直链单烯烃保留指数与分子结构关系的研究

根据同系直链单烯烃保留指数与同碳数正构烷烃保留指数差值与分子碳数间关系曲线拟合,提出预测同系直链单烯烃保留指数的准确公式。在ＳＱ,ＰＦＥ,ＰＥＧ－４０００上,利用所提出的公式对检验集中化合物保留指数预测值与实测值差的标准偏差在±０．９ｉ．ｕ．～±１．５ｉ．ｕ．之间。并且研究发现,同碳数、同几何构型直链单烯烃各位置异构体分子中双键位置与化合物保留指数具有指数关系。首次提出依据分子中双键位置预测其保留指数的准确公式。公式适合各种不同极性固定相。     

The impact of column inner diameter on chromatographic performance in temperature gradient liquid chromatography

The impact of column inner diameter on chromatographic performance in temperature gradient liquid chromatography has been investigated in the present study. Columns with inner diameters of 0.32, 0.53, 3.2 and 4.6 mm were compared with respect to retention and efficiency characteristics using temperature gradients from 30 to 90 degrees C with temperature ramps of 1, 5, 10 and 20 degrees C min(-1). The columns were all of 15 cm length and were packed with 3 microm Hypersil ODS particles. Alkylbenzenes served as model compounds, and the mobile phase consisted of acetonitrile-water (50:50, v/v). The study revealed that the column ID is not a critical limiting factor when performing temperature programming in LC, at least for columns narrower than 4.6 mm inner diameter in the temperature interval 30-90 degrees C. The retention times for all components on all columns were highly comparable, with similar peak profiles without any signs of peak splitting. The use of mobile phase pre-heating when using the larger bore columns was avoided by starting the temperature gradients close to ambient. However, the relative apparent efficiency was inversely proportional to column inner diameter, making the capillary columns generally more functional towards temperature gradients than the larger bore columns with respect to chromatographic efficiency. In addition, the capillary columns possessed higher robustness towards temperature programming than the conventional columns.     

Distribution coefficients of n-alkanes measured on wall-coated capillary columns

Distribution coefficients K of n-alkanes were determined in wide ranges of temperature and carbon numbers from gas chromatographic retention data measured on wall-coated poly(dimethylsiloxane) commercial capillary columns. A discussion is centered on how to mitigate the difficulties for an accurate determination of K when using weakly retentive columns, as those bearing very high phase ratios or short lengths. Particularly, the errors associated with the estimation of the gas hold-up and the phase ratio of the column are considered. The chromatographic importance for determining K of n-alkanes relies on the fact that these are the most commonly applied references for reporting relative thermodynamic parameters such as the Kovats Index and the relative retention. A great amount of information has been compiled in this form. If K of the reference is known, absolute values of distribution coefficients for a myriad of substances are readily obtainable. The knowledge of K(T) functions of solutes in wide ranges of temperature is a primary necessity in temperature-programmed gas chromatography. This knowledge is needed for the prediction of absolute retention times and for computing separation optimizations of mixtures containing several critical pairs of analytes.     

Retention in overloaded columns, an experimental approach

Employing a micro-bore silica capillary coated with Carbowax 20 M, the dependence of chromatographic retention upon operative variables was studied surpassing the sample capacity of the column. Solution thermodynamics in the non-linear range of the absorption isotherm of n-alkanes on poly(ethylene oxide) were analyzed interpreting the experimental data through a retention equation deduced in a preceding theoretical work. At 120 degrees C, and pressures up to 11 bar abs, deviations from the ideal-gas behavior are found to be negligible, either for the fluid dynamics of the carrier-gas, or the thermodynamics of solution of the n-alkanes. Within the experimental error, for all practical purposes the mobile phase can be treated as an ideal gas. This constraint allows studying a solute molecule placed in an environment ranging from solvent monomers only, to a mixture of varying composition of solvent and solute, avoiding effects from significant interactions in the gas phase. In the experimental conditions explored, the absorption isotherm can be represented by taking only two-terms of its power series development.     

