Pressure-programming of a binary,on-line mixed mobile phase for capillary column SFC using a packed column SFC pumping system

Summary Mixed mobile phase delivery for CSFC is served by a system previously designed for packed column SFC, where a back-pressure regulator controls pressure programming. Piston pumps deliver separate flows of carbon dioxide and modifier (2-propanol). The on-line mixed phase with the injected sample is split to the capillary column. Compared to a commercially available CSFC instrument the system shows no significant differences in resolution. Applications show the advantages of using modifiers in CSFC, such as separations of polar, ionic and high molecular weight compounds, influences on selectivity and shorter retention times.     

Devising an adjustable splitter for dual-column gas chromatography

A flow controlled adjustable splitter was configured from a Deans switch and employed in an automated dual column gas chromatographic (GC) system for analyzing mono-aromatic compounds. Volatile organic compounds (VOCs), thermally desorbed from the sorbent trap, were split by the adjustable splitter onto two columns of different phases for separation and then detection by flame ionization detection (FID). Unlike regular splitters in which the split ratio is passively determined by the diameter and/or length of the connecting columns or tubing, the split ratio in our adjustable splitter is controlled by the auxiliary flow in the Deans switch. The auxiliary flow serves as a gas plug on either side of the column for decreasing the sample flow in one transfer line, but increasing the flow in the other. By adjusting the auxiliary flow and therefore the size of the gas plug, the split ratio can be easily varied and favorable to the side of no auxiliary gas. As an illustration, two columns, DB-1 and Cyclodex-B, were employed in this study for separating benzene, toluene, ethylbenzene, xylenes, denoted as BTEX, in particular the structural isomers of o-, m-, p-xylenes. This configuration demonstrates that BTEX cannot be fully separated with either column, but can be deconvoluted by simple algebra if dual columns are used with a splitter. The applicability of the proposed concept was tested by analyzing a gas standard containing BTEX at different split ratios and with various sample sizes, all leading to a constant ratio of m-xylene versus p-xylene.     

Radio gas chromatography on capillary columns using a multi-column system (column-switching)

Summary A gas chromatographic system with capillary columns (fused silica) for the analysis of radiolabelled compounds is described. The system presented is based on a dual column gas chromatograph equipped with column switching facllity and a variable splitter at the column outlet combined with a dead-volume free adapter for the radioactivity monitor for continous measurement of radioactivity in the column effluent. The first column works as a separation column and the second is roughly shortened and used as a feed to the mass detector. The adjustment of the split ratio is regulated by the inlet pressures for the carrier gas supplying both columns. For mass detection all conventional systems can be used. Detection of radioactivity by a gas proportional counter (system based on a combustion technique). Three flow modes can be adjusted: a) total column effluent to the mass detector or b) to the radioactivity monitor, and c) simultaneous flow (dependent on the chosen split ratio) to mass-and radioactivity detectors. The system was developed for use in clinical chemistry and tested with labelled and unlabelled steroids. The method for peak identification by means of relative retention times and methylene units was possible also for radioactive peaks when a heart cutting technique was used. The radio gas chromatographic system presented allows the development of radiochromatograms with the same peak characteristics as in conventional capillary gas chromatography.Presented at the 14th International Symposium on Chromatography London, September, 1982     

Supercritical fluid injection of high-molecular-weight polycyclic aromatic compounds in capillary supercritical fluid chromatography

A sample introduction system for capillary supercritical fluid chromatography, which allows the dissolution of the sample in the supercritical mobile phase before being introduced into the column, was constructed and evaluated. Supercritical n-pentane was shown to solvate high-molecular-weight polycyclic aromatic compounds that could not be solvated using typical liquid solvents. In addition, split injection of a supercritical fluid solution was found to be more reproducible than split injections of a liquid solution. The potential of such an injection system was demonstrated, although further developments are needed in order to make the technique of practically utility.     

