Minimum column temperature required for concurrent solvent evaporation in coupled HPLC-GC

Concurrent solvent evaporation using the loop-type HPLC-GC interface requires that the GC oven temperature be above the eluent boiling point at the given carrier gas inlet pressure in order to prevent eluent flowing into the GC capillary column. Corresponding oven temperatures representing minimum oven temperatures for eluent transfer were experimentally determined for solvents and solvent mixtures of interest for use as HPLC eluents. Evaluation of eluents for concurrent evaporation is discussed. Recommended lengths of uncoated column inlets (pre-columns) are derived from the mechanisms involved in solvent evaporation. Temperatures listed as minimum column temperatures for concurrently evaporating HPLC eluents are also useful for estimating maximum applicable column temperatures when working with the conventional retention gap or partially concurrent solvent evaporation techniques in coupled HPLC-GC.     

Co-solvent effects for preventing broadening or loss of early eluted peaks when using concurrent solvent evaporation in capillary GC. Part 1: Concept of the technique

The concept and some first results of a method are described for evaporating large volumes of solvent in a relatively short pre-column (retention gap) in such a way that solvent trapping retains volatile components in the inlet up to completion of solvent evaporation. The method was developed for transferring large volumes (easily exceeding 1 ml) of HPLC eluent to GC when using on-line coupled HPLC-GC, but is equally suited for injecting large sample volumes (at least some 50 μl) and could be particularly useful for introducing aqueous solutions. Concurrent solvent evaporation allows introduction of very large volumes of liquid into GC. However, peaks eluted up to some 40–80° above the column temperature during introduction of the liquid are strongly broadened due to the absence of solvent trapping. On the other hand, previous retention gap techniques involving solvent trapping were not suited for transferring very large volumes of liquid into GC. Using a relatively high boiling co-solvent added to the sample or the HPLC eluent, advantages of concurrent solvent evaporation can be combined with solute reconcentration by solvent effects, allowing elution of sharp peaks starting at the column temperature during introduction of the sample.     

Concurrent solvent evaporation for on-line coupled HPLC-HRGC

A technique is proposed which allows introduction of very large volumes of liquid (10 ml were tested) into capillary columns equipped with short (1–2 m long) retention gaps. It is based on concurrent solvent evaporation, i.e. evaporation of the solvent during introduction of the sample. The technique presupposes high carrier gas flow rates (at least during sample introduction) and column temperatures near the solvent boiling point. The major limitation of the method is the occurrence of peak broadening for solutes eluted up to 30°, in some cases up to 100°, above the injection temperature. This is due to the absence of solvent trapping and a reduced efficiency of phase soaking. Therefore, use of volatile solvents is often advantageous. Application of the concurrent solvent evaporation technique allows introduction of liquids which do not wet the retention gap surface. However, the method is still not very attractive for analysis of aqueous or water-containing solutions (reversed phase HPLC).     

Early solvent vapor exit in GC for coupled LC-GC involving concurrent eluent evaporation

Use of early solvent vapor exits for concurrent eluent evaporation with the loop-type interface has two purposes: protection of the GC detectors from large amounts of solvent vapors and more efficient discharge of the vapors to accelerate eluent evaporation and help avoiding broad solvent peaks. Use of a retaining pre-column after the uncoated pre-column can rule out losses of solute materials that form sharp peaks.     

Column temperature during eluent transfer with concurrent eluent evaporation in coupled HPLC-HRGC

Concurrent solvent evaporation is suited for coupled HPLC-HRGC if solutes elute at intermediate to high column temperatures—otherwise retention gap techniques are more appropriate. Concurrent eluent evaporation using a loop-type interface requires that the GC oven temperature during eluent introduction be above the eluent boiling point at the carrier gas inlet pressure applied. An experimental background is given for facilitating selection of the appropriate column temperature.     

