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.     

Vaporising systems for large volume injection or on-line transfer into gas chromatography: Classification, critical remarks and suggestions

Techniques for large volume introduction of liquid samples into capillary gas chromatography (GC) follow a small number of principals. Vaporising systems, vapour discharge modes and methods for solvent-solute separation are classified and evaluated.

Presently, programmed temperature vaporising (PTV) solvent split injection is the preferred method if on-column techniques cannot be applied. Critical re-evaluation suggests, however, that solvent evaporation and solvent-solute separation should be performed in separate compartments and optimized individually. Permanently hot chambers offer the highest capacity for solvent evaporation. The preferred techniques for solvent-solute separation are stationary phase focusing in a coated capillary or solvent trapping in an uncoated capillary precolumn. The vaporising chamber-precolumn solvent split-gas discharge system is proposed for large volume injection and on-line transfer of water-containing solvent mixtures, and in-line vaporiser-precolumn solvent split-overflow system for most on-line transfer applications.     

Role of the retaining precolumn in large-volume on-column injections of volatiles to gas chromatography

In the present study the retaining precolumn, which is commonly used in a set-up for large-volume on-column injections, or when solid-phase extraction (SPE) or liquid chromatography is coupled to gas chromatography (CC), was removed after varying its length from the standard length of 3 m down to zero. A dramatic increase of the evaporation rate of the injected organic solvent was obtained from a typical value of 100 microl/min up to 300 microl/min. The increased evaporation rate allowed (i) injection of a larger volume in the same retention gap, (ii) faster injection/transfer of the organic solvent and (iii) reduction of the transfer temperature. As volatile compounds under partially concurrent solvent evaporation conditions are easily lost once the organic solvent has been removed via a solvent-vapour exit (SVE), the parameters for large-volume injection, i.e. the evaporation rate and injection speed, were optimised using accurate measurements of the real flow-rate of the carrier gas into the GC system. All these options have been evaluated over the last 4 years. In order to demonstrate that omitting the retaining precolumn had no effect on the application range of the on-column interface, analytes as volatile as benzene were injected into GC-MS using 50-200 microl of n-pentane solutions. Contaminants were extracted from river water and wastewater into n-pentane using in-vial liquid-liquid extraction. The detection limits for benzene, toluene, ethylbenzene and m-xylene were approximately 10 ng/l. To obtain optimum results the SVE had to be closed 1 s before the end of evaporation. Several brands of n-pentane were analysed to check for the presence of benzene. Most of them contained interfering compounds and benzene at the low microg/l level and therefore had to be cleaned by means of column chromatography. As another example C8-C17 alkylphenones were extracted from wastewater with n-hexane. Detection limits were 10-40 ng/l.     

On-line combination of liquid chromatography and capillary gas chromatography. Preconcentration and analysis of organic compounds in aqueous samples

This paper describes the design of a new, versatile, and low-cost on-line LC-GC interface that allows the fast and reliable introduction of large sample volumes onto a capillary GC column. The sample introduction procedure consists successively of: evaporation of the entire sample (LC fraction), selective removal of the solvent and simultaneously cold-trapping of the solutes, splitless transfer of the solutes to the GC column, on-column focusing, GC separation and detection. Quantitative and qualitative aspects of various experimental parameters are evaluated and optimum conditions are reported. The applicability of the method is demonstrated on a synthetic aqueous sample of chlorinated pesticides.     

Automated sample cleanup using on-line coupling of size exclusion chromatography to high resolution gas chromatography

A fully automated on-line sample cleanup system based on the coupling of size exclusion chromatography to high resolution gas chromatography is described. The transfer technique employed is based on fully concurrent solvent evaporation using a loop-type interface, early vapor exit and co-solvent trapping. Optimization of the LC-GC transfer was done visually via an all-glass oven door. To circumvent the problem of mixing within the injection loop, an adaptation was made to the standard loop-type interface. The determination of a series of additives in a polymer matrix is presented as one example of the vast range of applications opened up by this technique.     