Improving splitless injection with electronic pressure programming

An experimental injection port has been designed for split or splitless sample introduction in capillary gas chromatography; the inlet uses electronic pressure control, in order that the column head pressure may be set from the GC keyboard, and the inlet may be used in the constant flow or constant pressure modes. Alternatively, the column head pressure may be programmed up or down during a GC run in a manner analogous to even temperature programming. Using electronic pressure control, a method was developed which used high column head pressures (high column flow rates) at the time of injection, followed by rapid reduction of the pressure to that required for optimum GC separation. In this way, high flow rates could be used at the time of splitless injection to reduce sample discrimination, while lower flow rates could be used for the separation. Using this method, up to 5 μl of a test sample could be injected in the splitless mode with no discrimination; in another experiment, 2.3 times as much sample was introduced into the column by using electronic pressure programming. Some GC peak broadening was observed in the first experiment.     

Practical aspects of Pt/Ir effluent splitters for multidetector GC and pneumatic solute switching

Practical details are given to produce low dead volume Pt/Ir to glass connections and a variety of components for effluent splitting and pneumatic solute switching. It is shown that the column effluents from a glass capillary column can be split in any desired ratio and maintained constant, regardless of column flow rate. Bandbroadening in the splitter remains negligible even for very low flow rates. A complex pneumatic system to be used for heartcutting with two glass capillary columns is shown.     

Capillary Column Inlet System for the Gas Chromatography of Biological Samples

Abstract

A glass injection system consisting of a packed precolumn and gasphase splitter has been developed for the introduction of derivatized biological samples on glass capillary columns. The precolumn provides complete sample evaporation before the splitter zone, traps contaminating nonvolatiles, and prevents decomposition or adsorption of components. In addition, a carrier gas control system is described that permits stable and repeatable adjustment of split ratio and capillary inlet pressure.

The linearity of the splitter has been established using a hydrocarbon mixture covering the range of methylene unit (MU) values in which the steroid derivatives are eluted.     

Chromatographic peak broadening with a lab-made splitter: Reproducibility and accuracy studies

Sample inlet splitters are used in capillary column gas chromatography. We have investigated the sample band broadening in the splitter and studied reproducibility and accuracy in quantitative analysis using a simple lab-made splitter. It was found that the results were dependent upon the operating conditions such as split ratio, splitter temperature, glass wool plug, injected sample volume and solvent.     

Split flow and bypass flow systems for monolithic capillary columns in liquid chromatography

Split flow and bypass flow systems were assembled using Nano Y Connectors with low dead volume commercially available for capillary liquid chromatography (LC). The split ratio could be controlled by changing the dimension of restriction tubing and applied back pressure to the restriction tubing. The split flow system allowed us to use valve injectors and pumps commercially available for capillary LC. The reproducibility of the present split flow system was acceptable. The relative standard deviation for six successive measurements was 0.4% for the retention time, whereas that for the peak height and peak area was 1-3% depending on the analytes. The bypass flow system uses two Nano Y Connectors, where the eluent split at the first Nano Y Connector, which is located in the inlet of the separation column, is merged again into the effluent from the column at the second Nano Y Connector. The bypass flow system could avoid on-column detection and allowed us to use flow cells, leading to an approximate three times improvement in signal-to-noise. The present flow systems were evaluated by using aromatic hydrocarbons and alkylbenzenes as test analytes.     

Preparative gas chromatography with glass capillary columns

The present paper describes an automated system for preparative gas chromatography with glass capillary columns, controlled by a microprocessor. The effluent from the capillary column is divided by a pneumatically controlled splitter and any desired split ratio between traps and detector can be obtained. Moreover, a second pneumatic control allows instantaneous change-over to a different split ratio, thus minimizing loss of material during collection. The effluent containing the compounds of interest is passed through a multiple manifold and collected in coiled glass capillary traps. To ensure maximum trapping efficiency even for very small amounts of material, the inner walls of the capillary traps are wetted with a suitable solvent, which gives a quantitative recovery of micro- and nanogram amounts of material. After repetitive sampling, sufficient amounts of material can be obtained for NMR spectroscopy and possibilities exist to enrich trace components with the aid of a double column system. Two examples of such applications are given, employing mixtures of both synthetic and natural origin.     

