Matrix effects of easily ionizable elements and nitric acid in high-power microwave-induced nitrogen plasma atomic emission spectrometry

Matrix effects of Na, Ca and nitric acid in high-power microwave-induced nitrogen plasma atomic emission spectrometry were investigated with a number of atomic and ionic lines covering a wide range of energies. The plasma was sustained in an Okamoto cavity at atmospheric pressure by using nitrogen as the plasma gas as well as the nebulizer gas. Most of the atomic lines tested exhibited enhancement effects and all of the ionic lines gave suppression effects in the presence of Na and Ca. The effects decreased with increasing microwave power. There was good correlation between matrix effects, the excitation energy and energy sum of the spectral lines. The lower the excitation energy for atomic lines and the higher the energy sum for ionic lines, the stronger the matrix effects. Unlike Na and Ca, nitric acid showed suppression effect for all of the spectral lines and the effect was independent of the energy of the spectral lines and the microwave power. The effect of matrix on the excitation temperature and the electron number density was measured and the mechanism of matrix effect was discussed.     

Measured and modeled enhancement of transition metal emissions in the d.c. plasma jet

Enhancement of sensitive transition metal lines by a sodium matrix is measured in a 3-electrode d.c. plasma jet. Spiking with 0.43 M NaCI causes enhancement by factors of 1.85–2.92 in ionic lines and of 1.22–1.99 in atomic lines for eight of the structurally related analyte elements, but suppresses Zn I and Zn II emissions by about 25 %. Emission response to NaCI of lines within the same spectrum, or between different spectra of like ionization stage, can be simulated to 15 % and 20–25 %, respectively, by approximations linear in energy differences. For ionic lines these differences are the absolute value of the line excitation potential minus the energy of the ion state most readily pumped by Penning ionization by argon. For atomic lines it is the difference between emitting state excitation potential and the first ionization potential. Analyses of the experimental data strongly suggest that: (1) Na acts mainly to pertub radiative transfer rather than collisional redistribution processes; (2) population pumping of excited analyte states is largely driven by Penning ionization; (3) accelerated radiative cooling due to Na is manifested in a lowering of local kinetic temperature; (4) to a first-order of approximation, ambipolar diffusion, analyte-Na collisions of the second kind, and analyte ground state spin, do not influence emission line enhancement by easily ionized elements (EIE). Approximations are developed for predicting transition metal enhancements by arbitrary Na doping concentrations, and means are sketched for extending the method to other analyte group/EIE combinations. Practical implications of the work are noted.     

Investigations on cluster and molecular ion formation by plasma mass spectrometry

The formation of molecular and cluster ions of different inorganic materials in plasma mass spectrometry – spark source mass spectrometry (SSMS), radiofrequency glow discharge mass spectrometry (rf GDMS), laser ionization mass spectrometry (LIMS), inductively coupled plasma mass spectrometry (ICP-MS) and laser ablation ICP-MS (LA-ICP-MS) – was investigated and compared. Similar abundance distributions of cluster ions were observed for a graphite sample, for boron nitride/ graphite and for metal oxide/graphite mixtures using different plasma mass spectrometric methods. A correlation of intensities of metal argide ions in ICP-MS with their bond dissociation energies was used to estimate unknown dissociation energies of molecular ionic species. For the elements of the 2nd or 3rd period in the periodic table, the intensities of most argon molecular ions (ArX⁺) measured by ICP-MS rise with increasing atomic number in a similar manner to the theoretically calculated bond dissociation energies of argon molecular ions.     

Shannon information entropy of fractional occupation probability as an electron correlation measure in atoms and molecules

A new method to determine electron correlation energy is presented for atoms and molecules. This method is based on Shannon information entropy that is obtained by fractional occupation probabilities of natural atomic orbitals. It is indicated that the Shannon entropy increases as the number of electrons increases and thus can be considered as a possible measure for the electron correlation in atomic and molecular systems. For neutral atoms and singly charged positive ions we proposed an expression for correlation energy with explicit dependence on the Shannon entropy and atomic number. The obtained correlation energies have been used to compute the first ionization potentials of the ground state of the main group elements from hydrogen through krypton. The calculated ionization potentials are in reasonably good agreement with their corresponding experimental values.We also developed the additivity scheme to find a connection between Shannon entropy and molecular correlation energy. The estimated molecular correlation energies show an excellent agreement with those obtained by elaborate G3 method with R² = 0.990.     