On-line coupled HPLC-HRGC: A powerful tool for vapor phase FTIR analysis (LC-GC-FTIR)

The design of an on-line LC-GC-FTIR system using an on-column interface and partially concurrent solvent evaporation with early vapor exit is described. The integration of LC-GC coupling into vapor phase FTIR analysis enables problems of sensitivity encountered with HRGC-FTIR detection to be over-come. The applicability of the method is demonstrated by the identification and determination of citropten and bergapten in bergamot oil.     

Coupled LC-GC: A new method for the on-line analysis of organochlorine pesticide residues in fat

This paper describes the use of coupled LC-GC for the determination of organochlorine pesticide residues in fat samples. Organochlorine pesticide residues are preseparated from fat by LC on a short C-18 column using an organic solvent as the mobile phase. Evaporation of the LC eluent is achieved by a modified on-column interface, introducing an on-line vaporizer module using the fully concurrent evaporation technique.     

Analysis of volatile fatty acids in biological specimens by capillary gas chromatography

A method was developed to analyze and quantitate volatile fatty acids such as acetic, propionic, butyric, iso-butyric, valeric, and iso-valeric acid from samples of biological origin. A capillary column system including an automatic on-column injection device as well as a precolumn of larger internal diameter than the analytical column was elaborated for this purpose. In order to obtain well resolved and correctly quantifiable chromatographic peaks it turned out to be essential to work under acidic/aqueous conditions. To achieve a better sample transfer into the chromatographic system an organic solvent had to be used together with the aqueous milieu, thus improving wetting properties of the liquid sample plug introduced into the column. Cold on-column injection was applied in order to avoid discrimination of the various acids due to sample splitting and the automatic technique was chosen in view of the large number of samples from biological extractions which had to be analyzed.     

Behavior of heavy solutes on a precolumn in on-column gas chromatography

The behavior of heavy solutes present in a sample injected on the precolumn has not been clarified in on-column injection onto gas chromatograph having a capillary column coupled to an uncoated precolumn in an oven. To investigate this point, an on-column gas chromatograph has been developed which is constructed with a heatable precolumn outside the oven. Some experiments were carried out in order to evaluate the performance. As a result, it was found that heavy solutes reach the first part of the main capillary column after the solvent has gone, resulting in sharp peaks without solvent effects. The reason why sharp peaks appear for the heavy solutes is also discussed. The cold-trapping effect has been shown to play an important role in narrowing the band width of the heavy solutes. Some of the advantages of the gas chromatograph developed are also presented.     

Investigations on analyte losses in systems suited for large-volume on-column injections

Summary Use of a large-volume injection system with a solvent vapour exit (SVE) requires optimisation. An appropriate strategy is to determine the evaporation rate by increasing the injection time at a fixed injection speed, injection temperature and head pressure. When measuring the flow rate in the carrier gas supply line to the on-column injector, optimisation can be very rapid: some five injections of pure solvent will be sufficient. When working under partially concurrent solvent evaporation conditions, loss of volatiles is often observed if no retaining precolumn is used between the retention gap and the SVE. To investigate the requirements (length and stationary phase) of the retaining precolumn, C8–C18n-alkanes inn-hexane were used. The minimum length of the retaining precolumn (0.32 mm diameter) needed to prevent substantial losses of volatiles was 2 m. Experiments with retaining precolumns with and without stationary phase gave identical results. This shows that there is no need to coat the capillary as it only acts as a restrictor.     

Memory effects with the on-column interface for on-line coupled high performance liquid chromatography-gas chromatography: The Y-interface

When using the on-column interface for on-line high performance liquid chromatography (HPLC)-gas chromatography (GC), there is a memory effect typically equivalent to 0.5–3% of the previous transfer. The shape of peaks distorted as a result of incomplete reconcentration of the initial bands enabled mapping of the distribution of the solute material in the uncoated precolumn and deriving the mechanism which causes the memory effect. The relatively slow transfer of HPLC eluent causes liquid being sucked backwards into the narrow interspace between the transfer line and the precolumn wall. Solvent is evaporated into the passing carrier gas and is replaced by more eluent pulled into this zone, resulting in enrichment of solute material. At the end of the transfer, some of this solute material enters the transfer line and remains there up to the subsequent transfer of an HPLC fraction. This problem is avoided by replacing the on-column injector used as interface by a Y-piece in which the eluent flow from HPLC and the carrier gas are joined. The memory effect was reduced to below 0.02%.     