Injection temperature effects using On-column and split sampling in capillary gas chromatography

The effect of sample injection temperature on quantitation is examined for on-column and conventional split modes of sampling in capillary gas chromatography. Discrimination effects can be observed even with on-column injection if the injection temperature is too far above the boiling point of the solvent (or that of a major low boiling constituent(. This is attributed to higher boiling components being left behind in the syringe needle, and a set of simulation experiments are described to illustrate this effect. Various discrimination patterns using conventional split injection were observed, depending on temperature of injection. Apart from syringe needle effects, discrimination is probably due to the preferential venting of higher boiling components as liquid sample droplets, which can have a lifetime greater than the time of transit to the splitter. With such a two-phase system, involving variable droplet size, the flow distribution in the splitter will be critical to uniform sampling. The use of combination on-column/split sampling, with the appropriate temperature control to provide sample uniformity to the splitter is discussed as an advantageous alternative to conventional split sampling.     

Development of a custom high-throughput preparative liquid chromatography/mass spectrometer platform for the preparative purification and analytical analysis of compound libraries

Solution-phase parallel synthesis has had a profound impact on the speed of compound synthesis delivering relatively pure compounds (>80%) in short order. However, to develop structure activity relationships (SAR) for a compound series, each library member should preferably be >95% pure. Historically, achieving and quantifying such high-purity criteria for each library member proved to be the slow step for most lead discovery groups. To address this issue, significant modifications have been made to a commercial Agilent preparative LC/MS system to allow for the general mass-guided purification of diverse compound libraries. The custom modifications include (1) the "DMSO slug" approach for the purification of samples with poor solubility; (2) an active splitter to reduce system back-pressure, reduce the delay volume, and allow for a variable split ratio; (3) a sample loading pump for the quick purification of large, dilute samples; (4) a preparative column-selection valve to quickly change column selectivity or sample loading; and (5) an analytical injector with a separate flow path for crude reaction or fraction analyses.     

Computer controlled micropreparative isolation and enrichment from GC capillary columns and subsequent sample handling

This paper describes a computerized system for micropreparative enrichment of components from a mixture being separated on a capillary column. All electronic components are commercially available. Programming was performed in BASIC so that users can easily adapt the program to their own special needs. The enrichment procedure is based on a repetitive absorption of eluting compounds after separation from a capillary column, by sucking them through an absorption tube mounted at the outlet splitter. In order to perform this procedure reliably, the system searches for reference peaks which can stem from componetns of the mixture or from deliberately added reference compounds. Use of error diagnostics prevents contamination of the collected fractions in case of system failures. Optimization is easily performed during a supervised GC run in a semiautomatic mode. Using the function keys of the computer, the operator can adjust injection volume, measure the base line drift at any retention time, and perform reference peak allocation and peak switching until optimal conditions have been found. The system presented has been in operation for several years in our laboratory where, in some cases, the mixture had to be injected onto the capillary column over 500 times to obtain the amount of pure component (10–1000 μg) necessary for NMR structure elucidation. However, recording of meaningful spectra of such small amounts requires special precautions in sample handling. This technique can also be applied for sample recovery after analysis by non-destructive elucidation methods or by chemical microreactions without loss.     

Application of a capillary‐assembled microfluidic system for separation of cephalosporins

This paper demonstrates a simple and easy setting up of a fused‐silica capillary‐assembled microfluidic system (μCE). This system incorporates a split‐flow pressure injection of the sample into a microfluidic system made from PDMS and a short (～20 cm) length of fused‐silica capillary as a separation unit. The on‐capillary detection was carried out by fiber optic spectrometry. A mixture of six cephalosporin antibiotics was separated in the μCE system and the obtained results were compared to those achievable by conventional CE. The six components could be separated within 8.5 min with the number of theoretical plates around 10 000.     

A new configuration for coupling purge-and-trap/cryogenic focusing/capillary column gas chromatography with splitless injection mode

A new configuration for coupling a purge-and-trap unit to a capillary column gas chromatograph via a cryogenic focusing interface has been developed. In this configuration, the precolumn of the cryogenic focusing interface was inserted through the septum of a split/splitless injection port where it served as both sample transfer and carrier gas supply lines. The injection port of the gas chromatograph was modified by plugging the carrier gas and the septum purge lines. This configuration allowed for the desorption of analytes at high flow rates while maintaining low, analytical-column flow rates which are necessary for optimum capillary column operation. The capillary column flow rate is still controlled by the column backpressure regulator. Chromatograms of purgeable aromatics exhibited improved resolution, especially for early eluting components compared to those obtained by direct liquid injection using the normal splitless injection mode. Quantitative sample transfer to the analytical column afforded excellent linearity and reproducibility of compounds studied.     

