Improvements in thermospray flame furnace atomic absorption spectrometry

In thermospray flame furnace atomic absorption spectrometry (TS-FF-AAS) a nickel atomization tube is placed in the acetylene/air flame on a holder built onto a standard AAS burner head. The liquid to be analyzed is transported by a low or high-pressure pump through a very hot, simple, inexpensive ceramic capillary tip acting as a flame-heated thermospray into the flame furnace. This results in complete sample introduction and increases the residence time of the sample in the absorption volume. This leads for 17 elements to a 3-110-fold improvement in the power of detection compared to conventional flame AAS. The absolute detection limits (3s values, N=25) lie between 0.2 μg l⁻¹ (Zn) and 310 μg l⁻¹ (Se) according to the element. The R.S.D. (N=15) is 1.4-5.5% according to the element and applied concentration. TS-FF-AAS can easily be incorporated on any standard flame AAS instrument and can be automated with a standard autosampler.     

Comparison of tungsten coil electrothermal vaporization and thermospray sample introduction methods for flame furnace atomic absorption spectrometry

Flame furnace atomic absorption spectrometry (FF-AAS) is a newly developed flame atomic absorption spectrometric technique based on arranging a flame furnace onto the top of the flame burner head. In this fundamental investigation, 25 elements were carefully tested by using either thermospray FF-AAS or tungsten coil electrothermal vaporization FF-AAS, of which 15 volatile and semi-volatile elements (Cd, Tl, Ag, Pb, Zn, Hg, Cu, Sb, Bi, Te, In, As, Se, Sn and Au) exhibited better limits of detection compared to those by conventional FAAS; however, non-volatile or refractory elements (Fe, Co, Ni, Cr, Mn, Pd, Pt, Al, Be and V) showed inferior sensitivities by the proposed methods.     

Cloud point extraction–thermospray flame quartz furnace atomic absorption spectrometry for determination of ultratrace cadmium in water and urine

A simple, low cost and highly sensitive method based on cloud point extraction (CPE) for separation/preconcentration and thermospray flame quartz furnace atomic absorption spectrometry was proposed for the determination of ultratrace cadmium in water and urine samples. The analytical procedure involved the formation of analyte-entrapped surfactant micelles by mixing the analyte solution with an ammonium pyrrolidinedithiocarbamate (APDC) solution and a Triton X-114 solution. When the temperature of the system was higher than the cloud point of Triton X-114, the complex of cadmium-PDC entered the surfactant-rich phase and thus separation of the analyte from the matrix was achieved. Under optimal chemical and instrumental conditions, the limit of detection was 0.04 μg/L for cadmium with a sample volume of 10 mL. The analytical results of cadmium in water and urine samples agreed well with those by ICP-MS.     

Direct determination of Cu and Zn in fruit juices and bovine milk by thermospray flame furnace atomic absorption spectrometry

In the present work, thermospray flame furnace atomic absorption spectrometry (TS-FF-AAS) was employed for Cu and Zn determination in bovine milk and fruit juice samples without any pretreatment. TS-FF-AAS system was optimized and a sample volume of 300 μl was injected into the carrier stream (0.014 mol l⁻¹ HNO₃ at a flow rate of 0.4 ml min⁻¹), and it was introduced into a hot Ni tube. The detection limits obtained for Cu and Zn in aqueous solution were 2.2 and 0.91 μg l⁻¹, respectively, and 3.2 μg l⁻¹ for Cu in a medium containing water-soluble tertiary amines. The relative standard deviations varied from 2.7 to 4.2% (n=12). Sample preparation was carried out by simple dilution in water or in water-soluble tertiary amines medium. Accuracy was checked by performing addition-recovery experiments as well as by using reference materials (whole milk powder, non-fat milk powder, and infant formula). Recoveries varied from 97.7 to 105.3% for Cu and Zn. All results obtained for reference materials were in agreement with certified values at a 95% confidence level.     

