Use of a spectrally segmented photodiode-array spectrometer for inductively coupled plasma atomic-emission spectroscopy Examination of procedures for the evaluation of detection limits

The utility of a spectrally segmented photodiode array spectrometer was examined by using inductively coupled plasma atomic-emission spectrometry (ICP-AES). The spectrometer used in this study is capable of high resolution (reciprocal linear dispersion of approximately 0.08 nm/mm at 300 nm) over a wide spectral range (190-415 nm). The effect of using spectral peak areas instead of peak heights as a signal definition was examined by using the emission signals from 10 molybdenum lines obtained with various photodiode-array integration periods. In addition, a procedure to determine the detection limits obtainable with such a spectrometer is proposed. It was found that a signal definition involving a summation over a range of 5 pixels offered the best signal-to-noise ratio when the noise was defined as the standard deviation of the residual values from the line fitted to the sideband background level. A detection limit of 6 ng/ml was determined in this way for molybdenum. The multichannel capability of the spectrometer was found to permit continuous background correction, thereby reducing errors caused by low-frequency noise or plasma drift. The linearity of response was found to extend over three orders of magnitude with use of a single integration period. However, by use of different integration periods, the linear range of the detector could be extended to at least four orders of magnitude. The precision (RSD) of the spectrometer for a molybdenum concentration of 0.5 mug/ml was found to be about 3-4% for molybdenum peaks where the background emission was relatively low.     

Adaptation of an existing algorithm for automated continuum correction in inductively coupled plasma-atomic emission spectrometry using a photodiode array detector

The use of a photodiode array as a multichannel detector for off-line continuum correction in inductively coupled plasma-atomic emission spectrometry(ICP-AES)is demonstrated. The photodiode array allows concurrent data acquisition of all reference wavelengths necessary for the calculation of the background. An existing background correction algorithm that selects reference channels by a statistical criterion is modified for use with the photodiode array. The influence of several parameters on the performance of the algorithm is investigated with spectra derived from the photodiode array and from a conventional photomultiplier based system. It is shown that with either system background correction can be performed completely automated and sample adapted.     

Lineare Silizium-Photodioden-Matrizen mit parallelem Datenausgang als Strahlungsempfänger in der optischen Emissionsspektroskopie—II. Analytische anwendung in verbindung mit der glimmentladungslampe nach Grimm

A silicon photodiode array was placed in the focal plane of a spectrometer to detect the radiation. Three single diodes of the array, each 6.5 mm long and 19 μm wide, were connected to the three channels as described in Part I.The properties of the photodiodes were investigated with respect to their applicability in spectral analysis. Especially the spectral response, the dynamic range and the signal-to-noise ratio were of great interest. The quantum efficiency was found to be in the range from 300 to 800nm. The dynamic range, defined as the range from the noise level to the maximum permissible signal voltage extends over 5.5 decades and is a linear function of the radiative flux. The signal-to-noise ratio of systems using photodiodes depends on the wavelength of the radiation and, if high feedback resistors (≧10¹¹Ω) are used, reaches the same order of signal-to-noise ratio as that obtained by using a photomultiplier (EMI 9789 QB). To demonstrate the applicability of the photodiodes, the analysis of limestone was studied. The sample powder was mixed with copper powder, pressed into pellets, and the major constituents Al, C, Mg and Si were determined by means of a Grimm glow discharge lamp. The calibration curves obtained as well as the detection limits are reported.Furthermore, oxygen was determined in a series of synthetic samples using the line 777.2 nm.     

Cross-correlation masks for automatic detection of spectral information

Cross-correlation masks based on spectral intensities and binary spectral peak positions are evaluated for detection and quantitation of atomic emission spectra. The inductively coupled plasma emission spectra of nickel and vanadium as acquired with a photodiode array spectrometer were utilized to test and illustrate the cross-correlation procedures.     

