path cell designs have fixed pathlengths and
others have adjustable pathlengths. Adjustable pathlength gas cells are designed to
allow the number of beam reflections (and
hence the overall pathlength) to be controlled by an external mirror adjustment.
While this design is certainly more flexible
from an analytical standpoint there is usually
a loss in overall system alignment stability.
Figure 6. Overlaid spectra of 90.9 ppm methane in nitrogen taken with a spectral resolution setting of 0.125 cm-1 at pressures of 20 psia and 40 psia. Notice the peak broadening
at the higher pressure.
Figure 3. Laboratory product FTIR spectrometer fitted with a 10 meter gas cell.
Figure 4. Industrial online product FTIR spectrometer fitted with the same 10 meter gas
cell. This product is purpose-built for automated service and shown as a bench top unit
and as a fully enclosed rack mounted unit.
Figure 5. Illustration of a single
optical bounce in
a multiple bounce
/ folded path FTIR
gas cell. These
are also known as
“White cells”.
Gas cells most often used in FTIR gas
analytical systems are in the range of 2 to
10 meters in pathlength. A 10 meter cell
provides very high sensitivity (see Table 1
for some common compounds and their
detection limits). However, the cell volume
is normally rather high—on the order of 2
liters. If the sample flow through the system
is set to 3 liters per minute, a cell changeover
calculated purely by cell volume and accomplished by purge method requires about 40
seconds. A minimum of 3 to 6 cell changeovers will completely purge and refill a cell
of this volume. This dictates a 2 to 4 minute
delay time when the sample is changed or
the system set for re-zero. If a vacuum pump
is used to evacuate the cell and allow refill,
this time can be reduced to 30 to 60 seconds
delay time between sampling. Purge and fill
or evacuation and fill will have a significant
impact on the temporal resolution of the
system.
A two meter gas cell of the same design
will have limits of detection approximately
10 times higher than those shown for 10
meter cells in Table 1. However, the cell
volume is also reduced by a factor of 10
to approximately 200 mL. Therefore, for a
sample flow rate of 3 liters per minute the
same 3 to 6 cell changeovers can be accomplished in about 15 to 30 seconds. This is on
the same order of time whether a vacuum
purge is used or not.
Gas cells are designed to be resistant to
some level of reactive materials. Most gas
cells are stainless steel or of aluminum fabrication, possibly electropolished, with either
nickel or gold coatings. Cell window materials are available in a wide variety of materials
but the most popular are either coated KBr
or ZnSe materials. Both of these substances
have high infrared transmission profiles, are
resistant to most chemicals, and can tolerate heated cell temperatures. While most
cells are capable of temperatures as high as
185 C to 200 C, most applications benefit
oo
from only moderate heating, (temperature
