Dionex ASE 150, Руководство оператора

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E • Theory of ASE
Doc. 065207-02 9/08
149
Solvents that exhibit marginal results at ambient conditions may perform
adequately under ASE conditions. Most liquid solvents, including water
and buffered aqueous mixtures, can be used in ASE. Strong acids (HCl,
HNO
3
, H
2
SO
4
) are not recommended for use with stainless steel cells
because they may damage the cells. Small amounts of these acids can be
used with zirconium cells
only . When required, weak acids such as acetic
or phosphoric can be used. These should be added to aqueous or polar
solvents in the 1-10% (v/v) range.
For extraction of polymer samples, select a solvent that will extract the
additives but not the matrix itself. For example, isopropanol with small
levels of cyclohexane has been shown to work well for the extraction of
phenolic antioxidant additives in polymers.
Temperature
Temperature is the most important parameter used in ASE extraction. As
the temperature is increased, the viscosity of the solvent is reduced,
thereby increasing its ability to wet the matrix and solubilize the target
analytes. The added thermal energy also assists in breaking analyte-
matrix bonds and encourages analyte diffusion from the matrix surface.
When developing a new method, start at 100 °C, or if the target analytes
have a known thermal degradation point, start at 20 °C below this level.
Most ASE applications operate in the 75 to 125 °C range, with 100 °C the
most common temperature for environmental applications. If the sample
has a tendency to melt in the sample cell, a cellulose thimble can be used
to facilitate extraction and sample removal. An example of the effect of
temperature is shown below for the extraction of petroleum hydrocarbons
(TPH) from soil. Note that not only does the analyte recovery increase,
but the reproducibility improves as a function of temperature.
Temperature Recovery RSD (%)
27 °C 81.2 6.0
50 °C 93.2 5.0
75 °C 99.2 2.0
100 °C 102.7 1.0
Samples were analyzed by IR, with n = 5.
Table E-1. Effect of Temperature on TPH; Extraction from Soil