Statistical evaluation of the performance of a new programmed-scan monochromator for multielement determinations by atomic emission

The reproducibility of a programmed-scan monochromator with stationary dispersion optics was evaluated by means of analysis of variance (ANOVA). The spectrometer used an optical multiplexer coupled with glass-fiber optic light-guides to a multiple entrance-slit spectrometer employing a photomultiplier as the detector. With this spectrometer, 15 emission intensity measurements at the lithium resonance line wavelength (670.7 nm) were collected for five rotations of the optiplexer mirror under four different emission situations: flame background emission at 670.7 nm, lithium emission from an acetylene-air flame in the absence of an ionization buffer, lithium emission from an acetylene-air flame in the presence of an ionization buffer, and tungsten lamp emission at 670.7 nm. For all four situations, the ANOVA results showed that instrumental changes which occurred during mirror rotation in the optiplexer were a significant source of signal variation compared with factors not associated with mirror rotation, i.e., photon shot noise, source fluctuation noise, and electronic drift. The actual magnitude of the signal variability introduced during mirror rotation, however, was found to be quite small, producing an average relative standard deviation of only 0.76% for the signal.     

The rf input power dependence of chromium excited state level populations in the inductively coupled plasma

Excited state level populations have been measured for Cr(I) and Cr(II) in the inductively coupled plasma. The measurement was carried out using a 4096 pixel linear photodiode array spectrometer to detect emission intensities from 16 atomic and 16 ionic lines. The effect of changes in rf input power on the level populations was studied over the power range 0.75-1.75 kW. The results are consistent with a p-LTE model of excitation and ionization.     

光电二极管阵列等离子体原子发射光谱分析背景智能化实时校正研究

提出了用于光电二极管阵列等离子体原子发射光谱分析背景智能化实时校正的方法。采用二次微分结合原ICP光谱法,在不经微分噪声过滤的基础上,实现快速准确的谱线识别。简单背景采用多项式模型,结构背景及翼展干扰采用概率统计模型。通过对高浓度Ca存在下A₁及La的测定,表明本方法能够准确地校正背景干扰。     

A spatial study of electron density and analyte emission in a d.c. argon plasma

Spatially resolved electron density measurements have been performed on a three-electrode d.c. plasma using a linear photodiode array based spectrometer. The electron density values measured are between 1× 10¹⁵ and 1 × 10¹⁶ cm^?3 depending on spatial position. The spatial distribution of Ca I (422.7 nm) and Ca II (393.4 nm) emission has also been measured and the Ca II-Ca I emission intensity ratio evaluated. Using the n_e values measured, an analagous LTE ratio has been calculated and this has been compared to the experimental values. Measured ratios are found to be from 28 to 100 times less than LTE ratios. Some possible sources leading to these infrathermal ratios are discussed.     

Some theoretical and practical considerations to the application of linear photodiode arrays for inductively coupled plasma emission spectrometry

Expressions are developed and used to predict the performance of a linear photodiode array when used with a conventional dispersive spectrometer for inductively coupled plasma (ICP) atomic emission spectrometry in the 200 to 450 nm region. The photon flux from the ICP and the relative standard deviation (RSD) at high concentrations are used in the equations to calculate the degradation of performance to be expected at and above the detection limit for integration times up to 10 s. Significant degradation is predicted only below 230 nm, and this degradation is less than a factor of 10 at 200 nm for an ICP with an RSD of 0.2 %. One-second integration times are possible with noiser ICPs or at longer wavelengths without significant performance degradation by the photodiode array.     

A Novel Spectrometer for Simultaneous Chemical Analyses

A novel spectrometer incorporating a multichannel detector for simultaneous multi-element atomic emission spectrometry (AES) is described. The spectrometer consists of a multi-bandpass optical filter comprised of a concave and a fiat grating in substractive mode, and a high resolution stage which utilizes an echelle grating and an one inch 1024-channel photodiode array as the detector. Characterization with respect to the spectral resolution, the spectral response, and the capability of multi-element detection demonstrated the systems's potential for simultaneous chemical analyses without moving any mechanical parts. To adjust the system for the analyses of different sets of elements, the corresponding mask is simply placed in position.     

