Chapter: INJECTORS
Topic:
THERMAL DESORPTION
THERMAL
DESORBER
DETAIL
Tocolumn
Tocolumn
For
replacement
tubes.
ccraact:
SANDFlRE
SClENTIFlC LID.
(604) 826-5355
part
00.
5-9100
(pock
of
101Ub<»
10PORTVALVEDIAGRAM
"INJECT" POsmON
Sample
Desorbertube
'-cover hardware
to
PORT VALVEDIAGRAM
"LOAD" POSmON
+-
)/8-
O.D.glass
desorption
tube
1117('),,",,"><--
Gas
inlet
J-..
_~.
)/8-
graphite
ferrule
; .L•..
j~"E-
(CRS
part
#
211600)
+-
Glass YoIQOI
Carrier
in
Carrier
in
+-
Glass YoIQOI
~
Desorber
opening.
Located
on
lop
left side
,
of chromatograph.
HOT.
Desorber--.
With the SRI 8610 Thermal Desorption unit,
samples of soil or other solids can be analyzed for
organic compounds without any extraction orother
special sample preparation. The sample being tested is
placed directly into the
3/8"
O.D. machine glass
desorption tube. The bottom end of the tube is plugged
with glass
wool.
This holds the sample in place without
restricting gas flow.
A
onegramsample weight is
adequate. The open end of the tube is then packed with
glass wool
to
secure the sample
and
inserted
into the
opening of the
3/8"
stainless steel Swagelok* hardware
attached to the pigtailed gas tubing. This hardware is
the desorber
tube
cover and sealsthe organics
in
until
desorbed. The
gas
tubing supplies the carrier gas. The
sample
tube
is then inserted
into
the
heated
desorption
chamber
and
secured by the
3/8"
Swagelok* nut,
When the sample
is
in
place,
the
injection valve is
rotated (eithermanually orautomatically,
if
so
equipped),
and
the volatized organics flow into the
column on the carriergas.
Historically. samples
in
soil have required
solvent extraction
with
methlyene chloride, hexane,
carbon disulfide orothers prior
to
injection
into
a
gas
chromatograph. Unfortunately, solvent extraction often
dilutes the sample and adversely affects
detection
limits. The detection limit for diesel fuel in soil by
extraction is typically
10
ppm. When thermal
desorption is employed, 10
ppb
is attainable. With the
phasing outof theuseof CFCs such as freon and the
ever-increasing
scrutiny
of laboratory solvent usage,
the stripping of analytes from the soil by and into the
column by thermal desorption is a practical (and
sensible) alternative.
In
the past, direct thermal desorption of average
soil samples had been difficult due to the massive
amounts ofwater liberated. This tended to extinguish
the
flame of the
FJD
detector (typical detector for
hydrocarbon analysis). Water elutes along with the
early gasolioe components
and
may interfere with the
quantitation ofbenzene and toluene. Water does not
interfere,
however, with
diesel
quantitation because
the
diesel compooents elute well after the water.
The AD detector-equipped SRI 8610
gas
chromatograph is supplied with an
advanced
desigo
Desorber
ceramic ignitor which can benm hot continuously,
thus re-igniting the AD flame should it momentariIy be
affected
by the
passing water vapor. This minimizes
the water interference
and
flame-out difficulties
normally experieoced with
high
moisture content
samples analyzed with an ADdetector.
