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        Spectrum detection

          Shortly afterwards the blackening of photographic coatings was used as a means of detection and for intensity measurements. Since it was discovered that photographic emulsions are sensitized by pigments in the spectral range in which their absorption lies, it has been possible to carry out spectroscopy by photography in the visible range and also in the far UV range.

          Sensitive photoelectric coatings for all spectral ranges and amplification of the weak photoelectron currents in the CCD have made it possible to replace photographic intensity measurements- which were highly error-prone- with photoelectric methods which show the intensity value direct.

          A basic advantage of photographic spectrum detection is the instantaneous and permanent documentation of all the information contained in the spectrum. When the composition of the sample is completely or partly unknown this is of great importance in order to arrive at an overall analysis. Later checks are possible. One basic disadvantage is the long time required but with the development of computer-controlled microphotometers the time required for spectra read-out is being considerably reduced. Measurement accuracy is not improved since it is still basically determined by the film or photographic plate and, further, when the composition of the sample is largely unknown, the time required for sample preparation is often itself long.

          A basic advantage of photoelectric spectrum detection with simultaneous or sequential spectrometers is the accuracy and rapidity of spectrum detection and readout. The basic disadvantage is that these instruments can only be used for a predetermined number and type of analytes and concentration on ranges-the latter limitation being due to the fact that one and the same spectral line can only be used within a certain concentration range. Instantaneous and permanent documentation of the whole spectrum is not provided because only individual lines are measured.

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