Microwave-induced plasma emission spectrophotometer combined with a photodiode array monitor for capillary column gas chromatography

A microwave-induced plasma emission spectrophotometric detector (MIPD) was used as an element-specific detector for capillary column gas chromatography. The atmospheric pressure microwave helium plasma generated with an original device called a SURFATRON was used as an atomization and excitation source. Combining a photodiode array spectrophotometer with the above system made the emission spectrophotometric detector very powerful. A wide range of spectra could be instantly monitored without any mechanical device. However, the spectrum of atmospheric helium emission plasma was complicated by the presence of air around the plasma discharge. An on-line background correction scheme was developed to handle such complicated spectra.     

Lineare Silizium-Photodioden-Matrizen mit parallelem Datenausgang als Strahlungsempfänger in der optischen Emissionsspektroskopie—I. Messanordnung und Analysen mit Funkenstrahlungsquelle

A linear silicon photodiode array has been used which consists of 5 photodiodes each having a separate output. Because of the dimensions of a single photodiode, i.e. 3mm long and 20 μm wide, they are capable of replacing the exit slits of a spectrometer. The read out system having 3 channels is described in detail. In each channel a FET operational amplifier with a low input bias current served as a current-to-voltage converter.The signal-to-noise ratios of systems using the photodiode array and of a photomultiplier (EMI 9789 QB) respectively are compared. The dynamic range of the photodiode is linear and extends over 5 $\text{1}\text{2}$ decades referred to the noise level. As an analytical example copper was determined in aluminium alloys using medium voltage spark excitation in an argon atmosphere. The limit of detection was c = 4.8 × 10^?3%, the relative standard deviation as a measure of the precision was s_rel = 0.014 for concentrations above 0.08%.In part II the application of recently produced photodiode arrays on the analysis of powders with a glow discharge will be described.     

Absolute characterization of laser-induced breakdown spectroscopy detection systems

A quantitative comparison of the performance of four different laser-induced breakdown spectroscopy detection systems is presented. The systems studied are an intensified photodiode array coupled with a Czerny–Turner spectrometer, an intensified CCD coupled with a Czerny–Turner spectrometer, an intensified CCD coupled to an Echelle spectrometer, and a prototype multichannel compact CCD spectrometer system. A simple theory of LIBS detection systems is introduced, and used to define noise-equivalent spectral radiance and noise-equivalent integrated spectral radiance for spectral detectors. A detailed characterization of cathode noise sources in the intensified systems is presented.     

Optical characteristics of an afterglow extracted from an inductively coupled plasma

An inductively coupled plasma is extracted into a quartz vacuum chamber through an orifice to observe optical characteristics of the extracted afterglow. The Mach disk, the barrel shock and the zone of silence, which are familiar in supersonic molecular beam experiments with neutral gases, are clearly observed. Axial profiles of the emission of various lines are measured with a photodiode array spectrometer. Intensities of ionic lines as well as atomic lines are stronger at the Mach disk than in the zone of silence. The location of the Mach disk with varying chamber presure is in conformity with an experimental equation obtained in supersonic molecular beam experiments.     

Time-resolved long-lived luminescence imaging method employing luminescent lanthanide probes with a new microscopy system

Superior fluorescence imaging methods are needed for detailed studies on biological phenomena, and one approach that permits precise analyses is time-resolved fluorescence measurement, which offers a high signal-to-noise ratio. Herein, we describe a new fluorescence imaging system to visualize biomolecules within living biological samples by means of time-resolved, long-lived luminescence microscopy (TRLLM). In TRLLM, short-lived background fluorescence and scattered light are gated out, allowing the long-lived luminescence to be selectively imaged. Usual time-resolved fluorescence microscopy provides fluorescence images with nanosecond resolution and has been used to image interactions between proteins, protein phosphorylation, the local pH, the refractive index, ion or oxygen concentrations, etc. Luminescent lanthanide complexes (especially europium and terbium trivalent ions (Eu3+ and Tb3+)), in contrast, have long luminescence lifetimes on the order of milliseconds. We have designed and synthesized new luminescent Eu3+ complexes for TRLLM and also developed a new TRLLM system using a conventional fluorescence microscope with an image intensifier unit for gated signal acquisition and a xenon flash lamp as the excitation source. When the newly developed luminescent Eu3+ complexes were applied to living cells, clear fluorescence images were acquired with the TRLLM system, and short-lived fluorescence was completely excluded. By using Eu3+ and Tb3+ luminescent complexes in combination, time-resolved dual-color imaging was also possible. Furthermore, we monitored changes of intracellular ionic zinc (Zn2+) concentration by using a Zn2+-selective luminescent Eu3+ chemosensor, [Eu-7]. This new imaging technique should facilitate investigations of biological functions with fluorescence microscopy, complementing other fluorescence imaging methodologies.     