Optimization of the porous structure and polarity of polymethacrylate-based monolithic capillary columns for the LC-MS separation of enzymatic digests

The porous structure as well as the polarity of methacrylate ester-based monolithic stationary phases has been optimized to achieve the separation of various peptides originating from enzymatic digestion. The porous structure, determined by the size of both pores and microglobules, was varied through changes in the composition of porogenic solvents in the polymerization mixture, while the polarity was controlled through the incorporation of butyl, lauryl, or octadecyl methacrylate in the polymer backbone. Both the morphology and the chemistry of the monoliths had a significant effect on the retention and efficiency of the capillary columns. The best resolution of peptidic fragments obtained by digestion of Cytochrome c with trypsin in solution was obtained in a gradient LC-MS mode using a monolithic capillary column of poly(lauryl methacrylate-co-ethylene dimethacrylate) featuring small pores and small microglobules. Raising the temperature from 25 to 60 degrees C enabled separations to be carried out at 40% higher flow rates. Separations carried out at 60 degrees C with a steeper gradient proceeded without loss of performance in half the time required for a comparable separation at room temperature. Our preparation technique affords monolithic columns with excellent column-to-column and run-to-run repeatability of retention times and pressure drops.     

Analytical aspects of capillary gas chromatography of lower fatty acids [up to C18]

The principal aspects influencing analytical capillary gas chromatography of fatty acids up to C18 have been evaluated. Selected fundamental problems of interlaboratory exchange of retention data were problems of defined temperature in commercial thermostated air baths, of capillary tubing, and of stationary phases. A modification of commercial thermostats has been proposed in order to secure a defined temperature for glass capillary columns. It has been found that retention data of fatty-acid methyl esters can be measured under standard conditions with the same accuracy as retention data of hydrocarbons on squalane. Metal capillary columns coated with Apiezon L were found to be unsuitable for the analysis of fatty-acid methyl esters when compared with the results of their quantitative analysis in packed Apiezon L and polar capillary columns. Possibilities of a nontraditional statistical evaluation of the results of measurements are suggested. A program in FORTRAN IV language is given for the calculation of Kovats' retention indices for fatty-acid methyl esters.     

Fast temperature programming on a stainless-steel narrow-bore capillary column by direct resistive heating for fast gas chromatography

A direct resistive-heating fast temperature programming device for fast gas chromatography was designed and evaluated. A stainless-steel (SS) capillary column acted both as a separation column and as a heating element. A fast temperature controller with the deviation derivative proportional-integral-derivative (DDPID) control algorithm, which was suitable for ramp control using ramp-to-setpoint function, was used to facilitate the fast pulse heating. The SS resistive-heating column can generate linear temperature ramps up to 10 degrees C/s and can re-equilibrium from 250 degrees C down to 50 degrees C within 30s. With n-alkanes as the test analytes, the relative standard deviations (RSDs) of retention time were between 0.19 and 0.59% and the RSDs of their peak areas were less than 4% for all but one. The results indicated that this technique could be used for both qualitative and quantitative analysis. Phenolic and nitroaromatic compounds were also analyzed by using the SS resistive-heated system. The combination of a short narrow-bore SS column and rapid heating rates provides sufficient separation efficiency for relatively simple mixtures at drastically reduced analysis time. The total analysis time including equilibration time was less than 2 min for all test mixtures in this study.     

溶胶-凝胶法制备用于分离极性有机化合物的毛细管气相柱

 采用溶胶 凝胶法制备了用于分离有机极性化合物的毛细管气相柱 ,其制备工艺简单 ,制柱时间较传统工艺大为缩短。该柱涂层与毛细管内壁间形成的化学键使得其热稳定性好。游离的脂肪酸、有机碱可直接在该柱上得到很好的分离 ,其他极性化合物也在该柱上得到极好的分离。该类柱不仅在同一柱上显示了良好的分离重复性 ,而且不同柱间在容量因子、柱效、对称性及姆氏常数上也显示了良好的重复性。     

Influence of temperature on the behaviour of small linear peptides in capillary electrochromatography

The influence of temperature, T, on the retention times, peak widths, peak symmetry coefficients and theoretical plate numbers of two small linear peptides, [Met5]enkephalin and [Leu5]enkephalin, has been studied with capillary electrochromatography (CEC) capillary columns of 100 microm I.D. and 250 mm packed length with a total length of 335 mm, containing 3 microm Hypersil n-octadecyl bonded silica. With increasing column temperature from 15 to 60 degrees C, the electroosmotic flow (EOF) and the column efficiencies increased, whereas the retention coefficients (Kcec) of both peptides decreased. A linear relationship was found between the EOF value and the square root of the temperature over this temperature range, with a linear regression correlation of 0.998. Non linear Van 't Hoff plots (In Kcec versus 1/T) were observed for these peptides between 15 and 60 degrees C, suggesting that a phase-transition occurred with the n-octadecyl chains bonded on the silica surface, affecting the CEC retention behaviour of these peptides. In CEC systems, the Kcec values of peptides incorporate contributions from both electrophoretic migration and chromatographic retention. Positive and negative Kcec values can, in principle, thus arise with these charged analytes. However, the Kcec values of the enkephalin peptides under all temperature conditions studied were positive with an eluent composed of water-50 mM NH4OAc/AcOH, pH 5.2-acetonitrile (5:2:3, v/v) and therefore the chromatographic component dominates the retention process with these small peptides under these conditions.     

