Advantages of the iridium permanent modifier in fast programs applied to trace-element analysis of plant samples by electrothermal atomic absorption spectrometry

The application of a fast program combined with the advantages of the iridium permanent modifier is proposed for trace element analysis of plant samples by electrothermal atomic absorption spectrometry (ETAAS). For two volatile elements (Cd, Pb) and two mid-refractory elements (Cr, Ni) it was demonstrated that coating of the platform or of the tube atomization area with Ir is an efficient means of improving the accuracy and precision of results. A detailed study of interference from individual main matrix components and from composite plant matrices has confirmed the usefulness of the whole approach. The validity of the method has been confirmed by analysis of eight reference plant materials.     

Cloud point extraction of trace metals from seawater and determination by electrothermal atomic absorption spectrometry with iridium permanent modifier.

A simple method is described for preconcentration and separation of trace metals such as Ag, Co, Cr, Cu, Fe, Mn, Ni and Pb simultaneously from seawater using a cloud point extraction (CPE) procedure. Triton X-114 nonionic surfactant and ammonium pyrrolidine dithiocarbamate (APDC) have been used as an extraction medium and a chelating extractant, respectively. The amounts of Triton X-114 and APDC and the pH value necessary for extraction were carefully optimized. The preconcentration factor of about 200 is achieved for all the studied metals. Electrothermal atomic absorption spectrometry (ETAAS) with an Ir coated graphite tube as permanent chemical modifier has been used for determination. The limits of detection of Ag, Co, Cr, Cu, Fe, Mn, Ni and Pb were 0.003, 0.008, 0.003, 0.006, 0.015, 0.002, 0.009 and 0.01 ng ml-1, respectively. Certified reference materials such as CASS-4 and NASS-5 (seawater) and NIST-1640 (natural water) have been used for validation of the new method. The relative standard deviation (%) obtained for all the metals are in the range 0.8 - 3.6% for natural water and 11-25% for seawater materials, except for Co in NASS-5 for which it was 50%.     

Direct determination of arsenic and antimony in naphtha by electrothermal atomic absorption spectrometry with microemulsion sample introduction and iridium permanent modifier

This paper reports the determination of arsenic and antimony in naphtha by employing electrothermal atomic absorption spectrometry (ETAAS) as the analytical technique. In order to promote the direct determination of the analytes in the very volatile naphtha, the formation of a microemulsion with different surfactants (Triton X-100 and Brij-35) and different chemical modification strategies were tested. The results indicated that Triton X-100 is the best emulsification agent for naphtha in both As and Sb determination when it is employed at a concentration of 1% w/v in the microemulsion. Under these conditions, the microemulsion was stabile for at least 2 h. By using Brij-35 it was possible to achieve good stability only in the first 15 min. Among all chemical modification approaches investigated (Ir permanent modifier, W-Ir permanent modifier, and Pd modifier), the Ir permanent modifier provided better sensitivity for both analytes and allowed a higher pyrolysis temperature, which decreased the background signals at lower levels. Under the best conditions established in this work, an RSD of 4.6% (20 g L^–1) and a detection limit of 2.7 g L^–1 were observed for arsenic. For antimony, an RSD of 4.0% (20 g L^–1) and a detection limit of 2.5 g L^–1 were obtained. The accuracy of the procedure was assessed by analyzing spiked samples of naphtha from different origins.     

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 in biological samples solubilized with tetramethylammonium hydroxide by electrothermal atomic absorption spectrometry, using ruthenium as permanent modifier

The feasibility of Ru as a permanent modifier for the determination of Cd in biological samples treated with tetramethylammonium hydroxide (TMAH) by ET AAS was investigated. The tube treatment with Ru was carried out only once and lasted for about 300 atomization cycles. The pyrolysis and atomization temperatures, 750 °C and 1300 °C, respectively, were chosen from the temperature curves. The sample dissolution procedure was very simple: a sample aliquot was mixed with a small volume of a 25% m/v TMAH solution, the volume was made up to 50 ml and the mixture was kept at 60 °C for 1 h. Six certified biological reference materials were analyzed and the obtained Cd concentrations are within the 95% confidence interval of the certified values, proving the accuracy of the proposed procedure for a variety of biological samples. The calibration curve, with correlation coefficient higher than 0.99, was established for a working range up to10 μg l⁻¹. The precision was good as demonstrated by relative standard deviations below 3%, except for one sample. The limit of detection (3σ) was 0.05 μg l⁻¹ and the characteristic mass was 1.30 pg, obtained in the presence of the Ru modifier.     

