Determination of As, Cd, Pb and Se in DORM-1 dogfish muscle reference material using alkaline solubilization and electrothermal atomic absorption spectrometry with Ir+Rh as permanent modifiers or Pd+Mg in solution

In this work, tetramethylammonium hydroxide (TMAH) was used to solubilize the DORM-1 dogfish muscle certified reference material as a model substance for the determination of As, Cd, Pb and Se by electrothermal atomic absorption spectrometry (ET AAS). The sample was mixed with a small amount of TMAH and heated to 60 °C for 10 min in a water bath. After dissolution, As and Se were determined using palladium and magnesium nitrates as a chemical modifier added in solution. For Cd and Pb, best results were obtained with a mixture of 250 μg of each of iridium and rhodium as permanent modifiers. In both cases, the calibration was performed with aqueous solutions in 0.2% v/v HNO₃. The temperature program for each analyte was optimized using pyrolysis and atomization curves established with the fish reference material. The detection limits in dry samples and the characteristic mass values were: Cd 0.005 μg g⁻¹ and 0.9 pg; Pb 0.04 μg g⁻¹ and 7.6 pg; As 0.4 μg g⁻¹ and 13 pg and Se 0.6 μg g⁻¹ and 20 pg, respectively. Results from the determination of these elements in the DORM-1 certified fish reference material were within the 95% confidence interval of the certified values.     

Electrothermal atomic absorption spectrometric determination of cadmium and lead with stabilized phosphate deposited on permanently modified platforms

Some drawbacks of the phosphate modifier such as reagent blank contribution and background absorption in electrothermal atomic absorption spectrometric determination of cadmium and lead are substantially alleviated by application of small amounts of phosphate, approximately 0.2 mumol (25 mug of NH(4)H(2)PO(4) or (NH(4))(2)HPO(4)), on the integrated platform of transversely heated graphite atomizer pre-treated with 2.7 mumol of Zr (250 mug) or W (500 mug) and 0.1 mumol of Ir (20 mug). Pyrolysis temperatures for Cd and Pb are up to 900 and 1100 degrees C for aqueous solutions and within 400-600 degrees C (Cd) and 750-850 degrees C (Pb) for biological fluids (urine, blood) and tissues (hair, liver, muscle) solubilized with tetraethylammonium hydroxide. The thermally stabilized phosphate on Zr-Ir or W-Ir treated platforms serves as a permanent modifier in analyses of environmental waters by multiple hot injections of large sample aliquots. Applications to water and biological certified reference materials are tabulated and show good agreement with certified values. Characteristic masses are 0.7-1.0 pg for Cd and 26-31 pg for Pb.     

Direct determination of selenium in a wild fruit juice by electrothermal atomic absorption spectrometry

A matrix modifier composed of platinum and nickel is proposed for the determination of selenium in a wild fruit juice made from Lantingguo (Vuccinium uliginosam). Five matrix modifiers (copper/nickel, palladium/magnesium, platinum/magnesium, platinum/nickel and platinum/copper) for suppressing the interference effects of seven co-existing elements (potassium, phosphorus, calcium, magnesium, manganese, zinc and iron) in a wild juice were studied and a matrix modifier composed fro;m 10 mug of platinum and 200 mug of nickel was found to give the best performance. Selenium in three juices was determined by electrothermal atomic absorption spectrometry employing the proposed matrix modifier without matrix preseparation. The relative standard deviation was 14% for 0.20 mg l(-1) of selenium. The recoveries were 95-110%. A characteristic mass was 28 pg.     

Determination of cadmium and lead in biological samples by Zeeman ETAAS using various chemical modifiers

The electrothermal atomic absorption spectrometric determination of cadmium and lead in biological certified reference materials (CRMs) has been carried out by using NH(4)H(2)PO(4), Ni, Pd, Ni+NH(4)H(2)PO(4), Pd+NH(4)H(2)PO(4) and Ni+Pd+NH(4)H(2)PO(4) as chemical modifiers. A comprehensive comparison was made among the modifiers in 1% Triton X-100 plus 0.2% nitric acid as diluent and without modifier. Zeeman background correction and graphite tubes inserted with platforms were used. Comparison was made in terms of pyrolysis and atomization temperatures, atomization and background absorption profiles. Ni+Pd+NH(4)H(2)PO(4) modifier mixture was found to be preferable for the determination of Cd and Pb. Pyrolysis temperatures of analytes were increased up to 900 degrees C for Cd and 1250 degrees C for Pb by using Ni+Pd+NH(4)H(2)PO(4) in 1% Triton X-100 plus 0.2% nitric acid diluent solution. Biological CRMs were analyzed to verify the accuracy and precision of this method. Depending on the biological sample type, the percent recoveries were increased from 62 to 102% for Cd and from 58 to 106% for Pb by using the proposed modifier mixture. The detection limits of Cd and Pb were found to be 0.04, 0.92 mug l(-1), respectively.     

