Universal Gas Law Parameters; Pressure; Vial Pressurization - Teledyne Tekmar Versa User Manual

UniVersaL gas Law Parameters

Since the headspace that is sampled using this technique is a gas, its characteristics are dictated by the Universal Gas
Law, PV=nRT where P is Pressure, V is Volume, n is Number of Moles or Mass, R is a constant and T is temperature. In
headspace analysis "n" equals number of moles, which directly correlates to mass or sensitivity. If the Universal Gas Law
is solved for "n", you will see that pressure and volume have a direct effect on the amount of mass, and temperature
has an indirect effect on mass. In the Versa, the user has control of all of these functions.

Pressure

Control of the Universal Gas Law is mainly used for the control of the mass in the loop of the sample that is actually
transferred to the GC column. The one exception to this is the pressure component of the equation. Since pressure is
controlled in two distinct areas of the headspace system, it influences the amount of mass transferred in two ways. The
first pressure step is the pressurization of the vial and the second pressure step is the pressure within the loop prior to
transfer to the GC for analysis. The effects of pressure on both of these modes is explained below.

Vial Pressurization

Vial pressurization mode pressurizes the vial with pressurize gas prior to allowing the sample to flow through the
loop. The vial needs to have enough pressure to allow it to adequately and repetitively fill the sample loop. Typically,
the sample will generate what is called a static vial pressure during the sample equilibration step. This pressure will
normally range from between 0.5 to 11psig depending on the sample. To determine the pressurization setting,
the static vial pressure needs to be determined. Allow a minimum of three vials to equilibrate using the platen
temperature and thermal exposure times that will be used to analyze samples. A minimum of three samples should be
used to get the average Static Vial pressure. Once the samples have been cycled through the Versa, the user can check
the schedule or the system history in the TekLink™2G software and view the recorded static vial pressure. After the
average static vial pressure has been calculated, a vial pressurization should be selected that is 2-4psi above the static
vial pressure.
NOTE: The pressurization gas that is being added is a dilution gas thus the lower the value, the higher the sensitivity.
In rare cases, samples within a lot do not generate the same static vial pressure, resulting in differing amounts of
pressurization gas being added to each vial. This results in different dilution factors being applied to the samples
making the data incomparable. In this rare case, a volumetric pressurization should be selected. This can be done by
selecting the maximum pressurization setting (28psig) and a relatively short pressurization time 0.5 – 1.0 minute. This
will make sure a consistent volume of gas is added to each vial, which is equal to 20mL/min. pressurization time.
NOTE: If the pressure setting is reached prior to the pressurization time, the unit will step to the next mode. If
the pressure setting is not reached within the pressurization time, the unit will step to the next mode upon
completion of the pressurization time.
Versa User Manual
Method Development • Page 6-5
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