强极性高聚物毛细管柱的制备与性能评价

以强极性的丙烯腈为单体,以二乙烯苯为交联剂,采用原位聚合法一步反应制备出了强极性的ＯＰＰＬＯＴ－Ａ型高聚物毛细管柱。该柱的极性大于早先制备的ＯＰＰＬＯＴ－Ｑ,－Ｒ,－Ｓ型柱。对柱子的色谱性能进行了评价。该柱可实现对醛、酮、醇、酯、醚、腈等类化合物的分离,尤其对腈、醛的分离更好。对柱子的耐水性也进行了考察,水对柱子的保留行为不产生影响。由于该柱极性较大,因此耐温性较前述柱低,最高使用温度为２００℃。还对柱子的其它分离特性进行了考察。     

Poly(cyclooctene)-based monolithic columns for capillary high performance liquid chromatography prepared via ring-opening metathesis polymerization

Monolithic columns for capillary HPLC were prepared via ring-opening metathesis polymerization (ROMP) from cis-cyclooctene (COE), tris(cyclooct-4-enyl-1-oxy)methylsilane (CL) as monomers, 2-propanol and toluene as porogens and RuCl(2)(Py)(2)(IMesH(2))(CHC(6)H(5)) (Py=pyridine, IMesH(2)=1,3-dimesityl-4,5-dihydroimidazolin-2-ylidene) as initiator within the confines of 200 microm i.d. fused silica columns. For evaluation of the novel monolithic capillary HPLC columns, a protein standard consisting of six proteins in the molecular weight range of 5800-66000 g/mol, i.e. ribonuclease A, insulin, albumin, lysozyme, myoglobin and beta-lactoglobulin, was used. Reproducibility of synthesis was checked by determining the relative standard deviation (RSD) in retention times (t(R)), which was found to be in the range of 2.9-3.9% for all analytes. Variations in polymer kinetics were realized by adding different amounts of free pyridine and had a significant influence on the monolith's morphology, the backpressure and retention times. On the contrary, variations in monomer content and COE to CL ratio showed only minor changes on these parameters. Long-term stability of 1000 runs at 50 degrees C showed excellent stability of the columns and no significant alteration in separation performance was observed in combination with slightly decreased retention times (approx. 1.6-7.2% for all analytes).     

Prediction of programmed temperature retention times on linked capillary columns of different polarity

Systems formed by serial connection of capillary columns of different polarity were studied with methods previously used to predict the behavior of linked capillary columns under isothermal conditions and to obtain programmed temperature gas chromatography (PTGC) retention times of the individual columns starting from isothermal data. The two calculation methods were simultaneously applied in order to predict PTGC retention times of the series system starting from isothermal data obtained on the two individual columns. Experimental retention values measured using different temperature programs on the individual columns and on the series systems were found to agree with those calculated.     

Evaluation of copper-1,3,5-benzenetricarboxylate metal-organic framework (Cu-MOF) as a selective sorbent for Lewis-base analytes

The metal-organic framework copper-1,3,5-benzenetricarboxylate (Cu-BTC) was evaluated for its ability to selectively interact with Lewis-base analytes by examining retention on gas-chromatographic columns packed with Chromosorb W HP that contained 3.0% SE-30 along with various loadings of Cu-BTC. Scanning electron microscopy images of the support material showed the characteristic Cu-BTC crystals embedded in the SE-30 coating on the diatomaceous support. The results indicated that the Cu-BTC-containing stationary phase had limited thermal stability (220 °C) and strong general retention for analytes. Kováts index calculations showed selective retention (amounting to about 300 Kováts units) relative to n-alkanes for many small Lewis-base analytes on a column that contained 0.75% Cu-BTC compared with an SE-30 control. Short columns that contained lower loadings of Cu-BTC (0.10%) allowed elution of nitroaromatics; however, selectivity was not observed for aromatic compounds (including nitroaromatics) or nitroalkanes. Observed retention characteristics are discussed.     

