A DETECTOR FOR MONITORING LOW MOLECULAR WEIGHT COMPOUNDS

Abstract: A detector for monitoring low molecular weight compounds and especially the alcohols ethanol and methanol in environments where higher molecular weight contaminants may also be present. Samples of gas are passed through a porous adsorber which has relatively low retention for low molecular weight compounds such as methanol and ethanol and relatively high retention for higher molecular weight species. The portion of the sample not retained by the adsorber is then passed through a reagent which undergoes a colour change with low molecular weight compounds the extent of colour change being indicative of the amount of low molecular weight compound present. The invention is particularly useful for monitoring the concentration of low molecular weight organic alcohols such as methanol and ethanol.

Description: A Detector for Monitoring Low Molecular Weight Compounds The invention relates to the detection of low molecular weight compoundsr especially ethanol andmethanol/ in the presence of highermolecularweight compounds and is particularly but not exclusively related to a methanol detector for atmosphere monitoring in closed environments. Methanol (CH30H) isa toxicsubstanceandwhenpresentinclosedenvironments thereisaneedtoroutinelymonitortheair. Amoni ormustbeableto signal that the ambient level of methanol is less than a maximum permissible concentration. In one known detector arrangement a commercially availableDraeger detector tube, Methanol 50/a, can be used tomonitor methanol in the range 50 to 3000 ppm. Other tubes are available for themonitoringof other compounds. The detector itself consists of a narrow bore tube containing a precleansing layer of drying agent followed by crystals impregnated with an orange Chromium (Cr(VI)) compound. In order tomonitormethanol a sample of air of standard volume is drawn through the detector tube by means of a bellows pump. The presence of methanol in that sample causes a colour change in the tube fromorange to black. Thelength of theblack stain thendetermines the amount of methanol present in the sample for the given volume of air. The detection process is illustrated by: CH30H + Cr6+ > Reaction products (black) This monitoring arrangement, however, suffers from cross sensitivity to higher molecular weight alcohols and some other organic compounds, in particular n octane. According to the manufacturer, a concentration of500 ppm of n octane "impurity" gives an indication of approximate 2500 ppm of methanol. This makes the use of methanol Draeger tubes in closed environments impractical because many closed environments contain a large number of organic pollutants including octane which would interfere with methanol monitoring. The problem of cross sensitivity may also existwhen monitoring other low molecular weight organic compounds and possibly also with low molecular weight inorganic compounds. SUBSTITUTE SHEET The inventorhasbeeninvolved indevelopmentof a samplingprocedureforthe quantitative collection of trace organic contaminants in air using a coπroerciallyavailableporouspolymerTenax. Tenaxhasdifferingretention characteristics for different chemical species and inparticular retention of methanol and ethanol are low compared with n octane. The breakthrough volumes formethanol, ethanol andnoctane sampledonTenaxare0.3 lg-1, 1.8 lg-1 and770 lg-1 respectivelyat20oc. Tenaxisavailablecommerciallyin two formsir TenaxTAandTenaxGC. For the invention inquestioneither form is suitable. An object of the present invention is to provide a means to detect low molecular weight compounds when present in a sample with higher molecular weightcontaminants. Afurtherobjectofthepresentinventionistoprovide ameanstomonitormethanol inthepresenceofnoctane. Ayetfurtherobject is to provide a means to detect ethanol in the presence of other organic species. The invention provides a detector for monitoring low molecular weight compounds in the presence of higher molecular weight species comprising: a porous adsorber having relatively low retention for the low molecular weight coπpounds and relatively high retention for highermolecularweight species; and a reagent which undergoes a colour change on reaction with low molecular weight compounds; the arrangement being such that the adsorber and reagent are contained such thata sampleofair canbepassedthroughtheadsorberbeforepassingthrough the reagent, th amount of low molecular weight compound being calculable from the amount of reagent which changes colour. Theterm "lowmolecularweightcompound" canbe referingtoeitherorganicor inorganic compounds. Preferably the low molecular weight compounds are organic Compounds. Preferablytheorganiccompoundsareeithermethanol or ethanol. Where the lowmolecularweightcompound isanorganiccompound itpreferably contains between1 and5 carbonatomsandmaybe straightchainoroptionally branched. The compounds may also be substituted. Typical groups of compounds would be Cj to C5 alkanes, alkenes, amines, alcohols, aldehydes, ketones organo-halides, such as vinyl chloride, etc. Where the low SUBSTITUTE SHEET molecular weight compound is inorganic it will preferably comprise 2 to 4 atoms and typical examples could includeNCfc, S02, C02, NO, CO etc. Higher molecularweight compounds referstocompoundscontainingmore than5 atoms. Thesevalues are thosewhichare generally applicable to theporous adsorber Tenax andwhere other adsorbers are used thetypesofcompoundsand retention on the adsorber on the basis ofmolecularweightmayvary. These limitationsdonotrepresent clearboundariesforallcompoundswhichwillbe able to pass through the porous adsorber Tenax or be adsorbed by it but generallyit isfoundthat thelargerthemolecularweightofthemoleculethe more difficult it tends tobe for it topass through theporous adsorber and hence molecules containing 5 carbon atoms may take quite some time to pass through if they do at all. Furthermore where Tenax is theporous adsorber used polarity becomes a criteria in whether a compound will be adsorbed or not.. In this case there is a preference to adsorb polar compounds. To enable the invention to work it is necessary that the low molecular weight compound is in the vapour phase for passing through the porous adsorber. Preferablythe reagent isas specificaspossibletothecompoundsorclasses of compounds which require detection. Where a detector for methanol or ethanol is required thenpreferably the reagent is a Chromium (VI) compound which changes colour from orange to black on reaction with methanol or ethanol. Advantageously the Chromium (VT) compound is impregnated on crystalscontainedwithinatubewithwiregauzesbeingprovidedtoretainthe crystals. A drying agent may also be included in the tube to removewater from the air sample before passing over the reagent. Advantageously the adsorber is a porous powdered polymer, poly 2,6 dipheιτl-p-pheιτylene oxide which may be retained by glass wool plugs. The adsorber may be contained withinthesametubeasthereagentorinasecondtubewhichisconnectableto thereagenttube. Inapreferredarrangementthereagentandadsorbermaybe sealed before air sairpling and a pumpmeans is provided to draw a standard volume of air sample through the detector. The inventionwill nowbe described byway of example onlywith referenceto theaccompanyingFigurewhichshowsamethanoldetectorwithseparatereagent and adsorber tubes 10 and 11. The reagent tube 10, available from Draeger, is a narrow-bore tube partly filled with orange Chromium (VI) compound 12, retained by plugs 13 ofwire. SUBSTITUTE SHEET Onthe air inlet side14 ofthetube10 aporousdryingagent15 isprovidedand theoutlet16 isconnectabletoapumpfordrawingastandardisedvolumeofair through the reaction tube 10. Joined to the reaction tube inlet 14 by any knownmeansistheadsorbertube11whichcontains aporouspolymeradsorber, (poly-2,6-dipheϊψl-p-pheπylene oxide) TENAX (Trade Mark) 17 retained in position by porous glass wool plugs 18. Thereagent tube10 andadsorber tube11 areinitiallysealedattheairinlet 21 totheadsorber tube11 andtheairoutlet16 fromthereagent tube10. In use, the seals arebroken, apump is connected to the reagent tube outlet16 and a standard volume of air is drawn through the tubes. The Tenax is effectivein adsorbing organicmolecules >C ~ - i n a closed environmentwhile methanol isnot retained. Inparticular, theTenaxadsorbsnoctanewhichis present in the air. The amount of methanol in the sampled air is then proportional to the length of the chromium (VI) column which changes from orange to black. Although the invention has beendescribedashavingtwoseparate reagentand adsorbertubes, itwill be apparent tothoseskilled intheartthatasingle tubecouldbeused, includingsuccessivequantitiesofadsorber, dryingagent andChromium(VI) reagent. TheChromium (VI) reagentwill alsoreactwiththe same sensitivity to ethanol. Tfous the invention could alsobe used for the detection of et-hanol where there is no methanol contamination. ThetableshowstheresultsofoperationoftheChrcmium (VI) Draegerreagent tube, with and -without a Tenax adsorber pre-cleanse tube in atmospheres containing known amounts of methanol and or n octane. Each atmospherewas sampled by drawing 2500 ml of air through thedetector. This is five time the sampling volume for which the Draeger tube is calibrated. Thέobservedreadingsquotedaboveallowforthis. Theresults SUBSTITUTE SHEET show that Tenax selectively removes the n octane, thereby removing amajor cross sensitivity problem when detecting methanol. SUBSTITUTE SHEET

