SKC Limited AirChek 3000 Deluxe Pump Step By Step Guide - Manual (Page 15)

www.skcltd.com 224-G1 Issue D Page 13
Sorbent Tube Features
Gas & Vapour Sampling
To sample for gases or vapours, especially for
longer periods such as 8 hour TWA, the use
of sorbent tubes is a widely accepted method.
A Sorbent is like a sponge, which soaks up
the molecules of the material being sampled
(which can subsequently be extracted and
analysed). Activated charcoal is the most
commonly used material but it cannot adsorb
every substance, which is why other materials
such as Silica Gel and Tenax are often used.
Once the pollutant has been trapped by a
sorbent tube it then needs to be removed for
analysis. This is achieved using either a solvent
wash or heat, which drives the collected
chemicals off the sorbent and into an analytical
instrument such as a Gas Chromatograph.
Most tubes have two layers of sorbent in them,
the smallest of which (usually situated nearest
the pump) is known as the backup. Analysis
of the backup sorbent reveals whether
breakthrough has occurred (see figure below).
Breakthrough occurs when the main layer of
sorbent (nearest the open end of the tube)
has become flooded with the contaminant
chemicals which then overflow into the backup
layer.
Gas is sampled in one of the two following
ways using a sorbent:
1. Active sampling using pumps and tubes.
2. Passive (or Diffusive) sampling using
badges.
There are many different types of sorbents
and an even larger number of different sorbent
tubes. Each tube is designed and validated
for a specific substance. Sorbent tubes vary
in size from the Standard Charcoal Tube (6 x
70mm) to ''Jumbo Tubes'' (10mm x 110mm)
and they have different separators between
the sorbent beds.
NIOSH approved sealing caps
prevent contamination
High purity glass wool
precise amount for uniform
pressure drop
Glass tube
drawn to very close tolerances
for repeatable results
Backup sorbent layer
detects sample breakthrough
Precision lockspring
holds sorbent layers securely
in place to prevent sample
channeling; allows transporting
without damaging sample
(patented)
Sorbent layer
precisely controlled surface
area, pore size, adsorptive
characteristics, particle size
Foam separator
for uniform pressure drop
Precision sealed tips
permit safe, easy breaking to the
specified opening size
Contaminant
saturates sorbent
leading to
break through
Sorbent Break Through
All contaminant
adsorbed by
sorbent