Amplifier design considerations for blood cell counter sampling probes

Blood cell counters that operate on the Coulter principle of an electrical resistance change when a cell passes through a small sampling orifice are especially sensitive to electrical noise. The sampling probe is immersed in an electrolyte (isotonic diluent for blood cells), which in itself presents an electrochemically noisy environment. The probe, owing to its large size, acts as an antenna for environmental electrical noise up to 60 Hz. Additionally, the basic Coulter method for cell counting requires a dc potential across the sampling orifice electrodes contained within the probe. This potential produces electrolysis of the diluent and generation of gas bubbles at the electrode surfaces. Sampling (counting) time must be short (less than 30 seconds) to avoid sample heating and an intolerably high noise level as a consequence of ionic motion and gas bubble generation. The resistance change that takes place when a cell passes through the sampling orifice is only a small fraction of a percent, thus noise is a serious problem. Electric noise produces false counts and general degradation of counting function. This paper presents a discussion of a currently used method for signal acquisition and some of the problems encountered in the clinical laboratory. A novel alternative design has been implemented using integrated circuit components, which eliminates many of the problems associated with the use of small bench-type counters. Design philosophy is discussed in detail including presentation of the final circuitry developed. Performance characteristics of the signal acquisition circuitry are presented.(ABSTRACT TRUNCATED AT 250 WORDS)