Fine chemical manipulations of microscopic liquid samples. 2. Consuming and nonconsuming schemes

Microscopic liquid particles can be manipulated chemically using a suitable diffusional microburet (DMB), whose tiny tip plugged with a diffusion memb rane acts as a well-defined diffusional transfer channel. In part 1 of this work (Gratzl et al, Anal. Chem, 1999, 71, 2751-2756), we discussed the sim plest DMB-based operation: addition, i.e., loading a droplet with a chemica l that accumulates there without any chemical reaction occurring. Since in this process no consumption of the delivered molecules in the target drople t takes place, addition is a nonconsuming scheme. In this work, another typ e of nonconsuming scheme is explored, which is the subtraction of a substan ce from droplets via a DMB. This process has no analogy among macroscopic c hemical operations, Both addition and subtraction occur according to an exp onential asymptotic process when diffusion is at quasisteady state inside t he DMB tip. These nonconsuming operations were characterized using the tran sport of microscopic quantities of Lucifer Yellow CH, a fluorescent dye, un der a fluorescent microscope. The third basic type of chemical manipulation is when the substance delivered by a DMB is consumed in the target droplet instantaneously by a fast chemical reaction, This consuming scheme was stu died by delivering EDTA into droplets containing Pb2+ ions and a color indi cator. These microscopic titrations were monitored using gray scale transmi ttance images of the droplets as recorded versus time. A unified theory of the three basic DMB operations is also presented.