Current infrared instrumentation permits the measurement of low concentrations or small quantities of material by electronic amplification of the signal and/or by utilizing infrared microscopes, beam condensers, and micro cells. It is now possible to achieve further sensitivity than permitted by these techniques. Using a computer of average transients (CAT) in conjunction with the spectrophotometer, a signal masked by noise can be increased with respect to the noise by a summation or averaging technique. In this method a region of the spectrum is scanned repetitively and the recorder input signal of each scan fed to the computer. A trigger circuit is utilized to synchronize the instrument and computer scans and provide a reference signal to the computer. The portion of the recorder input signal due to sample absorption is phase-locked to the reference signal, while the random component due to noise is not. When a number of input signals are added together, the phase-locked portions of the signal add arithmetically while the random noise adds out of phase and, therefore, tends to average out to zero. The improvement in the signal-to-noise ratio is proportional to the √N, where N is the number of scans summed. This technique has been applied recently to enhance the sensitivity of nuclear magnetic resonance and election spin resonance measurements (1).
Aaron L. Bluhm, John A. Sousa, and Julius Weinstein, "Increased Sensitivity of Infrared Absorption Measurements by Computer Averaging," Appl. Spectrosc. 18, 188-189 (1964)