Internal standard flame photometer

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. 

Dissolution was carried out with the paddle method according to USP XXI, using a Prolabo dissolution tester. The dissolution medium was 1000 ml of distilled water at 37 0.5°C and 50 rev min-1. At appropriate time intervals, 5 ml of sample was withdrawn and an equal volume of medium was added to maintain a constant volume. Sample were filtered, diluted with lithium carbonate solution as an internal standard, and analysed using a Dr Lange MD 70 flame photometer. Each dissolution profile is the average of six separate tablets. 

The first commercially available flame photometer was introduced in the 1940s by the Perkin-Elmer Corporation. In 1948, Beckmann Instruments, Inc., introduced a flame attachment that could be used with their popular model D.U. spectrophotometer. By the late 1950s, instruments had been developed that used lithium as an internal standard to maximize precision. Autodilution features and microprocessor-controlled operations became widely used options in the 1970s. The most recent significant development was the introduction of cesium as the internal standard, by Instrumentation Laboratory, Inc. (Figs. 1-3). This development makes concurrent lithium determinations more practical. 

Fig. 1 Flame photometer using cesium as the internal standard. (Courtesy of Instrumentation Laboratory, Inc.)...

The widely used clinical flame photometers have been described in the section above on flame spectrometers and filter photometers. Analyte concentrations and their inherent line intensities are such that samples may be diluted greatly and various difficulties avoided. The burners are small and round so that self-absorption is slight and calibration curves nearly linear. Use of lithium added at a high concentration as the internal standard helps to correct for some of the uncontrolled variables. 

In clinical chemistry the determination of sodium, potassium, and calcium is well standardized. In the past decades flame photometry has been reputed to be the mediod of choice in the analysis of biological samples. The advantages of this procedure are a short requirement of time and materials for sample preparation, short duration of the analytical procedure, and the possibility of automation. The procedure became improved due to the application of lithium as internal standard. Excellent precision and accuracy could be obtained in sodium and potassium determination. In calcium determination inaccuracy occurs due to the matrix. It is of disadvantage that only the determination of total calcium and not the differentiation between free and protein bound calcium is possible. Furthermore special equipment (flame photometer) is necessary. 

Several instrument manufacturers supply flame photometers designed specifically for the determination of sodium, potassium, lithium, and sometimes calcium in blood serum, urine, and other biological fluids. Single-channel and multichannel (two to four channels) instruments are available for these determinations. In the multichannel instruments, each channel can be used to determine a separate element without an internal standard, or one of the channels can be reserved for an internal standard such as lithium. The ratios of the signals from the other channels to the signal of the lithium channel are then taken to compensate for flame noise and noise from fluctuations in reagent flow rate. Flame photometers such as these have been coupled with flow injection systems to automate the sample-introduction process (see Section 33B-3). Typical precisions for flow-injection-analysis-based flame photometric determinations of lithium, sodium, and potassium in serum are on the order of a few percent or less. Automated flow injection procedures require l/KIO the amount of sample and 1/10 the time of batch procedures. -... 

A flame emission photometer consists of an atomizer by which the sample is transformed into an aerosol spray before it is introduced into the flame. In the flame the metal atoms emit light of a wavelength characteristic to that element. The light passes through filters or diffraction gratings which isolate a single spectral line, and its intensity is measured by a photoelectric device. Internal standards can be used to compensate for variations in the intensity of the flame, e.g. lithium if sodium and potassium are being measured. 

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. 

The standard Cone Calorimeter (Section 14.3.3.2.1) described in ASTM E 1354 includes a smoke photometer to measure light extinction in the exhaust duct. The system is based on a laser light source. The same system is also standardized internationally, although it is described in a separate document from the main Cone Calorimeter standard (ISO 5660-2). Smoke measurements are reported in terms of the average specific extinction area (ASTM E 1354 and ISO 5660-2) and the smoke production rate and total smoke production for the period prior to ignition and the flaming period (ISO 5660-2). 

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