Continuum Source Flame Atomic Absorption Research Articles

Determination of Silicon in Vegetable Oil and Biodiesel by High-Resolution Continuum Source Flame Atomic Absorption Spectrometry Using Sample Dilution with Xylene

In this paper, a simple procedure for the determination of Si in vegetable oils and biodiesel of different origins by high-resolution continuum source flame atomic absorption spectrometry is proposed. Samples were diluted with xylene, and external calibration was performed with calibration solutions prepared with xylene and mineral base oil used for viscosity matching. The accuracy was assessed using recovery tests and comparing the results for a set of vegetable oils and biodiesel to those determined by inductively coupled plasma–optical emission spectroscopy (ICP–OES). Recoveries at 3.0 and 5.0 mg L–1 levels using organic Si standard-spiked solutions ranged from 93 to 113%. The limit of detection at the optimized conditions in the original sample was approximately 0.60 mg kg–1, which is adequate for the analysis of vegetable oils and biodiesel according to the Brazilian legislation.

Fast sequential determination of antimony and lead in pewter alloys using high-resolution continuum source flame atomic absorption spectrometry

A simple method has been developed to determine antimony and lead in pewter alloy cups produced in Brazil, using fast sequential determination by high-resolution continuum source flame atomic absorption spectrometry. The samples were dissolved in HCl and H2O2, employing a cold finger system in order to avoid analyte losses. The main resonance line of lead at 217.001 nm and a secondary line of antimony at 212.739 nm were used. The limits of detection for lead and antimony were 0.02 and 5.7 mg L−1, respectively. The trueness of the method was established by recovery tests and comparing the results obtained by the proposed method with those obtained by inductively coupled plasma optical emission spectrometry. The results were compared using a student's t-test and there was no significant difference at a 95% confidence interval. With the developed methods, it was possible to determine accurately antimony and lead in pewter samples. The lead concentration found in the analysed samples was around 1 mg g−1, which means that they are not lead free; however, the content was below the maximum allowed level of 5 mg g−1. The antimony content, which was found to be between 40 and 46 mg g−1, is actually of greater concern, as antimony is known to be potentially toxic already at very low concentrations, although there is no legislation yet for this element.

Solid-phase extraction and separation procedure for trace aluminum in water samples and its determination by high-resolution continuum source flame atomic absorption spectrometry (HR-CS FAAS)

In the present study, a separation/preconcentration procedure for determination of aluminum in water samples has been developed by using a new atomic absorption spectrometer concept with a high-intensity xenon short-arc lamp as continuum radiation source, a high-resolution double-echelle monochromator, and a charge-coupled device array detector. Sample solution pH, sample volume, flow rate of sample solution, volume, and concentration of eluent for solid-phase extraction of Al chelates with 4-[(dicyanomethyl)diazenyl] benzoic acid on polymeric resin (Duolite XAD-761) have been investigated. The adsorbed aluminum on resin was eluted with 5mL of 2molL(-1) HNO(3) and its concentration was determined by high-resolution continuum source flame atomic absorption spectrometry (HR-CS FAAS). Under the optimal conditions, limit of detection obtained with HR-CS FAAS and Line Source FAAS (LS-FAAS) were 0.49μgL(-1) and 3.91μgL(-1), respectively. The accuracy of the procedure was confirmed by analyzing certified materials (NIST SRM 1643e, Trace elements in water) and spiked real samples. The developed procedure was successfully applied to water samples.

Method development for the determination of fluorine in toothpaste via molecular absorption of aluminum mono fluoride using a high-resolution continuum source nitrous oxide/acetylene flame atomic absorption spectrophotometer

Fluorine was determined via the rotational molecular absorption line of aluminum mono fluoride (AlF) generated in C2H2/N2O flame at 227.4613nm using a high-resolution continuum source flame atomic absorption spectrophotometer (HR-CS-FAAS). The effects of AlF wavelength, burner height, fuel rate (C2H2/N2O) and amount of Al on the accuracy, precision and sensitivity were investigated and optimized. The Al–F absorption band at 227.4613nm was found to be the most suitable analytical line with respect to sensitivity and spectral interferences. Maximum sensitivity and a good linearity were obtained in acetylene-nitrous oxide flame at a flow rate of 210Lh−1 and a burner height of 8mm using 3000mgL−1 of Al for 10–1000mgL−1of F. The accuracy and precision of the method were tested by analyzing spiked samples and waste water certified reference material. The results were in good agreement with the certified and spiked amounts as well as the precision of several days during this study was satisfactory (RSD<10%). The limit of detection and characteristic concentration of the method were 5.5mgL−1 and 72.8mgL−1, respectively. Finally, the fluorine concentrations in several toothpaste samples were determined. The results found and given by the producers were not significantly different. The method was simple, fast, accurate and sensitive.

