Picomole analysis of alkali metals by flameless atomic absorption spectrophotometry

The chemical analysis of sodium and potassium concentrations in picoliter to nanoliter quantities of physiologic fluid is critical to the understanding of potassium and sodium transport within the kidney. In 1965, Drs. Gerald Vurek and R. L. Bowman [1] developed the helium glow photometer, an instrument for analyzing sodium and potassium in picomole amounts. Unfortunately, this instrument is not commercially available, and its place has been largely supplanted by flameless atomic absorption spectrophotometry (AA). Although flame and flameless AA have been commercially feasible for sometime, it is suprising how few micromethods exist for alkali metal analysis. Indeed, a recent reference text on the subject [2] has over 300 references for sodium and potassium analysis, but only ten deal with micromethods. Moreover, none of these micromethods are designed to measure such minute quantities of sodium and potassium that are present in 1 to 20nl of physiologic aqueous solutions. To our knowledge, only one other ultramicroassay for Na and K has been devised [3]. We were encouraged by the scientists at Allied Analytic (Timothy Corum and Michael Duffy) to persue a different method of analysis.Using this approach, we have developed a simple and highly reliable method of measuring sodium and potassium in volumes as small as 5nl. Multiple analyses can be performed on individual samples, and the samples may be saved for a week without deterioration. By choosing more sensitive spectral lines, it is possible to measure samples of 100 pl. By combining this technique and electrotitration for chloride, we can determine sodium, potassium, and chloride concentrations in the same sample