Application of plasma gas modulation technique for improvement of the measurement of Mn emission intensity in ICP-AES

A phase-sensitive detection technique associated with a digital lock-in amplifier was applied for an improvement of the detection in ICP-AES. The lock-in amplifier works as an extremely narrow band pass filter. It can pick up the modulated signal, which has the same frequency as the reference signal, from any noise and thus it can improve the signal-to-noise ratio. Modulation of the ICP can be performed by mixing small amounts of air to argon as the outer gas cyclically, because the emission intensities of ionic lines are enhanced by using the mixed gas. An electromagnetic valve, which is placed in the outer-gas flow path, causes periodic variation in the air gas in the outer-gas flow, and thus switching the valve on/off can modulate the ICP. By choosing the appropriate conditions, the addition of air gas enhances the emission intensity of ionic lines more than that of the background, thus leading to improved signal-to-background ratios. At the same time the lock-in amplifier further enhances the ionic emissions because it picks up only the modulated part of the signal. By applying the plasma gas flow modulation technique the detection and the determination limits of the Mn II 257.610 nm line are improved in comparison with the conventional method. A change in plasma shape corresponding to the modulation frequency is observed when the ICP is modulated.     

Emission intensity modulation of radio-frequency helium glow-discharge emission source by laser ablation.

A novel emission excitation source comprising a high repetition rate diode-pumped Q-switched Nd:YAG laser and a Grimm-style glow-discharge lamp is described. Laser-ablated atoms are introduced into the He glow discharge plasma, which then give emission signals. By using phase-sensitive detection with a lock-in amplifier, the emission signal modulated by the pulsed laser can be detected selectively. It is possible to estimate only the emission intensity of sample atoms ablated by laser irradiation with little interference from the other species in the plasma.     

A simultaneous multielement non-dispersive atomic-fluorescence spectrometer using modulated sources and frequency discrimination of fluorescence signals

Commercial radiofrequency-excited electrodeless discharge lamps can be run from a square-wave modulated power supply so as to give a low level of continuous emission when modulated in the frequency range 3–10 kHz. Use of a different modulation frequency and lock-in amplifier for each lamp allows multielement non-dispersive atomic-fluorescence spectrometry to be performed. Very low detection limits have been obtained for arsenic, selenium, tin and mercury. The use of low-cost electronic components in the system largely offsets the high cost of the individual excitation power supplies and tuned a.c. detectors.     

Optical emission spectroscopy of CF4+O2 plasmas using a new technique

Optical emission intensities in the sheath region are not the same as those in the plasma region. This is because not only the electron density, but also the electron temperature, is different between the two regions. In this study a Cu rod is inserted into the plasma, and the rod potential is altered from the ground potential to a negative potential with a frequency of 20 Hz. The optical emission ray comes from the sheath region when the negative potential is applied, but comes form the plasma region at the ground potential. We can immediately detect the difference of the emission intensities between the plasma and the sheath regions by a lock-in amplifier. The pre-amplifier is placed prior to the lock-in amplifier. By using this pre-amplifier the output signal of the lock-in amplifier can be adjusted to zero for any emission line. the emission spectra from a CF₄+O₂ plasma are measured. A small amount of Ar gas and/or N₂ gas is added and the output signal of the lock-in amplifier is adjusted to zero for either the Ar emission line or the N₂ emission line. In a fluorine-contained plasma the F emission intensity normalized by the Ar one has been widely used in order to obtain the F density. This validity is confirmed by the present experiment. It is also confirmed that the CO emission intensity normalized by that of N₂ is proportional to the CO density. The metastable states play an important role in the optical emission intensities of CO and N₂ molecules.     

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 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.     

