DIFFUSIONAL MICROTITRATION - ACID-BASE TITRATIONS IN PICOLITER AND FEMTOLITER SAMPLES

Diffusional microtitration employs controlled diffusion to deliver reagents into ultramicrosamples. Precise acid/base, complexometric, and precipitate titrations in the microliter sample range have been performed with this technique earlier. In this work, a further decrease in titratable sample size by 7 orders of magnitude has been achieved. A diffusional microburet is made from a glass capillary pipet by placing a miniature agar gel plug in its tip (1-2-mum outer diameter) and back-filling the pipet with the reagent. With this device, acid/base titrations of femtomole amounts in femto- and picoliter samples have been performed. Sharp end point detection was achieved with a color indicator mixture. Reagent delivery by the microburet as a function of time was linear (regression coefficient, r2 greater-than-or-equal-to 0.99) over a broad sample range. An amount as small as 29 fmol of nitric acid could be precisely determined in a 1.9-pL droplet. The smallest sample volume successfully titrated was 700 fL containing 42 fmol of the acid. Typical precision of the technique is about 2% full-scale error.