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The principle of hydrodynamic focusing is applied to flowing gas streams for sample concentration in ultrasensitive analysis. In a concentric nozzle system, samples introduced from the inner nozzle in the converging section of the outer nozzle are focused to streams 50–250 μm in diameter. In some cases diameters of ~100 μm are maintained for several centimeters downstream from the exit of the outer nozzle. This technique is particularly useful for concentrating samples to match the beam waist of focused laser beams. Applications to resonance ionization mass spectrometry are discussed.
Keller et al. (Sun,) studied this question.