George L. Donati

Simultaneous determination of the Lanthanides by tungsten coil atomic emission spectrometry

The fourteen Lanthanides are determined by tungsten coil atomic emission spectrometry. Twenty-five microlitre sample aliquots are placed directly on the coil. A simple constant current power source carefully dries the sample prior to analysis. During this dry step, the voltage is monitored to prevent over heating. This allows for shorter atomization programs, while improving sensitivity and coil lifetime. During the 5 s high temperature atomization step, the emission signals for as many as seven Lanthanides are determined simultaneously in the same 55 nm spectral window. The analytical figures of merit for all 14 natural Lanthanides are reported and compared with nitrous oxide flame atomic emission spectrometry. Tungsten coil atomic emission concentration detection limits are in the range 0.8 (Yb) to 600 (Pr) µg l−1, and are lower than those for the flame in most cases. The absolute detection limits are near or below the ng level: significantly lower than the flame detection limits due to the smaller sample volume required. A three-fold improvement in detection limit may be realized by combining the signals for multiple emission lines for a single element. The method is applied to the determination of seven Lanthanides in a soil sample acquired from the National Institute of Standards and Technology. After a simple acid extraction, the measured values agree with the reported values with 95% confidence in all cases but one, Yb. Finally, a conditioning program for new tungsten coils enhances reproducibility and maximizes the emission signal.

Determination of Cd in urine by cloud point extraction-tungsten coil atomic absorption spectrometry.

Cadmium concentrations in human urine are typically at or below the 1 microgL(-1) level, so only a handful of techniques may be appropriate for this application. These include sophisticated methods such as graphite furnace atomic absorption spectrometry and inductively coupled plasma mass spectrometry. While tungsten coil atomic absorption spectrometry is a simpler and less expensive technique, its practical detection limits often prohibit the detection of Cd in normal urine samples. In addition, the nature of the urine matrix often necessitates accurate background correction techniques, which would add expense and complexity to the tungsten coil instrument. This manuscript describes a cloud point extraction method that reduces matrix interference while preconcentrating Cd by a factor of 15. Ammonium pyrrolidinedithiocarbamate and Triton X-114 are used as complexing agent and surfactant, respectively, in the extraction procedure. Triton X-114 forms an extractant coacervate surfactant-rich phase that is denser than water, so the aqueous supernatant is easily removed leaving the metal-containing surfactant layer intact. A 25 microL aliquot of this preconcentrated sample is placed directly onto the tungsten coil for analysis. The cloud point extraction procedure allows for simple background correction based either on the measurement of absorption at a nearby wavelength, or measurement of absorption at a time in the atomization step immediately prior to the onset of the Cd signal. Seven human urine samples are analyzed by this technique and the results are compared to those found by the inductively coupled plasma mass spectrometry analysis of the same samples performed at a different institution. The limit of detection for Cd in urine is 5 ngL(-1) for cloud point extraction tungsten coil atomic absorption spectrometry. The accuracy of the method is determined with a standard reference material (toxic metals in freeze-dried urine) and the determined values agree with the reported levels at the 95% confidence level.

Determination of Cr, Ni, Pb and V in gasoline and ethanol fuel by microwave plasma optical emission spectrometry

Simple procedures for the determination of Cr, Ni, Pb and V in gasoline and ethanol fuel by microwave plasma optical emission spectrometry (MIP OES) are described. Ethanol fuel samples were simply diluted in an aqueous 1% v/v HNO3 solution. For gasoline, microemulsions in n-propanol were prepared. The MIP OES instrument requires no separate gas source since a N2 gas generator coupled to a simple air compressor is sufficient to maintain the microwave-induced plasma. An external gas control module (EGCM) is used to inject air into the plasma in order to minimize background emissions and avoid carbon deposition on the torch and on the pre-optical window. The Flow Blurring Nebulizer (FBN) technology was also employed to ensure more efficient and homogeneous sample aerosols, which contributes to plasma stability and lower limits of detection (LODs). Nebulizer gas pressure and plasma viewing position were optimized separately for each analyte. Gasoline and ethanol fuel samples were analyzed and the calculated LODs were in the range of 0.3–60 μg L−1 or 4–1700 μg kg−1 for all analytes. The accuracy was checked by spike experiments and recoveries between 84 and 123% were obtained.

