Article pubs.acs.org/ac
Ring-Oven Based Preconcentration Technique for Microanalysis: Simultaneous Determination of Na, Fe, and Cu in Fuel Ethanol by Laser Induced Breakdown Spectroscopy Juliana Cortez and Celio Pasquini* Chemistry Institute, Department of Analytical Chemistry, UNICAMP, Caixa Postal 6154, CEP: 13087-971, Campinas , SP, Brazil ABSTRACT: The ring-oven technique, originally applied for classical qualitative analysis in the years 1950s to 1970s, is revisited to be used in a simple though highly efficient and green procedure for analyte preconcentration prior to its determination by the microanalytical techniques presently available. The proposed preconcentration technique is based on the dropwise delivery of a small volume of sample to a filter paper substrate, assisted by a flow-injection-like system. The filter paper is maintained in a small circular heated oven (the ring oven). Drops of the sample solution diffuse by capillarity from the center to a circular area of the paper substrate. After the total sample volume has been delivered, a ring with a sharp (c.a. 350 μm) circular contour, of about 2.0 cm diameter, is formed on the paper to contain most of the analytes originally present in the sample volume. Preconcentration coefficients of the analyte can reach 250-fold (on a m/m basis) for a sample volume as small as 600 μL. The proposed system and procedure have been evaluated to concentrate Na, Fe, and Cu in fuel ethanol, followed by simultaneous direct determination of these species in the ring contour, employing the microanalytical technique of laser induced breakdown spectroscopy (LIBS). Detection limits of 0.7, 0.4, and 0.3 μg mL−1 and mean recoveries of (109 ± 13)%, (92 ± 18)%, and (98 ± 12)%, for Na, Fe, and Cu, respectively, were obtained in fuel ethanol. It is possible to anticipate the application of the technique, coupled to modern microanalytical and multianalyte techniques, to several analytical problems requiring analyte preconcentration and/or sample stabilization. efore being submitted to the final stage of an analytical process, aiming at the determination of the species of interest (the analytes), laboratory samples frequently need to be conditioned or processed to achieve some desired characteristics appropriate to the selected analytical technique. It is common, for instance, that the sample needs to be processed to increase the original concentration of analytes up to, or exceeding, the limit of quantification of the analytical technique employed in the analysis. Several techniques aiming at extracting and concentrating the analytes present in a liquid sample prior to measurement are described in the literature.1−7 Frequently, these techniques are dependent on the relative affinity of the analytes for the sample matrix and extracting medium. In consequence, only a few methods are capable of simultaneously concentrating analytes with very distinct chemical properties, such as alkaline metals (e.g., Na and Li) and transition metals, such as Fe and Cu, present in aqueous samples. On the other hand, modern techniques can achieve very high selectivity and preconcentration coefficients (up to 1500-fold) and employ very low volumes of sample and extracting solvents.1−7 However, one of the major difficulties to concentrate samples at the microscale is in the reproducible handling of small quantities of the concentrated material. This fact has led to automation of the preconcentration procedure.3,5,8
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© 2012 American Chemical Society
Few preconcentration techniques are aimed at the determination of analytes by modern simultaneous multielementar microanalytical techniques that are capable of producing quantitative results directly from tiny quantities of the concentrate sample. The microanalytical technique can generate reproducible measurements directly from the concentrated sample, causing no loss in the preconcentration coefficient due to additional analyte manipulation. However, the virtuous combination of the preconcentration and microanalytical methods presently available has not been yet intensively exploited. Ring-Oven Technique. The ring-oven procedure was proposed and developed in 1954 as a microanalytical technique by Herbert Weisz.9 The technique elicited widespread interest in the 1960s through the 1970s, when it was recognized as a relevant improvement of the spot tests proposed by Feigl,10,11 because the technique imparts higher detectivity to microscale qualitative chemical assays. The ring-oven technique employs a simple apparatus (the oven) in which a disc of filter paper is adapted. As described by Weisz,9,12−14 the oven is heated to 105−110 °C, when working Received: September 21, 2012 Accepted: December 20, 2012 Published: December 20, 2012 1547
dx.doi.org/10.1021/ac302755h | Anal. Chem. 2013, 85, 1547−1554
Analytical Chemistry
Article
decreasing its precision. When a liquid sample is probed by the laser pulse, a great part of the energy is spent in vaporization of the solvent. The reduced energy of the laser pulse available for vaporization and excitation decreases analytical detectivity.29 The use of a liquid jet, produced by pumping the liquid sample at high flow rates through a narrow capillary, can contribute to minimize some of these problems.29,31,32 However, it employs larger volumes of sample (which need to be pumped continuously) and can not exploit the advantages of analyte preconcentration although, for some specific analytes and instrumentation, very low detection limits have been reported, as when the laser wavelength is strongly absorbed by some constituent added to the sample.33 Even so, the detection limits achieved are not uniform, and values obtained for some elements are still very high (typically greater than 5 μg mL−1). The detection limits for several elements reported in the literature are contradictory. The estimated range goes from 0.01 to120 μg mL−1, considering several elements, measured in steady state and jet liquid samples using diverse experimental arrangements and