Plasma manipulator

Details Plasma manipulator Plasma manipulator

1996-02-22

1998-03-24

Evans, F. L.

Optics: measuring and testing

By dispersed light spectroscopy

With sample excitation

G01N 2173

Patent

active

057318727

DESCRIPTION:

BRIEF SUMMARY
The present invention relates generally to a device for shaping the observation zone of a plasma region for optical emission spectrum analysis.
It is desired that a device is to separate the region which guarantees low intensities of baseline radiation at the highest possible analytical emission signal from a non-conducting plasma containing the analysis sample. Furthermore, emission signals are to be stabilized against fluctuations and the ambient atmosphere is to be kept apart from the light path between the observation zone of the plasma and the spectrometer.
Various plasma radiation sources suitable for use in emission analysis are known. Using a carrier gas, analysis samples are commonly introduced as an aerosol into the plasma, where they are vaporized, dissociated, partly ionized, and the free atoms and ions are excited to a state of optical emission. Measurement of the emitted radiation usually takes place in an electrically non-conducting region of the plasma to keep influences of the sample matrix to a minimum. Examples of such sources of radiation include disc-stabilized arcs, inductively coupled plasma (ICP) and microwave induced plasma.
The analytical capability of the emission radiation sources described above increases with the signal-to-noise ratio (SNR), the quotient of the spectral line intensity of the verifiable species and the spectral baseline emission occurring in the spectrum interval in question. Through numerous experiments it has been established that SNR depends greatly on the type of optical observation. For example, observation of ICP along the vertical plasma axis shows that at a certain distance from the excitation coil, with said distance being dependent on the analysis element viewed, SNR is maximized (P. W. J. M. Boumans, Ed., Inductively Coupled Plasma Spectroscopy, John Wiley & Sons, New York, 1987). If ICP is observed in the direction of the plasma axis (end-on), the maximum SNR is reached in the center of the plasma and attention must be paid to the appropriate masking off of the hotter outer regions of the plasma having a radius of approximately 2 mm (D. R. Demers, "Evaluation of the axially viewed (end-on) Inductively Coupled Argon Plasma Source for Atomic Emission Spectroscopy", Applied Spectroscopy v. 33 (1979), p. 584).
Conventionally, the available methods of observation for electrically non-conducting plasmas mask off only a specific portion of the interfering baseline radiation. The spectrally continuous baseline radiation results mainly from interaction between plasma ions and electrons. In addition, the emission of molecules contributes to an increase in baseline radiation. The intensities of both kinds of baseline radiation increase with the rising temperature of the plasma at a greater rate than the desired light intensity. Optimal temperatures for various emission characteristics of atoms and ions used for the analysis vary considerably. When viewing the plasma either vertically or parallel to the axis, different temperature zones are registered. This factor limits the attainable SNR.
It is the object of the present invention to manipulate electrically non-conducting plasmas such that baseline radiation is reduced while the signal-to-noise ratio of the analytical measurement is increased.
This objective is accomplished by masking off that part of the region that is optimal for emission measuring by carrying out the observation in a volume area protected to a large extent from ambient influences and by making it possible to additionally optimize the composition of the plasma with electric or magnetic fields. The invention significantly improves the detection capability of the analysis procedure.
To this end, a cooled diaphragm is mounted concentric with the axis of the plasma in the flow direction. The diaphragm allows only the central part optimal for emission measurements and, in particular, only the optical part to pass through. In the direction of the flow behind the plasma there is a chamber which allows the plasma region selected to be observed along

REFERENCES:
patent: 4395091 (1983-07-01), Russo
patent: 4902099 (1990-02-01), Okamoto et al.
patent: 5483337 (1996-01-01), Barnard et al.
P.W.J.M. Boumans, editor, "Inductively Coupled Plasma Emission Spectroscopy," John Wiley & Sons, New York, 1978.
D.R. Demers, "Evaluation of the Axially Viewed (End-on) Inductively Coupled Argon Plasma Source for Atomic Emission Spectroscopy," Applied Spectroscopy, vol. 33, No. 6, pp. 584-591 (1979).

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