Pulsed-valve chemical ionization for gas chromatography/fourier-transform mass spectrometry

The pressure requirements for chemical ionization g.c./F.t.m.s. which restrict mass resolution and accuracy are overcome through use of a pulsed valve that provides momentary reagent gas pressures. For alternate electron impact (EI)/chemical ionization (c.i.) g.c./F.t.m.s., similar resolution for both e.i. and c.i. data is demonstrated. The efficiency of chemical ionization with the pulsed valve is similar to static high pressure c.i. measurements of several model compounds. Results from the analysis of peppermint oil and a fuel additive illustrate the potential information available from a single g.c./F.t.m.s. experiment.     

Spectrophotometric method for the determination of 1,2-propylene glycol

A spectrophotometric method is proposed for the determination of 1,2-propylene glycol. It is based on the ADH/ AlDH catalyzed oxidation of propylene glycol by NAD⁺. The NADH formed is measured at 340 nm. Alcohol dehydrogenase from equine liver was found to be much more effective than that of yeast. No enantiomeric selectivity for s(+) propylene glycol was observed. A linear relation was found in the concentration range from 5 to 50 mg/L. The method achieves a correlation coefficient of r = 0.996 and a relative standard deviation of 1.37%. The limit of quantitation was calculated to 9.6 mg/L. Since the total reaction volume was restricted to 800 μL, only 2.8 units of AlDH and 8 units of ADH were sufficient to develop the final absorption within 30 min.     

Spectrophotometric method for the determination of 1,2-propylene glycol

A spectrophotometric method is proposed for the determination of 1,2-propylene glycol. It is based on the ADH/ AlDH catalyzed oxidation of propylene glycol by NAD⁺. The NADH formed is measured at 340 nm. Alcohol dehydrogenase from equine liver was found to be much more effective than that of yeast. No enantiomeric selectivity for s(+) propylene glycol was observed. A linear relation was found in the concentration range from 5 to 50 mg/L. The method achieves a correlation coefficient of r = 0.996 and a relative standard deviation of 1.37%. The limit of quantitation was calculated to 9.6 mg/L. Since the total reaction volume was restricted to 800 μL, only 2.8 units of AlDH and 8 units of ADH were sufficient to develop the final absorption within 30 min. Received: 16 April 1997 / Revised: 19 June 1997 / Accepted: 25 June 1997     

Coupling of narrow-bore liquid chromatography to thin-layer chromatography : Part 2. Application of fluorescence-based spectroscopic techniques for off-line detection

The effluent from a narrow-bore liquid chromatographic (l.c.) separation can be immobilized on thin-layer chromatographic (t.l.c.) plates with little loss of resolution. The deposited compounds are then available for further inspection. For off-line detection, direct fluorescence emission, fluorescence excitation emission spectra, and fluorescence line-narrowing spectroscopy are investigated with tetracene and benz[k] fluoranthene as model compounds. Detection based on direct emission measurements and on measurements for which complete spectra are obtained for the separated compounds, is suitable for identification and determination. Detection limits are of the same order of magnitude as those for on-line detection in narrow-bore l.c. The fluorescence spectra of immobilized compounds can be obtained with a conventional fluorescence spectrometer equipped with a solid-sample accessory. No other special apparatus is needed. The immobilized chromatogram is also suitable for techniques incompatible with flow systems, e.g., fluorescence line-narrowing spectroscopy, which yields fluorescence spectra via laser excitation of low-temperature solid samples. Very selective narrow-line fluorescence spectra were obtained for tetracene deposited on t.l.c. plates in amounts down to the low picogram level.     

Identification and determination of metabolites in plant cell biotechnology by gas chromatography and gas chromatography/mass spectrometry : Application to Nonpolar Products of Chrysanthemum cinerariaefolium and Tagetes Species

