Comparison of spark OES methods for analysis of inclusions in iron base matters

A statistical model for the evaluation of the emission intensities of single spark discharge spectra on three different spark OES instruments (ARL 4460, OBLF-QSN 750, SpectroLab) was developed for the analysis of non-metallic inclusions in steel. In total, the raw measurement data consists of several thousand data points (intensities), about 2000 per element channel. A histogram (frequency distribution of pulse height intensities) of inclusion-forming elements like Al, Ti and Mn performed in intensity classes and the related frequencies contain the information of the measurement.Measurement of a pure iron sample with slag powder rubbed into the surface by single spark emission spectrometry changes the originally (nearly) symmetrical histogram for inclusion related element signals to higher intensities. Thus, the known asymmetry of the histograms for steel samples is explained by the presence of inclusions.A sum of Gaussian fits representing the histogram was expected to express the different elemental species and inclusion sizes by using the variables mean, standard deviation and area of the peak containing the information of the measurement. Even a less complex approximation using a sum of two Gaussian fits is not practicable for the Al, Ti and Mn channel and the three spectrometers due to too many variables in the fitting function resulting in a large scattering. Coincidences of high signals in different element channels could be seen only in terms of MnS.A further attempt was to form the second differential of the logarithmic normal distribution and to compare the resulting partial areas below the functional graph to get information about the asymmetry of the fitting function of the original histogram. Two different procedures to evaluate the effect of the inclusion concentration on the size of the partial areas were applied to measurements on a set of synthetic samples with different concentrations of aluminium oxide in micro alloy steel. The results of both evaluation procedures correlate significantly with the inclusion concentrations. Electronic Supplementary Material is available if you access this article at http://dx.doi.org/10.1007/s00216-002-1595-1. On that page (frame on the left side), a link takes you directly to the supplementary material.     

内标法在ICP－AES中的应用研究──Ⅰ．信号的相关性与精密度的改善

本文介绍了选择一系列具有不同物理性质的元素谱线来研究分析元素与内标元素间的内在联系。分别研究了背景之间、各谱线的信号与背景之间、分析信号与内标线信号之间的相关性。结果表明，只有当连续辐射背景较强时前两者才有较高的相关性。采用分析线信号与内标线的信号比可提高分析精密度，而内标元素的物理性质不必与分析元素的相匹配。同时详细分析了信号和背景波动的机制。     

Dual-pulse Laser Induced Breakdown Spectroscopy for analysis of gaseous and aerosol systems: Plasma-analyte interactions

Dual-pulse LIBS has been previously investigated to a large extent on solid and liquid phase analytes, where it has been demonstrated to significantly enhance atomic emission signal intensity, and more importantly, to enhance the analyte peak-to-base and signal-to-noise ratios. This study focuses on the effects of an orthogonal dual-pulse laser configuration on the atomic emission response for both purely gaseous and calcium-based aerosol samples. The gaseous sample consisted of purified (i.e. aerosol free) air, from which nitrogen and oxygen spectral emission lines were analyzed. Measurements for the gaseous system resulted in no notable improvements with the dual-pulse configuration as compared to the single-pulse LIBS. Experiments were also conducted in purified air seeded with calcium-rich particles, which revealed a marked improvement in calcium atomic emission peak-to-base (∼ 2-fold increase) and signal-to-noise ratios (∼ 4-fold increase) with the dual-pulse configuration. In addition to increased analyte response, dual-pulse LIBS yielded an enhanced single-particle sampling rate when compared to conventional LIBS. Transmission measurements with respect to the plasma-creating laser pulse were recorded for both single and dual-pulse methods over a range of temporal delays. In consideration of the spectroscopic and transmission data, the plasma-analyte interactions realized with a dual-pulse methodology are explained in terms of the interaction with the initially expanding plasma shock wave, which differs between gaseous and particulate phase analytes, as reported in a recent study [V. Hohreiter, D.W. Hahn, Calibration effects for laser-induced breakdown spectroscopy of gaseous sample streams: analyte response of gas-phase species versus solid-phase species, Anal. Chem. 77 (2005) 1118–1124].     

