gas can be added directly to the hydrogen flame gas for flame
ionization detectors (FID), nitrogen phosphorous detectors
(NPD), and flame photometric detectors (FPD) or added to
the column effluent by an adaptor fitting. However,GCs such
as Perkin-Elmer and Fisons do not require make-up gas.
Combustion type detectors (FID, NPD, FPD) use three gases:
make-up, hydrogen (fuel gas), and air (combustion/oxidizing
gas). For non-combustion detectors, such as the thermal
conductivity detector (TCD), electron capture (ECD), and
photo ionization detector (PID), only carrier and make-up
gases are required. In the case of the electrolytic conductivity
detector (ELCD), the make-up gas is hydrogen, as a reaction
gas in the halogen and nitrogenmode or air in the sulfur
mode. Table I shows recommended gases for various detectors.
Table I - Carrier and detectorfuel
gases used with various GC detectors:
Detector
TCD ECD FID NPD PPD ELCD PID
Carrier
He He
He
He
He
He
He
Gases
H2
H2
H2
H2
H2
H2
N2
N2
N2
N2
N2 N2
Combustion/Reaction
H2
H2
H2
H2
Gases Air Air Air
Make-up N2 N2 N2 N2 N2
N2
Gases He ArCH4
He
He
He
He
Recommended Gas Purities
Gas purity is very important. The expense of using high
purity gases in combination with carrier gas purifiers will be
offset by longer column lifetime and less GC maintenance.
Carrier gas should contain less than lppm of oxygen,
moisture, or other trace contaminants to prevent column
degradation, increase column lifetime, and decrease station-
ary phase bleed. Carrier gas impurities can also contribute to
detector noise. Figure 3 illustrates 0, contamination on a
sensitive ECD and shows how an impure carrier gas can
affect detector performance. Contaminants such as trace
hydrocarbons can be detected by an FID during a temperature
programmed run, causing ghost peaks to appear. Make-up
and fuel gases should be contaminant-free to reduce baseline
fluctuations and excessive detector noise.
Figure 3
1
Impure carrier gas
1. endrin
2. endrin aldehyde
instability on sensitive
3. endrin ketone
!I
detectors.
I
I
I
I
I
I
min.4
8
12
16
:o ;4 :8 42 46 10
PlumbingGases to theGC
Once the proper carrier and detector gases are selected, they
must be connected to the instrument. The gas flow path
travels through regulators, pigtails, tubing, valves, and traps
(Figure 4). Each component in the flow pathwill be dis-
cussed in sequencewith helpful hints on their proper use.
Oxygen Trap:
cat.# 20824
Gas Sources
Gases are most often supplied to the
instrument by gas cylinders. Begin installa-
tion at the cylinder andwork towards the
GC. Cylinders are under very high pressure
and dropping one could result in an ex-
tremely dangerous situation. All cylinders
(full or empty) should be securely chained
to a wall or bench for safety. Any spare
cylinders should also be chained to awall
i:
their storage locationwith the valve cap
intact. It is advisable to have back-up cylinders on all gas
lines to avoid any interruption of flow. This is particularly
important for carrier gas. Should you lose carrier flowwhile
the column is being heated, irreversible column damagemay
occur. Two-stage pressure regulators are used with gas
cylinders to reduce the pressure of a gas supplied from a higl
pressure source to a desiredworking pressure. For safety
n
h
reasons,
when
removing a
regulator
from a
cylinder,
always
position
yourself so
that the
regulator is
pointing
away from
you.
L
4
4
1998