Analytical laser ionization spectrometry in flame: Choice of the analytical form for the determination of rare earth elements

Based on the ionization potentials and dissociation energies of rare earth monoxides, these molecules are classified into two groups. For the molecules of the first type, the ionization potential is lower than the dissociation energy. Laser excitation of these molecules leads to their predominant ionization. For the molecules of the second type, the ionization potential is higher than the dissociation energy. Depending on the absolute values of the latter, atomic lines can be observed in the ionization spectrum. The elements that form molecules of the first type in flame should be determined as monoxide molecules. For the elements forming molecules of the second type, determination in the atomic form gives better results.     

From Electronegativity towards Reactivity—Searching for a Measure of Atomic Reactivity

Pauling introduced the concept of electronegativity of an atom which has played an important role in understanding the polarity and ionic character of bonds between atoms. We set out to define a related concept of atomic reactivity in such a way that it can be quantified and used to predict the stability of covalent bonds in molecules. Guided by the early definition of electronegativity by Mulliken in terms of first ionization energies and Pauling in terms of bond energies, we propose corresponding definitions of atomic reactivity. The main goal of clearly distinguishing the inert gas atoms as nonreactive is fulfilled by three different proposed measures of atomic reactivity. The measure likely to be found most useful is based on the bond energies in atomic hydrides, which are related to atomic reactivities by a geometric average. The origin of the atomic reactivity is found in the symmetry of the atomic environment and related conservation laws which are also the origin of the shell structure of atoms and the periodic table. The reactive atoms are characterized by degenerate or nearly degenerate (several states of the same or nearly the same energy) ground states, while the inert atoms have nondegenerate ground states and no near-degeneracies. We show how to extend the use of the Aufbau model of atomic structure to qualitatively describe atomic reactivity in terms of ground state degeneracy. The symmetry and related conservation laws of atomic electron structures produce a strain (energy increase) in the structure, which we estimate by use of the Thomas-Fermi form of DFT implemented approximately with and without the symmetry and conservation constraints. This simplified and approximate analysis indicates that the total strain energy of an atom correlates strongly with the corresponding atomic reactivity measures but antibonding mechanisms prevent full conversion of strain relaxation to bonding.     

Amplification of noble gas ion lines in the afterglow of a pulsed hollow cathode discharge and possible benefit for analytical glow discharge mass spectrometry

A high-current pulsed hollow cathode discharge was used to study the role of atomic and ionic metastables involved in ionization plasma processes. We observed the enhancement of the spectral emission lines of noble gas ions in the afterglow. A study of the processes that involve atomic and ionic metastables is of great interest since it should lead to a better understanding of and enhanced control over the ionization mechanisms crucial to analytical glow discharge mass spectrometry (GDMS) analysis. Figure Time profile of Ti, Ti⁺, and Ne⁺ spectral lines     

Ionization and Coulomb explosion of small group 10 transition metal oxide clusters in strong light fields

The ionization properties of small group 10 metal oxide clusters are explored using ultrafast pulses centered at 624 nm. Maximum atomic charge states resulting from Coulomb explosion were observed to be Ni(3+), Pd(3+), Pt(5+), and O(2+) species with similar ionization potentials ～30-35 eV. Ion signal as a function of laser intensity of each charge state of Ni, Pd, Pt, and O resulting from Coulomb explosion was mapped and compared to that predicted from semi-classical tunneling theory using sequential ionization potentials to quantify observed enhancements in ionization. The saturation intensity (I(sat)) of each charge state is measured and compared to previous studies on group 5 transition metal oxides. The atomic charge states of nickel showed a large enhancement in ionization compared to palladium and platinum, reflective of the differing bonding properties of each metal with oxygen. Results indicate that nickel oxide clusters undergo a greater extent of ionization enhancement as a result of multiple ionization mechanisms. The ionization enhancement behavior of each metal oxide species is explored herein.     

