User`s guide

Method Development
236
Four major techniques are commonly used to introduce standards
into desorption tubes.
Vaporizing a Liquid Standard
In the first method, standards are introduced in the vapor phase into a
sample tube in the following way:
A liquid sample is placed in a packed-column GC injector. The
liquid sample is then vaporized in the injector and the vapor is swept
onto the tube packing in the flow of carrier gas.
This method is ideal for applications where components are sampled
onto adsorbents from the vapor phase - for example, during
atmospheric monitoring.
Observe the following information when you are using this method
to load calibration sample tubes.
• Where possible, use a solvent for the standard solution that is not
retained by the adsorbent that has been selected for the
components of interest. If you are able to use a solvent that is not
retained by the adsorbent selected, then you must allow
sufficient time for the solvent to pass completely through the
adsorbent bed before disconnecting the tube from the injector.
Example: Select methanol when you are analyzing aromatics
using Tenax tubes.
• Set the injector to a temperature slightly lower than the boiling
point of the solvent. Do not set the injector temperature any
lower than 50°C below the boiling point. The injector
temperature must also be above approximately
150°C of the
boiling point of the least volatile component of interest.
NOTE: The injector should be operated at ambient temperature
when using volatile solvents such as methanol.
• Set the carrier gas flow rates to between 50 and 100 mL/min.
• Standard injection volumes of 5 µL can usually be introduced
more accurately than 1 µL injections.
• Do not heat the GC oven while you are using the injector to load
samples.
• This method works best for dilute solutions (<0.1%) when
applied to components boiling above approximately 200°C.