Microsampling Flame Atomic Absorption Spectrometry Research Articles

Mineral Contents of Some Canned, Jarred, and Packaged Foods Commonly Consumed in South-East Nigeria

Background: Minerals are required for human body to function properly. The purpose of this study was to find if these canned, jarred, and packaged foods are a good source of sodium (Na), calcium (Ca), magnesium (Mg), and potassium (K) in Nigerian diets.
 Methods: Totally, 188 samples of canned, packaged, and jarred foods were collected from malls, local markets, and street vendors; and then they were divided into eight food groups. After microwave-assisted digestion; the Na, K, Ca, and Mg contents of canned, packaged, and jarred foods commonly consumed in South-East Nigeria were determined using micro-sampling Flame Atomic Absorption Spectrometry. The results obtained were subjected to ordinary statistical analysis and presented as box plots. All statistical analysis was done using the Statistical Package for Social Sciences, version 20 software.
 Results: Ca was the most abundant micronutrient in all samples, reaching levels above 0.7 g/100 g in the milk and milk products group, followed by Mg at levels above 0.2 g/100 g. Na was the scarcest micronutrient with levels ranging from 0.040 to 0.065 g/100 g across food groups. K level also varied reaching levels above 0.070 g/100 g in the beverages (alcoholic and non-alcoholic). The mean concentration (g/100 g) of Ca (0.782±0.313) and Mg (0.202±0.044) in milk and milk products were within the national reference standard by National Agency for Food and Drug Administration and Control for such micronutrients. However, the Na, K, Ca, and Mg concentrations of the various food groups were all lower than the international standards.
 Conclusion: It is concluded that some efforts are necessary to improve the nutritional quality of canned, jarred, and packaged foods distributed in Nigeria.

Risk assessment of trace metals in sunblock creams using DABCOnium-based ionic liquid-functionalized magnetic nanoparticles.

In the present study, we synthesized DABCOnium-based-Brønsted acidic ionic liquid-functionalized magnetic nanoparticles (Fe3O4@(SU-DBC) NPs). Their structure was characterized using various morphological and physicochemical techniques such as SEM, powder-XRD, XPS, FTIR, VSM, and BET. The Fe3O4@(SU-DBC) NPs have remarkable magnetic recovery, extensive colloidal stability, and excellent recyclability. The fabricated ionic liquid-modified magnetic NPs show capability for magnetic dispersive micro-solid-phase extraction (MD-μ-SPE) of trace metals (Cd, Cr, Ni, and Pb) from sunblock cream samples. Micro-sampling flame atomic absorption spectrometry (MS-FAAS) was used to analyze the analytes. For evaluating the simultaneous effects of various parameters on the extraction efficiency, a central composite design was employed. In the method validation, the recoveries ranged from 97.84 to 102.36%, with relative standard deviations of 0.97% to 3.27%. The detection limits of the proposed method ranged between 0.067 and 0.715 μg kg-1. The developed method showed high sensitivity and precision, and stable recovery. Health risks were evaluated using the margin of safety (MoS), hazard quotient (HQ), hazard index (HI), and lifetime cancer risk (LCR). Sunblock creams' MoS, HQ, and HI values were within permissible ranges, but LCR values were higher than permitted.

An effervescence-assisted switchable hydrophobicity solvent microextraction before microsampling flame atomic absorption spectrometry for copper ions in vegetables