A Study on Determination of Some Trace Nonmetals by Microwave Induced Plasma Atomic Emission Spectrometry

In general, atomic emission spectrometry (AES) is an excellent technique for determination of metal elements. However, its capability of determining nonmetals has not been developed well. The major reasons are the resonance lines of most nonmetals lie in the vacuum ultraviolet spectral region and the ionic lines of these elements are difficult to be used because the ionization potentials of the elements are very high. And furthermore only He plasma can efficiently excit those ionic resonance lines. The practical application of HeICP-AES to determining the nonmetals is also difficult because its operating and perchace costs are very high. In contrast to HeICPAES, the costs of He microwave induced plasma (MIP)-AES are relatively low, HeMIP has a high excitation capability which can excite ionic lines of various nonmetals and the nonmetals can be determined by HeMIP-AES with a high sensitivity.     

Use of spatial emission profiles and a nomenclature system as aids in interpreting matrix effects in the low-power argon inductively coupled plasma

An automated profiling system was used to determine vertical intensity distributions for atomic and ionic lines of several elements in the ICP and to measure the effect of matrix components on those distributions. Most atomic lines show maximum signal (not necessarily maximum signal/noise) rather low in the plasma. Ion lines predominate in the plasma region used most frequently for analysis. In the region of high atomic emission, enhancements are observed for both atom and ion lines with many analytes when matrix elements are added. The enhancements either disappear or become much less severe in the region of high ionic emission normally viewed. The zone where the interferences occur can be shifted higher or lower in the plasma depending primarily on the central gas flow and the power level. Plasma structure can be used to predict regions of high interference. A Nomenclature System for the plasma zone is used as an aid in comparing plasma conditions.     

A theoretical study of the photoelectron spectra of dichloroketene with accurate computation of ionization energies via complete basis set limit extrapolation

We calculated the equilibrium geometries and harmonic vibrational frequencies of the ground state and five cationic states of dichloroketene using (TD-)B3LYP, PBE0, and M06/M06-2X approaches. The photoelectron spectra of dichloroketene were simulated by computing Franck-Condon factors. The ionization energies were computed using the CCSD(T) approach with extrapolation to the complete basis set (CBS) limit. We propose two new CBS energy formulas (E = E_CBS + Aexp(-x) + B/(x−1) ⁿ, n = 2 or 3) and compare the performance of different CBS approaches. A new ionic state of dichloroketene belonging to the C_s point group is reported. This state is identified as the first excited state of Cl₂CCO⁺ having a double-well potential-energy curve along the CCO bending mode with a barrier height of 1.335 eV. The simulated photoelectron spectra are in agreement with the experiment. The vertical ionization energies calculated via spectral simulation are more accurate compared with those obtained at the ground-state structure. Among the CBS formulas used, the proposed ansatz with n = 2 performs best, with a mean absolute error of 0.021 and 0.012 eV for the adiabatic and vertical ionization energies, respectively.     

Factors affecting the formation of the radiation pre-peak at the operation of a Grimm-type source in pulsed DC mode

The plasma emission pre-peaks of many atomic and ionic spectral lines of Cu and Ar were systematically investigated in a Grimm-type pulsed glow discharge (PGD). To register the pre-peaks with sufficient time resolution, a monochromator with photomultiplier detection was used. When the applied power exceeded a specific threshold, pre-peaks were found in all spectral lines investigated, and it was revealed that the electrical pre-peak was the cause of the atomic emission pre-peak. The form and intensity of the pre-peak radiation were, however, found to be different for different atomic emission lines. The excitation energy of the upper energy level of the atomic line transition, and factors related to recombination and self-absorption, were found to affect the emission pre-peak. Pre-peaks observed when using pulsed DC and pulsed radio-frequency power were compared. This investigation provides insight into best practice when selecting spectral lines most suitable for analytical spectrometry using PGD.     