Analysis of petroleum fractions by on-line micro HPLC-HRGC coupling,involving increased efficiency in using retention gaps by partially concurrent solvent evaporation

Two-dimensional chromatography of gasoline by on-line coupled HPLC-HRGC, as described in this paper, allows separate GC analysis of paraffins and aromatics. The GC system contains a retention gap of only 10 m length for introducing HPLC fractions of 100 μl volume. This becomes possible through evaporation of part of the solvent during introduction of the HPLC eluent. This “partially concurrent solvent evaporation” technique allows transfer of large volumes of HPLC eluent into relatively short retention gaps, maintaining the full efficiency of the solvent effects in reconcentrating the bands of the early eluted solutes.     

Co-solvent effects for preventing broadening or loss of early eluted peaks when using concurrent solvent evaporation in capillary GC. Part 2: n-Heptane in n-pentane as an example

Co-solvent effects are applied to allow use of concurrent solvent evaporation for applications requiring analysis of compounds eluted less than some 50° above the column temperature during sample introduction, i.e. at oven temperatures below some 100–120°C. Required conditions such as GC even temperature, concentration of the co-solvent and length of the uncoated pre-column (retention gap) are studied theoretically as well as experimentally for the case of n-heptane as co-solvent in n-pentane.     

Coupled LC-GC: Evaporation rates for partially concurrent eluent evaporation using an early solvent vapor exit

Partially concurrent eluent evaporation presupposes an eluent evaporation rate in the GC pre-column that approaches the LC flow rate. Discharging the vapors through the whole GC column, evaporation rates reach 10–30 μl/min, i.e. are suitable just for LC flow rates typical for packed capillary LC columns. With an early vapor exit, evaporation rates are increased to 100–200 μl/min (under extreme conditions to some 800 μl/min), thus fitting the LC flow rates of 2 mm i.d. columns. Evaporation rates were measured for a standard set of pre-columns and conditions. The dependence of the evaporation rate on temperature, inlet pressure, carrier gas, and internal diameter of the retaining pre-column are discussed in order to allow the design of a GC system producing a desired evaporation rate.     

Variable splitter for regulation of the solvent evaporation rate in the coupling of liquid chromatography with gas chromatography

A system is described which accelerates the solvent evaporation rate in the retention gap. The evaporation is due to a saturation effect of the carrier gas stream, and a considerable increase in evaporation rate is obtained by inserting a split outlet between the retention gap and the capillary separation column in the gas chromatograph. By varying the backpressure of the spliter device, the flow rate through the retention gap can be adjusted and so too the evaporation rate. The evaporation process was monitored by inserting a dectecter in the split outlet line. The technique was applied to the on-line LC trace enrichment/GC analysis of water containing a mixture of polycyclic aromatic hydrocarbons.     

Analysis of fossil-fuel fractions by on-line coupled microcolumn HPLC capillary column GC-MS

A multidimensional LC-GC-MS system was developed and used to analyze complex fossil-fuel samples. The electron-acceptor stationary phase, 5 μm 2,4-dinitrophenylmercaptopropylsilica, was packed into a 0.32-mm × 350-mm microcapillary liquid chromatography (μLC) column. Aromatic fractions eluted from the μLC system at 12 μL/min and were diverted to the gas chromatograph mass spectrometer (GC-MS) by a ten-port switching valve with 50 and 7.6-μL loops. Concurrent cosolvent evaporation occurred in a 0.32-mm × 3-m precolumn ahead of a 0.25-mm × 30-m DB-5 analytical column. Solvent vapors exited through an open-split interface. The utility of the μLC-GC-MS system was demonstrated through the analyses of solvent refined coal, kerosene, and crude oil.     

Comprehensive on-line HPLC-GC for screening potential migrants from polypropylene into food: The effect of pulsed light decontamination as an example

