Patent Application: US-22414394-A

Abstract:
apparatus for preconcentrating trace amounts of organic vapors in a sample of air for subsequent detection , comprising a metallic substrate ; a thin film of fullerenes deposited on the metallic substrate for adsorbing the organic vapors on the thin film of fullerenes , thereby preconcentrating the organic vapors ; and apparatus for heating the metallic substrate to a predetermined optimum temperature for desorbing the vapors from the thin film of fullerenes to form desorbed organic vapors for subsequent detection .

Description:
returning to the discussion of fullerenes begun above in my summary of invention , it is known that fullerenes are soluble in benzene or toluene and chlorinated aromatics , and that such fullerenes can readily be deposited as thin films on clean metal surfaces , ( e . g . nichrome wire ). once deposited , and thermally treated , the fullerenes adhere tenaciously to the metal surface and are difficult to remove . they are chemically and physically stable , up to temperatures on the order of 600 ° c . i have found that such thin films of fullerenes act as excellent adsorbers and collectors of organic vapours , in particular those of nitroaromatics or alkylnitrates ( i . e . common explosives ), such as illustrated in fig1 . because of their low thermal mass , the thin fullerene films can be rapidly heated , by resistive electrical heating of their substrate , to quickly desorb the organic vapours on their surface . this is an important advantage over prior art adsorber materials with respect to their application to explosive vapour detection , where time of measurement is often a critical factor . an additional characteristic of thin fullerene films is that they act in a catalytic fashion , to promote the decomposition of the adsorbed vapours at temperatures which are lower than would otherwise have been required . for the purposes of the present invention , this is a very useful characteristic for the development of a portable explosive detector having low operating power requirements . most nitroaromatics and alkylnitrate based explosives undergo decomposition during heating and release no 2 , no and other pyrolysis products ( see references 9 - 11 ). the pyrolytic release of no and no 2 from explosive molecules is described by the following reaction , referred to herein as reaction 1 : ## str1 ## where hc is the hydrocarbon fragment of the molecule . the ratio of no 2 to no is dependent on the temperature and the availability of oxygen for the oxidation of no to no 2 . the no 2 / no ratio is also dependent on the type of explosive being pyrolyzed . alkylnitrate , such as egdn and petn produce more no 2 fragments , whereas , nitro - toluene explosives produce more no fragment . similarly , all nitrogen containing compounds , such as cocaine and heroin can be pyrolyzed in the presence of oxygen ( air ) to produce molar amounts of no and no 2 ( see reaction 2 , below ), which are thus , detected with the electrochemical sensor . ## str2 ## where hc is the hydrocarbon fraction , comprising co 2 and h 2 o . the adsorption / desorption characteristics of fullerenes coated &# 34 ; wire tubes &# 34 ; will now be discussed with reference to fig2 . a collector / adsorber is shown in the form of a &# 34 ; wire tube &# 34 ; comprising a glass tube 1 containing a densely packed double helical coil of fine metal wire 2 , which serves as a substrate . the ends of the wire coil 2 are fixed to two metal end caps 3 of the tube , thus permitting electrical contact for electrical heating of the wire . a vapour generator , described in reference 12 was used to generate a known concentration of egdn in the parts - per - trillion levels . coated and uncoated tubes were used for sampling the vapour source for a fixed sampling time of 15 seconds . the sampler used in all tests , consisted of a diaphragm pump and an electrical timer for fixing the sampling at 15 seconds . each sample tube was placed in turn in the sampler , and analyzed on an explosive vapour detector , model evd - 8000 . the concentration of the vapour generator was ascertained by liquid injection of standard amounts of egdn on a tenax gc sample tube and from vapour sampling with the same tube and analysis in a model evd - 1 detector . the model evd - 1 detector is a portable gas chromatograph , equipped with a sensitive electron - capture detector ( see reference 13 ). similarly , the model evd - 8000 detector is a portable gc / ecd system , but having the capability of rapid thermal desorption of the wire tube . samples of c 60 and c 70 were obtained from aldrich chemicals and used without further purification . these samples were dissolved in toluene , and solutions of 0 . 1 mg / cc were used for coating the wire tubes . coating was effected by dipping the wire coils 2 into the solution and subsequent solvent evaporation by air drying at 70 ° c . sooting of the wire tubes was carried out by aspiration of toluene soot through the wire tube , followed by rinsing with acetone and air drying at 70 ° c . this left a residue of multiple fullerenes , as a thin film , on the wire coils 2 . sampling results of 500 ppt ( v / v ) of egdn from the vapour generator are shown below in table 1 for uncoated , sooted tubes , and c 60 and c 70 wire tubes . table 1__________________________________________________________________________ table i uncoated sooted sooted sootedrun # tube tube 1 tube 2 tube 3 c60 c70__________________________________________________________________________1 0 203 211 208 90 602 0 247 312 305 110 703 0 255 289 257 103 654 0 234 269 289 111 815 ( ave .) 