High-speed, temperature programmable gas chromatography utilizing a microfabricated chip with an improved carbon nanotube stationary phase

A new growth recipe for producing carbon nanotubes (CNTs) combined with a new bonding technique was implemented in a microfabricated gas chromatography (micro-GC) chip. Specifically, the micro-GC chip contained a 30-cm (length) microfabricated channel with a 50 μm × 50 μm square cross-section. A CNT stationary phase “mat” was grown on the bottom of the separation channel prior to the chip bonding. Injections onto the micro-GC chip were made using a previously reported high-speed diaphragm valve technique. A FID was used for detection with a high-speed electrometer board. All together, the result was a highly efficiency, temperature programmable (via low thermal mass, rapid on-chip resistive heating) micro-GC chip. In general, the newly designed micro-GC chip can be operated at significantly lower temperature and pressure than our previously reported micro-GC chip, while producing excellent chemical separations. Scanning electron microscopy (SEM) images show a relatively thin and uniform mat of nanotubes with a thickness of ∼800 nm inside the channel. The stationary phase was further characterized using Raman spectroscopy. The uniformity of the stationary phase resulted in better separation efficiency and peak symmetry (as compared to our previous report) in the separation of a mixture of five n-alkanes (n-hexane, n-octane, n-nonane, n-decane and n-undecane). The on-chip resistive heater employing a temperature programming rate of 26 °C/s produced a peak capacity of eight within a 1.5-s time window.     

Investigation of high-speed gas chromatography using synchronized dual-valve injection and resistively heated temperature programming

High-speed temperature programming is implemented via the direct resistive heating of the separation column (2.3m MXT-5 Silicosteel column with a 180 microm I.D. and a 0.4 microm 5% phenyl/95% dimethyl polysiloxane film). Resistive temperature programming was coupled with synchronized dual-valve injection (with an injection pulse width of 2 ms), producing a complete high-speed gas chromatography (GC) system. A comparison of isothermal and temperature programmed separations of seven n-alkanes (C(6) and C(8)-C(13)) shows a substantial improvement of peak width and peak capacity with temperature programming. The system was further implemented in separations of a mixture of analytes from various chemical classes. Separations of the n-alkane mixture using three different temperature programming rates are reported. A temperature programming rate as high as 240 degrees C/s is demonstrated. The method for determination of temperature programming rate, based on isothermal data, is discussed. The high-speed resistive column heating temperature programming resulted in highly reproducible separations. The highest rate of temperature programming (240 degrees C/s) resulted in retention time and peak width RSD, on average, of 0.5 and 1.4%, respectively, for the n-alkane mixture. This high level of precision was achieved with peak widths-at-half-height ranging from 13 to 36 ms, and retention times ranging from 147 to 444 ms (for n-hexane to n-tridecane).     

Temperature programmed retention indices: Calculation from isothermal data. Part 1: Theory

Direct conversion of isothermal to temperature programmed indices is not possible. In this work it is shown that linear temperature programmed retention indices can only be calculated from isothermal retention data if the temperature dependence of both the distribution coefficients and the column dead time are taken into account. Procedures are described which allow calculation of retention temperatures and from these, accurate programmed retention indices. Within certain limits the initial oven temperature and programming rate can be chosen freely. The prerequisite for this calculation is the availability of reliable isothermal retention data (retention times, retention factors, relative retention times, or retention indices) at two different temperatures for one column. The use of compiled isothermal retention indices at two different temperatures for the calculation of retention temperatures and thus temperature programmed indices is demonstrated. For the column for which programmed retention indices have to be determined, the isothermal retention times of the n-alkanes and the column dead time as a function of temperature have to be known in addition to the compiled data for a given stationary phase. Once the programmed retention indices have been calculated for a given column the concept allows the calculation of temperature programmed indices for columns with different specifications. The characteristics which can be varied are: column length, column inner diameter, phase-ratio, initial oven temperature, and programming rate.     

The absolute retention index in chromatography. Part 1

A definition for an absolute retention index is given. This index is independent of whether the process is isothermal or temperature programmed. The relation between this index and Kovats's index is discussed. General equations are derived for the retention times of homologous series, from which the position of the air peak or missing members can be determined. Methods used in the literature for air peak determination are improved and extended to other cases of interest. The retention index in temperature programmed chromatography is reexamined in the light of recent publications on temperature programming.     