High performance liquid chromatography (HPLC) was used for the fractionation of extracts from polypropylene (PP) films and coupled on-line to gas chromatography (GC) with automated transfer of the complete HPLC fractions (comprehensive on-line HPLC-GC, i.e. HPLCxGC). Flame ionization detection (FID) was used for the estimation of concentrations, mass spectrometry (MS) for identification work. This method was applied to investigate whether pulsed light (PL) treatment for the microbiological decontamination of polypropylene packaging materials produces reaction products requiring an evaluation to meet regulatory requirements. To demonstrate the safety of PL treatments with regard to the formation of reaction products, i.e. that no component is formed that could endanger human health, basically comprehensive analysis of components potentially migrating into food is required, but comprehensiveness cannot be proven and remains an approximation. The threshold concentration in the film was estimated either from the conventional European non-detection limit of 0.01 mg/kg food or the concept of the threshold of toxicological concern (TTC) for an unknown substance, i.e. an exposure to 0.15 μg per person and day. PL treatment of the films containing Irgafos 168 produced several new components exceeding these limits, i.e. a toxicological safety assessment would probably be required. No such peaks were detected for Tinuvin 326, Irganox 1076 and Chimassorb 81. No degradation of the polymer was detected.     

Determination of solvent recondensation or evaporation in the GC-precolumn by measurement of temperature changes on the column wall

Temperature measurements on the column outer well were used for detecting recondensation or evaporation of solvent inside the precolumn during injection or on-line transfer of large solvent volumes. This facilitates the choice of the most critical parameter of these techniques, i.e. oven temperature. When using the vaporizer/precolumn solvent split/gas discharge system, the dew point of the solvent is determined, either to just prevent solvent recondensation or to limit it to the capacity of the precolumn to retain liquid. In concurrent eluent evaporation through the loop type LC-GC interface, temperature measurement enables the determination of whether or not the flooded zone exceeds a given limit. Fanally, when solvent trapping is used (on-column injector/partially concurrent solvent evaporation evaporation or vaporizer/partial recondensation), temperature measurement near the front end of the flooded zone is used as a signal for accurate closure of the vapor exit shortly before the end of solvent evaporation.     

Polar solvents for the desorption of a liquid chromatographic precolumn coupled on-line with a capillary gas chromatograph

Coupling column liquid chromatography and gas chromatography on-line is becoming more important in analytical chemistry. Especially when large amounts of polar solvents can be introduced into the gas chromatograph without any problem, the technique will offer new possibilities. With a DPTMDS retention gap, evaporation rates and flooded zones of some solvents have been determined. Two modes of operation using partially concurrent solvent evaporation conditions are discussed: (1) injecting a sample via a loop of an LC valve followed by introduction into the gas chromatograph with an LC pump; (2) trace enrichment on a precolumn followed by on-line desorption with n-propanol into the gas chromatograph. Preliminary results for a splitter system, inserted between the retention gap and the analytical column which allows a considerable increase of the evaporation rate are also presented.     

Theory of a reversible redox reaction in an ionic liquid that is coupled to an ion transfer across the aqueous electrolyte/ionic liquid interface

A theory is provided for a reversible electro-oxidation of a neutral redox probe dissolved in room-temperature ionic liquid, which is sandwiched between an electrode surface and an aqueous solution as a thin film. If the peak potentials in cyclic voltammetry depend on the bulk concentration of electrolyte in water, the oxidation is most probably coupled to the transfer of anions from water into ionic liquid; but if the peak potentials are independent of the electrolyte concentration, the transfers of anions from water into ionic liquid and cations from ionic liquid into water are equally probable. Dedicated to Professor Dr. Yakov I. Tur’yan on the occasion of his 85th birthday.     

Direct synthesis of hierarchically porous TS‐1 through a solvent‐evaporation route and its application as an oxidation catalyst

Synthesis of hierarchically porous zeolites has drawn intensive interest because of their improved catalytic performance. It is highly desirable to find ways to generate these materials in a low‐cost and scalable way for their commercial applications. A solvent evaporation route has been established to synthesize hierarchically porous titanosilicalite‐1 (TS‐1). In the protocol, hexadecyltrimethoxysilane was added to an ethanolic solution of titanium isopropoxide, tetraethyl orthosilicate and tetrapropylammonium hydroxide, i.e. the embryo solution of TS‐1. The solution was subjected to solvent evaporation‐induced self‐assembly to afford an ordered dry gel. Subsequent steam‐assisted crystallization converted the dry gel into a hierarchically porous TS‐1. X‐ray powder diffraction (XRD), UV–visible diffusive reflectance spectroscopy, N₂ physisorption and electron microscopic characterizations have been employed to elucidate the structure. Ti is incorporated into the tetrahedral sites of the MFI structure and mesopores around 20 nm penetrating the crystalline framework are formed. Hexadecyltrimethoxysilane plays a key role in creating mesopores as well as increasing the crystal size. The hierarchically porous TS‐1 exhibits improved activity in styrene oxidation and phenol hydroxylation. Copyright © 2014 John Wiley & Sons, Ltd.     

