Reduction of nonpolar amino acids to amino alcohols to enhance volatility for high-precision isotopic analysis

Amino acids are routinely derivatized using carbon-containing groups prior to gas chromatography continuous-flow isotope ratio mass spectrometry (GCC-IRMS). Derivative C contaminates analyte C because the entire derivatized compound is combusted to CO2. Correction procedures are required to extract the analyte isotope ratio. We present a method for reduction of six nonpolar amino acids to their corresponding amino alcohols, demonstrate a GC strategy to produce acceptable peak shapes from the resulting strongly H-bonding analytes, and present isotopic analysis for amino acids and their corresponding amino alcohols to evaluate any possible isotopic fractionation. Alanine, valine, leucine, isoleucine, methionine, or phenylalanine was reduced using NaBH4 in THF with I2 as an electrophile. Reactions were performed with 2 g of analyte to permit isotopic analysis by conventional elemental analysis-IRMS. All reactions were quantitative as assessed by IR spectra, melting points, and GC. Recovery from the reaction mixture was 60-84%. GC separation of a mixture of the six amino alcohols was achieved using a thick stationary-phase (5 microm) capillary column to avoid tailing due to hydrogen bonding to the walls of the fused-silica capillary. The reproducibility of GCC-IRMS determinations of amino alcohols averaged SD(613C) = 0.25 +/- 0.19%. The absolute differences between delta13C of amino acids measured by an elemental analyzer coupled to IRMS and amino alcohols measured by GCC-IRMS was delta613C = 0.14% and showed no general trend. Reactions performed with 2 mg of analyte yielded equivalent chromatograms. These data indicate that the reduction method does not induce isotopic fractionation and can be used for continuous-flow isotopic analysis to avoid addition of contaminating carbon.     

Using minimum deviation of a secondary rainbow and its application to water analysis in a high-precision, refractive-index comparator for liquids

A new method for measuring the refractive-index difference of a liquid has been developed. The liquid to be measured is contained in a 60-mm-diameter, cylindrical glass cell, and a He-Ne laser light is passed into the cell so that the laser light incidence fulfills the condition of minimum deviation. In this condition, the beam emerging from the cell has a fine interference fringe. The position of the interference fringe is read out as a marker to measure the deflection of the laser light. Directly reading the peak shift of the interference fringe makes it easy to obtain the refractive index difference of the liquid with a fairly high precision of at least 6 x 10(-6). Further high precision is potentially expected to be realized by use of an improved data analysis treatment of the overall interference fringe pattern.     

Chemical conversion of nitrate and nitrite to nitrous oxide for nitrogen and oxygen isotopic analysis in freshwater and seawater

We present a novel method for nitrogen and oxygen natural isotopic abundance analysis of nitrate and nitrite of seawater and freshwater at environmental concentrations. The method involves the reduction of nitrate to nitrite using spongy cadmium with further reduction to nitrous oxide using sodium azide in an acetic acid buffer. For separate nitrite analysis, the cadmium reduction step is simply bypassed. Nitrous oxide is purged from the water sample and trapped cryogenically using an automated system with subsequent release into a gas chromatography column. The isolated nitrous oxide is then analyzed on a continuous flow isotope ratio mass spectrometer via an open split. This paper describes the basic protocol and reaction conditions required to obtain reproducible natural abundance level nitrogen and oxygen isotopic ratios from nitrate, nitrite, or both, and the results obtained to support these conclusions. A standard deviation less than 0.2 per thousand for nitrogen and 0.5 per thousand for oxygen was found for nitrate samples ranging in concentration from 40 to 0.5 microM (better for nitrite), with a blank of 2 nmol for 50-mL samples. Nitrogen and oxygen isotopic fractionation and oxygen atom exchange were consistent within each batch of analysis. There was no interference from any seawater matrixes. Only one other method published to date can measure the nitrate oxygen isotopic abundance in seawater and none that do so for nitrite alone in the presence of nitrate. This method may prove to be simpler, faster, and obtain isotopic information for lower concentrations of nitrate and nitrite than other methods.     

