Self-absorption effects in radially and axially viewed inductively coupled plasma-atomic emission spectrometry – the key role of the operating conditions

Self-absorption effects leading to curvatures of the upper part of calibration graphs were investigated in multichannel detection ICP–AES. A dual view Optima 3000 ICP system was used to enable the simultaneous determination of 38 lines for both radial and axial viewing. Resonance and non-resonance lines were selected for both atomic and ionic lines. The concentrations of 22 standards were in the range 0.1–100 mg L^–1 and two sets of operating conditions, namely power and carrier gas flow rate, were used to evaluate their influence. It was found that these two conditions, and in particular the carrier gas flow rate, play a major role in self-absorption effects. Except for strongly absorbing lines, it was possible, under suitable conditions, to reduce or to suppress differences between self-absorption effects in radial and axial viewing, enabling extension of the range of linearity of axial viewing to higher concentrations. A diagnostic tool, based on emission line ratios, is proposed for detection of self-absorption. A calibration procedure is given for strongly absorbing lines affected by self-absorption even when operating conditions were optimized.     

Boundary‐Layer Model to Predict Chemically Reacting Flow within Heated,High‐Speed,Microtubular Reactors

Chen nozzle experiments are used to study the early‐stage unimolecular decomposition of larger molecules and, sometimes, the following chemistry. The nozzle itself is typically a small‐diameter (order millimeter), short (order 20–50 mm), heated (order 1700 K) tube (nozzle) that exhausts into a vacuum chamber where a variety of diagnostics may be used to measure gas‐phase composition. Under typical operating conditions, the velocities are high and the exhaust flow is near sonic. Quantitatively interpreting the measurements requires a model for flow within the nozzle that is coupled with reaction kinetics simulations. The present model shows that the flow can produce significant radial and axial variations in both the thermodynamic conditions and species concentrations. Thus, plug‐flow models may not be appropriate. Results show that using He as a carrier gas produces much more plug‐like flow than is the case with Ar as the carrier gas. The boundary‐layer model provides a computationally efficient approach to modeling detailed chemical kinetics within Chen nozzles. Results are illustrated using acetaldehyde decomposition kinetics.     

Performance evaluation of an axially viewed horizontal inductively coupled plasma for optical emission spectrometry

A performance evaluation of a horizontal axially viewed inductively coupled plasma (ICP) for optical emission spectrometry is presented. The main contribution of this work is the elucidation of the sources of analytical performance differences using practical diagnostics in the comparison of axial and conventional radial viewing of the ICP. Figures of merit such as detection limit, background equivalent concentration, precision, and dynamic range are compared for both viewing arrangements. The detection limit improvements with axial viewing, known from previous work in the literature, are shown to be understood in the context of the signal-to-background-ratio relative-standard-deviation-of-the-background (SBR-RSDB) theory. The usefulness of the SBR-RSDB approach as a diagnostic tool for understanding the detection limit improvement and identifying performance differences is demonstrated. This approach can be further utilized for quality control and quality assurance of instrument performance and detection limit results. Other characteristic differences between axial and radial viewing are presented including matrix effects on line signals and the magnitudes of spectral interferences from OH bands. An overall improvement factor of five in detection power was observed when using axial viewing compared with radial viewing.     

Emission Characteristics of Jet Configurations for Jet-Boosted Glow Discharge Atomic Emission Spectrometry

Comparisons of emission characteristics and analytical performance have been made between three types of jet configuration. These include cone-jet configuration, six-jet configuration, and cylinder-jet configuration. The cone-jet configuration shows the highest emission intensity among all jet configurations. Regardless of the jet configuration, the Cu II 224.70 nm emission line was found to be the most dominant of all Cu emission lines. The intensity ratios of the resonance line Cu I 324.75 to a nonresonance line Cu I 282.44 nm was 1.3 for the six-jet configuration, 2.3 for the cylinder jet, and 2.8 for the cone jet. This may indicate that self-absorption was significant in six-jet configuration. The effects of main and auxiliary gas flow rates on the emission characteristics for the cone jet configuration were also investigated. The intensity of Cu I at 324.75 and 327.40 nm decreases about 30% when the gas flow rate increased from 0 to 150 ml/min, while the intensities of Cu II 224.70 nm and the UV–Visible Cu I 510.55-, 515.32-, and 521.82-nm lines increased by a factor of 2 to 3. The decrease in intensity of the resonance line relative to the Cu II and Cu I green lines may be caused by self-absorption. The cone-jet and six-jet configurations show comparable values of precision, linearity, and detection limits, while the cylinder-jet configuration provides the worst analytical performance among three types of jet configurations. The linearity of the calibration curve was the worst in the six-jet configuration due to self-absorption.     