Matrix effect of barium on spectral line intensities and detection limits in inductively coupled plasma atomic emission spectroscopy

The effect of four concentrations of Ba (as nitrate) on emission intensities of La(II) 337.7, Mg(I) 285.2, Mg(II) 279.1, Cd(I) 228.8, Cd(II) 226.5, Zn(I) 213.8 and Zn(II) 206.2 nm as a function of power and height above the load coil is studied. The detection limits of atomic and ionic lines of La, Mg, Zn and Cd with/without addition of Ba are reported. Low Ba concentration (0.001M) decreases the detection limit of the element of low ionization energy (La) while high Ba concentration (0.3M) decreases detection limits of the elements of high ionization energies (Cd and Zn) and increases the detection limit of the element of medium ionization energy (Mg).     

Evaluation of the detection capability of a U-shaped d.c. arc for spectrometric analysis of solutions

A gas-stabilized arc with aerosol supply, originally designed for atomic absorption studies, has been used for emission spectrometric analysis of solutions. A characteristic of the arc is a fairly long horizontal part of the arc column which makes possible “end-on” observation of the spectral emission and selection of a well-defined region of the arc column for analysis. The most intense emission of continuum is at the arc column axis while the emission maxima of the nebulized elements are located at different distances from the axis, which mainly depend on the ionization potential of the corresponding element.The presence of alkali elements in the arc plasma enhance considerably the spectral emission of the elements with medium and low ionization potentials. The effect depends mainly on the first ionization potential of the element and its extent is approximately the same for atom and ion lines. In the case of potassium chloride the intensity increase approaches a plateau at a concentration of 2.5 mg ml^?1. The magnitude of the effect justifies the use of potassium chloride as a spectroscopic buffer.Detection limits obtained with this source on 60 spectral lines are compared with those found in inductively coupled plasmas and in an inverted V-arc echelle spectrometer system. Comparison reveals that inductively coupled plasmas yield consistently lower detection limits with the ion lines used, while with the atom lines it retains the advantage only for elements having a high ionization potential.     

Spectral line selection for time-resolved investigations of laser-induced plasmas by an iterative Boltzmann plot method

The aim of this work is to provide a procedure to determine time-resolved electron temperatures with minimized relative errors by the Boltzmann plot method. The applied procedure consists of two parts, a systematic theoretical spectral line selection and an iterative Boltzmann plot algorithm. After a pre-selection of an appropriate non-disturbed or overlapped set of spectral lines of a particular atomic or ionic species Boltzmann plots are generated using experimentally recorded data for every time window and laser pulse energy of interest. Spectral lines with the highest average deviations from the regression function are assumed as being not representative for the considered ensemble of spectral lines and are therefore discarded gradually until a threshold value for the coefficient of determination is exceeded. Laser-induced breakdown spectroscopy (LIBS) is applied for time-resolved and spatially integrated investigations of plasmas on 1.1750 C75 steel alloy samples with laser pulse energies ranging between 200 µJ and 2 mJ. For the specific chemical composition of these samples a selection of atomic and ionic Fe spectral lines has been carried out. In spite of the fact that only laser pulse energies in the low millijoule regime are applied the final sets of spectral lines comprise in total 61 Fe I and 12 Fe II emission lines. By applying this method electron temperatures can be determined with averaged relative errors of down to 1.8% for Fe I and 4.4% for Fe II emission lines.     

原子电负性和极化度对卤代甲烷C 1s电子电离能的影响

以原子的电负性χP和极化度α为基本参数, 估算卤代甲烷CHnY4−n−mZm(Y, Z=F, Cl, Br, I) C 1s电子电离能的电荷效应和松弛效应. 电荷效应由C—H和C—Y(Z)键两端原子的电负性差来度量, 松弛效应由碳原子带的电荷乘上氢和卤素原子极化度来衡量, 进而用电荷效应和松弛效应一起表达卤代甲烷中C 1s电子电离能的静电-松弛屏蔽效应ΔSi. 将ΔSi代入类-Slater模型, 得到卤代甲烷中C 1s电子电离能E1,C的估算方程, 该方程的相关系数r=0.99987, 对27个卤代甲烷的计算值与实验值之间的平均绝对误差仅为0.038 eV, 小于实验误差0.1 eV. 同时, 用留一法(leave-one-out)进行交叉验证(相关系数rcv=0.99977, 预测值与实验值之间的绝对平均误差只有0.049 eV), 结果表明所得方程具有良好的预测能力和稳定性.     