single pulse LIBS.31,33−39 Although these detection limits are enough for a number of applications, several of them are above the values required for many others. The best detection limits were found using more expensive dual-pulse systems or integration of many spectra (e.g., 3000).34,40−43 The use of LIBS for liquid sample analysis with laser induced fluorescence can also reach better detection limits. However, the complexity and cost of the equipment is also increased.44,45 Transfer of the Analyte from the Liquid Phase to the Solid Phase. Several papers have reported on liquid-to-solid transfer of analytes to overcome several limitations found in the direct analysis of liquids by LIBS.46−51 This type of transfer can be, in many cases, accompanied by analyte preconcentration. Preconcentration techniques can help to overcome the problems associated with the relative low detectivity of singlepulse LIBS, frequently found when applied directly to liquid samples. Electrolytic deposition of mercury on a copper solid electrode can improve the detection limit for mercury in water down to 11 ng mL−1 50. However, the sample volume is high (400 mL), and the method is limited to species that can be electrolytically reduced and deposited on the electrode surface. Inert or chemically active solid substrates can also be employed to transfer and/or concentrate the analyte. Charcoal,51 carbon planchets,46 ion exchange membranes,48 wood,47 and filter paper49 have been employed. Although the detection limits obtained are distributed over a wide range (up to 80 μg mL−1) for different analytes, values as low as 10 ng mL−1 have been reported for single pulse LIBS after analyte transfer to a solid substrate.46 Perhaps the major limitation of these approaches are in the dependence of analyte adsorption on sampling the matrix and the competition for adsorbing sites that can be found between the analyte and major constituent species present in the liquid sample. Ethanol as Fuel. Fuel ethanol is employed directly or in mixtures with gasoline to propel vehicles in Brazil and several other countries. It is constituted of a material of high purity because it corresponds to the azeotropic fraction obtained by distillation of the product resulting from the fermentation of sugar cane juice. The azeotrope, basically water and ethanol, contains 92.8% (m/m) of ethanol and 7.2% (m/m) of water. Some metals can be present at trace levels in the fuel ethanol, most originating from the sugar cane and carried through during the production process, although some metals may
with aqueous solutions. One or more drops of the sample are manually delivered to the center of the heated filter paper. The aqueous sample diffuses as a circular stain by capillarity, and the solvent simultaneously evaporates, leaving the nonvolatile analytes on the paper. To perform qualitative tests, Weisz first added some drops of a washing solution (usually diluted hydrochloric acid) followed by spraying some selective reactant on the filter paper to produce a substance easily identified by its color. The gain in detectivity arises from the fact that the washing process carries the analytes to the border of the circular wetted area, reaching and touching the inner border of the upper metallic part of the oven, where residues of the solvent are evaporated from the filter paper. The process generates a thin ring-like region on the filter paper where the analytes are concentrated and where the color reaction was developed. Weisz also proposed the quantitative use of the ring-oven technique, by direct comparison of the ring color intensities produced by samples with those produced by standards.9 Of course, at the time the ring-oven technique was proposed and developed, quantitative microanalytical techniques capable of fully exploiting the analyte preconcentration directly in the ring were still under development or, even, not yet conceived. Nowadays, there are several analytical techniques that can probe very tiny samples or a very small area of solid samples, aiming at quantitative microanalysis. Among these techniques, one can mention micro-X-ray fluorescence (XRF), microRaman, near infrared (NIR) hyperspectral imaging, microinfrared (IR) spectrometry, laser ablation inductively coupled plasma optical emission spectroscopy (ICP OES)/ICPMS, and laser induced breakdown spectroscopy (LIBS). Therefore, a new and yet unexploited field for the ring-oven technique arises when its characteristics of preconcentration of analytes is complemented by the use of a microanalytical technique in the measurement stage. Laser Induced Breakdown Spectroscopy (LIBS). The LIBS technique was introduced in the 1960s15,16 but stood up as a viable analytical method under the scenario of modern laser and detection instrumentation in the years since 1980.17,18 Presently, the technique has gained widespread interest due its quasi nondestructive, portable, stand-off, multielemental, and microanalytical characteristics.19−26 As a microanalytical technique, LIBS can probe very small areas on the order of few tens of square micrometers (typically, 80 μm2 for a 10 μm diameter spot of a focused Nd:YAG laser pulse).24−26 The technique can be employed with samples in any physical state (solid, liquid, and gaseous, including aerosols),27−29 with excellent absolute detectivity. However, the relative detectivity of LIBS is not among its more notable characteristics, especially with liquid samples. Therefore, the possibility of preconcentrating the analytes present in liquid samples before their measurement by LIBS is attractive.30 Direct Analysis of Liquid Samples by LIBS. Although the primary target for the LIBS applications is in the analysis of solid samples, a number of papers have proposed the use of the technique for liquid analysis. However, there are several disadvantages associated to the direct determination of liquid samples by the technique. If performed by focusing the laser into the bulk of the liquid, the signal intensity decreases. The emission is quenched resulting in short-lived plasmas (
