Microliter Sample Introduction Technique for Microwave-Induced Plasma Mass Spectrometry

Abstract

Nitrogen-MIP-MS has an advantage over conventional inductively coupled plasma mass spectrometry (ICPMS) in that it is free from Ar-related spectral interference when measuring the concentration of elements, for example As and Se. Therefore, MIP-MS is a powerful technique for simultaneous determination of multiple elements. However, the ionization efficiencies in the MIP are lower for elements with high values of ionization potential (IP). Our goal was to improve the sensitivity of MIP-MS for As and Se by optimizing the sample introduction technique. A new nebulizer, called the sonic spray nebulizer (SSN), was developed to introduce a liquid sample into the MIP-MS with a flow rate at the microliter per minute level. Our results showed that plasma characteristics could be significantly affected by the sample introduction method used. Compared with a conventional concentric nebulizer (CCN), the detection limits were improved. By using a sample uptake rate that was only one-seventh of that for the CCN, i.e., a SSN rate of 50 μL/min, detection limits were improved by a factor of about 7 for As, Se, and Au, which have very high IP values. Detection limits were also improved for Te, Be, Zn, Sb, and Cd but remained almost the same for elements with low ionization potentials. A mechanism that accounts for this sensitivity enhancement was discussed. Typically, relative standard deviation was found to be less than 2%. In addition, the formation of oxide ions of rare earth elements was reduced by a factor of more than 100 when the SSN was used. The SSN system is capable of introducing samples at a rate as low as 1 μL/min with good precision, which is highly desirable in many cases.