Background of the Invention Combinatorial chemistry is a technique whereby very many different chemical compounds are produced by multiple chemical reactions. A library of chemical compounds may be formed on solid phase supports. The solid phase supports are commonly known as beads. Beads of the size approximately a tenth of a millimetre in diameter are typically used in such reactions. Molecules can be attached to the beads by way of chemical hooks.

To form a library of chemical compounds, it is usual to start with a large number of beads. In order to illustrate a combinatorial process, let us assume that the beads are divided into 3 groups. A different reagent (A, B and C) may then be added to each group of beads. There are now 3 types of molecules attached to the beads: beads in group 1 have a molecule of compound A attached to them, beads in group 2 have a molecule of compound B, and beads in group 3 a molecule of compound C. Next, the 3 groups of beads are pooled, mixed up, and again split up into 3 groups. Three more separate reactions are then carried out. This results in a combination of 3 reactions in the first stage of the process, and 3 reactions in the second stage, producing 9 different species of molecule. If the groups of beads are again pooled, mixed up, and split up into 3 further groups, 27 different compounds are generated.

The library of compounds created by the above steps is known as a 3 x 3 x 3 library.

Nine individual (3 + 3 + 3) reactions have been carried out, and 27 (3 x 3 x 3) different compounds have been generated. Other sizes of library may be created by varying the number of reaction stages, and by varying the number of groups of beads. Libraries containing many thousands of compounds, such as (50 x 50 x 50) or (20 x 20 x 20 x 20) are envisaged.

It is desirable to label each chemical compound of such a library. Until recently this has been a very difficult task; typically additional sophisticated chemical processing stages have been undertaken to add a"chemical tag"to the bead. Such chemical tags have proved difficult to identify and, moreover, restrict the chemical processes which may be employed in the formation of desired compounds.

International Patent Application No. WO-A3-9712680 (IRORI) discloses methods of data storage and retrieval technology for use in molecular tracking and identification.

Products for such tracking and identification are also described, along with assays, diagnostics and screening protocols that use the products.

UK Patent Application GB-A-2 306 484 discloses a method of labelling the beads by attaching a bar code to each bead to identify the particular reactions that the bead has undergone. A chemical compound of interest may be identified by reading the bar code attached to a bead. However, the disadvantage of this method is that each bead must be labelled at each stage of the combinatorial process. This may be a time consuming, and therefore expensive, task.

Another method for forming libraries of compounds is to retain groups of beads in containers having porous walls. Chemical reagents are able to permeate the walls of the container and interact with the beads. Each container bears a unique tag, which may be used to identify the container and the reactions in which the container has taken part. Such containers are commonly known as micro-reactors. An advantage of using a micro-reactor in multiple chemical reactions is that large numbers of beads may be more conveniently processed by enclosing them in the micro-reactor.

One such type of container is currently sold under the Trade Mark MICROKANS. MICROKANS''M are well known in combinatorial chemistry as a means of containing sets of beads as they pass through various stages of a combinatorial chemical process.

MICROKANTM micro-reactors are designed to be loaded with solid phase resin beads.

Chemical synthesis takes place in the micro-reactor by allowing reagents to flow through its outer mesh walls. By combining and dividing micro-reactors (rather than

individual solid phase resin beads) by a process known as"directed sorting", one discrete compound is synthesized in each reactor.

A miniature radio-frequency (RF) tag is contained within each micro-reactor. The RF tag is a unique label that is used to identify the micro-reactor during sorting processes that occur between chemical synthesis steps. The RF tag is encased in glass, and provides a unique ID for each micro-reactor and hence each compound. This unique ID allows each micro-reactor to be identified during the combinatorial"directed sorting" process.

However, one disadvantage of NUCROKANTM micro-reactors is that they expensive.

This is limiting when synthesising libraries containing many thousands of compounds.

Another disadvantage is that the beads used within a MICROKANTM micro-reactor are not individually identified, or labelled. Hence, there is no easy way of sub-dividing a library of such beads and maintaining knowledge of the chemical compound attached to each individual bead.