Synthesis and application of imprinted polyvinylimidazole-silica hybrid copolymer for Pb determination by flow-injection thermospray flame furnace atomic absorption spectrometry

A novel ion imprinted polyvinylimidazole-silica hybrid copolymer (IIHC) was synthesized and used as a selective solid sorbent for Pb²⁺ ions preconcentration using an on-line solid phase extraction (SPE) system coupled to TS-FF-AAS. The ionic hybrid sorbent was prepared using 1-vinylimidazole and 3-(trimethoxysilyl)propylmethacrylate as monomers, Pb²⁺ ions as template, tetraethoxysilane as reticulating agent and 2,2′-azobis-isobutyronitrile as initiator. The best on-line SPE conditions concerning sorption behavior, including sample pH (6.46), buffer concentration (9.0 mmol L⁻¹), eluent (HNO₃) concentration (0.5 mol L⁻¹) and preconcentration flow rate (4.0 mL min⁻¹), were optimized by means of full factorial design and Doehlert matrix. The analytical curve ranged from 2.5 to 65.0 μg L⁻¹ (r = 0.999) with limit of detection of 0.75 μg L⁻¹; the precision (repeatability) calculated as relative standard deviation (n = 10) was 5.0 and 3.6% for Pb²⁺ concentration of 10.0 and 60.0 μg L⁻¹, respectively. From on-line breakthrough curve, column capacity was 3.5 mg g⁻¹. Preconcentration factor (PF), consumptive index (CI) and concentration efficiency (CE) were 128.0, 0.16 mL and 25.6 min⁻¹, respectively. The selective performance of the sorbent, based on relative selectivity coefficient, was compared to NIC (non imprinted copolymer) for the binary mixture Pb²⁺/Cd²⁺, Pb²⁺/Cu²⁺ and Pb²⁺/Zn²⁺. The results showed that ion imprinted polyvinylimidazole-silica hybrid polymer had higher selectivity for Pb²⁺ than NIC at 64.9, 16.0 and 8.8 folds. The developed method was successfully applied for highly sensitive and selective Pb²⁺ determination in different kinds of water samples, parenteral solutions and urine. Accuracy was also assessed by analyzing certified reference fish protein (DORM-3) and marine sediment (MESS-3 and PACS-2) with satisfactory results.     

Acid extraction and cloud point preconcentration as sample preparation strategies for cobalt determination in biological materials by thermospray flame furnace atomic absorption spectrometry

A thermospray flame furnace atomic absorption spectrometer (TS-FF-AAS) was employed for Co determination in biological materials. Cobalt presents a high atomization temperature and consequently poor sensitivity is obtained without changing its thermochemical behavior. The effect of different complexing agents on sensitivity was evaluated based on the formation of Co volatile compounds. A cloud point procedure was optimized for Co preconcentration for further improvement of sensitivity. Samples were treated with 1 mol l^− 1 hydrochloric acid solution for quantitative extraction of Co without simultaneous extraction of Fe, since it is a strong interferent. After the extraction and preconcentration steps, a sample volume of 150 μl was introduced into the hot Ni tube using air as carrier at a flow-rate of 0.4 ml min^− 1. The best sensitivity was attained using ammonium pyrrolidinedithiocarbamate (APDC) and Triton X-114 was employed for implementation of the cloud point procedure. The detection limit obtained for Co was 2.1 μg l^− 1 and the standard deviation was 5.8% for a solution containing 100 μg l^− 1 (n = 10). Accuracy was checked using two certified reference materials (tomato leaves and bovine liver) and results were in agreement with certified values at a 95% confidence level. Employing the developed procedure, Co were quantified in different biological materials (plant and animal tissues). The proposed method presents suitable sensitivity for cobalt determination in the quality control of foods.     

