Simple flame photometer

These two typical instruments shall be discussed briefly here highlighting their various components and procedural details. 

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. 

B = Drain outlet (to maintain constant pressure head in the mixing Chamber), 

F = Mixing Chamber for Fuel Gas, Compressed Air, and Atomized Liquid Sample, 

K = Optical filter to transmit only a strong-line of the element, and 

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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. 

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

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

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). 

In a simple flame (emission) photometer an interference filter (Section 17.7) can be used. In more sophisticated flame emission spectrophotometers which require better isolation of the emitted frequency, a prism or a grating monochromator is employed. 

Its rapidity and detection limits, which are in the order of a few ppt (10-12) for many elements, make atomic emission one of the best techniques currently available for elemental analysis. These sophisticated instruments, however, are not intended to replace the flame photometers that are still used for many simple measurements. 

Figure 10 The essential components of a simple filter flame photometer...

Flame photometry (see also p. 168) is almost exclusively used for the determination of alkali metals because of their low excitation potential (e.g. sodium 5.14eV and potassium 4.34 eV). This simplifies the instrumentation required and allows a cooler flame (air-propane, air-butane or air-natural gas) to be used in conjunction with a simpler spectrometer (interference filter). The use of an interference filter allows a large excess of light to be viewed by the detector. Thus, the expensive photomultiplier tube is not required and a cheaper detector can be used, e.g. a photodiode or photoemissive detector. The sample is introduced using a pneumatic nebulizer as described for FAAS (p. 172). Flame photometry is therefore a simple, robust and inexpensive technique for the determination of potassium (766.5 nm) or sodium (589.0nm) in clinical or environmental samples. The technique suffers from the same type of interferences as in FAAS. The operation of a flame photometer is described in Box 26.2. 

Where only the determination of alkali metals is desired, a simple instrument can be devised in which selection of a resonance line can be obtained with the help of color filters. However, at present such instruments would not appear to have an appreciable advantage over flame emission methods, considering the sensitivity of the latter and the simplicity of presently available flame photometers. Appreciable advantages, on the other hand, are inherent in absorption over emission methods in the determination of the alkaline earths and magnesium. Since these metals have simple emission spectra, the use of filters, notably interference filters, would be feasible in instruments limited to the determination of these elements. 

To simplify these calculations, the capital cost of the instrument may be amortized over 5 or 6 years and maintenance costs ignored. The average daily cost can then be calculated and will be the same whether the instrument is used or not. Reagent costs are simple to calculate and are usually small in relation to other costs. Examples of labor and equipment costs of 5 commercial flame photometers, used to measure plasma sodium and potassium simultaneously, were given by Broughton and Dawson (B18). With small numbers of analyses, the least expensive instrument was the cheapest to run, but despite wide differences in capital outlay and labor requirements, the cost per analysis for the 5 instruments... 

A simple emission flame photometer is adequate for Na and K while a more selective emission/absorbance system is necessary for Ca, Mg, and trace metals. The range of trace metals which can be analyzed (e.g., Cu, Zn, Fe, As, Pb, Co, Mo, Se, Cd, Hg) with an instrument depends on the efficiency of atomization, excitation, and light collection, as well as the intensity and stability of the background. Owing to the difficulty of obtaining complete stability of baseline and sensitivity, frequent standardization of instruments is usually necessary. This can... 

For routine flame-emission determinations of alkali metals and alkaline earth elements, simple filter photometers often suffice. A low-temperature flame is employed to prevent excitation of most other metals. As a consequence, the spectra are simple, and interference filters can be used to isolate the desired emission lines. Flame emission was once widely used in the clinical laboratory for the determination of sodium and potassium. These methods have largely been replaced by methods using ion-selective electrodes (see Section 2 ID). 

To determine the proportions of the various chemical elements present in the human body is comparatively simple. All that is needed is an incinerator, a balance and the variety of techniques that analytical chemistry has developed to deal with the simpler problems presented by minerals. For instance, a flame photometer can recognize the metallic elements by the difference in the colour of the flame produced when a small sample of each is burnt in air. There are also specific ion electrodes that can quantify the amount of particular elements present in a solution. 

A salt aerosol is generated in an enclosure, usually up to a concentration of 13 mg m 3, with a particle mass median diameter of 0.3 to 1.3 pm, depending on the aerosol generator used. The subject dons the respirator, enters the enclosure and performs a set of simple exercises while the salt concentration inside and outside the respirator are measured by flame photometry. The Dstl, Porton Down uses a specially built high-sensitivity flame photometer to measure very low salt concentrations (and hence to confirm very high PFs). 

FES, which is used for measuring a small number of elements, is of much simpler design. For example, when sodium is analysed with a flame photometer whose flame attains 2000 °C, the sodium atoms are practically the only ones emitting radiation. To measure the emitted light, a simple optical filter put between the flame and the detector, in the absence of a monochromator, isolates a rather broad spectral band including the yellow radiation emitted by the element. 

