COMPLEXOMETRIC DETERMINATION OF METAL-IONS BY MICROSCOPIC DIFFUSIONALTITRATION

Acid/base titrations of pico- and femtoliter microsamples have been pe rformed previously using a diffusional microburet (DMB) for reagent de livery in a simple droplet-heptane system (Gratzl, M.; Yi, C. Anal. Ch em. 1993, 65, 2085-2088). The lowest delivery rate achieved with a DMB was about 6 fmol/s, which would correspond to about a 1 mu L/year vol umetric flow rate with a hypothetical equivalent mechanical delivery s cheme (Yi, C.; Gratzl, M. Anal. Chem. 1994, 66, 1976-1982). In this wo rk, the feasibility of complexometric titrations in microscopic sample s is explored. Stability of pH in the microdroplets required for diffe rent determinations and the effects of DMB shank geometry on titration characteristics are also studied. Diffusional microtitrations of Fe(I II), Zn(II), and Cu(II) have been performed with EDTA. Xylenol orange and Eriochrome Black T provide clear color changes at the end point of the respective titrations, despite the microscopic size of the sample s (between 16 and 1570 pL, corresponding to diameters between 30 and 1 44 mu m). Random errors of the determinations relative to full scale w ere 6.6% for Fe(III), 5.8% for Cu(II), and 7.9% for Zn(II). The pH req uired for EDTA titrations of the individual metal ions stays stable in the acidic range. This makes the microscopic titration of a number of metal ions, such as Fe(III), Fe(II), Cu(II), and Pb(II), feasible in a simple droplet-heptane system without any modification. With a highe r density of strongly alkaline buffer droplets (about 100 droplets/mm( 2)) sprayed on the bottom of the Petri dish, or by flushing N-2 above the heptane, the microscopic samples can also be kept alkaline despite ambient CO2 present. In this way, Zn(II) can also be titrated in micr odroplets, requiring a pH around 10. This work renders it possible to perform a variety of complexometric titrations and other chemical mani pulations in microdroplets even if they need to be kept alkaline. Simi lar titrations in single biological cells to assess intracellular buff er capacities of different metal ions, such as Ca(II) and Mg(II), are underway.