Pharmaceutical applications on columns packed with sub-2 microm particles

During the last few years, there has been a great interest in the development of fast liquid chromatography. Among the reported approaches, the reduction of the particle size to attain sub-2microm diameter represents a good solution for achieving both increased separation power and faster analysis time. This paper demonstrates the chromatographic performance of such supports using plate-height curves and reveals the possibility for obtaining ultra-fast or highly efficient separations, according to the column geometry and system pressure limitations. The stability of these columns is initially evaluated using a system suitability experiment. The chromatographic performance remains stable in terms of retention, efficiency, and pressure for more than 1700 injections with pressure conditions ranging from 200 to 800 bar. Several fast and ultra-fast pharmaceutical applications are reported. In isocratic mode, a 5- to 10-fold reduction in analysis time is obtained with limited influence on efficiency and resolution. The run time is further reduced by 30-fold with the shorter available columns (i.e., 30 mm length). In gradient mode, the separation of a complex mixture containing an active pharmaceutical compound and related impurities is significantly improved with column length equal to 100 mm, to increase peak capacity and resolution.     

Prediction of retention times in linear gradient temperature and pressure programmed analysis on capillary columns

The simultaneous temperature and inlet pressure programming (TPP) in gas chromatographic analysis decreases the retention time and the maximum value of temperature required for the elution of high boiling substances. Therefore, compounds sensitive to thermal degradation can be better analysed and column ageing is reduced. However, the empirical choice of proper analysis conditions requires many preliminary runs; this paper describes a procedure for the theoretical prediction of retention times in TPP using few preliminary runs carried out in isobaric and isothermal conditions. The used program permits the prediction of the retention times of the compounds analysed with any different TPP run carried out within the temperature and pressure ranges investigated with the preliminary runs. The influence of various analytical parameters on the accuracy of the prediction values was investigated. The proposed model also predicts the relative position in the chromatogram of closely eluting peaks and the possible coincidence of retention times or inversion of the elution order with changing temperature. It is also possible to foresee the analytical conditions, which offer a baseline separation of all of the peaks of the sample.     

Gas chromatographic system for the identification of halogenated pesticides by retention indices using n-alkanes as standards

A gas chromatographic system for the evaluation of linear temperature-programmed retention indices allowing n-alkanes to be adopted as the reference retention markers for any type of analyte, irrespective of the atoms present in their molecules, is described. It is based on the simultaneous use of two different detectors (a flame ionization detector and a specific detector suitable for the sample components), both connected (in parallel) to the same column outlet. The performance of this system has been tested by measuring the retention indices of fifteen chlorinated pesticides under conditions of linear programming temperature, by adopting an electron-capture detector as the specific detector. The reliability of the retention indices thus determined has been proven by verifying that they can be reproduced under different chromatographic conditions.     

On-line liquid backflush—a new technique for GC analysis of samples containing nonvolatile material

Deteriorated performance in capillary GC is often caused by the deposition of nonvolatile components in capillary GC inlets. In many cases this is a limiting factor in trace analysis of complex samples. Volatile solutes of the sample interact with the high boiling material, which leads to broadened and unsymmetrical peaks. This paper describes a novel technique which utilizes independent temperature programs of a retention gap and the column as well as a liquid backflush of the retention gap. Samples that normally contaminate the retention gap after a few injections can be repeatedly injected over extended periods of time, while the chromatographic resolution is retained.     

重整生成汽油样品中烃的类别分析

用气相色谱以程序升温方式分析了重整生成汽油,并将各组分升温保留时间转换为恒温保留指数。以各组分在ＯＶ－１和ＳＥ－５４固定相上,同一柱温下的保留指数差及在各柱上的温度系数为三因素进行斜交因子分析和本征矢量旋转,给出了重整生成汽油样品中烷烃、烯烃、环烷烃、芳烃值,经气相色谱－质谱分析验证了结果的正确性,为重整成成汽油样品中烃的类别分析提供了一种新方法。     

烷基化汽油中烃的类别分析

在程序升温条件下，用气相色谱法分析烷基化汽油样品，将各组分的程序升温保留时间转化的恒温保留指数。根据各组分在SE-54固定相和OV-1固定相上的气相色谱保留指数差对其进行类别定性。利用色谱-质谱分析法进行了验证，结果表明该方法简便可行。     

Optimization and identification in any kind of multi-step temperature programmed gas chromatography

The theory which predicts the retention time, retention temperature, and peak width for any kind of multi-step TPGC and the principle of optimization has been described. Software for optimization and identification in TPGC has been developed on the basis of this theory. It has also been proven that the relationship between peak width in TPGC and the derived or “invented” retention time is similar to that between peak width and retention time in isothermal processes. The validity of the software has been proved by using it to predict retention temperature, retention time, and peak width for any kind of temperature programming and to predict the optimum temperature program for separation of a multihomolog mixture of industrial alcohols and 15 enantiomeric pairs of amino acids.     

