Photometer flame

Example of the reduction of an arsenic salt by sodium horohydride  

The metallic hydrides, which are easily thermolysed at around 1000 K, liberate the corresponding elements in the atomic state. An electrodeless lamp is preferably used as a light source. 

As for mercury, it is not transformed to a hydride but rather remains in the metallic state (Hg°). Consequently, a special cell that does not require to be put into the flame is used. This is called the cold vapour method, and requires specialized instruments (reduction by SnCl2). 

Measurements by flame photometry are carried out either using atomic absorption spectrometers with a burner (but without the light source), or flame photometers. The latter are less sophisticate instruments whose price is ten time less that atomic absorption spectrometers. These photometers are designated to make measurements of only five or six elements. They include interchangeable coloured 

CHAPTER 13 - ATOMIC ABSORPTION AND FLAME EMISSION SPECTROSCOPY 

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Commercial Hquid sodium alumiaates are normally analyzed for total alumiaa and for sodium oxide by titration with ethylene diaminetetraacetic acid [60-00-4] (EDTA) or hydrochloric acid. Further analysis iacludes the determiaation of soluble alumiaa, soluble siHca, total iasoluble material, sodium oxide content, and carbon dioxide. Aluminum and sodium can also be determiaed by emission spectroscopy. The total iasoluble material is determiaed by weighing the ignited residue after extraction of the soluble material with sodium hydroxide. The sodium oxide content is determiaed ia a flame photometer by comparison to proper standards. Carbon dioxide is usually determiaed by the amount evolved, as ia the Underwood method. 

The purpose of the nebuliser-burner system is to convert the test solution to gaseous atoms as indicated in Fig. 21.2, and the success of flame photometric methods is dependent upon the correct functioning of the nebuliser-burner system. It should, however, be noted that some flame photometers have a very simple burner system (see Section 21.13). 

An example of a modem instrument of this type is the Coming Model 410 flame photometer. This model can incorporate a lineariser module which provides a direct concentration read-out for a range of clinical specimens. Flame photometers are still widely used especially for the determination of alkali metals in body fluids, but are now being replaced in clinical laboratories by ion-selective electrode procedures (see Section 15.7). 

Although flame emission measurements can be made by using an atomic absorption spectrometer in the emission mode, the following account refers to the use of a simple flame photometer (the Coming Model 410 flame photometer). Before attempting to use the instrument read the instruction manual supplied by the manufacturers. 

Potassium in potassium sulphate. Weigh out accurately about 0.20 g potassium sulphate and dissolve it in 1 L de-ionised water. Dilute 10.0 mL of this solution to 100 mL, and determine the potassium with the flame photometer using the potassium filter. 

NOTE The moisture content of steam can be measured by means of a throttling calorimeter or by analysis of the sodium content in a sample of condensed steam, using perhaps a specific ion electrode or flame photometer. 

At present, calcium and magnesium are estimated almost exclusively by atomic absorption (36). Present instrumentation permits the dilution of the specimen to approximately 1 - 100 for calcium and even higher for magnesium. For many instruments, the two elements are not read out simultaneously such as is practicable for sodium and potassium with the flame photometer. The lower limit of serum volime at present, for the practical assay for calciim and magnesiim in the laboratory of Neonatology, is approximately 10 ul The instruments are very readily automated, and it is not uncommon for results to be available at the rate of 240 per hour in the routine laboratory, where a typical atomic absorption instrument such as a Perkin-Elmer has been attached to an automatic feed system. 

It has been long believed that a lithium ion-selective electrode would render obsolete the flame photometer in the clinical laboratory. Lithium is administered to manic depressive psychiatric patients. Since the therapeutic range (0.5-1.5 mM) is quite close to the toxic range (>2 mM), it must be closely monitored. Most of the iono-phores propo d to date have not met the Li" /Na selectivity required for an interference-free assay. However, it has been reported that calibration in the presence of 140 mMNa permitted the analysis of Li in serum The errors observed are due to fluctuations in the Na concentrations in the sample. More selective ionophores would certainly improve the accuracy of this method. 

Many sophisticated analytical techniques have been used to deal with these complex mixtures.5,45,46 A detailed description is not possible here, but it can be noted that GLC, often coupled with mass spectrometry (MS), is a major workhorse. Several other GLC detectors are available for use with sulfur compounds including flame photometer detector (FPD), sulfur chemiluminescence detector (SCD), and atomic emission detector (AED).47 Multidimensional GLC (MDGC) with SCD detection has been used48 as has HPLC.49 In some cases, sniffer ports are provided for the human nose on GLC equipment. 

The detailed design and construction of instruments used in the various branches of spectrometry are very different and at first sight there may seem to be little in common between an X-ray fluorescence analyser and a flame photometer. On closer examination, however, certain common features emerge and parallels between the functions of the various parts of the different instruments are observed. The basic functions of any spectrometer are threefold (Figure 7.4) ... 

Flame photometer or spectrophotometer incorporating nebulizer and burner, filters, prism or grating monochromator, photocell or photomultiplier detection system. 

There are two types of Flame Photometers that are used invariably in Flame Emission Spectroscopy (FES), namely ... 

The line-sketch of a simple flame photometer is shown in Figure 25.2. 

In general, Flame Photometers are designed and intended mainly for carrying out the assay of elements like Sodium, Potassium, Calcium, and Lithium that possess the ability to give out an easily excited flame spectrum having sufficient intensity for rapid detection by a photocell. 

Figure 25.2 Layout of a simple Flame Photometer [Coming model 410 Flame Photometer is based on this pattern].

The layout of an internal standard flame photometer is illustrated in Figure 25.3. 

The use of an internal standard flame photometer not only eliminates the visible effects of momentary fluctuations in the flame characteristics produced by variations in either the oxidant or under full pressures, but also the errors caused due to differences in surface tension and in viscosity are minimised to a great extent. 

In short, an internal-standard flame photometer provides a direct and simultaneous result with respect to the ratio of intensities. 

A flame photometer (Figure 2.31) is designed to cause atomic excitation of the analyte and subsequently to measure the intensity of the emitted radiation. A monochromating system is essential to distinguish between the emission of the test element and other radiation from the flame. 

Any of the monochromating systems described for absorptiometers may be used, although the cheaper models of flame photometer usually employ filter systems. In these cases interference from other elements at wave-... 

In addition to the emission due to the test element, radiation is also emitted by the flame itself. This background emission, together with turbulence in the flame, results in fluctuations of the signal and prevents the use of very sensitive detectors. The problem may be appreciably reduced by the introduction into the sample of a constant amount of a reference element and the use of a dual-channel flame photometer, which is capable of recording both the test and reference readings simultaneously. The ratio of the intensity of emission of the test element to that of the reference element should be unaffected by flame fluctuations and a calibration line using this ratio for different concentrations of the test element is the basis of the quantitative method. Lithium salts are frequently used as the reference element in the analysis of biological samples. 

Sodium and potassium were determined using Jenway PFP-7 model flame photometer and sodium or potassium chloride to prepare the standards. Calcium, iron,... 

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