Effect of mobile phase density gradients on efficiency in packed column supercritical fluid chromatography

Summary The efficiency of packed columns was measured as a function of flow rate, temperature, outlet density, and the density differential across the column, unsing pure carbon dioxide as the mobile phase. Although density differentials are often blamed for a serious loss in efficiency in packed column supercritical fluid chromatography, the results show that efficiency was not a function of the density differential. Peak shapes suggest that apparent loss in efficiency is actually due to inadequate solubility of the solute in carbon dioxide.     

Capillary and packed column SFC/MS

A frit restrictor interface for capillary column supercritical fluid chromatography/mass spectrometry (SFC/MS) has been constructed and used for the analysis of high boiling point alkanes. Packed column SFC/MS is described using both a moving belt liquid chromatographic/mass spectrometric interface and a thermospray source in the filament-on mode.     

Quantitative analysis of delta9-tetrahydrocannabinol in preserved oral fluid by liquid chromatography-tandem mass spectrometry

A rapid and sensitive method for the analysis of delta9-tetrahydrocannabinol (THC) in preserved oral fluid was developed and fully validated. Oral fluid was collected with the Intercept, a Food and Drug Administration (FDA) approved sampling device that is used on a large scale in the U.S. for workplace drug testing. The method comprised a simple liquid-liquid extraction with hexane, followed by liquid chromatography-tandem mass spectrometry (LC-MS-MS) analysis. Chromatographic separation was achieved using a XTerra MS C18 column, eluted isocratically with 1 mM ammonium formate-methanol (10:90, v/v). Selectivity of the method was achieved by a combination of retention time, and two precursor-product ion transitions. The use of the liquid-liquid extraction was demonstrated to be highly effective and led to significant decreases in the interferences present in the matrix. Validation of the method was performed using both 100 and 500 MicroL of oral fluid. The method was linear over the range investigated (0.5-100 ng/mL and 0. 1-10 ng/mL when 100 and 500 microL, respectively, of oral fluid were used) with an excellent intra-assay and inter-assay precision (relative standard deviations, RSD <6%) for quality control samples spiked at a concentration of 2.5 and 25 ng/mL and 0.5 and 2.5 ng/mL, respectively. Limits of quantification were 0.5 and 0.1 ng/mL when using 100 and 500 microL, respectively. In contrast to existing GC-MS methods, no extensive sample clean-up and time-consuming derivatisation steps were needed. The method was subsequently applied to Intercept samples collected at the roadside and collected during a controlled study with cannabis.     

Retention in coupled capillary column supercritical fluid chromatography with lipids as model compounds

Summary We have investigated to which extent retention data, acquired on single capillary columns, can be used for predicting retention factors in a coupled column system. For this purpose we utilized a model mixture of 18 lipid components with widely different vapor pressures and polarities. The sample was chromatographed on two columns, SB-biphenyl-30 (70% methyl-30% biphenylpolysiloxane) and SB-cyanopropyl-50 (50% methyl-50% cyanopropylsiloxane). Experimental retention factors, acquired in coupled column systems with two columns connected in different order, were thus compared with values calculated from runs on each single column. The agreement between calculated and experimental values generally was better than 5% without any pressure drop correction.To study the possibility of predicting retention behavior in a wide pressure range from a limited number of experiments, we also investigated the relation between solute retention and mobile phase density. We found that all data could be fitted to second order equations, which gives the possibility to optimize the resolution with respect to pressure from a limited number of runs at different pressures.     

A two-step supercritical fluid extraction of polycyclic aromatic hydrocarbons from roadside soil samples

A two-step procedure for the supercritical fluid extraction (SFE) of polycyclic aromatic hydrocarbons from soil samples was developed. The procedure consists of a static supercritical fluid treatment in a closed extraction cell at a high temperature (T=250 or 340degreesC for 20 min) and an SFE with a solvent trapping. During the static phase, the sample is exposed to a supercritical organic solvent (methanol, toluene, dichloromethane, ACN, acetone, and hexane). The solvent penetrates particles of the matrix to substitute strongly bonded molecules and dissolves the analytes in the supercritical phase. At ambient temperature, supercritical fluids became liquid and lost their solvation abilities. Most of the analytes condense on the surface of the particles or on the extraction cell walls without forming strong bonds or penetrating deep into the matrix. Thus, the pretreatment liberates the analytes and they behave similar to those in freshly spiked samples. The common SFE with toluene-modified CO2 as an extraction fluid follows the static phase. With the use of the most suitable extraction phases (toluene, ACN), the extraction efficiency of the combined procedure is much higher (approximately100%). The results of the combined procedure are compared to the SFE procedure of the same untreated sample (difference less than 5%) and to the Soxhlet extraction. The extracts were analyzed using a GC with the flame ionization detection.     

