The modulator according to the present invention can be used in a traditional gas chromatographic apparatus to improve its sensitivity, by reducing the peak widening, when it is disposed directly upstream the detector, or to focalize the injected analytes when it is placed immediately downstream the injector. However it is especially intended for two-dimensional gas chromatography, the so called comprehensive 2D GC.

Comprehensive two-dimensional gas chromatography represents a gas chromatographic technique in which the sample is initially separated in a first conventional high resolution GC capillary column, of normal internal diameter, according to the usual temperature programming method. All the effluent of this first column is then focalized in a large number of extremely close, adjacent fractions, (<100ms), at regular short intervals, which are afterwards injected into a second capillary column, which is short and of reduced internal diameter to allow very rapid separations.

PRIOR ART TECHNIQUE Two-dimensional gas chromatography can be interpreted as the in- series arrangement of two gas chromatographic systems by means of a so-called modulation system (fig. 1). The first is a conventional GC system including an injector, while the second is a GC system that is usually approximately 50 times faster than the first one. This is achieved by using a short column of small internal diameter to obtain very close peaks, with widths at the base of 100-200ms. The modulation system

allows the injection of correspondingly close pulses to be obtained so that there is no sample loss during transfer between the chromatographic dimensions. In this way, the technique of two- dimensional gas chromatography allows the attainment of a considerably greater separation power than that of conventional capillary gas chromatography, together with improved sensitivity, with better identification of the peaks and other advantageous characteristics. (Z. Y. Liu. JB. Philips, J. Chirom. Sci. , 1991,29, 227-231; USP 5. 196. 039).

As previously mentioned, to carry out two-dimensional gas chromatography it is necessary to activate a so-called modulation system between the first and the second capillary column to hold and focalize close effluent fractions of the first column and inject the same at intervals into the second column. The most widely used modulators are of thermic type, which make use of a thermic effect on a column section to trap and release the fractions to be injected into the second column. This thermic effect was initially based on local heating, but for various reasons, which there is no need to examine here, so-called cryogenic modulators or cooled modulators, in which the thermic effect is obviously based on cooling, were more recently used.

For example these modulators consist in a cold trap which moves sequentially in the two directions along the initial part of the second capillary column, cryogenically trapping and focalizing part of the analytes, on the first section of the second column, when these elute from the first column. (R. M. Kinghom, P. J. Marriott, J. High Resolut.

Chromatogr., 1998,21, 620-622). When the cryogenic system moves away from the zone in which the analytes were trapped, the air surrounding the GC oven rapidly heats the trapped analytes and

moves them, carrying out their re-injection into the remaining part of the second column. The main drawback of this system is caused by very frequent breakages of the fused silica section of the capillary column where the cold trap moves, due to the formation of ice, between the cold trap and the column.

In order to eliminate these disadvantages, the use of two cooling gas jets (liquid C02) has been proposed (PCT/1301/02253), said jets being placed at two points spaced out along the column section intended for modulation and being control led so to obtain an alternate emission of the jets to cool such points of the column and as a consequence hold and focalize the analytes. Then said points are heated by the oven environment during interruption of the jet, so as to release the fractions held beforehand. This system eliminates the problems of column breakage, since there are no moving parts and the jet do not create permanent formations of ice.

SUMMARY OF THE INVENTION A new modulator for gas chromatography is now proposed according to the present invention, specially for comprehensive two-dimensional gas chromatography, which, though having moving parts, nevertheless allows the elimination of problems concerning column breakage and, achieves a correct modulation without the need to use precise timings of the control valves of jet delivery.

The main characteristic and further characteristics of the modulator according to the present invention are mentioned in claim 1 and respectively in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be now described with reference to an embodiment of the same reported, only by way of an example, in the enclosed figures, wherein:

Figure 1 is a diagram of a two-dimensional gas chromatography system, Figure 2 is a schematic drawing of a modulator according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to the drawings, figure 1 outlines the components of a known two-dimensional gas chromatographic system preferably housed in a single oven 1.

The system includes a first section of"normal"gas chromatography 2 and a second section, downstream the first one in the direction of elution, of"fast"gas chromatography 3. The"normal"section 2 includes an injector 4 and a first capillary column 5, while the"fast" section 3, in series as regards the previous one, includes a modulator 6, a second column 7 and a detector 8. The modulator is disposed at an initial section of the second column 7 to execute there in the modulation of the solutes exiting from the first column 5, as described previously.

In figure 2 is shown an example of a modulator according to the invention, housed inside the oven 1. The modulator operates on a section 9 of column, corresponding to an initial section of the second column 7, and essentially comprises a spray nozzle 10 able to deliver a jet 11 of cryogenic gas, for example obtained from a source of liquid C02, which feeds the nozzle 10 through a valve 13 and a flexible tube 14.

According to the invention, the nozzle 10 is provided with controlled speed movement along the column section 9 in the same direction of elution X, maintaining constant the distance between the source of the jet 11 and the column section 9, a distance which in any event will avoid any problems due to the formation of ice between the nozzle

and the column.

To obtain such relative movement between the nozzle 10 and the column section 9, various solutions can be provided, preferably centered on the fact of moving the nozzle 10.

In figure 2, the body of the nozzle 10 is for example assembled sliding in the two directions (arrow Y) along a track 15 installed inside the oven 1 and moved in the direction of elution X under the effect of a servo control 16 for example connected to the body of the nozzle 10 by means of a Bowden cable 17 or other mechanical connection. The servo control 16 pulls the nozzle 10 until the end the track 15 at a controlled speed, to carry out said modulation, after which the nozzle 10 is sent back to its initial position for example by the action of a spring 18 connected to the body of the nozzle 10 and to a fixed part 19 inside the oven. The recovery speed in the direction opposite to X can be controlled too and in case is greater than the controlled speed in the X direction.

Other solutions are possible, both to return the nozzle 10 to the start of the track 15, as well as to support and actuate movements of the nozzle 10, which can act indifferently on a horizontal, vertical or variously inclined column section 9.

Assembly of the nozzle 10 can also be provided on an arm pivoted to a fixed point inside the oven and actuated to move in the two directions or in one direction along an arc parallel to the column section 9, always under the effect of a suitable servo control.

The delivery of C02 to the nozzle 10 can be kept unchanged basically for the entire duration of an analysis, keeping the corresponding valve 13 open. Alternatively, it will also be possible to interrupt the delivery of gas during the return movement of the nozzle 10, but in this case arrangements must be made for a valve that is timed or interlocked to the movement of the nozzle 10.