Rugged, Portable Tungsten Coil Atomic Emission Spectrometer

Tungsten coil atomic emission spectrometry is an ideal technique for field applications because of its simplicity, low cost, low power requirement, and independence from cooling systems. A new, portable, compact design is reported here. The tungsten coil is extracted from an inexpensive 24 V, 250 W commercial light bulb. The coil is housed in a small, aluminum cell. The emission signal exits from a small aperture in the cell, while the bulk of the blackbody emission from the tungsten coil is blocked. The resulting spectra exhibit extremely low background signals. The atomization cell, a single lens, and a hand-held charge coupled device (CCD) spectrometer are fixed on a 1 × 6 × 30 cm ceramic base. The resulting system is robust and easily transported. A programmable, miniature 400 W solid-state constant current power supply controls the temperature of the coil. Fifteen elements are determined with the system (Ba, Cs, Li, Rb, Cr, Sr, Eu, Yb, Mn, Fe, Cu, Mg, V, Al, and Ga). The precision ranges from 4.3% to 8.4% relative standard deviation for repetitive measurements of the same solution. Detection limits are in the 0.04 to 1500 μg/L range. Accuracy is tested using standard reference materials for polluted water, peach leaves, and tomato leaves. For those elements present above the detection limit, recoveries range from 72% to 147%.

Application of the interference standard method for the determination of sulfur, manganese and iron in foods by inductively coupled plasma mass spectrometry

The interference standard method (IFS) is evaluated to improve the accuracy of the determination of S, Mn and Fe in meat and grain samples by inductively coupled plasma quadrupole mass spectrometry (ICP-QMS). Due to ICP-QMS relatively low resolution, polyatomic interferences caused by (16)O(2)(+), ((16)OH)(2)(+), (40)Ar(14)NH(+), and (40)Ar(16)O(+), for example, can compromise determinations at m/z 32, 34, 55, and 56, respectively. In IFS, differently from traditional internal standard methods, plasma naturally occurring species are used to correct for variations in the interference signal rather than the analyte signal. The method is based on the hypothesis that the interfering ion and the IFS probe present similar behaviors in the plasma, and that by using the analytical (analyte plus interference)/IFS signal ratio one could reduce variations due to interference and, consequently, improve accuracy. In this work, this strategy is evaluated in real sample applications and significant improvements on accuracy are observed for (32)S, (34)S, (55)Mn, and (56)Fe determinations. Recoveries ranging from 72% for Mn to 105% for Fe in two different standard reference materials are obtained using the (38)Ar probe. These analytes are successfully determined in meat and grain samples with concentrations ranging from 4.42 μg g(-1) for Mn in corn to 7270 μg g(-1) for S in chicken liver. The method is compared with other strategies such as internal standardization and mathematical correction. No instrumental modification or introduction of foreign gases is required, which is especially attractive for routine applications.

A new atomization cell for trace metal determinations by tungsten coil atomic spectrometry

A new metallic atomization cell is used for trace metal determinations by tungsten coil atomic absorption spectrometry and tungsten coil atomic emission spectrometry. Different protecting gas mixtures are evaluated to improve atomic emission signals. Ar, N(2), CO(2) and He are used as solvents, and H(2) and C(2)H(2) as solutes. A H(2)/Ar mixture provided the best results. Parameters such as protecting gas flow rate and atomization current are also optimized. The optimal conditions are used to determine the figures of merit for both methods and the results are compared with values found in the literature. The new cell provides a better control of the radiation reaching the detector and a small, more isothermal environment around the atomizer. A more concentrated atomic cloud and a smaller background signal result in lower limits of detection using both methods. Cu (324.7 nm), Cd (228.8 nm) and Sn (286.3 nm) determined by tungsten coil atomic absorption spectrometry presented limits of detection as low as 0.6, 0.1, and 2.2 μg L(-1), respectively. For Cr (425.4 nm), Eu (459.4 nm) and Sr (460.7 nm) determined by tungsten coil atomic emission spectrometry, limits of detection of 4.5, 2.5, and 0.1 μg L(-1) were calculated. The method is used to determine Cu, Cd, Cr and Sr in a water standard reference material. Results for Cu, Cd and Cr presented no significant difference from reported values in a 95% confidence level. For Sr, a 113% recovery was obtained.

Standard Dilution Analysis

Standard dilution analysis (SDA) is a novel calibration method that may be applied to most instrumental techniques that will accept liquid samples and are capable of monitoring two wavelengths simultaneously. It combines the traditional methods of standard additions and internal standards. Therefore, it simultaneously corrects for matrix effects and for fluctuations due to changes in sample size, orientation, or instrumental parameters. SDA requires only 200 s per sample with inductively coupled plasma optical emission spectrometry (ICP OES). Neither the preparation of a series of standard solutions nor the construction of a universal calibration graph is required. The analysis is performed by combining two solutions in a single container: the first containing 50% sample and 50% standard mixture; the second containing 50% sample and 50% solvent. Data are collected in real time as the first solution is diluted by the second one. The results are used to prepare a plot of the analyte-to-internal standard signal ratio on the y-axis versus the inverse of the internal standard concentration on the x-axis. The analyte concentration in the sample is determined from the ratio of the slope and intercept of that plot. The method has been applied to the determination of FD&C dye Blue No. 1 in mouthwash by molecular absorption spectrometry and to the determination of eight metals in mouthwash, wine, cola, nitric acid, and water by ICP OES. Both the accuracy and precision for SDA are better than those observed for the external calibration, standard additions, and internal standard methods using ICP OES.