Investigation of hydroxyl-mediated spectral interference from easily ionized elements in laser-enhanced ionization spectrometry in the 281–285 nm range

The spectral background from 281 to 285 nm in the laser-enhanced ionization (LEI) spectrum of aqueous samples containing easily ionized elements (EIE) at concentrations similar to those found in blood was investigated. A complex, structured spectral background was observed, which appears in the presence of Na or K, but does not match the spectral signature of either element. The same behavior was also observed for Li. It was established that this background originates from an energy transfer between laser-excited hydroxyl (OH) molecules and ground-state EIEs. The intensity of this spectral feature was found to increase with EIE concentration and applied electrode voltage. This unexpected source of spectral interference may complicate the determination of trace metals by LEI in the presence of EIEs, since it can not be prevented by simply avoiding interference from atomic lines.     

The influence of ethanol addition on the determination of trace elements in aqueous solutions by ICAP

The effect of the addition of ethanol to aqueous solutions of various trace elements (Ca, Cd, Mg, Cu and Ni) on their spectral line intensities in inductively coupled argon plasma (ICAP) was investigated. Using a relatively low ethanol content (up to 10%, v/v) an enhancement of both atomic and ionic lines was found but to a different extent. By optimization of experimental parameters, an improvement of the sensitivity of the multielemental analysis was obtained.     

Consideration on excitation mechanisms in a high-power two-jet plasma

The study of excitation mechanisms in the region before the jet confluence of a high-power two-jet plasma used for analysis of different powders has been undertaken. Distribution of excited levels of Fe atoms and ions according to the Boltzmann population was found. Measuring Fe atomic and ionic excitation temperatures showed their considerable difference (≈ 2000–2500 K). The effect of argon on line intensities of a wide range of elements was investigated by the experiment with argon covering. A negligible effect of argon covering on line intensities of atoms with ionization energy of < 8 eV allows one to assume their predominant excitation by electron impact. The argon participation in excitation of atoms having ionization energy of > 8 eV was revealed. This is likely to be due to Penning ionization by metastable argon followed by ion recombination with an electron and stepwise de-excitations. A more pronounced effect of argon covering was observed for ionic lines of investigated elements with total excitation energy ranging from 11 to 21 eV. Penning ionization followed by electron impact is believed to be a probable mechanism for ion excitation. The contribution of metastable argon to excitation processes results in departure from local thermodynamic equilibrium and different atomic and ionic excitation temperatures.     

强短脉冲供电时空心阴极灯的放电特性研究

空心阴极灯是原子光谱分析中常用的线光源。对它的光谱和放电特性的研究已有报道,但多为针对直流供电或一般脉冲供电状态时的空心阴极灯。黄本立等报道了市售空心阴极灯在强短脉冲供电时的时间分辩光谱特性及其作为高强度离子线或原子线光源的可行性。本文通过对Eu空心阴极灯在强短脉冲供电时的伏安特性,以及对直流和强短脉冲供电时空心阴极灯发射光谱差异的研究,探讨了强短脉冲供电时空心阴极灯的放电机制,讨论了离子线和原子线强度增强的过程。 1 实验部分 1.1 仪器 1.0m平面光栅光谱仪(中国科学院长春应用化学研究所),光栅刻线1200条/mm,入射和出射狭缝宽度分别为10和35μm,一级光谱倒色散率0.8nm/mm;MF-1A型脉冲发生器(南通电子仪器厂);自制HCL供电电源和Boxcar积分器;BS-5504型双踪示波器(40 MHz,韩国安罗);瞬态记录仪频率为20 MHz(中国科技大学快电子学实验室),1P28A和EMI 6265型PMT;LM-15型X-Y记录仪;所用HCL为市售商品灯。 1.2 实验装置 研究空心阴极灯(HCL)光谱特性的装置见图1.由脉冲发生器产生的脉冲信号通过灯电源控制HCL,由HCL发射的光经单色仪后由光电倍增管(PMT)将光信号转化为电信号,经前置放大器以及Boxcar积分器或瞬态记录仪后采集并记录脉冲信号。图1中PG为脉冲发生器,HCL Dr