Alkylating tryptic peptides to enhance electrospray ionization mass spectrometry analysis

A major limitation of mass spectrometry-based proteomics is inefficient and differential ionization during electrospray ionization (ESI). This leads to problems such as increased limits of detection and incomplete sequence coverage of proteins. Incomplete sequence coverage is especially problematic for analyses that require the detection and identification of specific peptides from a protein, such as the analysis of post-translational modifications. We describe here the development and use of aldehyde-based chemistry for the alkylation of peptide primary amines to increase peptide hydrophobicity, providing increased ionization efficiency and concomitant signal enhancement. When employed to modify the peptide products of protein tryptic digests, increased sequence coverage is obtained from combined modified and unmodified digests. To evaluate the utility of alkylation of peptides for selected reaction monitoring (SRM) assays, we alkylated a peptide from the protein Oct4, known to play a role in regulating stem cell differentiation. Increased chromatographic retention and ionization efficiency is observed for the alkylated Oct4 peptide compared to its unmodified form.     

Status and prospects for the development of high-precision dilatometry

Translated from Izmeritel'naya Tekhnika, No. 6, pp. 26–27, June, 1989.     

An extraction chromatography method for Hf separation prior to isotopic analysis using multiple collection ICP-mass spectrometry

A novel method for the single-step separation of Zr + Hf from all matrix elements of geological samples has been developed for Hf isotopic measurements using multiple collector-ICP-mass spectrometry. The method combines an effective sample decomposition by LiBO2 fusion with a selective separation of Hf + Zr by a solid-phase extraction material based on dipentyl pentyl phosphonate, commercially available as U-TEVA.Spec. Using this simple and rapid procedure, Hf and Zr can be isolated in a single separation step with good recoveries (>90%) and satisfactory blank levels (approximately 55 pg of Hf), so that a subsequent isotopic measurement with ICPMS is possible. An excellent separation from rock-forming constituents is achieved, including those elements (Al, P, Ti, Cr, Fe, Mo, etc.) known to interfere in conventional separation methods based on ion-exchange techniques. The potential of this new method for Hf isotopic analysis is demonstrated by replicate MC-ICPMS measurements of 176Hf/177Hf ratios in seven international reference materials of silicate rocks, spanning a range of Hf contents and bulk compositions.     

Methods for the stable isotopic analysis of chlorine in chlorate and perchlorate compounds.

Chlorate and perchlorate compounds, used as herbicides, solid fuel propellants, and explosives, are increasingly recognized as pollutants in groundwater. Stable isotope characterization would permit both environmental monitoring of extent of remediation and forensic characterization. Stoichiometric reduction to chloride (greater than 98% yield), by Fe(II) for chlorate and alkaline fusion-decomposition for perchlorate, allows analysis by standard methods to give highly reproducible and accurate delta37Cl results (0.05/1000, 2 x standard error). Analysis of various compounds from different suppliers yielded delta37Cl values for chlorate samples near to +0.2/1000 (SMOC), but one has within-sample heterogeneity of 0.5/1000, possibly due to crystallization processes during manufacture. Results for perchlorate samples also are generally near +0.2/1000, but one is +2.3/1000 (SMOC). The initial results suggest that both forensic and environmental applications might be feasible.     

Tags for the stable isotopic labeling of carbohydrates and quantitative analysis by mass spectrometry

Although stable isotopic labeling has found widespread use in the proteomics field, its application to carbohydrate quantification has been limited. Herein we report the design, synthesis, and application of a novel series of compounds that allow for the incorporation of isotopic variation within glycan structures. The novel feature of the compounds is the ability to incorporate the isotopes in a controlled manner, allowing for the generation of four tags that vary only in their isotopic content. This allows for the direct comparisons of three samples or triplicate measurements with an internal standard within one mass spectral analysis. Quantitation of partially depolymerized glycosaminoglycan mixtures, as well as N-linked glycans released from fetuin, is used to demonstrate the utility of the tetraplex tagging strategy.     

