WO2005119245A1

WO2005119245A1 - Hybridisation of analytical pervaporation and dynamic headspace techniques

Info

Publication number: WO2005119245A1
Application number: PCT/ES2005/000309
Authority: WO
Inventors: José Luis GOMEZ ARIZA; Tamara Garcia Barrera; Fernando Lorenzo Garcia
Original assignee: Universidad De Huelva
Priority date: 2004-06-01
Filing date: 2005-05-31
Publication date: 2005-12-15
Also published as: ES2245236A1; ES2245236B1

Abstract

The invention relates to an analytical process for the separation of volatile species, which can be used with liquid samples, solid samples or a mixture of both. The invention is characterised in that it comprises the hybridisation of the processes involved in analytical pervaporation and dynamic headspace techniques, as well as the design of a basic module for performing said analytical separation.

Description

HYBRIDIZATION OF DYNAMIC HEAD SPACE TECHNIQUES AND ANALYTICAL PEREVAPORATION.

FIELD OF THE TECHNIQUE

The present invention relates to the field of art of analytical methods of analyte separation and detection.

STATE OF THE PREVIOUS TECHNIQUE

The head space is an analytical technique widely used for the separation of analytes from solid or liquid samples, prior to analysis by gas chromatography. It is based on the heating of a liquid or a solid in a tightly closed and partially filled container, in this way the so-called head space originates. The volatile analyte (s) is (or are) released from the matrix and pass into the gas phase, establishing a balance between both phases. A part of this gas phase is extracted with a syringe and injected into the chromatographic system. Used in this way the technique is called "static head space". If the mixture of volatile compounds that originate are carried with a carrier gas, we have a second variant called "dynamic head space", which implies better volatilization. In this second case, the entrainment gas with the analytes must be passed through an adsorbent solid (Chromosorb, Tenax, and others), for its fixation and subsequent elution suddenly, before introducing it into the gas chromatograph.

Purge-and-entrapment is also a separation technique that is used for liquid samples. Use a container for heat treatment of the sample, which is connected to a refrigerant where water vapor condenses and drips on the problem solution. The refrigerant has a mini-column attached where the analyte (s) is (or is) retained. Analytical pervaporation is a special case of volatile substance separation. It is carried out in a tightly closed container, which is constituted by two chambers and a membrane located between them, which is the one that exerts the discriminating effect on the matrix. The liquid or solid sample is introduced into the lower chamber and subjected to heating until a balance is reached between the two phases. The analyte (s) are separated by difference in vapor pressure and by the selectivity introduced by the membrane. Finally, a carrier gas drags the analyte (s) that have passed through the membrane to the gas chromatograph. This technique has also been coupled to liquid chromatography. This process allows the instrumental coupling of a minicolumn for the retention of the analyte (s). Its use is very scarce in the analytical field. Perevaporation consists in the use of a hygroscopic membrane formed by two concentric tubes, the innermost one made of a hygroscopic material, preferably said material is a copolymer that is composed of tetrafluoroethylene and 3,6-perfluoro-4-dioxa- acid 7-methyl-octene-sulfonic, in a particular embodiment said material is Nafion®. It is through this inner tube that the wet gas mixture containing the analytes flows. Between the walls of both tubes flows a drying gas that can be nitrogen, air, argon or others, which drags (perevapora) the matrix towards the outside of the mambrana. The co-polymer of tetrafluoroethylene (Teflon) and 3,6-perfluoro-4-dioxa-7-methyl-octene-sulfonic acid, is very resistant to chemical attack and the presence of its free sulfonic groups gives it a hygroscopic character. This absorption process is a kinetic reaction. First order so the balance is fast. These membranes are designed for drying gases or as anion exchange resins in liquid phases.

DETAILED DESCRIPTION OF THE INVENTION

The problem to be addressed by the present invention is to provide an analytical process of separation of volatile species, in liquid or solid samples, which achieves greater efficiency and sensitivity with respect to head space, analytical pervaporation and purge and entrapment techniques. The solution proposed by the present invention is based on the fact that the present inventors have identified that by means of the suitable coupling of the modules of analytical separation of dynamic head space and perevaporation, it is possible to enhance the numerous positive aspects inherent to each of these techniques and minimize the inconvenience and limitations of both.

In accordance with a first aspect of the present invention, an analytical process of separation of volatile species for application to liquid, solid samples or mixture of a solid with a liquid is provided, by hybridizing the processes of the headspace techniques dynamic and analytical perevaporation as an alternative to other analytical techniques of the same purpose. According to a preferred embodiment of the present invention, the analytical process of separation of volatile species for application to liquid, solid samples or mixing of a solid with a liquid, by hybridizing the processes of dynamic head space and perevaporation techniques analytical, includes the stages of: a. establishment of the head space of a liquid or solid sample by heating it; b. drag with a stream of gas carrying the analytes contained in the head space; c. introduction of the gas phase into a perevaporation membrane. d. elimination of the matrix of the gas stream containing the analytes of interest, e. fixation of the analytes in an adsorbent material placed inside the gas chromatograph, or other preconcentration systems, at the exit of the membrane, before its introduction into the chromatographic column. The completion of this last stage depends on the need for sensitivity analysis.