Evaluation of ammonia as diluent for serum sample preparation and determination of selenium by graphite furnace atomic absorption spectrometry

A method for the determination of total selenium in serum samples by graphite furnace atomic absorption spectrometry was evaluated. The method involved direct introduction of 1:5 diluted serum samples (1% v/v NH₄OH+0.05% w/v Triton X-100^®) into transversely heated graphite tubes, and the use of 10 μg Pd+3 μg Mg(NO₃)₂ as chemical modifier. Optimization of the modifier mass and the atomization temperature was conducted by simultaneously varying such parameters and evaluating both the integrated absorbance and the peak height/peak area ratio. The latter allowed the selection of compromise conditions rendering good sensitivity and adequate analyte peak profiles. A characteristic mass of 49 pg and a detection limit (3s) of 6 μg 1⁻¹ Se, corresponding to 30 μg l⁻¹ Se in the serum sample, were obtained. The analyte addition technique was used for calibration. The accuracy was assessed by the determination of total selenium in Seronorm™ Trace Elements Serum Batch 116 (Nycomed Pharma AS). The method was applied for the determination of total selenium in ten serum samples taken from individuals with no known physical affection. The selenium concentration ranged between 79 and 147 μg l⁻¹, with a mean value of 114±22 μg l⁻¹.     

Ti and Ni tubes combined in thermospray flame furnace atomic absorption spectrometry (TS-FF-AAS) for the determination of copper in biological samples

This study presents an evaluation of Cu determination in thermospray flame furnace atomic absorption spectrometry (TS-FF-AAS) using Ni and Ti metal tube atomizers. The TS-FF-AAS system was equipped with Ti tubes inserted inside Ni tubes (called Ni/Ti), and also different configurations of Ti tube atomizers placed on an oxidizing air/acetylene flame. This new arrangement combining both tubes permitted an increased sensitivity (approximately 4 times) when it was compared with single Ni or Ti tubes. This high sensitivity is due to the formation of TiO₂ inside the Ti tubes (shown by X-ray diffractometry and microanalyses), improving the Cu atomization through the corresponding oxide. The estimated gaseous phase temperatures for the tubes (Ni, Ti and Ni/Ti) were of the same magnitude (varied from 1400 to 1800 °C). Tests with concomitants (Na, K, Ca and Mg) showed similar behavior of Ni and Ni/Ti tubes. The differences between Ni and Ni/Ti tube atomizers for Cu sensitivity were not related to differences between the tube atomizers' internal volumes. The method accuracy was verified using certified reference materials (CRM's) of biological samples prepared with a closed-vessel microwave system using diluted HNO₃. A 2 µg L^− 1 Cu detection limit was obtained using a Ni/Ti tube atomizer. The recoveries for Cu from the CRM's were about 100%. Finally, the use of a Ti tube inside a Ni tube increased the Ti tube lifetime.     

On-line hyphenation of capillary electrophoresis with flame-heated furnace atomic absorption spectrometry for trace mercury speciation

Capillary electrophoresis (CE) was directly interfaced to flame-heated furnace atomic absorption spectrometry (FHF-AAS) via a laboratory-made thermospray interface for nanoliter trace element speciation. The CE-FHF-AAS interface integrated the superiorities of stable CE separation, complete sample introduction, and continuous vaporization for AAS detection without the need of extra external heat sources and any post-column derivation steps. To demonstrate the usefulness of the developed hybrid technique for speciation analysis, three environmentally significant and toxic forms of methylmercury (MeHg), phenylmercury (PhHg), and inorganic mercury (Hg(II)) were taken as model analytes. Baseline separation of the three mercury species was achieved by CE in a 60 cm long x 75 microm inner diameter fused-silica capillary at 20 kV and using a mixture of 100 mM boric acid and 10% v/v methanol (pH 8.30) as running electrolyte. The precision (relative standard deviation, RSD, n = 7) of migration time, peak area and peak height for the mercury species at 500 microg x L(-1) (as Hg) level were in the range of 0.9-1.2%, 1.5-1.9%, and 1.4-2.0%, respectively. The detection limit (S/N = 3) of three mercury species was 3.0 +/- 0.15 pg (as Hg), corresponding to 50.8 +/- 2.4 microg x L(-1) (as Hg) for 60 nL sample injection, which was almost independent on specific mercury species. The developed hybrid technique was successfully applied to the speciation analysis of mercury in a certified reference material (DORM-2, dogfish muscle).     