An aim of the present invention is to provide a method of synthesizing a number of different chemical compounds which may easily be identified.

Summary of the Invention According to the present invention there is provided a method of synthesizing a plurality of products using a process, the method including the steps of : 1) providing a plurality of articles on which the product is to be formed; 2) dividing the articles into a plurality of groups of articles and introducing each group of articles into respective micro- reactors; 3) subjecting the articles therein to a first synthesis step of the process; and 4) subjecting said articles to at least a second synthesis step of the process, characterised in that at least some of the articles are removed from at least one micro-reactor at the penultimate synthesis step and are labelled with an identifying code indicating the synthesis history which the articles in the micro-reactor have undergone.

The articles may be returned to the same or a different micro-reactor after they have been labelled, and the micro-reactor may be subjected to a final synthesis step of the

process. Alternatively, the articles may be retained so that the chemical compounds formed thereon (or attached thereto) may be analysed. The micro-reactors may then take part in a further synthesis step of the process.

The identifying code preferably also indicates the final synthesis step of the process to which the articles are to be subjected.

Preferably the process has at least 3 synthesis steps. For example, in a 3 stage synthesis, some of the articles are removed on completion of the second stage of the process.

These articles are then labelled with an identifying code. The identifying code records the synthesis stages that the articles have undergone in stages 1 and 2, and also the synthesis steps that the articles will undergo in the final stage. Thus, in a 3 stage synthesis, labelling of the articles (or a sub-set of the articles) occurs only once, that is, on completion of the second synthesis stage.

The process may be a combinatorial process. Preferably the micro-reactors are grouped together and then divided into a plurality of groups before each synthesis step of the process, as in a conventional combinatorial process.

Some, or all of the articles may be marked with a code before the first step of the combinatorial process, i. e., before they have been introduced into the micro-reactor (s).

These marked articles may be removed from the micro-reactor (s), and the code read, at the penultimate synthesis step. The articles may be returned to the micro-reactor (s) after this has occurred, or may be reserved for future use.

The micro-reactor may be a container having porous walls. The container may, for example, resemble a tea bag. The container may bear an identifying label, or tag.

Preferably the identifying code is applied to an article using a laser. Thus a"physical" tag is formed on the article. The use of such a tag does not restrict the chemical processes which may be employed in the formation of desired chemical compounds.

Preferably the identifying code includes an alphanumeric code. The alphanumeric code enables both the chemical synthesis steps that an article has undergone while enclosed in

the micro-reactor, and a subsequent synthesis step, to be easily recorded. Thus the chemistry carried on the article may be identified.

According to another aspect of the invention, there is provided an apparatus for labelling an article. The apparatus comprises a laser beam arranged to be directed onto the article so as to label the article, means for isolating an individual article, and means for directing the beam with respect to the surface of the article so as to form a label thereon.

The article may also be known as a bead.

Ideally, beads labelled using this system are standard beads known in the field of combinatorial chemistry. However, non-standard beads or other solid phase supports may be used.

The beads may be formed from a highly cross-linked, lightly derivatised resin. This may be used for the laser marked beads, so that the label is retained throughout all the processing stages. A less cross-linked, highly derivatised resin may be used for the beads which are not labelled during the combinatorial process.

According to a further aspect of the invention there is provided a product which is synthesized according to the aforedescribed method. The product is preferably a chemical compound.

Brief Description of the Drawings An number of embodiments of the invention will now be described, by way of example only, with reference to the accompanying Figures, in which:- Figure 1 shows a diagrammatical cross-section through an embodiment of a micro- reactor; Figure 2 shows a diagrammatical representation of a first stage synthesis of a combinatorial process according to the invention; and Figure 3 shows a diagrammatical representation of a second stage synthesis of a combinatorial process according to the invention.