Evaluation of lines of boron, phosphorus and sulfur by high-resolution continuum source flame atomic absorption spectrometry for plant analysis

The wavelength-integrated absorbance (WIA) and summation of absorbance (∑ lines) of different lines were evaluated to enhance sensitivity and determine B, P and S in medicinal plants by HR-CS FAAS. The lowest LOD for B (0.5mgL−1) and P (13.7mgL−1) was obtained by integration of lines 249.773nm (3pixels) and 247.620nm (5pixels), respectively. The ∑ lines for CS at 257.595nm and 257.958nm furnished LOD=30.5mgL−1, ca. 10% lower than the LOD obtained for the WIA using 257.595nm and 5pixels. Data showed the advantage of WIA over ∑ lines to improve sensitivity for all analytes. Under optimized conditions, calibration curves in the 1.0–100mgL−1 B and 50.0–2000mgL−1 P, S ranges were consistently obtained. Results obtained with the HR-CS FAAS method were in agreement at 98% and 95% confidence level with certified values for B and P, respectively. And results for S were in accordance to non-certified values. Concentrations of B, P, and S in 12 medicinal plants analyzed by the proposed method varied within the 19.4–34.5mgkg−1 B, 719–3910mgkg−1 P and 1469–7653mgkg−1 S ranges.

Internal standardization combined with dilute-and-shoot preparation of distilled alcoholic beverages for Cu determination by high-resolution continuum source flame atomic absorption spectrometry

Internal standardization (IS) and dilute-and-shoot preparation of distilled alcoholic beverages were evaluated for the direct determination of Cu by high-resolution continuum source flame atomic absorption spectrometry. Among Bi, In, Sn and Te evaluated as internal standards, Bi and Te furnished best results. Calibration curves were built up by plotting ACu/ABi and ACu/ATeversus Cu concentration (0.05–4.0mgL−1) and linear correlation coefficients were 0.9994 and 0.9990, respectively. Contents of Cu in 22 commercial distilled beverages analyzed by the proposed method varied in the 0.029–3.608mgL−1 interval. These results were in agreement (paired t-test) at 95% confidence level with those obtained by line source FAAS using standard addition calibration. Recoveries improved from 77–83% (without IS) to 99–101% (with IS) intervals. The relative standard deviation (n=12) was 0.8–3.7% (IS-Bi), 0.4–4.4% (IS-Te), and 0.5–9.1% (without IS) and the limit of detection was ca. 5μgL−1 using Bi or Te.

A practical method for the determination of sulphur in coal samples by high-resolution continuum source flame atomic absorption spectrometry

Sulphur in coal was determined using a high-resolution continuum source flame atomic absorption spectrophotometer (HR-CS-FAAS) with actylene/air flame. The C–S absorption band at 258.056nm was found the most suitable analytical line with respect to sensitivity and spectral interferences. The instrumental parameters were optimized. The coal samples were dried and dissolved using microwave-assisted digestion technique. The validity of the method was tested using standard reference material and certified values were found in the limits of 95% confidence level. Since the concentrations of matrix elements of coal other than carbon are low enough not to cause any spectral interferences, the linear calibration method was applied in all quantifications without any problem. The calibration standards were prepared in sulphuric acid. The method was accurate, fast, simple and sensitive. The limit of detection (LOD, 3δ, N=10) and the limit of quantification (LOQ, 10δ, N=10) were found to be 0.01 and 0.03% (w/w), respectively. The sulphur concentrations of various kinds of the coal samples received around Turkey were determined. The sulphur contents of the coal samples were ranged from ≤LOQ to 1.2%.

