Thermo Scientific TriPlus RSH, Руководство пользователя

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Getting Familiar with Your TriPlus RSH
MHE Station
50 TriPlus RSH User Guide Thermo Scientific
The effect of the sample matrix is eliminated by extracting the entire amount of the analyte.
Contrary to the situation that exists in single-headspace extraction, it is therefore possible to
use calibration standards that are
not prepared with the same matrix as the actual samples. For
MHE to provide meaningful results, the only prerequisite is that equilibrium must exist with
respect to the analytes in terms of the distribution between the phases in the headspace vial.
Instrumentation in MHE
In normal extraction using headspace gas chromatographic technique with a gas-tight syringe,
the following steps are employed in order to sample an aliquot of the headspace gas:
The vial is thermostatted in order for equilibrium to be established between the phases in the
headspace vial.
The gas-tight syringe is used to introduce an aliquot of headspace gas to the GC.
Loop-injection headspace samplers use a pressurization step to start the headspace cycle. The
vial is pressurized to a preselected value, usually using Helium gas, which is also often the
carrier gas. The pressure is selected so that it is always greater than the pressure increase due to
sample heating and vapor pressure of the sample.
The sample is introduced onto the GC column for analysis and the system is ready for the
next sampling after cleaning.
In MHE, equilibrium must be reestablished in the headspace following each extraction. With
a syringe-based system the following steps would normally be employed:
• The vial is thermostatted in order for the equilibrium to be established between the
phases in the headspace vial.
• The vial is pressurized.
• The gas-tight syringe is used to introduce an aliquot of headspace gas to the GC.
• The vial is vented. This is accomplished by a specially designed venting tool that
punctures the septum.
The pressurized headspace gas vents through the MHE Adapter needle (venting tool) to
atmospheric pressure;
• Equilibrium is established again before the next analysis (Step 1).
• This procedure is repeated a number of times; typically three to five samplings are
performed from a single vial (analytical sample and standard).
• The main steps are described in the section “MHE Cycle Step-by-Step” on page 54 .
Examples
In the following examples, we will briefly go through the basics of multiple headspace
extraction (MHE).