Sampling modulation technique in radio-frequency helium glow discharge emission source by use of pulsed laser ablation

An emission excitation source comprising a high-frequency diode-pumped Q-switched Nd:YAG laser and a radio-frequency powered glow discharge lamp is proposed. In this system sample atoms ablated by the laser irradiation are introduced into the lamp chamber and subsequently excited by the helium glow discharge plasma. The pulsed operation of the laser can produce a cyclic variation in the emission intensities of the sample atoms whereas the plasma gas species emit the radiation continuously. The salient feature of the proposed technique is the selective detection of the laser modulation signal from the rest of the continuous background emissions, which can be achieved with the phase sensitive detection of the lock-in amplifier. The arrangement may be used to estimate the emission intensity of the laser ablated atom, free from the interference of other species present in the plasma. The experiments were conducted with a 13.56 MHz radio-frequency (rf) generator operated at 80 W power to produce plasma and the laser at a wavelength of 1064 nm (pulse duration:34 ns, repetition rate:7 kHz and average pulse energy of about 0.36 mJ) was employed for sample ablation. The measurements resulted in almost complete removal of nitrogen molecular bands (N₂⁺ 391.44 nm). Considerable reduction (about 75%) in the emission intensity of a carbon atomic line (C I 193.03 nm) was also observed.     

Emission characteristics of Cu in Dc voltage modulation applied glow discharge optical emission spectrometry.

In order to obtain the depth profile of a thin film, we investigated the emission characteristics of a voltage modulation glow discharge to optimize the modulation parameters (modulation voltage, offset voltage, and modulation frequency). In this study, a phase-sensitive detection method with a lock-in amplifier to the modulation technique led to a higher sensitivity and a larger signal-to-noise ratio in the emission analysis compared to the normal dc amplification method. Upon increasing the maximum voltage, the emission intensity of the Cu atomic line (CuI 239.34 nm) increased linearly at a modulation voltage of 400 V and an offset voltage of 300 V. On the other hand, the emission intensity was gradually reduced when a modulation frequency increased. It is advantageous for surface analysis that the voltage modulation technique gives a lower sputtering rate rather than the conventional dc discharge.     

Real-time analysis of CuO by inductively coupled plasma emission without external calibration

The study of a method, devoted to real-time detection of metallic pollutants present in stack gas, is investigated. This method is based on spectroanalysis using an inductively coupled plasma (ICP) emission system without external calibration. The fluidized bed technology is employed to inject metallic species into the ICP emission. The mass fluxes of copper oxide (CuO) are then determined by using the intensity ratios of the metallic element spectral lines with those of the plasma gas element (argon or dry air). These ratios and the plasma characteristics (atomic excitation temperature, degree of thermal disequilibrium θ=T_e/T_h) are inserted into a calculation code of plasma composition to determine the mass flux. The results are in good agreement using either argon plasma or dry air plasma. A study of the fluidized bed properties is made to compare our values with those resulting from the elutriation calculation of the copper oxide.     

悬浮液直接进样ICP–OES法测定高纯氧化铝中的铁、钛、硅、铬

采用悬浮液直接进样电感耦合等离子体发射光谱法(ICP–OES)测定高纯氢氧化铝中铁、钛、硅、铬的含量。悬浮液用电磁搅拌器搅拌,均匀地分散在溶液中,通过仪器蠕动泵进入雾化室,均匀无阻地导入ICP光源。Fe,Ti,Si,Cr的分析谱线分别为259.940,336.112,251.611,205.552 nm;RF功率为1 300W,等离子体气流量为13.0 L/min,雾化器气体流量为0.60 L/min,辅助气流量为1.00 L/min。Fe,Ti,Si,Cr的质量浓度分别在0.0~30.0,0.0~15.0,0.0~90.0,0.0~15.0μg/m L范围内与信号强度呈良好的线性,线性相关系数均大于0.999,方法的检出限为0.027 6~0.993 9μg/m L,测量结果的相对标准偏差为0.65%~6.84%(n=11),回收率为95.0%~104.8%。该法抗干扰能力强、线性范围宽,适用于高纯氢氧化铝中铁、钛、硅、铬含量的分析。     

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.     