Provenance study of volcanic glass using 266–1064 nm orthogonal double pulse laser induced breakdown spectroscopy

Double pulse laser induced breakdown spectroscopy in orthogonal configuration was used for the analysis of twelve samples of volcanic glass. Raw material and artifact samples originated from Czech, Slovak, German, Hungarian, Greek, Turkish, and Ukrainian sites. The primary 266 nm laser beam was focused onto a sample area of about 0.1 mm in diameter at the optimised energy of 10 mJ resulting in only very slight sample damage, almost unrecognizable even by a microscope. The secondary 1064 nm laser beam, positioned parallel to the sample surface and focused onto the intersection with the primary beam, induced a spark with enhanced radiation at the optimised energy of 100 mJ. Measurement of emission lines selected on basis of chemical composition, signal intensity, signal-to-background ratio, and minimum interference from the surrounding spectra: Si(I) 288.16 nm, Mg(II) 279.55 nm, 280.27 nm, Mg(I) 285.21 nm, Ca(II) 317.93 nm, Na(I) 589.59 nm, Al(I) 308.22 nm, Fe(II) 259.94 nm, Ti(II) 334.94 nm, Sr(II) 407.77 nm, Ba(II) 455.40 nm, K(I) 769.90 nm, provided experimental data sufficiently sensitive to differentiate the properties of the studied samples. Rare earth elements were not detected even though the double pulse technique is more sensitive than the single pulse variant. Visualisation methods of multidimensional statistical analyses such as radar chart, Chernoff faces, scatterplots, and the Spearman correlation matrix provided successful differentiation of the sample groups and/or particular samples by their origin.     

Peak purity assessment by matrix projection for spectral line selection and background correction in inductively coupled plasma optical emission spectrometry

This paper describes an improvement of the procedure, proposed previously (Spectrochim. Acta, Part B, 48 (1993) 1517), to provide information about multiplicative and spectral interferences at lines selected for the analysis of samples. The new version of the procedure only requires intensity scans across an analyte line when three solutions are aspirated: a standard solution, the sample solution and the sample spiked with the analyte standard solution. The third solution can be omitted if multiplicative interference is not of interest. Like the initial method, the present procedure does not require or assume information about the sample composition nor is the preparation of solutions of suspected interferents required. Figures of merit, namely the structured background factor, and the overall spectral interference have been devised and are used with the true detection limit at a line for line evaluation and selection. As the nonanalyte concomitant contribution to the measured spectrum is estimated, automatic background correction is possible. The proposed method not only allows selection of the spectral line, but can give a good estimate of the analyte concentration prior to a full quantitative analysis.     

Comparison of univariate and multivariate calibration for the determination of micronutrients in pellets of plant materials by laser induced breakdown spectrometry

The application of laser induced breakdown spectrometry (LIBS) aiming the direct analysis of plant materials is a great challenge that still needs efforts for its development and validation. In this way, a series of experimental approaches has been carried out in order to show that LIBS can be used as an alternative method to wet acid digestions based methods for analysis of agricultural and environmental samples. The large amount of information provided by LIBS spectra for these complex samples increases the difficulties for selecting the most appropriated wavelengths for each analyte. Some applications have suggested that improvements in both accuracy and precision can be achieved by the application of multivariate calibration in LIBS data when compared to the univariate regression developed with line emission intensities. In the present work, the performance of univariate and multivariate calibration, based on partial least squares regression (PLSR), was compared for analysis of pellets of plant materials made from an appropriate mixture of cryogenically ground samples with cellulose as the binding agent. The development of a specific PLSR model for each analyte and the selection of spectral regions containing only lines of the analyte of interest were the best conditions for the analysis. In this particular application, these models showed a similar performance, but PLSR seemed to be more robust due to a lower occurrence of outliers in comparison to the univariate method. Data suggests that efforts dealing with sample presentation and fitness of standards for LIBS analysis must be done in order to fulfill the boundary conditions for matrix independent development and validation.     

