Chemistry Reference
In-Depth Information
polar analytes from aqueous matrices, and in the synthesis of class-selective immuno-sorbents (ISPEs) and
molecular imprinted polymers (MIPs) allowing an increased selectivity of the retention process and so
contribute to simplify the subsequent clean-up and/or detection steps [20, 21]. As an example, in a recently
published study, five ionic liquid-modified porous polymers with different imidazolium-based functional
groups were obtained and a new molecular imprinting technique was introduced to form the ordered
functional groups in the porous structure [22]. The sorbent finally selected allowed selective SPE of
tanshinone commercial functional groups.
The progress achieved in past decades in the synthesis of sorbent materials allowed obtaining very pure
small-size sorbent particles in a very reproducible way. These small particles (ca. 40
m) provided higher
retention capacities than conventional-size ones, an interesting feature that promoted the use of smaller SPE
cartridges without a significant lost of retention efficiency and with adequate breakthrough volumes. In fact,
reducing the size of the conventional 1-6 ml SPE syringe barrels to the 10 mm × 1-2 mm i.d. of the so-called
Prospeckt-type cartridges used in the hyphenated systems led to a reduction of sample volumes from
0.5-1.0 l to less than 50-100 ml. Actually, quite often, even 5- 10 ml suffices to obtain similar LOD of
0.01-0.1
μ
g l
−1
with SPE-LC and 1000-fold lower with SPE-GC that previously required 100-fold larger
volumes. More importantly, quantitative elution of the analytes can be achieved with 50-100
μ
l of the
appropriate solvent, that is, with a volume small enough to allow complete transfer to the instrument selected
for final determination. This fact promoted the development of hyphenated and automated systems for
(unattended) and fast (ca. up 20 min) treatment of aqueous samples with minimal solvent consumption and
wastes generation [23]. A scheme of an on-line SPE-GC-MS system is shown in Figure 17.3.
In principle, the small volumes and short analytical times involved in these valve-based hyphenated
miniaturized SPE systems should made method development faster than in conventional approaches. The
experimental parameters to consider during method optimization are the same as for conventional SPE,
namely the nature and amount of sorbent, the nature of the solvents used in the different SPE steps and, in
particular, their flow rates. The main reasons for low analyte recoveries are in general also similar: reduced
sorbent capacity or too strong retention, slow kinetic of the sorption process (or, in other words, too high
sample and/or solvent flow rates), and a possible adsorption of the analytes in the tube used to connect the
different parts of the system. On the other hand, in these close systems the risk of analyte degradation and
oxidation is greatly reduced as compared to the (open) conventional approaches. Finally, the exposition of
operator to hazardous solvents is greatly reduced.
Accepting that the hyphenation of SPE with LC and GC can nowadays be considered achieved goals, at
present, development in this area is mainly orientated to the progressive reduction of the sample amount
required for accurate determination of trace compounds and to the further simplification of the sample
treatment methodologies. Both aspects usually rely on the use of any of the high-capacity or highly selective
sorbents nowadays available and/or the use of very powerful MS- [25] or, preferably, MS/MS-based [26]
detectors. Such approaches have been demonstrated to be useful for extremely fast determinations that could
hardly be carried out with analysis times similar to those involved by conventional sample treatment
procedures. For example, replacing the SPE-LC part for a single short column, SSC (1-2 cm length), and
using MS-MS as detection system facilitated the real-time study of analyte degradation at the trace level [26,
27], with LC run times of frequently, only some 3 min. A slight modification of the conventional valves
configuration typically adopted for miniaturized SPE to introduce a filter before the SPE cartridge allowed
the direct injection of soil and sediment slurries in the SPE-GC-MS system. This set-up allowed in-deep
evaluation of the so-called fast adsorption of pesticides in these complex matrices by simultaneous analysis
of both phases (i.e. water and soil/sediment) in less than 45 min and with a single injection of the mixture in
the system [28]. Finally, the progressive reduction of the sample size required to perform these types of
hyphenated analyses has made possible, in some cases, the direct injection of the aqueous sample [29] or of
the aqueous extract obtained from fruits and vegetables [30], with LODs low enough to consider the methods
μ