Determination of selenium by tungsten coil atomic absorption spectrometry using iridium as a permanent chemical modifier

Permanent chemical modifiers have been shown to prolong graphite tube lifetime while reducing the furnace cycle time, thus improving cost-effectiveness. In this work, iridium is used as a permanent chemical modifier for the first time in the determination of selenium by tungsten-coil atomic absorption spectrometry (W-Coil AAS). The iridium modifier is thermally coated onto the tungsten coil. After coating, the coil can be used for 300–400 firings without further application of the modifier. Thermal treatment with iridium permits operating with higher pyrolysis temperature and coil lifetime is extended up to 1600 firings. The sensitivity and linearity of the method is improved, and the analytical procedure allows the use of analyte solutions containing up to 8% nitric acid. The short-term stability of the absorbance measurements is demonstrated by the reproducibility in the measurements of a Se amount (6 ng) 30 times higher than the limit of detection (0.2 ng). A 7% relative standard deviation (R.S.D.) was observed for 10 consecutive measurements of 6 ng Se. The long-term stability is almost as good: less than 9% R.S.D. over a 3-week period and 1500 firings. The surface of the tungsten-coil treated with iridium is examined before and after intensive use by scanning electron microscopy. Finally, the thermal treatment of the tungsten-coil with iridium appears to delay the appearance of selenium atoms by approximately 0.2 s although the integrated absorbance measurements are unaffected. The magnitude of delay decreases with coil age.     

Performance of permanent iridium modifier in the presence of corrosive matrix in graphite furnace atomic absorption spectrometry

The influence of up to 16% HNO3, 28% HCl, and the mixture of both acids in aqua regia on the analytical performance of electrodeposited modifiers (Ir or Ir+Pd) was evaluated and discussed. Cadmium was used as an example of volatile elements often determined by graphite furnace atomic absorption spectrometry (GF AAS). In the presence of HCl, the maximum pyrolysis temperature that could be applied was found to be 600 degrees C. In the presence of HNO3 and aqua regia, both modifiers stabilized cadmium up to 800 degrees C. The long-term performance of electrodeposited Ir or Ir+Pd was not influenced by mineral acids; moreover the tube lifetime was significantly prolonged compared with a non-modified tube.     

Iridium as permanent modifier in the determination of lead in whole blood and urine by electrothermal atomic absorption spectrometry

The behavior of Iridium as a thermally deposited permanent modifier for the determination of lead by GFAAS was studied. The iridium coating procedure was optimized using a two-level factorial design. Using the optimized coating procedure, up to 1100 firings were possible with the same coating without sensitivity losses. The system was used in the determination of lead in whole blood and urine. A mixture of 0.1% Triton X-100 and 0.2% nitric acid was used as diluent. Pyrolysis temperature was 800°C for both matrices. Spiked, low-level samples were used for calibration. Under these conditions, no significant difference was found in comparison to the results obtained using a validated conventional modification procedure employing phosphate as modifier. Also, good agreement between found and certified/reference values were observed in the analysis of certified and commercial quality control materials, respectively. Such agreement was observed even using a 1100 times fired Ir coated platform–graphite tube assembly.     

Direct determination of aluminium in serum and urine by electrothermal atomic absorption spectrometry using ruthenium as permanent modifier

Ruthenium (Ru), thermally deposited on a integrated platform graphite furnace, was investigated as a permanent modifier for the determination of Aluminum (Al) in blood serum and urine by electrothermal atomic absorption spectrometry (ETAAS). The platform was treated with 500 μg of Ru as previously described. The pyrolysis and atomization temperatures for each material were of 1300 and 2300 °C, for serum sample and of 1000 and 2400 °C, for urine. The characteristic mass were of 31 and 33 pg for Al in serum sample and urine, respectively (recommended of 31 pg for Al in nitric acid 0.2% (v/v)). For this reason, the calibration was made with aqueous solutions for both the samples. Calibration curves presented r of 0.99145 and 0.99991 for serum and urine, respectively. With the optimized temperatures, being analyzed eight spiked blood serum samples, the recovery was between 95.90 and 113.50%. Two certified urines samples were analyzed with good agreement between experimental and reference values. In both the samples the R.S.D. were <5% (n=3). The detection limit (k=3, n=10) was of 0.40 μg of Al per liter for both the samples. The absorption pulses obtained were symmetrical, with very low background and without interferences. The life time of the tube-platform was higher than 600 cycles of atomizations for both the urine and serum samples.     

