Reading Spectrometers

The results can be placed on any one of several read-out devices, a strip chart recorder, a digital voltmeter, or a printed report from a computer. 

Almost all companies producing spectrometers offer direct readers. They vary from quite small instruments, capable of determining 6-12 elements simultaneously, to large dispersion instruments that can determine as many as 50 elements simultaneously. Many instruments also can interchange photographic recording and direct reading. 

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Compared to flame excitation, random fluctuations in the intensity of emitted radiation from samples excited by arc and spark discharges are considerable. For this reason instantaneous measurements are not sufficiently reliable for analytical purposes and it is necessary to measure integrated intensities over periods of up to several minutes. Modern instruments will be computer controlled and fitted with VDUs. Computer-based data handling will enable qualitative analysis by sequential examination of the spectrum for elemental lines. Peak integration may be used for quantitative analysis and peak overlay routines for comparisons with standard spectra, detection of interferences and their correction (Figure 8.4). Alternatively an instrument fitted with a poly-chromator and which has a number of fixed channels (ca. 30) enables simultaneous measurements to be made. Such instruments are called direct reading spectrometers. 

Other desirable features of a monochromator are stability and multi-element capabUity. Initially, direct reading spectrometers, based on a polychromator, were used for simultaneous multi-element analysis, although these were expensive, bulky and generally limited to specific elements. The development of rapid-scanning monochromators under... 

Spatial Devices. Spatial devices use detection elements which are separated from each other in space. Although the photographic plate may be considered the original spatial detector, this discussion will be limited to electronically-based detection systems. Such systems include the direct-reading spectrometer and various solid-state array detector spectrometers. 

The use of analytical atomic spectroscopy in clinical chemistry has developed rapidly over the last 20 years and there is now adequate knowledge and instrumentation available for the measurement of a wide range of elements (C12, H25, M4, W25) in concentrations as low as 1 ng/ml or amounts as small as 10" g. The cost of the instruments ranges from 100 ( 240) for the simplest flame photometer to 50,000 ( 120,000) for an advanced direct reading spectrometer with data handling facilities. 

Baird, Davis. 2000a. "Encapsulating Knowledge The Direct Reading Spectrometer." Foundations of Chemistry, 2(1) 5-45. 

ICP/OES can be conducted either simultaneously or sequentially. Simultaneous instruments rely on a polychromator or direct-reading spectrometer to read up to 60 elements from the same sample excitation. Sequential analyses use a computer-controlled, scanning monochromator system. The light emitted by the sample in the plasma source is focused on the entrance slit of the monochromator and the spectrum is scanned through the region of interest. Typically, it is possible to determine several elements per minute in the sample in a sequential spectrometer. 

The simplest application of these multiplexing methods involves the so-called direct-reading spectrometer , which was used with some success for a short period in atomic spectroscopy [42]. This instrument consists of a dispersion system with an array of exit silts arranged at appropriate locations. Behind each silt Is a photodetector —usually a photomultiplier. These multiplexing methods have also been used In UV-vIsIble spectroscopy, although to a lesser extent they have been Implemented on automatic discrete analysers featuring an optical system of this type with 5-10 channels or wavelengths... 

The dispersed radiation is photographically recorded on film or plate providing a permanent record for analysis. The slowness of the photographic process and subsequent evaluation led to the development of instruments known as direct-reading spectrometers. 

Proposed Spectrochemical Method of Test for Wear Metals in Used Diesel Lubricating Oils by a Rotating-Disk Electrode Technique Using a Direct-Reading Spectrometer, in Methods for Emission Spectrochemical Analysis, 6th ed., Philadelphia American Society for Testing and Materials, 1971, pp 375-82, ASTM D-2-... 

A variation of the same optical system, as shown in Figure 4-14, is used in the Consolidated Electrodynamics direct reading spectrometers. The optical system is folded, with the use of a folding mirror, to make the instrument more compact. Using a 30,000 lines/in. grating provides a linear reciprocal diversion of about 2.78 A/mm. Spectral intensities are determined by using photomultiplier tubes mounted at the exit slits. 

Fassel and Kniseley reported experimentally determined detection limits for 61 elements that compare favorably with detection limits obtained by atomic absorption, atomic fluorescence, and flame emission methods. Interelement interferences are lower than by other methods and interferences due to PO4" and apparently are negligible. The technique seems well adapted to simultaneous multielement analyses through use of direct reading spectrometers. 

Light-sensitive phototubes also can be used to determine relative spectral line intensities. Two approaches are used for this purpose. The large, direct reading spectrometers use a battery of phototubes, one for each spectral line desired, located at the individual focal points. Usually the output of the phototube is collected over a specified time interval and stored in a capacitor. After exposure the capacitor is discharged into some type of read-out device. This method integrates the total energy over a time interval to provide a measure of spectral energy. 

For routine analytical control the direct reading spectrometers described in Chapter 4 are ideal. These are special-purpose instruments that can be adjusted for specific purposes. Once adjusted they can provide a rapid, precise method for the analysis of the chosen elements in a single matrix. The instruments use photomultiplier tubes, one for each element to be determined, mounted along the focal plane of the spectrometer, immediately back of a carefully adjusted exit slit. The number of elements that can be determined simultaneously can be as many as 30-35. One channel usually is used... 

Direct reading spectrometers can provide analytical data on routine samples in 2-3 min with an average error of 1-2 %. In the metallurgical industries the direct reader easily provides valuable data while a metal is in the molten state so composition can be adjusted as desired. 

Metals and alloys as well as the raw materials from which they are made are analyzed primarily by spectroscopic methods. The iron and steel as well as the aluminum industries rely on spectroscopic analysis in all steps of their processes. Many of the analyses are needed in a very short time thus these industries make wide use of multielement direct reading spectrometers to provide needed analytical data. It is important to control the composition of the molten metals before further processing and the direct reading spectrometers can supply routine analytical information in less than 2 min, something impossible with photographic recording or with chemical wet methods. Some control laboratories have direct readers capable of simultaneously determining 30 elements. 

Starting with the rather unreliable comparison by human eye and then by photography, emission spectrum evaluation made spectacular improvements as a result of electronics and instrumentation, via photoelectric line densitometers introduced by Lundegardh in 1929, to the commercial manufacture of today s direct reading spectrometers. 

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