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.     

On-line measurements of delta(15)N in biological fluids by a modified continuous-flow elemental analyzer with an isotope-ratio mass spectrometer

A modified continuous-flow elemental analyzer coupled to an isotope-ratio mass spectrometer (modified EA-IRMS) was tested for on-line delta(15)N measurement on urea solution and biological fluids (e.g. urine). The elemental analyzer configuration was adapted by adding a U-shaped cold trap and an X-pattern four-way valve for on-line trapping/venting of water from the liquid samples. Results indicate that the delta(15)N ratios show little variation (standard deviation (SD) = 0.05 per thousand) with a sample size above the equivalent N yield of 0.2 mg urea (0.092 mg N) when the mass spectrometer conditions were carefully optimized. By contrast, a significant logarithmic decrease in delta(15)N with sample size was observed but this can be offset by applying a linearity correction or blank correction when the sample size is between equivalent N yields of 0.05 and 0.2 mg urea. The blank corrected delta(15)N ratios give an overall precision of approximately 0.16 per thousand whereas the average precision for delta(15)N corrected using combined linearity and shift correction is 0.05 per thousand. The relatively large variation in blank corrected delta(15)N values may be attributed to the variability of the blank delta(15)N in the sequence. Therefore, the blank correction should be carefully performed in routine measurements. As a result, the linearity range of a modified EA-IRMS can be extended to a minimum sample size of 0.023 mg N. In addition, the reproducibility of the new system is good, as indicated by the precision (<0.2 per thousand) for a set of standards and unknowns. The data show that fluids containing nitrogen can be successfully analyzed in the modified EA-IRMS.     

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.     

Noise in atomic absorption flame spectrometry—a comparison between single beam and double beam instruments

Noise levels have been measured under a variety of conditions using comparable single and double beam instruments. At low absorbance, in the vicinity of the detection limit, the major types of noise are photon noise, flame transmission noise and lamp flicker noise. The precision at low absorbance is slightly better with single than double beam providing a blank correction is made before and after each sample measurement. At higher absorbances, analyte absorption noise is dominant and there is no difference in precision between single and double beam.     

Influence of the spectral bandwith of the spectrometer on the sensitivity using continuum source AAS

The influence of the spectral bandwidth of the spectrometer on the performance of the spectrometer is of great importance in atomic absorption spectrometry using continuum source (CS-AAS). For a theoretical analysis of the detection of small absorbances two cases have to be taken into account: as long as the limiting noise of the signal is given by the detector noise an increase in spectral bandwidth of the spectrometer implies an improvement in the limit of detection. In contrast, if the noise of the signal is dominated by the photon shot noise the detection limit should become independent of the spectral bandwidth. In this case the spectral bandwidth of the spectrometer should be chosen equal to the bandwidth of the absorption line to avoid spectral interferences. Theoretical calculations are presented for the dependence of the characteristic mass on the spectral bandwidth in case of CS-AAS measurements. The results are compared with experimental measurements for six different elements using continuum source as well as line sources. The investigations were done using a double echelle monochromator (DEMON) which offers a high spectral resolution lambda/Deltalambda of about 140,000.     

A new membrane inlet interface of a vacuum ultraviolet lamp ionization miniature mass spectrometer for on-line rapid measurement of volatile organic compounds in air

A novel membrane inlet interface coupled to a single-photon ionization (SPI) miniature time-of-flight mass spectrometer has been developed for on-line rapid measurement of volatile organic compounds (VOCs). The vacuum ultraviolet (VUV) light source for SPI was a commercial krypton discharge lamp with photon energy of 10.6 eV and photon flux of 10(10) photons/s. The experimental results showed that the sensitivity was 5 times as high as obtained with the traditional membrane inlet. The enrichment efficiency could be adjusted in the range of 10 to 20 times for different VOCs when a buffer cell was added to the inlet interface, and the memory effect was effectively eliminated. A detection limit as low as 25 parts-per-billion by volume (ppbv) for benzene has been achieved, with a linear dynamic range of three orders of magnitude. The rise times were 6 s, 10 s and 15 s for benzene, toluene and p-xylene, respectively, and the fall time was only 6 s for all of these compounds. The analytical capacity of this system was demonstrated by the on-line analysis of VOCs in single puff mainstream cigarette smoke, in which more than 50 compounds were detected in 2 s.     

Concept for a single-shot mid-infrared spectrometer using synchrotron radiation

Several attempts have been made to extend time-resolved mid infrared spectroscopy to higher time resolution. Such methods are either limited to specific samples that are cyclic and therefore allow the reaction under investigation to be repeated multiple times in the same manner, or they lack spectral resolution or sufficient signal-to noise ratio. Here, we report on a single-shot spectrometer concept which overcomes the aforementioned limitations utilizing fast linear detector arrays and highly brilliant infrared synchrotron radiation. The spectrometer may find applications, beside others, for the investigation of irreversible cascades of structural alterations in proteins.     

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.     

