Alemayehu Asfaw

Dual mode sample introduction for multi-element determination by ICP-MS: the optimization and use of a method based on simultaneous introduction of vapor formed by NaBH4 reaction and aerosol from the nebulizer

A method has been developed utilizing dual mode sample introduction of a commercial Multi Mode Sample Introduction System (MSIS™) for simultaneous multi-element (As, Bi, Cd, Co, Cu, Ni, Pb, Sb, Zn) determination by ICP-MS. Vapor formed by NaBH4 reaction and aerosol from the nebulizer were introduced simultaneously into the plasma. The effects of parameters that are expected to affect hydride generation of As, Bi and Sb, i.e. concentrations of HNO3, NaBH4 and thiourea, plus flow rates of sample and NaBH4, were evaluated using Plankett–Burman experimental design. Then, the most significant of the parameters were optimized using a central composite design. Further, as optimum values were not the same for all the elements, a response optimizer was used to obtain a compromised optimum. Except for Pb, the sensitivity of the ‘classical’ hydride generating elements (As, Bi and Sb) increased significantly using the dual mode sample introduction, compared to pneumatic nebulization. The factors of sensitivity increase for As, Bi and Sb were 77, 33 and 56, respectively. For the other elements (Cd, Co, Cu, Ni and Zn), that are also reported to react with NaBH4 forming volatile species, no significant change in sensitivity was observed. The limits of detection obtained for As, Bi, and Sb using dual mode sample introduction were 7, 15 and 10 pg mL−1, respectively. The use of thiourea as a pre-reducing agent and masking agent was also investigated. The accuracy of the method was verified by analyzing various certified reference materials (CRMs): SRM1572 ‘Citrus leaves’, SRM1575 ‘Pine Needles’, SRM1643e ‘Trace Elements in Water’, GBW07602 ‘Bush Branches and Leaves’ and GBW07601 ‘Human hair’.

Combination of a multimode sample introduction system with a pre-evaporation tube to improve multi-element analysis by ICP-OES

A pre-evaporation tube (heated to 400 °C) was inserted between a multi-mode sample introduction system (MSIS) and the plasma torch in inductively coupled plasma optical emission spectrometry (ICP-OES) to improve the simultaneous determination of hydride-forming and other elements. Multivariate optimisations were conducted to find operating conditions improving plasma robustness. Under these conditions and compared to conventional sample introduction with a pneumatic nebulizer and spray chamber, using MSIS with a pre-evaporation tube (PET) improved sensitivity and detection limits for hydride-forming elements (As, Sn, Hg, Sb and Bi) and other elements as well. In order to obtain a significant improvement for all elements, irrespective of hydride formation or not, the pneumatic nebulizer was replaced by an ultrasonic nebulizer (USN) and the same experimental conditions were used for the simultaneous introduction of aerosol and hydrides (from the MSIS) into a PET. With USN-MSIS-PET, the figures of merit of ICP-OES improved by over an order of magnitude for both hydride-forming elements and other elements compared to those with a conventional sample introduction system. Furthermore, using either PN-MSIS-PET or USN-MSIS-PET increased plasma robustness, likely as a result of the hydrogen gas by-product of hydride generation, which was reported to increase energy transfer within the plasma, and from the introduction of water vapor (with PET), which was reported to improve plasma stability as well as sensitivity. Good agreement with certified values was obtained when this approach was applied to the determination of hydride-forming and other elements in environmental certified reference materials.