Development of an Analytical Method Based in the Slurry Sampling for Iron Determination in Fortified Milk Powder by HR-CS FAAS

The use of slurry sampling as the procedure for sample preparation provides simplicity, speed, and low consumption of reagents in analytical methods. In this paper, a method based on slurry sampling for the determination of iron in samples of fortified milk powder by high-resolution continuum source flame atomic absorption spectrometry was developed. Multivariate design techniques were applied for the optimization of experimental conditions of the method for a sample mass of 100 mg, final volume of slurry of 10 mL, and using the absorbance signal as response. Initially, a two-level full factorial design was used for the preliminary evaluation of the variables involved in slurry preparation: concentration hydrochloric acid, Triton X-100 concentration, and sonication time. Then, the Doehlert matrix was applied for the determination of the critical conditions: 2.5 mol L−1 hydrochloric acid and sonication time of 20 min. External calibration technique with aqueous standard was used for the quantification of iron. This way, the method allowed iron determination with limits of detection and quantification of 0.9 and 3.0 μg g−1, respectively. The precision expressed as the relative standard deviation was evaluated under repeatability and reproducibility conditions, being 3.2% and 4.0%, respectively. Addition/recovery test was used for assessing the accuracy of the method, and the recovery values achieved were in the range of 90–110%. The method was applied for iron determination in eight samples of fortified milk powder, and the obtained concentrations varied from 95.4 to 295.6 μg g−1. The results were compared with those obtained after acid digestion, and no significant difference was observed applying t test at the 95% confidence level.

Intra- and interspecific mineral composition variability of commercial instant coffees and coffee substitutes: Contribution to mineral intake

The present paper reports the amount and estimated daily mineral intake of nine elements (Ca, Mg, K, Na, P, Fe, Mn, Cr and Ni) in commercial instant coffees and coffee substitutes (n=49). Elements were quantified by high-resolution continuum source flame (HR-CS-FAAS) and graphite furnace (HR-CS-GFAAS) atomic absorption spectrometry, while phosphorous was evaluated by a standard vanadomolybdophosphoric acid colorimetric method.Instant coffees and coffee substitutes are rich in K, Mg and P (>100mg/100g dw), contain Na, Ca and Fe in moderate amounts (>1mg/100g), and trace levels of Cr and Ni. Among the samples analysed, plain instant coffees are richer in minerals (p<0.001), except for Na and Cr. Blends of coffee substitutes (barley, malt, chicory and rye) with coffee (20–66%) present intermediate amounts, while lower quantities are found in substitutes without coffee, particularly in barley.From a nutritional point of view the results indicate that the mean ingestion of two instant beverages per day (total of 4g instant powder), either with or without coffee, cannot be regarded as important sources of minerals to the human diet, although providing a supplementation of some minerals, particularly Mg and Mn in instant coffees. Additionally, and for authentication purposes, the correlations observed between some elements and the coffee percentage in the blends, with particular significance for Mg amounts, provides a potential tool for the estimation of coffee in substitute blends.

Determination of Silicon in Lubricant Oil by High-Resolution Continuum Source Flame Atomic Absorption Spectrometry Using Least-Square Background Correction and Internal Standardization

The least-squares background correction (LSBC) and internal standardization procedures were combined to eliminate spectral interferences caused by the CS molecular band (251.602 nm) and transport effects for determining Si in sulfuric acid digests of lubricant oil by high-resolution continuum source flame atomic absorption spectrometry. Aluminum, Ba, Ti, V, and W were tested as internal standard (IS) candidates, and W provided the best results. For absorbance measurements of solutions containing 0.5–5.0 mg L−1 Si in the presence of 25 mg L−1 W (at the wavelength integrated absorbance equivalent to 3 pixels), the correlation coefficient for the ratio of absorbance of Si to absorbance of W vs. analyte concentration was 0.9978. Fluctuations in analytical signals due to variations in sulfuric acid concentrations or acetylene/nitrous oxide flow-rate ratios were corrected by using this calibration plot. Relative standard deviations varied from 1.9 to 7.2% and 2.1 to 5.4% (n = 12) with and without LSBC/IS, respectively. Recoveries for samples spiked with 2.0 mg L−1 Si in 5.0% (v/v) sulfuric acid were within the 72.5–82.5% and 94.0–99.0% ranges without correction and by LSBC associated with internal standardization procedure, respectively. Accuracy of the proposed method was checked for the determination of Si in commercial lubricant oils and results obtained with internal standardization were better than those without correction.

