The TSI Liquitrak interferometer

The TSI Liquitrak interferometer [195-197] uses a dual beam interferometer to detect slight differences in the refractive indices of particles relative to the surrounding media. It is less sensitive to vibration than the Mach Zehnder type of interferometer beeause it does not separate the two light beams. Instead the beams are overlapped and polarized at 90° to each other, hence the beams do not interfere and are effectively separated from each other until combined by the second beam splitter. The narrow separation of the beams reduces vibration sensitivity dramatically because any vibration interference affects both beams equally. 

The dual beam interferometer also has the advantage of allowing flow-rate measurement. The instrument s flow-rate ranges from 4 to 40 ml min of which 1/200 is examined. Flow-rate is measured each time a particle signal is processed hence particle concentration can be measured in a fluctuating flow. 

The interferometer has several advantages over dark field scattering instruments. Because it is a bright field instrument it is less sensitive to the stray light scattered by interfaces between the instrument s capillaiy cell wall and the liquid medium. The instrument can also identify signals created by bubbles thus avoiding false counts. It can also measure flow-rate and it is more sensitive to particles with a refractive index near that of the liquid than dark field instruments because it can look at forward light without noise interference from the incident laser beam. 

A drawback is that its inspected volume is small (0.5%) compared to the full flow stream since a highly intensive beam is required in the 

An evaluation of this procedure is available in articles by Blackford and Grant [198] and Grant [199] and the instrument is available from Thermal Systems Inc. as Model 7750. 

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