Claims: £____ι 1. Adetectorformonitoringlowmolecularweightcompoundsinthepresence of higher molecular weight species comprising: a porous adsorber having relatively low retention for the low molecular weight compounds and relativelyhigh retention for highermolecularweight species; and a reagent which undergoes a colour change on reaction with low molecular weight compounds; the arrangement being such that theadsorberand reagentarecontainedsuch thatasampleofaircanbepassedthroughtheadsorberbeforepassingthrough the reagent, the amount of lowmolecular.weight compound beingcalculable from the amount of reagent which changes colour. 2. A detector as claimed in claim 1 wherein the low molecular weight compounds are organic compounds. 3. Adetectorasclaimedinclaim2whereintheorganiccompoundsareeither methanol or ethanol. 4. A detector as claimed in claim 2 or 3 wherein the organic compounds contain between 1 and 5 carbon atoms. 5. Adetectorasclaimedinclaim1whereinthelowmolecularweightcompound is inorganic and comprise 2 to 4 atoms. 6. Adetector as claimed in claim3 wherein the reagent isaChrcmium (VI) compoundwhichchangescolourfromorangetoblackonreactionwithmethanol or ethanol. 7. A detector as claimed in claim6 wherein theChromium (VI) compound is impregnatedimpregnatedoncrystalscontainedwithinatubewithwiregauzes being provided to retain the crystals. 8. Adetectorasclaimedinanyoneof thepreviousclaimswhereinadrying agent is included in the tube to remove water from the air sample before passing over the reagent. SUBSTITUTE SHEET 9. A detector as claimed in any one of the previous claims wherein the adsorber is a porous powdered polymer, poly 2,6 dipheπyl-p-phenylene oxide optionally retained by glass wool plugs. 10. Adetectorasclaimedinanyoneofthepreviousclaimswhereinthereagent and adsorber are sealed before air sampling and a pumpmeans isprovided to draw a standard volume of air sample through the detector. 11. A detector substantially as herein described and with reference to the accαπpaηying drawings. SUBSTITUTE SHEET

Inventors: WOODS STEPHEN CLIFFORD, WOODS, STEPHEN, CLIFFORD
Applicants: SECR DEFENCE BRIT, THE SECRETARY OF STATE FOR DEFENCE IN HER BRITANNI
Application year: 1990
Main classification: G01N1
IPC codes: G01N1, G01N31, G01N33