Evaluation of Lines of Phosphorus and Potassium by High-Resolution Continuum Source Flame Atomic Absorption Spectrometry for Liquid Fertilizer Analysis

Evaluation of Lines of Phosphorus and Potassium by High-Resolution Continuum Source Flame Atomic Absorption Spectrometry for Liquid Fertilizer Analysis

Slurry sampling and high-resolution continuum source flame atomic absorption spectrometry using secondary lines for the determination of Ca and Mg in dairy products

The present paper proposes a simple and fast analytical procedure for the sequential multi-element determination of Ca and Mg in dairy products employing sampling slurry and high resolution-continuum source flame atomic absorption spectrometry (HR-CS FAAS). Considering the high concentration of these species in these matrices, the analytical measurements were carried out at the secondary lines of 239.856 and 202.852 for Ca and Mg, respectively. The experimental conditions established for the preparation of the slurries during the optimization step were: 2.0 mol L − 1 hydrochloric acid, sonication time of 20 min and sample mass of 1.0 g for a slurry volume of 25 mL. Experiments demonstrated that the analytical curves can be established using the external calibration technique employing aqueous standards. The method allows the determination of Ca and Mg with limits of quantification of 0.038 and 0.016 mg g − 1 , respectively. The precision was evaluated under reproducibility and repeatability conditions and expressed as relative standard deviation. The results varied from 2.7 to 2.9% (all tests with n = 10) and using a yogurt sample containing Ca and Mg concentrations of 1.40 and 0.13 mg g − 1 , respectively. The accuracy was confirmed by the analysis of a certified reference material of non-fat milk powder furnished by the National Institute of Standard and Technology. The proposed method was applied for the determination of Ca and Mg in yogurt, cow milk and milk powder samples. The samples were also analyzed after complete acid digestion and Ca and Mg determination by HR-CS FAAS. No statistical difference was observed between the results obtained by both of the procedures performed.

Multi-element determination of Cu, Fe, Ni and Zn content in vegetable oils samples by high-resolution continuum source atomic absorption spectrometry and microemulsion sample preparation

The aim of this work was to evaluate the microemulsification as sample preparation procedure for determination of Cu, Fe, Ni and Zn in vegetable oils samples by High-Resolution Continuum Source Flame Atomic Absorption Spectrometry (HR-CS FAAS). Microemulsions were prepared by mixing samples with propan-1-ol and aqueous acid solution, which allowed the use of inorganic aqueous standards for the calibration. To a sample mass of 0.5g, 100μL of hydrochloric acid and propan-1-ol were added and the resulting mixture diluted to a final volume of 10mL. The sample was manually shaken resulting in a visually homogeneous system. The main lines were selected for all studied metals and the detection limits (3σ, n=10) were 0.12, 0.62, 0.58 and 0.12mgkg−1 for Cu, Fe, Ni and Zn, respectively. The relative standard deviation (RSD) ranged from 5% to 11 % in samples spiked with 0.25 and 1.5μgmL−1 of each metal, respectively. Recoveries varied from 89% to 102%. The proposed method was applied to the determination of Cu, Fe, Ni and Zn in soybean, olive and sunflower oils.

Determination of copper at wide range concentrations using instrumental features of high-resolution continuum source flame atomic absorption spectrometry

This work describes a method to determine Cu at wide range concentrations in a single run without need of further dilutions employing high-resolution continuum source flame atomic absorption spectrometry. Different atomic lines for Cu at 324.754 nm, 327.396 nm, 222.570 nm, 249.215 nm and 224.426 nm were evaluated and main figures of merit established. Absorbance measurements at 324.754 nm, 249.215 nm and 224.426 nm allows the determination of Cu in the 0.07 – 5.0 mg L-1, 5.0 – 100 mg L-1 and 100 – 800 mg L-1 concentration intervals respectively with linear correlation coefficients better than 0.998. Limits of detection were 21 μg L-1, 310 μg L-1 and 1400 μg L-1 for 324.754 nm, 249.215 nm and 224.426 nm, respectively and relative standard deviations (n = 12) were £ 2.7%. The proposed method was applied to water samples spiked with Cu and the results were in agreement at a 95% of confidence level (paired t-test) with those obtained by line-source flame atomic absorption spectrometry.

