at a maximum, of complete system
control and data output. Within each
part of the sampling system there are
multiple requirements for subsystems
to monitor and control the stable
operation of the overall system. To
understand specific operations we
will discuss these in the following
sections.
Gas Analytical System
Design: Gas Sample Handling
Overview
Perhaps the most important part of
any FTIR gas analytical system is the
sampling system itself. A simple basic
requirement, yet one which is quite
often initially overlooked, is that the
system must be non-reactive to the
analytes of interest in the gas stream.
If the sample integrity can’t be maintained between the origination point
and the FTIR it cannot accurately be
measured. In some applications, such
as engine exhaust analysis, the sampling lines between the origination
point and the FTIR system are heated
to prevent any possible condensation
of analytes prior to analysis. In applications such as these, the sampling
line and the gas sample cell itself may
be heated to temperatures as high as
185oC to 200oC. Two variations on a
design are shown in Figures 1 and 2.
Figure 1 shows a system design
for multiple compressed gas cylinder
samples including a sample divider/
diluter to assist in the generation of
calibration curves. Each cylinder is
connected to a high purity gas regulator that connects to a main sampling
manifold by a control valve. These
valves are of the simple on/off style
and can either be manual or controlled via an automated system.
After pressure reduction and sample
introduction into the manifold, the
gas proceeds through a mass flow
controller that establishes a controlled flow of gas through the gas
sample cell. The actual flow through
the cell will need to be fast enough
to provide a sample flush (or turn-over) that matches analytical speed
Figure 1. Typical FTIR gas analytical system for automated analysis of pressurized source samples.
Figure 2. Typical FTIR gas analytical system for analysis of ambient or below
ambient pressure source samples.
requirements, but slow enough to
not contribute to noise in the measured spectrum. The analytical speed
of the system can be defined as the
“temporal resolution”. In other words,
how long it takes to cycle the system
through from the end of one analysis
to the end of a new analysis. After
the mass flow controller, the sample
travels through a sample preheater
into the gas cell in the FTIR. The gas
then passes out of the cell through
a digital back pressure regulator
that controls sample pressure in the
cell and vents from the system in an
appropriate manner (with scrubber
technologies if necessary for environmental concerns).
Figure 2 shows a similar system but
instead of pressure driving the sample
through the system, a vacuum pump
pulls the sample from an ambient
pressure condition. This configuration is especially useful for ambient air
sampling, automotive exhaust sampling, and smoke stack sampling.
Having seen the basic layout of a
typical FTIR gas analytical system, we
can now examine each individual part
in greater detail and discuss operation
and requirements. For the purposes
of this discussion we will consider the