Matrix effects in the analysis of biological matrices by axial view inductively coupled plasma optical emission spectrometry

We are reporting observations of positive and negative variations of emission line intensities during the determination of boron and titanium in biological matrices by axial view inductively coupled plasma optical emission spectrometry with segmented charge-coupled device detection. The study included the testing of several elements (yttrium, palladium and platinum) and analytical wavelengths for internal standardization, aiming to compensate for variations in signal recovery due to matrix interferences. Human albumin was chosen as principal matrix component to assess the effect of variable chemical and instrumental operating conditions on boron response. A parametric study was performed by considering the application of two different nebulizer–aerosol chamber systems, the effect of plasma operating conditions on analyte and internal standard signals and the influence of common blood plasma electrolytes, added as salts of alkaline or alkaline earth elements. The pneumatic injection systems tested were a standard cross-flow nebulizer with a Scott type spray chamber and a concentric Meinhard type device coupled to a glass cyclonic spray chamber. The change from standard (i.e. medium RF power and relatively high aerosol carrier gas flow rate) to robust (i.e. higher RF power and lower carrier gas flow rate) conditions contributed to large, non-correlated variations in boron intensities and in some of the analyte/internal standard ratios. Significant memory effects were observed for injection of boron solutions prepared with boric acid and containing small amounts of acid, but those effects were negligible when the boron carrier compound was boronophenylalanyne. The injection of titanium solutions did not produce analyte carry-over effects. When internal standards were employed, a less effective signal compensation was consistently observed for boron at higher albumin concentrations when the difference in energies of the lines was between 4.5 and 6 eV. This effect was enhanced for some line pairs when robust conditions are employed. Differences in the response between nebulizers were minor, with a slight advantage in sensitivity for the cross-flow/Scott system. Yttrium was found to be useful for signal compensation in the determination of boron and titanium in blood and human plasma provided that the equivalent concentration of albumin in the nebulized sample dilutions was kept below 0.2% w/v. Simultaneous measurement of a reference strontium line was found to be useful as an additional verification of the response of yttrium as internal standard.     

Non-linear and non-constant variance calibration curves in analysis of volatile organic compounds for testing of water by the purge-and-trap method coupled with gas chromatography/mass spectrometry

Currently used operating conditions for analysing volatile organic compounds (VOCs) by purge-and-trap gas chromatography/mass spectrometry (GC/MS) produced non-linear calibration curves with non-uniform variance. Second-order polynomial models therefore had to be used in weighted regression analysis of measurements of replicates spiked at various concentrations. A transparent procedure based on a reported method of very low computational complexity allowed calculation of the parameters of second-order models, confidence bands of regression lines, prediction bands, and confidence intervals of discriminated analyte concentrations. Tolerance intervals were introduced for this last purpose. Critical, detection and quantification levels drawn from the calibration curves were compared with those calculated by the EPA method.     

A novel bottom-viewed inductively coupled plasma-atomic emission spectrometry

A novel optical configuration for inductively coupled plasma (ICP)-atomic emission spectrometry is presented. Plasma emission is measured axially via the bottom end of the ICP torch. Analytical performance, such as increase in signal-to-background ratio (SBR) over radially viewed ICP and linear dynamic range, is comparable to that of end-on axially viewed ICP reported in the literatures. Under typical ICP operating conditions (forward power=1.0–1.6 kW, central channel gas flow rate=0.8–1.4 l/min), SBR is generally five times or more that of radial-viewing mode (observation heights=3–20 mm) for atomic lines of elements of low to medium ionization potential (Na, K, Sr and Ba). The enhancement factor in SBR is two to four times for ionic lines (e.g. MgII) and atomic lines of elements of high ionization potential (Zn). The influence of ICP forward power and carrier gas flow rate on analyte emission intensity and SBR were also studied. Similar to radially viewed ICP, as forward power increases, the net emission intensity increases and SBR decreases. Using a constant flux of analyte aerosols, the net intensity decreases as the central channel gas flow rate increases. No trend of SBR vs. central channel gas flow rate, however, is found. The linear dynamic range starts and ends at analyte concentration 0.5–1 order of magnitude lower than the corresponding radial-viewing mode. As a result, the span of linear dynamic range is similar for all viewing modes. Matrix effects of K and Ca on atomic lines are different from those reported for end-on axially viewed ICPs, probably due to the difference in the plasma regions that were probed. The matrix effects on ionic lines, however, are similar in magnitude.     

