Magnesium determination phosphate interferences

There are a number of interferences in magnesium determination in the air-acetylene flame, the best known being silicate, phosphate, and aluminium, so a releasing agent, e.g. lanthanum at a final dilution of 5 mg ml" must always be used. 

Willis (Wll), using a potassium hollow cathode tube instead of the commonly employed discharge lamp, determined potassium in blood serum. At the 1 50 dilution no interference was encountered from calcium, magnesium, and phosphate at serum levels, but sodium gave a small enhancement. The sodium interference was controlled by the addition in excess of sodium chloride or of the disodium salt of EDTA to samples and standards alike. 

Several papers deal with magnesium determination in blood and urine. Willis (WIO) analyzed serum in the air-acetylene flame and found no effect from the presence of sodium, potassium, calcium, or phosphate, but states that an enhancement was seen in serum diluted with water only, probably due to serum proteins. This interference was controlled by addition of strontium or EDTA. Sensitivities were the same in the eoal gas-air and air-acetylene flame, indicating complete atomization of magnesium. In urine (W13) no interference was encountered and determinations were performed on samples directly diluted with water. 

Kopito and Shwachman (K7) have described a method for the analysis of lead in either freshly voided or partially decomposed urine. The lead in 25 or 50 ml of urine is coprecipitated on bismuth hydroxide by adding bismuth nitrate and ammonia. After centrifuging, the precipitate is dissolved in acid to a final volume of 5 ml, and this solution is aspirated. With a 25-ml sample, 0.05-0.2 ppm of lead can be determined. The bismuth does not interfere with the lead absorption, and it suppresses interferences from sodium, potassium, calcium, magnesium, and phosphates. This procedure appears to offer sufficient sensitivity and the advantages of freedom from interference and simplicity in operation. Control of pH is not critical. Kopito and Shwachman claim that in... 

Phosphorous. The presence of trace amounts of phosphorus in metals and semiconductors is known to affect material properties. The produced in the (n,y) reaction is a pure emitter and has to be separated and rigorously purified. Paul (1998,2000) developed a method for P determination in steels and other high-temperature refractory alloys of interest to the aircraft industry. Irradiated samples were dissolved passed through cation-exchange columns to remove Co, Ni, and Cr followed by repeated precipitations of magnesium ammonium phosphate and ammonium phosphomolybdate. One of the major advantages of this technique is the determination of the chemical yield by gravimetry. Phosphorus was determined by INAA in matrices other than metals, e.g., polymers. In this case, the beta spectrum was corrected for interferences and self-absorption (St-Pierre and Kennedy 1998). A modified version of this procedure has been used to certify implanted phosphorus in silicon (Paul et al. 2003). 

Phosphate, arsenate, and vanadate interfere. Borate, fluoride, and large amounts of aluminium, calcium, magnesium, and the alkali metals have no effect in the determination, but large amounts of iron (> 5 per cent) appear to produce slightly low results. 

Procedure (determination of inorganic phosphate (a) in the acetic acid extract). The 8-hydroxyquinoline forms a precipitate in acidic ammonium molybdate solution, which will interfere unless the aliquot is <5 ml. It should therefore be removed by ignition as follows. Transfer 10 ml acetic acid extract to a 45-ml silica basin, add 0.5 ml 1 M magnesium acetate and evaporate to dryness on a water-bath. (Note do not use magnesium nitrate, which reacts adversely on heating with 8-hydroxyquinoline.)... 

This is by far the most frequently encountered interference in AAS. Basically, a chemical interference can be defined as anything that prevents or suppresses the formation of ground state atoms in the flame. A common example is the interference produced by aluminium, silicon and phosphorus in the determination of magnesium, calcium, strontium, barium and many other metals. This is due to the formation of aluminates, silicates and phosphates which, in many instances, are refractory in the analytical flame being used. 

Magnesium, like calcium, is subject to the effect of anions, although to a lesser degree. The most serious interference derives from refractory acidic oxides formed in the flame from a number of elements, particularly aluminum and silicon. The effect of phosphate and sulfate is much less marked than with calcium (Fig. 17), hut if aluminum or silicon is also present in the solutions, magnesium depression is much more severe than with either of these interfering agents alone. These interferences can be overcome by the addition of strontium, lanthanum, or calcium. Leithe and Hofer (L4, L5) showed that magnesium could be determined... 

Zettner and Seligson (Z4) determined magnesium in serum, using an air-acetylene flame. A study of interferences showed that phosphate and... 

Chromium was determined by Williams et cH. (W7) in animal feces samples to study pasture intakes. In the air-acetylene flame the sensitivity limit was 0.15 ppm. Of a variety of substances tested individually, only calcium, silicate, and phosphate depressed chromium absorption. However, when interferences were studied following treatment of solutions with phosphoric acid, manganese sulfate, and potassium bromide, depression was caused by silicon and aluminum, but calcium and magnesium enhanced absorption. Calcium was also capable of abolishing the effect of silicon and aluminum. 

