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Hydride / Cold Vapor

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    75023

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    Solutions, bulk solids, powders, gases. These sample forms are commonly introduced to elemental analysis. Absorbance, emission, or mass spectrometry can be performed to detect free atoms produced by graphite furnaces, assorted types of plasmas, or flames. However, for several specific elements, Hg, Se, As, Te, and Sb, the combination of elementary chemistry and physics makes an alternative sampling and atomization approach preferable. At 25°C, Hg has a vapor pressure of 1.85×10-3 torr, which is 10-7 mol L-1. Suppose one is trying to analyze mercury at a few parts per billion in some sample, say, coal. 1 ng Hg/g of coal is 5 picomoles Hg/g. If the mercury were reduced to neutral Hg and those vapors concentrated into a tube with V = 1 mL, would all the Hg be in the gas phase at room temperature? Sure enough, 5 picomoles/10-3 L = 5 nM, a factor of 20 below the equilibrium vapor pressure. One needs no heat source such as a flame or plasma to quantify trace mercury -- one need only reduce Hg to its elemental, unionized form and confine it to a small observation volume with well-defined path length in which absorption can be measured. Because the measurement is at low temperature, Doppler broadening is minimal. Interferences are minimal (since most other elements have negligible vapor pressure). Cold vapor generation is the most common method for measuring trace levels of mercury.

    The other elements do not have significant vapor pressure at room temperature. However, NaBH4 or LiAlH4 can reduce these elements to hydrides, H2Se, AsH3, H2Te and SbH3 respectively. These gas-phase compounds can be wafted into a quartz tube that, in turn, can be heated sufficiently to decompose these hydrides into the elements, in turn detectable by atomic absorption. The chemical selectivity of elementary reduction minimizes interelement effects, though work continues in finding chemical conditions conducive to unbiased, simultaneous determination of these environmentally important elements. An example of such compromise conditions (ironically, linked to plasma emission!) can be found in C. Dietz, Y. Madrid, C. Camara, and P. Quevauviller, "Simultaneous Determination of As, Hg, Se and Sb by Hydride Generation-Microwave Induced Plasma Atomic Emission Spectrometry After Preconcentration in a Cryogenic Trap," J. Analyt. Atomic Spectrom. 14(9), 1349-1355 (1999).

    This page titled Hydride / Cold Vapor is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Alexander Scheeline & Thomas M. Spudich via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.