Interferences with flame methods

Barium is present at very low concentrations in most environmental samples. Thus, in spite of the availability of a detection limit of only a few ng ml 1 by flame AES, the element is rarely determined by flame methods AAS with electrothermal atomization or ICP-AES is more commonly used. A notable exception is in the determination of the element in barium-rich geological deposits.8 Another exception is in the analysis of formation waters from offshore oil wells.9 However, in this matrix, inter-element interferences are encountered from alkali and alkali-earth elements. These could be effectively eliminated by the addition of 5 g 1 1 magnesium and 3 g 1 1 sodium as a modifier.9... 

Sodium and Potassium. Sodium and potassium can be deterrnined by either atomic emission or absorption. Large concentrations of sodium can interfere with the potassium deterrnination in either of these methods. Excess sodium can be added to both the potassium standards and samples to minimize any variations in the samples. Proper positioning of the flame helps reduce sodium interference in atomic absorption. 

Two colorimetric methods are recommended for boron analysis. One is the curcumin method, where the sample is acidified and evaporated after addition of curcumin reagent. A red product called rosocyanine remains it is dissolved in 95 wt % ethanol and measured photometrically. Nitrate concentrations >20 mg/L interfere with this method. Another colorimetric method is based upon the reaction between boron and carminic acid in concentrated sulfuric acid to form a bluish-red or blue product. Boron concentrations can also be deterrnined by atomic absorption spectroscopy with a nitrous oxide—acetjiene flame or graphite furnace. Atomic emission with an argon plasma source can also be used for boron measurement. 

Van Hall, S afr anko, and Stenger [ 51 ] have also pointed out that strong brines interfere with the method by producing fogs which maybe counted as carbon dioxide, while in cases where the flame ionisation detector is being used, large spikes appear in the recorded curve [105]. 

Metal salts may be used in the treatment of wool. Flame methods for the determination of aluminium [185], barium, chromium, copper, mercury, strontium, tin, zinc [186] and zirconium [187] in wool have been published. Standard additions to wool cleaned by soaking and washing it with disodium EDTA (800 ml of 0.5 M for 30g wool with soaking for 3 days and double washing) was used as the calibration technique. This compensated for interferences from hydrochloric acid and amino-acids. The samples were equilibrated to a constant humidity for 24 h and then 0.3 g sealed with 5 ml of constant boiling point hydrochloric acid in a glass tube. The tubes were placed in an oven at 110UC for 20 h. The nitrous oxide/acetylene flame was used for the determination of aluminium and zirconium. Sulphate, phosphate, citrate and silicate have been found to interfere in the determination of titanium and zirconium in fire-proofed wool [188], These flame... 

The presence of sodium compounds in certain flame pyrotechnics is to be avoided on account of the masking influence of the sodium flame and also on account of the hygroscopic nature of sodium nitrate which might interfere with the proper functioning of the p>rotechnic. The sodium nitrate can be estimated by the method of Ball. Wher a mixture of potassium nitrate, bismuth nitrate and caesium nitrate in nitric acid is added to a dilute solution of sodium nitrate, even in presence of large an>ount3 of potassium salts, the sodium is precipitated... 

With this method heavy metals are atomized electrothermally. The detection limits are about 100-times lower than those for the Flame-AAS. Problems with interference signals at this very low measuring range (below lp/1) can be overcome by using the Zeemaim correction technique [2]. 

The initial assessment of the accuracy of a new method should also include a detailed investigation of its speciflcity to determine the possible effects of interference from other naturally occurring compounds or from drugs and their metabolites it is possible for two methods, one with a lower degree of specificity than the other, to provide results that have comparable levels of accuracy. For instance, MacIntyre (Ml) showed that the differences between the flame spectrophotometric method and the less specific Kramer-Tisdall (K2) method in the determination of calcium were less than might have been predicted this occurred because losses of calcium revealed by the use of Ca in the assessment of the latter... 

The preferred sample preparation method for residual solvent analysis of pharmaceuticals is direct injection of the dissolved sample (11,60). With this technique, the recovery is most reliable because there is no opportunity for recovery loss due to adsorption or entrapment. The other techniques involve a separation of the volatiles before the GC injection and there is a risk that the volatile will be trapped. Typical solvents for this analysis are water, dimethyl sulfoxide, benzyl alcohol, and dimethylformamide (11,12,61). The three latter solvents are chosen because they are higher-boiling than commonly used pharmaceutical solvents and thus elute after them and do not interfere with the analysis. Water offers the advantage that it contributes little interference with a flame ionization detector. 

This is a most elegant and attractive method, which is likely to be used increasingly in flame photometric analysis. As will be discussed later, interference effects are not inherently absent with this method anionic interference is equally or more troublesome with this method than with emission ffame work, while cationic interference is less prominent, not because of the inherent nature of the method, but because of the lower flame temperatures usually used with this method. Its attraction is that it has some claims to be an absolute method in certain cir-... 

Sample preparation with flame methods can often be kept to a minimum. As long as chemical or spectral interferences are absent, essentially all that is required is to obtain the sample in tbe form of a diluted and filtered (for particulates) solution. It often makes no difference what the chemical form of the analyte is because it will be dissociated to the free elemental vapor in the flame. Thus, several elements can be determined in blood, urine, cerebral spinal fluid, and other biological fluids by direct aspiration of the sample. Usually, dilution with water will be required to prevent clogging of the burner. 

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. 

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