环境空气 挥发性有机物的测定 美国EPA Method TO-1

METHOD TO-1 Revision 1.0April, 1984 METHOD FOR THE DETERMINATION OF VOLATILE ORGANIC COMPOUNDS IN AMBIENT AIR USING TENAX® ADSORPTION ANDGAS CHROMATOGRAPHY/MASS SPECTROMETRY (GC/MS)1.Scope1.1The document describes a generalized protocol forcollection and determination of certain volatile organiccompounds which can be captured on Tenax® GC (poly(2,6-Diphenyl phenylene oxide)) and determined by thermaldesorption GC/MS techniques. Specific approaches usingthese techniques are described in the literature (1-3).1.2This protocol is designed to allow some flexibility inorder to accommodate procedures currently in use.However, such flexibility also results in placement ofconsiderable responsibility with the user to documentthat such procedures give acceptable results (i.e.,documentation of method performance within eachlaboratory situation is required). Types ofdocumentation required are described elsewhere in thismethod.1.3Compounds which can be determined by this method arenonpolar organics having boiling points in the range ofapproximately 80E - 200E C. However, not all compoundsfalling into this category can be determined. Table 1gives a listing of compounds for which the method hasbeen used. Other compounds may yield satisfactoryresults but validation by the individual user isrequired.2.Applicable Documents2.1ASTM Standards:D1356Definitions of Terms Related to AtmosphericSampling and Analysis.E355Recommended Practice for Gas ChromatographyTerms and Relationships.2.2Other documents:Existing procedures (1-3).U. S. EPA Technical Assistance Document (4).3.Summary of Protocol3.1Ambient air is drawn through a cartridge containing -1-2grams of Tenax and certain volatile organic compounds aretrapped on the resin while highly volatile organiccompounds and most inorganic atmospheric constituentspass through the cartridge. The cartridge is thentransferred to the laboratory and analyzed.3.2For analysis the cartridge is placed in a heated chamberand purged with an inert gas. The inert gas transfersthe volatile organic compounds from the cartridge onto acold trap and subsequently onto the front of the GCcolumn which is held at low temperature (e.g., -70E C).the GC column temperature is then increased (temperatureprogrammed) and the components eluting from the columnare identified and quantified by mass spectrometry.Component identification is normally accomplished, usinga library search routine, on the basis of the GCretention time and mass spectral characteristics. Lesssophisticated detectors (e.g., electron capture or flameionization) may be used for certain applications buttheir suitability for a given application must beverified by the user.3.3Due to the complexity of ambient air samples only highresolution (i.e., capillary) GC techniques are consideredto be acceptable in this protocol.4.Significance4.1Volatile organic compounds are emitted into theatmosphere from a variety of sources including industrialand commercial facilities, hazardous waste storagefacilities, etc. Many of these compounds are toxic;hence knowledge of the levels of such materials in theambient atmosphere is required in order to determinehuman health impacts.4.2Conventional air monitoring methods (e.g., for workspacemonitoring) have relied on carbon adsorption approacheswith subsequent solvent desorption. Such techniquesallow subsequent injection of only a small portion,typically 1-5% of the sample onto the GC system.However, typical ambient air concentrations of thesecompounds require a more sensitive approach. The thermaldesorption process, wherein the entire sample isintroduced into the analytical (GC/MS) system fulfillsthis need for enhanced sensitivity.5.DefinitionsDefinitions used in this document and any user prepared SOPs should be consistent with ASTM D1356(6). All abbreviations and symbols are defined with this document at the point of use.6.Interferences6.1Only compounds having a similar mass spectrum and GCretention time compared to the compound of interest willinterface in the method. The most commonly encounteredinterferences are structural isomers.6.2Contamination of the Tenax cartridge with the compound(s)of interest is a commonly encountered problem in themethod. The user must be extremely careful in thepreparation, storage, and handling of the cartridgesthroughout the entire sampling and analysis process tominimize this problem.7.Apparatus7.1Gas Chromatograph/Mass Spectrometry system - should becapable of subambient temperature programming. Unit massresolution or better up to 800 amu. Capable of scanning30-400 amu region every 0.5-1 second. Equipped with datasystem for instrument control as well as dataacquisition, processing and storage.7.2Thermal Desorption Unit - Designed to accommodate Tenaxcartridges in use. See Figure 2a or b.7.3Sampling System - Capable of accurately and preciselydrawing an air flow of 10-500 ml/minute through the Tenaxcartridge. (See Figure 3a or b.)7.4Vacuum oven - connected to water aspirator vacuum supply.7.5Stopwatch.7.6Pyrex disks - for drying Tenax.7.7Glass jar - Capped with Teflon-lined screw cap. Forstorage of purified Tenax.7.8Powder funnel - for delivery of Tenax into cartridges.7.9Culture tubes - to hold individual glass Tenaxcartridges.7.10Friction top can (paint can) - to hold clean Tenaxcartridges.7.11Filter holder - stainless steel or aluminum (toaccommodate 1 inch diameter filter). Other sizes may be used if desired. (optional)7.12Thermometer - to record ambient temperature.7.13Barometer (optional).7.14Dilution bottle - Two-liter with septum cap for standardspreparation.7.15Teflon stirbar - 1 inch long.7.16Gas-tight glass syringes with stainless steel needles -10-500 µ1 for standard injection onto GC/MS system.7.17Liquid microliter syringes - 5.50 µL for injecting neatliquid standards into dilution bottle.7.18Oven - 60 + 5E C for equilibrating dilution flasks.7.19Magnetic stirrer.7.20Heating mantel.7.21Variac7.22Soxhlet extraction apparatus and glass thimbles - forpurifying Tenax.7.23Infrared lamp - for drying Tenax.7.24GC column - SE-30 or alternative coating, glass capillaryor fused silica.7.25Psychrometer - to determine ambient relative humidity.