Magnesium atomic absorption

International Standard Organization. 1986. Water quality. Determination of calcium and magnesium. Atomic absorption spectrometric method. ISO 7980. International Organization for Standardization, Case Postale 56, CH-1211, Geneva 20 Switzerland. 

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. 

The alkah metals are commonly separated from all other elements except chlorine before gravimetric determination. In the absence of other alkaUes, sodium maybe weighed as the chloride or converted to the sulfate and weighed. WeU-known gravimetric procedures employ precipitation as the uranyl acetate of sodium—2inc or sodium—magnesium. Quantitative determination of sodium without separation is frequently possible by emission or atomic-absorption spectrometric techniques. 

Magnesium may conveniently be determined by atomic absorption spectroscopy (Section 21.21) if a smaller amount (ca 4 mg) is used for the separation. Collect the magnesium effluent in a 1 L graduated flask, dilute to the mark with de-ionised water and aspirate the solution into the flame of an atomic absorption spectrometer. Calibrate the instrument using standard magnesium solutions covering the range 2 to 8 ppm. 

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. 

Figure 24, The basic principle used in atomic absorption. The sample is sprayed into the flame, and the calcium and magnesium emission from the lamp is absorbed. The extent of absorption is measured on the detector arm translated in terms of concentration.

We have found that the use of serum standards for standardizing the instrument in the laboratory is useful. However, the serum standards cannot be used for urines. In urines, one runs into other problems and needs to use aqueous standards. Therefore, at present, while atomic absorption is the instrument of choice, there is much to be desired for the determination of calcium and magnesium in the routine laboratory of clinical chemistry. 

Magnesium deficiency has been long recognized, but hypermagnesia also occurs (Anderson and Talcott 1994). Magnesium can be determined in fluids by FAAS, inductively coupled plasma atomic emission spectrometry (ICP-AES) and ICP-MS. In tissue Mg can be determined directly by solid sampling atomic absorption spectrometry (SS-AAS) (Herber 1994a). Both Ca and Mg in plasma/serum are routinely determined by photometry in automated analyzers. 

Atomic absorption spectrophotometry [165,166] has been used in the determination of calcium and magnesium in seawater. 

The chemiluminescence technique has been used to determine trivalent chromium in seawater. Chang et al. [187] showed Luminol techniques for determination of chromium (III) were hampered by a salt interference, mainly due to magnesium ions. Elimination of this interference is achieved by seawater dilution and utilising bromide ion chemiluminescence signal enhancement (Fig. 5.7). The chemiluminescence results were comparable with those obtained by a graphite furnace flameless atomic absorption analysis for the total chromium present in samples. The detection limit is 3.3 x 10 9 mol/1 (0.2 ppb) for seawater with a salinity of 35%, with 0.5 M bromide enhancement. 

Atomic absorption spectrometry has been used to determine magnesium in seawater [413-415]. 

Elution volume calibrations were performed using radioactive tracers of the rare earth elements and 133Ba, with atomic-absorption or flame-emission analysis of iron, sodium, potassium, calcium, and magnesium. As shown in Fig. 5.14, any barium added to the second columns is eluted at the start of the light rare earth element fraction . To ensure barium removal the sample can be put through the first column again. 

Petit [563] has described a method for the determination of tellurium in seawater at picomolar concentrations. Tellurium (VI) was reduced to tellurium (IV) by boiling in 3 M hydrochloric acid. After preconcentration by coprecipitation with magnesium hydroxide, tellurium was reduced to the hydride by sodium borohydrate at 300 °C for 120 seconds, then 257 °C for 12 seconds. The hydride was then measured by atomic absorption spectroscopy. Recovery was 90 - 95% and the detection limit was 0.5 pmol/1. 

Andreae [564] coprecipitated tellurium (V) and tellurium (VI) from seawater and other natural waters with magnesium hydroxide. After dissolution of the precipitate with hydrochloric acid, the tellurium (IV) was reduced to tellurium hydride in 3 M hydrochloric acid. The hydride was trapped inside the graphite tube of a graphite furnace atomic absorption spectrometer, heated to 300 °C, and tellurium (IV) determined. Tellurium (VI) was reduced to tellurium (IV) by boiling with hydrochloric acid and total tellurium determined. Tellurium (VI) was then calculated. The limit of detection was 0.5 pmol per litre and precision 10-20%. 

Tony et al. [951] have discussed an online preconcentration flame atomic absorption spectrometry method for determining iron, cobalt, nickel, magnesium, and zinc in seawater. A sampling rate of 30 samples per hour was achieved and detection limits were 4.0,1.0,1.0,0.5, and 0.5 xg/l, for iron, cobalt, nickel, magnesium, and zinc, respectively. 

Tominaga et al. [682,683] studied the effect of ascorbic acid on the response of these metals in seawater obtained by graphite-furnace atomic absorption spectrometry from standpoint of variation of peak times and the sensitivity. Matrix interferences from seawater in the determination of lead, magnesium, vanadium, and molybdenum were suppressed by addition of 10% (w/v) ascorbic acid solution to the sample in the furnace. Matrix effects on the determination of cobalt and copper could not be removed in this way. These workers propose a direct method for the determination of lead, manganese, vanadium, and molybdenum in seawater. 

The application of palladium and magnesium nitrate matrix modifier for graphite furnace atomic absorption spectrometry has been discussed in detail [686]. The work has shown that a mixture of palladium and magnesium... 

Several recent determinations of the alkali and alkaline earth metals in serum or urine have been reported. Barrett 29) determined potassium, sodium, and calcium in semm by diluting the samples with lanthanum chloride solution. Suttle and Field 3°) used atomic absorption spectroscopy to determine potassium and magnesium in sheep plasma. 

Jones and Isaac 16 ) compared atomic absorption spectroscopy and spark emission spectroscopy for the determination of several elements in plant tissue. By comparing results statistically using a t-test, no significant differences were found for calcium, manganese, iron, copper, zinc, and aluminium, but significant differences were found for potassium and magnesium at the 0.01 % level. Breck162) made a similar comparison study for 15 elements. 

Schall192) recommended that the atomic absorption determination of magnesium, calcium, manganese, iron, and copper in fertilizers should be adopted as official, first action. 

Air-dry soil is mixed with 0.02 M calcium chloride solution (1 2 ratio, for instance, 10-g soil 20 mL 0.02 M CaCl2 solution) and mixed for 1 hour. The pH of the suspension can be measured directly. In addition, the solution can be filtered for the determination of aluminum or magnesium by atomic absorption spectroscopy (AAS) or inductively coupled plasma (ICP) spectroscopy (adapted from Reference 5). 

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. 

Procedure (determination). Magnesium is determined by atomic absorption spectrophotometry (see Method 5.2, Measurement of calcium and magnesium by AAS ). 

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