Direct determination of Cr and Cu in urine samples by electrothermal atomic absorption spectrometry using ruthenium as permanent modifier (R1)

In this study Ru, deposited thermally on an integrated platform pyrolytic graphite tube, is proposed as a permanent modifier for the determination of Cu and Cr in urine samples by electrothermal atomic absorption spectrometry. The samples were diluted 1:1 with nitric acid (1% v/v). Pyrolysis and atomization temperatures for spiked urine samples were 1,100 degrees C and 1,900 degrees C respectively for Cu, and 1,400 degrees C and 2,500 degrees C respectively for Cr. For comparison purposes, the conventional modification with Pd+Mg was also studied. The sensitivity for Ru as permanent modifier was higher for the two analytes. The characteristic masses were 7.3 and 17.7 for Cr and Cu. The detection limits (3sigma) were 0.22 and 0.32 microg/L, for Cr and Cu, respectively. Good agreement was obtained with certified urine samples for the two elements.     

Methods of calibration in the direct analysis of solid samples by electrothermal atomic absorption spectrometry

One of the major problems involved in the direct analysis of solid samples by electrothermal atomic absorption spectrometry (ETAAS) lies in the calibration step because non-spectral interference effects are often pronounced. Three standardization techniques have been described and used in solid sampling-ETAAS: (i) standard additions method; (ii) calibration relative to a certified reference material; and (iii) calibration curve technique. However, an adequate statistical evaluation of the uncertainty in the analyte concentration in the solid sample is most frequently neglected, and reported errors may be seriously underestimated. This can be attributed directly to the complexity of the statistical expressions required to accurately account for errors in each of the calibration techniques mentioned above, and the general lack of relevant reference literature. The object of this work has been to develop a computer package which will perform the necessary statistical analyses of solid sampling-ETAAS data; the result is the program “SOLIDS” described here in the form of an electronic publication in Spectrochimica Acta Electronica, the electronic section of Spectrochimica Acta Part B. The program could also be useful in other analytical fields where similar calibration methods are used. The hard copy text, outlining the calibration models and their associated errors, is accompanied by a diskette containing the program, some data files, and a manual. Use of the program is exemplified in the text, with some of the data files discussed included on the diskette which, together with the manual, should enable the reader to become familiarized with the operation of the program, and the results generated.     

Evaluation of different permanent modifiers for the determination of arsenic in environmental samples by electrothermal atomic absorption spectrometry

Single noble metal permanent modifiers such as, Rh, Ir, and Ru, as well as mixed tungsten plus noble metal (W-Rh, W-Ru, W-Ir) permanent modifiers thermally deposited on the integrated platform of transversally heated graphite atomizer were employed for the determination of arsenic in sludges, soils, sediments, coals, ashes and waters by electrothermal atomic absorption spectrometry. Microwave digests of solid samples and water samples were employed for obtaining the analytical characteristics of the methods with different permanent modifiers. The performance of the modifiers for arsenic determination in the real samples depended strongly on the type of permanent modifier chosen. The single noble metal (Rh, Ir and Ru) permanent modifiers were suitable for the analyte determinations in simpler matrices such as waters (recoveries of certified values 95-105%), but the analyte recoveries of certified values in sludges, soils, sediments, coals, and ashes were always lower than 90%. On the other hand, for the determination of arsenic, using W-Rh, W-Ru, and W-Ir permanent modifiers presented recoveries of certified values within 95-105% for all the samples. Long-term stability curves obtained for the determination of arsenic in environmental samples with different permanent modifiers (Rh, Ir, Ru, W-Rh, W-Ir, W-Ru) showed that the improvement in the tube lifetime depends on the tungsten deposit onto the platform. The tungsten plus noble metal permanent modifier presents a tube lifetime of at least 35% longer when compared with single permanent modifier. The results for the determination of As employing different permanent modifiers in the samples were in agreement with the certified reference materials, since no statistical differences were found after applying the paired t-test at the 95% confidence level.     

Determination of tin in marine sediments by slurry sampling atomic absorption spectrometry with electrothermal atomization using a permanent modifier

A molecularly imprinted polymer (MIP) was synthesized by the precipitation polymerization method using triazophos (TAP) as the template. The MIP can selectively absorb TAP from sample solutions. The absorbed TAP strongly enhances the chemiluminescence (CL) of luminol-H₂O₂. This was applied to design a method for the on-line enrichment and detection of TAP. The change in CL intensity is linearly proportional to the concentration of TAP in the range from 1.0 μM to ~4.0 nM, and the detection limit is 2.5 nM (at an S/N of 3σ). This is about one order of magnitude lower than that of the conventional CL assay. The method was successfully applied to the determination of TAP in vegetable samples.     

