Anions detection limits

TABLE 17-1 Amperometric anion detection limits using polypyrrole... 

Anion detection limit (in M) minimum detectable amount when signal is twice noise level. Pulsed potential E, +0.40 V E2, -1.0 V. ti = t2 = 60 ms. Current sampled for 16 ms at end of pulse. Ej, 1 M glycine eluent. Flow rate, 1 mL/m. Injection volume, 50 /iL. 

Anions of another group were derivatized with formation of gaseous chemiluminescing species. Chemical reaction - gas extraction has been used with chemiluminescence detection in the stream of canier gas in on-line mode. Rate of a number of reactions has been studied as well as kinetic curves of extraction of gaseous products. Highly sensitive and rapid hybrid procedures have been developed for the determination of lO, BrO, CIO, CIO, NO,, N03, CrO, CIO, Br, T, S, 803 with detection limits at the level of pg/L, duration of analysis 3 min. 

The ion pair extraction by flow injection analysis (FIA) has been used to analyze sodium dodecyl sulfate and sodium dodecyl ether (3 EO) sulfate among other anionic surfactants. The solvating agent was methanol and the phase-separating system was designed with a PTFE porous membrane permeable to chloroform but impermeable to the aqueous solution. The method is applicable to concentrations up to 1.25 mM with a detection limit of 15 pM [304]. 

Isoihamnetin la 323 Isothiazolone, microbiocidal la 45 Isothiocyanates la 75 lb 312 Isothiocyanate anions lb 307 Isotopes, detection limits la 41 Itaconic acid, reduction la 61 lodazide reaction lb 301,303... 

Cyanide and thiocyanate anions in aqueous solution can be determined as cyanogen bromide after reaction with bromine [686]. The thiocyanate anion can be quantitatively determined in the presence of cyanide by adding an excess of formaldehyde solution to the sample, which converts the cyanide ion to the unreactive cyanohydrin. The detection limits for the cyanide and thiocyanate anions were less than 0.01 ppm with an electron-capture detector. Iodine in acid solution reacts with acetone to form monoiodoacetone, which can be detected at high sensitivity with an electron-capture detector [687]. The reaction is specific for iodine, iodide being determined after oxidation with iodate. The nitrate anion can be determined in aqueous solution after conversion to nitrobenzene by reaction with benzene in the presence of sulfuric acid [688,689]. The detection limit for the nitrate anion was less than 0.1 ppm. The nitrite anion can be determined after oxidation to nitrate with potassium permanganate. Nitrite can be determined directly by alkylation with an alkaline solution of pentafluorobenzyl bromide [690]. The yield of derivative was about 80t.with a detection limit of 0.46 ng in 0.1 ml of aqueous sample. Pentafluorobenzyl p-toluenesulfonate has been used to derivatize carboxylate and phenolate anions and to simultaneously derivatize bromide, iodide, cyanide, thiocyanate, nitrite, nitrate and sulfide in a two-phase system using tetrapentylammonium cWoride as a phase transfer catalyst [691]. Detection limits wer Hi the ppm range. 

Simultaneous determination of both cations and anions in acid rain has been achieved using a portable conductimetric ion-exclusion cation-exchange chromatographic analyzer.14 This system utilized the poly(meth-ylmethacrylate)-based weak acid cation exchange resin TSK-Gel OA-PAK-A, (Tosoh , Tokyo, Japan) with an eluent of tartaric acid-methanol-water. All of the desired species, 3 anions and 5 cations, were separated in less than 30 minutes detection limits were on the order of 10 ppb. Simultaneous determination of nitrate, phosphate, and ammonium ions in wastewater has been reported utilizing isocratic IEC followed by sequential flow injection analysis.9 The ammonium cations were detected by colorimetry, while the anions were measured by conductivity. These determinations could be done with a single injection and the run time was under 9 minutes. 

Applications Applications of IC extend beyond the measurement of anions and cations that initially contributed to the success of the technique. Polar organic and inorganic species can also be measured. Ion chromatography can profitably be used for the analysis of ionic degradation products. For example, IC permits determination of the elemental composition of additives in polymers from the products of pyrolysis or oxidative thermal degradation. The lower detection limit for additives in polymers are 0.1% by PyGC... 

CE has been used for the analysis of anionic surfactants [946,947] and can be considered as complementary to HPLC for the analysis of cationic surfactants with advantages of minimal solvent consumption, higher efficiency, easy cleaning and inexpensive replacement of columns and the ability of fast method development by changing the electrolyte composition. Also the separation of polystyrene sulfonates with polymeric additives by CE has been reported [948]. Moreover, CE has also been used for the analysis of polymeric water treatment additives, such as acrylic acid copolymer flocculants, phosphonates, low-MW acids and inorganic anions. The technique provides for analyst time-savings and has lower detection limits and improved quantification for determination of anionic polymers, compared to HPLC. 