According to another even more preferred embodiment of the present invention, the analytical separation process is carried out by preconcentrating the analytes in the headspace starting from the introduction of a large volume of liquid sample that recirculates through the lower part of the camera. In accordance with a second aspect of the present invention, a basic module is provided for the analytical process of separation of volatile species for application to liquid, solid samples or mixing of a solid with a liquid, by hybridizing the processes of the techniques of dynamic head space and analytical perevaporation.

According to a preferred embodiment of the present invention, means for favoring the recirculation of the liquid sample in said chamber is coupled to the lower chamber of the dynamic head space module.

According to another even more preferred embodiment, said means for favoring the recirculation of the liquid sample is a pump driving peristaltic of the liquid sample thus automating the sample introduction process. In this way we can preconcentrate the analyte (s) in the headspace starting from the introduction of a large volume of liquid sample that recirculates through the bottom of the chamber. In this case we will obtain a high sensitivity that is difficult to match that achieved by other analytical extraction techniques.

According to another preferred embodiment of the present invention, stirring means are used in the lower chamber of the dynamic head space module to favor the extraction of the analytes from the sample, being the preferred means, although it should not be limited thereto, a magnetic stirrer According to a third aspect of the present invention, this refers to the use of the basic module described above in the analytical process of separation of volatile species for application to liquid, solid samples or mixing of a solid with a liquid. By hybridizing the processes of the dynamic head space and perevaporation technique, as described in the present invention, the following advantages are achieved with respect to analytical pervaporation: "The perevaporation process that takes place in the art The proposal is applied in bulk to the matrix of the sample and not to the analyte (s) in question, as is the case of analytical pervaporation, which allows a much more efficient separation that results in increased sensitivity of the analysis since the interferences from the sample are eliminated in this way, and since in the samples there is more quantity of matrix than of analyte (s), it is easier to eliminate that which is in greater proportion. Pervaporation is based on the opposite process, limiting the passage of the matrix and favoring that of the analyte (s) that is sometimes found at the trace level. Perevaporation, a process that takes place in the proposed technique, is therefore based on the conjunction of the use of a membrane of an efficient material in the elimination of the matrix of the sample (such as Nafion) and an adequate configuration, in concentric ducts which incorporates a surrounding current to favor the passage of the matrix outwards. Thus, perevaporation has a great advantage, the elimination of the influence of the matrix of the sample, with a consequent increase in sensitivity, and with it, the possibility of coupling chromatographs with apolar columns or with mass detectors, over which the introduction of water or polar chemical species has a very negative effect. This effect is one of the problems commonly encountered when using pervaporation or dynamic head space.

• In the proposed technique, the perevaporation membrane is far from the heat source which prevents its deterioration.

"With the new technique we avoid deterioration of the perevaporation membrane, by contact with the solid or liquid sample. Only the gaseous phase comes into contact with it. The dynamic nature of the carrier gas also contributes to lengthen the life of the membrane avoiding its saturation

• The cylindrical configuration of the perevaporation membrane, and the dynamic nature of the drying gas increases the effectiveness of the separation process. "Considerable increase in sensitivity since the perevaporation process may be lengthened until a maximum concentration of the analyte (s) is obtained to introduce into the instrument since no saturation of the membrane takes place, as occurs in the systems of pervaporation. ""

The advantages with respect to dynamic head space are the following: "It eliminates water at the entrance of the chromatographic system, which constitutes a serious problem for the coupling of this analytical technique to gas chromatography systems, especially chromatographs of Mass gases increasingly used in laboratories and essential for many analyzes.

It eliminates numerous components of the gas mixture from the head space, which suppose an interference in the analytical measurement. In short, it virtually eliminates the entire matrix.

• The preconcentration of the analyte (s) in the injector of the chromatographic system itself implies a decrease in the analysis time, as well as a greater reproducibility with respect to traditional systems which are based on the use of minicolumns filled with the adsorbent material suitable.

The advantages with respect to the purge-and-entrapment of the proposed technique are: • Simplification of the equipment by replacing the refrigerant with the hygroscopic membrane. "Automation of the introduction of the sample into the module by coupling a flow system.

The process object of the invention consists in the partial evaporation of a liquid sample, or heating in the case of a solid, in order to reach a balance between the phase in which the sample is and the space that exists on it. After a time, a stream of carrier gas is passed that will drag the analyte (s) contained in the head space of this container and in turn, will favor the establishment of successive balances that will allow the extraction of a greater amount of them. In a second stage of this analytical process, the gas phase obtained is introduced into a perevaporation membrane in order to remove the matrix (mainly water and polar organic components if Nafion is used). This process takes place simultaneously with the dynamic head space and the concentration of the analyte (s) in the gas chromatograph injector for its total introduction of the analyte (s) extracted from of the sample. Finally the chromatographic analysis takes place.

