User`s guide

Sampling Techniques
339
NOTE: Complete recovery of analytes can be verified during
method development by redesorbing the same sample to
ensure that none of the components of interest remain.
Calibration of the entire analytical system can be performed using a
simple external standard procedure. Refer to
Calibrating Thermal
Desorption Systems
on page 235 for the details.
Some applications, for example flavor and fragrance profiling, do
not require 100% volatile extraction, but simply that a reproducible
and representative aliquot is desorbed under a given set of
conditions. This produces a profile or fingerprint of the product
against which other samples are compared in order to identify
variations from the control material.
In these cases, a large surface area is not essential and the only
sample requirements are that the sample mass that can be placed in
the tube will:
• allow the carrier gas to pass through
• be representative of the product
• not contain more than 30 to 40 mg of water.
Contamination of the instrument at high concentration levels can be
very time-consuming to remove. Choose a sorbent which allows the
analytes of interest, but which does not allow higher boiling
compounds to pass. Typically, Tenax TA is a good general adsorbent
for these applications.
Example: Using the following analysis parameters, you will be
sampling for residual chloroform:
• Expected concentration: 2%
• Typical mass taken: 20 mg
• A typical sample will contain 400 µg of chloroform
CAUTION
Using a small adsorbent bed inside the tube is highly
desirable to prevent high-boiling components of the sample
(e.g., resins) from getting into the ATD/TD plumbing.