Palladium and magnesium nitrates,a more universal modifier for graphite furnace atomic absorption spectrometry

A mixture of palladium and magnesium nitrates was found to be a very powerful modifier for the determination of As, Bi, In, Pb, Sb, Se, Sn, Te and Tl in graphite furnace atomic absorption spectrometry. Thermal pretreatment temperatures of 900-1400°C can be used with the proposed modifier. This is in most cases substantially more than what can be applied with the modifiers recommended up to now, so that separation of the analyte from the concomitants should be easier. This is shown to be true for the determination of lead in sea water and of selenium in biological materials. Optimum atomization temperatures are more uniform and typically around 2000°C for the investigated elements when the palladium and magnesium nitrates mixed modifier is used. This modifier therefore allows the use of common conditions for all the investigated elements with a minimum sacrifice in sensitivity, an important pre-requirement for multi-element furnace techniques. The proposed mixed modifier also minimizes the risk of contamination because palladium as well as magnesium nitrate can be obtained in high purity, and both elements are infrequently determined in the graphite furnace.     

Direct sample introduction of wines in graphite furnace atomic absorption spectrometry for the simultaneous determination of arsenic, cadmium, copper and lead content

A multi-element graphite furnace atomic absorption spectrometry (GFAAS) method was elaborated for the simultaneous determination of As, Cd, Cu, and Pb in wine samples of various sugar contents using the transversally heated graphite atomizer (THGA) with end-capped tubes and integrated graphite platforms (IGPs). For comparative GFAAS analyses, direct injection (i.e., dispensing the sample onto the IGP) and digestion-based (i.e., adding oxidizing agents, such as HNO(3) and/or H(2)O(2) to the sample solutions) methods were optimized with the application of chemical modifiers. The mixture of 5 microg Pd (applied as nitrate) plus 3 microg Mg(NO(3))(2) chemical modifier was proven to be optimal for the present set of analytes and matrix, it allowing the optimal 600 degrees C pyrolysis and 2200 degrees C atomization temperatures, respectively. The IGP of the THGA was pre-heated at 70 degrees C to prevent the sputtering and/or foaming of sample solutions with a high organic content, dispensed together with the modifier solution, which method also improved the reproducibility of the determinations. With the digestion-based method, the recovery ranged between 87 and 122%, while with the direct injection method it was between 96 and 102% for Cd, Cu, and Pb, whereas a lower, compromise recovery of 45-85% was realized for As. The detection limits (LODs) were found to be 5.0, 0.03, 1.2, and 0.8 microg l(-1) for As, Cd, Cu, and Pb, respectively. The characteristic mass (m(0)) data were 24 pg As, 1.3 pg Cd, 13 pg Cu, and 35 pg Pb. The upper limits of the linear calibration range were 100, 2, 100, and 200 microg l(-1) for As, Cd, Cu, and Pb, respectively. The precisions were not worse than 4.8, 3.1, 3.7, and 2.3% for As, Cd, Cu, and Pb, respectively. For arsenic, a higher amount of the modifier (e.g., 20 microg Pd plus 12 microg Mg(NO(3))(2)) could be recommended to overcome the interference from the presence of sulphate and phosphate in wines. Although this method increased the sensitivity for As (m(0)=20 pg), it also enhanced the background noise, thus only a slight improvement in the LOD of As (3.9 microg l(-1)) was realized. For the 35 red and white wine samples studied, the highest metal contents were observed for Cu ranging from 20 to 640 microg l(-1) (average: 148 microg l(-1)), followed by Pb from 6 to 90 microg l(-1) (average: 32.3 microg l(-1)), and Cd from 0.05 to 16.5 microg l(-1) (average: 1.06 microg l(-1)), whereas the As content was below the LOD. This wide fluctuation in the trace metal content could be associated with the origin of wines from various regions (i.e., different trace metal level and/or quality of soil, and/or anthropogenic impact), and with diverse materials (e.g., additives and containers) involved in the wine production processes. The Cu content of wine samples was significantly correlated with Pb, whereas its weak anti-correlation was found with Cd. Interestingly, the level of Pb was anti-correlated with the year of production of the wines. This is likely due to the gradual decrease in the Pb content of soils of vineyards by time, which certainly causes less Pb-uptake of the grape plant, thus a decrease in the Pb content of wines as well.     