Identification of diethylene glycol monobutyl ether as a source of contamination in an ion trap mass spectrometer

Tandem liquid chromatography-mass spectrometry coupled to online radioactive material detection (LC/RAM/MS/MS) is a technique that is used routinely for in vivo and in vitro drug metabolism studies and allows for a simultaneous correlation between radiochemical peaks and mass spectral data. The compound diethylene glycol monobutyl ether (DGBE), a component of a commercially available scintillation cocktail for RAM analysis, was identified as a source of overwhelming chemical noise in a mass spectrometer which was used in an LC/RAM/MS/MS configuration. In this report, we describe the identification of DGBE as the source of the chemical noise and the methods that were used to minimize the exposure of the mass spectrometer to volatile components of the scintillation cocktail.     

Application of Response Surface Methodology to laser-induced breakdown spectroscopy: Influences of hardware configuration

Response Surface Methodology (RSM) was employed to optimize LIBS analysis of single crystal silicon at atmospheric pressure and under vacuum conditions (pressure  10^− 6 mbar). Multivariate analysis software (StatGraphics 5.1) was used to design and analyze several multi-level, full factorial RSM experiments. A Quality Factor (QF) was conceived as the response parameter for the experiments, representing the quality of the LIBS spectrum captured for a given hardware configuration. The QF enabled the hardware configuration to be adjusted so that a best compromise between resolution, signal intensity and signal noise could be achieved. The effect on the QF of simultaneously adjusting spectrometer gain, gate delay, gate width, lens position and spectrometer slit width was investigated, and the conditions yielding the best QF determined.     

Influence of the spectral bandwith of the spectrometer on the sensitivity using continuum source AAS

The influence of the spectral bandwidth of the spectrometer on the performance of the spectrometer is of great importance in atomic absorption spectrometry using continuum source (CS-AAS). For a theoretical analysis of the detection of small absorbances two cases have to be taken into account: as long as the limiting noise of the signal is given by the detector noise an increase in spectral bandwidth of the spectrometer implies an improvement in the limit of detection. In contrast, if the noise of the signal is dominated by the photon shot noise the detection limit should become independent of the spectral bandwidth. In this case the spectral bandwidth of the spectrometer should be chosen equal to the bandwidth of the absorption line to avoid spectral interferences. Theoretical calculations are presented for the dependence of the characteristic mass on the spectral bandwidth in case of CS-AAS measurements. The results are compared with experimental measurements for six different elements using continuum source as well as line sources. The investigations were done using a double echelle monochromator (DEMON) which offers a high spectral resolution / of about 140,000.     

Cryogenic instrumentation and detector limits in FTS

The desire to measure atmospheric emission spectra of the upper atmosphere has led to the development of cryogenically cooled infrared Fourier transform spectrometers. These spectrometers combine the sensitivity advantage of a Fourier transform spectrometer with the sensitivity gains obtainable with modern infrared detectors operated at low photon backgrounds. The temperature required to obtain maximum sensitivity is primarily determined by the wavelength region of interest, longer wavelengths requiring lower temperatures. In order to take full advantage of reduced background operation, the preamplifier must be cooled in order to reduce the noise. Low noise preamplifiers capable of operation down to liquid helium temperatures have been developed. Three cryogenically cooled spectrometers of significantly different design are discussed. The first instrument is a standard Michelson interferometer cooled to 10 K with supercritical helium and capable of 1 wave number resolution. This interferometer has been successfully flown on a rocket and obtained measurements of atmospheric limb emission at tangent altitudes from 70 km to 150 km. The second is a field widened prism interferometer which operates at liquid nitrogen temperature and has been flown on a rocket and obtained measurements of atmospheric emission in the 2 to 8 micrometer region during an aurora. The third is a balloon borne liquid nitrogen cooled interferometer which incorporates cat's eye retroreflectors for end mirrors and is capable of 0.1 wave number resolution. This spectrometer has successfully flown on a balloon several times and obtained measurements of atmospheric limb emission at tangent altitudes from 0 km to 30 km.     

Photon-by-photon determination of emission bursts from diffusing single chromophores

One of the difficulties in diffusion-type single-molecule experiments is the determination of signal amid photon-counting noise. A commonly used approach is to further average the noisy time trace by binning, followed by placing a threshold to discriminate signal from background. The choice of smoothing parameters and the placement of the threshold may impact on the efficiency with which the information-rich region can be harvested, among other potential complications. Here we introduce a procedure that operates on the data sequence photon by photon, thereby relieving the incertitude in choosing binning-thresholding parameters. We characterize this procedure by detecting the two-photon emission bursts from diffusing single gold nanoparticles. The results support our burst-finding procedure as a reliable and efficient way of detecting and harvesting photon bursts from diffusing experiments.     

