Beenakker cavity

In a typical MIP-MS instrument, the ICP portion is replaced with one of a variety of microwave discharge sources, usually a fairly standardised (modified) Beenakker cavity connected to a microwave generator. The analytical MIP at intermediate power (<500 W) is a small and quiet plasma source compared with the ICP. The mass spectrometer needs no major modifications for it to be interfaced with the MIP. With MIP used as a spectroscopic radiation source, typically consisting of a capillary (1mm i.d.), a power of 30-50 W and a gas flow below 1 L min 1, multi-element determinations are possible. By applying electrodeposition on graphite electrodes, ultratrace element determinations are within reach, e.g. pg amounts of Hg. 

Figure 10.2 is a schematic diagram of a helium MIP-MS system, with gaseous sample introduction, developed by the Caruso group. This is the most popular method of sample introduction to date for MIP-MS analysis as the MIP at low pressures is not tolerant to liquid samples. A commercial ICP-MS system may be modified by mounting an MIP discharge source in place of the ICP source. A Beenakker cavity is commonly used as the microwave source and serves to focus the microwave energy. Cavity construction and dimensions have been described in detail by Evans et al. [18]. 

Richts U., Broekaert J. A. C., Tschopel P. and Tolg G. (1991) Comparative study of a Beenakker cavity and a surfatron in combination with electrothermal evaporation from a tungsten coil for microwave plasma optical emission spectrometry (MIP-AES), Talanta 38 863-869. 

An atmospheric pressure helium MIP is generated using a 2.45 GHz microwave generator and an electromagnetic cavity resonator, called a Beenakker cavity. The hehum gas is passed through a discharge tube placed in the cavity, as seen in Fig. 7.48. The plasma is initiated by a spark from a Tesla coil. The electrons produced by the spark oscillate in the... 

Figure 3 Microwave induced piasma schematic iiiustrating the Beenakker Cavity design.

The analysis system consisted of a Shimsdzu QC-6A gas chromatograph, a chemically deactivated four-way valve for solvent ventilation, a heated transfer tube interface, a Beenakker-type TM0i0 microwave resonance cavity, and an Ebert-type monochromator (0.5m focal length). 

Fig. 12.26 Schematic diagram of a Beenakker resonant cavity for microwave induced plasmas.

The cavities most often used in earlier work were either the shortened 3/4-wave or 1/4-wave coaxial types, but these were unable to sustain an atmospheric helium plasma. Fehsenfeld introduced a shortened 1/4-wave radial cavity that was improved by Beenakker and with certain minor modifications this proved to be a successful design for sustaining an atmospheric helium plasma due to its strong electrical field. The cavity, shown in Figure 1, was designed to have its resonant frequency at 2450 MHz, with a minimal cavity volume so that a high-energy density... 

Fig. 1.1. The Beenakker microwave plasma cavity. (Reproduced by permission, from J. Chromatog., 1982, 239, 181 copyright 1982, Elsevier Science Publishers.)...

The first GC-microwave-induced plasma emission system was reported in 1965 [23]. During the past two decades GC-plasma emission systems have gained in popularity and have been used for the identification and quantification of mercury, lead, tin, selenium, and arsenic compounds [13]. The most frequently used plasma source is the microwave-induced plasma operated either at reduced pressure or at atmospheric pressure with helium or argon as the plasma gases at powers of 100 to 200 W The Beenakker cylindrical resonance cavity introduced in 1976 [24], and since then modified to achieve better detection limits, is most frequently used in the GC-microwave-induced plasma emission systems that are easily adaptable to capillary GC operation. These microwave-induced plasma detectors respond to non-metals (H, D, B, C, N, O, F, Si, F S, Cl, As, Se, Br, I) and metals, with absolute detection limits in... 

---