Minimization of lead and copper interferences on spectrophotometric determination of cadmium using electrolytic deposition and ion-exchange in multi-commutation flow system

A new strategy for minimization of Cu(2+) and Pb(2+) interferences on the spectrophotometric determination of Cd(2+) by the Malachite green (MG)-iodide reaction using electrolytic deposition of interfering species and solid phase extraction of Cd(2+) in flow system is proposed. The electrolytic cell comprises two coiled Pt electrodes concentrically assembled. When the sample solution is electrolyzed in a mixed solution containing 5% (v/v) HNO(3), 0.1% (v/v) H(2)SO(4) and 0.5 M NaCl, Cu(2+) is deposited as Cu on the cathode, Pb(2+) is deposited as PbO(2) on the anode while Cd(2+) is kept in solution. After electrolysis, the remaining solution passes through an AG1-X8 resin (chloride form) packed minicolumn in which Cd(2+) is extracted as CdCl(4)(2-). Electrolyte compositions, flow rates, timing, applied current, and electrolysis time was investigated. With 60 s electrolysis time, 0.25 A applied current, Pb(2+) and Cu(2+) levels up to 50 and 250 mg l(-1), respectively, can be tolerated without interference. For 90 s resin loading time, a linear relationship between absorbance and analyte concentration in the 5.00-50.0 mug Cd l(-1) range (r(2)=0.9996) is obtained. A throughput of 20 samples per h is achieved, corresponding to about 0.7 mg MG and 500 mg KI and 5 ml sample consumed per determination. The detection limit is 0.23 mug Cd l(-1). The accuracy was checked for cadmium determination in standard reference materials, vegetables and tap water. Results were in agreement with certified values of standard reference materials and with those obtained by graphite furnace atomic absorption spectrometry at 95% confidence level. The R.S.D. for plant digests and water containing 13.0 mug Cd l(-1) was 3.85% (n=12). The recoveries of analyte spikes added to the water and vegetable samples ranged from 94 to 104%.     

Direct determination of bismuth, indium and lead in sea water by Zeeman ETAAS using molybdenum containing chemical modifiers

Direct determination of Bi, In and Pb in sea water samples has been carried out by ETAAS with Zeeman background correction using molybdenum containing chemical modifiers and tartaric acid as a reducing agent. Maximum pyrolysis temperatures and the effect of mass ratios of the mixed modifier components on analytes have been investigated. Mo+Pd+TA or Mo+Pt+TA mixture was found to be powerful for the determination of 50 mug l(-1) of Bi, In and Pb spiked into synthetic and real sea waters. The accuracy and precision of the determination were thereby enhanced. The recoveries of analytes spiked were 94-103% with Mo+Pd+TA or Mo+Pt+TA and they are only 49-61% without modifier.     

Determination of lead in drinking waters by hydride generation and atomic-absorption spectroscopy, and three other methods

Simultaneous presence of copper and nickel in potable waters interferes with the determination of lead at trace levels by the hydride-atomic-absorption spectrophotometric method. This interference is eliminated by co-precipitating lead with manganese dioxide from acidic solution. The precipitate is dissolved in 0.85% nitric acid and analysed by the automated hydride-atomic-absorption method. This method has been applied to 22 representative water samples and results compared with those obtained by using differential pulse anodic-stripping voltammetry, flame atomic-absorption and graphite-furnace atomic-absorption spectrophotometry. The precision of the three methods is reported and their accuracy checked by the analysis of reference standard water samples. The sensitivity of the three methods is of the order of 1 mug/l., compared to 100 mug/l. for flame atomic-absorption. The merits of each method are discussed.     