Choline chloride based deep eutectic solvent as an efficient solvent for the benzylation of phenols

Deep eutectic solvents (such as the combination of urea and choline chloride) are found to be promising solvent and phase-transfer-media for benzylation of phenol. These methods avoided the complexity of multiple alkylations giving selectively O-alkylated aromatic products. Good to excellent yields of the corresponding benzyl phenyl ether were obtained. The non-toxic, biodegradable, inexpensive, and recyclable nature of DES make this protocol green and cost-effective.     

Epoxidized soy bean oil migrating from the gaskets of lids into food packed in glass jars. Analysis by on-line liquid chromatography-gas chromatography

The migration of epoxidized soy bean oil (ESBO) from the gasket in the lids of glass jars into foods, particularly those rich in edible oil, often far exceeds the legal limit (60 mg/kg). ESBO was determined through a methyl ester isomer of diepoxy linoleic acid. Transesterification occurred directly in the homogenized food. From the extracted methyl esters, the diepoxy components were isolated by normal-phase LC and transferred on-line to gas chromatography with flame ionization detection using the on-column interface in the concurrent solvent evaporation mode. The method involves verification elements to ensure the reliability of the results for every sample analyzed. The detection limit is 2-5 mg/kg, depending on the food. Uncertainty of the procedure is below 10%.     

GC column effluent splitter for problematic solvents introduced in large volumes: Determination of di-(2-ethylhexyl) phthalate in triglyceride matrices as an application

Introduction of solutions of up to several milliliters by on-column injection of large volumes or by coupled HPLC-GC may cause problems with GC detectors (FID, AFID, MS). For instance, dichloromethane forms large amounts of hydrochloric acid and carbon black in FIDs. A column effluent splitter was developed for keeping the major portion of the solvent vapors away from the detector; approximately 99% of the vapor is vented while the remaining 1% of vapor is used for detecting the widths of the solvent peaks. During analysis, the split ratio is reversed by a strong increase of the resistance to the gas flow through the split exit line. The system was used for the determination of di-(2-ethylhexyl)-phthalate (DEHP) in triglyceride matrices of various foods. Direct determination by HPLC is not sufficiently sensitive, whereas direct analysis by GC is hindered by the triglycerides. Solutions of fats or oils were pre-separated on a silica column using dichloro-methanelcyclohexane 1:l with addition of 0.05 % acetonitrile as eluent. The HPLC fraction containing the DEHP was transferred to GC through a loop-type interface using concurrent solvent evaporation. Detection limits were around 0.1 ppm.     

PTV vapor overflow — principles of a solvent evaporation technique for introducing large volumes in capillary GC

The concept of a GC solvent evaporation technique is outlined that involves a modified Programmed Temperature Vaporizing (PTV) injector. The vapor overflow technique is intended for introducing samples in large volumes of solvent by syringe injection of strongly diluted samples or by coupled LC-GC. The liquid is introduced into a packed vaporizing chamber kept above the solvent boiling point at a pressure which is near or below ambient. The carrier gas is essentially switched off. Evaporation and discharge of the solvent vapors occurs by expansion of the vapors, driven by the solvent vapor pressure. For transferring the sapmple into the column, the carrier gas is switched on again and the vaporizing chamber heated. Compared to PTV solvent split injection, vapor overflow offers the following advantages: It automatically optimizes operational parameters, therefore facilitating its application. Losses of volatile materials are minimized by a minimal flow rate through the injector. Vapor overflow is a promising technique for transferring watercontaining eluents in coupled LC-GC since no wettability is required and leaching of pre-column surfaces is avoided.