A new demountable hydrofluoric acid resistant triple mode sample introduction system for ICP-AES and ICP-MS

A new sample introduction system with a triple mode function was developed by modifying a commercially available cyclonic spray chamber and combining it with a commercial parallel path nebulizer. The system can be used for nebulization only, vapor generation (hydride or cold vapor) only and both together (dual mode). The introduction system is very practical in use, as it can be dismantled from the bottom side and all the inserts can be removed and cleaned easily after the end of analysis. Unlike another commercial multimode system, this new one is HF-resistant and can be used with all acids. The analytical performance of the new HF-resistant triple mode sample introduction system (HFR-TMSIS) was studied by coupling it with ICP-AES and ICP-MS and comparing the performance with the conventional single mode HF-resistant sample introduction systems: i.e. a V-groove nebulizer with Sturman–Masters spray chamber for ICP-AES and cross flow nebulizer with double pass spray chamber for ICP-MS. The performance of the new system was studied using the various modes, determining As, Cd, Cu, Fe, Mn and Pb using ICP-AES and Cd, Cu, Hg and Pb using ICP-MS. In the first case NaBH4 was used for hydride generation (of As), and in the second case SnCl2 was used for generation of cold vapor (Hg). For both ICP-AES and ICP-MS, the use of HFR-TMSIS improved the analytical performance of both As and Hg in hydride and cold vapor generation mode, respectively, and also in the dual mode compared to the conventional single mode systems. For the other elements the figure of merits of nebulization and dual mode were comparable with the conventional systems. Cold vapor generation mode (for the determination of Hg) and dual mode (for simultaneous determination of cold vapor of Hg together with other elements) were used for the analysis of various CRMs (digested with HF + HNO3 + H2O2 or HNO3 + H2O2) by ICP-MS.

Solid sampling electrothermal vaporization inductively coupled plasma optical emission spectrometry for discrimination of automotive paint samples in forensic analysis

Pieces of automotive paint often constitute trace evidence at crime scenes, especially in hit-and-run cases. As minute amounts of sample generally are available for forensic analysis, sensitive direct analysis methods are preferable to multi-step methods that may lead to sample contamination and/or analyte loss. Such a simple method was developed for discrimination of automotive paint samples. It involves solid sampling electrothermal vaporization (ETV) coupled to inductively coupled plasma optical emission spectrometry (ICP-OES) and multivariate analysis. A total of 18 automotive paint fragments were collected by scraping six red-colored car wrecks from three sides (front, side and back). Two replicates of each paint fragment were analysed. In each case, 0.8–2.0 mg paint was directly weighed onto an ETV graphite boat, which was then inserted into the ETV furnace for vaporization at 2200 °C. The vaporization products were carried into the ICP by Ar carrier gas containing dichlorodifluoromethane reactant gas. For classification of paint samples according to their sources, several elements were simultaneously monitored. No sample pretreatment or quantification of the elements was needed. Point-by-point normalization of the emission signal of each analyte to an Ar emission line was carried out, followed by integration of the resulting transient signal. The data matrices composed of the integrated areas (divided by sample mass) for the different elements were then subjected to principal component analysis (PCA), cluster analysis and linear discriminant analysis (LDA). After identifying the major contributing variables by PCA, LDA was performed and showed that a combination of three/four variables from Cr, Pb, Sn, and Zn correctly assigned each of 18 red paint samples to the correct source car. Hence, solid sampling ETV-ICP-OES, which only requires a very small amount of sample, is a simple technique for forensic automotive paint analysis, i.e. for the identification of the make and model of a car in question.

Ultrasonic nebulization with an infrared heated pre-evaporation tube for sample introduction in ICP-OES: application to geological and environmental samples