Inductively Coupled Plasma Mass Spectrometry for Sequential Determination of Trace Metals in Rain and River Waters Using Electrothermal Vaporization

Abstract

The sequential determination of 14 trace metals, Al, V, Cr, Mn, Fe, Ni, Cu, Zn, As, Se, Mo, Cd, Sb and Pb, in rain and river water samples has been investigated using an inductively coupled plasma mass spectrometry (ICP-MS) with a graphite rod electrothermal vaporizer (ETV) in the presence of the mixed modifier of palladium nitrate and magnesium nitrate. The sensitivity enhancements due to the presence of the modifier were observed for all analyte elements. Detection limits as high as 0.52, 0.13, 0.89, 0.35, 1.76, 0.5, 0.9, 0.5, 0.04, 1.03, 0.28, 0.07, 0.1 and 3.78 pg, respectively, for Al, V, Cr, Mn, Fe, Ni, Cu, Zn, As, Se, Mo, Cd, Sb and Pb have been achieved. For the determination of trace metals in both rain and river water samples by this method, the repeatibility of sample solution were very good, i.e. from 1% to 7% (as a coefficient variation) and the recoveries of elements were good enough, i.e. from 81% to 106%, by using a standard addition method. There was no difference between the results obtained by nebulizer ICP-MS and those obtained by this method, except for zinc and arsenic.     

Use of iridium plus rhodium as permanent modifier to determine As, Cd and Pb in acids and ethanol by electrothermal atomic absorption spectrometry

The most severe interferences in atomic absorption spectrometry are caused by the presence of anions when they are in different concentrations in the samples and in the calibration solutions. The analyte addition technique or matrix matching calibration can be employed to minimize or compensate the non-spectral interferences, but they are time consuming or difficult to be carried out. The use of chemical modifiers usually allows higher pyrolysis temperatures and consequently the removal of components of the sample matrix, equalizing the analyte signal in the sample and in the calibration solution. In this work, a mixture of Ir and Rh is proposed as permanent modifier to determine As, Cd and Pb in diluted hydrochloric, sulfuric and phosphoric acids and in ethanol and methanol by electrothermal atomic absorption spectrometry (ET AAS) with calibration against 1% v/v nitric acid aqueous solutions. The performance of the proposed permanent modifier was compared to that of Pd plus Mg nitrates in solution. Better recoveries, low background levels and faster analysis were obtained with the permanent modifier. The permanent modifier was also successfully employed for the determination of As, Cd and Pb in different concentrations of sulfuric and hydrochloric acids. For the phosphoric acid, the proposed modifier was only efficient for acid concentrations up to 2% v/v for As and up to 5% v/v for Cd and Pb. The precision, expressed as the relative standard deviation (n=3), was lower than 10%, for all samples, including ethanol and methanol.     

Determination of cadmium, copper and lead in soils, sediments and sea water samples by ETAAS using a Sc + Pd + NH(4)NO(3) chemical modifier

Cadmium, copper and lead in soils, sediments and spiked sea water samples have been determined by electrothermal atomic absorption spectrometry (ETAAS) with Zeeman effect background corrector using NH₄NO₃, Sc, Pd, Sc + NH₄NO₃, Pd + NH₄NO₃, Sc + Pd and Sc + Pd + NH₄NO₃ as chemical modifiers. A comprehensive comparison was made among the modifiers and without modifier in terms of pyrolysis and atomization temperatures, atomization and background absorption profiles, characteristic masses, detection limits and accuracy of the determinations. Sc + Pd + NH₄NO₃ modifier mixture was found to be preferable for the determination of analytes in soil and sediment certified and standard reference materials, and sea water samples because it increased the pyrolysis temperature up to 900 °C for Cd, 1350 °C for Cu and 1300 °C for Pb. Optimum masses of mixed modifier components found are 20 μg Sc + 4 μg Pd + 8 μg NH₄NO₃. Characteristic masses of Cd, Cu and Pb obtained are 0.6, 5.3 and 15.8 pg, respectively. The detection limits of Cd, Cu and Pb were found to be 0.08, 0.57 and 0.83 μg l⁻¹, respectively. Depending on the solid sample type, the percent recoveries were increased up to 103% for Cd, Cu and Pb by using the proposed modifier mixture. The accuracy of the determination of analytes in the sea water samples was also increased.     

