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Publication Date: 2015-Sep-03
Document File: 7 page(s) / 281K

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The Prior Art Database

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Miniaturized Coil for Microwave Plasma Generation

I.          Background

This article discloses a new microplasma source that may be utilized for molecular and atomic excitation of a gaseous sample or stream.  The microplasma source may be utilized, for example, as a component of devices utilized for analyte identification through methods such as atomic emission detection (AED) or ion mass spectrometry (MS).  The identification of components of effluent gas emitted from a gas chromatograph (GC) is envisioned as a particularly well suited application of this invention. 

An example is the Agilent G2350A / HP 5921A AED for GC.  In this instrument, a key component is a microwave-powered (~70 W) helium (He) plasma whose function is to produce atomic emission from analytes contained in the gaseous GC effluent.  This plasma is generated in a microwave cylindrical cavity which is fed by a magnetron at 2.45 GHz.  The gas tube (1-mm i.d, 4-mm length) is placed at the center of the cylindrical cavity where the electric field is the most intense (TM010 fundamental mode) in order to efficiently couple the electromagnetic energy to the gas.  The analyte enters the tube through the GC capillary column (~1 sccm) and gets mixed with additional makeup gas before it enters the microwave cavity.  Figure 1 below is a cross-sectional view of the microwave cavity (1) and surrounding region of this instrument.  The GC effluent enters the gas tube (12) from a capillary (21).  Input tubes (23) and (24) supply the helium utilized to form the plasma as well as the makeup gas.  Light emitted from the analytes as a result of interacting with the plasma passes through a window (18) to a spectral analyzer (not shown).

A common problem with this instrument and similar instruments is the efficiency of the analyte interaction with the plasma.  Analyte deposition on the tube walls (which necessitates the need to use reagent gases to clean the tube) can be an indication of an inefficient interaction of the analyte within the bulk plasma.  This can be attributed to the shape of the plasma, which either uniformly fills the entire cross-section of the torch, or is hotter at the center (“bullet” plasma).  The result is that the analyte (or a significant fraction of the analyte) flows around the plasma and is deposited on the tube walls.  This is due to the fact that in gases, viscosity increases with temperature making the hotter regions more viscous and correspondingly harder to penetrate with external gas flows (such as the GC analyte stream).

In addition, in the Agilent G2350A / HP 5921A AED, a high amount of makeup gas (~50 sccm, compared to ~1 sccm for the GC output) needs to be added to the GC output in order to maintain a stable plasma.  This causes analyte dilution which can affect the sensitivity of the instrument.  This also places a larger demand on resources required for instrument operation (most notably purified helium gas).  Also, the mi...