Principles of Infrared Analysis:
Infrared (IR) absorption (or reflection for solids) is a technique that can be used successfully for continuous chemical analysis of a process. The IR region of the electromagnetic spectrum is generally considered to cover wavelengths from 0.8 to 1000µ m. These limits, expressed in frequency terms (cm-1, wave numbers or the number of waves per cm) are 12,500 cm-1to 10 cm-1.
The region of the IR most used by process analyzers is broken into two parts: the near IR (12,500 to 4000 cm-1) and the mid IR (4000 to 650 cm-1). Except for a small overlap region, sources and detectors that are needed in the near IR will not work in the mid IR and vice versa. Most laboratory IR spectrophotometers work form 4000 cm-1 to between 650 and 200 cm-1,
Infrared radiation interacts with all molecules [except the homonuclear diatomics oxygen (O2), nitrogen (N2), hydrogen (H2), chlorine (Cl2), etc.] by exciting molecular vibrations and rotations. The oscillating electric field of the IR wave interacts with the electric dipole of the molecule, and when the IR frequency matches the natural frequency of the molecule, some of the IR power is absorbed. The pattern of wavelengths, or frequencies, absorbed identifies the molecules in the sample. The strength of absorption at particular frequencies is a measure of their concentration. Analytical laboratory IR is largely concerned with identification, or qualitative analysis, while process IR is concerned with quantitative analysis.
Fundamentals of Infrared Analysis :
Particular groups of atoms tend to absorb at the same time frequency with very little influence from the rest if the molecule. These group frequencies are a great help in identifying molecules from the IR spectra. On the other hand, similar molecules, such as a series of homologous hydrocarbons, have very similar IR spectra. Infrared analysis is, therefore, most straightforward when the component molecules of the sample have significantly different atomic groupings. A mixture of aliphatic hydrocarbons would be better analyzed by another technique, such as gas chromatography. The part of the spectrum offering the best discrimination between molecules is between 7 and 15 µm, the so-called finger-print region.
The starting point for quantitative analysis is the Beer-Lambert law, which relates the amount of light absorbed to the sample’s concentration and path length.
A= abc=log10 I0 / I
Where:
A= absorbance
I= IR power-reaching detector with sample in beam path
I0= IR power-reaching detector with no sample in beam path
a= absorption coefficient of pure components of interest at analytical wavelength; the units depend on those chosen for b and c
b= sample path length
c= concentration of sample component
The law states that concentration is directly proportional to absorbance at a given wavelength and path length and a specified temperature and pressure.
Calibration plots of A versus C can be made up using known samples and used to analyze unknown ones. The Beer-Lambert law is also helpful in choosing the optimum sample path length for accurate analysis.
The law states that concentration is directly proportional to absorbance at a given wavelength and path length and a specified temperature and pressure.
Single-Beam Dual Wavelength Configuration:
Single-beam analyzers are provided with two optical filters one for the sample, the other as a reference. The reference filter is selected in a region where the components of interest are not absorbing, while the measuring filter is chosen to be absorbing in the spectral region of interest. As the chopper alternatively spins one filter or the other into the optical path, the difference or ratio in the energies received at the detector will be a function of the concentration of the component of interest.
Single beam dual wavelength (SBDW) infrared analyzers are able to make measurements using one source, one measurement cell and one detector. Typically, a lens is used to focus the light for a straight pass through the cell. Thus, the SBDW analyzer does not depend on internal reflections to increase energy throughput or increase effective path length. In a practical sense, effects of component aging and window contamination are minimized, since aging and contamination effects both the measurement and reference wavelengths equally. In fact using the SBDW principle an infrared analyzer can perform to specifications with up to a 50% coating on the windows. After this point, energy transmission falls to a point where noise in the data results in decreasing analytical precision.
Another advantage of this analyzer is the split architecture of the analyzer, with the cell being separate from the source and detector modules. This facilitates ease of maintenance and separates the process containing component from the electronics, which is always a good analyzer system design practice. Next, the cells use mechanical seals (o-ring grooves, Bevel washers), which impart excellent pressure ratings (up to 1000 psig). Variable path lengths are available, as are heated cells.
The primary drawback of this design is the low spectral resolution available. Optical filter bandwidths are typically 1-2% of the actual wavelength at half height. This can result in interferences, as will be seen for the CO application. Interferences can occur both for the measure wavelength (typically a positive interference) and for the reference wavelength (typically a negative interference). One way to compensate for this problem is to make a thorough stream survey, either with grab samples and process knowledge or in-situ, perhaps with a temporary installation of a process FTIR.
Dual-Beam Configuration:
In the dual-beam configuration the IR radiation is allowed by the chopper to pass alternately through the sample and the reference tube . The reference tube provides a true zero reference, as it is filled with nonabsorbing gases. A narrow bandpass optical filter is placed in front of the detector to limit the IR energy it receives to the wavelength which is characteristic of the component of interest. Therefore if the sample contains the component of interest, this will attenuate the magnitude of the detected signal in the absorption band of the bandpass filter. The use of the reference cell in the dual-beam configuration reduces the drift causes by power supply or temperature fluctuations. The use of collimating optics also eliminates the need for internal reflection from the interior surfaces of the tubes, thus simplifying their construction and elimination the associated drift.
The classical dual beam infrared analyzer can use one or two sources (both are available today), a measurement or sample cell, a sealed reference cell and a gas filled Luft detector. This analyzer is spectroscopically a high resolution analyzer, with excellent sensitivity and background rejection. The background rejection is conferred both by the gas fill in the detector itself, which is usually the component of interest (in this case CO) and by the sealed reference cell, which is used to balance the optical energy through the two beams of the analyzer. The double layer Luft detector design imparts additional desirable characteristics, such as extended range and stability. Rejection ratios can be on the order of 50,000:1 or greater, depending on the application. Also, sensitivity has been shown as low as 0-1 ppm full scale for semiconductor applications. The combination of sensitivity and rejection ratio available in the Luft type analyzer is almost unmatched by other techniques.
The most common stated drawbacks of the Luft design are effects of mechanical vibrations and temperature on the detector itself . These drawbacks can be overcome with good installation practices. Unfortunately, other drawbacks exist, primarily due to instrument design choices made by analyzer suppliers, which have prevented even wider use of the Luft design in actual process analysis. These center on the 19 inch rack mount design, with a low pressure cell being mounted inside the analyzer with the electronics. The low pressure cell is a problem as it limits options for sample return into the process. The low pressure ratings of most commercially available also cause concern when flammable (ethylene) or toxic (phosgene, HCl, chlorine, etc.) sample require analysis. These frequently use windows which are held in place with an adhesive, rather than use of mechanical seals such as o-rings and Bevel washers. Finally, many Luft analyzers have cells which are designed to be internally reflecting (i.e., gold sputtered on Ni electroplated on steel). This is to maximize energy throughput for an unfocused set of optics, and to increase effective path length. The drawback to this type of cell is the fouling and corrosion/pitting associated with many process samples. These effects can drastically cut throughput and have the additional effect of changing effective cell path length, resulting in a span change and a decrease in precision.
