Consideration of some sampling problems in the on-line analysis of batch processes by low-field NMR spectrometry

A low-field medium-resolution NMR spectrometer, with an operating frequency of 29 MHz for 1H, has been assessed for on-line process analysis. A flow cell that incorporates a pre-magnetisation region has been developed to minimise the decrease in the signal owing to incomplete polarisation effects. The homogeneous esterification reaction of crotonic acid and 2-butanol was monitored using a simple sampling loop; it was possible to monitor the progression of the reaction through changes in CH signal areas of butanol and butyl crotonate. On-line analysis of heterogeneous water-toluene mixtures proved more challenging and a fast sampling loop system was devised for use with a 5 L reactor. The fast sampling loop operated at a flow rate of 8 L min(-1) and a secondary sampling loop was used to pass a sub-sample through the NMR analyser at a slower (mL min(-1)) rate. It was shown that even with super-isokinetic sampling conditions, unrepresentative sampling could occur owing to inadequate mixing in the reactor. However, it was still possible to relate the 1H NMR signal obtained at a flow rate of 60 mL min(-1) to the composition of the reactor contents.     

Ultrasonic nebulization extraction coupled with on-line gas chromatography for determination of trans-anethole in spices

Ultrasonic nebulization extraction (UNE) coupled with on-line gas chromatography (GC) was proposed for the determination of trans-anethole in fruits of Illicium verum Hook. f. and Foeniculum vulgare Mill. The extraction was performed in a common self-made extraction system. In the UNE the analyte was transferred and enriched from the solid sample to gas phase. The sample gas containing analyte obtained by UNE was introduced into the sampling loop with the purging gas (N₂). And then the sample gas in the sampling loop was introduced into the GC column. Several experimental parameters of on-line UNE-GC, including sampling time, flow rate of purging gas, standstill time and temperature of tubing, were optimized. The calibration curve ranging from 0.05 to 1.5 mg g⁻¹ for determining the trans-anethole was obtained. The recoveries for determining trans-anethole are between 99.2% and 111.2% and RSDs are less than 8.3% when the UNE was applied. The analytes can rapidly be extracted and transferred from the solid sample to gas phase. The analytes in the gas phase are easier to be introduced into GC system than those in the solid and liquid phase. Compared with off-line systems, the proposed on-line system is more suitable to detect volatile compounds.     

Sampling frequency,response times and embedded signal filtration in fast,high efficiency liquid chromatography: A tutorial

With increasingly efficient columns, eluite peaks are increasingly narrower. To take full advantage of this, choice of the detector response time and the data acquisition rate a.k.a. detector sampling frequency, have become increasingly important. In this work, we revisit the concept of data sampling from the theorem variously attributed to Whittaker, Nyquist, Kotelnikov, and Shannon. Focusing on time scales relevant to the current practice of high performance liquid chromatography (HPLC) and optical absorbance detection (the most commonly used method), even for very narrow simulated peaks Fourier transformation shows that theoretical minimum sampling frequency is still relatively low (<10 Hz). However, this consideration alone may not be adequate for real chromatograms when an appreciable amount of noise is present. Further, depending on the instrument, the manufacturer's choice of a particular data bunching/integration/response time condition may be integrally coupled to the sampling frequency. In any case, the exact nature of signal filtration often occurs in a manner neither transparent to nor controllable by the user. Using fast chromatography on a state-of-the-art column (38,000 plates), we evaluate the responses produced by different present generation instruments, each with their unique black box digital filters. We show that the common wisdom of sampling 20 points per peak can be inadequate for high efficiency columns and that the sampling frequency and response choices do affect the peak shape. If the sampling frequency is too low or response time is too large, the observed peak shapes will not remain as narrow as they really are – this is especially true for high efficiency and high speed separations. It is shown that both sampling frequency and digital filtering affect the retention time, noise amplitude, peak shape and width in a complex fashion. We show how a square-wave driven light emitting diode source can reveal the nature of the embedded filter. We discuss time uncertainties related to the choice of sampling frequency. Finally, we suggest steps to obtain optimum results from a given system.     

Ermittlung der analysenfrequenz bei diskontinuierlicher prozessanalytischen verfahren: Establishment of the frequency of analysis for discontinuous measurements in process control

Establishment of the frequency of analysis for discontinuous measurements in process control.Application of sampling theory to industrial chemical processes involves special problems arising from the peculiarities of the processes and of the analytical work. Apart from in the process itself, it is necessary to consider the statistical error of the correlation function caused by the finite length of the observed signal. Knowledge of the correlation time of the stochastic signal makes it possible to estimate the optimal time-difference between two analyses, depending on the variance of the process and the allowed analytical tolerance.     

Multi-frequency S2FTIR PA spectral depth profiling by use of sinusoidal phase modulation and harmonic demodulation up to 2250 Hz (10th harmonic)

Multi-frequency phase modulation for step-scan Fourier transform infrared photoacoustic spectroscopy (S²FTIR PAS) is demonstrated using a single frequency sinusoidal modulation of the interferometer movable mirror with modulation amplitude 5.2 λ_He-Ne. A digital signal processing (DSP) lock-in amplifier is used to demodulate the photoacoustic response at the fundamental phase modulation frequency and its harmonic frequencies. Because the phase modulation is sinusoidal and of the appropriate amplitude, all the even and odd harmonics (up to the 10th harmonic) can be detected with good signal-to-noise ratio (SNR). Applications of this multi-frequency phase modulation approach for photoacoustic depth profiling are demonstrated by the study of a simple two-layered polymer sample. The sampling depth multiplexing advantage (as compared to that obtained with other types of phase modulation) is clearly demonstrated with the analysis of the phase and magnitude of signals at the fundamental frequency and all the harmonic frequencies. This is the first report of using the sinusoidal phase modulation method to easily obtain multi-frequency and high-frequency light modulation for FTIR PAS depth profiling. Although these data have been obtained sequentially, it would be possible (with considerable economy of time) to obtain them simultaneously by use of the internal digital signal processing capability of the instrument.     

多相催化反应的在线光谱-质谱探测系统

在研究多相催化反应中,建立和发展了一套在线高灵敏的共振增强多光子电离-飞行时间质谱检测系统。采用光纤作为传输介质,消除了激光在电离池内的吸收;自制了高输入阻抗的电荷灵敏放大器,提高了信号的稳定性;改进了飞行时间质谱仪中脉冲阀的进气方式,减少了死区,实现了质谱的快速采样。实验结果证明与传统的催化检测仪器相比,本系统具有灵敏度和采样率高,在线实时检测等特点。     

Indirect atomic absorption spectrometric determination of perchlorate by liquid-liquid extraction in a flow-injection system

The determination is based on the continuous liquid-liquid extraction of the copper(I)/6-methylpicolinealdehyde azine/perchlorate ion-pair into 4-methyl-2-pentanone in a flow-injection manifold. The organic fills a loop of an injector situated in an integrated feed system of an atomic absorption spectrometer. Measurement of the copper atomic absorption signal from the organic phase allows the indirect determination of 0.5–5 μg ml^?1 perchlorate, giving a 3-fold increase in sensitivity over the conventional spectrophotometric method. The detection limit is 70 ng ml^?1. The sampling frequency is 45 ± 5 h^?1. The method is highly selective, and has been used for the determination of perchlorate added to serum and urine samples.     

微流动注射进样-加热雾室-等离子体质谱测定血清中的铂

开发了一种基于雾化室加热的微流动注射进样系统,并用于血清中Pt的测定。该进样系统由微量毛细管雾化器、加热微型雾化室、八通道十六孔多功能旋转阀、蠕动泵和注射泵组成。研究了雾化室尺寸、加热温度和采样环体积对信号强度的影响。当雾化室内径为9 mm、加热段长度为6 cm,雾化室温度90 ℃,采样环体积为5 μL时,195Pt的信号强度提高了2.31倍,同时信号精密度从5.1%降至2.2%,并得到峰形良好的信号峰。该进样系统的试样消耗小、灵敏度和检出限均优于常规进样系统。10次测定10 μg/L的Pt标准溶液和血清样品溶液,峰高的RSD分别为2.9%和3.3%。该进样系统测得10个血清中的Pt含量与常规进样系统的测试结果无显著差异,在样品量稀少的情况下具有良好的应用价值。     

Sample preconcentration by continuous flow extraction with a flow injection atomic absorption detection system

The flow manifold described allows automatic extraction of metal ions in aqueous samples into 4-methyl-2-penthanone with ammonium pyrrolidinedithiocarbamate as an extracting agent. The organic extract is led into the loop of an injector situated in an integrated feed system of an atomic absorption spectrometer. No dispersion of the injected organic extract plug, 110 μl, occurs in the aqueous feed stream and the resulting signal from the spectrometer is a peak. An increase in sensitivity of 15–20 is achieved for copper, nickel, lead and zinc in comparison with direct aspiration of the aqueous samples. The sampling frequency is 40 h^?1 and the consumption of 4-methyl-2-pentanone is typically 0.3 ml min^?1. The detection limit for copper is about 1 μ l^?1.     

Spatially resolved surface enhanced Raman spectroscopy: Feasibility,intensity dependence on sampling area and attomole mass sensitivity

Spatially resolved surface enhanced Raman scattering (SR SERS) from μm sized sampling areas has been observed in the electrochemical environment for the first time. Analysis of the equation for SR SERS intensity reveals that the detected signal should be area independent. This is demonstrated experimentally for the aqueous pyridine/Ag model system using 200 kW cm⁻² peak laser irradiance. The mass detection limits for SR SERS are found to be on the order of 10⁵ molecules or 0.17 attomoles!     

干湿循环下混凝土中钢筋腐蚀的电化学检测

应用电化学噪声技术,结合电化学阻抗谱研究了干湿循环条件下3种不同pH值的3.5%NaCl溶液中混凝土钢筋的腐蚀过程.结果表明,钢筋的腐蚀分为3个阶段:钝化膜的溶解期、腐蚀活化期和腐蚀产物的累积期.在pH1的溶液中经过30个干湿循环后主要以均匀腐蚀为主,而对pH3和pH7溶液,则主要以点蚀为主,散粒噪声分析证实了混凝土中钢筋在强酸性溶液中更容易发生腐蚀.电化学阻抗谱分析也进一步说明了混凝土中钢筋的腐蚀经历了3个阶段:第1阶段Nyquist谱图中出现两个时间常数,高频区为混凝土层的容抗性质,低频区为钢筋与混凝土界面的电荷转移电阻;第2阶段,中频区增加一个时间常数,这与腐蚀产物的累积有关;第3阶段,随着腐蚀产物向混凝土中的扩散,中频区容抗弧有所减小,特别是pH1体系,中频弧几乎消失.SEM形貌表征观察到随着溶液酸度的增加钢筋表面的锈层明显增多.     

A high-speed,high-precision data acquisition and processing system for experiments producing steady-state periodic signals

A wide variety of experiments investigate the response of a sample to a steady-state periodic excitation. We describe a computerized data acquisition and processing system for experiments of this type assembled from relatively inexpensive, commercially available components which has been optimized for highly precise determinations of the magnitude and relative phasing of drive and response signals having frequencies from 1 μHz to approximately 1 MHz. Through the use of timing control with a precision of 1 part in 10⁹ and high speed on-line signal processing, the system combines a very high rate of data acquisition with extended signal averaging times, enabling it to acquire an uninterrupted data stream consisting of at least 10⁹ samples. Furthermore, the system acts as a digital filter which can be tuned to completely reject unwanted signals of any frequency other than the exact signal frequency. This allows accurate characterization of sample response even when obscured by other, extraneous signals of substantially larger magnitudes as long as they have at least somewhat different frequencies. Oscillatory experiments on very dilute polymer solutions, including oscillatory flow birefringence, dynamic viscoelasticity, oscillatory electric birefringence and dielectric measurements, are important examples of this type of measurement situation, in which the experiment depends critically on accurate determination of the magnitude and phasing of weak response signals, frequently under extremely poor signal-to-noise conditions. The system has an aggregate sampling rate of 1 MHz and up to four input channels. The potential for distortion of the results by aliasing frequencies may limit the input signal frequency to less than 500 kHz for one channel. © 1994 John Wiley & Sons, Inc.     

Effect of plasmatron excitation conditions on the determination of traces

Optimal analysis conditions can be selected and errors caused by variations in the excitation conditions can be decreased or corrected based on the results of studying the temperature distribution along the plasma torch height and correlations between the analytical signal Y and the correcting signal Z that follows the temperature variation in the plasma torch. The obtained correlations can be used to create automated feedback between the spectrum recording system and the mode control system of the plasmatron.     

Accelerating Diffusion‐Ordered NMR Spectroscopy by Joint Sparse Sampling of Diffusion and Time Dimensions

Diffusion‐ordered multidimensional NMR spectroscopy is a valuable technique for the analysis of complex chemical mixtures. However, this method is very time‐consuming because of the costly sampling of a multidimensional signal. Various sparse sampling techniques have been proposed to accelerate such measurements, but they have always been limited to frequency dimensions of NMR spectra. It is now revealed how sparse sampling can be extended to diffusion dimensions.     

Implementation of the EU Water Framework Directive in monitoring of small water bodies in Hungary, I. Establishment of surveillance monitoring system for physical and chemical characteristics for small mountain watercourses

The EU Water Framework Directives (WFD) prescribes to achieve a good status of surface waters by 2015. To keep this deadline of a new three level (surveillance, operational and investigative) monitoring system is required, including hydro-morphological, biological and chemical elements. The status of a water body should be related to the reference conditions, depending on surface water types. In Hungary, the standard monitoring network for surface water quality has been operated for rivers and large lakes; however no satisfactory information is available for small water bodies. Therefore, there is an urgent need to design a methodology to develop a monitoring network for small watercourses. For this purpose a slightly impacted mountain stream (Nagy Stream in Mátra Mountains), was selected and intensively monitored during three years (May 2002 → May 2005). Hydrological, biological and chemical parameters required by WFD were monitored in this program. Variability of parameters vs. time and space was also investigated. The minimum frequency of sampling was determined for chosen physico-chemical components to meet the WFD standards for confidence. It was found that the minimum frequency of sampling suggested by WFD in space and time may not be adequate for precise and accurate characterization of water quality.     

Instabilities in electric fields of nematic liquid crytals with positive dielectric anisotropy: domains,loop domains and reorientation

Instabilities in electric fields are reported for p,p′-di-n-butyl azoxybenzene (DIBAB), a new room temperature nematic with a positive dielectric anisotropy. The threshold voltage is found to be almost independent of the frequency up to 100 kHz. However, unlike in nematics with negative dielectric anisotropy usually no normal domains are observed. The instabilities take the form of a reorientation, while loop domains are observed as a transiet effect. These loop domains indicate that the instability is not purely dielectric in origin.     

Recent trends in microdialysis sampling integrated with conventional and microanalytical systems for monitoring biological events: A review

Microdialysis (MD) is a sampling technique that can be employed to monitor biological events both in vivo and in vitro. When it is coupled to an analytical system, microdialysis can provide near real-time information on the time-dependent concentration changes of analytes in the extracellular space or other aqueous environments. Online systems for the analysis of microdialysis samples enable fast, selective and sensitive analysis while preserving the temporal information. Analytical methods employed for online analysis include liquid chromatography (LC), capillary (CE) and microchip electrophoresis and flow-through biosensor devices. This review article provides an overview of microdialysis sampling and online analysis systems with emphasis on in vivo analysis. Factors that affect the frequency of analysis and, hence, the temporal resolution of these systems are also discussed.     

On‐Line Monitoring of Chemical Reactions by using Bench‐Top Nuclear Magnetic Resonance Spectroscopy

Real‐time nuclear magnetic resonance (NMR) spectroscopy measurements carried out with a bench‐top system installed next to the reactor inside the fume hood of the chemistry laboratory are presented. To test the system for on‐line monitoring, a transfer hydrogenation reaction was studied by continuously pumping the reaction mixture from the reactor to the magnet and back in a closed loop. In addition to improving the time resolution provided by standard sampling methods, the use of such a flow setup eliminates the need for sample preparation. Owing to the progress in terms of field homogeneity and sensitivity now available with compact NMR spectrometers, small molecules dissolved at concentrations on the order of 1 mmol L^?1 can be characterized in single‐scan measurements with 1 Hz resolution. Owing to the reduced field strength of compact low‐field systems compared to that of conventional high‐field magnets, the overlap in the spectrum of different NMR signals is a typical situation. The data processing required to obtain concentrations in the presence of signal overlap are discussed in detail, methods such as plain integration and line‐fitting approaches are compared, and the accuracy of each method is determined. The kinetic rates measured for different catalytic concentrations show good agreement with those obtained with gas chromatography as a reference analytical method. Finally, as the measurements are performed under continuous flow conditions, the experimental setup and the flow parameters are optimized to maximize time resolution and signal‐to‐noise ratio.     

Mobile, outdoor continuous-flow isotope-ratio mass spectrometer system for automated high-frequency 13C- and 18O-CO2 analysis for Keeling plot applications

A continuous-flow isotope-ratio mass spectrometer (CF-IRMS, custom-made GasBenchII and Delta(plus)Advantage, ThermoFinnigan) was installed on a grassland site and interfaced with a closed-path infrared gas analyser (IRGA). The CF-IRMS and IRGA were housed in an air-conditioned travel van. Air was sampled at 1.5 m above the 0.07-m tall grassland canopy, drawn through a 17-m long PTFE tube at a rate of 0.25 L s(-1), and fed to the IRGA and CF-IRMS in series. The IRMS was interfaced with the IRGA via a stainless steel capillary inserted 0.5 m into the sample air outlet tube of the IRGA (forming an open split), a gas-tight pump, and a sample loop attached to the eight-port Valco valve of the continuous-flow interface. Air was pumped through the 0.25-mL sample loop at 10 mL s(-1) (a flushing frequency of 40 Hz). Air samples were analysed at intervals of approx. 2.8 min. Whole system precision was tested in the field using air mixed from pure CO2 and CO2-free air by means of mass flow controllers. The standard deviation of repeated single measurements was 0.21-0.07 per thousand for delta13C and 0.34-0.14 per thousand for delta18O of CO2 in air with mixing ratios ranging between 200-800 micromol mol(-1). The CO2 peak area measured by the IRMS was proportional to the CO2 mixing ratio (r2 = 1.00), allowing estimation of sample air CO2 mixing ratio from IRMS data. A 1-day long measurement cycle of CO2, delta13C and delta18O of air sampled above the grassland canopy was used to test the system for Keeling plot applications. Delta18O exhibited a clear diurnal cycle (4 per thousand range), but short-term (1-h interval) variability was small (average SD 0.38 per thousand). Yet, the correlation between delta18O and CO2 mixing ratio was relatively weak, and this was true for both the whole data set and 1-h subsets. Conversely, the delta13C of all 541 samples measured during the 25.2-h interval fitted well the Keeling regression (r2 = 0.99), yielding an intercept of -27.40 per thousand (+/-0.07 per thousand SE). Useful Keeling regressions (r2 > 0.9, average r2 = 0.96) also resulted from data collected over 1-h intervals of the 12-h long twilight and dark period. These indicated that 13C content of ecosystem respiration was approx. constant near -27.6 per thousand. The precision of the present system is similar to that of current techniques used in ecosystem studies which employ flask sampling and a laboratory-based CF-IRMS. Sampling (and measurement) frequency is greatly increased relative to systems based on flask sampling, and sampling time (0.025 s per sample) is decreased. These features increase the probability for sampling the entire CO2 range which occurs in a given time window. The system obviates sample storage problems, greatly minimises handling needs, and allows extended campaigns of high frequency sampling and analysis with minimal attendance.