Chemistry Reference
In-Depth Information
high sample volumes should be used [2]. On the other hand, high selectivity can be reached by appropriately
choosing working conditions and using a few ml of sample volume [9] in one step procedure without
derivative reagents. Nevertheless, SPMS in batch mode is time and labor consuming due to bead
manipulation stages (load, unload, filtration, transferring to cell, etc.). With a set of six cells, about 10-12
analysis can be performed per hour working with 10 ml sample volume. After measurement, the beads are
usually discarded, thus involving the use of large amounts of solid support. In this sense, the implementation
of SPMS with flow analysis methodology became one of the most relevant advances in SPMS. The solid
microbeads are placed inside an appropriate flow cell, so that successive determinations are performed on
the same beads, as the solid phase can be regenerated after each measurement. Therefore, important saving
of reagents, sample solution and waste generation is achieved. Moreover, relevant improvements in
accuracy, reliability, precision, sample throughput and versatility are other remarkable features of this
implementation.
This concept, first introduced by Ruzicka and Hansen [16] as the 'flow-through optosenso´, has miniaturized
the SPMS methodology and its analytical potential has been widely demonstrated, including easy-to-perform
determinations on a same sample in very different fields of the chemical analysis [17, 18, 19]. It works
similarly to a liquid chromatographic process in which the molecular spectroscopic detector is continuously
monitoring the signal on the separation column itself: separation and detection processes are integrated and
they occur at the same place and at the same time.
Taking into account these features, flow-based SPMS is a more environmental friendly principle than
SPMS in batch mode. A key aspect of flow SPMS to be pointed out, is the reutilization of the solid microbeads,
which allows to perform a high number of successive determinations, that is, the need to achieve the retention
of the analyte to be reversible, so that the active sites of the sensing surface keep ready for the next
determination. Thus, a few mg (typically 30-50 mg) of beads can be reused hundreds of times, drastically
reducing the amount of solid phase required.
There are two ways of performing the regeneration (elution step) of the solid surface: (1) after the signal
reaches its maximum value, an eluent solution (either via injection of a known volume of eluent or using
a selection valve) is passed through it and (2) to choose an appropriate carrier able to also elute by itself
the retained species [11], so originating a transitory signal. The first option shows a lower sample throughput
and in addition, a more reduced lifetime of the solid support. Both drawbacks are overcome by using the
second option.
The most usual assembly for implementing SPMS with flow analysis is the conventional flow injection
analysis based approach (FIA), and, in a minor extension, the multicommutation principle (MC). These two
implemented flow-SPMS methodologies are particularly attractive from the GAC point of view when
detection is based on monitoring of an intrinsic property. In the following section, these two approaches will
be presented and discussed along with alternative flow SPMS systems such as Sequential Injection Analysis
(SIA) and Bead Injection Spectroscopy (BIS).
12.4.1
Monitoring an intrinsic property
When an intrinsic analyte property is monitored, flow-SPMS is a valuable contribution to GAC. For instance,
measurements in UV region in conventional homogeneous solution exhibit scarce selectivity. Nevertheless,
to perform the on-line spectroscopic measurements directly on the solid phase, increases drastically the
selectivity towards the targeted species compared to respective homogeneous solution measurements. The
selected operating conditions (nature of solid support, pH, polarity, retention behavior, etc.) provide
appropriate selectivity conditions, contributing to the development of single and rapid procedures fulfilling
the GAC requirements [20].
For this purpose, the more common approach is FIA-SPMS. The first attempt was the flow solid phase
spectrophotometric determination of Cu(II) ion based on the use of a cation-exchange resin as solid phase and