Gas-liquid chromatography (g.l.c.) and gas-liquid chromatography/mass spectrometry (g.c./m.s.) were used for profile analysis and for identification, confirmation and determination of traces of nonpolar metabolites synthesized in plant tissue, calli and in suspension-cultured cells of Chrysanthemum cinerariaefolium (pyrethrum) or Tagetes species (marigold). When dried leaf samples (1–20 mg) were tested, six pyrethrins were detected simultaneously at the picogram level with an electron capture detector. This method permits the early selection of high yielding tissues and calli for the development of plant cell cultures. α-Terthienyl was quantified at the nanogram level with the electron capture detector; calibration graphs were linear between 0.2 and 20 ng and the minimum detectable quantity was about 0.1 ng. The concentration of α-terthienyl in air-dried roots of T. erecta was 140 μg g^?1. However, this procedure used alone is not reliable, because α-terthienyl and the pyrethrin Cinerin I have the same retention time. Mass spectrometry is needed for identification and confirmation. Profile analysis of a leaf extract of T. minuta by high-resolution g.c./m.s, and accurate mass measurements showed 13 nonpolar compounds, of which 9 were tentatively identified; they include 2 sesquiterpenoids, 3 thiophenes, 2 sterols with a precursor and ethyl linolenate. Analyses of T. minuta indicated that in calli and cell suspensions, plant sterols are synthesized but sesquiterpenoids or thiophenes are not produced.     

Determination of tin in the ng g−1 range differential pulse polarography

A sensitive determination method of tin by differential pulse polarography is described. Addition of tropolone to acetate supporting electrolyte at about pH 4.7 provides a 30-fold signal enhancement, giving a sensitivity comparable to that obtained in anodic stripping voltammetry, but without the need for enrichment by pre-electrolysis. The response is linear over more than three orders of magnitude (1 ng ml^?1?5 μg ml^?1 Sn), allowing measurements within a wide concentration range without changes in sample preparation or measuring conditions. Large amuonts of Ti, W, Cr or Mo interfere, but can be removed completely by pre-separation of tin based on extraction with tropolone/toluene and back-extraction into the supporting electrolyte. Results for tin in water and fruit juice are consistent with those obtained by other techniques.     

Improved micro-flame detection method for gas chromatography

A previously developed micro-counter-current flame method is modified to provide both sensitive photometric and ionization detection for gas chromatography (GC). A stainless steel capillary (0.254 mm i.d.) supplying oxygen functions as a burner, which supports a compact flame that burns in a counter-flowing excess of hydrogen. In the “micro-flame photometric detector” (μFPD) response mode, the background emission level is reduced by over an order of magnitude compared to previous experiments using a fused silica capillary burner, resulting in greatly improved detection limits. Chemiluminescent sulfur response in the μFPD is quadratic over 3.5 orders of magnitude, yielding a detection limit of 3 × 10⁻¹¹ gS/s, while that of phosphorus is linear over 5 orders of magnitude down to a minimum detectable limit of 3 × 10⁻¹² gP/s. Tin response is examined for the first time in the μFPD and yields a blue luminescence (ascribed to SnO*) on the surface of the quartz flame enclosure. Although this emission provides a sensitive detection limit near 6 × 10⁻¹⁵ gSn/s, due to the limited surface area within the detector cell it only produces a linear response range of one order of magnitude. Ionization response toward hydrocarbons is also obtained from the hydrogen-rich micro-counter-current flame. A linear response is produced over five orders of magnitude, yielding a detection limit toward carbon of 2 × 10⁻¹⁰ gC/s. Analysis of a simple gasoline sample is used to demonstrate that the device can successfully operate as both a selective and universal GC detector. Results indicate that this micro-counter-current flame method yields comparable performance to conventional flame photometric and flame ionization detectors, making it potentially useful for adaptation to micro-analytical devices and portable GC units.     

Flame photometric detection inside of a capillary gas chromatography column

A novel micro-flame photometric detector (FPD) employing a miniature counter-current flame is described. The micro-FPD flame, encompassing a volume of about 30 nL, is operated inside the end of a capillary gas chromatography column (i.e. on-column) or inside of a quartz capillary after the column (i.e. post-column). Either air or oxygen can support a hydrogen flame in the device, although oxygen is far preferable. The detector can be operated for several hours without any observed degradation in performance or flame stability. The optimal gas flows established for the detection of sulfur and phosphorus are in the range of 4 mL min(-1) of oxygen and 9 to 13 mL min(-1) of hydrogen. The fuel-rich micro-FPD flame generates chemiluminescent blue S2* emission for sulfur and green HPO* emission for phosphorus, similar to a conventional FPD. Sulfur response in the micro-FPD is quadratic over nearly 3 orders of magnitude while that of phosphorus is linear over nearly 5 orders of magnitude. The micro-FPD detection limit for sulfur is 1 x 10(-9) g S s(-1), and that of phosphorus is 2 x 10(-10) g P s(-1). The properties established for the initial prototype of the micro-FPD make this counter-current flame method potentially suitable for integration with on-chip gas chromatography or other micro-analytical devices where flame-based detection methods are desirable.     