Coupling of ultrasonic nebulization to flame furnace atomic absorption spectrometry—new possibilities for trace element determination

The use of ultrasonic nebulization (USN) with desolvation system for sample introduction in flame atomic absorption spectrometry (F AAS) and flame furnace atomic absorption spectrometry (FF AAS) with a nickel tube is described. Polytetrafluorethylene (PTFE) adaptors were built to replace the pneumatic nebulizer for USN-F AAS measurements. For USN-FF AAS analysis, an alumina injector allowed the direct introduction of the dry aerosol into the nickel tube. The analytical performance of both systems is shown for Ag, Bi, Cd, Cr, Cu, Mn, Pb, Sb, Se, Tl and Zn. The results demonstrate that a sensitivity gain of up to 39 times can be achieved using USN-FF AAS, mainly due to the increase in residence time and to the absence of dilution of the analyte by the flame gases, as the atomization takes place inside the nickel tube. However, elements that require higher atomization temperatures, such as Cr and Mn, are more efficiently determined using USN-F AAS. To evaluate the accuracy of the proposed methods for the determination of trace elements, five certified reference samples were analyzed, and good agreement was, in general, achieved between certified and determined values at a 95% confidence level. The relative standard deviation was frequently below 5%, demonstrating good precision, particularly for USN-FF AAS. In this sense, coupling of USN with F AAS and especially with FF AAS has proved to be simple, safe, with high precision and good accuracy, also maintaining some of the most important features of F AAS, such as the high analytical frequency and the low running cost.     

Hollow fiber supported liquid membrane extraction coupled with thermospray flame furnace atomic absorption spectrometry for the speciation of Sb(III) and Sb(V) in environmental and biological samples

A new method of hollow fiber supported liquid membrane extraction (HF-SLME) coupled with thermospray flame furnace atomic absorption spectrometry (TS-FF-AAS) for the speciation of Sb(III) and Sb(V) in environmental and biological samples has been developed. The method is based on the complex of Sb(III) with sodium diethyldithiocarbamate (DDTC). The formed hydrophobic complex is subsequently extracted into the lumen of hollow fiber, whereas Sb(V) is remained in aqueous solutions. The extraction organic phase was injected into TS-FF-AAS for the determination of Sb(III). Total Sb concentration was determined after reduction of Sb(V) to Sb(III) in the presence of l-cysteine and the extraction procedure mentioned above. Sb(V) was calculated by subtracting of Sb(III) from the total Sb. DDTC was used as complexing reagent. 1-Octanol was immobilized in the pores of the polypropylene hollow fiber as liquid membrane and also used as the acceptor solution. Some parameters that influenced extraction and determination were evaluated in detail, such as concentration of sodium diethyldithiocarbamate (DDTC), type of organic solvent, pH of samples, stirring rates, extraction time, as well as interferences. Under optimized conditions, a detection limit of 0.8 ng mL⁻¹ and an enrichment factor of 160 were achieved. The relative standard deviation (RSD) was 6.2% for Sb(III) (50 ng mL⁻¹, n = 5). The proposed method was successfully applied to the speciation of Sb(III) and Sb(V) in environmental and biological samples with satisfactory results.     

Determination of thallium by furnace atomic absorption spectrometry

The analytical conditions for the determination of thallium by graphite furnace atomic absorption spectrometry were studied and optimized using the peak-height mode. The charring-atomization curves for thallium from different atomization surfaces were constructed and the optimum charring and atomization conditions were established. These atomization surfaces included pyrolytic graphite-, tantalum-, zirconium- and tungsten-coated graphite tubes. The effects of different inorganic acids on the absorbance of thallium from different surfaces were studied. Using tungsten carbide-coated tubes, the interference effects due to hydrochloric and perchloric acids were eliminated. The matrix modification technique was also investigated for increasing the maximum permissible charring temperature for thallium. The matrix modifiers used included tungsten, zirconium, nickel and tantalum. The effect of adding these modifiers were studied in the presence of different acids. Tungsten increased the maximum permissible charring temperature from 400 to 1000 °C.     