On-line packed column supercritical fluid chromatography--microwave-induced plasma atomic emission

Interfacing and evaluation of packed column supercritical fluid chromatography (SFC)-microwave-induced plasma atomic emission detection (AED) is described. Via a flow splitter and an integral restrictor, efficient transfer of solutes from column to detector without band broadening is obtained. Variation of CO2 flow-rate during pressure gradients has little influence on both AED signal and baseline drift while it provides similar sensitivity as in capillary SFC. Continuous introduction of CO2 in the plasma reduces the available range of emission domains; nevertheless the region of detection which is free of CO2 interferences allows selective detection of C1 and Br as reported in this paper. reserved.     

On-line hydrogenation of fatty acid methyl esters in capillary gas chromatography

An injection splitter in front of a glass capillary column was used for the hydrogenation of fatty acid methyl esters (FAME) mixtures. Hydrogenation followed by gas chromatographic analysis on capillary columns permitted detection and identification in complicated natural mixtures of branched fatty acids, showing minor structural differences, in quantities down to 10^?8g. The technique described, apart from its suitability for FAME analysis, shows promise for structure determination studies of other classes of compounds.     

The contribution of the pressure drop to analyte retention in coupled column supercritical fluid chromatography of lipids

Summary This paper reports the qualitative and quantitative effects of the column pressure drop on the retention of lipid components in a serially coupled capillary column SFC system. The contribution of the pressure drop consists of two components, the density effect and the flow effect. The magnitude of the flow effect,i. e. the change in retention which results from changes in the flow-rate when column pressures are changed, is determined by the difference in single column analyte k values. The effect will be positive compared with the uncorrected retention values when the column with largest k value is closest to the injector. With the columns in reversed order, the effect will be negative. The contribution from the density effect always resulted in larger coupled column k values and was in most instances of more significance than the flow effect component. Values calculated with and without pressure drop correction have been compared and it has been shown that for most of the eighteen model lipid compounds investigated, the deviations from the experimental retention factors were smaller when pressure drop corrections were made.     

Flame ionization detection after splitting the water effluent in subcritical water chromatography

The coupling of subcritical water separation with flame ionization detection (FID) in the split mode has been investigated in this study. In order to keep the FID system stable during subcritical water separation, a Tee union was connected between the separation column and the FID system to split the water flow. The ratio of the water flow to the FID system over the flow-rate to a waste bottle varied depending on the dimension of capillary tubings and the total water flow-rate used. Separations of several carbohydrates, carboxylic acids, and amino acids were performed on commercially available columns using a laboratory-made subcritical water chromatography-FID system. The FID system was very stable in this split mode even at total flow-rate as high as 1.24 ml/min. The linear dynamic range was up to three orders of magnitude and the limit of detection (LOD) ranged from 38 to 111 ng (306-925 ng/microl injected) with split ratios of approximately 1:10 to approximately 1:17 (FID/waste bottle) for several analytes studied. However, the LOD can be significantly lowered by adjusting the dimensions of the restrictors to allow a higher percentage of the total flow to the FID system.     

Dynamic control of split flow in packed column supercritical fluid chromatography using dual resistively heated restrictors

Remote control of the vent/detector split flow ratio in packed column supercritical fluid chromatography (pSFC) with flame ionization detector (FID) is demonstrated using a dual heated restrictor method. Restrictors stemming from a Tee at the separation column outlet were, respectively, fixed into an FID and a vent port, and their individual temperatures were controlled using resistively heated wires. Subsequently, both system pressure and split flow could be manipulated. For example, for applied restrictor temperatures examined up to 600°C, corresponding vent/FID split flow ratios between 2 and 7 were observed depending on the port heated. As well, column pressures around 16–23 MPa were also achievable over the same range. Conversely, isobaric altering of the split flow ratio was possible when opposing positive and negative temperature gradients were applied at the two restrictors. Under these conditions, the system pressure varied less than 1% RSD over a 10 min period. As an application, the method was used to establish stable detector operation in the analysis of n‐alkanes under pSFC‐FID conditions that initiated flame instability. Results indicate that this technique could be a relatively simple and inexpensive means of controlling system pressure and detector split flow ratios in pSFC‐FID.