Matrix interferences in the determination of trace elements in waste waters by inductively coupled plasma atomic emission spectrometry with ultrasonic nebulization

The use of inductively coupled plasma atomic emission spectrometry with ultrasonic nebulization (USN-ICP-AES) for determining Ag, Al, As, Ba, Bi, Cd, Co, Cr, Cu, Fe, Hg, Mg, Mn, Ni, Pb, Sb, Sr, V and Zn in complex matrices of Ca, Na, K and P in waste waters is described. Generally, depressions in the analyte emission intensity occur in the presence of concomitants. Matrix interferences can be minimized by increasing the operating power and lowering the carrier gas flow rate. However, the enhancement of the signal-to-background ratios (SBRs) shows an opposite trend. Therefore, routine analyses were performed at a compromise power setting of 1,350 W, a carrier gas flow rate of 0.8 L min(-1) and an observation height of 14 mm above the load coil and using a matrix matched calibration procedure. Limits of detection (LODs) at chosen operating conditions were at microg L(-1) levels for most of the elements studied, including mercury when KBr is added to the analyte solution to enhance sensitivity. LODs were not significantly changed in the presence of matrix elements. Recoveries for the majority of added elements from spiked waste water samples are between 93 and 105% using a matrix matched calibration.     

Inductively coupled plasma atomic emission spectrometry — Optimization of the operating conditions in the determination of trace of elements in line-rich emission matrices

Radial viewing 40.68 MHz inductively coupled plasma atomic emission spectrometer was used in the determination of Y, Sc and rare earth elements in Eu₂O₃ or Lu₂O₃ as pure rare earth matrices. The Mg II 280.270 nm/Mg I 285.213 nm line intensity ratio was measured to evaluate the robustness of the operating conditions. The operating conditions were affected by varying the incident power and sheathing gas flow rate. The carrier gas flow rate remained a constant value. The relationship between the Mg II 280.270 nm/Mg I 285.213 nm ratio and the excitation temperature was obtained. A dependence of the magnesium ratio in the pure solvent and the corresponding values in the presence of the above matrices was established.     

流动注射分光光度法测定砷的研究

本文采用自行设计制造的氢化物发生及吸收装置,将流动注射技术同氢化物发生分光光度法结合在一起,设计了一种不使用载气的流动注射分析系统并对该系统的测定条件进行了实验研究,实验用HNO_3-AgNO_3-聚乙烯醇-乙醇混合液作吸收液,结果证明该系统具有操作简便,成本低廉,分析速度快(30次/小时),灵敏度高,重现性好及线性范围可调的优点。     

小型化尖端放电原子发射光谱检测微量碘

采用尖端放电激发源和电荷耦合检测器(CCD)组装了小型化原子发射光谱仪,并将其用于检测微量碘。在酸性条件下以H_2O_2氧化碘离子产生碘蒸气,并由氩气载入尖端放电光源中,激发后产生碘原子发射光谱信号,用CCD光谱仪在206 nm处检测。经优化得到的最佳实验条件为:H_2O_2浓度为2%,尖端放电电压为90 V,载气流速为450 m L·min~(-1);检测信号稳定,相对标准偏差为2.5%,标准曲线的线性范围为0.5~500 mg·L~(-1),线性相关系数大于0.99,碘的检出限低至0.1 mg·L~(-1)。该小型化原子发射光谱仪具有简单、高灵敏度、便携式适用于现场分析等优点。     

Matrix interferences in the determination of trace elements in waste waters by inductively coupled plasma atomic emission spectrometry with ultrasonic nebulization

