The PinnacleTM II Phenyl phase, possessing hydrophobic dipolar
heads, provided enhanced selectivity for unsaturated compounds.
– interactions with the phenyl moieties of the analytes occurred
during the chromatographic run. The satisfactory performance of
this column, in terms of efficiency and analysis time, is demonstrated
by the data reported in Fig. 2 and Table 2.
3.2. Increasing sensitivity: on-column focusing
The reduction of the column ID offers an interesting advantage
in relation to the analytical sensitivity. This effect is mainly due to
the reduction of chromatographic peak dilution and to increased
efficiency [18,19]. However, the lowinjection volumes employed in
all miniaturized techniques, determines an obvious loss in detection
sensitivity, representing amajor challenge in the development
of a new method.
Limits of detection (LOD) measured, analyzing the pesticide
mixture using the PinnacleTM II Phenyl stationary phase under the
experimental conditions reported in Table 1 and Fig. 2, were in the
0.42 and 0.65g/mL range. Such values were not satisfactory for
trace analysis, and therefore other analytical options were investigated.
Sensitivity can be enhanced by using different approaches;
mass-spectrometric hyphenation, for instance, ensures a higher
sensitivity, but is not always applicable. A valid alternative is
represented by the possibility to perform large-sample volume
injections, following specific procedures that prevent column
overloading. The use of micro-pre-columns coupled to switching
systems [27] and on-column peak compression (also defined “oncolumn
focusing”) are the most applied techniques, of rather recent
description.
The first study dealing with on-column focusing, dates back
to the 1980s. The approach enables the injection of large sample
volumes, increasing sensitivity with no influence on efficiency. As
previously reported [28–30], the sample is diluted in a solvent with
lower elution strength, than that of the mobile phase. Following
the injection, analytes are concentrated (or “focused”) at the head
of the column, generating a narrow band adsorbed on the packing
material. Afterwards, an appropriate mobile phase is selected to
selectively release each compound, minimizing any band broadening
effects.
On the basis of these considerations, on-column focusing was
chosen as the most suitable methodology to enhance sensitivity.
The injection volume was increased from 60L to 1L, while
water (as a non-eluting phase) appeared to be the most appropriate
solvent for the sample injection plug. Moreover, based on
an interesting study described by El Rassi and Tegeler [31], showing
the usefulness of the presence of small amount of ACN in the
mobile phase in on-column focusing, the OPPs were dissolved in
water/ACN mixtures, with the organic modifier amounts changed
in the 0–40% (v/v) range.
The PinnacleTM II Phenyl phase, possessing hydrophobic dipolar
heads, provided enhanced selectivity for unsaturated compounds.
– interactions with the phenyl moieties of the analytes occurred
during the chromatographic run. The satisfactory performance of
this column, in terms of efficiency and analysis time, is demonstrated
by the data reported in Fig. 2 and Table 2.
3.2. Increasing sensitivity: on-column focusing
The reduction of the column ID offers an interesting advantage
in relation to the analytical sensitivity. This effect is mainly due to
the reduction of chromatographic peak dilution and to increased
efficiency [18,19]. However, the lowinjection volumes employed in
all miniaturized techniques, determines an obvious loss in detection
sensitivity, representing amajor challenge in the development
of a new method.
Limits of detection (LOD) measured, analyzing the pesticide
mixture using the PinnacleTM II Phenyl stationary phase under the
experimental conditions reported in Table 1 and Fig. 2, were in the
0.42 and 0.65g/mL range. Such values were not satisfactory for
trace analysis, and therefore other analytical options were investigated.
Sensitivity can be enhanced by using different approaches;
mass-spectrometric hyphenation, for instance, ensures a higher
sensitivity, but is not always applicable. A valid alternative is
represented by the possibility to perform large-sample volume
injections, following specific procedures that prevent column
overloading. The use of micro-pre-columns coupled to switching
systems [27] and on-column peak compression (also defined “oncolumn
focusing”) are the most applied techniques, of rather recent
description.
The first study dealing with on-column focusing, dates back
to the 1980s. The approach enables the injection of large sample
volumes, increasing sensitivity with no influence on efficiency. As
previously reported [28–30], the sample is diluted in a solvent with
lower elution strength, than that of the mobile phase. Following
the injection, analytes are concentrated (or “focused”) at the head
of the column, generating a narrow band adsorbed on the packing
material. Afterwards, an appropriate mobile phase is selected to
selectively release each compound, minimizing any band broadening
effects.
On the basis of these considerations, on-column focusing was
chosen as the most suitable methodology to enhance sensitivity.
The injection volume was increased from 60L to 1L, while
water (as a non-eluting phase) appeared to be the most appropriate
solvent for the sample injection plug. Moreover, based on
an interesting study described by El Rassi and Tegeler [31], showing
the usefulness of the presence of small amount of ACN in the
mobile phase in on-column focusing, the OPPs were dissolved in
water/ACN mixtures, with the organic modifier amounts changed
in the 0–40% (v/v) range.
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