1
Getting Familiar with Your TriPlus RSH
MHE Station
Thermo Scientific
TriPlus RSH User Guide
51
We will look at four examples
situations
of equilibrium between the condensed (solid or
liquid) phase and the gas phase (the headspace). The examples will be shown in increasing
complexity with respect to the equilibrium distribution and removal of the headspace between
subsequent extractions.
In each of the four cases described below, the complexity of the equilibrium distribution and
of the removal of the headspace between subsequent extractions will be increased. The final
conclusion is that the MHE method will work in most cases, with the only prerequisite that
the distribution of the analyte between the gas and the condensed phases is actually governed
by an equilibrium.
Situation 1
The equilibrium between the solid phase and the gas phase is assumed to be a
50/50-distribution. In the course of each extraction,
all
of the headspace is
removed/exchanged.
The measured area of the analyte peak is
A
.
A(ref )
— Area we would measure if all of the analyte were in the headspace (corresponding to
the measured signal from a similar gas standard).
A(0)
— Measured area of the analyte peak from the first extraction.
With 50% of the analyte present in the headspace, this amounts to:
A(0) = ½ * A(ref )
The total area that is determined over a number of subsequent extractions is:
A(tot) = A(0) + ½ * A(0) + ¼ * A(0) + 1/8 * A(0) + ……, or
A(tot) = A0 / (1-½) = 2 * A0
However, A(0) = ½ * A(ref ) and therefore:
A(tot) = A(ref )
Thus, it would be correct to determine the total amount of analyte by comparing the
measured sum of analyte areas to a calibration measurement with data from a single
determination of a gas standard.
Situation 2
The equilibrium between the condensed phase and the headspace is assumed to be 25/75
(with 75% in the gas phase). Furthermore, we assume that the entire headspace is exchanged
during the course of each extraction step.
The measured area of the analyte peak is
A
.
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