Renata S. Amais

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.

Determination of P, S and Si in biodiesel, diesel and lubricating oil using ICP-MS/MS

The feasibility of inductively coupled plasma tandem mass spectrometry (ICP-MS/MS) to overcome polyatomic interferents in severely affected isotope determination is demonstrated here. This instrument arrangement presents an octopole reaction/collision system (ORS) in between two quadrupole mass analyzers. The first quadrupole operates as a mass filter which allows just the target mass-to-charge ratio (m/z) to enter the ORS and rejects all other species with m/z different from the target analytes m/z. Once in the ORS, analytes react with the reaction gas introduced into the ORS, and the second quadrupole selects the product of the reactions m/z (mass shift operation mode). We describe here a procedure for accurate determination of P, S and Si in biodiesel, diesel and lubricating oil by ICP-MS/MS. Fuel standard reference materials (SRMs) were microwave-assisted acid-digested using nitric acid and hydrogen peroxide. The oxygen gas flow rate in the ORS was optimized considering the limit of detection (LOD) values and accuracies reached for P, S and Si determinations in the lubricating oil SRM and the best oxygen gas flow rate was 0.75 mL min−1. Adopting optimized conditions, limits of detection were 1.2, 0.49, 0.31, 0.33 and 0.78 μg L−1 for 28Si16O+, 29Si16O+, 31P16O+, 32S16O+ and 34S16O+, respectively. Adequate precision, accuracy and sensitivity were obtained when using the mass shift mode and recoveries for biodiesel, diesel and lubricating oil digests ranged from 95.0 to 113%. No significant differences were observed between the certified values and the ones obtained using ICP-MS/MS at a 95% confidence level.

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 novel strategy to determine As, Cr, Hg and V in drinking water by ICP-MS/MS

This work demonstrates the potential of inductively coupled plasma tandem mass spectrometry (ICP-MS/MS) in trace element analysis. Aiming Hg determination in water, sample preservation requires chloride addition, which presents formation of polyatomic interfering ions as a drawback for the determination of other analytes (e.g. As and V). Thus, the effect of chloride on As, Cr, V and Hg determination and the feasibility of using ICP-MS/MS to circumvent the interference were evaluated. A standard reference material (NIST SRM 1643e) and recoveries from tap water spiked with As, Cr, Hg and V were used for accuracy assessment and for the evaluation of matrix effects. The reaction between Hg and oxygen gas is not favorable in the octopole reaction system (ORS3). Therefore, Hg was accurately determined as a single atom ion (202Hg+) under the same analytical conditions adopted for As, Cr and V determination by monitoring 75As16O+, 52Cr16O+ and 51V16O+ while the ORS3 was pressurized with oxygen gas. The oxygen reaction mode significantly improved the accuracy for As, Cr and V, especially at the lowest spiked level, which is strongly affected by matrix effects. The signal gain in m/z 75 for a solution containing 1 μg L−1 As in 0.48 mol L −1 chloride medium was 1974% when employing single quadrupole mode. On the other hand, the signal increased only 7% when 0.75 mL min−1 O2 was used in mass shift mode proving the efficiency of the MS/MS mode. Thus, ICP-MS/MS and ORS3 pressurized with O2 gas effectively reduced the interference, and enhanced the accuracy and sensitivity for some specific isotopes in a complex medium. The limits of detection for V (51V16O+), Cr (52Cr16O+), As (75As16O+) and Hg (202Hg+) were estimated to be 2, 3, 1.6 and 38 ng L−1 at the 99.7% confidence level, respectively.

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.

Determination of sulfur in biodiesel microemulsions using the summation of the intensities of multiple emission lines

A method for the determination of sulfur in biodiesel samples by inductively coupled plasma optical emission spectrometry which uses microemulsion for sample preparation and the summation of the intensities of multiple emission lines has been developed. Microemulsions were prepared using 0.5 mL of 20% v/v HNO(3), 0.5 mL of Triton X-100, 2-3 mL of biodiesel sample, and diluted with n-propanol to a final volume of 10 mL. Summation of the emission intensities of multiple sulfur lines allowed for increased accuracy and sensitivity. The amounts of sulfur determined experimentally were between 2 and 7 mg L(-1), well below legislative standards for many countries. Recoveries obtained ranged from 72 to 119%, and recoveries obtained for the 182.562 nm line were slightly lower. This is most likely due to its lower sensitivity. Using microemulsion for sample preparation and the summation of the intensities of multiple emission lines for the successful determination of sulfur in biodiesel has been demonstrated.