Large volume injection techniques in capillary gas chromatography

Large volume injection (LVI) is a prerequisite of modern gas chromatographic (GC) analysis, especially when trace sample components have to be determined at very low concentration levels. Injection of larger than usual sample volumes increases sensitivity and/or reduces (or even eliminates) the need for extract concentration steps. Also, an LVI technique can serve as an interface for on-line connection of GC with a sample preparation step or with liquid chromatography. This article reviews the currently available LVI techniques, including basic approaches to their optimization and important real-world applications. The most common LVI methods are on-column and programmed temperature vaporization (PTV) in solvent split mode. Newer techniques discussed in this article include direct sample introduction (DSI), splitless overflow, at-column, and "through oven transfer adsorption desorption" (TOTAD).     

Characterization of complex hydrocarbon mixtures using on-line coupling of size-exclusion chromatography and normal-phase liquid chromatography to high-resolution gas chromatography

An on-line coupling of size-exclusion Chromatography (SEC), normal-phase liquid Chromatography (NPLC), and gas Chromatography (GC) for the characterization of complex hydrocarbon mixtures is described. The hyphenated system separates according to size, polarity, and boiling point. The use of size exclusion as the first separation step allows for the direct injection of complex (“dirty”) samples withont prior clean-up. SEC-NPLC coupling was realized using an on-line solvent evaporator based on fully concurrent solvent evaporation (FCSE) using a modified loop-type interface, vapor exit and co-solvent trapping. Complete reconcentration of the analytes was realized by the introduction of a cryogenic cold trap. For the subsequent hydrocarbon group-type separation an ammo-silica column with n-heptane as eluent was used. The NPLC-GC coupling was based on an on-column interface using partially concurrent solvent evaporation (PCSE) and an early vapor exit. Initial results obtained on the analysis of a residue from the atmospheric crude-oil distillation (a so-called long residue) are presented as an example of the enormous separation power of the SEC-NPLC-GC system. The application of the system for quantitative analysis has not yet been studied.     

Separation of metal complexes by on-line coupled LC-GC

The on-line coupled LC-GC technique was applied to the analysis of several metal chelates of N,N-diethyldithiocarbamic acid. A 10-port valve interface was used to couple the LC and GC instruments. Conditions during sample transfer into the GC gave fully concurrent solvent evaporation. The chelates investigated were separated with a short apolar fused silica column. LC preseparation was made with cyano or amino phases using a hexane/dichloromethane mixture as eluent. On-line LC-GC combination seems to be very suitable for the separation of the metal chelates studied.     

Loop-type interface for concurrent solvent evaporation in coupled HPLC-GC. Analysis of raspberry ketone in a raspberry sauce as an example

Presently, two coupling techniques are used for directly introducing HPLC fractions into capillary GC: The retention gap technique (involving negligible or partially concurrent solvent evaporation) and fully concurrent solvent evaporation. While the former involves use of a conventional on-column injector, it is now proposed that concurrent solvent evaporation technique be carried out using a switching valve with a built-in sample loop. The technique is based on the concept that the carrier gas pushes the HPLC eluent into the GC capillary against its own vapor pressure, generated by a column temperature slightly exceeding the solvent boiling point at the carrier gas inlet pressure. Further improvement of the technique is achieved by flow regulation of the carrier gas (accelerated solvent evaporation) and backflushing of the sample valve (improved solvent peak shape). Concurrent solvent evaporation using the loop-type interface is easy to handle, allows transfer of very large volumes of HPLC eluent (exceeding 1 ml), and renders solvent evaporation very efficient, allowing discharge of the vapors of 1 ml of solvent through the column within 5–10 min.     