Purification methodologies for estrogens have been carried out by SPE with C18 [53, 54], polymeric [56, 60-62], silica [56-59] and anion exchange materials [59], preparative LC [53, 62], gel permeation chromatography (GPC) [54, 59, 97], liquid-liquid extraction [56], combination of them, or simple filtration [51]. However, the latter corresponds to the method with the highest reported method detection limits (MDL) [up to 175 ng/g dry weight (dw)]. 

A flow injection optical fibre biosensor for choline was also developed55. Choline oxidase (ChOX) was immobilized by physical entrapment in a photo-cross-linkable poly(vinyl alcohol) polymer (PVA-SbQ) after adsorption on weak anion-exchanger beads (DEAE-Sepharose). In this way, the sensing layer was directly created at the surface of the working glassy carbon electrode. The optimization of the reaction conditions and of the physicochemical parameters influencing the FIA biosensor response allows the measurement of choline concentration with a detection limit of 10 pmol. The DEAE-based system also exhibited a good operational stability since 160 repeated measurements of 3 nmol of choline could be performed with a variation coefficient of 4.5%. 

A. Malon, A. Radu, W. Qin, Y. Qin, A. Ceresa, M. Maj-Zurawska, E. Bakker, and E. Pretsch, Improving the detection limit of anion-selective electrodes an iodide-selective membrane with a nanomolar detection limit. Anal. Chem. 75, 3865-3871 (2003). 

To date, a few methods have been proposed for direct determination of trace iodide in seawater. The first involved the use of neutron activation analysis (NAA) [86], where iodide in seawater was concentrated by strongly basic anion-exchange column, eluted by sodium nitrate, and precipitated as palladium iodide. The second involved the use of automated electrochemical procedures [90] iodide was electrochemically oxidised to iodine and was concentrated on a carbon wool electrode. After removal of interference ions, the iodine was eluted with ascorbic acid and was determined by a polished Ag3SI electrode. The third method involved the use of cathodic stripping square wave voltammetry [92] (See Sect. 2.16.3). Iodine reacts with mercury in a one-electron process, and the sensitivity is increased remarkably by the addition of Triton X. The three methods have detection limits of 0.7 (250 ml seawater), 0.1 (50 ml), and 0.02 pg/l (10 ml), respectively, and could be applied to almost all the samples. However, NAA is not generally employed. The second electrochemical method uses an automated system but is a special apparatus just for determination of iodide. The first and third methods are time-consuming. 

Iodide was determined by an iodide-selective electrode (Ag2S/AgI) after other anions were separated by a rhodium nitrate element [101]. However, the electrode that was stabilised by 0.5 xm iodide responded to chloride ions in seawater, and the detection limit of iodide was 22 xg/l. 

Isaeva [181] described a phosphomolybdate method for the determination of phosphate in turbid seawater. Molybdenum titration methods are subject to extensive interferences and are not considered to be reliable when compared with more recently developed methods based on solvent extraction [182-187], such as solvent-extraction spectrophotometric determination of phosphate using molybdate and malachite green [188]. In this method the ion pair formed between malachite green and phosphomolybdate is extracted from the seawater sample with an organic solvent. This extraction achieves a useful 20-fold increase in the concentration of the phosphate in the extract. The detection limit is about 0.1 ig/l, standard deviation 0.05 ng-1 (4.3 xg/l in tap water), and relative standard deviation 1.1%. Most cations and anions found in non-saline waters do not interfere, but arsenic (V) causes large positive errors. 

Anion Preconcentration method Analytical finish Detection limit (pg/1) Section Reference... 

Methods of improving the detection limits for anions by preconcentration are reviewed in Table 2.6. 

Howard [27] determined dissolved aluminium in seawater by the micelle-enhanced fluorescence of its lumogallion complex. Several surfactants (to enhance fluorescence and minimise interferences), used for the determination of aluminium at very low concentrations (below 0.5 pg/1) in seawaters, were compared. The surfactants tested in preliminary studies were anionic (sodium lauryl sulfate), non-ionic (Triton X-100, Nonidet P42, NOPCO, and Tergital XD), and cationic (cetyltrimethylammonium bromide). Based on the degree of fluorescence enhancement and ease of use, Triton X-100 was selected for further study. Sample solutions (25 ml) in polyethylene bottles were mixed with acetate buffer (pH 4.7, 2 ml) lumogallion solution (0.02%, 0.3 ml) and 1,10-phenanthroline (1.0 ml to mask interferences from iron). Samples were heated to 80 °C for 1.5 h, cooled, and shaken with neat surfactant (0.15 ml) before fluorescence measurements were made. This procedure had a detection limit at the 0.02 pg/1 level. The method was independent of salinity and could therefore be used for both freshwater and seawater samples. 