Retention behaviour of carboxylic acid methyl esters in supercritical fluid chromatography

The study on retention behavior in supercritical fluid chromatography (SFC) is necessary to understand the mechanism of the various interactions in SFC. The retention of SFC in carboxylic acid methyl ester/polymethylsiloxane/CO2 system was studied systematically and the retention behavior of this kind of compounds under various typical operation conditions was described using the method of an alternative unified theory of chromatographic retention. The results illustrated that expression: Ink.= a + b/T + cp + dp/T + ep2/T can be used to describe quantitatively the retention behavior of carboxylic acid methyl ester/polymethylsiloxane/CO2 system in the ranges of reduced density from 0.549 to 1.411. It was also found that the entropy of solute in stationary phase is dependent on the density of supercritical fluid (SF) under typical operating conditions of SFC.     

Separation of fatty acid methyl esters by comprehensive two-dimensional supercritical fluid chromatography with packed columns and programming of sampling duration

Fatty acid methyl esters from various fats and oils were separated by comprehensive two-dimensional supercritical fluid chromatography with conventional packed columns and FID detection. The first dimension was a silica gel column and the second dimension was an ODS column. This combination was largely orthogonal for the separation of fatty acid methyl esters. The first dimension separations were primarily based on the number of double bonds while the second dimension separations were based on the chain length. The highly-ordered chromatograms and improved resolution allowed the easy detection and identification of minor components. Although the first dimension separations were performed under isobaric conditions where the peak width increased in proportion to the retention, the programming of the sampling duration allowed us to maintain the optimum re-injection frequency (3–4 times) per peak into the second dimension and so to minimize the total analysis time without deteriorating the resolution.     

共边排样件激光切割路径的规划

排样软件的应用使材料利用率得到了很大提高，然而后续切割软件若不能保证有效地切割零件、保证零件质量及提高生产率，则排样软件在材料利用率上获得的收益将丧失。基于图论学理论，建立了规则与非规则零件共边排样时激光切割路径规划的数学模型，给出了在充分考虑加工质量、加工效率、制造成本情况下的激光切割路径优化目标：打孔点最少以及切割中割嘴空行程最短。提出了满足激光切割工艺要求的三个切割路径优化算法：用于求解理想情况下共边切割路径优化问题的一个新的欧拉回路算法；基于奇度顶点完全图最小权最大匹配算法来求解一般情况下共边切割路径优化问题的算法；利用废料区域进一步减少打孔点的处理策略与求解算法。给出了各种算法的运行实例，验证了所提出的算法的有效性。     

Flow and heat transfer with prescribed skin friction in a rotating fluid

This paper investigates the rotating flow and heat transfer of a viscous fluid induced by a stretching surface. The nonlinear problem subject to a given skin friction at the boundary is solved. Analytic solution is obtained using homotopy analysis method. The velocity, temperature, and stretching velocity is calculated for different values of the rotation parameter (λ). The obtained results are compared with the well known results of rotating flow induced by a stretching surface by using four sets of boundary conditions. Copyright © 2011 John Wiley & Sons, Ltd.     

Simultaneous effects of induced magnetic field and heat and mass transfer on the peristaltic motion of second‐order fluid in a channel

In this article, we investigate the influence of heat and mass transfer on the peristaltic flow of magnetohydrodynamic second‐order fluid in a channel when the induced magnetic field effects are present. Problem formulation in a wave frame of reference is presented. The governing nonlinear analysis is carried out under the assumption of small wave number. Explicit expressions of the pressure gradient, the stream function, the magnetic force function, the axial induced magnetic field, the current density distribution, the temperature, and the concentration distribution are derived. The effects of embedded parameters are also examined. Copyright © 2011 John Wiley & Sons, Ltd.