Quantification of organic carbon in water by inductively coupled plasma atomic emission spectrometry

Abstract

The presence of carbon in an inductively coupled plasma (ICP) gives rise to strong light emission at 193.09 and 247.86 nm. Although the emitted radiation is potentially useful for the quantification of carbon by atomic emission spectrometry (AES), the dissolved organic carbon (DOC) content of water samples cannot be measured by traditional ICP-AES using the aspiration of aqueous samples into a nebulizer. There are two main difficulties with this method of sample introduction: (1) the aerosol formation in an AES nebulizer is affected by physical sample properties, such as viscosity and surface tension, which in turn are influenced by the presence of dissolved organic compounds, and (2) volatile compounds which evaporate from the aerosol droplets and pass into the gas phase are carried to the plasma with a much higher efficiency than the droplets themselves. Thus, the nebulization process differentiates organic compounds according to their physical properties and can lead to different emission signals for samples with the same DOC content. For example, the traditional ICPAES analysis of a solution containing 1 g/1 C as n-butanol produced a signal six times higher than a 1 g/1 KH-phthalate standard. In addition, excessively long equilibration and washout times of the nebulizer were observed with standards of volatile organic compounds. The approach taken in this study to overcome those problems was to convert the organic carbon in water samples to a common chemical form (CO2) by combustion in a heated quartz tube filled with copper oxide and to carry the reaction products with a stream of argon into an ICP torch. Ten microliter samples were injected into the combustion tube through a system consisting of a peristaltic pump, an 8port sampling valve and miscellaneous gas flow control valves. The entire sample injection system was controlled by an electronic timer. The combustion tube had an i.d. of 12 mm and a length of 44 cm. It was filled to 1/2 of its length with copper oxide (wire form, Merck) and heated to 850~ in a vertical tube oven, Argon, pre-cleaned by a 5 ~ molecular sieve trap, was passed through the quartz tube at a rate of 140 ml/min. After leaving the oven and before entering the plasma, the reaction products were accelerated by an auxiliary argon stream of 500 ml/min. Water vapor formed during combustion of the samples was largely removed from the gas stream by a simple condenser system.