Specific rotation measurements from peak height data, with a Gaussian peak model

A method is described whereby a sensitive laser-based polarimeter can be used to make very accurate and precise specific rotation measurements on microgram quantities of optically active materials. Flow-injection or liquid chromatography systems provide reproducible introduction of the sample into the polarimetric system. If a Gaussian distribution of the analyte concentration is assumed, the peak height can be used in the determination of specific rotation. This method provides a direct calibration with an absolute standard which yields more accurate and precise results than those obtainable by using peak area.     

In-situ determination of cross-over point for overcoming plasma-related matrix effects in inductively coupled plasma-atomic emission spectrometry

A novel method is described for overcoming plasma-related matrix effects in inductively coupled plasma-atomic emission spectrometry (ICP-AES). The method is based on measurement of the vertically resolved atomic emission of analyte within the plasma and therefore requires the addition of no reagents to the sample solution or to the plasma. Plasma-related matrix effects enhance analyte emission intensity low in the plasma but depress the same emission signal at higher positions. Such bipolar behavior is true for all emission lines and matrices that induce plasma-related interferences. The transition where the enhancement is balanced by the depression (the so-called cross-over point) results in a spatial region with no apparent matrix effects. Although it would be desirable always to perform determinations at this cross-over point, its location varies between analytes and from matrix to matrix, so it would have to be found separately for every analyte and for every sample. Here, a novel approach is developed for the in-situ determination of the location of this cross-over point. It was found that the location of the cross-over point is practically invariant for a particular analyte emission line when the concentration of the matrix was varied. As a result, it is possible to determine in-situ the location of the cross-over point for all analyte emission lines in a sample by means of a simple one-step sample dilution. When the original sample is diluted by a factor of 2 and the diluted sample is analyzed again, the extent of the matrix effect is identical (zero) between the original sample and the diluted sample at one and only one location — the cross-over point. This novel method was verified with several single-element matrices (0.05 M Na, Ca, Ba and La) and some mixed-element matrices (mixtures of Na–Ca, Ca–Ba, and a plant-sample digest). The inaccuracy in emission intensity due to the matrix effect could be as large as − 30% for conventional measurements in the normal analytical zone, but is reduced to within 5% with this new method. The major currently known limitation is that the accuracy of the method is highly sensitive to fluctuations and noise in the vertical emission-intensity profile, so the stability of the ICP system must be controlled to preferably within 1%.     

Interference from easily ionizable element matrices in inductively coupled plasma emission spectrometry—a spatial study

Utilizing a photodiode array based spatial profiling spectrometer an extensive spatial characterization of the effect of an excess of an easily ionizabie element (EIE) on analyte emission in the inductively coupled plasma has been carried out. Significant spatial shifts, enhancements and depressions are induced on analyte emission by the presence of an EIE and it is shown that limited fixed height measurements may be seriously misleading with respect to interpretation of the effects of ElEs. In general, the addition of excess EIE enhances emission in the lower regions of the analyte channel for both atom and ion lines and in the upper regions of the analyte channel, emission intensity is depressed for both atom and ion species. Radial emission maps (for CaI and CaII) reveal that the enhancements low in the discharge occur along the off axis boundary of the analyte channel and the plasma discharge. Higher in the discharge CaI emission is generally depressed but CaII emission undergoes a central axis depression coupled with an off axis enhancement. Overall the data suggests that low in the discharge enhancement appears to be the result of increased collisional excitation and that higher in the discharge ambipolar diffusion may exert an influence on the spatial distribution of analyte species.     