In 1939 Griggs " and Ells and MarshalE introduced Lundegardh s method into the United States. In 1945 Barnes et al reported on a simple filter photometer for alkali metal determinations. The Perkin-Elmer Corporation apparently was the first American corporation to market such an instrument, which was similar to that developed by Barnes. In 1948 Beckman Instruments, Inc. made a flame excitation attachment available for their spectrophotometer. 

The high stability of the flame source, when compared to arc or spark excitation, was recognized as the key to the construction of simple instruments for the determination of easily excited elements, such as the alkali metals. Thus the first flame photometer produced in the U.S. in 1945 by Barnes used filters rather than a prism or grating, and used a modified Meeker burner as the flame excitation source. The instrument was especially useful for sodium and potassium determinations and was also soon utilized for calcium and magnesium analyses despite the handicap of poor detection limits. 

The ideal flame photometer is one which comprises a monochromator with motorised wavelength control, and to which can be attached any type of source unit. The detectors should be capable of responding to emission from 2,000A to 8,000A, and these should feed into an amplifier of wide sensitivity range. The output can be either read on a meter or passed into a recorder. This opinion is not intended to decry the use of simple equipment, but indicates what is required for the estimation of a large number of elements in all kinds of samples. 

Instead of Indicating a wavelength, a simple emission spectrometer (flame photometer ) may only be supplied with a so-called Ca filter. Make sure that this Is an interference filter, since glass filters are not selective enough. 

In this simple technique, the metal to be determined, in the form of a solution of a suitable compound, is sprayed into a flame. As in atomic absorption, when the solvent evaporates in the flame, the solid obtained is atomised and a gaseous metal ion is excited to a higher electronic level. When this drops to a lower level, a line spectrum is emitted and its intensity is measured. Flame photometers rely on the use of filters to isolate the line emitted, which is detected by a photocell and its output is measured by a calibrated galvanometer. The method is applicable to 16 metals. Reliable results are only obtainable by careful control of the experimental conditions. These depend on temperature (i.e. the type and rate of flow of the flammable gas and the oxidant which is usually air), the rate of flow of the solution to the flame as well as the compound tested and solvent used. A method used to minimise the effects of these variables is to add a known constant amount of an internal standard of a compound of a metal other than the metal to be determined but with similar excitation characteristics. Ihe ratio of the intensities of the standard and the test sample is determined. A calibration plot of the logarithm of the intensity ratio and the logarithm of the concentration of the test element is drawn. The concentration of an unknown is found by interpolation of the calibration plot. Alternatively, the standard additions method as in Sec.2.4.3 is used. In all cases, allowance should be made for any dilution effects. 

When the requirement is for many routine analyses of sodium and potassium, a simple filter fiame photometer burning a low temperature flame should be purchased. Many such models are on the market. On the other hand, if analysis for calcium and magnesium in biological fluids is also required, then only a fairly complex instrument with monochromator, photomultiplier, and high-temperature flame is satisfactory (Fig. 4). Compromise instruments between these two extremes lose the simplicity of the first type without gaining the versatility of the second. 

Because these spectra are simple, basic tiller photometers can be quite adequate for routine determinations of the alkali and alkaline-earth metals. A low-lemperalure flame is used lo avoid excitation of mosl... 

The atomic absorption method for determining the concentration of metallic elements has now gained wide acceptance. Instrumentation is relatively inexpensive and simple to use. Analytical interferences are less prevalent than with most other techniques means of recognizing and combating the interferences that do exist are described. The article discusses the basic principles of atomic absorption and also describes the fundamental design and modern improvements in the major components of instrumentation hollow-cathode lamps, burners, photometers, and monochromators. Atomic absorption is compared with some of its rival techniques, principally flame emission and atomic fluorescence. New methods of sampling and the distinction between sensitivity and detection limit are discussed briefly. Detection limits for 65 elements are tabulated. 

Most alkali and alkaline-earth metal ions that are found dissolved in water are readily quantitated by flame atomic emission spectroscopy (FUAES). This determinative technique has been, in the past, termed flame photometry. A simple photometer uses cutoff filters to isolate the wavelength,... 

Flame infrared emission (FIRE) spectrometry is a new technique that is useful in determining FAC in liquid bleach. In the FIRE method, solutions of sodium hypochlorite are acidified to produce aqueous CI2 (reactions [I] and [II] and Figure 1). Dissolved CI2 is liberated from solution in a purge tube and converted to vibrationally excited HCl molecules in a hydrogen-air flame. The intensity of the P-branch of the HCl stretching vibration at 3.8 pm is monitored with a simple filter infrared photometer that employs a lead selenide detector. 

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