Dionex ASE 150, Operator'S Manual

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ASE 150 Operator’s Manual
150
Doc. 065207-02 9/08
There appears to be confusion about the purpose of the preheat function
available on ASE systems. When this is used, a cell is placed in the oven
and heated without any solvent being pumped into the cell. This function
was originally developed to allow pre-extraction derivatization of
analytes or the drying of samples by blowing nitrogen through the cell
while it is heated. The preheat function should
not be used in routine ASE
operation. In general, this should always be set to zero minutes. If used,
recoveries of more volatile compounds (for example, organochlorine
pesticides) will be greatly reduced.
Pressure
The effect of pressure is to maintain the solvents as liquids while above
their atmospheric boiling points, and to rapidly move the fluids through
the system. The pressures used in ASE are well above the thresholds
required to maintain the solvents in their liquid states, so pressure
adjustments for changing solvents are not required. Changing the pressure
will have very little impact on analyte recovery, and it is not considered a
critical experimental parameter. ASE extractions are performed at
1500 psi (10 MPa).
Cycles
The use of static cycles was developed to introduce fresh solvent during
the extraction process, which helps to maintain a favorable extraction
equilibrium. This effectively approximates dynamic extraction conditions
without the need for troublesome flow restrictors to maintain pressure.
When more than one cycle is used in a method, the rinse volume is
divided by that number. When the first static time is complete, the divided
portion of the rinse volume is delivered to the cell, with the “used”
solvent directed to the collection vessel. The system then holds the
sample and solvent for a second static period. The nitrogen purge step is
initiated only after the final static cycle. Because the original rinse
volume has only been divided, no additional solvent is used for the
extraction. Static cycles have proven to be useful for sample types with a
very high concentration of analyte, or samples with difficult to penetrate
matrices. The static time can be adjusted to minimize the total extraction
time. For example, three 3-min static cycles can be used in place of one
10-min static step. When low temperature extractions are desired
(< 75 °C), multiple static cycles should be used to compensate for the