Determination of Ca,Er, Eu,Ga, In,K, Na,Mo and W by atomic absorption spectrometry with an electrothermal graphite braid atomizer

The limits of detection lie in the range 10^-10–10^-12 g and are usually similar to the values achieved with other electrothermal atomizers. Except for molybdenum and tungsten, calibration graphs are linear over two orders of magnitude.     

Chemiluminescence for the determination of traces of cobalt(II) by continuous flow and flow injection methods

Flow injection analysis, with chemiluminescence detection, is used to determine traces of cobalt(II) by means of the gallic acid—hydrogen peroxide—sodium hydroxide system containing a small amount of methanol to increase the sensitivity. This permits the determination of cobalt(II) more selectively than any other chemiluminescent system with a detection limit of 0.04 μg l^-1 (continuous sample flow) or 0.04 ng (10-μl sample injection). The linear range is 3 orders of magnitude, the sampling rate is 20 h^-1, and the relative standard deviation is 5.9% for 0.06 ng Co(II) (n = 10). Silver(I), the strongest enhancer after cobalt(II), provides a signal 1.3% of that for Co(II). A few precipitants and complexing agents suppress the signal.     

Voltammetric on-line analysis for some sulphur-containing drugs

Some 2-thiobarbituric acid and thioamide drugs are determined voltammetrically in a flow-through cell. Both direct oxidation in the d.c. mode and cathodic stripping on a mercury-coated glassy carbon electrode are examined for the determination of these drugs. The thiobarbiturie acids can be determined by both methods; the linear range covers 1–2 orders of magnitude with a detection limit in the range 10^-3–10^-6 M depending on the method used and the compound. The thioamides do not plate satisfactorily on the mercury film when operated in the on-line mode and can only be determined by the d.c. anodic oxidation method; the linear range covers at least two orders of magnitude with a detection limit of 5×10^-8–5×10^-7 M depending on the compound.     

Quantitative determination of polar and ionic compounds in petroleum fractions by atmospheric pressure chemical ionization and electrospray ionization mass spectrometry

The capabilities of atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI) methods for quantitative analysis of polar and ionic compounds in petroleum fractions have been examined. The requirements of the analysis for sensitivity, linear dynamic range, and structural characterization have been discussed. ESI was found to be approximately two orders of magnitude more sensitive than APCI and is most suitable for the detection of analytes in weak concentrations. Equivalent relative linear dynamic ranges were observed by the two methods (at least three orders of magnitude). For the relatively high analyte concentrations examined here (e.g., 1-100 ppm or higher), the absolute area counts increased linearly with the analyte amount only in APCI, making this method more attractive for quantitative liquid chromatography/mass spectrometry (LC/MS) applications. Nevertheless, a wider range of ionic compounds can be detected by ESI than by APCI.     

Determination of arsenic in glycerine by flow injection,hydride generation and inductively-coupled plasma/atomic emission spectrometry

The determination of low concentrations of arsenic in glycerine is investigated with flow injection for the introduction of analyte and tetrahydroborate reagent for arsine generation, separation in a vapor-liquid flow cell, and inductively-coupled plasma/atomic emission spectrometry (i.c.p./a.e.s.). Peak areas are used to quantify arsenic in the concentration range 0.12–3.0 μg ml^?1. Peak areas for prepared standards exhibited average deviations of about 1.4% for this concentration range. The slope and intercept for a least- squares fit of area (nanocoulombs) vs. concentration (μg ml^?1) were 10.9 ± 0.17 and 0.46 ± 0.3, respectively.     

Determination of trace amounts of highly hydrophilic compounds in water by direct derivatization and gas chromatography — mass spectrometry

A new procedure has been developed to derivatize a large set of highly hydrophilic substances ((poly) hydroxy and/or (poly)carboxylic acids, glycols and dihydroxybenzenes) directly in water. The key element of the method is the derivatizing agent used, n-hexyl chloroformate, which proved to be much more effective than other alkyl and aryl chloroformates. Detection limits in the low μg/L range were found for most of the compounds studied, using positive ion chemical ionization mass spectrometry as the detection technique. Calibration curves exhibited a good linearity over 2–3 orders of magnitude, so that quantitative determinations were possible. Among the experimental parameters tested, it was found crucial to introduce the chloroformate slowly and under sonication. The whole derivatization procedure takes only 2–3 min from sample collection to injection into the gas chromatograph.     