Falkner and Edmond [334] determined gold at femtomolar quantities in seawater by flow injection inductively coupled plasma quadrupole mass spectrometry. The technique involves preconcentration by anion exchange of gold as a cyanide complex, [AulCNjj], using 195Au radiotracer (ti/2 = 183 days) to monitor recoveries. Samples are then introduced by flow injection into an inductively coupled plasma quadrupole mass spectrometer for analysis. The method has a detection limit of 10 fM for 4 litres of seawater preconcentrated to 1 ml, and a relative precision of 15% at the 100 fM level. 

Bhat et al. [199] used complexation with the bis(ethylenediamine) copper (II) cation as the basis of a method for estimating anionic surfactants in fresh estuarine and seawater samples. The complex is extracted into chloroform, and copper is measured spectrophotometrically in the extract using l,2(pyridyl azo)-2-naphthol. Using the same extraction system these workers were able to improve the detection limit of the method to 5 pg/1 (as linear alkyl sulfonic acid) in fresh estuarine and seawater samples. 

HTAC cationic micelles also markedly enhance the CL intensity of fluorescein (FL) in the oxidation of hydrogen peroxide catalyzed by horseradish peroxidase (HRP) [39], However, no CL enhancement was observed when anionic micelles of sodium dodecyl sulphate (SDS) or nonionic micelles of polyoxyethylene (23) dodecanol (Brij-35) were used (Fig. 9). CL enhancement is attributed to the electrostatic interaction of the anionic fluorescein with the HTAC micelles. The local concentration of fluorescein on the surface of the micelle increases the efficiency of the energy transferred from the singlet oxygen (which is produced in the peroxidation catalyzed by the HRP) to fluorescein. This chemiluminescent enhancement was applied to the determination of traces of hydrogen peroxide. The detection limit was three times smaller than that obtained in aqueous solution. 

With the newly proposed detector, Dadoo et al. [84] adapted this bioluminescence reaction to determine ATP. A selective and sensitive determination is achieved because the use of CE as a separation technique minimizes the effect of several interfering substances such as some anions (e.g., SCN, I ) that inhibit the reaction decreasing the luminescence emission, and even some nucleotides that generate light in this reaction but with lower intensity. A detection limit of 5 nM, approximately 3 orders of magnitude lower than using UV detection, was obtained. 

CL sensors based on immobilization of nonenzyme reagents have been extensively studied in recent years. Nakagama et al. [63] developed a CL sensor for monitoring free chlorine in tap water. This sensor consisted of a Pyrex tube, packed with the uranine (fluoresceine disodium) complex immobilized on IRA-93 anion-exchange resin, and a PMT placed close to the Pyrex tube. It was used for monitoring the concentration of free chlorine (as HCIO) in tap water, up to 1 mmol/L, with a detection limit of 2 nmol/L. The coefficient of variation (n =... 

Zhang s group [82] recently presented a novel CL sensor combined with FIA for ammonium ion determination. It is based on reaction between luminol, immobilized electrostatically on an anion-exchange column, and chlorine, electrochem-ically generated online via a Pt electrode from hydrochloric acid in a coulometric cell. Ammonium ion reacts with the chlorine and decreases the produced CL intensity. The system responds linearly to ammonium ion concentration in a range of 1.0-100 pM, with a detection limit of 0.4 pM. A complete analysis can be performed in 1 min, being satisfactorily applied to the analysis of rainwater. 

A 10 g sample is roasted at 650°C and decomposed with hydrochloric acid/hydrogen peroxide. The Pt and Pd in the solution is pre-concentrated using adsorbent materials which are composed of active charcoal and anion resin. The adsorbent materials are washed sequentially with 2% ammonium bifluoride, 5% hydrochloric acid and distilled water, and subsequently ashed in a muffle furnace at 650°C. The total residue of ca. 0.25 mg is dissolved with 2 ml fresh aqua regia, then diluted to 5ml using 10% hydrochloric solution, and determined using ICP-MS, which has a detection limit of 0.2 ppb for Pt and Pd. The residue can also be mixed with a spectral buffer, and determined by DC-arc ES, which has detection limits of 0.3 ppb for Pt and 0.2 ppb for Pd. 

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