High‐throughput gas chromatography for volatile compounds analysis by fast temperature programming and adsorption chromatography

The synergy of combining fast temperature programming capability and adsorption chromatography using fused silica based porous layer open tubular columns to achieve high throughput chromatography for the separation of volatile compounds is presented. A gas chromatograph with built‐in fast temperature programming capability and having a fast cool down rate was used as a platform. When these performance features were combined with the high degree of selectivity and strong retention characteristic of porous layer open tubular column technology, volatile compounds such as light hydrocarbons of up to C₇, primary alcohols, and mercaptans can be well separated and analyzed in a matter of minutes. This analytical approach substantially improves sample throughput by at least a factor of ten times when compared to published methodologies. In addition, the use of porous layer open tubular columns advantageously eliminates the need for costly and time‐consuming cryogenic gas chromatography required for the separation of highly volatile compounds by partition chromatography with wall coated open tubular column technology. Relative standard deviations of retention time for model compounds such as alkanes from methane to hexane were found to be less than 0.3% (n = 10) and less than 0.5% for area counts for the compounds tested at two levels of concentration by manual injection, namely, 10 and 1000 ppm v/v (n = 10). Difficult separations were accomplished in one single analysis in less than 2 min such as the characterization of 17 components in cracked gas containing alkanes, alkenes, dienes, branched hydrocarbons, and cyclic hydrocarbons.     

Development of membrane extraction with a sorbent interface-micro gas chromatography system for field analysis

The commercially available portable gas chromatographs have a rather limited scope of applications, typically allowing analysis of gaseous samples only, and having relatively poor sensitivity. Combination of those instruments with modern sampling/sample preparation techniques can remedy these problems. A Chrompack micro-GC system equipped with a thermal conductivity detector has been coupled to membrane extraction with a sorbent interface (MESI). The sorbent trap has replaced the GC injector. The design of the trap was also modified in order to enhance the preconcentration of analytes. The use of a thin flat sheet membrane reduces the response time, and decreases the memory effect of the system. Rapid separation times were achieved, and the sensitivity was significantly improved. MESI enables semi-continuous monitoring of both gaseous and aqueous samples, owing to the selectivity of the membrane material. The system does not use moving parts, therefore being reliable. The sensitivity of the micro-GC system was increased by a factor of more than 100 by the addition of the MESI system, even with a preconcentration time as short as 1 min. Chloroform, having a concentration lower than 1 ppb, was detected in tap water. A cup system was used to allow headspace sampling of volatile organic compounds from aqueous matrices, keeping the membrane away from interfering species that could be present in water, and improving the mass transfer. A linear calibration line was obtained, and the estimated limit of detection was 60 ppt. This represents a great improvement for the sensitivity of the micro-GC system.     

Determination of calibration function in thermal field flow fractionation under thermal field programming

A new procedure for determining the calibration function able to relate retention and operative parameters to molecular weight of the species in thermal field flow (ThFFF) under thermal field programming (TFP) conditions is presented. The procedure involves determining the average values of retention parameters under TFP and determining a numerical function related to the temperature variations that occur during TFP. The calibration parameters are obtained by a procedure fitting the retention and operative parameters that hold true at the beginning of the TFP. The procedure is closely related to the one previously developed to calibrate the retention time axis under TFP ThFFF and, together, they constitute a full calibration procedure. Experimental validation was performed with reference to polystyrene (PS)-decalin and PS-THF systems. The calibration functions here obtained were compared to those derived by the classical procedure at constant thermal field ThFFF to obtain the calibration function at variable cold wall temperatures. Excellent agreement was found in all cases proving "universality" of the ThFFF calibration concept, i.e. it is independent of the particular system on which it was determined and can thus be extended to ThFFF operating under TFP. The new procedure is simpler than the classical one since it requires less precision in setting the instrumentation and can be obtained with fewer experiments. The potential applications for the method are discussed.     

Resolution prediction and optimization of temperature programme in comprehensive two-dimensional gas chromatography

A model is developed for predicting the resolution of interested component pair and calculating the optimum temperature programming condition in the comprehensive two-dimensional gas chromatography (GC x GC). Based on at least three isothermal runs, retention times and the peak widths at half-height on both dimensions are predicted for any kind of linear temperature-programmed run on the first dimension and isothermal runs on the second dimension. The calculation of the optimum temperature programming condition is based on the prediction of the resolution of "difficult-to-separate components" in a given mixture. The resolution of all the neighboring peaks on the first dimension is obtained by the predicted retention time and peak width on the first dimension, the resolution on the second dimension is calculated only for the adjacent components with un-enough resolution on the first dimension and eluted within a same modulation period on the second dimension. The optimum temperature programming condition is acquired when the resolutions of all components of interest by GC x GC separation meet the analytical requirement and the analysis time is the shortest. The validity of the model has been proven by using it to predict and optimize GC x GC temperature programming condition of an alkylpyridine mixture.     