Isotactic sequence lengths by ozonation of poly(propylene oxide)

Ozonation followed by lithium aluminum hydride reduction cleaved high molecular weight isotactic poly(propylene oxide) to crystalline polyglycols. From the melting point and molecular weight of the latter, the molar freezing point depression produced by end groups is found to be ca. 18°C./mole, as compared to that estimated for poly(ethylene glycols), K_f = 12°C./mole, from earlier data. By assuming syndiotactic placements (or other irregularities) would produce the same molar depression, the melting point of isotactic poly(propylene oxides) produced by various catalysts has been used to estimate the isotactic sequence lengths.     

Identification of barbiturates from extracts of urine,stomach fluid,liver and kidney by high-performance liquid chromatography and field desorption mass spectrometry1,2

The characteristic behaviour of a series of important barbiturates in field desorption mass spectrometry (f.d.m.s.) is described. Detection limits are reported for three standard barbiturates by f.d.m.s. and high-performance liquid chromatography (h.p.l.c.). The off-line combination of h.p.l.c. and f.d.m.s. is used in forensic investigations to identify these drugs unequivocally in body fluids such as urine and stomach fluids and in extracts from human liver and kidney tissues. Eighteen barbiturates were tested. The use of f.d.m.s. as an off-line h.p.l.c. detector provides detection limits (S/N =3:1) ranging from 1 ng g^-1 to 10 ng g^-1, depending on the compound and on the preliminary clean-up used.     

氧化锆负载Pd-Cu双金属催化剂上山梨醇选择性氢解合成乙二醇和丙二醇

山梨醇和木糖醇等多元醇是可再生生物质转化合成液体燃料和化学品的重要平台分子,其中,可通过选择氢解反应一步制备乙二醇和丙二醇等重要化工原料,有望代替从乙烯和丙烯制备二元醇的传统石油化工工艺.目前文献中多元醇氢解反应主要使用Ru基和Ni基催化剂等,但是不可避免地生成C?C键非选择性断裂的副产物甲烷等.与之相比,非贵金属Cu基催化剂往往具有较优异的选择性,但其活性较低和水热稳定性较差.因此,到目前为止研制具有高活性和选择性、以及良好水热稳定性的Cu基催化剂用于生物质基多元醇氢解反应仍然存在挑战.在本文中,我们采用贵金属修饰的方法提高Cu基催化剂在山梨醇选择氢解反应中的活性和水热稳定性.通过分步浸渍法合成了1%Pd-3%Cu/ZrO2、1%Pt-3%Cu/ZrO2和1%Ru-3%Cu/ZrO2双金属催化剂,并比较了它们在山梨醇氢解反应中的催化性能.在相同的反应条件下,上述催化剂中1%Pd-3%Cu/ZrO2(Cu/Pd =5)具有最优的活性及乙二醇、丙二醇和甘油的总选择性.以固体碱La(OH)3为助剂,1%Pd-3%Cu/ZrO2的山梨醇氢解活性高达20.3 h-1,是单金属1%Pd/ZrO2(8.7 h-1)和3%Cu/ZrO2(6.5 h-1)催化剂活性的2-3倍,也高于含有相同Pd、Cu含量的1%Pd/ZrO2和3%Cu/ZrO2机械混合体系的活性(12.2 h-1).而且, Pd-Cu/ZrO2双金属催化剂对C2-C3低碳多元醇的选择性也明显优于Pd/ZrO2和Cu/ZrO2以及二者的机械混合体系.这些结果说明Pd对Cu/ZrO2的促进作用不仅仅需要Pd与Cu两种金属的共同存在,还需要它们两者之间的相互作用.进一步发现, Pd-Cu/ZrO2双金属催化剂在Cu/Pd比为1.5-10.0的较宽范围内都表现出了较高的反应活性(17.8-20.3 h-1)以及乙二醇、丙二醇和甘油的总选择性(57.3%-62.8%),说明较低含量Pd的存在就能够有效地改善Cu催化剂的催化性能.在493 K和5.0 MPa H2的反应条件下,以1%Pd-3%Cu/ZrO2为催化剂,在山梨醇接近完全转化时,获得了61.7%的乙二醇、丙二醇和甘油的总选择性.同时, Pd的加入还能有效地抑制水热反应条件下Cu粒子的团聚,使得Pd-Cu/ZrO2催化剂在山梨醇氢解反应中具有优良的水热稳定性和循环使用性能.在5次循环实验中1%Pd-3%Cu/ZrO2的活性和选择性基本保持不变; X-射线粉末衍射结果表明,反应后的催化剂上未观察到Cu的特征衍射峰, Cu粒子仍然保持良好的分散状态.而对于没有Pd修饰的单金属3%Cu/ZrO2催化剂,经5次循环使用后山梨醇氢解反应的活性则下降了42%;在循环反应中Cu粒子显著地聚集而长大到~30 nm. CO吸附漫反射红外光谱结果揭示了Cu粒子倾向于在Pd粒子表面沉积,随着Cu/Pd原子比的增大, Cu粒子逐渐稀释并覆盖Pd的表面位点,说明Pd与Cu粒子之间存在紧密的接触.氢气程序升温还原结果表明,可能与氢溢流有关, Pd的加入促进了CuO的还原.然而,不同于Pd/ZrO2和Cu/ZrO2机械混合样品的TPR图谱,其显示PdO和CuO各自的还原峰, Pd-Cu双金属催化剂则只存在一个宽化的还原峰,这说明了Pd-Cu之间结构上的紧密接触使得两种金属之间存在强相互作用,其中可能存在从Pd向Cu的电子转移.综合这些结构和电子效应,可以推测Pd的存在促进了Cu粒子对山梨醇的脱氢能力和不饱和中间体的加氢能力,进而提高了Cu基催化剂在山梨醇氢解反应中的活性及目标产物的选择性.同时Pd-Cu之间的强相互作用和氢溢流效应抑制了Cu粒子在水热反应条件下的聚集,提高了催化剂的稳定性.这些结果和认识有助于指导人们为多元醇氢解和其它生物质基化学品的转化反应设计具有更高效率和水热稳定性的新型Cu基催化剂.     