海水槽控温波动性及均匀性的概率密度分布分析

针对海水槽控温波动性和均匀性所满足概率分布的推断问题,提出一种基于卡方检验的概率密度函数统计推断方法。该方法由波动性和均匀性测量数据的直方图获取其概率分布的直观印象,进而确定概率分布函数的数学形式。采用最大似然法对函数未知参数进行估计;由卡方检验法实现对其概率分布的严格统计推断。通过测量数据的分析,得到海水槽控温波动性和均匀性的概率密度分布函数,避免在海水槽测量不确定度评定中主观确定概率分布函数等问题。     

Error analysis of high-precision DEM generated from airborne dual-antenna interferometric SAR data

结合机载双天线干涉合成孔径雷达(InSAR)正侧视成像模型,分析了机载双天线InSAR数据处理中存在的影响数字高程模型(DEM)高程精度和点位精度的主要因素,包括平台高度、斜距、干涉相位、基线长度、基线倾角、中心多普勒频率以及载机姿态,着重分析了平台高度、基线长度、基线倾角以及侧视角等因素的变化与DEM误差之间的关系.选用机载双天线InSAR数据进行了干涉实验,并对生成的DEM进行了误差分析,实验分析结果和误差定量分析的结果相一致.对机载双天线InSAR数据处理进行误差分析有利于提高DEM的高程和点位精度,也有利于机载双天线InSAR系统设计及飞行试验设计,因而具有重要实用意义.     

基于有限元分析的高精密柔性夹具优化方案设计

针对高精密加工过程中柔性夹具的变形问题进行理论计算和实验分析.首先应用CAE软件对现有柔性夹具进行有限元分析,分析夹具变形问题.然后通过CAD软件改变该夹具模型的结构参数和尺寸参数,优化夹具构造,以减少定位过程的夹具变形,并再次通过CAE软件进行分析,验证优化效果.通过增加夹具体主平面刚度、增加夹具体肋板刚度和改变夹具定位点位置等方法,实现夹具体的最大变形量满足加工精度的要求.实验分析以定位点变形为研究对象,测量得到4个定位点在夹具装夹过程中的变形量.实验结果基本符合理论计算结果,得到最终优化方案的4个定位点变形量均小于001 mm,满足了该高精密加工中对夹具体的精度要求.优化方案在实际生产中得到实施,效果良好.     

Improved interpolation method suitable for very high-precision data applied to ge resistance thermometry

An improved method is proposed for eliminating the commonly occurring spurious ripples which arise from interpolation fits to very accurate data and have an unknown functional form. This method is illustrated by fitting an interpolation curve to a commercial Ge resistance thermometer characteristic. For data with a rms scatter of ± 0.8 mK, an interpolation accuracy of about ± 0.2–0.4 mK is achieved. This accuracy is about four times higher than that obtained from the generally used high-order, polynominal, least-squares fitting method. It is also explicitly shown that the proposed method is superior to other interpolation methods.     

Carbon position-specific isotope analysis of alanine and phenylalanine analogues exhibiting nonideal pyrolytic fragmentation

