Interphase device for the direct coupling of liquid...

Liquid purification or separation – With means to add treating material – Chromatography

Reexamination Certificate

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C210S656000, C210S659000, C096S101000, C096S103000, C096S104000, C422S070000

Reexamination Certificate

active

06402947

ABSTRACT:

PRIOR ART
The use of a direct coupling of high efficiency liquid chromatography and gas chromatography is very useful for analyzing complex mixtures. The advantages in using this multidimensional system basically centre on the possibility of combining the liquid chromatography potential as a sample preparation technique with that of gas chromatography in relation to the system's efficiency (Grob. K.,
On
-
Line Coupled LC
-
GC.,
Hüthig, Heidelberg, Germany, 1991; Mondello, L.,; Dugo, G.; Bartle, K. D.,
J. Microcol,
Sep., 1996, 8, 275-310). In this way, it is possible to avail of analysis methods which do not require the use of conventional sample preparation procedures which, apart from being laborious and unreliable, have the great disadvantage of calling for the use of relatively high volumes of polluting organic solvents.
A particularly problematic aspect in connection with the use of direct coupling of liquid chromatography and gas chromatography relates to the characteristics of the interface necessary to make this coupling possible. This is an aspect which displays the difficulty in making two essentially different systems, where the operating parameters are substantially different, compatible. The interfaces initially developed only allowed the use of a normal phase in the pre-separation performed by liquids since, in this case, the small volumes of vaporization produced during the transfer do not lead to any additional difficulties. This is why different interfaces (“autosampler”, “on-column”, “loop type”) enabling direct coupling to be carried out between liquid chromatography in the normal phase and gas chromatography (Grob, K. J.
Chromatogr. A
1995, 703, 265-76; Vreuls, J. J.,; de Jong, G. J.; Ghijsen, R. T.; Brinkman, U. A. Th.
J. AOAC. Int
1994, 77, 306-27) have been designed and used.
However, it is necessary in many cases to turn up to the use of the reverse phase in the liquid chromatography stage in order to achieve a certain separation and, consequently, the extension of the field of applicability of direct liquid chromatography and gas chromatography coupling requires the development of suitable interfaces for carrying out direct coupling between liquid chromatography in a reverse phase and gas chromatography (Se{overscore (n)}oráns, F. J.,; Villén, J; Tabera, J.; Herraiz, M. J.
Agric. Food Chem.
1998, 46, 1022-27. Villén, J; Blanch, G. P.; Ruiz del Castillo, M. L.; Herraiz, M.
J. Agric. Food Chem.
1998, 46, 1027-31. With this aim in mind, several systems have been proposed over the last few years (“retention gap”, “concurrent solvent evaporation”, “open tubular trap”, etc.) (Grob, K.
J. Chromatogr. A
1995, 703, 265-76; Vreuls, J. J.; de Jong, G. J.; Ghijsen, R. T.; Brinkman, U. A. Th.,
J. AOAC. Int.
1994, 77, 306-27) although the limitations involved in using polar eluents (fundamentally the high volumes of vaporization produced during transfer and the difficulty of suitably focussing the chromatographic band) have prevented the development of interfaces meeting the required conditions as regards simplicity, reliability, versatility and possibility of automation.
Interfaces described up to date in literature have limitations in some of the following aspects: The liquid chromatography fraction's volume that can be transferred, the flow rate at which this fraction can be transferred, the impossibility of making the transfer when liquid chromatography is performed in the reverse phase, or the system cannot be automated.
The liquid chromatography fraction volume normally transferred with the systems described is about 500 microliters and rarely exceeds one milliliter and then only by a little. This makes it necessary to transfer only a part of the liquid chromatography fraction of interest or leads to working with very small diameter columns in liquid chromatography (liquid microchromatography), which means experimental difficulties and a loss of sensitivity. Some interface allowing much greater volumes to be transferred has been described, but it cannot be automated. In the device which is the subject of the invention, the volume of the fraction transferred is unlimited. Up to 100 ml have been inserted, a much higher volume than any liquid chromatography fraction it is desired to transfer with analytical ends, but this volume can be far exceeded when required.
The flow rate at which the transfer is performed is limited by the solvent evaporation rate in most of the systems described. This also makes it necessary to work in liquid microchromatography or to use an interface having a system which collects the liquid chromatography fraction and transfers it to gas chromatography at a lower flow rate at which it elutes from liquid chromatography, and this increases the complexity of the interface. Some interface has been described which allows transfer to be made at a much greater flow rate than the rest, but it cannot be automated. In the device which is the subject of the invention, the flow rate at which the liquid chromatography fraction is transferred is far higher than any of the systems described which may be automated. Transfers have been made at 5 milliliters per minute, a far higher rate than that necessary to transfer a liquid chromatography fraction with analytical ends, but this rate may be far exceeded if required.
Most of the interfaces described only allow the transfer to be made when liquid chromatography is performed in a normal phase. This means a limitation in itself, which is substantial if it is borne in mind that the immense majority of liquid chromatography separations have been carried out in the reverse phase. The interfaces described which allow liquid chromatography fractions to be transferred in the reverse phase make a change in solvent during the transfer or cannot be automated or have the limitations as discussed earlier. In the device which is the subject of this invention, liquid chromatography can be performed both in the normal and in reverse phases. It also allows very high volumes of aqueous solution to be transferred at a high flow rate.
The difficulty in achieving direct coupling of liquid chromatography and gas chromatography lies in the fact that the maximum volume of sample that can be inserted into a gas chromatography capillary column is in the order of one microliter, whilst a liquid chromatography fraction of interest normally has a volume between about 100 microliters and a few milliliters. Therefore, the interface employed for this direct coupling has to evaporate the solvent to the volume admissible in gas chromatography, retain the solutes of interest and transfer them to the gas chromatography column occupying a narrow band of the column, so that chromatographic separation is effective.
An increase of analysis sensitivity is achieved by introducing high volumes of sample or extract in gas chromatography. Due to this increase in sensitivity, the extraction and concentration processes prior to the gas chromatography analysis may be replaced in many cases by introducing high volumes of sample. The difficulty with this technique is basically the same as that of liquid chromatography and gas chromatography coupling, i.e. to evaporate the solvent from the high volume inserted until the solution has a volume lower than the maximum admissible by the gas chromatography capillary column, retain the solutes and move them into the gas chromatography column.
Therefore, if there is no specific instrumental difficulty, any device suited for using as an interface for direct liquid chromatography and gas chromatography coupling is also suited to the introduction of high volumes of sample in gas chromatography, and vice-versa.
DESCRIPTION OF THE INVENTION
The device according to the present invention is built on the basis of the scheme of a PTV (programmed temperature vaporizer) injector in which the system for inserting the sample, the hydraulic gas system and the operating mode have been modified. Thus, the present invention provides an interface device for direct coupling of liquid chromatography and ga

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