Determination of trace sulfur in biodiesel and diesel standard reference materials by isotope dilution sector field inductively coupled plasma mass spectrometry.

A method is described for quantification of sulfur at low concentrations on the order of mgkg(-1) in biodiesel and diesel fuels using isotope dilution and sector field inductively coupled plasma mass spectrometry (ID-SF-ICP-MS). Closed vessel microwave-assisted digestion was employed using a diluted nitric acid and hydrogen peroxide decomposition medium to reduce sample dilution volumes. Medium resolution mode was employed to eliminate isobaric interferences at (32)S and (34)S related to polyatomic phosphorus and oxygen species, and sulfur hydride species. The method outlined yielded respective limits of detection (LOD) and limits of quantification (LOQ) of 0.7 mg kg(-1) S and 2.5 mg kg(-1) S (in the sample). The LOD was constrained by instrument background counts at (32)S but was sufficient to facilitate value assignment of total S mass fraction in NIST SRM 2723b Sulfur in Diesel Fuel Oil at 9.06±0.13 mg kg(-1). No statistically significant difference at a 95% confidence level was observed between the measured and certified values for certified reference materials NIST SRM 2773 B100 Biodiesel (Animal-Based), CENAM DRM 272b and NIST SRM 2723a Sulfur in Diesel Fuel Oil, validating method accuracy.

Interference standard: a new approach to minimizing spectral interferences in inductively coupled plasma mass spectrometry

Spectral interferences are the most critical issue in inductively coupled plasma mass spectrometry determinations. The effects of isobaric polyatomic ions on accuracy often represent an important limitation to the application of this technique to more complex matrices. This aspect is especially critical in quadrupole-based instruments and the most common approaches to minimizing the problem are based on collision-reaction cells and interfaces. In this work a new strategy based on minimizing the interfering ion signal variation is used to improve accuracy in K, As, P and Si determinations. Different from traditional internal standard methods, argon species naturally present in the plasma are used as probes to correct fluctuations in the interfering ion rather than in the analyte signal. Accuracy is significantly improved with no instrumental modification or post-plasma reactions. At the best conditions, recoveries ranging from 86 to 107% were obtained for all elements evaluated in different certified reference materials and spiking studies. Results were comparable and sometimes better than recoveries obtained with a collision–reaction interface. Adequate precision, accuracy and sensitivity were obtained while using the 38Ar+ probe even for isotopes prone to severe isobaric interferences such as 28Si.

Interference standard applied to sulfur determination in biodiesel microemulsions by ICP-QMS

Determining sulfur by inductively coupled plasma mass spectrometry (ICP-MS) is not a trivial task especially due to the intense spectral interferences caused by polyatomic ions such as 16O2+, (16OH)2+, 14N16OH2+ and 14N18O+. In this work, argon species naturally present in the plasma (interference standards, IFS) are used to minimize spectral interferences on sulfur determination in biodiesel microemulsions. An external calibration method using ratios between analytical and IFS signals is employed in all determinations. The limits of detection (LOD) calculated for the 32/36, 34/36, 34/37 and 34/38 signal ratios were 4.0, 1.0, 1.0 and 0.8 mg L-1, respectively. A standard reference material (Lubricating Oil, SRM 1848) and spiking studies in biodiesel samples were employed to check the method accuracy. Recoveries varied from 77 to 105% while using any of the IFS probes (36Ar+, 36ArH+ or 38Ar+).

Greener procedures for biodiesel quality control

Biodiesel analysis often requires large amounts of chemicals and organic solvents with consequent generation of large waste volumes. However, several environmentally friendly alternatives have been presented for assay of biodiesel quality control parameters. These approaches include direct analysis with reagentless procedures, improved sample preparation by dilution or analyte extraction with non-toxic solvents, preparation of emulsions or microemulsions, and sample mineralisation under mild conditions as well as strategies for minimization of reagent consumption and waste generation. These greener alternatives are critically reviewed, highlighting the advantages and limitations of each approach in biodiesel analysis. Alternative analytical techniques (e.g. flow analysis and electrochemical detection), predictive studies, and measurement of physicochemical parameters are also discussed.