Silica versus polymer-based stationary phases for reversed-phase high-performance liquid chromatographic analyses of rat insulin biosynthesis. A comparison of resolution and recovery

Because of the problems caused by the irreversible binding of insulins and proinsulins to several silica-based reversed-phase columns, the use of polymeric reversed-phase columns was investigated for the analysis of rat islet polypeptides involved in insulin biosynthesis. No irreversible binding of insulins and proinsulins was observed for the polymeric reversed-phase columns, probably due to the absence of silanol groups. The six polypeptides involved in insulin biosynthesis in rat islets were equally well resolved in shallow trifluoroacetic acid-acetonitrile gradients on the silica-based Nucleosil 300-5C4 column (45 degrees C), the polymer-based Asahipak C4P-50 (25 and 45 degrees C), and ODP-50 columns (45 degrees C). In shallow triethylammonium phosphate-acetonitrile gradients (25 degrees C) satisfactory resolution of the two rat proinsulins was only obtained on the polymer-based Asahipak C4P-50 and C8P-50 columns. Increasing the separation temperature to 45 degrees C improved the separation of the two insulins and the two proinsulins in all cases. The shifts in retention times for the individual islet polypeptides observed in relation to the increased separation temperature were found to be different for the silica C4 and the polymer C4 columns. Recoveries of rat islet polypeptides were comparably high from the silica- and the polymer-based C4 columns and linear load-response curves were obtained in the microgram to picogram mass range on both columns.     

Separation of polyethylene glycol oligomers using inverse temperature programming in packed capillary liquid chromatography

Inverse temperature programming in packed capillary liquid chromatography coupled to evaporative light-scattering detection has been used to resolve native polyethylene glycol (PEG) oligomers. The model compound, PEG 1000, was separated on a 300 mm x 0.32 mm I.D. capillary column packed with 3 microm Hypersil ODS particles with acetonitrile-water (30:70, v/v) as mobile phase. The retention of the PEG oligomers increased with increasing temperature, different from what is commonly observed in liquid chromatography. The retention times of the oligomers were approximately doubled for each 25 degrees C increment of the column temperature in the temperature range 30-80 degrees C. The oligomers were almost unretained and co-eluted at a column temperature of 30 degrees C. At 80 degrees C a baseline separation of more than 22 peaks was obtained, but the last eluting peaks were severely broadened and all oligomers did not elute. When a negatively sloped temperature ramp from 80 to 25 degrees C at -1.5 degrees C/min was applied, the peak shapes were improved, additional peaks were detected and the analysis time was reduced by 48%. In the temperature programming mode, the intra-day precision of the retention times ranged from 0.5 to 5.8% (n=5).     

Practical aspects of using methacrylate-ester-based monolithic columns in capillary electrochromatography

Methacrylate-ester-based monoliths containing quaternary ammonium groups were prepared in situ in capillary columns and in simultaneous experiments in vials, employing thermal initiation. The chromatographic properties of the monoliths were determined with capillary electrochromatography (CEC), and their morphology was studied with mercury-intrusion porosimetry on the bulk materials. Materials with different, well repeatable pore-size distributions could be prepared. A satisfactory column-to-column and run-to-run repeatability was obtained for the electro-osmotic mobility, the retention characteristics (k-values) and the efficiency on the columns prepared and tested in the CEC mode. A relatively high electro-osmotic flow was observed in the direction of the positive electrode. The electro-osmotic mobility was found to be influenced only marginally by mobile-phase parameters such as the pH, ionic strength, and acetonitrile content. The retention behavior of the monolithic columns was similar to that of columns packed with C18-modified silica particles. Columns could be prepared with optimum plate heights ranging from 6 microm for unretained compounds to 20 microm for well retained (k=2.5) polyaromatic hydrocarbons. However, for specific analytes a - still unexplained - lower chromatographic column efficiency was observed.     

Temperature effects on separation on zirconia columns: applications to one- and two-dimensional LC separations of phenolic antioxidants

The effects of temperature and mobile phase on LC chromatographic separation of phenolic antioxidants on zirconia-based columns were investigated. Unlike silica-based materials, zirconia columns show excellent thermal stability over a wide range of temperatures and enable high-temperature separations. Enthalpic and entropic contributions to the retention of phenolic compounds on ZR-Carbon and ZR-Carbon C18 columns were determined from retention versus temperature plots in order to elucidate the retention mechanism of sample compounds over the temperature range up to 14 degrees C. High-temperature liquid chromatography on ZR-Carbon columns was used for comprehensive LC x LC two-dimensional separation systems based on the different selectivity of a Zorbax SB micro-column used in the first dimension and a ZR-Carbon column used in the second dimension. Two-dimensional LC x LC systems employing a setup with two alternately operated parallel ZR-Carbon columns in the second dimension were used for the analysis of phenolic antioxidants in beer and wine samples.     