An inductively coupled plasma-time-of-flight mass spectrometer for elemental analysis. Part II: Direct current quadrupole lens system for improved performance

An electrostatic quadrupole lens has been substituted for a cylindrical lens system used in the original inductively coupled plasma-time-of-flight mass spectrometer (ICP-TOFMS). With an improved vacuum system also installed, the cylindrical and quadrupole lenses are compared to each other and to the performance of the prototype ICP-TOFMS. The quadrupole lens requires no tradeoff between ion throughput and resolving power as was encountered with cylindrical lenses. The background noise in both ion-optical systems is within the same order of magnitude. Images of the ion beam formed by each ion-optical system have been obtained on a microchannel plate-phosphor screen. The quadrupole lens shows a higher ion-beam flux and produces a slitlike focus required in the orthogonal ICP-TOFMS instrument. Signal-to-noise ratios in the ICP-TOFMS can be improved by using a technique called pulsed-ion injection that is particularly convenient with the quadrupole lens. In this technique, one quadrupole electrode is pulsed to prevent ions from entering the extraction zone except when an ion packet is to be extracted for mass analysis. This technique significantly reduces the noise over continuous ion injection. In the orthogonal ICP-TOFMS with pulsed-ion injection, 0.5 frnol of analyte could be detected in 1.4 ms with a proper data acquisition system. Overall, the combination of a quadrupole lens and pulsed-ion injection may provide detection limits for the ICP-TOFMS that are competitive with those of quadrupole inductively coupled plasma-mass spectrometry instruments.     

A computer controlled photon counting flame emission spectrometer

An automated emission spectrometer has been developed around a simple commercial instrument. The manner in which a computer is used for control and the way data are acquired and assimilated are discussed. The interfacing to the computer is described, sample programs are illustrated and methods of analyses are given. Sensing is achieved through photon counting. The linearity of the system has been examined and it was found that, for lithium, quantitative results can be obtained with an accuracy of 1 per cent for a concentration range of four orders of magnitude. The time required for analysis is discussed and, although the spectrometer is serial in nature, the increased information rate obtained through photon counting results in high speed data acquisition.     

Theoretical multiplex gain in a UV/VIS Hadamard transform spectrometer utilizing a uniformly imperfect encoding mask

The effect of a uniformly imperfect (non-ideal) encoding mask on the signal-to-noise ratio improvement in a Hadamard transform spectrometer utilizing photon detection is theoretically studied. General equations are developed for the calculation of the multiplex gain (the Fellgett advantage) under conditions of limitation by shot noise and source-fluctuation noise. It is shown that for both cases, the multiplex gain depends on the transmission properties of the encoding mask, the UV/VIS spectrum impinging upon the mask, and the multiplex size, N. It is demonstrated that a uniformly imperfect encoding mask allows sufficient multiplex gain, and that photon-detection in Hadamard transform spectrometry can have advantages in some spectroscopic applications. In addition, comparisons are made between the multiplex gain advantages present in UV/VIS FTS and those present in UV/VIS HTS.     

EMISSION SPECTRUM OF THE MICROSOMAL CHEMILUMINESCENCE OF A PROXIMATE CARCINOGEN, 7,8-DIOL-BENZO[a]PYRENE, DETERMINED WITH A WEDGE INTERFERENCE FILTER SPECTROMETER*

Abstract— The emission spectrum of the microsome-mediated chemiluminescence of a proximate carcinogenic metabolite of benzo[a]pyrene, 7,8-diol-benzo[a]pyrene. has been measured with a wedge interference filter spectrometer calibrated with known chemiluminescent and bioluminescent reactions. This instrument has a noise equivalent signal at 490 nm (bandpass 24 nm) of 10⁴ photon s-¹ at 20°C. Spectral and kinetic data indicate that the microsomal chemilurninescence of this metabolite is an exiplex chemiluminescence that proceeds through a dioxetane intermediate.     

催化裂解-金汞齐富集-冷原子吸收光谱仪测定土壤样品中总汞含量

为避免汞消解过程中产生的蒸发损失及前处理过程中产生器具与样品污染,通过配制汞总量为0～2、0～15和25～1 023 ng的三种不同汞浓度系列的标准工作曲线,选取9个土壤样品、3种国家土壤有证标准物质,同一样品分别进行6组平行测定,并抽取3个土壤样品进行3种不同浓度加标回收实验,以对其方法精密度和准确度进行论证,建立了催化裂解-金汞齐富集-冷原子吸收光谱法即直接测汞仪测定土壤样品中汞含量的方法。结果显示,仪器信号值与Hg总量之间均呈良好的线性关系。根据仪器多次测定空白数据结果,按照称样量0.1 g计算,方法检出为0.09 ng/g;平行测定结果相对标准偏差均小于10%,土壤标准物质测定值与参考值均相符,不同浓度的加标回收率范围为69.0%～97.0%。方法可用于批量土壤样品中汞含量的快速测定,精密度和准确度可满足测定要求,且实验过程中无需前处理消解,操作方便、快速高效。     

Laser pulse fluorometry as a tool in polymer research

A time-resolved fluorescence spectrometer with laser excitation and picosecond time-correlated single photon counting detection is described. The parameter recovery from experimental data is discussed in detail. Correction methods for errors in the time-correlated single photon counting technique are indicated. Applications of this photophysical tool to polymer research are given.