ABSTRACT A novel method of an effervescence-assisted switchable hydrophilicity-solvent based liquid-phase microextraction (EA-SHS-LPME) was developed to preconcentration of copper traces in organic and conventional growth vegetable samples prior to determination by flame atomic absorption spectrometry with micro-sampling (µS-FAAS) system. The methodology is based on the change of decanoic acid’s hydrophobicity with changing pH. Decanoic acid is solid under room conditions, so phase separation is effortless and highly efficient. In the proposed study, 8-Hydroxyquinoline (8-HQ) was used as a chelating agent, Na2CO3 was used as a CO2 source, and H2SO4 was used as a proton donor. Several factors affecting the microextraction efficiency, such as pH, 8-HQ amount, decanoic acid amount, Na2CO3 and H2SO4 amount and concentration, and extraction temperature, were investigated and optimised. Optimal conditions were determined as pH:6, 100 μL of 0.1% (w/v) 8-HQ, 75 μL decanoic acid, each 500 μL of 0.1 M Na2CO3 and H2SO4, and 40°C. Under the optimal conditions, the enhancement factor (EF) was 52, the limit of detection (LOD) was 0.75 µg L−1, the limit of quantification (LOQ) was 2.52 µg L−1, and relative standard deviation for 25 μg L−1 (n = 10) was 0.94, respectively. The matrix effect was investigated by comparing the solvent-based calibration curve with the matrix-matched calibration curve. The developed method was successfully applied to the extraction and determination of copper in certified reference material and organic and conventional grown vegetable samples. The results were compared with a statistical evaluation method, which is ANOVA. The development EA-SHS-LPME method is a sensitive, repeatable, economical, environmentally friendly, fast, and easy-to-apply to enhance and determine Cu(II) contents in vegetable samples. The LOD values obtained with the proposed method make FAAS compatible with the instruments, which are relatively more expensive and more difficult to use.

Determination of prohibited lead and cadmium traces in hair dyes and henna samples using ultrasound assisted-deep eutectic solvent-based liquid phase microextraction followed by microsampling-flame atomic absorption spectrometry.

In this study ultrasound assisted-deep eutectic solvent-based liquid phase microextraction followed by microsampling-flame atomic absorption spectrometry was developed to determine prohibited lead and cadmium traces in hair dye and henna samples. For this purpose, deep eutectic solvent, prepared from choline chloride and phenol, was used as an extraction solvent, dithizone was used as a complexing agent, and THF was used as an aprotic solvent. All parameters that affect extraction efficiency, such as pH, the DES volume and composition, the extraction time, the amount of dithizone, were optimized. Under the optimal conditions, for Pb(ii) and Cd(ii), enhancement factors of 92 and 57, LODs of 2.5 μg L-1 and 0.75 μg L-1, LOQs of 7.8 μg L-1 and 2.5 μg L-1, linear working ranges of 10-250 μg L-1 and 2.5-50 μg L-1, were obtained, respectively. Relative standard deviation (n = 10) was calculated to be 2.7 for 100 μg L-1 of Pb(ii) and 2.1 for 25 μg L-1 of Cd(ii). The matrix effect was investigated by comparing the solvent-based calibration curve with the matrix-matched calibration curve. The determination of lead and cadmium in hair dye and henna samples without being affected by the sample matrix was successfully performed. The lead content was between 1.3 and 6.5 μg g-1, and the cadmium content was between 0.028 and 0.54 μg g-1 for the selected hair dye and henna samples.

Magnetic dispersive micro-solid phase extraction merged with micro-sampling flame atomic absorption spectrometry using (Zn-Al LDH)-(PTh/DBSNa)-Fe3O4 nanosorbent for effective trace determination of nickel(II) and cadmium(II) in food samples

In this work, Zn-Al layered double hydroxide (Zn-Al LDH) combined with polythiophene (PTH)/sodium dodecyl benzene sulfonate (DBSNa) and Fe3O4 was synthesized and used in a centrifugeless ultrasound-assisted dispersive micro solid phase extraction (UA-D-μSPE) technique for pre-concentration and separation of Ni2+ and Cd2+ ions. The final analysis of the analytes was carried out by the micro sampling flame atomic absorption spectrometry (MS-FAAS). The influences of several effective analytical parameters, including pH value, amount of nanosorbent, sonication time, and eluent volume, on the extraction efficiency were studied by experimental design method. In the optimal conditions, the linearity ranges of 5–300 and 2.5–100 ng mL−1 for the Ni2+and Cd2+ ions were attained, respectively. The limits of detection (LODs) of the method were 1.3 and 0.7 ng mL−1 for the Ni2+ and Cd2+ ions, respectively. The proposed method demonstrated to be suitable for concurrent extraction of studied metals in various vegetable and meat samples with high accuracy and enrichment factors.

