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When coupled to a TOF mass spectrometer this soft ionization source has demonstrated signal-to-noise (S/N) ratios from 7 to 730 at 100 pg per injection (depending on the compound), and a dynamic range of three orders of magnitude (100 pg to 100 ng) for some of the test compounds.

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This work investigates resonances in a capacitively coupled, low pressure krypton microdischarge operated at 2.5 GHz. A circuit model for the device, which has a length dimension of approximately 1 mm, calculates impedance values for a range of electron densities. The model results predict several “parallel” and “series” resonances at the driving frequency when the electron density is approximately 8 × 1011 cm−3 and 5 × 1012 cm−3. The series resonance occurs when the resistance approaches the output impedance of the radio-frequency signal source, minimizing the reflected power. These resonances explain an experimentally observed jump in intensity with increasing input power.

Microplasma sources typically take advantage of pd (pressure × size) scaling by increasing pressure to operate at dimensions as small as tens of microns. In many applications, low pressure operation is desirable, which makes miniaturization difficult. In this paper, the characteristics of low pressure microplasma sources excited by microwave power are discussed based on results from experimental and computational studies. The intended application is production of VUV radiation for chemical analysis, and so emphasis in this study is on the production of resonant excited states of rare gases and radiation transport. The systems of interest operate at a few to 10 Torr in Ar and He/Ar mixtures with cavity dimensions of hundreds of microns to 1 mm. Power deposition is a few watts which produces fractional ionization of about 0.1%. We found that production of VUV radiation from argon microplasmas at 104.8 nm and 106.7 nm saturates as a function of power deposition due to a quasi-equilibrium that is established between the electron temperature (that is not terribly sensitive to power deposition) and the population of the Ar(4s) manifold.