Dionex ASE 150, Руководство оператора, Страница: 159 / 176

E • Theory of ASE
Doc. 065207-02 9/08
151
smaller amount of solvent being introduced during the heat-up step, as the
static valve pulses to regulate the pressure.
Time
Certain sample matrices can retain analytes within pores or other
structures. Increasing the static time at elevated temperatures can allow
these compounds to diffuse into the extraction solvent. The effect of static
time should always be explored in conjunction with static cycles, in order
to produce a complete extraction in the most efficient way possible.
Generally, ASE extractions are completed in less than 20 minutes, and if
times longer than this are necessary to get complete extraction, then a
higher temperature, different solvent, or multiple static cycles should be
explored to reduce the overall extraction time.
E.4 Method Development
A representative sample should be prepared as outlined in the discussion above on
sample preparation. Select the sample cell size that most closely matches the
sample size. The sample cells do not need to be filled completely; however, a full
cell will use less solvent in the extraction process than a partially filled one. A
recommended set of conditions to begin method development are as follows:
pressure=1500 psi (10 MPa), temperature=100 °C (or 20 °C below analyte
decomposition temperature), static time=5 min, rinse volume=40%, purge
time=60 s (up to 300 s for 100-mL cell), and static cycle=1. If desired, a sequence
can be created to extract each of a series of samples by a different set of conditions
and collected in separate collection vessels. Samples can also be extracted
multiple times to determine extraction completeness. If analyte is present in the
second or third vessel, then the initial conditions need to be changed.
The order of change would be (1) change the solvent, (2) increase the temperature,
(3) add a second or third cycle, and (4) increase the time. In this way, one can
quickly determine the optimized extraction conditions for a particular analyte and
matrix.