一种直热式快速气相色谱快速升温装置的设计

用大电流脉冲直接加热不锈钢毛细管柱, 将脉冲间隔调整到正好使柱管局部完成热平衡, 用快速PID技术控制脉冲频率和宽度, 设计了一种直热式快速升温装置. 该装置最高升温速率可达到5 ℃/s, 升温范围 40~150 ℃, 程序升温线性相关系数大于0.9996, 最大功耗74 W, 加热平均功耗小于50 W, 在34 s内完成nC8~nC17 10种正构烷烃的分离, 保留时间重复精度误差RSD在0.22%~0.55％之间, 降温和平衡时间仅为30 s. 与常规气相色谱仪相比, 该装置分析挥发性和半挥发性有机物速度可提高20倍以上, 专用于快速气相色谱仪.     

Temperature programming elution in capillary supercritical fluid chromatography

Summary Effects of column temperature on the retention behaviour of aromatic hydrocarbons and dialkyl phthalates were investigated in capillary supercritical fluid chromatography (SFC) with carbon dioxide as the mobile phase. Negative temperature programming could partly replace pressure programming. Positive temperature programming was applicable to solutes with proper volatility, in which gas chromatography-like retention mechanism (partition process) was involved.     

GC behaviour of symmetrical n-dialkyl sulphides under isothermal and temperature programming conditions

Retention indices were determined for a homologous series of n-dialkyl sulphides on three stationary phases (SE-30, OV-17 and XE-60) under isothermal and linear temperature programming conditions. Under these two different GC conditions, equations were derived for each of the three stationary phases which showed the dependence of retention index on the number of carbon atoms and the boiling points for a homologous series of n-dialkyl sulphides. The equation for the correlation isothermal retention index was shown to be applicable to the identification of n-dialkyl sulphides using linear temperature programming. It was found that the GC behaviour of n-dialkyl sulphides makes these compounds suitable for use as a standard series instead of n-alkanes for the calculation of retention indices in GC analysis in which detectors insensitive to n-alkanes are employed. The use of the homologous series of n-dialkyl sulphides for the calculation of sulphide retention indices can be great practical importance in the microanalysis of natural compounds. We have used this method successfully in the analysis of pesticides containing S-atoms.     

气相色谱分析过程的计算机仿真模拟与优化

以色谱法的ＬＥＳ模型作为气相色谱分析过程的计算机仿真模拟与优化的理论基础，提出了把两次多阶程度升温实验保留时间作为基本数据进行模拟优化的方法，拓宽了模拟优化方法在复杂组分样品中的应用。软件编程采用了计算数学方法，提高了模拟优化的精度和速度，节省了分析样品的时间。     

Effect of high-temperature on high-performance liquid chromatography column stability and performance under temperature-programmed conditions

Six commercially available analytical (4.1 or 4.6 mm i.d.) columns were evaluated under temperature-programmed high-temperature liquid chromatography (HTLC) conditions to access their stability and performance at extreme temperatures. Seven components consisting of acidic, basic and neutral compounds were analyzed under temperature-programmed conditions and solvent gradient conditions using three different mobile phase compositions (acidic, basic and neutral). Each column was checked with a two-component test mix at various stages of the evaluation to look for signs of stationary phase collapse. Three zirconia based stationary phases studied exhibited column bleed under temperature-programmed conditions. The other three columns, a polydentate silica column, a polystyrene-divinylbenzene (PS-DVB) polymeric column, and a graphitic carbon column performed well with no evidence of stationary phase degradation. The R.S.D. for the retention times and efficiencies were less than 10% for most conditions, and not more than 15% during the course of the evaluation for each column. The polydentate silica stationary phase was temperature programmed to 100 degrees C, the PS-DVB stationary phase was temperature programmed up to 150 degrees C, and the graphitic carbon column was used with temperature programming up to 200 degrees C. Comparable peak capacities and similar retention behaviors were observed under solvent gradient and temperature-programmed conditions. Temperature programming with dynamic mobile phase preheating can replace solvent gradient analysis without a loss of peak capacity when used with 4.1 or 4.6 mm columns.