Needle hub in-syringe solid phase extraction based a novel functionalized biopolyamide for simultaneous green separation/preconcentration and determination of cobalt, nickel, and chromium (III) in food and environmental samples with micro sampling flame atomic absorption spectrometry

In this research study, facile needle hub in-syringe solid phase extraction coupled with micro-sampling flame atomic absorption spectrometry (NH-IS-SPE -µS-FAAS) was used for simultaneous separation/preconcentration and determination of cobalt, nickel, and chromium (III). The functionalized curdled milk N-acetic acid as a novel functionalized biopolyamide was proposed into syringe needle hub and the extraction process was carried out by pushing and pulling the syringe plunger to treat the biosorbent with the analytes. The influential parameters of pH, amount of functionalized curdled milk N-acetic acid, pushing and pulling cycles, elution solvent, sample volume, and ionic strength were investigated. Under optimum conditions, the limits of detection (LODs) (n = 10) of 2.95, 2.74, and 3.10 µg L−1, the linear rage of 10–200 µg L−1, 10–250, 10–200 µg L−1, and the enhancement factors of 206, 205, 207 were obtained for cobalt, nickel, and chromium (III) respectively. The relative standard deviations (RSD% (n = 7)) of 3.5 for cobalt and nickel and 3.2 for chromium were obtained. The sensitive and high potential NH-IS-SPE-μS-FAAS was successfully applied in food and environmental samples analysis.

Novel magnetic nanoparticles as adsorbent in ultrasound-assisted micro-solid-phase extraction for rapid pre-concentration of some trace heavy metal ions in environmental water samples: desirability function

In this research, a simple, rapid, and ecofriendly magnetic nanoparticles (MNPs)-based ultrasound-assisted micro-solid-phase extraction (US-µ-SPE) method has been proposed for simultaneous determination of five heavy metal ions [Cu(Π), Ni(Π), Pb(Π), Co(Π), Cd(Π)] in environmental water samples. Here, CoO–Fe2O3@SiO2@TiO2 NPs were synthesized as an efficient adsorbent with excellent adsorption capacity, high dispersion ability and super-magnetic property for the adsorption of the ions. The parameters influencing the extraction efficiency were evaluated and optimized by statistical experimental design. The pH of sample solution, sonication time, dosages of adsorbent, and the volume, concentration and type of elute are 6.50, 14.2 min, 5.5 mg; 300 µL, 2 mol L−1 of HNO3, respectively. Moreover, the reusability for MNPs was six times. The target ions were successfully quantified using a micro-sampling flame atomic absorption spectrometry (MS-FAAS) technique in five different real water samples: tap water, lake water, river water, well water and distilled water. For all studied metal ions, the proposed method provided high recoveries (94.6–103%), high R2 (0.9993–0.9998), low LODs (0.03–0.18 µg L−1) and repeatability (RSD% = 1.49–3.59).

Solid-phase extraction of copper as 1-(2-pyridylazo)-2-naphthol (PAN) chelates on Coprinus atramentaria

ABSTRACT A solid-phase extraction method for separation-preconcentration of copper as 1-(2-pyridylazo)-2-naphthol (PAN) on Coprinus atramentaria has been presented. The copper concentration was determined by micro-sampling flame atomic absorption spectrometry (FAAS). It could be discussed that the developed method has low LOD in addition to high preconcentration factor. By considering the over advantages of recommended method, we can suggest the use of its to preconcentration of Cu(II) by routinly. The various analytical factors affecting the efficiency of extraction were optimised. Copper(II)-PAN chelate was quantitatively recovered at pH 6.0 on Coprinus atramentaria and eluted with 2 M HNO3. The limit of detection, limit of quantification, inter-day and intra-day precision values of copper were 0.7 μg L−1, 2.33 µg L−1, 5.31% and 5.15%, respectively. The pre-concentration factor was 70. The method was confirmed by applying the method to certified reference materials and addition–recovery tests prior to application to the determination of copper ions in various samples recovery results. The solid-phase extraction with FAAS is a successful method for the determination of copper in different samples.

Tandem dispersive liquid–liquid microextraction coupled with micro-sampling flame atomic absorption spectrometry for rapid determination of lead(II) and cadmium(II) ions in environmental water samples