Mathematical equation correction to spectral and transport interferences in high-resolution continuum source flame atomic absorption spectrometry: determination of lead in phosphoric acid

In this work, a new mathematical equation correction approach for overcoming spectral and transport interferences was proposed. The proposal was applied to eliminate spectral interference caused by PO molecules at the 217.0005 nm Pb line, and the transport interference caused by variations in phosphoric acid concentrations. Correction may be necessary at 217.0005 nm to account for the contribution of PO, since Atotal 217.0005 nm = A Pb 217.0005 nm + A PO 217.0005 nm. This may be easily done by measuring other PO wavelengths (e.g. 217.0458 nm) and calculating the relative contribution of PO absorbance (APO) to the total absorbance (Atotal) at 217.0005 nm: A Pb 217.0005 nm = Atotal 217.0005 nm - A PO 217.0005 nm = Atotal 217.0005 nm - k (A PO 217.0458 nm). The correction factor k is calculated from slopes of calibration curves built up for phosphorous (P) standard solutions measured at 217.0005 and 217.0458 nm, i.e. k = (slope217.0005 nm/slope217.0458 nm). For wavelength integrated absorbance of 3 pixels, sample aspiration rate of 5.0 ml min-1, analytical curves in the 0.1 – 1.0 mg L-1 Pb range with linearity better than 0.9990 were consistently obtained.

Determination of macro- and micronutrients in plant leaves by high-resolution continuum source flame atomic absorption spectrometry combining instrumental and sample preparation strategies

A method for determination of B, Ca, Cu, Fe, K, Mg, Mn, Mo, P, S and Zn in plant tissues by high-resolution continuum source flame atomic absorption spectrometry (HR-CS FAAS) is proposed. This method is based on special features of HR-CS-AAS, such as side pixel registration, wavelength integrated absorbance, and molecular absorption bands, for determining macro- and micronutrients in foliar analysis without requiring several different strategies for sample preparation and adjustment of the analytes concentration ranges. Plant samples were analyzed and results for certified materials were in agreement at a 95% confidence level (paired t-test) with reference values. Recoveries of analytes added to plant digests varied within the 82–112% interval. Relative standard deviations ( n = 12) were lower than or equal to 5.7% for all analytes in all concentration ranges.

Determination of copper at wide range concentrations using instrumental features of high-resolution continuum source flame atomic absorption spectrometry

This work describes a method to determine Cu at wide range concentrations in a single run without need of further dilutions employing high-resolution continuum source flame atomic absorption spectrometry. Different atomic lines for Cu at 324.754 nm, 327.396 nm, 222.570 nm, 249.215 nm and 224.426 nm were evaluated and main figures of merit established. Absorbance measurements at 324.754 nm, 249.215 nm and 224.426 nm allows the determination of Cu in the 0.07 - 5.0 mg L-1, 5.0 - 100 mg L-1 and 100 - 800 mg L-1 concentration intervals respectively with linear correlation coefficients better than 0.998. Limits of detection were 21 µg L-1, 310 µg L-1 and 1400 µg L-1 for 324.754 nm, 249.215 nm and 224.426 nm, respectively and relative standard deviations (n = 12) were £ 2.7%. The proposed method was applied to water samples spiked with Cu and the results were in agreement at a 95% of confidence level (paired t-test) with those obtained by line-source flame atomic absorption spectrometry.

Mathematical equation correction to spectral and transport interferences in high-resolution continuum source flame atomic absorption spectrometry: determination of lead in phosphoric acid

In this work, a new mathematical equation correction approach for overcoming spectral and transport interferences was proposed. The proposal was applied to eliminate spectral interference caused by PO molecules at the 217.0005 nm Pb line, and the transport interference caused by variations in phosphoric acid concentrations. Correction may be necessary at 217.0005 nm to account for the contribution of PO, since Atotal217.0005 nm = A Pb217.0005 nm + A PO217.0005 nm. This may be easily done by measuring other PO wavelengths (e.g. 217.0458 nm) and calculating the relative contribution of PO absorbance (A PO) to the total absorbance (Atotal) at 217.0005 nm: A Pb217.0005 nm = Atotal217.0005 nm - A PO217.0005 nm = Atotal217.0005 nm - k (A PO217.0458 nm). The correction factor k is calculated from slopes of calibration curves built up for phosphorous (P) standard solutions measured at 217.0005 and 217.0458 nm, i.e. k = (slope217.0005 nm/slope217.0458 nm). For wavelength integrated absorbance of 3 pixels, sample aspiration rate of 5.0 ml min-1, analytical curves in the 0.1 - 1.0 mg L-1 Pb range with linearity better than 0.9990 were consistently obtained. Calibration curves for P at 217.0005 and 217.0458 nm with linearity better than 0.998 were obtained. Relative standard deviations (RSD) of measurements (n = 12) in the range of 1.4 - 4.3% and 2.0 - 6.0% without and with mathematical equation correction approach were obtained respectively. The limit of detection calculated to analytical line at 217.0005 nm was 10 µg L-1 Pb. Recoveries for Pb spikes were in the 97.5 - 100% and 105 - 230% intervals with and without mathematical equation correction approach, respectively.