Life-time studies with capillary electrochromatography columns operated under different conditions

A test system has been established to permit the monitoring of the life-time performance of several reversed- phase capillary electrochromatography (CEC) columns. The retention factors, k(cec), peak symmetry coefficients, lambda(sym), and column efficiencies, N, of three neutral n-alkylbenzene analytes, namely ethyl-, n-butyl- and n-pentylbenzenes, were determined for Hypersil 3 microm n-octylsilica and n-octadecylsilica packed into CEC capillary columns of 100 microm I.D., with a packed length of 250 mm, and a total length of 335 mm. The performances of these CEC capillary columns were examined for a variety of eluents with pH values ranging between pH 2.0 - 8.0, similar to those employed to study the retention behaviour of peptides that we have previously reported. The relative standard deviation (RSD) of the retention factors (k(cec) values) of these n-alkylbenzenes, acquired with an eluent of (25 mM Tris-HCl, pH 8.0,)-acetonitrile (1:4, v/v), when the CEC capillary columns were used for the first time (virgin values), were 4% (based on data acquired with 4 CEC capillary columns) for the n-octyl bonded silica capillary columns, and 6% (based on 8 columns) for n-octadecyl bonded silica capillary columns. The RSD values of the k(cec) values of the n-alkylbenzenes for one set of replicates (n=6) with one CEC capillary column was < 0.5%. The theoretical plate numbers, N, for the virgin CEC capillary columns were ca. 60,000, whilst the observed N values for all new CEC capillary columns were > or = 40,000 for n-octyl bonded silica capillary columns and > or = 50,000 for n-octadecyl bonded silica capillary columns. The peak symmetry coefficients, lambda(sym), of the n-alkylbenzenes for virgin CEC capillary columns and for CEC capillary columns used for more than 1,000 injections were always in the range 0.95-1.05. The experimental results clearly document that the life-time performance of the CEC capillary columns depends on the eluent composition, as well as the nature of the analytes to which the CEC capillary columns are exposed.     

An accurate and easy procedure to obtain isothermal Kováts retention indices in gas chromatography

Isothermal Kováts retention indices may be used in GC for identification purposes, but they are also useful in characterisation of stationary phases and for studying structural and physico-chemical properties of both the analyte and the stationary phase. They are currently reported as whole numbers with an accuracy of one index unit for non-polar stationary phases. The method recommended for their calculation uses a linear regression model, although non-linear models have also been applied with good results. In both cases, a computer programme and the retention times of a series of n-alkanes that elute correctly and resolved from the other compounds are needed, conditions which cannot always be fulfilled. However, it is possible to calculate retention indices with an accuracy of 0.1 retention index units (0.2 units for packed columns) with the help of only three n-alkanes using just a pocket calculator or a computer spreadsheet. The main requirement is that at least one of the n-alkanes has a retention index differing by no more than one hundred retention index units from that of the analyte being investigated. Examples are given for different stationary phases.     

Joule heating in packed capillaries used in capillary electrochromatography

Effective heat dissipation is critical for reproducible and efficient separations in electrically driven separation systems. Flow rate, retention kinetics, and analyte diffusion rates are some of the characteristics that are affected by variation in the temperature of the mobile phase inside the column. In this study, we examine the issue of Joule heating in packed capillary columns used in capillary electrochromatography (CEC). As almost all commonly used CEC packings are poor thermal conductors, it is assumed that the packing particles do not conduct heat and heat transfer is solely through the mobile phase flowing through the system. The electrical conductivity of various mobile phases was measured at different temperatures by a conductivity meter and the temperature coefficient for each mobile phase was calculated. This was followed by measurement of the electrical current at several applied voltages to calculate the conductivity of the solution within the column as a function of the applied voltage. An overall increase in the conductivity is attributed to Joule heating within the column, while a constant conductivity means good heat dissipation. A plot of conductivity versus applied voltage was used as the indicator of poor heat dissipation. Using theories that have been proposed earlier for modeling of Joule heating effects in capillary electrophoresis (CE), we estimated the temperature within CEC columns. Under mobile and stationary phase conditions typically used in CEC, heat dissipation was found to be not always efficient. Elevated temperatures within the columns in excess of 23 degrees C above ambient temperature were calculated for packed columns, and about 35 degrees C for an open column, under a given set of conditions. The results agree with recently published experimental findings with nuclear magnetic resonance (NMR) thermometry, and Raman spectroscopic measurements.