Morphological and spectroscopic investigation of the behavior of permanent iridium modifier deposited on pyrolytic graphite coated and zirconium treated platforms in electrothermal atomic absorption spectrometry

In order to better characterise a permanent modifier based on iridium deposited on zirconium or tungsten treated platforms of transversely heated graphite atomizer, and to gain additional information about its chemical behavior directed to an eventual further optimization, a series of experiments were carried out, both by surface techniques such as scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS or ESCA) and X-ray fluorescence (XRF) and by electrothermal atomic absorption spectrometry on the iridium release from unmodified and various other modified pyrolytic graphite platforms. Special attention was paid to the influence of the amount of iridium, zirconium carbide coating of the platform surface and the presence of citric acid on the iridium vaporization during pyrolysis and atomization. The processes of iridium losses during pyrolysis and atomization and peak maximum alignment depend on the amount of the iridium deposited on the pyrolytic graphite coated platforms in the presence of nitric acid. A fractional order of release which suggests an atom vaporization from the surface or edges of the iridium islands was estimated. In the presence of citric acid, mass independence and zero order of the atom release were found. The zirconium treatment of the platform results in change of the spatial distribution of iridium and hence its vaporization. Vaporization temperatures as high as 2100°C, and first order of the process of atom generation were obtained. While it was possible to study the iridium atomization from uncoated and zirconium coated surfaces, evidencing a different order for the release process, the same was not possible for the tungsten coated platforms due to an ‘overstabilization’ that brought the iridium atomization temperature out of the working range of the instrument used. The different chemical behavior of tungsten and zirconium was also confirmed by XPS investigations. With tungsten, evidence of both W---C and W---O bonding was found, while zirconium on the contrary shows only Zr---O bonding and no evidence of carbide bonding. The SEM revealed a highly dispersed distribution of spot-like features whose smallest average diameter was of the order of 0.1 μm. The XRF asserted the confinement of iridium in these features and a strict association with zirconium in the case of zirconium treated surfaces. It is worth mentioning that such structure was preserved also after 400 thermal cycles simulating an atomization step at 1900°C despite a quite evident deterioration of the graphite surface, thus confirming the excellent durability of this modifier.     

Semi-permanent lutetium modifier for the determination of beryllium in urine by electrothermal atomic absorption spectrometry

The determination of beryllium using electrothermal atomic absorption spectrometry with deuterium background correction in the presence of various isomorphous metals and Mg(NO₃)₂ was studied. While, Eu, Ir and Sm had no effect on the transient signals, the addition of Lu and Mg(NO₃)₂ improved the sensitivity of the beryllium signal with respect to that obtained in the absence of modifier. Although, Mg(NO₃)₂ has improved the signal with respect to its sensitivity, it also increased the tail and the background (BG) signals, specially when urine samples are under study. Whereas, when Lu was used the analytical signal is virtually free of BG interference indicating that the urine matrix interference was almost eliminated. Besides, the addition of 6 μg of Lu ensured that the signals were effectively constant for five firings following the furnace program, which included: three drying, and the pyrolysis, atomization, cleaning and cooling steps. The effect of some components, likely to interfere in the accurate determination of beryllium (such as: Al, Ca, Cl, Co, Cr, Fe, Mg and Mn) were investigated. At the physiological levels, most of these elements had no effect, except in the case of chloride when Mg(NO₃)₂ was used as modifier. In this case, the tolerance limit was of 3000 mg Cl⁻ l⁻¹. The characteristic masses were 1.19, 0.45 and 0.48 pg, when integrated absorbance was measured for beryllium without the addition of any modifier and in the presence of Lu and Mg(NO₃)₂, respectively. The limits of detection (3σ) were 85, 19 and 58 fg, respectively. The accuracy and precision with the use of Lu and Mg(NO₃)₂ was tested for the direct determination of beryllium in urine samples. Quantification was performed with aqueous standards. The results obtained for the determination of beryllium in reference materials (Trace Elements in Urine), together with good recovery of spiked analyte, using either Lu or Mg(NO₃)₂ modification demonstrate the applicability of the procedure to the analysis of real samples. However, Lu provided the most accurate results. Also, the addition of Lu enhanced the precision of the measurements to levels of 1.8% relative standard deviation instead of 5.6 and 3.3% for the case of beryllium alone and with the addition of Mg(NO₃)₂.     

