Direct-reading spectrometers

Baird Associates. 1956. "Advertising Bulletin 42 Spectromet Direct Reading Analysis on the Plant Floor." Privately produced advertising brochure, Cambridge, MA Baird Associates. (In the possession of the author.)... 

Three different types of grating spectrometer detection sterns are used (Figure 3) sequential (slew-scan) monochromators, simultaneous direct-reading polychroma-... 

Prior to the use of plasma excitation, arc and spark sources were used on multichannel spectrometers, the so-called direct-reading instruments. 

Diphenylcarbazide as adsorption indicator, 358 as colorimetric reagent, 687 Diphenylthiocarbazone see Dithizone Direct reading emission spectrometer 775 Dispensers (liquid) 84 Displacement titrations 278 borate ion with a strong acid, 278 carbonate ion with a strong acid, 278 choice of indicators for, 279, 280 Dissociation (ionisation) constant 23, 31 calculations involving, 34 D. of for a complex ion, (v) 602 for an indicator, (s) 718 of polyprotic acids, 33 values for acids and bases in water, (T) 832 true or thermodynamic, 23 Distribution coefficient 162, 195 and per cent extraction, 165 Distribution ratio 162 Dithiol 693, 695, 697 Dithizone 171, 178... 

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... 

Further development of direct reading instruments for analysis may be expected, for example in polarographs, x-ray and emission spectrometers. 

Recently, a very important development has been made to enhance the ease of manipulation and the range of applicability of emission spectroscopy to trace metal analysis. This development is the plasma source which can be employed as an. accessory source in most direct reading emission spectrometers in place of the arc or spark or may be incorporated directly in the design of the spectrometer by the manufacturer. This development has been discussed in detail in the recent literature ( 9,53-58). 

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 function of the spectrometer is to accept as much light from the source as possible and to isolate the required spectral lines. This may be impossible where there is a continuous spectrum in the same region as the analytical line for example, the magnesium line of 286.2 nm coincides with a hydroxyl band. In direct reading instruments, electronic devices may be used to supplement the resolution of the spectrometer by modulating the intensity of the analytical signal. In absorption and fluorescence the light source is modulated in emission the spectral line is scanned (816) or the sample flow modulated (M23). 

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. 

The instrumentation employed by Stapleton and Bowie was an ion cyclotron resonance mass spectrometer that had been modified to permit computer control of all drift voltages and to allow direct reading of the ion transit time (typically 10 to 10 seconds) similar to that described by McMahon and Beauchamp An emission current of about 0.2 microampere and a nominal 70 eV electron beam produced ion currents of 10 to 10 A at source pressures of approximately 10 Torr. The mass spectra were measured by magnetic field modulation. 

The nicest possible summary of the transformation in instrumentation and objectivity can be read off a 1959 advertisement. In 1959 BA advertised their spectrometers by comparing analytical methods, wet chemistry, spectrographic methods, and direct reading (figure 6.1). An iconic summary was presented for each approach. As with any effective ad, the visual point is made quickly and clearly wet chemical analysis takes more steps than spectrographic analysis, which itself takes more steps than spectrometric analysis (using a direct reader). Furthermore, the steps involved are easier with spectrographic methods than with wet chemical analysis, and easier still with spectrometric methods. 

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. 

Many spectrometers are equipped with facilities to monitor and regulate the temperature within a probe head. Usually the sensor takes the form of a thermocouple whose tip is placed close to the sample in the gas flow used to provide temperature regulation. However, the readings provided by these systems may not reflect the true temperature of the sample unless they have been subject to appropriate calibration. One approach to such calibration is to measure a specific NMR parameter that has a known temperature dependence to provide a more direct reading of sample temperature. Whilst numerous possibilities have been proposed as reference materials [41], two have become accepted as the standard temperature calibration samples for solution spectroscopy. These are methanol for the range 175-310 K and 1,2-ethanediol (ethylene glycol) for 300-400 K. 

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