Anion Interferences

If the analyte element forms a stable molecule with an anion, or with some other substance present in the flame, the analyte element will not be available for the atomic excitation process. Several systems show this effect. 

FIGURE 9-13. Effects of phosphorus and boron on the flame emission signal of calcium. [From a Kansas State University Ph.D. dissertation by Mitchell E. Doty.] 

---

The anion interference with cationic response (or vice versa) ... 

Biosensors fabricated on the Nafion and polyion-modified palladium strips are reported by C.-J. Yuan [193], They found that Nafion membrane is capable of eliminating the electrochemical interferences of oxidative species (ascorbic acid and uric acid) on the enzyme electrode. Furthermore, it can restricting the oxidized anionic interferent to adhere on its surface, thereby the fouling of the electrode was avoided. Notably, the stability of the proposed PVA-SbQ/GOD planar electrode is superior to the most commercially available membrane-covered electrodes which have a use life of about ten days only. Compared to the conventional three-dimensional electrodes the proposed planar electrode exhibits a similar... 

The use of less polar solvents increases the preference for monovalent ions over divalent ones, and decreases the anion interference and the response time [154]. 

An electrode with a plastic membrane containing valinomycin as the active carrier is now predominantly used in clinical analyzers. Nearly four decades of experience with this sensor have proven that it fulfils all demands concerning sensitivity, selectivity and lifetime. An anionic interference that can be observed during measurements in undiluted urine may be eliminated by the use of silicone rubber instead of polyvinyl chloride in the membrane or by pre-dilution of urine. Despite some experimental trials, no other ionophore has replaced valinomycin as the active compound in potassium ISEs. This is basically due to the better stability and lipophilicity of this compound in comparison to the others proposed. 

Surfactants Surfactants are molecules made up of a polar hydrophilic head group and a hydrophobic hydrocarbon tail, which leads these molecules to favor as being at the interface between two phases.31 Membranes that were previously used, such as solvent-cast cellulose acetate72 or dip-coated polyurethane membranes,73 can be modified by surfactants to decrease biofouling. Lindner et al. showed that membranes with lower surfactant plasticizer ratios exhibited lower anion interference and better biocompatibility.74... 

In dicyanocobalt(III) a,b,c,d,e,g-hexamethyl-f-stearylamide cobyrinate (derivative 3) the six peripheral amide groups of vitamin B12 have been replaced with methyl ester groups, and the proximal base of the vitamin at the f-position with a stearylamide group (11). Electrodes prepared with this ionophore and DOS as the plasticizer were also selective for thiocyanate and nitrite over the rest of the anions tested. The main anionic interferent was salicylate. In all cases, the response of the electrodes to the preferred anions was sub-Nemstian. Overall, the selectivity pattern obtained with ionophore 3 is similar to that of 2 and to that of the hydrophobic cobyrinate-based electrodes reported previously (5, 12, 13). This observation suggests that in all cobyrinate ionophores the anions interact with the cobalt(III) center, and that the side chains of the corrin ring have a small effect on the selectivity of this interaction. 

Figure 4.21 Use of negatively charged polymeric films for excluding anionic interferences.

EDTA addition can prevent anion interferences other than chloride, phosphate and per-oxydisulfate. 

Organic acids (and their anions) interfere with the normal course of analysis. Thus in the presence of oxalic, tartaric or citric acids, the addition of the group reagent NH4C1 and NH3 solution might fail to cause the precipitation of the hydroxides of Fe, Al, and Cr. Furthermore, if oxalic acid is present, the oxalates... 

Although the method is rapid and straightforward, the use of free terbiiun as a sensing tool has several weaknesses, such as the potential for false positives or false negatives through com-plexation of anionic interferents to the exposed tripositive cation. 

We have seen similar improvements in the dipicolinate system in terms of increased resistance to common cationic and anionic interferents. The inclusion of D02A improved Tb-DPA binding in the presence of a wide array of interfering ions, most up to concentrations five orders of magnitude greater than that of DPA (92). This indicates that [Tb(D02A)] is able to bind... 

W4. West, A. C., and Cooke, W. D., Elimination of anion interferences in flame. spectroscopy. Use of (ethylenedinitrilo)tetraacetic acid. Anal. Chem. 32, 1471-1475 (1960). 

Yl. Yofe, J., and Firrkelstein, R., Elimination of anionic interference in flame photometric determination of calcium in the presence of phosphate and sulfate. Anal. Chim. Acta 19, 166-173 (1958). 

The voltammetric determination of nitrate following a preconcentration step using Permion 1025 was developed by Cox, Lundquist and Washinger (33). Nitrate is exchanged from the sample solution through the membrane into a small volume cell contain ing 0.1 M potassium chloride and 0.01 M lanthanum chloride. The method eliminates cationic interferences with the voltammetry but does not successfully deal with anionic interferences such as a sulfate or nitrite. 

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-... 

---