This invention relates to inductively coupled plasma spectroscopy, and more particularly to the means for sustaining, and possibly generating an inductively coupled plasma in a spectrometer for use in chemical analysis. It will be convenient to hereinafter describe the invention with particular reference to mass spectrometers, but the invention is not limited to that application and may be used in other spectrometers such as optical emission spectrometers.
An inductively coupled plasma mass spectrometer should be able to detect the presence of very low levels of chemical elements in samples being tested. To achieve this end, the apparatus is provided with an inductively coupled plasma torch and induction coil means for sustaining a plasma by coupling radio frequency electro-magnetic energy into a suitable gas, typically argon, flowing through the plasma torch. The combination of an inductively coupled plasma torch and induction coil means for sustaining a plasma can be referred to as an inductively coupled plasma source.
A sample for analysis in a mass spectrometer is introduced into the inductively coupled plasma by known means. Chemical elements in the sample are atomised and ionized, and the apparatus responds to the ions so produced by generating signals which are characteristic of particular chemical elements. The output of the apparatus also contains a background signal which may arise from a number of effects including light, excessively energetic ions or excited neutral atoms reaching the detector of the mass spectrometer, and background electrical signals inevitably present in such apparatus.
To permit the detection of very low levels of chemical elements in samples being tested, the ion signals produced by chemical elements should be large compared to the background signal in the output of the apparatus, so that signals arising from chemical elements may readily to distinguished from fluctuations in the background signal.
Various inductively coupled plasma sources are known to be useful in inductively coupled plasma mass spectrometers, and are distinguished principally by the configuration of the induction coil means used for sustaining the plasma. In one particular prior art arrangement, the radio frequency energy is applied to one end of the coil and the other end of the coil, nearest the mass spectrometer interface, is grounded. This is known as the front-grounded coil. In another prior arrangement, the coil is supplied with radio frequency energy at its two ends and is grounded at the centre. This latter arrangement is sometimes referred to as a centre-grounded coil and is described in U.S. Pat. No. 4,501,965.
The prior induction coils referred to above are not entirely satisfactory in operation. In particular, the prior coils retune at a relatively slow rate under some operating conditions, and do not have sufficiently efficient power coupling characteristics. Both aspects are thought to be due to the relevant coil having an inadequate electrical coupling coefficient.