The use of inductively coupled plasma atomic emission spectrometry with ultrasonic nebulization (USN-ICP-AES) for determining Ag, Al, As, Ba, Bi, Cd, Co, Cr, Cu, Fe, Hg, Mg, Mn, Ni, Pb, Sb, Sr, V and Zn in complex matrices of Ca, Na, K and P in waste waters is described. Generally, depressions in the analyte emission intensity occur in the presence of concomitants. Matrix interferences can be minimized by increasing the operating power and lowering the carrier gas flow rate. However, the enhancement of the signal-to-background ratios (SBRs) shows an opposite trend. Therefore, routine analyses were performed at a compromise power setting of 1350 W, a carrier gas flow rate of 0.8 L min^–1 and an observation height of 14 mm above the load coil and using a matrix matched calibration procedure. Limits of detection (LODs) at chosen operating conditions were at μg L^–1 levels for most of the elements studied, including mercury when KBr is added to the analyte solution to enhance sensitivity. LODs were not significantly changed in the presence of matrix elements. Recoveries for the majority of added elements from spiked waste water samples are between 93 and 105% using a matrix matched calibration. Received: 13 January 2000 / Revised: 10 April 2000 / Accepted: 18 April 2000     

Low flow,externally air cooled torch for inductively coupled plasma atomic emission spectrometry with axial viewing

The torch wall is cooled largely by air passing through a cooling jacket added to the outside of a Fassel torch. The plasma is viewed axially through a cooled cone interface centered on the axial channel. The outer argon gas flow can be reduced to 7 l min⁻¹ with no compromise in performance or torch lifetime. The plasma exhibits the same ‘robustness index’ and interference effects from Na as the conventional, high-flow ICP supplied with the particular spectrometer used. Detection limits (DL) for lines at ∼200 nm are poorer by approximately a factor of two, while those for lines at ∼400 nm are actually better than values typically seen for the same lines by axial viewing of a conventional, high-flow ICP.     

Influence of the Shroud Gas Injection Configuration on the Characteristics of a DC Non-transferred Arc Plasma Torch

The characteristics of the plasma jet emanating from a dc non-transferred plasma torch is affected by many factors including arc current, type of gas, gas flow rate, gas injection configuration and torch geometry. The present work focuses on experimental investigation of the influence of shroud gas injection configuration on the I–V characteristics and electro-thermal efficiency of a dc non-transferred plasma torch operated in nitrogen at atmospheric pressure. The plasma gas is injected into the torch axially and shroud gas is injected through three different nozzles such as normal, sheath and twisted nozzles. The effects of flow rates of plasma/axial gas and arc current on I–V characteristics and electro-thermal efficiency of the torch holding different nozzles are investigated. The I–V characteristics and electro-thermal efficiency of the torch are found to be strongly influenced by the shroud gas injection configuration. The effect of arc current on arc voltage decreases with increasing the axial gas flow rate. At higher axial gas flow rate (>?45 lpm), the I–V characteristics of the plasma torch are similar irrespective of the nozzle used. The variation of electro-thermal efficiency with arc current is almost similar to that of arc voltage with arc current. As expected, the electro-thermal efficiency is increased when the axial gas flow rate is increased and at higher axial gas flow rate, it is not influenced by the arc current and shroud gas configuration. The plasma torch with normal nozzle may be better in the range of operating conditions used in this study.     

二乙基二硫代氨甲酸银体系流动注射法测定砷

本文设计了一种ＡｇＤＤＣ流动注射－分光光度法测定砷的系统。该系统采用自制的氢化物发生及吸装置，将液体流路和载气流路结合在一起。     

从基体干扰角度进行电感耦合等离子体原子发射光谱分析条件的优化

本文探讨了不同分析条件下基体干扰效应的分布规律,从消除基体干扰效应角度对ICP-AES操作条件的优化进行了讨论。结果表明,在一般分析条件下,典型基体元素的零干扰点主要出现于10～15mm的观察区域。在此区间仔细选择观察高度,同时结合入射功率、载气流量的调整及加入基体缓冲剂,可以将基体的影响减至最小。     

Evaluation of self-absorption of manganese emission lines in Laser Induced Breakdown Spectroscopy measurements

