting it form
the online minicolumn, being eluted by the carrier itself. This strategy is used when the first analyte
cannot be retained on the sensing support and its concentration is much higher than the second one [38].
Sequential determinations
Unlike simultaneous determinations, in sequential determinations, one injection
per each analyte is needed. Different systems for the determination of two analytes have been described. The
described approaches are based on the following strategies:
