LCMS/MS and TOF-SIMS identification of the color bodies on the surface of a polymer

The source of discoloration on a polymer surface can often be identified by washing the surface of the discolored polymer to collect the color bodies, then analyzing the washings using liquid chromatography-mass spectrometry (LCMS), with an in-line ultraviolet (UV) detector set at the optimum wavelength for the particular color bodies. A reference sample having no discoloration is also analyzed in the same way. In this paper, results from this methodology are compared with direct time of flight-secondary ion mass spectrometry (TOF-SIMS) analysis of a discolored polymer. The benefits and shortcomings of each methodology are discussed.     

A review and tutorial discussion of noise and signal-to-noise ratios in analytical spectrometry—III. Multiplicative noises

In this review, signal-to-noise ratios are discussed in a tutorial fashion for the case of multiplicative noise. Multiplicative noise is introduced simultaneously with the analyte signal and is therefore much more difficult to reduce than additive noise. The sources of noise, the mathematical representation of noise, and the major types of noises in emission and luminescence spectrometry are discussed. If the limiting source of noise is multiplicative white noise, the signal-to-noise ratio for optimal sampling time τ_s increases as the square root of the response or integration time of the readout and is independent of the rate at which sample and reference are measured. The variation of multiplicative flicker noise with variation in sampling time, τ_s, time interval between sample and reference measurements, T, and response (τ_r) or integration (τ_i) time is discussed in some detail. The optimal system for the case of multiplicative noise is a dual channel approach in which the sample and reference are measured simultaneously and a ratio of signals is taken. Although the best reference in most cases of interest to analytical chemists is a calibration standard, it is often impossible to measure a sample and a calibration standard simultaneously and so an internal standard, a detector monitoring the source intensity, etc., may be useful.     

Determination of pesticides and other physiologically active compounds by capillary gas chromatography with an atomic-emission detector without using standard reference materials

It was shown that organic substances can be determined by gas chromatography with an atomicemission detector without using standard reference materials of these substances or information on their toxicity, but using reference materials of other compounds containing similar hetero-atoms     

Investigation of chromatographic peak broadening in supercritical fluid chromatography/atmospheric pressure chemical ionization mass spectrometry

Multimode ionization source allows for switching between different ionization techniques, for example, electrospray and atmospheric pressure chemical ionization, within a single analysis. Supercritical fluid chromatography can handle a wide polarity range of substances from hydrophilic to lipophilic in a single run and can undoubtedly benefit from versatility of this ion source. Nevertheless, we observed a significant chromatographic peak broadening effect in atmospheric pressure chemical ionization mode during supercritical fluid chromatography‐mass spectrometry analysis of volatile flavor compounds with a dual ion source named ESCi (Waters). Surprisingly, this effect was not related to the separation process but was triggered solely by the ion source conditions. Neither of photodiode array detector, electrospray mode nor a dedicated atmospheric pressure chemical ionization source suffered from such a phenomenon. Chromatographic peak profiles of ten test substances obtained with the dual ion source were compared with photodiode array detector data as a reference. The broadening effect was more pronounced for volatile compounds with low polarity. Dependence of peak broadening on the ion source settings was systematically investigated. Tuning of desolvation gas flow and its temperature dramatically reduced peak distortion and increased detection sensitivity.     

Ion detection limitations to mass resolution in matrix-assisted laser desorption time-of-flight mass spectrometry.

The ion detection process in a discrete-dynode electron multiplier can result in significant mass resolution losses in time-of-flight mass spectrometry (TOF-MS) for higher mass-to-charge (m/z) ion species. This resolution loss is attributed to propagation time delays and signal broadening in the ion detector. This is presumed to be due to the generation of a distribution of secondary ion species produced initially upon impact of a primary ion with the first dynode surface of the ion detector. Comparisons are made between the signals produced by a standard discrete dynode ion detector (which amplifies the negatively charged species produced by impact of a primary ion) and a detector modified to respond to only the positively charged secondary ion species produced by a primary ion impact. Ion signals for higher m/z ions with the standard detector geometry are seen to be due to a narrow signal component, most likely due to the generation of secondary electrons and/or very low mass secondary ions (H-), and a broad signal component, apparently due to secondary ions which take significant amounts of time to traverse the low potential fields between the first and second detector dynode. This results in ion signal tailing for higher m/z ion species. Numerical subtraction of the ion signal obtained with the standard and modified detector geometries (singly protonated molecular ion species of equine myoglobin) results in an improvement in mass resolution, such that a new adduct ion species (from trifluoroacetic acid) can be resolved.     

Building an oxidation reactor in Taiwan: From volatile organic compounds to secondary organic aerosols

Large amounts of volatile organic compounds (VOCs) are emitted into the atmosphere from both human and natural sources. A significant portion of VOCs would be oxidized via their reactions with atmospheric oxidants like OH, NO₃, ozone, etc. The products of the oxidation reactions are often of low volatility and may condense to form secondary organic aerosols (SOA). To study the effect of VOC oxidation in aerosol formation, we are building an oxidation flow reactor system, which consists of (1) a 22-l aluminum chamber, (2) an ozone source with an ozone detector, (3) a UV-C (254 nm) lamp, (4) a photoionization detector to measure the effective VOC concentration, (5) various flow/concentration controlling apparatuses, and (6) a scanning mobility particle sizer to monitor the generated particles. Under the conditions of high UV and ozone levels, the oxidation process can be speeded up by orders of magnitude in this reactor. We hope to use this reactor: (i) to learn the “potential” mass of SOA that can be formed from a given VOC source like a traffic or industry site; (ii) to trace back the SOA source by utilizing the shortened reaction times; (iii) to learn the trends from VOC to SOA.     

A Simple and Cheap Device for Colorimetric Determination of Serum Iron

A simple and cheap device for colorimetric determination of serum iron and TIBC (total iron‐binding capacity) was devised. The proposed device employs an LED as a light source and a common light dependent resistor (LDR) as a detector. This device functions on the basis of the light intensity received by LDR, connected to a digit multimeter, yielding resistance readings. The serum, standard, and blank solutions were prepared according to the kits instructions and introduced into the colorimeter with a disposable syringe. The iron content of the serum was calculated from the resistance difference of serum and standard solutions. The precision of the method was assessed with two commercially available serum‐based chemistry controls. The values obtained with the proposed device indicated that the serum iron concentrations correlated well with the values obtained with the commercial automated analyzer. The calibration graph was linear for iron concentrations up to 90 μmol/L (500 μg/dL). The proposed fabricated colorimeter is considerable cheaper, requires smaller sample volumes, and is suitable for serum iron assay.     

Evaluation of the applicability of backscattered light measurements to the determination of microbial cell densities in microtiter plates

The turbidity of a microbial suspension sample is routinely determined by measuring the optical density (often referred to as the “absorbance”). This method requires a dilution step at moderate and high cell densities in order to ensure that measurements fall within the region where biomass concentration is linearly correlated to optical density. The measurement of backscattered light (often referred to as the “reflectance”), which has so far been mainly applied to large-scale stirred tanks, should also be applicable on the microscale. To evaluate the validity of this assumption, a standard fluorescence microplate reader was adapted to measure backscattered light. Backscattered light readings from undiluted microbial fermentation samples determined using this modified reader gave similar growth curves to optical density measurements from diluted samples determined in a standard cell photometer. Indeed, the fact that the dilution procedure is not necessary for backscattered light measurements gives them an important advantage over optical density measurements. Such an apparatus is not only suitable for manual operation, but also shows the potential for integration into fully automated robotic systems used for high-throughput experimentation.     

凝胶色谱-光散射联用法对壳聚糖及其产品的分子质量及分布的测量

采用国家基准物质氯化钠和甲苯校准了示差折光仪和增强光系统的仪器常数,用超纯水重量法校准了激光光散射仪到示差检测器之间的延时体积,用葡聚糖标准物质对各角度的光强进行归一化处理,研究了凝胶色谱-激光光散射联用方法中设备的校准和溯源。上述校准途径使仪器溯源到国际SI单位,从而确保了测量结果准确可靠。采用激光光散射仪测得壳聚糖及其产品术后防粘连隔离膜的光折射增量dn/dc值分别为0.118、0.092 mL/g,凝胶色谱-激光光散射联用方法测得壳聚糖及其产品术后防粘连隔离膜的重均分子质量分别为8.897×104、1.168×105g/mol;分子质量分布Mw/Mn分别为1.148±0.026和3.132±0.377。     

气相色谱-微池电子捕获器检测尿液中的三唑仑

建立了气相色谱-微池电子捕获检测器(GC-μECD)检测尿液中三唑仑的方法。筛选了pH值、提取溶剂、涡旋时间,优化了液-液萃 取条件。在0.2～50 ng/mL范围内,线性关系良好,相关系数为0.9995;方法的检出限为0.1 ng/mL,日内和日间测定的相对标准偏差分 别为4.17%和5.31%,平均回收率为93.9%。该方法操作简便、灵敏度高、线性范围广、回收率高,完全能够满足日常检测工作的需要。     

锂离子选择性光电二极管传感装置的研究及在血清分析中的应用

将锂离子选择性中性载体和亲脂性pH指示剂结合在增塑的PVC膜中,pH缓冲液中锂离子浓度的变化将引起膜颜色的改变.将该膜涂覆在光电二极管光敏区并与发出锐线光的发光二极管组装在一起,构成了一种新型锂离子选择性传感装置.报道了这一传感装置的响应特性,探讨了临床分析应用的可能性.     

Monte Carlo generated spectra for QA/QC of automated NAA routine

A quality check for an automated system of analyzing large sets of neutron activated samples has been developed. Activated samples are counted with an HPGe detector, in conjunction with an automated sample changer and spectral analysis tools, controlled by the Canberra GENIE 2K and REXX software. After each sample is acquired and analyzed, a Microsoft Visual Basic program imports the results into a template Microsoft Excel file where the final concentrations, uncertainties, and detection limits are determined. Standard reference materials are included in each set of 40 samples as a standard quality assurance/quality control (QA/QC) test. A select group of sample spectra are also visually reviewed to check the peak fitting routines. A reference spectrum was generated in MCNP 4c2 using an F8, pulse-height, tally with a detector model of the actual detector used in counting. The detector model matches the detector resolution, energy calibration, and counting geometry. The generated spectrum also contained a radioisotope matrix that was similar to what was expected in the samples. This spectrum can then be put through the automated system and analyzed along with the other samples. The automated results are then compared to expected results for QA/QC assurance.     

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.     

Optimization of the parameters affecting the solid state detector efficiency in alpha-spectrometry

In radiochemical analysis, specially by using high resolution alpha-spectrometry, a key issue is the determination of the radiochemical yield. Radiochemical yield allows to determine the concentration of the radionuclide of interest and the quality of the chemical separation. To determine the radiochemical yield it is necessary to know the solid state detector efficiency, which can be obtained by calibration with a circular calibrated source. In this paper the optimization of the parameters affecting both the calibration and measurement of a source by alpha-spectrometry is described. The optimization is based on two sets of data: experimental and theoretical. Experimental data were obtained from the calibration of the solid state detector with four calibrated ²⁴¹Am sources. Theoretical data were calculated by geometry formulas and were verified experimentally.     

Mass spectrometric characterization of different norandrosterone derivatives by low-cost mass spectrometric detectors using electron ionization and chemical ionization

The abuse of nortestosterone in sport is an important problem in doping-control analysis. In order to detect the main urinary metabolite of nortestosterone, norandrosterone (NA), sensitive and specific methodology is necessary. In this context the use of a low-cost mass spectrometric detector such as the Finnigan MAT ion-trap detector (ITD) was studied. The electron ionization (EI) and positive-ion chemical ionization (PICI) mass spectra of the methoxime-trimethylsilyl, trimethylsilyl-enol trimethylsilyl ether and pentafluoropropionic ester derivatives of NA are described. The limits of detection of these derivatives are compared with those obtained by the Hewlett-Packard mass selective detector (MSD), another low-cost mass spectrometric detector and operating only in the EI mode. For the derivatives of the reference standard of NA the ITD has in the EI mode the same limit of detection, in the range of 0.5 to 1 ng injected on the column, as the MSD. However, under these conditions the ITD provides more spectrometric information, because it gives full scan data. Moreover, with the same or even improved limits of detection the ITD can operate in the PICI mode. On the other hand, for the analysis of NA isolated from urine samples, the performance of the MSD was better than that of the ITD. The ion trapping technique is probably limited when the chemical background is high.     

Concentration independent calibration of β–γ coincidence detector using 131mXe and 133Xe

Absolute efficiency calibration of radiometric detectors is frequently difficult and requires careful detector modeling and accurate knowledge of the radioactive source used. In the past we have calibrated the β–γ coincidence detector of the Automated Radioxenon Sampler/Analyzer (ARSA) using a variety of sources and techniques which have proven to be less than desirable (Reeder et al., J Radioanal Nucl Chem, 235, 1989). A superior technique has been developed that uses the conversion-electron (CE) and X-ray coincidence of ^131mXe to provide a more accurate absolute gamma efficiency of the detector. The ^131mXe is injected directly into the beta cell of the coincident counting system and no knowledge of absolute source strength is required. In addition, ¹³³Xe is used to provide a second independent means to obtain the absolute efficiency calibration. These two data points provide the necessary information for calculating the detector efficiency and can be used in conjunction with other noble gas isotopes to completely characterize and calibrate the ARSA nuclear detector. In this paper we discuss the techniques and results that we have obtained.     

On the use of reference materials in gamma-ray spectrometric efficiency calibration for environmental samples

In order to calibrate a gamma-ray spectrometer for radioactivity measurements in environmental samples, it is essential to use standard radioactive sources in the appropriate geometrical configuration. However, the lack of standard sample due to any reason would necessitate the use of reference materials as reliable alternative. They often appear in many matrices and densities. Accordingly, special attention should be drawn towards optimizing the calibration. This work studies the errors encountered because of the attenuation process due to the density difference of the samples (including reference samples) aliquots with identical matrix contents. The highest effect was noticed in the lower energy regions. This study suggests that even for the reference samples with the same matrix, it is necessary to set a recommended density for the measurement process to avoid the errors due to the attenuation. If this is not practically possible it is recommended that attenuation correction due to matrix contents followed by another correction due to density correction would be a reliable approach to reach an optimum calibrating condition.     

Purity verification and measurement uncertainty

In chemical analysis, laboratories are required to verify the purity of reference material being used. Also, the contributions from the verification procedure, where significant, have to be included in the estimation of the total measurement uncertainties of the test results. One common verification procedure is to use another source of the same material if an appropriate certified reference material were not available. This involves a comparison test where the purity value of the reference standard is determined using a second source reference standard as the “calibrant”. In normal practice, the standard uncertainty of the purity value of a reference standard is estimated according to the probability distribution function (PDF) of the possible purity values of the reference standard concerned. With the use of Monte Carlo simulation technique, this paper attempted to study the effect of verification process on that PDF and thus the associated standard uncertainty as well. Also, the effects of parameters like the purity of the second source reference standard, the method precision and the acceptable range set for the comparison test on the verification outcome were discussed.     

Improvement of detection limits in flow injection analysis

Summary When using a conventional UV/VIS-Spectrophotometer with a large sample compartment as a detector for FIA, flow cells with longer pathlengths can be applied. Despite of a unavoidable band broadening a significant improvement of detection limit can be achieved compared to standard cells. Band broadening is compensated by slight modifications of the manifold, so that the dispersion is in the same range for long path cells and standard flow cells. With a 5 cm flow cell phosphate can be detected down to 0.005 mg/l P.