Flame photometry calibration

Figure 14-9 also shows a flowchart for analysis of wet and dry precipitation. The process involves weight determinations, followed by pH and conductivity measurements, and finally chemical analysis for anions and cations. The pH measurements are made with a well-calibrated pH meter, with extreme care taken to avoid contaminating the sample. The metal ions Ca, Mg, Na, and are determined by flame photometry, which involves absorption of radiation by metal ions in a hot flame. Ammorda and the anions Cl, S04 , NO3 , and P04 are measured by automated colorimetric techniques. 

In flame photometry, signal drift and lamp flicker require that one or a few unknowns be bracketed by calibrations. Here, independent measurements on the same solutions means repeating the whole calibration and measurement cycle. 

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. 

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. 

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. 

It should be noted that when we used methods of measurement needing inorganic reference materials for calibration (such as flame photometry or atomic absorption spectrometry) the uncertainty due to the reference materials was considerably lower than that due to the photometric device. On the contrary, when we used a clinical reference material certified for its glucose concentration with a 10% (rel) uncertainty, this uncertainty exceeded twice the uncertainty due to the spec-trophotometric device. When we determined Mg by a spectrophotometric method with Titan Yellow, we found that the uncertainty due to the reference material was approximately twice that due the device, as we used a very accurate spectrophotometer. 

A variety of electrochemical methods have been incorporated into automated systems. The most widely used Mec-trochemical approach involves ion-selective electrodes. These electrodes have replaced flame photometry for the determination of sodium and potassium in many analyzers and have lately found direct application in the measurement of other electrolytes and indirect application in the analysis of several other serum constituents. The operating principle of ion-selective electrodes is given in some detail in Chapter 4. The relationship between ion activity and the concentration of ions in the specimens must be established with calibrating solutions, and frequent recalibration must be done to compensate for alterations of electrode response. 

To measure the volume of the Sephadex at equilibrium the equilibrated samples were contained in a calibrated tube (0.1 mL/division). An apparent volume was obtained from visual observation of the boundary defined by the layer to obtain the true volume an aliquot of the equilibrated solution phase was analysed for sodium by flame photometry. Most of the supernatant solution was then removed until 2.0 zt 0.025 mL of solution remained above the gel-defined boundary. Exactly 1.00 mL of water was added and the mixture was stirred sufficiently to assure a homogeneous aqueous phase. The solution phase was sampled immediately for sodium analysis with the flame photometer. From the observed dilution of the aqueous phase the true volume of the Sephadex gel at equilibrium was obtained. A correction for the matrix volume was based on the monomeric molecular weight of the Sephadex ( 220 25 capacity of 4.5 0.5 meq/dry g). In our samples, which contained about 88% water or 0.12 g acid/g sample, a volume of about 0.13/g is estimated for the matrix by assuming a density of approximately 0.9 for the dehydrated Sephadex. 

Sodium and potassium content in polyethers is determined by flame photometry from aqueous solutions of polyethers disaggregated before, or directly from solutions of polyethers in ethanol. The determinations are based on calibration curves made with solutions having known amounts of sodium and potassium ions. The maximum content of Na and K ions in polyethers was around a maximum of 5-10 ppm. In the polyether polyols used for prepolymer manufacture, the maximum limit for Na and K content is accepted as a maximum of 2 ppm in order to avoid the trimerisation and gelation of the prepolymer during storage. 

Figure 2.2. Routine assay of (a) sulfate in soil extracts by spectrophotometry [355] and (b) of potassium in soil extracts by flame photometry [43] as performed by the Analytical Laboratory of the Centro de Energia Nuclear na Agricultura, Piracicaba, Sao Paulo, Brazil. Note that the large series of routine assays, all performed in duplicate, are bracketed by serial calibration of standards injected in triplicate.

The results obtained show that the calibration of the ion-selective electrodes for sodium and potassium with saline phosphate buffers at 0.16 ionic strength give a different measurement if compared with non-buffered solutions and provide inferior results in respect of the flame photometry or, anyway, of the stoichiometric quantity present in the solution. 

A successful method employs flame photometry, using an atomic absorption spectrophotometer. Basically, the sample is ashed at 800°C, converted to the chloride with HCl and diluted with ionized water to give a test solution. Calibration solutions containing 1.0, 2.5, 5.0 and 10.0 pg/cm of Na are made up from a commercially available Sodium Stock Solution (containing 1 mg/cm Na). A blank of deionized water/HCl is aspirated into an air/acetylene flame set up with the atomic absorption spectrophotometer, followed by the standard solutions and the sample. A portion of the radiation proportional to the concentration of the sodium is absorbed. The absorption is measured and the concentration can be determined. 

Flame photometry. The polymer is ashed in platinum with sulphur and a xylene solution of magnesium AC dope. A nitric acid solution of the residual ash is analysed by flame photometry. Calibration is achieved against standard solutions of sodium in nitric acid. 

The developments of Beckman, Orion and Technicon (Analyzers for Na , K" ", Ca and Crions) represent impressive examples of automation in clinical chemistry using ion-selective electrodes. Normally the result is reported in meq/1 within about 3 minutes, which is accurate to about 2%. These automated instruments operate at optimum temperatures (37°C for Ca, room temperature in other cases) and are self-standard-izing with the help of an inner calibration solution. As an indication of the reproducibility of these direct potentiometric methods as compared to other techniques (for example flame photometry in the case of cations or coulometry for chloride), the correlation coefficients lie above 0.98 in all cases [246,247]. The correlation coefficients between whole blood and serum samples are 0.961 for Na", 0.962 for K, 0.976 for Ca " and 0.991 for CP [248]. 

Quantitative methods using flame emission photometry cannot be absolute because an unknown, although relatively constant, proportion of the sample will reach the flame of which only a further small proportion of atoms will actually be excited and subsequently emit radiation. Hence it is essential to construct calibration curves for any analysis. The radiation emitted by the flame when pure solvent is sprayed is used to zero the instrument and the maximum reading set when the standard with the highest concentration is sprayed. 

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