Optics: measuring and testing – By dispersed light spectroscopy – With sample excitation
Patent
1997-06-20
1998-11-24
Evans, F. L.
Optics: measuring and testing
By dispersed light spectroscopy
With sample excitation
G01N 2168, G01J 3433
Patent
active
058415319
DESCRIPTION:
BRIEF SUMMARY
This invention relates to an inductively coupled plasma spectrometer. The invention is advantageously applied to an optical emission spectrometer (ICP-OES) wherein the plasma torch and the optical system of the spectrometer may be axially aligned and will be described herein in that context. Nevertheless it is to be appreciated that it is not thereby limited to such applications.
In emission spectrometers of the above type, radio frequency (RF) energy is inductively coupled into a gas, such as for example argon, which is caused to flow through the torch to generate a plasma discharge. The plasma is used to atomise and excite a sample that is injected into the plasma to cause the emission of light at wave lengths which are characteristic of the atomic composition of the sample. The emitted light is detected and measured to obtain an analysis of the sample.
Analytical detection limits are improved in optical emission spectrometers in which the cloud of excited atoms generated in the plasma is viewed by an optical detection system of the spectrometer axially along the central axis of the plasma torch rather than perpendicular to that axis as in some known instruments. However an axially aligned optical system needs to be protected from the heat and contaminants in the plasma exhaust. This may be done by interposing a shield that includes a sampling or viewing port or orifice between the plasma "tail" and the entrance end of the optical system. Such a shield is best if made of a conductive metal, as this allows it to be adapted, for example by the incorporation of a cooling system, to minimise damage which may be caused to the shield or its insulating support structure by heat from the plasma or hot gases leaving the plasma.
In inductively coupled plasma spectrometers the plasma acquires a radio frequency potential because of capacitive coupling between the induction coil and the plasma. This potential can cause an electrical discharge to occur from the plasma to the sampling shield, the possibility of which is increased if the shield is made of a conductive metal.
An object of the present invention is to provide an inductively coupled plasma spectrometer in which the problem of an electrical discharge occurring between the plasma and the sampling shield is eliminated or at least substantially ameliorated.
A similar discharge problem can occur between the plasma and a cone containing a sampling orifice in inductively coupled plasma mass spectrometers (ICP-MS). The problem may be addressed in ICP mass spectrometers by reducing the potential difference between the plasma and the sampling cone, for example by special induction coil arrangements as in U.S. Pat. Nos. 5,194,731 and 4,501,965 (RE 33386) or by biasing the cone as in U.S. Pat. No. 4,682,026. However, these are relatively costly solutions which may be commercially viable for ICP mass spectrometers in circumstances where the arcing problem is especially critical in these spectrometers. In other circumstances the present invention may offer a solution to the problem of arcing which is realisable in a simple and cost effective manner in ICP-OES as well as in ICP-MS instruments.
Accordingly, the invention provides an inductively coupled plasma spectrometer including shielding/sampling means located between a plasma torch and an optical system of the spectrometer, wherein said shielding/sampling means is associated with an enclosure for the plasma torch such that a relatively high impedance path is established for limiting flow of electrical current between said shielding/sampling means and said enclosure.
Flow of electrical current between the shielding/sampling means should be limited below a level which would sustain a discharge.
It has been shown that isolating the shielding/sampling means from the enclosure by an insulating medium does not provide the requisite high impedance as reactance of the capacitance between the shielding/sampling means and the enclosure at the frequency of the plasma RF source is typically too low.
Preferably the high impedance
REFERENCES:
patent: 5334834 (1994-08-01), Ito et al.
patent: 5367163 (1994-11-01), Otsuka et al.
Berkowitz Edward
Evans F. L.
Varian Associates Inc.
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