Advantages and effects of nitrogen doping into the central channel of plasma in axially viewed-inductively coupled plasma optical emission spectrometry

The effects and benefits of N₂ addition to the central channel of the ICP through the nebulizer gas used in ICP OES with axial view configuration were investigated in the present study. The N₂ flow rate, nebulizer gas flow rate, RF power and sample uptake rate were evaluated and compared for two sample introduction systems (pneumatic nebulization/aerosol desolvation and conventional pneumatic nebulization). It was observed that N₂ did not affect solution nebulization and aerosol transport but affects the ICP characteristics. The higher thermal conductivity of N₂ (in comparison with Ar) changes energy distribution in the ICP, observed by monitoring the signals of Ar emission lines and sodium emission. The ratio Mg(II)-280.270 nm/Mg(I)-285.213 nm was utilized as a diagnostic tool for plasma robustness. The addition of N₂ (20 mL min⁻¹) increased plasma robustness significantly and mitigated effects caused by Na, K and Ca. For 40 spectral lines evaluated, it was observed that the emission signals of ionic spectral lines were in general more affected by N₂ than those of atomic spectral lines. Detection limits, precision, sensitivity and linearity of calibration curves obtained using N₂-Ar-ICP were almost similar to those obtained using Ar-ICP. The analysis of 5 different reference materials revealed that accuracy was not degraded by adding N₂ to the Ar-ICP.     

Emission characteristics of nickel ionic lines excited by reduced-pressure laser-induced plasmas using argon, krypton, nitrogen, and air as the plasma gas

The emission characteristics of nickel ionic lines in low-pressure laser-induced plasmas are investigated when argon, krypton, nitrogen, or air gas was employed as the plasma gas. The spectrum patterns and the relative intensities of the ionic lines are measured with and without a blind cylinder surrounding the sample surface to separate the detected emission area into two portions roughly: an initial breakdown zone and an expansion zone of the plasma. Their emission intensities are strongly dependent on both the kind and the pressure of the plasma gas. Different major ionic lines are observed in the argon and the krypton plasmas: for example, the Ni II 230.010-nm line (8.25 eV) for argon and the Ni II 231.604-nm line (6.39 eV) for krypton. The excitation mechanism of these ionic lines is considered to be a resonance charge-transfer collision with argon or krypton ion due to good energy matching to the corresponding energy levels of nickel ion. These ionic lines measured with the blind cylinder at reduced pressures of around 1300 Pa give the largest signal-to-background ratios; therefore, the analytical application under such optimum plasma conditions is recommended.     

Enhancement of signal-to-noise level by synchronized dual wavelength modulation for light emitting diode fluorimetry in a liquid-core-waveguide microfluidic capillary electrophoresis system

The signal-to-noise level of light emitting diode (LED) fluorimetry using a liquid-core-waveguide (LCW)-based microfluidic capillary electrophoresis system was significantly enhanced using a synchronized dual wavelength modulation (SDWM) approach. A blue LED was used as excitation source and a red LED as reference source for background-noise compensation in a microfluidic capillary electrophoresis (CE) system. A Teflon AF-coated silica capillary served as both the separation channel and LCW for light transfer, and blue and red LEDs were used as excitation and reference sources, respectively, both radially illuminating the detection point of the separation channel. The two LEDs were synchronously modulated at the same frequency, but with 180°-phase shift, alternatingly driven by a same constant current source. The LCW transferred the fluorescence emission, as well as the excitation and reference lights that strayed through the optical system to a photomultiplier tube; a lock-in amplifier demodulated the combined signal, significantly reducing its noise level. To test the system, fluorescein isothiocyanate (FITC)-labeled amino acids were separated by capillary electrophoresis and detected by SDWM and single wavelength modulation, respectively. Five-fold improvement in S/N ratio was achieved by dual wavelength modulation, compared with single wavelength modulation; and over 100-fold improvement in S/N ratio was achieved compared with a similar LCW-CE system reported previously using non-modulated LED excitation. A detection limit (S/N = 3) of 10 nM FITC-labeled arginine was obtained in this work. The effects of modulation frequency on S/N level and on the rejection of noise caused by LED-driver current and detector were also studied.     

Inductively coupled plasma-ionic absorption spectrometry using a xenon arc lamp and an echelle monochromator

Ionic absorption spectrometry using the inductively coupled plasma (ICP) as an ionization source and a 150 W stable xenon arc lamp as a light source has been investigated. A high-resolution echelle spectrometer was used for the absorption measurement. Ionic absorption lines in the wavelength region above 320 nm showed high sensitivity and detection limits of Y, Ti, Zr, Sc, Ce and La were of the same order of magnitude as those attained by ICP atomic emission spectrometry. Linear dynamic ranges of three orders of magnitude were observed for most elements tested. Effects of the measurement conditions such as rf power, outer gas flow rate, inner gas flow rate and observation height on the transmittance of the beam through the plasma and the absorbed radiation power of the analytes are also given.     

