Matrix effects of carbon and bromine in inductively coupled plasma optical emission spectrometry

Abstract

In inductively coupled plasma (ICP) based techniques the signal enhancing effect of carbon on some elements like arsenic or selenium is well documented. However, there is a large spread in the reported magnitude of this effect and whether it can be observed for other elements too. In this investigation we studied the effect of larger amounts of carbon on a total of 157 emission lines of 36 elements. A strong instrument dependence of the “carbon enhancement effect” was encountered in inductively coupled plasma optical emission spectrometry (ICP-OES), despite the use of the same sample solutions and the same sample introduction system. Several potential enhancement sources (carbon in the form of methanol, phenylalanine and CO₂ as well as bromine) were compared. By tapping the high voltage power supply of the RF generator, current and voltage fed to the power oscillator could be recorded simultaneously with the emission line signal. From these data it was concluded that the carbon-based matrix effect is a combination of five factors: (1) depending on the source of carbon, changes in the sample nebulization; (2) carbon induced charge exchange reactions; (3) plasma impedance changes caused by the introduction of large quantities of carbon into the ICP: depending on the RF generator used, this effect causes power regulation problems and results in higher RF power coupled to the discharge; (4) thermal pinch effect – the ICP discharge shrinks and becomes smaller; (5) the state of matter (gaseous or liquid) of the introduced carbon sources is relevant to the magnitude of the carbon enhancement effect.