A processing method enabling the use of peak height for accurate and precise proton NMR quantitation

In NMR, peak area quantitation is the most common method used because the area under a peak or peak group is proportional to the number of nuclei at those frequencies. Peak height quantitation has not enjoyed as much utility because of poor precision and linearity as a result of inconsistent shapes and peak widths (measured at half height). By using a post‐acquisition processing method employing a Gaussian or line‐broadening (exponential decay) apodization (i.e. weighting function) to normalize the shape and width of the internal standard (ISTD) peak, the heights of an analyte calibration spectrum can be compared to the analyte peaks in a sample spectrum resulting in accurate and precise quantitative results. Peak height results compared favorably with ‘clean’ peak area results for several hundred illicit samples of methamphetamine HCl, cocaine HCl, and heroin HCl, of varying composition and purity. Using peak height and peak area results together can enhance the confidence in the reported purity value; a major advantage in high throughput, automated quantitative analyses. Published in 2009 by John Wiley & Sons, Ltd.     

The vertical spatial characteristics of analyte emission in the inductively coupled plasma

Utilizing a photodiode array based spatial profiling spectrometer vertical spatial profiles of analyte emission have been measured for a large number of neutral atom and ion lines in the inductively coupled plasma. The lines can be subdivided into two basic categories. One group (called “ soft ” lines after Boumans) have spatial emission behaviour that is very dependent on power, aerosol flow and analyte excitation and ionization characteristics. The second group (called “ hard ” lines after Boumans) have spatial emission behaviour that is relatively insensitive to all of the above parameters. In all cases ion lines have hard line behaviour as do the more energetic atom lines. Under fixed ICP conditions all hard lines have their peak emission at essentially the same position in the discharge which is always higher in the discharge than that for soft lines. It is also shown that the spatial behaviour of soft lines can be directly correlated with normal temperature.     

Spark ablation as sample introduction device for ICP-AES: Analysis of free cutting steels

A method for ICP analysis of free cutting steels after spark ablation is presented. The partial removal of the nonconductive inclusions by etching with hydrochloric acid leads to a sufficient conductivity of the sample surface and achieves a high efficiency of the presparking process. This is essential for the analytical evaluation of the spark. The etching time of 30 s enables removal of the sulfides in a layer smaller than the one concerned by the presparking process. The reproducibility of the measurements is within 2–5% depending on the element. After etching of the standard samples no significant differences in the results achieved by spark ablation-ICP and spark emission were observed. They were also in good agreement with the ICP analysis of solutions of the corresponding samples.Dedicated to Professor Günther Tölg on the occasion of his 60th birthday     

Effort on calibration of infrared spark ablation of copper with synthetic copper standards

Two types of copper samples, compact certified copper reference materials and calibration samples prepared from liquid doped, pressed copper powders, were studied in terms of accuracy of obtained calibration functions originating from infrared spark ablation. Additionally, corresponding particle size distributions of the aerosols from infrared spark ablation were recorded. It is shown that the differences in quantification results, originating from the two sets of calibration functions, could not mainly be ascribed to different particle size distributions of the two copper sample types. Possible other causes, as different ablation rates, parts of melting and differences of the chemical constitutions of the two sample types were explored.     

Some spatial effects observed in the axially viewed filament argon microwave induced plasma with solution nebulization

Spatial profiles of analyte emission in an axially viewed argon filament microwave induced plasma sustained in the TE₁₀₁ rectangular cavity have been measured along a discharge tube cross-section for neutral atoms as well as ion lines of several elements. The filament diameter was approximately 1 mm. The analyte solution was introduced by means of an ultrasonic nebulizer without desolvation. The radial emission distribution depends on the operating parameters and is different for each of the analytes examined. Spatial distributions of excitation temperature (4000–6000 K) measured with Ar I lines by the Boltzmann plot method as well as electron temperature (6000–8000 K) by line to continuum emission ratio measurements at Ar I 430 nm and electron number density (1–1.5×10¹⁵ cm⁻³) by the Stark broadening method of the H_β line were determined to support the evidence of plasma processes. In the presence of excess sodium the enhancement of emission intensity and its shift to the plasma center appears to be the result of increased analyte penetration to the plasma. Changes in spatial emission profiles for Ca atoms and ions suggest that for this element ambipolar diffusion may be important as an additional interference mechanism. A possibility of minimizing spectral interferences from argon emission lines by choosing an off-axis plasma region for emission intensity measurements is indicated.     