The unit (EVAPeREV) to carry out the proposed analytical technique consists of the following parts: a) A compartment manufactured in Teflon and consisting of two halves that are screwed in until a complete tightness is provided. The upper half has an inlet and an outlet to allow the flow of carrier gas that will transport the analyte (s) to the perevaporation membrane. The lower half is provided with two injection ports. One of them to introduce the liquid sample through a hypodermic syringe and the other to allow the recirculation of the liquid sample through the instrumental coupling of a pump peristaltic Solid samples are weighed inside the chamber. This compartment is heated to induce the separation of volatile species. b) A high pressure valve that allows the chamber to enter the membrane or not. c) A heat source consisting of a heating plate, microwave oven, or any other source that involves or does not modify the basic module. d) A magnetic stirrer inside the chamber allows the agitation of the liquid sample or mixture of solid sample with a liquid, which will make the extraction process of the analyte (s) more effective. e) A perevaporation membrane, located at the exit of the upper chamber described in section (a).

To facilitate a better understanding of the characteristics of the invention and as an integral part thereof, a set of figures is attached, where the following is merely illustrative and not limited:

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1.- Instrumental coupling of the dynamic head space module to the hygroscopic perevaporation membrane and the adsorbent material present in the gas chromatograph injector.

Figure 2.- Coupling of the proposed process with flow system.

The present invention is illustrated by way of practical example of embodiment by means of the detailed description of the aforementioned figures. EXAMPLES

With respect to figure 1, we can see the EVAPeREV unit in which the dynamic head space module (A), formed by a lower chamber where the sample is introduced and an upper one where the acceptor gas stream is incorporated. The introduction of the sample can be: (a) directly in the case of solid samples; In this case, the two injection ports (1) are closed tightly, (b) through one of the injection ports (1) in the case of liquid samples. In the lower chamber a magnetic stirrer (2) is introduced which favors, together with the temperature applied by introducing the unit in a hot water bath (3) or by using other heating sources, the fugitiveness of the analytes . Once the balance between the phase in which the sample and the gas is located has been created, the position of a high pressure valve (4) connected to the upper chamber where the head space is created is changed with what the passage of the analytes to the hygroscopic membrane (5), belonging to the perevaporation unit (B), through which the drying gas (6) is circulated through an outer tube and in the opposite direction to the passage of the sample is allowed . Subsequently, the analytes are transported to a gas chromatography system (C) through the injector, which contains an adsorbent material (7) suitable for the analytical case in question. Finally, the analytes are separated in a chromatographic column (8) and detected in the corresponding detector (D).

In figure 2, the possibility of connecting the module (A) to a peristaltic pump is shown in order to recirculate the liquid sample through the lower chamber. The number (1) corresponds to the vial where the sample is introduced which is connected by a tube system to the two injection ports of the lower chamber. Use is made of a peristaltic pump (2) with two channels (3) and (4) in order to boost the sample allowing a recirculation of it. The hot water bath where the unit is inserted is referenced with the number (5). The number (6) corresponds to the high pressure valve, the (7) to the hygroscopic membrane and the (8) to the adsorbent solid.

Claims

1. An analytical process of separation of volatile species for application to liquid, solid or mixed samples of a solid with a liquid, characterized by the hybridization of the processes of dynamic head space and analytical perevaporation techniques.

2. The process according to the preceding claim, characterized in that it comprises the steps of: a. establishment of the head space of a liquid or solid sample by heating it; b. drag with a stream of gas carrying the analytes contained in the head space; c. introduction of the gas phase into a perevaporation membrane; d. elimination of the matrix of the gas stream containing the analytes of interest; and. fixation of the analytes in an adsorbent material placed inside the gas chromatograph, or other preconcentration systems, at the exit of the membrane and before its introduction into the chromatographic column. The completion of this last stage depends on the need for sensitivity analysis.

The process according to any of the preceding claims 1 to 2, characterized in that a preconcentration of the analytes is carried out in the head space starting from of the introduction of a large volume of liquid sample that recirculates through the lower part of the chamber.

4. Basic module for the analytical process of separation of volatile species for application to liquid, solid or mixed samples of a solid with a liquid, characterized in that at the exit of the module used in the dynamic head space technique a membrane of analytical perevaporation.

5. The module according to claim 4, characterized in that said module has coupled to the lower chamber of the dynamic head space module means to favor the recirculation of the liquid sample in said chamber.

6. The module according to claim 5, characterized in that said means for favoring the recirculation of the liquid sample is a peristaltic pump.

7. The module according to any of the preceding claims 4 to 6, characterized in that agitation means are used in the lower chamber of the dynamic head space module to favor the extraction of the analytes from the sample.

8. Use of the module according to any of the preceding claims 4 to 7, in the analytical process of separation of volatile species for application to liquid, solid samples or mixing of a solid with a liquid.