Body Composition Assessment Using Siri Underwater Equivalent Percent Fat Assessed by Norland Equipment

Bismuth was evaluated as internal standard for Pb determination by line source flame atomic absorption spectrometry (LS FAAS), high-resolution continuum source flame atomic absorption spectrometry (HR-CS FAAS) and line source graphite furnace atomic absorption spectrometry (LS GFAAS). Analysis of samples containing different matrices indicated close relationship between Pb and Bi absorbances. Correlation coefficients of calibration curves built up by plotting APb/ABi versus Pb concentration were higher than 0.9953 (FAAS) and higher than 0.9993 (GFAAS). Recoveries of Pb improved from 52–118% (without IS) to 97–109% (IS, LS FAAS); 74–231% (without IS) to 96–109% (IS, HR-CS FAAS); and 36–125% (without IS) to 96–110% (IS, LS GFAAS). The relative standard deviations (n = 12) were reduced from 0.6–9.2% (without IS) to 0.3–4.3% (IS, LS FAAS); 0.7–7.7% (without IS) to 0.1–4.0% (IS, HR-CS FAAS); and 2.1–13% (without IS) to 0.4–5.9% (IS, LS GFAAS).

Fast sequential multi-element determination of Ca, Mg, K, Cu, Fe, Mn and Zn for foliar diagnosis using high-resolution continuum source flame atomic absorption spectrometry: Feasibility of secondary lines, side pixel registration and least-squares background correction

The fast sequential multi-element determination of Ca, Mg, K, Cu, Fe, Mn and Zn in plant tissues by high-resolution continuum source flame atomic absorption spectrometry is proposed. For this, the main lines for Cu (324.754 nm), Fe (248.327 nm), Mn (279.482 nm) and Zn (213.857 nm) were selected, and the secondary lines for Ca (239.856 nm), Mg (202.582 nm) and K (404.414 nm) were evaluated. The side pixel registration approach was studied to reduce sensitivity and extend the linear working range for Mg by measuring at wings (202.576 nm; 202.577 nm; 202.578 nm; 202.580 nm; 202.585 nm; 202.586 nm; 202.587 nm; 202.588 nm) of the secondary line. The interference caused by NO bands on Zn at 213.857 nm was removed using the least-squares background correction. Using the main lines for Cu, Fe, Mn and Zn, secondary lines for Ca and K, and line wing at 202.588 nm for Mg, and 5 mL min − 1 sample flow-rate, calibration curves in the 0.1–0.5 mg L − 1 Cu, 0.5–4.0 mg L − 1 Fe, 0.5–4.0 mg L − 1 Mn, 0.2–1.0 mg L − 1 Zn, 10.0–100.0 mg L − 1 Ca, 5.0–40.0 mg L − 1 Mg and 50.0–250.0 mg L − 1 K ranges were consistently obtained. Accuracy and precision were evaluated after analysis of five plant standard reference materials. Results were in agreement at a 95% confidence level (paired t-test) with certified values. The proposed method was applied to digests of sugar-cane leaves and results were close to those obtained by line-source flame atomic absorption spectrometry. Recoveries of Ca, Mg, K, Cu, Fe, Mn and Zn in the 89–103%, 84–107%, 87–103%, 85–105%, 92–106%, 91–114%, 96–114% intervals, respectively, were obtained. The limits of detection were 0.6 mg L − 1 Ca, 0.4 mg L − 1 Mg, 0.4 mg L − 1 K, 7.7 µg L − 1 Cu, 7.7 µg L − 1 Fe, 1.5 µg L − 1 Mn and 5.9 µg L − 1 Zn.