Simultaneous determination of uric acid, xanthine and hypoxanthine with an electrochemically pretreated carbon paste electrode

A simple method is presented for the simultaneous differential pulse voltammetric determination of uric acid, xanthine and hypoxanthine. It is based on the improved current responses of the three analytes at carbon paste electrodes polarized in a dilute alkaline medium (0.002 mol/l NaOH, 0.1 mol/l NaClO₄) at 1.3 V vs. SCE for a short time. Compared with the methods reported in the literature, this procedure has a much wider linear range (2 to 3 orders of magnitude in concentration), lower detection limits (5 to 10 g l^–1) and less interference by ascorbic acid. The electrochemical responses were found to be dependent on the pre-anodization potential and the time imposed on the electrodes as well as on the alkalinity of the supporting electrolyte. The proposed procedure was used to determine uric acid, xanthine and hypoxanthine in human urine without any preliminary treatment.     

Separation of trace rare earths and other metals from uranium by liquid-liquid extraction with quantitation by inductively-coupled plasma/mass spectrometry

N,N-Dihexylacetamide in toluene was used to extract uranium selectively from an aqueous phase containing 30 elements at 10 or 100 μg l^?1 concentrations. After three extractions, the uranium level fell from 119 000 mg l^?1 (0.5 M) to less than 3 mg l^?1. An inductively-coupled plasma/mass spectrometer (i.c.p./m.s.) was used to determine recoveries of the trace elements in the aqueous phase, which, in most cases, were in the range 90–110%. This combination of liquid-liquid extraction with i.c.p./m.s. offers determinations at the 10 ng g^?1 level in uranium for most of the elements studied.     

Marangoni effects in aqueous polypropylene glycol foams

The foam behavior of three polypropylene glycols covering the molecular weight range between 192 and 725 g/mol has been examined. Static and dynamic surface tension data, as well as bubble size distribution and retention time in the foam, were incorporated into a simple model of foam stability. The latter clearly indicates that surface tension differences between the plateau border and lamellar region adjacent to the bubble surface are the dominant factor in controlling foamability, causing liquid flow in the direction opposite to liquid drainage, a process termed the Marangoni effect.