Elucidation of the structure of drug degradation products by on-line coupled reversed phase HPLC-GC-MS and on-line derivatization

Degradation products obtained under severe alkaline stress of the drug substance ENA 713, a compound developed for the treatment of DAT (dementia of the Alzheimer type), have been identified by on-line coupled reversed phase HPLC-GC-MS as 3-hydroxyaceto-phenone and 3-hydroxystyrene. Their structures were proven by mass spectroscopic investigation of the products obtained by on-column derivatization during LC-GC transfer. On-column trimethyl-silylation and bromination of the degradation products during the transfer were achieved by the introduction of the reagent via the loop-type interface. The usefulness of the technique was further demonstrated by the on-column methylation of carboxylic acids with diazomethane solution (using naproxen as model compound). First results are also given of the use of the liquid-liquid extraction unit of the LC-GC interface for extractive derivatization during reversed phase HPLC-GC-MS transfer.     

Improvement in the solvent evaporation injection technique for preparative supercritical fluid chromatography

Summary In supercritical fluid chromatography, the sample load that can be injected using the sample solvent evaporation technique is limited by the volume of the loading precolumn. A multiple sample loading procedure is investigated in order to increase the injectable amount and to optimize injection conditions. This work also deals with optimization of solvent removal conditions.     

AT-column, a novel concentrating technique for large-volume injections in gas chromatography

Nowadays, large-volume injection is widely used for the GC determination of trace analytes, specifically to improve detectability. The most popular injectors for large-volume injections are the programmable temperature vaporisation (PTV) injector and the cold on-column (COC) injector, where each device has its own advantages and limitations. The novel AT-column concentrating technique combines features of two other injection techniques, loop-type large-volume and vapour overflow. AT-column injection is based on solvent evaporation in an empty liner with solvent vapour discharge via the split line. Little or no optimisation is required. The only relevant parameter is the injection temperature which can easily be calculated using the equation of Antoine. As an application, AT-column injection is combined with GC-MS for the trace-level determination of labile analytes and with GC-flame ionisation detection for the analysis of high molecular weight polymer additives. In summary, AT-column is an injection technique that combines the inertness of the COC, and the flexibility and robustness of the PTV large-volume technique.     

Pressurised hot water extraction coupled on-line with liquid chromatography-gas chromatography for the determination of brominated flame retardants in sediment samples

Pressurised hot water extraction (PHWE) was coupled on-line with liquid chromatography-gas chromatography (LC-GC) to determine brominated flame retardants in sediment samples. After extraction with pressurised hot water the analytes were adsorbed in a solid-phase trap. The trap was dried with nitrogen and the analytes were eluted to the LC column, where the extract was cleaned, concentrated and fractionated before transfer to the GC system. The fraction containing the brominated flame retardants was transferred to the GC system via an on-column interface. The PHWE-LC-GC method was linear from 0.0125 to 2.5 microg with limits of detection in the range 0.70-1.41 ng/g and limits of quantification 6.16-12.33 ng/g.     

Concurrent solvent recondensation large sample volume splitless injection

Concurrent Solvent Recondensation Large Sample Volume (CRS‐LV) splitless injection overcomes the limitation of the maximum sample volume to 1–2 μL valid for classical splitless injection. It is based on control of the evaporation rate in the vaporizing chamber, utilization of a strong pressure increase in the injector resulting from solvent evaporation, and greatly accelerated transfer of the sample vapors from the injector into the inlet of an uncoated precolumn by recondensation of the solvent. The sample vapors are transferred into the column as rapidly as they are formed in the injector (concurrent transfer). 20–50 μL of liquid sample is injected with liquid band formation. The sample liquid is received by a small packing of deactivated glass wool positioned slightly above the column entrance at the bottom of the vaporizing chamber. Solvent evaporation strongly increases the pressure in the injector (auto pressure surge), provided the septum purge outlet is closed and the accessible volumes around the vaporizing chamber are small, driving the first vapors into the precolumn. Transfer continues to be fast because of recondensation of the solvent, obtained by keeping the oven temperature below the pressure‐corrected solvent boiling point. The uncoated precolumn must have sufficient capacity to retain most of the sample as a liquid. The experimental data show virtually complete absence of discrimination of volatile or high boiling components as well as high reproducibility.