ABSTRACT Tandem dispersive liquid liquid microextraction coupled with micro - sampling flame atomic absorption spectrometry for rapid determination of lead2 and cadmium2 ions in environmental water samples. A simple method termed as tandem dispersive liquid–liquid microextraction coupled with micro-sampling flame atomic absorption spectrometry is used for determination of the lead(II) and cadmium(II) ions in different environmental water samples. According to the proposed method, the target analytes are extracted from an aqueous sample solution (10 mL) into a micro-volume of an organic solvent, and then they are selectively back-extracted into an aqueous acceptor solution (150 μL) to increase the compatibility of the extractant phase with a final analyser system and provide a suitable enrichment factor. The developed method is very fast, implemented in just about 7 min, and provides a high sample clean-up. The factors influencing the extraction efficiency including the type and volume of the organic solvent, pH and volume of the acceptor solution, and number of extractions are thoroughly examined and optimised. Under the optimal experimental conditions, the developed method provides a good linearity (in the range of 0.4–300 ng mL−1 (R2 ≥ 0.994)), and low limits of detection (in the range of 0.07–0.31 ng mL−1). Finally, the method is successfully applied for the direct determination of the understudied analytes in the river, dam, and well water samples.

A new amine based microextraction of lead (II) in real water samples using flame atomic absorption spectrometry

A new amine based microextraction procedure has been established for the separation and preconcentration of lead (II). N,N-dimethyl-n-octylamine was used as extraction solvent and chelating agent for lead (II). Microsampling flame atomic absorption spectrometry was used for the determination of lead. The analytical parameters including pH, amounts of N,N-dimethyl-n-octylamine, sample volume etc. were optimized. The influences of the matrix components were also investigated. The validation of the procedure was done by the analysis of TMDA 64.2 certified reference material. The Limit of detection (LOD) of developed method was 2.6 μg L−1. The developed microextraction procedure was successfully applied to natural water samples.

A magnetic graphitic carbon nitride as a new adsorbent for simple separation of Ni (II) ion from foodstuff by ultrasound-assisted magnetic dispersive micro solid-phase extraction method

In this research, a magnetic graphitic carbon nitride (g-C3N4-SnFe2O4) was successfully synthesized and utilized as an efficient adsorbent for nickel (Ni2+) separation/extraction from vegetable samples by ultrasound-assisted magnetic dispersive micro solid-phase extraction (UA-M-D-μSPE). After separation and preconcentration step, Ni ions were determined via micro-sampling flame atomic absorption spectrometry (MS-FAAS). A successful synthesis of g-C3N4-SnFe2O4 was investigated by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and vibrating sample magnetometer (VSM). The optimization of adsorption and desorption steps was effectively studied by the on-at-a time method. In addition, under the optimum experimental conditions, the limits of detection (LODs), the linear ranges (LR) and relative standard deviations (RSDs%, for n = 5) were obtained 1.0 μg.L−1, 4.0 ─ 500.0 μg.L−1, and 1.4 respectively.

Statistical evaluation of three kinds of sonochemically-prepared magnetic conductive polymer nanocomposites for ultrasound-assisted ligandless uptake of some deleterious metal ions in vegetable samples

In this work, three effectual nanoadsorbents in terms of both the synergically conductive and magnetic properties, named as PTh/surfactant/Fe3O4, PANI/surfactant/Fe3O4, and Ppy/surfactant/Fe3O4, were compared for the simultaneous ligandless preconcentration of Pb2+ and Ni2+ ions from some vegetable samples. The enrichment/detection process was performed by an ultrasound-ameliorated extraction method coupled with micro-sampling flame atomic absorption spectrometry, benefited by the ease of use, high speed, bio-compatibility, low cost, and high preconcentration. Also a simple and effective synthesis of magnetic nanocomposites by the co-precipitation method followed by an efficient in situ sonochemical oxidative polymerization embossed the current procedure. Under optimized conditions obtained by multivariate approaches, pH 5.5, 5 min ultrasonic time, 20 mg of the best nanoadsorbent (PTh/surfactant/Fe3O4), and 250 μL of the eluent resulted in high extraction percentages (85–95%). The calibration curves obtained showed the good liner ranges of 4–400 ng mL−1 and 4–650 ng mL−1 for Ni2+ and Pb2+ ions, respectively, accompanied with low detection limits ranging from 1 to 1.2 ng mL−1. The preconcentration factor was found to be 40. No significant interference of serious foreign agents in the trace analysis of the aforementioned metal ions indicated a good selectivity of the ligandless ultrasound-promoted microextraction procedure, which can be considerably owned by the versatile magnetic nanosorbent. On the other hand, the superb figures of merits demonstrated a great effectiveness of the ultrasonic irradiation in the impressive synthesis of the nanosorbent as well as the facile and efficient microextraction of trace amounts of Ni2+ and Pb2+ ions in complicated plant matrices.