On-line precipitation–dissolution in knotted reactor for thermospray flame furnace AAS for determination of ultratrace cadmium

A simple, environmentally friendly, cost-effective and sensitive method was developed for the determination of trace cadmium in rice and water by using flow injection (FI) on-line precipitation–dissolution in a knotted reactor (KR) as a preconcentration scheme for thermospray flame furnace atomic absorption spectrometry (TS-FF-AAS). The preconcentration was achieved by online merging the sample solution and the precipitating reagent in a KR and subsequently eluting the resultant precipitate of cadmium hydroxide with 1 mol/L HNO₃. The eluant was then introduced into TS-FF-AAS for the determination. A self-assembled FI system was employed to hyphenate the KR system with TS-FF-AAS. Under optimal chemical and instrumental conditions, a limit of detection of 0.04 μg/L and a sensitivity enrichment factor of 34 for cadmium was obtained with a total initial sample volume of 4 mL. The proposed method was applied to the determination of cadmium in certified reference rice and water samples with analytical results in good agreement with their certified values. Real rice samples and real water samples were also determined by the proposed method, with analytical results confirmed by inductively coupled plasma mass spectroscopy (ICP-MS).     

Analysis of submicroliter samples using micro thermospray flame furnace atomic absorption spectrometry

A recently described thermospray flame furnace atomic absorption spectrometric (TS-FF-AAS) system has been modified in order to extend the applicability of the method for the determination of elemental traces in very small sample volumes (microliter or submicroliter). As an easily available, effective thermospray vaporizer, a fused silica capillary was used and the liquid sample was transported by 1 MPa (10 bar) gas pressure delivered by a standard gas cylinder. A 0.3 microL sample volume can be analyzed with a higher power of detection than using 3 orders of magnitude larger sample volumes with conventional flame atomic absorption spectrometry. The relative standard deviations (N=12) for 0.3 microL volumes and 5 microg/mL Pb samples amount to 3.1% and 3.8% in signal height and signal area, respectively. The detection limit was found to be 69 ng/mL. Initial experiments with other elements (Cd, Hg, Tl, Zn) led to similar results.     

Cadmium determination in biological samples by direct solid sampling flame atomic absorption spectrometry

A direct solid sampling flame atomic absorption spectrometric procedure for trace determination of cadmium in biological samples has been developed. Test samples (0.05–2.00 mg) were ground and weighed into small polyethylene vials, which were connected to the device for solid sample introduction into a conventional air/acetylene flame. Test samples were carried as a dry aerosol to a quartz cell, placed between the burner and the optical path, which had a perpendicular entrance and a slit in the upper part. The atomic vapor generated in the flame produced a transient signal that was totally integrated within 1 s. The effect of operating conditions and the extent of grinding on the analytical signal were evaluated. Background signals were always low and a characteristic mass of 0.29 ng Cd was obtained. Calibration was performed using different masses of solid certified reference materials. Results obtained for certified and in-house reference materials were typically within the 95% confidence interval of the certified and/or reference value, and the precision, expressed as relative standard deviation, was between 3.8 and 6.7%. The proposed system is simple and it might be adapted to conventional atomic absorption spectrometers allowing the determination of Cd in more than 80 test samples per hour, excluding weighing.     

