Answers to Questions - Forensic Applications of Gas Chromatography - page 137

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Answers to Questions

Chapter 2

1. What can you determine from the van Deemter plot (Figure 2.2)
with regard to the choice of carrier gas?
Answer: The best compound separation is obtained with the low-
est value of HETP (millimetres). In addition, changing the linear
velocity of gas to a value that is too low or too high has a detrimen-
tal effect on HETP (and hence the resolution). Nitrogen has the
best column efficiency at the lowest linear velocity; however, the
minimum HETP occurs over a narrow range. Both helium and
hydrogen have a much broader range of linear velocities, giving low
values of HETP (i.e., greatest efficiency to separate compounds).

2. What is the optimal linear velocity for helium?
Answer: The optimal linear velocity for helium from the van Deemter
plot (Figure 2.2) is the point with the lowest HETP (millimetres).
On that basis, 20 cm/s is the optimal linear velocity for He.

3. What is a molecular sieve?
Answer: Molecular sieves are crystalline, highly porous, alumina
silicates. In this case, they are used to remove moisture from the
gas supply.

4. What issues would you need to consider when deciding whether to
use a cylinder of nitrogen versus a generator?
Answer: The main issues for use of a cylinder include the cost of the
purchase of gas, cylinder rental cost and delivery regime as well
as the capital cost of a pressure regulator. In the case of the nitro-
gen generator, the main issues relate to capital cost and regular
annual maintenance. In addition, it is also important to con-
sider the safe storage of pressurised cylinders in the laboratory
(or annexe to the laboratory or external to the building). From a
cost perspective, it is important to consider how many nitrogen
cylinders are required and how long each cylinder will last in
normal operation (depends upon how many GCs are in use from
a cylinder) versus the relatively high capital cost of purchasing a
nitrogen generator.