Direct determination of sodium, potassium, chromium and vanadium in biodiesel fuel by tungsten coil atomic emission spectrometry

High levels of sodium and potassium can be present in biodiesel fuel and contribute to corrosion, reduced performance and shorter engine lifetime. On the other hand, trace amounts of chromium and vanadium can increase the emission of pollutants during biodiesel combustion. Sample viscosity, immiscibility with aqueous solutions and high carbon content can compromise biodiesel analyzes. In this work, tungsten filaments extracted from microscope light bulbs are used to successively decompose biodiesels organic matrix, and atomize and excite the analytes to determine sodium, potassium, chromium and vanadium by tungsten coil atomic emission spectrometry (WCAES). No sample preparation other than simple dilution in methanol or ethanol is required. Direct analysis of 10-μL sample aliquots using heating cycles with less than 150 s results in limits of detection (LOD) as low as 20, 70, 70 and 90 μg kg(-1) for Na, K, Cr and V, respectively. The procedures accuracy is checked by determining Na and K in a biodiesel reference sample and carrying out spike experiments for Cr and V. No statistically significant differences were observed between reference and determined values for all analytes at a 95% confidence level. The procedure was applied to three different biodiesel samples and concentrations between 6.08 and 95.6 mg kg(-1) for Na and K, and between 0.22 and 0.43 mg kg(-1) for V were obtained. The procedure is simple, fast and environmentally friendly. Small volumes of reagents, samples and gases are used and no residues are generated. Powers of detection are comparable to other traditional methods.

Double tungsten coil atomic emission spectrometry: signal enhancement and a new gas phase temperature probe

A new double atomizer arrangement for tungsten coil atomic emission spectrometry is described. Two small constant current power supplies, a Czerny–Turner spectrograph, and a charge coupled device (CCD) detector are used to determine refractory elements in water samples. The analytical figures of merit for Ba, Sr, Ti, and V are reported and compared with the single coil arrangement. The use of two atomizers provides more energy for the atomization and excitation of the analytes and consequently improves sensitivity. Addition of a second coil improves limits of detection for refractory elements by a factor of 40 (V) and 5 (Ti). Using 25 µl sample volumes, the limit of detection for V is 10 µg l−1 at the 437.9 nm emission line, and 400 µg l−1 for the 399.9 nm Ti emission line. Vanadium is determined in a polluted water certified reference material, and the results agree with the certified value (250 mg l−1) at the 95% confidence level. For Ba and Sr, which emit strongly with a single coil, the double coil detection limits are improved by less than a factor of two: 0.007 µg l−1Ba and 0.4 µg l−1Sr. Emission signals for Ag, Cu and Sn are observed for the fist time with a tungsten coil atomic emission spectrometry (WCAES) system. A new method to determine the gas phase temperature by atomic emission spectrometry is also presented. Emission intensities for Dy (418.7 and 421.1 nm) and Eu (462.7 and 466.2 nm) are used to calculate Boltzmann temperatures for both the single and double coil arrangements. Temperature values calculated by this method agree with the traditional optical two-line absorption method using Sn (284.0 and 286.3 nm).

Tungsten coil electrothermal matrix decomposition and sample vaporization to determine P and Si in biodiesel by inductively coupled plasma mass spectrometry

A tungsten coil extracted from commercially available microscope light bulbs is used to decompose the organic matrix and vaporize biodiesel samples for P and Si determination by electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICP-MS). The vaporization cell is connected to a modified sheath gas tube so that sample vapors are mixed with a continuous aerosol flow exiting the ICP-MS nebulization chamber. Helium and H2 are used with a collision–reaction interface (CRI) to minimize spectral interferences and obtain low baseline spectra. These strategies contribute to improving repeatability and accuracy. A small solid state constant current power supply is used to resistively heat the vaporizer and 10 μl sample aliquots are introduced manually on the coil with a micropipette. Limits of detection of 0.4 and 0.1 mg kg−1 are obtained for P and Si, respectively. Reference biodiesel samples are used to check the accuracy of the procedure for P. A spike experiment is carried out to assess accuracy in Si determinations. No statistically significant differences are observed for reference and determined values by applying a t-test at a 95% confidence level. The direct analysis of biodiesel by tungsten coil ETV-ICP-MS is a fast and effective alternative to determine challenging elements like P and Si.