Recent advances in gas chromatography combustion-isotope ratio mass spectrometry (GCC-IRMS) has made compound-specific isotope analysis routine, but reports on position-specific isotopic analysis are still scarce. On-line GC-pyrolysis (Py) coupled to GCC-IRMS is reported here for isolation and isotopic characterization of alaninol and phenethylamine, analogues of alanine and phenylalanine, respectively. Ideally, pyrolytic fragments will originate from unique sites within the parent molecule, and isotope ratios for each position within the parent can either be measured directly or calculated from fragment isotope ratios without substantially degrading the analytical precision. Alaninol pyrolysis yielded several fragments, of which CO and CH4 were used for isotope ratio calculations. Isotope labeling experiments showed that CO derived entirely from the C(1) position, while all three positions of alaninol contributed to CH4 (29.0 +/- 0.3% from C(1), 3.6 +/- 0.2% from C(2), and 66.9 +/- 1.1% from C(3)). We demonstrate iterative use of mass balance to calculate isotope ratios from all positions despite the nonideal positional fidelity of CH4. Pyrolysis of phenethylamine generated benzene and toluene fragments. Benzene derived entirely from C(ring), and toluene was proportionately formed from C(3) and C(ring). Relative intramolecular isotope ratios (Deltadelta13C) were calculated directly from delta13C of fragments or indirectly by mass balance. Though the C(3) isotope ratio was calculated from the benzene and toluene fragments, propagation of errors showed that the final precision of the determination was degraded due to the small contribution that C(3) makes to toluene. Samples of each amino acid from four different vendors showed natural variability between sources, especially at the C(1) position of alaninol (range of Deltadelta13C approximately 50 per thousand). The average precision was SD(Deltadelta13C) < 0.20 per thousand for directly measured positions of alaninol and phenethylamine. The precision of indirectly measured positions was poorer (SD(Deltadelta13C) = 0.94 per thousand for alaninol, 6.54 per thousand for phenethylamine) due to propagation of errors. These data demonstrate that GC-Py-GCC-IRMS data can be used to extract high-precision isotope ratios from amino acids despite nonideal positional fidelity in fragments and that natural intramolecular variability in delta13C can be used to distinguish different sources of amino acids.     

跟踪测量视频的高精度同步采集系统

设计了一种无损实时跟踪测量视频和测控数据同步采集系统。根据测控信息同步信号及其对视频采集时刻的规定,以测控信息采集卡上高精度定时器为时钟,由视频采集卡通过软件修正摄像机曝光信号,实现对曝光时刻的精密调节。采用高速总线实现数据无失真存储和转储、AVI技术组织文件,充分利用了测量视频图像空间。     

高精度气体微流量计温度控制系统的研制

采用虚拟仪器技术开发了一个温度的数据采集系统.并在高精度气体微流量计中得到的应用.详细介绍了系统的硬件组成和软件的设计.系统的硬件部分使用研华公司的PCI-1716多功能数据采集卡和ADAM3013热电阻变送模块,软件部分采用美国NI公司的LabVIEW图形化编程语言创建,通过调用设计的温度计实现了温度的采集及数据的自动保存.并且界面简单、方便使用、解决了多通道监测、实时性等问题.     

Technical updates to the bacterial method for nitrate isotopic analyses

The bacterial conversion of aqueous nitrate (NO(3)(-)) to nitrous oxide (N(2)O) for isotopic analysis has found widespread use since its introduction (Sigman, D. M.; Casciotti, K. L.; Andreani, M.; Galanter, M.; Bo?hlke, J. K. Anal. Chem.2001, 73, 4145-4153; Casciotti, K. L.; Sigman, D. M.; Galanter Hastings, M.; Bo?hlke, J. K.; Hilkert, A. Anal. Chem.2002, 74, 4905-4912). The bacterial strain Pseudomonas aureofaciens (ATTC no. 13985) was shown to convert NO(3)(-) to N(2)O while retaining both N and O isotopic signatures, and automation of the isotopic analysis of N(2)O greatly increased the throughput of the method (Casciotti, K. L.; Sigman, D. M.; Galanter Hastings, M.; Bo?hlke, J. K.; Hilkert, A. Anal. Chem.2002, 74, 4905-4912). Continued development of the denitrifier method has led to increased precision and throughput of NO(3)(-) isotopic analysis. Presented here are several recent procedural modifications and the demonstration of their effectiveness.     

Spin filters and supermirrors: a comparison study of two methods of high-precision neutron polarisation analysis

The precise knowledge of the neutron polarisation is needed in tests of the electroweak Standard Model using angular correlations in polarised neutron beta decay. We performed an experimental comparison study of two different methods of polarisation analysis of a cold neutron beam which are based on spin-dependent reflection, respectively, transmission. The compared devices are a supermirror analyser used in prior neutron decay studies and an opaque transmission spin filter of either polarised ³He or polarised protons. The results of the neutron polarisation measured with the supermirror analyser and with the spin filter coincided in three different experiments within 0.1–0.2%.