Description of the Preferred Embodiment Referring to Figure 1, there is shown an embodiment of a micro-reactor. The micro- reactor 24 comprises mesh walls which allow reagents to flow into (and from) micro- reactor 24 in order to interact with the beads 20. Micro-reactor 24 contains a plurality of solid phase resin beads 20. Typically about 30 mg of resin beads 20 may be contained within micro-reactor 24. Micro-reactor 24 bears a tag 22 used to identify the chemistries of the beads 20 contained therein. Tag 22 may be a hand-written label, a bar code, an RF tag, or other identifying mark.

Each individual bead from a micro-reactor may be labelled using a laser system. The label creates a record of the synthesis that the bead has undergone, and the next stage of synthesis to which the bead will be subjected. The bead is then easily identifiable by reading the laser marked label. The label may, for example, be an alphanumeric code, or any other identifier. As the synthesis that the bead has undergone, (and will, in a further synthesis stage, undergo) is recorded on each bead, the library can be sub-divided in order to produce many sub-libraries. The micro-reactor is then available for re-use.

A typical sequence of events for labelling of the beads 20 is now described, by way of example only. Consider a 3 stage synthesis, for example, a 3 x 3 x 3 library of 27 chemical compounds. To form such a library on beads contained in micro-reactor 24 (or any other type of micro-reactor) would require 27 such micro-reactors and would be extremely expensive. In this method, the first two stages of synthesis are carried out using 9 micro-reactors, producing a set of 3 x 3 compounds carried on the beads. Stage 1 of the synthesis is illustrated by Figure 2, and stage 2 by Figure 3.

Figure 2 shows reaction vessels 30,32 and 34, each containing three micro-reactors.

Reaction vessel 30 contains micro-reactors 40,41, and 42 in contact with reagent A.

Reaction vessel 32 contains micro-reactors 43,44, and 45 in contact with reagent B.

Reaction vessel 34 contains micro-reactors 46,47, and 48 in contact with reagent C.

Prior to the second synthesis stage, the micro-reactors are pooled, and split up, as shown in Figure 3. Now, reaction vessel 30 contains micro-reactors 40,43 and 46 in contact with reagent A, reaction vessel 32 contains micro-reactors 41,44 and 47 in contact with

reagent B, and reaction vessel 34 contains micro-reactors 42,45 and 48 in contact with reagent C.

The beads contained in the micro-reactors have the chemistries shown in the Table below after two stages of synthesis.

Micro-reactor Chemistry <BR> <BR> <BR> <BR> <BR> <BR> 40 1A 2A<BR> <BR> <BR> <BR> <BR> <BR> <BR> 41 lA2B<BR> <BR> <BR> <BR> <BR> <BR> <BR> 42 1A 2C<BR> <BR> <BR> <BR> <BR> <BR> <BR> 43 1B 2A<BR> <BR> <BR> <BR> <BR> <BR> <BR> 44 1B 2B<BR> <BR> <BR> <BR> <BR> <BR> <BR> 45 1B 2C<BR> <BR> <BR> <BR> <BR> <BR> <BR> 46 1C2A<BR> <BR> <BR> <BR> <BR> <BR> <BR> 47 1C2B 48 1c2c Prior to the third synthesis stage, a number of the beads are extracted from each of the 9 micro-reactors and labelled with a laser (not shown). The laser marking indicates the processing that has been carried out in the micro-reactor, and the processing that is to be carried out in the next stage of the synthesis. For example, a bead from micro-reactor 40 <BR> <BR> <BR> may be marked 1 A 2, 38. This label indicates that the bead has undergone synthesis type A at the first stage, type A at the second stage (both while contained in micro- reactor 40), and is to undergo synthesis type B at the final stage-for example in a <BR> <BR> <BR> <BR> micro-reactor together with other beads such as 1 c 2 B 3 B. This step is performed for all of the beads taking part in the third stage synthesis. Screening for compounds of interest attached to each bead then follows. Variation may be made to the aforementioned embodiment without departing from the scope of the invention. For example, MicroTuberM micro-reactors may be employed.