Sodium flame photometry

The majoiity of the various analyte measurements made in automated clinical chemistry analyzers involve optical techniques such as absorbance, reflectance, luminescence, and turbidimetric and nephelometric detection means. Some of these ate illustrated in Figure 3. The measurement of electrolytes such as sodium and potassium have generally been accomphshed by flame photometry or ion-selective electrode sensors (qv). However, the development of chromogenic ionophores permits these measurements to be done by absorbance photometry also. 

Sodium, D. of as zinc uranyl acetate, (g) 467 by flame photometry, 812 Sodium arsenite solution prepn. of standard, 390... 

If serum sodium is measured by flame photometry ° Uncommon today with the use of ion (sodium) specific... 

Polarography has also been applied to the determination of potassium in seawater [535]. The sample (1 ml) is heated to 70 °C and treated with 0.1 M sodium tetraphenylborate (1 ml). The precipitated potassium tetraphenylborate is filtered off, washed with 1% acetic acid, and dissolved in 5 ml acetone. This solution is treated with 3 ml 0.1 M thallium nitrate and 1.25 ml 2M sodium hydroxide, and the precipitate of thallium tetraphenylborate is filtered off. The filtrate is made up to 25 ml, and after de-aeration with nitrogen, unconsumed thallium is determined polarographically. There is no interference from 60 mg sodium, 0.2 mg calcium or magnesium, 20 pg barium, or 2.5 pg strontium. Standard eviations at concentrations of 375, 750, and 1125 pg potassium per ml were 26.4, 26.9, and 30.5, respectively. Results agreed with those obtained by flame photometry. 

The ratio, Nj/N0, can therefore be calculated. For the relatively easily excited alkali metal sodium, it is 9.9 x 10 6 at 2000 °K and 5.9 x 10 4 at 3000 °K this latter temperature is about the highest commonly obtained with flames used for atomic absorption or emission work. Hence, only about 1(T3 % of the sodium atoms are excited at 2000 ° and 6 x 1(F2 % at 3000°. For an element such as zinc,Nf/N0 is 5.4 x 10"10 at 3000 and so only 5 x 10"8% is excited. In spite of the small fraction excited, good sensitivities can be obtained for many elements by flame photometry if a high temperature flame is used, because the difference between zero and a small but finite number is measured. For example, seventy elements can be determined by flame photometry using the nitrous oxide-acetylene flame 1H. 

Flame photometry is the name given to the technique that measures the intensity of the light emitted by analyte atoms in a flame. It is the oldest of all the atomic techniques. It is not highly applicable because of the low temperature of the flame. Only a handful of elements can be measured with this technique, including sodium, potassium, lithium, calcium, strontium, and barium. The technique was formerly used... 

All the cations of Group 1 produce a characteristic colour in a flame (lithium, red sodium, yellow potassium, violet rubidium, dark red caesium, blue). The test may be applied quantitatively by atomising an aqueous solution containing Group I cations into a flame and determining the intensities of emission over the visible spectrum with a spectrophotometer (flame photometry). 

The results obtained with ISEs have been compared several times with those of other methods. When the determination of calcium using the Orion SS-20 analyser was tested, it was found that the results in heparinized whole blood and serum were sufficiently precise and subject to negligible interference from K and Mg ([82]), but that it is necessary to correct for the sodium error, as the ionic strength is adjusted with a sodium salt [82], and that a systematic error appears in the presence of colloids and cells due to complexa-tion and variations in the liquid-junction potential [76]. Determination of sodium and potassium with ISEs is comparable with flame photometric estimation [39, 113, 116] or is even more precise [165], but the values obtained with ISEs in serum are somewhat higher than those from flame photometry and most others methods [3, 25, 27, 113, 116]. This phenomenon is called pseudohyponatremia. It is caused by the fact that the samples are not diluted in ISE measurement, whereas in other methods dilution occurs before and during the measurement. On dilution, part of the water in serum is replaced by lipids and partially soluble serum proteins in samples with abnormally increased level of lipids and/or proteins. 

Flame Photometry, Atomic Absorption, and Neutron Activation. Comparatively few substances amenable to measurement by these techniques are used therapeutically chief among those that are being sodium, potassium, lithium, calcium, magnesium, zinc, copper, and iron, for all of which one or other of the techniques is the method of choice. 