Ultrasound-assisted Supramolecular Microextraction of Copper in Water, Food, Hair, and Tobacco Samples Prior to Microsampling Flame Atomic Absorption Spectrometry

Ultrasound-assisted Supramolecular Microextraction of Copper in Water, Food, Hair, and Tobacco Samples Prior to Microsampling Flame Atomic Absorption Spectrometry

Dispersive Liquid-Liquid Microextraction of Lead(II) as Tropaeolin OOO Chelates From Environmental Samples Prior to Microsampling Flame Atomic Absorption Spectrometry

Dispersive Liquid-Liquid Microextraction of Lead(II) as Tropaeolin OOO Chelates From Environmental Samples Prior to Microsampling Flame Atomic Absorption Spectrometry

Magnetic nanocomposite of chitosan-Schiff base grafted graphene oxide for lead analysis in whole blood

A new Schiff base grafted graphene oxide-magnetic chitosan was utilized as a novel sorbent for extraction and quantification of lead ion in blood samples via dispersive magnetic solid phase extraction. The prepared nanocomposite sorbent was characterized by SEM, TEM, FT-IR, XRD, VSM and EDX and the quantification analysis was performed by microsampling flame atomic absorption spectrometry. The important parameters on the extraction efficiency were thoroughly optimized by means of experimental design. Under the optimized conditions, an aliquot of 50 mL of sample (pH 6.3) was extracted utilizing 60 mg of magnetic nanoparticles during 30 min. The sorbent was afterward desorbed using 1.0 mL of 0.8 mol L−1 HNO3 under fierce vortex for 6 min. A preconcentration factor of 20 and an absolute recovery of 40% were provided by the proposed method. The limits of detection (3S/N) and quantification (10 S/N) were 0.06 ng mL−1 and 2.0 ng mL−1, respectively. An excellent linearity was achieved within the range of (10–800) ng mL−1 and the regression coefficient was 0.9903. The intra- and inter-day RSDs% were found to be 1.8% and 7.0%, respectively. Furthermore, the method was applied for analysis of blood samples and good accuracies within the range of 97%–108% were obtained.

Development of effervescence-assisted liquid phase microextraction based on fatty acid for determination of silver and cobalt ions using micro-sampling flame atomic absorption spectrometry

In this work, coupling of effervescence-assisted dispersive liquid-liquid microextraction based on a medium-chain fatty acid (EA-DLLME-FA) with accurate micro-sampling flame atomic absorption spectrometric (MS-FAAS) analysis was developed for the first time as a swift, facile, and effective analytical procedure for the satisfactory analysis of Ag+ and Co2+ ions from different real samples. During the microextraction process, 1-phenyl-1,2-propanedione-2-oximethiosemicarbazone (PPDOT) was used as a proper chelating agent to form hydrophobic complex compounds, causing easy and effective penetration of metal ions into the organic extraction phase. On the other hand, by providing proper pH values via addition of an efficacious effervescent agent, the extraction medium could acquire a dual hydrophilicity/hydrophobicity nature, leading to a simple and efficient proceeding of the foresaid microextraction, followed by a quick and no labor intensive collection of the organic solvent (upper phase) in a special home-made extraction cell. Also simple, rapid, accurate, efficient, and inexpensive analyses via the MS-FAAS technique distinguished the determination of the foresaid metal ions. Under other optimum conditions enriched via the central composite design, the studied ions could be accurately determined in the wide linear dynamic ranges of 5.0–150ngmL−1 and 8.0–200ngmL−1 for the Ag+ and Co2+ ions, respectively, with the determination coefficients (R2s) higher than 0.98. The low limits of detection and the good percentage relative standard deviations were found to be in the span of 2.0–3.0ngmL−1 and 3.4–4.2%, respectively. The percentage extraction recoveries and the enrichment factors were found to be reasonably quantitative. These valuable results indicated the high quality of the developed procedure for the influential quantification of the aforementioned metal ions from cow milk, vitamin B12, orange juice, and tap water.