Bias-current modulation technique of a radio-frequency glow discharge plasma for atomic emission analysis associated with a fast Fourier transform analyzer

A measuring method using a fast Fourier transform (FFT) analyzer is suggested to estimate the emission intensity from a radio-frequency (RF)-powered glow discharge plasma for atomic emission analysis. The FFT analyzer has an ability to disperse the components by frequency from an overall signal, and thus works as a selective detector in modulation spectroscopy. In the RF glow discharge plasma, a dc bias current can be introduced by connecting an external electric circuit with the discharge lamp, which predominantly enhances the emission intensities. Further, the bias current can be pulsated with a switching device to modulate the emission intensities, and then the modulated component was selectively detected with the FFT analyzer. This method greatly improved the data precision. The emission intensity of the Cu I 324.75-nm line in an Fe-based alloy sample containing 0.043 mass% Cu could be estimated with a relative standard deviation of 0.20%. The 3σ detection limits of Cu in Fe-based alloys could be obtained to be 2.3 × 10^− 6 mass% Cu for Cu I 324.75 nm and 6.8 × 10^− 6 mass% Cu for Cu I 327.40 nm.     

The use of discharge lamps as directly modulated atom reservoirs for selective line modulation in atomic emission spectrometry

A simple method is described for selectively modulating the atomic or ionic resonance lines emitted by an excitation source. A modified discharge lamp with a cylindrical hollow cathode is used to generate a modulated atom cloud. The emission from the excitation source is then imaged through the modulated atom cloud within the cylindrical cathode lamp, to yield a selectively modulated signal. The ability of the system to reduce spectral interferences in inductively-coupled plasma emission spectrometry is assessed and discussed.     

Reminiscences with Martin P. Seah

Starting in the mid-1960s, the detection and display of peaks in Auger electron spectroscopy (AES) were improved by using modulation of the electron energy analyzer coupled with electron detection using a lock-in amplifier. This allowed a derivative of the electron energy distribution, N(E), to be obtained directly at the output of the lock-in amplifier thereby removing most of the effect from the relatively large, slowly varying, electron background signal due to secondary and backscattered electrons. For relatively low modulation amplitudes, the peak-to-peak intensity of the Auger features increased linearly with modulation amplitude (for a deflection-type analyzer), improving the signal-to-noise ratio. However, with relatively large modulations, the Auger peak shapes distorted, and the peak-to-peak heights eventually decreased in size, and this nonlinearity would cause problems in quantitative analysis. A universal curve was developed for singlet Auger peaks to approximate corrections due to this peak distortion, but an approach to exactly correct for such distortions was largely ignored by the AES community. This approach was called Dynamic Background Subtraction and is even relevant today as some Auger instruments using modulation and lock-in amplifiers are still being manufactured. This review paper describes approximate and exact corrections for modulation effects in AES data.     

Vertical spatial emission profiles in Ar-N2 mixed gas inductively coupled plasmas—I

The changes that occur in the spatial emission structure and characteristics of an inductively coupled plasma when the argon coolant is replaced by nitrogen have been measured in some detail. Analytes studied include Mg, Cd, Zn, Mn, Cu, Cr, Sr and Co. The basic behavior of both neutral atom and ion lines has been studied as a function of Ar-N₂ coolant gas composition and, as well, the dependence of mixed gas plasma emission characteristics on aerosol flow rate and power is presented. In general, as nitrogen is added to the coolant flow, the observation zone for maximum intensity shifts down towards the load coil and the intensity of ion line emission increases relative to that of neutral atom line emission. Also, so called soft lines show quite complex behavior changes with respect to the dependence of their emission intensity on power and aerosol flow rate as nitrogen is added to the coolant. In essence, the effect of these two plasma parameters on soft line emission in a 100% N₂ cooled ICP appears to be opposite to that in a 100% Ar cooled ICP.     

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.