Potassium and sodium are determined by flame photometry (see Method 5.2 Measurement of potassium and sodium by flame photometry ). 

Elemental composition K 23.41%, Br 47.85%, O 28.74%. Aqueous solution of the salt after sufficient dilution may be analyzed for its potassium content by AA, ICP, or flame photometry (see Potassium) and for bromate anion by ion chromatography. Also, bromate content can be measured by iodometric titration using a standard solution of sodium thiosulfate and starch as indicator. The redox reactions are as follows ... 

Sodium and potassium may be quantified by flame photometry, atomic absorption spectroscopy or ion specific electrodes. 

The content of aluminum in the reaction solution was determined at 50 p.p.m., the content of sodium at 100 p.p.m. Up to 95% conversion, the metal content in solution did not change, but at the end of the esterification about 50% of the aluminum had separated as an insoluble substance, and only 10% of the original concentration of the alkali still existed in solution. After removing the remaining alcohol and brightening the plasticizer with adsorbents, the content of the two metals lies with 0.07 p.p.m. sodium and 0.1 aluminum at the limit of detectability. The sodium was determined by flame photometry the determination of aluminum is discussed later. 

Conditioning of the manganese oxide suspension with each cation was conducted in a thermostatted cell (25° 0.05°C.) described previously (13). Analyses of residual lithium, potassium, sodium, calcium, and barium were obtained by standard flame photometry techniques on a Beckman DU-2 spectrophotometer with flame attachment. Analyses of copper, nickel, and cobalt were conducted on a Sargent Model XR recording polarograph. Samples for analysis were removed upon equilibration of the system, the solid centrifuged off and analytical concentrations determined from calibration curves. In contrast to Morgan and Stumm (10) who report fairly rapid equilibration, final attainment of equilibrium at constant pH, for example, upon addition of metal ions was often very slow, in some cases of the order of several hours. 

Major and minor elements in coal, having concentrations easily detectable by most modem analytical techniques, can be determined by a number of acceptable procedures. Various approaches, combining a. number of specific procedures, are frequently referenced in the literature. For example, the presently accepted procedure (ASTM D-2795) determines silicon, aluminum, iron, titanium, and phosphorus colorimetrically, calcium and magnesium chelatometrically, and sodium and potassium by flame photometry. This standard test method was withdrawn in 2001 but is still used in some laboratories. 

Fig. 17. Sodium (Na) and potassium (K) measured by flame photometry of eluates of strip fragments. The strip was cut transversely into sections. Buffers are either sodium or potassium Veronalate-Veronal buffers.

Flame photometry has promise for the measurement of sodium, lead, and potassium. An application to measurement of sodium and alkali metals has been reported. The continuous measurement of sulfur-containing particles has received considerable attention. The motivation for observation of sulfur-containing particles comes from concern about the potential hazard posed by sulfate in the atmosphere. 

In environmental analysis, flame photometry is most widely used for the determination of potassium, which emits at 766.5 nm. It is also often used for the determination of sodium at 589.0 nm, although spectral interference problems (see Chapter 3) then may be encountered in the presence of excess calcium because of emission from calcium-containing polyatomic species. Molecular species are more likely to be found in cooler flames than in hotter flames. Some instruments use single, interchangeable filters, while others have three or more filters, for example for the determinations of potassium, sodium and lithium,... 

Sodium is still often determined by flame photometry, measuring the emission intensity of the doublet at around 589 nm, but care is necessary to make sure that excess calcium does not cause spectral interference (from molecular emission). This is unlikely to be a problem if AES is used, with a narrow spectral band-pass, and the intensity of emission at 589.0 nm from an air-acetylene flame is measured. However, at low determinant concentrations it is then advisable to add 2-5 mg ml 1 potassium or caesium as an ionization buffer. This is even more true if a nitrous oxide-acetylene flame is used for FES, although its use is rarely justified in environmental analyses because the additional sensitivity gained is rarely necessary. 

Urine and serum samples are analyzed for uric acid (Uricaquant-method), creatinine (Jaffe reaction), sodium and potassium (flame photometry), calcium and magnesium (atom absorption method), and chloride (argentometry) as well as for osmolality. 

