and hence the speed of the analysis itself.
There is no limit to the spectral resolution
achievable by this detector, however the
slower acquisition speed leads to long data
accumulation times for high resolution
scans (0.5 cm-1 and below). Many FTIR gas
analytical systems have been designed and
constructed using this detector operating
at a 2 cm-1 resolution measurement setting.
They are robust, simple, and very effective.
MC T (mercury cadmium telluride) detectors are superior in several ways. However,
there is one serious drawback to their use
in an automated system. First, an MCT
detector exhibits superior signal-to-noise
ratios (typically 10 – 100 times greater)
than a DTGS detector. Second, it has a
very fast reaction time. Therefore, higher
resolution scans with better signal-to-noise
ratios can be completed much more quickly than DTGS scans because the moving
mirror in the interferometer can be set to
a faster scan speed. The drawback to the
MCT detector is the requirement for cryogenic cooling in the form of liquid nitrogen. MCT detectors must be cooled with
liquid nitrogen and should be allowed to
stabilize for at least 1-2 hours prior to use.
They must then be refilled every 4-16 hours
depending on the manufacturer and the
size of the Dewar mounted to the detector. Self-contained cooling units mated to
MCT detectors have been developed and
marketed; however, given the high cost
of these systems and the poor mean time
between failure (MTBF), these systems may
ultimately represent a poor choice.
One more consideration should be taken
into account with all FTIR spectrometers.
Systems of this type should be kept under a
dry gas purge at all times. Various components within the FTIR are subject to degradation under humid conditions (specifically the
beamsplitter that has a high replacement
cost). FTIR spectrometers are all sealed to a
greater or lesser degree and most have integral desiccant packages installed to assist in
atmospheric water removal, but ultimately
a purge will contribute to system sensitivity and longevity. If a cryogenic nitrogen
tank is installed or 160L to 180L Dewars of
nitrogen are available in a facility where an
FTIR system is installed, it is an efficient use
of head space nitrogen in these Dewars to
purge the FTIR system. This gas also serves
as an excellent zero gas for the infrared
measurements. If this option is not available, there are purge gas dryers available
commercially that can either dry standard
compressed air or act as fully self contained
systems. Although not as clean and therefore not preferable to dry nitrogen, they are
an acceptable alternative.
Gas Analytical System:
Computer Control
If the FTIR system is the heart of this analytical system, the computer control and
data acquisition unit is the brain. Some
companies market a software package
capable of running the FTIR, monitoring
temperatures and pressures, and controlling valve operation. Other companies rely
on third party software systems such as
the National Instruments LabView package that can be configured in an almost
infinite number of ways. Any system must
be programmed appropriately given the
considerations discussed in this article
to accomplish the tasks required, or the
system will not function properly and the
data will have no value. At a minimum, the
computer control system must be able to
monitor (and potentially adjust) pressures,
temperatures, and flow stream selection.
It must also be able to either control or
signal the FTIR to acquire and process data
and download that data into a format for
manipulation. The “perfect” system would
do all these things as well as maintain calibration curve data and reduce test data to a
formatted certification sheet with information on standards, calibration curve dates
and results, and limits of uncertainty.
With the theoretical rules for gas handling
established in Part 1, we have now discussed
the actual design considerations of a practical, efficient, and precise FTIR gas analytical system. Part 3 will be concerned with a
discussion of final performance parameters
and data reduction considerations.
Acknowledgements
We appreciate the assistance of the following companies and/or persons for contributing
graphics, data, or information to the current
article:
Gasera Ltd., Tykistokatu 4, 20520, Turku, Finland
( sales@gasera.fi) represented in the USA by Middleton Research, 8505 University Green, Suite
100, Middleton, WI, 53562. Contact: Dr. Gabor
Kemeny at Gabor@middletonresearch.com.
Gemini Scientific Instruments, 6061 Dale Street,
Unit #Q, Buena Park, California, 90621. Contact:
Dr. Steve Hanst at gemini@gascell.com.
Thermo Fisher Scientific, 5225 Verona Road,
Madison, WI, 53711. Contact: Ken Gowin at
ken.gowin@thermofisher.com.
DR. STEPHEN VAUGHAN IS THE FOUNDER
AND PRESIDENT AND
COO OF CUSTOM
GAS SOLUTIONS,
LLC, 1750 EAST
CLUB BOULEVARD,
DURHAM, NC
27704. DR.
VAUGHAN IS A
LEADING AUTHORITY
IN THE FOURIER
TRANSFORM INFRARED
SPECTROSCOPIC (FTIR) ANALYSIS OF GASES AND IN THE
DEVELOPMENT AND IMPLEMENTATION OF GAS MIXING AND
DELIVERY SYSTEMS FOR BOTH PROCESS AND ANALYTICAL
USE. DR. VAUGHAN HOLDS A B.S. DEGREE IN CHEMISTR Y
FROM THE UNIVERSIT Y OF NOR TH CAROLINA, CHAPEL HILL,
NOR TH CAROLINA, AND AN M. A. AND PH.D. IN PH YSICAL
CHEMISTRY FROM THE JOHNS HOPKINS UNIVERSITY,
BALTIMORE, MARYLAND. HE CAN BE REACHED AT 919-
220-2570 OR CUSTOMGAS@VERIZON.NET. SEE WEBSITE
WWW.CUSTOMGAS.COM.
KIP VAUGHAN IS CURRENTL Y IN HIS SENIOR YEAR AT
DURHAM ACADEMY
IN DURHAM, NOR TH
CAROLINA. HE HAS
WORKED FOR HIS
FATHER AT CUSTOM
GAS SOLUTIONS AS A
SUMMER INTERN FOR
THREE YEARS AND
PLANS TO PURSUE
A CAREERING THE
SCIENCES. HE ASSISTED IN THE RESEARCH AND THE GRAPHICS
FOR THIS AR TICLE.
1. Part III will appear in the March/April, 2009
issue of Gases & Instrumentation.
2. J. U. White. J. Opt. Soc. Am., Vol. 32, (1942)
p. 285.
