Optics: measuring and testing – By shade or color – With color transmitting filter
Patent
1987-02-27
1989-02-14
Evans, F. L.
Optics: measuring and testing
By shade or color
With color transmitting filter
356418, 356419, 250282, G01J 3433
Patent
active
048042714
DESCRIPTION:
BRIEF SUMMARY
The present invention concerns a process and apparatus for improving the selectivity of spectrometric measurements.
A substantial problem in the spectrometric measurements of the above-mentioned kind is that on the one hand the frequency response of known filter arrangements can only be narrowed to a minimum width, indeed both from physical and from technical grounds and, on the other hand, the energy passing through a very narrow band filter, which should subsequently be detected in a detector, is small. In addition, underlying problems with spectrometric measurements must be taken into account. In this connection, it is, in essence, not only a question of the signal influenced or caused by the test piece making a contribution to the test reading, but also of test piece-impurities making a substantial signal contribution-just the background. Finally, the measuring arrangements are also subject to difficulties, that is to say, disturbances occur between signal source and detector, which make a contribution to the test reading. It is necessary to eliminate all these disturbances as far as possible.
With optical emission spectroscopy, which is an important method of analysis for the determination of chemical elements in all possible matrixes, a plasma is nowadays used as emission source, in which the substance to be examined is stimulated by radiation. In order to obtain a particularly high resolution of the measurement, monochromatic illuminators which, because of their working principle, are voluminous and susceptible to failure as a result of temperature fluctuations and vibrations, are used in many cases today. These instruments are unsuited for field application. Moreover, the instruments are also very expensive, so that only intensive laboratory application is worthwhile (with several users). It is a question, for example, of only wanting to perform an optical spectrographic analysis for certain elements which, when necessary, were previously separated chromatographically, so that in many cases such an expensive monochromatic illuminator which, indeed, is capable of analyzing the whole spectrum, is not required. Compact smaller chromatic illuminators are however substantially less sensitive and also have a worse resolution. For clinically applied flame photometry for Li, Na, K and Ca-determination, fixed interference filters are used. However, these have a very large band width or half-width value of wave length transmission (best interference filters: 0.1 nm; monochromatic illuminator 0.01 nm). Thus a considerable proportion of the continuous light of each radiation source (flame background or plasma background), which varies the selectivity and the detection limit, arrives at the detector (photo cells or photo multipliers). The detection limit is in particular determined by the light intensity relationship between the analysis line and the background radiation. Weak line intensities, as they occur with trace analysis (environmental-trace-analysis) are then no longer filtered out from a statistically distributed background intensity.
A substantial problem with fixed wavelength instruments (also expensive monochromatic illuminators) is that, in certain circumstances, large measuring errors occur and remain unrecognized. This is always the case if a real test piece is introduced together with matrix material. These then change the background of the spectroscopic radiation source. With an estimate of this measuring error, the selectivity can be defined as follows: ##EQU1## at the relevant, fixed adjusted wavelength.
The selectivity of expensive monochromatic illuminators is not, as a rule, better than 10.sup.3, that is to say, 10.sup.3 times greaeer than for an impurity (for example, the clinical differentiation of Na from Li or Ca by chromatographical halogenide determination) causes a fault of 100%.
Nowadays, only selectivities under 100 and also several worse detection limits are obtainable with fixed interference filters.
The above-mentioned problems occur also with other spectrometric measurements,
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