0 235 270 265 104 69 ( s . d ) ( 23 ) ( 43 ) ( 43 ) ( 10 ) ( 9 ) amount 59 68 66 26 17egdncolledted . pg__________________________________________________________________________ vapour source concentration = 500 ppt ( v / v ) of egdn at 22 deg . c . and 760 mm hg . at low egdn vapour concentration , the uncoated tube showed no vapour adsorption of egdn at room temperature , whereas the fullerenes coated surfaces clearly showed adsorption of egdn vapours . the c 60 and c 70 coated tubes showed lower efficiency of trapping egdn in comparison with the sooted tubes , indicating that other active ingredients , presumably higher mass fullerenes , are responsible for part of the observed adsorption effect occurring on the coated wire / glass substrates , ( i . e . the adsorption action of the various fullerenes is additive ). it is , therefore , contemplated that this invention may be effected using higher order fullerenes as well , up to c 180 . the trapping efficiency of the soot coating on the nichrome wire is shown below in table ii . the collection efficiency of egdn on the wire tube was determined by placing a tenax packed tube in series with ( i . e . after ) the coated wire tube and analyzing the collected fraction on the tenax tube . repeated measurements showed that the average efficiency of the coated wire tube is about 47 % of that of the tenax tube ( known to be 100 % efficient ), with a standard deviation of only 8 %. the wire tubes analyzed on the evd - 8000 detector confirmed the efficiency to be in the range of 40 %- 51 %. table ii______________________________________ tenax gc tube % collec - in series with tenax gc tion ef - run # wire tube tube only ficiency______________________________________1 501 counts 1123 counts 482 530 1129 533 610 1101 454 561 1131 505 660 1126 416 661 1131 427 580 1113 48ave . ( sd ) 586 ( 62 ) 1122 ( 11 ) 47 ( 4 ) ______________________________________ vapour generator set at 250 ppt ( v / v ) of egdn . all analysis done on the evd1 ( g . c / ecd detectors ) corrections made for differences in sampling flowrate with the tenax tube in series with the wire tube the efficiency of collection for other volatile explosives , such as nitroglycerine , 2 , 4 dinitrotoluene and the mononitrotoluene isomers was found to be in the range of 39 - 49 %. other non - volatile explosives , such as tnt , petn and rdx ( fig1 ) were sampled by particulate collection or from swabbing techniques . thermal desorption of the swab vapourized these explosives and allowed collection by condensation on the fullerenes coated wire tube . considering the very small mass of the fullerenes film relative to the tenax in the comparative collector , one can conclude that the fullerene films are remarkably efficient adsorbers of organic vapours arising from explosives . as has been explained above , the significance of the small mass of the collector material is that it permits a very rapid desorption of the adsorbed vapours , with a low energy burden . fullerenes from toluene soot were deposited on a quartz glass tube by suction through a toluene flame . the tube was rinsed with acetone and air dried at 70 ° c . a second quartz tube was similarly prepared , but uncoated . the coated tube was placed in a sge pyrojector , equipped with a temperature controller up to 900 ° c . the product of decomposition ( no 2 ) was monitored with a lma - 3 , no 2 chemiluminescence detector . the lma - 3 has a detection limit down to 10 pptv of no 2 . known amounts of egdn from a standard solution were directly injected in the heated quartz tube . the no 2 signal was measured on an external strip chart recorder . the pyrolyzer temperature was varied from 200 ° to 450 ° c . for the coated and uncoated quartz tubes . the percentage decomposition of egdn as a function of pyrolyzer temperature is shown in fig3 . the coated tube shows some enhanced decomposition at lower temperatures in comparison to the uncoated quartz tube . this effect , although not shown here , is considerably more pronounced in plastic explosives , such as petn and rdx ( fig1 ). one specific embodiment of this invention is illustrated by fig4 . a sample pump 10 draws a sample of air 12 containing organic vapours to be detected , through a preconcentrator - adsorber 13 which consists of a metal substrate , coated with a thin film of fullerenes . a power supply 14 causes current to pass through the substrate , quickly heating it to a temperature which is optimum for the desorption and decomposition of the organic vapours which have been adsorbed on the concentrator . a teflon valve 15 then switches over so that a second pump 16 draws in carrier gas 17 and with it the desorbed and decomposed vapours . this combined gas then passes by a membrane 18 which selectively allows one or more of the desired vapours to pass to a detector 19 . the rejected vapours and carrier gas are then vented 20 . the specific decomposition products of interest in respect of organonitrates are no 2 and no , either separately or in combination ( see reaction 1 ). the specific decomposition products of interest in respect of illicit drugs are also no and no 2 either separately or in combination ( see reaction 2 ). the detector 19 should be selected so as to be as sensitive and specific as possible for the detection of the desired decomposition product vapour or vapours . for example , for no and no 2 , the detector may be an electrochemical or chemiluminescent - based sensor . for parent drug molecules , ( i . e . not decomposed ) the detector may be a surface ionization detector . of course , it is not necessary that the target organic vapours be decomposed on desorption , providing that the membrane 18 and detector 19 are selected so as to each be reasonably specific to the target vapours . for example , for organonitrates , the detector 19 may be an electron capture detector . other embodiments and modifications of the invention are possible . for example , the metallic substrate may be in the form of a flat ribbon rather than a helical coil 2 , as shown in fig2 . all such embodiments and modifications are believed to be within the sphere and scope of the claims appended hereto . 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