Enhanced fluorescence detection of cis-combretastatins by post-column photolysis

A method is described to enhance the sensitivity of fluorescence detection of cis-combretastatins using a short post-column photolysis coil with a mercury lamp, by inducing the rapid conversion to the trans isomer. Although all the compounds studied showed enhanced fluorescence after photolysis, there were large differences in the absolute level, with the inherent response of the catechol CA1 being much lower than the corresponding phenolic CA4. Brief exposure to the deuterium lamp in a photodiode array detector also resulted in significant enhancement.     

Simultaneous multielemental determination using a low-resolution inductively coupled plasma spectrometer/diode array detection system

This work presents a versatile method for simultaneous multielemental analysis using detection with photodiode arrays and multivariate calibration techniques. A multidetection system based on an array of 1024 photodiodes was built and adapted to a commercial plasma emission scanning spectrometer. Spectral data were acquired at low resolution, allowing simultaneous monitoring of a broad spectral range and resulting in very informative but considerably overlapped spectra. Partial least squares and principal component regressions were employed to minimize overlapping problems. A numerical procedure for window selection was also developed.

The new method was applied to the simultaneous determination of manganese, molybdenum, nickel, chromium and iron in steel samples, resulting in average relative prediction errors of 2.1% for Mn, 3.4% for Mo, 0.6% for Cr, 1.5% for Ni and 0.8% for Fe. These errors are comparable to those observed with conventional scanning detection systems and separate univariate calibrations, but the new method allows simultaneous determination of the five elements, with data acquisition significantly faster than in scanning instruments.     

Capabilities of a photodiode array based system for cataloguing the atomic emission spectra of the inductively coupled plasma

This paper describes a photodiode array based system and discusses its capabilities for cataloguing the atomic emission spectra emitted from an inductively coupled plasma. Dynamic range and line coverage are assessed and compared with those achievable with Fourier transform spectrometry.     

An electron bombardment CCD-camera as detection system for an echelle spectrometer

The electron bombardment CCD (EB-CCD)-camera system is a very sensitive detector designed mainly for low-light imaging applications. The major advantage compared to a microchannel plate intensified CCD system is the superior spatial resolution due to the special amplification system. This property would make it an ideal detection system for a high-resolution echelle spectrometer, where high sensitivity in combination with high spatial resolution is required. However, we found that part of the light focused on the photocathode of the EB-CCD is transmitted through it and detected on the CCD chip as a diffuse spot. Thus, it lowers the signal to background ratio and can lead to ghost peaks. The problem becomes particularly serious when detecting emission lines of very different intensities.     

Development of a pulsed-laser,fiber-optic-based fluorimeter: determination of fluorescence decay times of polycyclic aromatic hydrocarbons in sea water

A fiber-optic-based system for remote measurement of time-resolved fluorescence emission spectra is described and characterized. A pulsed nitrogen laser is used to induce fluorescence and a time-gated, one-dimensional photodiode array is used to measure the decay of the fluorescence emission spectra. The results compare favorably with reported values for well characterized compounds having fluorescence decay times in the range 4–50 ns. The potential of using time-resolved fluorimetry (TRF) over fiber-optic cables as a means of improving the specificity of remote fluorescence determinations of spectrally similar polycyclic aromatic hydrocarbons in sea water is demonstrated.