The present invention relates to a plasma mass spectrometer. In such a spectrometer, a plasma containing the sample material is generated by a plasma torch, and is passed to a mass analyzer.
In a plasma mass spectrometer, the plasma is generated by the application of high frequency power to an aerosol of the sample material. That aerosol is generated by spraying a solution of solvent containing the sample material from a nebulizer into a spray chamber. The solvent is removed from the aerosol by condensation in the spray chamber, and the sample material is passed to the plasma torch. In the plasma torch, the sample is converted to a plasma by high frequency power. A gas is also supplied to the plasma torch.
The most common form of such a plasma mass spectrometer makes use of radio frequency power. The power is supplied to a coil surrounding the plasma torch, and the power is inductively transferred to the plasma. Such an arrangement is known as inductively coupled plasma (ICP) mass spectrometry. The radio frequency power may have a frequency of 10 to 100 MHz and typical values used are in the range 27-40 MHz. With an inductively coupled plasma spectrometer, it is necessary to supply an inert gas to the plasma torch, and that inert gas is normally argon (Ar).
A disadvantage of such a plasma mass spectrometer is that the measurement of a sample may be affected by the presence in the plasma of ions other than those of the sample. The influence of such ions is referred to as a background spectrum, or interference. It is thus important that the interference is minimized.
Interference in an inductively coupled plasma mass spectrometer is described, for example, in the article entitled "Background Spectal of Features in Inductively Coupled Plasma/Mass Spectrometry" by S. H. Tan and G. H. Horlick in Applied Spectroscopy, Vol. 40, No. 4, 1986, pages 445-460, and the article entitled "Studies with Desolvation in Inductively Coupled Plasma-Mass Spectrometry" by R. T. Tsukahara and M. K. Kubota in . . . pages 581 to 589. In particular, it is found that the use of argon as the inert gas supplied to the plasma torch causes the disadvantage that the interference caused by the argon renders difficult or impossible the measurement of certain ions. Thus, as is evident from the article by S. H. Tan et al, the use of argon may render impossible measurements on calcium (CA), Iron (Fe) or chromium (Cr).
Therefore, proposals have been made to make use of microwave frequency power. Microwave frequency power generally has a higher frequency, e.g. greater than 100 MHz, than radio frequency power, and typical proposals have been of the order of 2.45 GHz. The use of such microwave frequency power has the advantage that the gas to be supplied to the plasma torch is not limited to an inert gas, and nitrogen, oxygen, or even air can be used. Of course, an inert gas such as Argon can also be used. The use of such microwave frequency power is known as microwave induced plasma (MIP) mass spectrometry.
Examples of such spectrometry are disclosed in an article entitled "A Moderate-Power Nitrogen Microwave-Induced Plasma as an Alternative Ion Source for Mass Spectrometry" by W. L. Shen et al in Applied Spectroscopy, and Vol. 44, No. 6, 1990, pages 1003-1010, the article entitled "Annular-Shaped Microwave-Induced Nitrogen Plasma at Atmospheric Pressure for Emission Spectrometry of Solutions" by Y. Okamoto in Analytical Sciences, April 1991, Vol. 7, pages 2382-88. The interference which then occurs is disclosed in, for example, the article entitled "Background Spectral Features for Moderate-Power Nitrogen Microwave-Induced Plasma Mass Spectrometry" by W. L. Shen et al in Applied Spectroscopy, Vol. 44, No. 6, 1990, pages 1011-1014.