The problem of arsenic interference in the analysis of Cd to evaluate its extractability in soils contaminated by arsenic

The arsenic (As) spectral interference observed in the determination of cadmium (Cd) by inductively coupled plasma atomic emission spectrometry (ICP-AES) was studied in atomic absorption spectrometry (AAS) using flame (FAAS) and graphite furnace (GFAAS) as atomizers. The soils of 15 kitchen gardens located near two smelters in the North of France were selected according the ratio As/Cd. Four different extracting solutions usually used to evaluate the mobility of Cd were chosen to extract Cd from these soil samples: citric acid 0.11 M, acetic acid 0.11 M, calcium chloride 0.01 M and water. The quantitative determinations of Cd in the 15 soils for each solvent were investigated by ICP-AES at two lines (228.802 and 214.438 nm) and by FAAS or GFAAS with two-way background compensation. Compared to the Cd concentrations measured in the acid solutions and in the CaCl₂ solution after the addition of a chemical modifier, it was clearly demonstrated that the high-speed self-reversal background compensation (HSSR-method) was the method of choice to eliminate the spectral interference of As during Cd determination by FAAS and by GFAAS. In water, it was shown that the deuterium lamp used for the background compensation (D₂-method) was able to eliminate the most of the As interference. In comparison with Cd concentrations in water after adding a chemical modifier, those obtained with the HSSR-method were similar and a very good correlation was obtained between these two methods (R² = 0.995). It was therefore established that the HSSR-method would be able to replace the chemical modifiers to eliminate As interference in the determination of Cd-extractable from As contaminated soils.     

Transverse heated filter atomizer for electrothermal atomic absorption spectrometry

The filter furnace atomization concept was applied for the transverse heated atomizer. A graphite filter with graphite fiber reeled onto it was inserted into the tube of the standard transverse heated graphite atomizer (THGA) in the place of the platform. Automatic plugging of the sampling hole was applied during the atomization stage. The performance of the filter atomizer (THFA), compared with the THGA, was tested for the determination of Ag, As, Au, Bi, Cd, Cu, Ga, In, Mn, Pb, Sb, Se and Tl. The analytical performances of the THFA displayed some advantages in comparison with the THGA. The sampling volume varied in the range of 5–90 μl, while drying time for any volume was less than half of that used for the THGA. Owing to the reduced diameter of the analytical zone (2 mm) along the filter axis, a sensitivity improvement was observed for all elements, 1.3–2.8-fold without plugging and 4.3–4.8-fold for Bi, Cd, Pb and Tl with plugging of the dosing hole. An increased peak width (by two to five times for the elements tested) limited the determination of less-volatile metals. The intensity of light decreased by 20–30% in comparison with the THGA. Taking into account the sensitivity, sampling volume, light loss and signal width, the calculated gain in relative detection limit is substantial (about 2.5–7 times) only for volatile elements when the plugging is applied. The pyrolysis temperatures for Ag, As, Au, Cd, Cu and Se in the THFA without addition of modifier were by 200–600°C higher than in the THGA using Pd/Mg modifier. The lifetime of THFA tubes was similar to that of THGA tubes.     

Large-volume injection electrothermal AAS combined with tungsten-treated pyrolytic graphite furnace: determination of cadmium in tap, snow and river water samples with phosphate modifier.

A large-volume (100 microl) injection-ETAAS with W-treated PG furnace combined with a phosphate modifier was applied to the determination of unpolluted levels of Cd in tap, snow and river-water samples. The limit of detection of 1.1 ng l(-1) was observed for a 4 w/v% NH4H2PO4 modifer. Matrix interference studies were tested for major ion species well found in fresh water. The direct determination of Cd in certified river water (12 +/- 2 ng l(-1)) was carried out and the observed value was in agreement with the certified one. The good recoveries of Cd added to real environmental water samples were also observed. This method was applied to the determination of Cd in unpolluted environmental water samples.     

Effects of ammonium nitrate on sensitivity for determinations of copper,iron, and manganese in sea water by atomic absorption spectrometry with pyrolytically coated graphite tubes

Ammonium nitrate, used as a matrix modifier in sea-water analysis to eliminate the interference of sodium chloride, degrades the pyrolytic coating on graphite-furnace tubes. The initially increased sensitivities for copper, manganese and iron are maintained for up to 15 atomizations; there is then a rapid decline to a constant lower sensitivity. The characteristics depend strongly on the particular lot of furnace tubes. To decrease the NaCl interference without using matrix modifier, estuarine samples must be diluted (1 + 1) with pure water. Blanks and standards are prepared and diluted with Gulf Stream water containing low amounts of trace metals to match the estuarine matrix.     

