Choosing the Proper GCGases
Using the correct carrier and detector gases are an important
factor in installing a new GC. The five gases commonly used
as carrier gas and detector fuels in capillary gas chromatogra-
phy are helium, hydrogen, nitrogen, argon-methane, and air.
The types of gases necessary are partly determined by the
detection system used. Factors to consider for each individual
gas are discussed below.
Carrier gas choice
Carrier gases that exhibit a broad minimum on a van Deemter
profile are essential in obtaining optimum performance.
Volumetric flow through a capillary column is affected by
temperature.When temperature programmingfrom ambient
to 3OO”C, the flow rate can decrease by 40 percent. A carrier
gas that retains high efficiency over a wide range of flow
rates and temperatures is essential in obtaining good resolu-
tion throughout a temperature programmed run. Figure 1
shows the vanDeemter profile for hydrogen, helium, and
nitrogen carrier gases.
Exert Caution when Using Hydrogen as a Carrier
Gas
Hydrogen is explosivewhen concentrations exceed 4% in air.
Proper safety precautions should be utilized to prevent an
explosionwithin the column oven.Most gas chromatographs
are designedwith spring loaded doors, perforated or corru-
gatedmetal column ovens, and back pressure/flow controlled
pneumatics tominimize the hazardswhen using hydrogen
carrier gas. Additional precautions include:
l
Frequently checking for leaks using an electronic leak
detector (Restek’s Leak Detective’“, cat.# 21607, 110 volts/
cat.# 21609,220 volts).
l
Using electronic sensors that shut down the carrier gas flow
in the event of pressure loss.
l
Minimizing the amount of carrier gas that could be expelled
in the column oven if a leakwere to occur by installing a
flow controller (needle valve) prior to the carrier inlet
bulkhead fitting to throttle the flow of gas (for head
pressure controlled systems only) as shown inFigure 2.
Figure
1 -
vanDeemter Profile of Hydrogen,
Helium, and Nitrogen
1.5 -
C-17at160°C(k=5)
z
30m,
0.25m
m
1.0
-
ID,
0.25pm
FIti-
.z.
.
:
2 os-
0°0
,
10
2 0
3 0
4 0
5 0
6 0
7 0
6 0
9 0
Average
Linear Velocity (cmlsec.)
Hydrogen is the fastest carrier gas (Uopt,), with an optimum
linear velocityof 40cm/sec, and exhibits the flattest van
Deemter profile. Helium is the next best choice; with an
optimum linear velocity of uopt = 20cm/sec. Nitrogen’s
performance is inferior with capillary columns because of its
slow linear velocity, Uopt = 12cm/sec. Argon-methane has a
slower optimum linear velocity than nitrogen and is not
recommended for use as a carrier gas with capillary columns.
Air is not recommended as a carrier gas because it can cause
stationary phase oxidation.
With hydrogen and helium as carrier gases, theminimum
H.E.T.P. values can bemaintained over a broader range of
linear velocities thanwith nitrogen, and high linear velocities
can be usedwithout sacrificing efficiency. Nitrogen is
beneficial onlywhen analyzing highly volatile gases under
narrow temperature ranges where increasing stationary phase
interaction is desirable. Otherwise, the use of N2 results in
longer analysis times and a loss of resolution for compounds
analyzed on a wide temperature range.
Figure 2 -
Use
a
flow
controller or needle valve to
throttle
the
flow
of H, carrier gas should a leak occur.
Pressure
Flow controller regulator
Needle
(needle valve)
valve
I
I
IT
I
/
Optional
safety
device
-
Needle
vabe
Inlet seal
Inlet sleeve
3
To detector
Purge
vent
Split
vent
Fully open the flow controller (needle valve) and obtain the
proper column head pressure, split vent flow, and septum
purge flow rates. Decrease the needle valve flow rate until the
head pressure gauge begins to drop (throttle point). Next,
increase the flow controller (needle valve) setting so that the
right amount of flow is available to the system. Should a leak
occur, the flow controllerwill throttle the flow, preventing a
large amount of hydrogen from entering the oven.
Make-up and Detector Fuel Gases
Choosing the correctmake-up and detector gaseswill depend
on
both the detector and application. Most GC detectors
operate best with a total gas flow of approximately 30ml/min.
to ensure high sensitivity and excellent peak symmetry. Refer
to yourGCmanual for optimum flow rates on different
instruments. Carrier gas flows for capillarycolumns range
from 0.5 to lOml/min. which arewell below the rangewhere
most detectors exhibit optimal performance.Tominimize
detector dead volume,make-up gas is often added at the exit
end of the column to increase the total flow entering the
detector.Make-upgas helps to efficiently sweepdetector
dead volume thereby enhancing detector sensitivity.Make-up
3
3
1998
