Background It is difficult to selectively detect small amounts of a gaseous analyte in air, and it is particularly important to do so when the location of dangerous chemicals is to be determined.

It is desirable to have a method for the selective detection of small amounts of an analyte in air for many applications, e. g. in the process industry for the detection of specific contamination or leakage, in the airport safety control, customs and police work for the detection of narcotics and explosives, and in clearing the terrain of mines and bombs after a war situation.

One important element in such a method is collecting the small amounts of gaseous analyte from the air surrounding a suspected location and transforming the collected material into a concentrated sample for analysis. This may be achieved by use of a thermostable adsorption/desorption filter, such as an adsoption/desorption filter creating a non-laminar gas flow comprising a winding in a housing of a resistance wire coated with an active material and electric heating of the wire for desorption of the analyte followed by reconditioning of the filter, as disclosed in the US patent 6,192, 766.

It would be desirable to have a thermostable adsorption/desorption filter that is cheap and disposable so that no reconditioning is needed.

Description of the invention The present invention provides a thermostable coated filter disk for adsorption/- desorption of gaseous analytes from air suitable in a method of concentrating small amounts of gaseous analytes from air for further analysis.

The filter disk is primarily intended for a single use, i. e. it is disposable, and thus reconditioning will not be necessary.

The invention is thus directed to a thermostable coated filter disk for adsorption/desorption of gaseous analytes from air, wherein the filter disk is a filter of a thermostable material with a particle retaining porosity prior to coating and a coating on the filter pore walls of a thermostable polymer in an amount reducing the particle retaining porosity and improving the uptake of gaseous analytes.

The thermally stable filter material is e. g. glass fiber, ceramics, metal or high- temperature resistant polymer.

The thermostable polymer coating must be thermally stable so that contaminating impurities desorbed along with the analyte will not disturb the final analysis. Thus, the coating should not decompose upon heating to temperatures used in an adsorption/desorption process and should preferably not form monomers when thermally decomposed. The thermostable polymer should preferably be soluble in a solvent to facilitate the coating process.

In an embodiment the thermostable material is glass fiber and the thermostable polymer is selected from the group consisting of Eudragites, poly ethylene oxide of different molecular weights, poly (ethylene oxide-co-propylene oxide) of different mixtures and polysiloxanes having various structures and crosslink densities, and poly-(2, 6-diphenyl-1, 4- phenylene oxide).

In a preferred embodiment the poly ethylene oxide has a molecular weight of 20 000 and the polysiloxanes are selected from dicyanomethylsilicone and polydimethylsiloxane. The presently most preferred thermostable polymer is polyethylene oxide, MW= 20 000 (Carbowax).

The amount of coating is suitably in the order of 10 % by weight of the final filter disk, such as 5-15 % by weigh, preferably 8-12 % by weight.

The invention is also directed to a method of concentrating small amounts of gaseous analytes from air for further analysis, comprising the steps of directing a stream of air through a filter according to the invention for absorption of analytes from the air stream, transferring the filter to a position where the filter is heated by an external heating source, such as a eatable metal plate or a heating lamp, to a temperature high enough to evaporate the analytes from the filter but low enough to avoid decomposition of the analytes and the filter coating, and collecting the desorbed gaseous analytes by condensation. Suitable desorption temperatures for explosives may be approximately 200°C, and suitable desorption temperatures for narcotics may be approximately 275°C.

Examples of analytes that can be collected from air with, and evaporated from, the filter disks of the invention are explosives and narcotics, and therefore the invention is also directed to the use of a thermostable coated filter disk according to the invention for adsorption and desorption of gaseous analytes, wherein the analytes are selected from the group consisting of explosives, such as trinitrotoluene (TNT), dinitrotoluene (DNT), hexahydro-1, 3,5-trinitro- 1,3, 5-triazine (RDX), octahydro-1, 3,5, 7-tetranitro-1, 3,5, 7-tetrazine (HMX), pentaerythritol

tetranitrate (PETN), and nitroglycerine (NG), and narcotics, such as cocaine, heroine, amphetamine, methamphetamine, cannabinols, tetrahydrocannabinols (THC), and methylenedioxy-N-methylamphetamine (extacy).