Dissolved carbon dioxide flotation-assisted in-syringe dispersive liquid–liquid microextraction coupled with microsampling flame atomic absorption spectrometry for selective determination of palladium in water samples

Dissolved carbon dioxide flotation-assisted in-syringe dispersive liquid–liquid microextraction (DCF-IS-DLLME) followed by microsampling flame atomic absorption spectrometry was developed as a simple, inexpensive and fast method for extraction and determination of Pd(II). In the proposed approach, N,N′-bis (naphthylideneimino) diethylenetriamine (NAPdien) was utilized as a selective complexing reagent for Pd(II) ion. Several influential factors on the extraction efficiency including types and volumes of extraction and disperser solvents, pH of the sample solution, concentration of NAPdien and interfering ions were studied. By applying the optimal conditions, a preconcentration factor of 28.7 and limit of detection of 2.5 ng mL−1 were provided by the proposed method. Linearity was in the range of 10–400 ng mL−1 with a correlation coefficient (R 2) of 0.9968. Intra-day RSD% values for five repetitive measurements of the spiked solutions at the concentrations of 20 and 100 ng mL−1 were 5.2 and 2.4%, respectively, whereas it was obtained within the range of 3.6–18.6% for the real samples. Inter-day RSD% values of the spiked solutions were found to be 9.6 and 8.7%, respectively. The results demonstrated that except for Fe2+ and Fe3+, no remarkable interfering effect was created by the other studied ions for determination of Pd(II) so that the tolerance limits (W Ion/W Pd(II)) of the major cations and anions were in the range of 1000–10,000. Finally, DCF-IS-DLLME was successfully applied for determination of Pd(II) in different water samples and the obtained relative recoveries in the range of 94.5–105% illustrated favorable accuracies for the proposed method.

Dissolvable layered double hydroxide as an efficient nanosorbent for centrifugeless air-agitated dispersive solid-phase extraction of potentially toxic metal ions from bio-fluid samples

In the present work, a novel nanosorbent namely layered double hydroxides with 4-amino-5-hydroxyl-2,7-naphthalendisulfonic acid monosodium salt interlayer anion (Mg-Al-AHNDA-LDH) was synthesized and applied as a dissolvable nanosorbent in a centrifugeless ultrasound-enhanced air-agitated dispersive solid-phase extraction (USE-AA-D-SPE) method. This method was used for the separation and preconcentration of some metal ions including Cd2+, Cr6+, Pb2+, Co2+, and Ni2+ prior to their determination using the micro-sampling flame atomic absorption spectrometry (MS-FAAS) technique. The most interesting aspect of this nanosorbent is its immediate dissolvability at pH values lower than 4. This capability drastically eliminates the elution step, leading to a great improvement in the extraction efficiency and a decrease in the extraction time. Also in this method, the use of a syringe nanofilter eliminates the need for the centrifugation step, which is time-consuming and essentially causes the analysis to be off-line. Several effective parameters governing the extraction efficiency including the sample solution pH, amount of nanosorbent, eluent condition, number of air-agitation cycles, and sonication time were investigated and optimized. Under the optimized conditions, the good linear dynamic ranges of 2–70, 6–360, 7–725, 7–370, and 8–450 ng mL−1 for the Cd2+, Cr6+, Pb2+, Co2+and Ni2+ ions, respectively, with the correlation of determinations (R2s) higher than 0.997 were obtained. The limits of detection (LODs) were found to be 0.6, 1.7, 2.0, 2.1, and 2.4 for the Cd2+, Cr6+, Pb2+, Co2+, and Ni2+ ions, respectively. The intra-day and inter-day precisions (percent relative standard deviations (%RSDs) (n = 5)) were below 7.8%. The proposed method was also successfully applied for the extraction and determination of the target ions in different biological fluid and tap water samples.

Supramolecular Solvent-based Microextraction of Copper at Trace Levels Before Determination by Microsampling Flame Atomic Absorption Spectrometry

Supramolecular Solvent-based Microextraction of Copper at Trace Levels Before Determination by Microsampling Flame Atomic Absorption Spectrometry

A green, novel and simple microprecipitation technique for separation and preconcentration of cadmium with 1-(2-thiazolylazo)-2-naphthol in food samples and determination by microsampling flame atomic absorption spectrometry

A microprecipitation is developed by using Cd(ii)–1-(2-thiazolylazo)-2-naphthol (TAN) microprecipitate for the preconcentration of cadmium at trace levels.