Monitoring of selenium in water samples using dispersive liquid-liquid microextraction followed by iridium-modified tube graphite furnace atomic absorption spectrometry

A simple and powerful microextraction technique was used for determination of selenium in water samples using dispersive liquid-liquid microextraction (DLLME) followed by graphite furnace atomic absorption spectrometry (GF AAS). DLLME and simultaneous complex formation was performed with rapid injection of a mixture containing ethanol (disperser solvent), carbon tetrachloride (extraction solvent) and ammonium pyrrolidine dithiocarbamate (APDC, chelating agent) into water sample spiked with selenium. After centrifuging, fine droplets of carbon tetrachloride, which were dispersed among the solution and extracted Se-APDC complex, sediment at the bottom of the conical test tube. The concentration of enriched analyte in the sedimented phase was determined by iridium-modified pyrolitic tube graphite furnace atomic absorption spectrometry. The concentration of selenate was obtained as the difference between the concentration of selenite after and before pre-reduction of selenate to selenite. Some effective parameters on extraction and complex formation, such as extraction and disperser solvent type and their volume, extraction time, salt effect, pH and concentration of chelating agent were optimized. Under the optimum conditions, the enrichment factor of 70 was obtained from only 5.00 mL of water sample. The calibration graph was linear in the range of 0.1-3 μg L^− 1 with detection limit of 0.05 μg L^− 1. The relative standard deviation (RSDs) for ten replicate measurements of 2.00 μg L^− 1 of selenium was 4.5%. The relative recoveries of selenium in tap, river and sea water samples at spiking level of 2.00 μg L^− 1 were 106, 96 and 98%, respectively.     

Studies towards understanding the mechanism of organic reagent enhancement in flame and plasma atomic absorption spectrometry

A series of organic reagents have been tested in atomic absorption measurement for signal enhancement of metal elements. Organic reagents like tetrabutylammonium bromide are demonstrated to enhance the absorption sensitivity to some specific elements such as calcium and chromium. A group of amines were found to have significant enhancement for chromium and calcium measurements. The function of organic reagents in flame and plasma atomic absorption spectrometry (AAS) was investigated in this work with emphases on mechanism of signal enhancement and interference suppression. An alternative mechanism of organic reagent enhancement in flame and plasma AAS has been suggested based on the experimental results obtained in this work. The reduction environments in flame and plasma produced by the organic reagents are considered as major reason for the signal enhancement.     

Quantitative element analysis by isotope dilution in diode laser graphite furnace atomic absorption spectrometry

The paper reviews the application of the isotope dilution technique in optical atomic absorption spectrometry by use of a low-pressure graphite tube furnace as atomizer and diode lasers as radiation sources. The principles and the methodology to obtain accurate quantitative results despite of the occurrence of interferences are presented. The successful application of different Doppler-limited and Doppler-free spectrometric techniques is also presented. The perspectives but also the limitations of this new method are discussed.     

Low volume microwave digestion for the determination of selenium in marine biological tissues by graphite furnace atomic absorption spectroscopy

A microwave digestion method for the determination of marine biological tissues has been developed to allow determination of selenium in small sample sizes (< 0.1 g). The benefits of this technique include maintaining concentrates in extracts without the subsequent over dilution encountered when using larger vessels, increased sample throughput and reduced loss of volatile material. Freeze dried biological material (< 0.1 g) and nitric acid (1 ml) were placed into 7 ml screw top Teflon vessels which are completely sealed on capping. Two 7 ml vials were placed into larger 120 ml vessels fitted with a Teflon spacer and 10 ml of distilled water. The effects of microwave power and time, and sample mass on selenium recovery from three marine standard reference materials (NIST SRM 1566a Oyster Tissue, NRCC DORM-1 Dogfish Muscle and NRCC TORT-1 Lobster Hepatopancreas) were examined. The optimum conditions: 600 W, 2 min; 0 W, 2 min; 450 W, 45 min, allowed quantitative recoveries of selenium from these and three other standard reference materials (NRCC DOLT-1 Dogfish liver, NIST RM-50 Albacore tuna and IAEA MA-A-2 fish flesh). Studies on sample mass showed that the analysis of sample masses from 0.025 to 0.1 g gave selenium concentrations within the certified range. Six species of selenium: selenite, selenate, selenomethionine, selenocysteine, selenocystamine, and trimethyl selenonium were added to oyster, dogfish, and lobster tissues. Recoveries were near quantitative for all species (94–105%) except trimethyl selenonium (90–101%).