The invention will now be further illustrated by description of embodiments and experiments, and drawings showing results from experiments.

Description of drawings Fig. 1 is a staple diagram that shows the GC signal output in Hz after filter extraction corresponds to the amount of 2, 6-DNT vapour collected during 1 minute on uncoated and Carbowax coated filter disks, 10 replicates.

Fig. 2 is a staple diagram that shows the GC signal output in Hz after filter extraction corresponds to the amount of 2, 4-DNT vapour collected during 1 minute on uncoated and Carbowax coated filter disks, 10 replicates.

Fig. 3 is a staple diagram that shows the GC signal output in Hz after filter extraction corresponds to the amount of 2, 4,6-TNT vapour collected during 1 minute on uncoated and Carbowax coated filter disks, 10 replicates.

Description of embodiments and experiments Commercially available glass fiber filters can be used for collection and concentration of particles as well as gaseous substances occurring at low concentrations. Since uncoated glass fiber filters are optimized for collecting particles they are not the best choice for collecting gaseous substances. Coating these filters with different adsorbent materials is here shown to increase the uptake of gaseous drugs and nitroaromatic compounds such as 2,4, 6- TNT, 2,4-DNT and 2,6-DNT from air.

Materials and methods Filters-Glass fiber filters were from Camfil, Sweden with a thickness of 0.3 mm. As delivered from the manufacturer they were precoated with polyacrylate. Before changing to another coating material, the filter sheets were treated at 400°C for 2 hours followed by soxhlet extraction in acetone for 12 hours and then dried at 400°C for 2 hours to remove the polyacrylate coating. The sheets were then soaked in a 2 % (w/w) solution of the specific coating material dissolved in methylene chloride (CH2C12) for about 1 minute. The amount of coating put on the filters should be enough to create a thin layer on the glass fibers.

Too much coating material will present problems in the desorption process by leaving impurities in the analyte or making the filter stick to the hot plate. A suitable level of the amount of coating material is approximately 10 % of the final filter. The coated sheets were then dried for 24 hours at 20°C. Filters were cut in disks with a diameter of 30 mm. Each

filter disk weighed 40 mg. The filter disks were then mounted in rings made of brass or thermoplastic glue.

Coating materials-The following coating materials were tested: Eudragites, Poly ethylene oxide MW=20 000 (Carbowax), poly-(2, 6-diphenyl-1, 4-phenylene oxide), poly (ethylene oxide-co-propylene oxide (Pluronics), polysiloxanes having various structures and crosslink densities, e. g dicyanomethylsilicone, and polydimethylsiloxane (PDMS), and uncoated glass.

Collection of vapors-Filters were placed in a holder connected to an air pump (Siemens) working at maximum speed. The flow rate was approximately 0. 5 Ils. The filter holder with the filter was positioned 5 cm above a glass jar with the substance and air was collected during 60 seconds.

Filter analysis-The filter disks that had been used for vapor collection were removed from the ring and then extracted with 0.5 ml of acetonitrile for 5 minutes. The filter extract was subsequently analyzed using a Gas Chromatograph GC/ECD (HP 6890 series) with a Hewlet Packard HP-1 column.

Results Results obtained with the method presented above are shown in Figures 1,2 and 3. The figures show that for volatile substances with high vapor pressures, like 2,4-DNT and 2,6-DNT, the presence of a Carbowax coating on the filter enhances the uptake of these substances on the filter almost one order of magnitude. For less volatile substances, like 2,4, 6- TNT, the effect is not as pronounced. These results are corroborated when amphetamine and cocaine vapors are collected. A stationary phase on the filter enhances drug uptake.

Similar test results were obtained with the other polymer coatings, which indicates that the chemical nature of the coating is of less importance. All coatings tested so far, differing greatly in chemical nature, enhanced the uptake of volatile substances.

A coating also increases the lifetime of the substances on the filters, especially volatile or sensitive substances like the nitrotoluenes. This is important when the filter is not to be analyzed immediately but stored for some time before analyses.