Silicon determination in milk by electrothermal atomic absorption spectrometry using palladium as chemical modifier

A direct method for silicon determination in milk samples by Electrothermal Atomic Absorption Spectrometry was developed. Palladium was used as chemical modifier at a concentration of 610 mg L(-1); with this modifier, silicon was stable up to 1800 degrees C. The precision and accuracy of the method were investigated. The detection limit was 16.2, 2.7 and 7.2 micro g L(-1) for cows' milk, human milk and infant formula, respectively. The method was applied to silicon determination in 17 infant formula samples, 13 human milk samples and 12 cows' milk samples.     

Rhodium as permanent modifier for atomization of lead from biological fluids using tungsten filament electrothermal atomic absorption spectrometry

Rhodium (Rh) was investigated as a permanent modifier for the atomization of Pb from biological fluids in W-filament atomic absorption spectrometry (AAS). Heating the W-filament with a Rh solution provided a protective coating for subsequent determinations of Pb in blood and urine matrices. The W-filament AAS instrumentation used was based on a prototype design that utilized self-reversal background correction scheme and peak area measurements. We found that Rh not only stabilized Pb during the pyrolysis step, but also facilitated the removal of carbonaceous residues during the cleaning step, requiring much less power than with phosphate modifier. Thus, the filament lifetime was greatly extended to over 300 firings. Periodic reconditioning with Rh was necessary every 30 firings or so. Conditioning the filament with Rh also permitted direct calibration using simple aqueous Pb standards. The method detection limit for blood Pb was approximately 1.5 μg dl⁻¹, similar to that reported previously. Potential interferences from concomitants such as Na, K, Ca and Mg were evaluated. Accuracy was verified using lead reference materials from the National Institute of Standards and Technology and the New York State Department of Health. Blood lead results below 40 μg dl⁻¹ were within ±1 μg dl⁻¹ of certified values, and within ±10% above 40 μg dl⁻¹; within-run precision was ±10% or better. Additional validation was reported using proficiency test materials and human blood specimens. All blood lead results were within the acceptable limits established by regulatory authorities in the US. When measuring Pb in urine, sensitivity was reduced and matrix-matched calibration became necessary. The method of detection limit was 27 μg l⁻¹ for urine Pb. Urine lead results were also validated using an acceptable range comparable to that established for blood lead by US regulatory agencies.     

Determination of beryllium by electrothermal atomic absorption spectrometry using tungsten surfaces and zirconium modifier

Electrothermal atomization of beryllium from graphite and tungsten surfaces was compared with and without the use of various chemical modifiers. Tungsten proved to be the best substrate, giving the more sensitive integrated atomic absorption signals of beryllium. Tungsten platform atomization with zirconium as a chemical modifier was used for the determination of beryllium in several NIST SRM certified reference samples, with good agreement obtained between the results found and the certified values. The precision of the measurements (at 10 μg L⁻¹), the limit of detection (3σ), and the characteristic mass of beryllium were 2.50%, 0.009 μg L⁻¹ and 0.42 pg, respectively.     

Determination of lead in fish samples by slurry sampling electrothermal atomic absorption spectrometry

Ultrasonic slurry sampling electrothermal atomic absorption spectrometry (USS-ETAAS) was been applied to the determination of lead in several fish samples. The influences of instrument operating conditions and slurry preparation on the signal were examined. Palladium and ammonium nitrate were used as the modifier to improve the signal. Since the sensitivity to lead in various fish slurries and aqueous solutions was different, the standard additions method was used for the determination of lead in these fish samples. The method was applied to the determination of lead in dogfish muscle reference material (DORM-2) and a swordfish muscle sample purchased from the local market. The analysis results agreed with the reference value. The accuracy was better than 6%. The precision between sample replicates was better than 16% with the USS-ETAAS method. The detection limit of lead estimated from standard additions curve was about 0.053-0.058 microgram g-1 in different samples.     

Prospects and problems in absolute analysis by electrothermal atomic absorption spectrometry

The most significant achievements in the development of methods of absolute analysis in electrothermal atomic absorption spectrometry (ET AAS) made in the recent five years are discussed. Problems requiring further investigation and the significance of the concept of absolute analysis in the evolution of ET AAS are pointed out.