Ionized calcium and total calcium measurements are both performed with PVC-type electrodes. Membranes based on lipophilic alkyl phosphates with phosphonate plasticizers have only marginal selectivity over magnesium but have been used in the past to determine ionized calcium (i.e., free calcium) in undiluted blood samples (M8). Interference from protons at low pHs prevents such membranes from being employed for total calcium determinations on samples diluted with acid. Use of ionophore ETH 1001 (see Fig. 2) overcomes any concerns about selectivity, whether from magnesium or pH, and is now the neutral carrier system most often utilized within analyzers to detect ionized or total calcium. 

Phosphate. Because of interference by uranium in the usual magnesium ammonium orthophosphate procedure,5 uranium is first removed by a cation-exchange procedure. A column (2.5 by 36 cm.) of 50 to 75 cc. of Dowex-50 resin is washed with 200 ml. of Z N hydrochloric acid and then 200 ml. of water at flow rates of 2 to 3 ml./min. An acidic solution of the product, containing 100 to 150 mg. of orthophosphate, is passed through the column at a rate of 1 ml./min. The column is washed with water until the washings are neutral to litmus, and phosphate is determined in the combined eluate and washings by the usual gravimetric procedure.6 Uranium can be eluted from the... 

Chemical interference, or the chemical combination of the element of interest with other elements in the sample or the flame, is probably the most important interference in flame methods. It directly affects the efficiency of production of neutral atoms in the flame and hence affects both absorption and emission in a similar manner. One of the most common types of chemical interference is the formation of refractory compounds with the test element, usually by an anion in the aspirated solution. The result is a decreased signal. For example, phosphate will react with calcium ions to produce calcium pyrophosphate in the flame. Less frequently, the presence of another cation may result in a decreased signal. For example, aluminum causes low results in the determination of magnesium, owing to the formation of a heat-stable aluminum-magnesium compound. Occasionally, a positive interference will occur in the presence of an interfering substance. The mechanism is not clearly understood, but has to do with the formation of a compound more volatile than the test element. 

Typical calibration graphs are presented in Figure 98. Sulfate has been determined by titrating with magnesium ions (Figure 98A). The sulfate ion concentration may vary between 1 and 20/i.gl . Phosphate and silicate interfere, and these ions should be removed before the determination. 

Flame atomic absorption spectrometry The determination of magnesium by FAAS is performed in diluted samples using the resonance line 285.2 nm and a stoichiometric air-acetylene flame. The interference due to phosphate is eliminated by the addition of LaCls. FAAS is used for the most reliable determination of total magnesium in physiological samples and has been chosen as the basis for the reference method. 

The advantage of discrete analyzers is that sample crossover in the system itself is the lowest possible. Volumes of 75 pi of reagent and sample volumes as large as 100 pi are sufficient. In an automated system with a throughput of 200 determinations per hour in the same sample 6 to 10 components (such as ammonium, alkalinity, aluminum, boron, bromide, calcium, chloride, chromium(VI), cyanide, fluoride, iron, magnesium, nitrate, nitrite, phosphate, etc.) can be determined. In discrete analyzers normally conventional spectrophotometric methods are used. These methods are prone to interference of the matrix of the sample. As a good concept for interference studies still is not available, interferences are as yet not sufficiently studied systematically even for routine analyses. 

Protective agents prevent interference by forming sfable but volatile species with the analyte Three common reagents for this purpose are elhylenedi-aminetetraacetic acid (EDTA). 8-hydroxyquinoline, and APCD, which is the ammonium salt of 1-pyrro-lidinecarbodithioic acid. The presence of EDTA has been shown to eliminate the interference of aluminum, silicon, phosphate, and sulfate in the determination of calcium. Similarly. 8-hydroxyquinoline suppresses the interference of aluminum in the determination of calcium and magnesium. 

General methods for the estimation of bismuth present in quantity include direct weighing as sulphide precipitation as sulphide, conversion to carbonate and ignition to oxide and precipitation as phosphate. Recently the use of EDTA as a titrant for bismuth has considerably simplified the determination of this metal in pharmaceutical mixtures. Bismuth forms a complex at pH 1 to 2 and at this degree of acidity few other metals likely to be encountered (with the exception of iron) interfere bismuth may therefore be selectively titrated in the presence of, say, aluminium, magnesium, or calcium. Various methods available have been discussed by Brookes and Johnson and the following general procedure is recommended ... 

Flame photometric determination cannot be recommended except for solutions containing upwards of 10 p.p.m. of magnesium because the resonance line occurs at 2852 A in the middle of an intense OH molecular band system and correction for this background is difficult even when a recording spectrophotometer is used. A hot flame is required and it has been reported by Knutson that some increase in sensitivity is obtained if an oxy-acetylene flame containing 55 per cent by volume of acetylene is used instead of the stoichiometric level of 29 per cent by volume. The resonance line suffers no serious spectral interference from other elements present in the sample solution but aluminium, chromium and molybdenum cause varying degrees of depression. Silicate, phosphate and sulphur ions depress the emission seriously and other anions less seriously and so standards should be made up to contain the same acids in the same concentrations as the samples. 

The atomic absorption technique provides one of the most sensitive and convenient methods of determining magnesium in solution. Allan, David and Willis have shown that a sensitivity limit of about OT p.p.m. can be reached and there is little difficulty in determining 1 p.p.m. or even less. The only serious interferences are from silicate, phosphate, aluminium and sulphate but these are eliminated according to David if calcium (or presumably any other alkaline earth element) is present in the solution. 

---