Use of tandem calibration for analyzing steels and alloys by inductively coupled plasma atomic emission spectrometry

A method is proposed for plotting calibration graphs by reference analyte solutions (tandem calibration) for the analysis of steels and alloys by inductively coupled plasma atomic emission spectrometry (ICP-AES) with spark ablation. The use of spectral lines of analytes and an internal standard with similar values of sums of ionization and excitation energy ensures an excellent repeatability and accuracy of the results of analysis using tandem calibration. A nebulizer chamber for the simultaneous introduction of solutions and solid sample aerosols into inductively coupled plasma (ICP) is designed. The optimal parameters of the introduction of sample aerosols and a method of plasma observation are chosen; the requirements for the choice of the analytical lines of analytes are proposed.     

Rapid aqueous sample extraction of VOCs: effect of physical parameters

Rapid aqueous sample extraction (RASE) devices were constructed and characterized using m-xylene as a test analyte. Extraction of m-xylene from aqueous samples was studied under many different conditions, independently varying extractor volume, extraction gas flow rate, temperature, pressure, sample volume, and sample concentration. Gas samples were analyzed as controls to determine the non-extraction (transport) component of the analyte pulse width. The extraction of analyte from water to the gas phase took proportionately longer (compared to transport) for RASE apparatus that had a volume greater than 10 ml. An order of magnitude change in RASE volume resulted in larger than an order of magnitude change in extraction time and total analyte pulse width. The flow rate of the extraction gas had a much larger effect on a RASE apparatus with a volume greater than 10 ml. For these large extractors, both extraction time and total analyte pulse width decreased by a factor of 4 for a flow increase from 40 to 120 ml min(-1). There was little change at higher flow rates, or for extractors with smaller volumes. Temperatures below 40 degrees C resulted in large increases in the pulse duration due to broadening during transport. The temperature effect on extraction time was only a factor of 2 over a range from 25 to 85 degrees C. Pressure also had only a relatively small effect, increasing extraction time and total pulse width by a factor of 2 over a range from 12 to 34 PSI. There was no observed change in either extraction time or total pulse width when the sample volume injected varied from 10 to 1000 mul, or over a concentration range from 170 to 17 000 mug l(-1). RASE apparatus were capable of complete extraction of analyte from water in less than 5 s under optimized conditions.     

Determination of arbitrary concomitant absorption and emission distributions in spark discharges using the Abel inversion

Neglect of absorption in high temperature spark discharges can result in severe degradation of computational fidelity of spatially-resolved emission profiles. To deal with this problem, an algorithm particularly well suited for array detectors is introduced, which allows diagnosis of emission profiles and deconvolution of absorbance and emission distributions. With this development, more accurate spatial distributions of both emitting and absorbing species can be obtained than with previous techniques. An example making use of literature data is given.     

Effects of matrix on spatial profiles of emission from an inductively coupled plasma

Effects of easily ionizable elements and other concomitants on spatial profiles of analyte emission were studied in an inductively coupled plasma. The measuring system consists of a self-scanning linear photodiode array mounted vertically in the focal plane of a monochromator and a microcomputer for data acquisition and processing. Easily ionizable elements had complex effects on spatial profiles of analyte emission depending on the excitation characteristics of the spectral lines and the operating parameters of the plasma. On the other hand, sulfuric acid simply depressed the emission over the whole observation height independent of the operating parameters. These data are useful for a better understanding of the complex interference effects found in optimisation experiments and may add to an understanding of the excitation mechanisms responsible for them.     

火花直读光谱法测定铝及铝合金中的钒含量

通过对测定条件的优化,采用高钒系列标准样品建立分析程序,完成类型标准化,校验后再进行测定,建立了火花直读光谱法对铝及铝合金中的钒含量进行测定的方法。选用E2235高钒标准样品进行标准化,所得测定值与标准值基本一致,相对标准偏差为2.9%(n=8)。结果表明,当测定元素含量不超过0.020%时,方法准确、可靠,适合于铝及铝合金中微量钒含量的测定。