This paper is part of a more general study aimed to the determination of the best experimental procedures for reliable quantitative measurements of Fe–Mn alloys by LIBS. In this work, attention is pointed on the self-absorption processes, whose effect deeply influences the LIBS measurements, reflecting in non-linear calibration curves. The effect of self-absorption on the line intensity can be quantified by defining a self-absorption coefficient, that measures the deviation of the line intensity from the linear extrapolation of the curve of growth in the optically thin regime. The authors demonstrated in a previous paper that self-absorption coefficients could be calculated once the electron density of the plasma is known and the Stark coefficients of the lines are available. However, when the Stark coefficients of the lines of interest are not known, a different approach is needed. In this work a new method for evaluation of self-absorption coefficients in LIBS measurements is presented, which does not require the knowledge of Stark coefficients. In order to understand the basic principles and setting out the theoretical tools that will be used for the analysis of the alloys, a preliminary study was done on pure Mn; LIBS spectra were acquired in different experimental conditions, at different laser energies and different delays after the laser irradiation of the sample. Moreover, collinear double pulse measurements were also performed. Analytical relations were derived and experimental procedures devised for evaluation of the self-absorption coefficients of several Mn lines, which are important for characterization and control of the experimental conditions in which the analysis is performed.     

Solid-phase microextraction and gas chromatography-mass spectrometry for the determination of polycyclic aromatic hydrocarbons in environmental solid matrices

A solid-phase microextraction (SPME) and gas chromatography-mass spectrometry method for determining polycyclic aromatic hydrocarbons (PAHs) in environmental solid matrices is developed. Investigated matrices include seaweed (Undaria pinnatifida and Himanthalia elongata), humic substances (isolated from a wetland out-flow and purchased from Aldrich), and soil. Optimal conditions for a good SPME efficiency of 16 hydrocarbon compounds are obtained using a 100- micro m polydimethylsiloxane fiber directly immersed in aqueous carrier medium. The method is remarkable for presenting short extraction times and considerably high sensitivities. The SPME results obtained by using internal calibration give the total analyte concentration based on the identical partitioning behavior of native and spiked pollutants. The detection limits range from 0.001 to 0.1 mg of PAH per kilogram of dry matrix. SPME external calibration provides information regarding freely dissolved analytes. The detection limits range from 0.001 to 0.05 micro g of PAH per liter of carrier medium. The SPME with external calibration procedure can be applied to measure free concentrations of a target compound spiked into a carrier medium and onto a matrix. Based on a comparison of results obtained for the two samples, the partitioning of the target analyte between the matrix and the carrier medium is calculated.     

Multivariate optimization of an axially-viewed inductively coupled plasma multichannel-based emission spectrometer for the analysis of environmental samples

An experimental design procedure was applied to optimize the operating conditions of an axially-viewed inductively coupled plasma emission spectrometer instruments equipped with echelle optics with cross dispersion and charge transfer device. The multivariate effect of carrier gas flow rate and r.f. power on several analytical figures was investigated and discussed. Both ultrasonic and pneumatic nebulization were used. For the final choice of the optimum, different criteria were taken into account, mainly plasma robustness, instrumental precision, analyte and background net emission, detection limits and signal-to-background ratios. It was found that the use of moderate power (1100W) and mean carrier gas flow rate (0.75 L/min) allows to obtain sufficient plasma robustness, satisfactory precision, and excellent signal-to-background ratios and limits of detection, favorable for ultratrace element determinations in environmental matrices.     

微波消解-微波等离子体炬原子发射光谱测定啤酒中的铜、锌、铁、锰、硒、锶

建立了微波消解-微波等离子体炬原子发射光谱(MPT-AES)法测定啤酒中微量元素。考察了微波前向功率、工作气流量、载气流量等参数,确定了MPT-AES法测定各元素的最佳实验条件。在该条件下铜、锌、铁、锰、硒、锶的检出限分别为7ng.mL-1、46ng.mL-1、13ng.mL-1、8ng.mL-1、1.2ng.mL-1、5.6ng.mL-1。相对标准偏差(RSD)均在0.9%～4.8%之间,线性范围分别为0.1～100μg.mL-1、0.5～100μg.mL-1、0.5～100μg.mL-1、0.1～100μg.mL-1、0.01～10μg.mL-1、0.05～100μg.mL-1,加标回收率均在96%～110%之间。