Determination of cadmium, lead and copper in water samples by flame atomic-absorption spectrometry with preconcentration by flow-injection on-line sorbent extraction

Cadmium, lead and copper were determined in synthetic sea-water, drinking water and the NBS 1643b Trace Elements in Water standard reference material at mug/l. levels by flame atomic-absorption spectrometry after on-line preconcentration by sorbent extraction with a flow-injection system. Bonded silica with octadecyl functional groups packed in a micro column of 100 mul capacity was used to collect diethylammonium diethyldithiocarbamate complexes of the heavy metals in the aqueous samples. The sample loading time was 20 sec at a flow-rate of 3.3 ml/min. Ethanol or methanol was used to elute the adsorbed analytes into the spectrometer. The sample loading rate, elution rate and pH were optimized. Enrichment factors of 19-25 for Cd, Pb and Cu were achieved at sampling frequencies of 120/hr with precisions of 1.4, 1.0 and 1.3% rsd (n = 11), respectively. The detection limits (3sigma) for Cd, Pb and Cu were 0.3, 3 and 0.2 mug/l., respectively. Determination of Cd, Pb and Cu in NBS SRM 1643b showed good agreement with the certified values. Recoveries of Cd and Pb added to sea-water were 95 and 102%, respectively.     

Simultaneous determination of Cu, Cd and Pb in drinking-water using W-Coil AAS

An inexpensive, multi-element, W-coil atomic absorption spectrometer has been developed. Atomization occurs on W-coils extracted from commercially available slide projector bulbs. The system has minimal power requirements, 120 ACV and 15 A. A small, computer controlled CCD spectrometer is used as the detector. A multi-element Cu, Cd and Pb hollow cathode lamp is used as the source. 20 mul volumes are deposited on the coil and atomized at 6.7 A or approximately 2200 degrees C. Cu, Cd and Pb were simultaneously determined in tap water, drinking water and a quality control sample. The instrument detection limits are 0.8, 0.2 and 3.0 mug/l for Cu, Cd and Pb, respectively.     

Determination of fluoride in sea-water by molecular absorption spectrometry of aluminium monofluoride after removal cation and anion interferences

Three procedures are proposed for the determination of trace levels of fluoride in sea-water, based on the formation of aluminium monofluoride in an electrothermal graphite furnace, followed by measurement of its molecular absorption at 227.45 nm. They involve the use of dilution, a matrix modifier, or a matrix modifier and an ion-exchange resin, and are all acceptably sensitive and specific for fluoride. Interferences from cations and anions are removed by a simple 20-fold dilution of the sample. At 10-fold sample dilution, chloride interference can be removed by adding 0.3M ammonium nitrate together with 0.01M aluminium + 0.01M strontium as a matrix modifier. The same matrix modifier is valid for use with 5-fold sample dilution and a cation-exchange step to avoid matrix affects from cations and chloride. The detection limit is about 8-10 ng/ml fluoride and the determination limit is 20 ng/ml. The precision of peak-height measurement at 0.2 mug/ml is 5-7%.     

Preconcentration of trace elements on a support impregnated with sodium diethyldithiocarbamate prior to their determination by inductively coupled plasma-atomic emission spectrometry

A procedure is developed for determination of As, Co, Se, Cr, Pb, Zn, Cu, Mn, Cd, Sb, and Sn in water by ICP-AES analysis of alcohol eluates after pre-concentration of the samples. The pre-concentration is performed on a sodium diethyldithiocarbamate supported soft polyurethane foam. The sorbed elements are subsequently eluted with 1-propanol and the alcohol eluates are analysed by ICP-AES. A eight-fold concentration is achieved. An increased sensitivity in the analysis of propanol-water (30:70, v/v) solution is established as compared with aqueous solutions. The strongest effect is observed for As, Se, Pb, Cr, Sn, and Cd-increasing is more than twice. For other elements the matrix influence is by a factor of 1.45 (Cu), 1.36 (Sb), 2.08 (Zn). The method is applied to the analysis of natural water samples.