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Table III. Some Process Applications of IR Application
Measurements
Phase
A. Using Transmission Cells Dry-cleaning solvent stream Flare gas Nuclear industry Film thickness Film analyzer Carbon air filter Gas stream Process stream Gas stream
0.1-2% Oil contaminant 0-50% Propylene 0-25 ppm CH4 in argon 10-25% Coating weight Residual vinyl acetate monomer 0-10 ppm Vinyl chloride 0-1000 ppm N0 2 0.05-0.3% Silicone oil 0-12% Ethylene oxide in freon
Liquid Gas Gas Solid Solid Gas Gas Liquid Gas
B. Using MIR Cells Hydrogénation of vegetable oil Polymer process Process stream Process stream Fertilizer process Wastewater spill detection
0-60% Trans-isomer Hydroxyl number 4 - 9 % Ethoxy group content 10-25% HOC2HsNH2 in water 0 - 2 % H2NCONH2 0 - 2 % CH3COOH
Liquid Liquid Liquid Liquid Liquid Liquid
0.1-mm cell, a variation of 0.01 mm causes a 10% error in the measured absorbance. It is difficult to maintain these close tolerances with even small fluctuations in sample pressure or temperature. Further, thin cells require high pressure to move fluids through them, complicating the thickness variation problem. Worse still, in the process situation, thin cells can be easily clogged, requiring frequent maintenance. T o avoid these problems, we have been applying to process applications a technique used in the laboratory since the 1950s—multiple internal reflection (MIR), also known as attenuated total reflection (ATR). T h e MIR technique is illustrated in Figure
4. If the infrared beam enters the crystal at an angle greater than the "critical" grazing angle (which is determined by the ratio of the indices of refraction of the crystal and the sample), it will be reflected a number of times before it leaves the crystal at the other end. Somewhat surprisingly to those not familiar with the technique, the emerging beam will be attenuated by absorption in the sample. Moreover, the attenuation will occur to a different extent at different wavelengths. At each reflection the beam, in effect, penetrates into the sample a short distance, depending on the angle of incidence and the relative refractive indices. In general, the apparent penetration depth will be less than half of
Sample Stream To IR Detector
From IR Source
MIR Crystal
f
Sample Stream
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Figure 4. Schematic illustration of the principle of multiple internal reflection, as used in a process stream At each reflection, the beam penetrates a short distance into the sample, giving results that are similar to those obtained by transmission
CIRCLE 128 ON READER SERVICE CARD
1338 A • ANALYTICAL CHEMISTRY, VOL. 52, NO. 12, OCTOBER 1980