An improved sample introduction system was developed for inductively coupled plasma optical emission spectrometry (ICP-OES). The heater/condenser (HC) of a commercial ultrasonic nebulizer (USN) system was replaced by a pre-evaporation tube (PET) and a sheathing device. The aerosol exiting the USN was heated to 400 °C by infrared (IR) heating. Multivariate optimizations were conducted under robust conditions to find operating conditions improving the detection limit. Under optimum conditions and compared to conventional pneumatic nebulization, 10–25 fold improvement in sensitivity and detection limit was obtained for a range of elements using USN-PET(IR), including elements that are removed in the HC (such as Hg and B) of USN-HC. Compared to USN-PET where the PET is heated with heating tape, and to conventional USN-HC, USN-PET(IR) provided a two-fold improvement in sensitivity and detection limit. The improvement was more significant for ionic emission lines than atomic emission lines. Plasma robustness, measured by the Mg II/Mg I intensity ratio, also increased significantly, enabling the accurate multi-element analysis of geological and environmental samples using an external calibration, with an Ar emission line for internal standardization. Moreover, compared to an identical USN-PET set-up operated with the same aerosol carrier gas flow rate, but with heating tape, USN-PET(IR) significantly reduced the washout time during the analysis of geological materials, making it essentially independent of the element (with the possible exception of Hg) and analyte concentration (at least up to 400 mg L−1), which suggests that it now mostly arises from the dead volume of this sample introduction system.

Solid sampling ETV-ICP-OES to study the distribution of elements in clay and soil samples for mineral exploration

A simple and fast method, using solid sampling electrothermal vapourization inductively coupled plasma optical emission spectrometry (ETV-ICP-OES), was developed to determine the distribution of elements in clay separates and soil samples from across the Talbot Lake VMS Cu-Zn prospect, in the Flin Flon-Snow Lake terrane, Manitoba, Canada in order to locate the undercover ore deposit, which is buried under Palaeozoic dolomites and Quaternary till. In the development of the method, the mass of sample, the mass of carrier agent (polytetrafluoroethylene, PTFE) or flow rate of reactant gas (dichlorodifluoromethane (R12)), the carrier and bypass gas flow rates and the temperature program were optimised. Under optimal conditions and with a four-step ETV temperature program, the distribution of the pathfinder elements (Zn, P, S and I) in clay separates and soils showed clear anomalies at 400 and 650 m. The results for Zn and P are in very good agreement with results obtained, following aqua regia (AR) digestion, by ICP mass spectrometry (ICP-MS) by Anglo American Exploration Division (AA-ED). Moreover, the distributions of S and I could be precisely determined (these elements were not reported in the AA-ED study). Using 0–4 mg of AA-ED S5 standard mixed with 2 mg PTFE or with 4.1 ml/min R12 as reactant gas, and using internal standardisation with an argon emission line, calibration curves were obtained that, when applied to Talbot clay separates and soil samples, yielded Zn, S and P concentrations in agreement with AR-ICP-MS results previously obtained by AA-ED. Hence, ETV-ICP-OES completely eliminates the need for clay separation and for extraction or digestion of samples prior to analysis, which significantly simplifies the analysis of geochemical exploration samples.

Towards the use of ICP-OES for the elemental analysis of organic compounds such as glucosamine

Inductively coupled plasma optical emission spectrometry with a long demountable torch (whose outer tube is 2.5 cm longer than a standard torch) and a conventional pneumatic nebulisation system was applied to the determination of C, H and O concentrations from an organic compound dissolved in water. The sample solution was simply aspirated directly into the plasma without any sample pre-treatment (such as desolvation or degassing). The long torch was required to significantly decrease the background arising from entrained air. Good linearity was obtained for C, N, H and O upon calibration with standard solutions prepared from ultrapure tris(hydroxymethyl)aminomethane. A weighed blank correction was applied to compensate for the contribution from water, especially in the cases of H and O. Under these conditions, the detection limits for C, N, H and O were, respectively, 0.2, 50, 1000 and 2000 μmol of analyte per g of solution. Accurate concentrations, according to a Students t test at the 95% confidence level, were measured for C, H and O in a solution of D-glucosamine hydrochloride. No internal standardisation was necessary. However, the N concentration was biased high, irrespectively of the N emission line used (from 149.262 to 593.178 nm), which rules out spectroscopic interference and will be further investigated. Nonetheless, the possibility of accurately determining C, H and O simultaneously with trace elements makes the approach quite promising. It is much simpler than alternate methods that require derivatisation of the compound prior to detection.