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. 

Herrmann and Lang (H3) studied various atomizers and recorded best results with a laboratory-built high pressure vaporizer. No ionization interference was seen in an air-propane flame and calibration curves were straight from 1 to 10 mg sodium per liter. Determinations were performed on serum diluted 1 20-1 200 and results agreed well with those concurrently obtained by emission flame photometry. 

Highly specific sodium electrodes have been developed in which the selectivity for sodium may be 10 times greater than that for potassium (C3, M19). With urine, the pH and potassium concentration should preferably be controlled, but this is unnecessary for blood. The potassium glass electrode is less selective and responds to NH/ and Na. Its selectivity may vary with age (M19). It can be used with blood only if corrections are made for sodium concentration according to Eq. (2) (M19, N2, N3), but when this is done, the electrode shows a linear response to potassium concentration. The precision of serum sodium and potassium measurements with electrodes was found to be better than those obtained by flame photometry (M19, N3). To compare the accuracy of the two methods, the results by flame photometry must be converted to concentrations in serum water. For most specimens, it was found that concentrations could be calculated satisfactorily from activity measurements and results by the two methods agreed (N3), but differences were noted with some samples. So far the cause of this has not been resolved, but it is possible that in future ionic activity will be recognized as a better diagnostic feature than ionic concentration (N3). 

The concentration of sodium ions was determined by flame photometry. Analysis of nickel, calcium, and magnesium ion concentration was carried out by atomic absorption spectrophotometry (Pye Unicam 8800, United Kingdom). The concentration of sulfate ions was determined by titration with barium chloride (BaCl2) solution in the presence of rhodizonate as indicator. Chloride ions were determined using ion-selective electrodes (manufacmred by Radelkis, Hungary). 

The use of flame photometry as a quantitative tool can be traced to work by Kirchhoff and Bunsen in the early 1860sJ Its modern history begins, however, in the 1940s, when instruments became available that successfully addressed the problems of reproducible sample introduction and detection. Flame photometry soon developed into a reliable analytical technique for the determination of several cations of pharmaceutical interest, notably sodium, potassium, and lithium. The technique is useful in the analysis of bulk drugs, dosage forms, and clinical samples such as blood and urine. 

The general viability of low-temperature flame photometry depends on two factors. First, the alkali and alkaline earth metals of analytical interest (sodium, potassium, lithium, cesium, rubidium, magnesium, calcium, strontium, and barium) reach their excited states at relatively lower temperatures than do most other elements. Second, the emission wavelengths offer enough resolution such that optical filtering can be accomplished at a relatively low cost. 

The analysis of clinical samples represents a typical application of flame photometry. Concentrations of sodium, potassium, and lithium in blood and urine are well within instrument working ranges. The specificity of the technique is a distinct advantage. Automated models of flame photometers, available during the past 25 years, are typically designed to serve the needs of the clinical chemist. Instrument calibration protocols are built into instruments to facilitate the timely analysis of sodium, potassium, and lithium in clinical samples. 

There are at least 14 USP, NF, or BP bulk drug monographs that use flame photometry either to control sodium potassium as an impurity or to assay for the primary ion (Table An external standard... 

There are at least 25 USP or BP formulation monographs that use flame photometry to assay ions of interest (Table This technique is applicable to a variety of situations because of the relatively low cost per sample (in analyst time, instrument capital expense, and testing supplies) reasonable precision (typical relative standard deviation values are 0.6% for sodium, 1% for potassium, and 2% for lithium) low sample volume requirements (as low as 10 pi in some cases) and ease of operation. 

With the development of ion-selection electrode technology (ISE), a means became available to directly measure (no dilution) sodium and potassium in the presence of clinical samples containing a significant amount of protein or lipids. Because of non-aqueous components in the sample matrix, the volume occupied by sodium and potassium ions is less than the total volume of the sample. When using a technique that requires dilution (flame photometry) or utilizes dilution (indirect-ISE), a lower concentration is observed than that obtained with direct-ISE. In as much as the bias can be clinically significant (up to 1% in some instances) it is important that the method used be taken into accoimt. ... 

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