(optional)8.Reagents and Materials8.1Empty Tenax cartridges - glass or stainless steel (seeFigure 1a or b).8.2Tenax 60/80 mesh (2,6-diphenylphenylene oxide polymer).8.3Glasswool - silanized.8.4Acetone - Pesticide quality or equivalent.8.5Methanol - Pesticide quality or equivalent.8.6Pentane - Pesticide quality or equivalent.8.7Helium - Ultra pure, compressed gas. (99.9999%)8.8Nitrogen - Ultra pure, compressed gas. (99.9999%)8.9Liquid nitrogen.8.10Polyester gloves - for handling glass Tenax cartridges.8.11Glass Fiber Filter - one inch diameter, to fit in filterholder. (optional)8.12Perfluorotributylamine (FC-43).8.13Chemical Standards - Neat compounds of interest. Highestpurity available.8.14Granular activated charcoal - for preventingcontamination of Tenax cartridges during storage.9.Cartridge Construction and Preparation9.1Cartridge Design9.1.1Several cartridge designs have been reportedin the literature (1-3). The most common (1)is shown in Figure 1a. This design minimizescontact of the sample with metal surfaces,which can lead to decomposition in certaincases. However, a disadvantage of this designis the need to rigorously avoid contaminationof the outside portion of the cartridge sincethe entire surface is subjected to the purgegas stream during the desorption process.Clean polyester gloves must be worn at alltimes when handling such cartridges andexposure of the open cartridge to ambient airmust be minimized.9.1.2 A second common type of design (3) is shown inFigure 1b. While this design uses a metal(stainless steel) construction, it eliminatesthe need to avoid direct contact with theexterior surface since only the interior ofthe cartridge is purged.9.1.3The thermal desorption module and samplingsystem must be selected to be compatible withthe particular cartridge design chosen.Typical module designs are shown in Figure 2aand b. These designs are suitable for thecartridge designs shown in Figures 1a and b,respectively.9.2Tenax Purification9.2.1Prior to use the Tenax resin is subjected to aseries of solvent extraction and thermaltreatment steps. The operation should beconducted in an area where levels of volatileorganic compounds (other than the extractionsolvents used) are minimized.9.2.2All glassware used in Tenax purification aswell as cartridge materials should bethoroughly cleaned by water rinsing followedby an acetone rinse and dried in an oven at250E C.9.2.3Bulk Tenax is placed in a glass extractionthimble and held in place with a plug of cleanglasswool. The resin is then placed in thesoxhlet extraction apparatus and extractedsequentially with methanol and then pentanefor 16-24 hours (each solvent) atapproximately 6 cycles/hour. Glasswool forcartridge preparation should be cleaned in thesame manner as Tenax.9.2.4The extracted Tenax is immediately placed inan open glass dish and heated under aninfrared lamp for two hours in a hood. Caremust be exercised to avoid over heating of theTenax by the infrared lamp. The Tenax is thenplaced in a vacuum oven (evacuated using awater aspirator) without heating for one hour.An inert gas (helium or nitrogen) purge of 2-3ml/minute is used to aid in the removal ofsolvent vapors. The oven temperature is thenincreased to 110E C, maintaining inert gas flowand held for one hour. The oven temperaturecontrol is then shut off and the oven isallowed to cool to room temperature. Prior toopening the oven, the oven is slightlypressurized with nitrogen to preventcontamination with ambient air. The Tenax isremoved from the oven and sieved through a40/60 mesh sieve (acetone rinsed and ovendried) into a clean glass vessel. If theTenax is not to be used immediately forcartridge preparation it should be stored in aclean glass jar having a Teflon-lined screwcap and placed in a desiccator.9.3Cartridge Preparation and Pretreatment9.3.1All cartridge materials are pre-cleaned asdescribed in Section 9.2.2. If the glasscartridge design shown in Figure 1a isemployed all handling should be conductedwearing polyester gloves.9.3.2The cartridge is packed by placing a 0.5-lcmglasswool plug in the base of the cartridgeand then filling the cartridge to withinapproximately 1 cm of the top. A 0.5-1cmglasswool plug is placed in the top of thecartridge.9.3.3The cartridges are then thermally conditionedby heating for four hours at 270E C under aninert gas (helium) purge (100 - 200 ml/min).9.3.4After the four hour heating period thecartridges are allowed to cool. Cartridges ofthe type shown in Figure 1a are immediatelyplaced (without cooling) in clean culturetubes having Teflon-lined screw caps with aglasswool cushion at both the top and thebottom. Each tube should be shaken to ensurethat the cartridge is held firmly in place.Cartridges of the type shown in Figure 1b areallowed to cool to room temperature underinert gas purge and are then closed withstainless steel plugs.V MAX 'VbxW9.3.5The cartridges are labeled and placed in atightly sealed metal can (e.g., paint can orsimilar friction top container). Forcartridges of the type shown in Figure 1a theculture tube, not the cartridge, is labeled.9.3.6Cartridges should be used for sampling within2 weeks after preparation and analyzed withintwo weeks after sampling. If possible thecartridges should be stored at -20E C in aclean freezer (i.e., no solvent extracts orother sources of volatile organics containedin the freezer).10.Sampling10.1Flow Rate and Total Volume Selection10.1.1Each compound has a characteristic retentionvolume (liters of air per gram of adsorbent)which must not be exceeded. Since theretention volume is a function of temperature,and possibly other sampling variables, onemust include an adequate margin of safety toensure good collection efficiency. Someconsiderations and guidance in this regardareprovided in a recent report (5).Approximate breakthrough volumes at 38E C(100E F) in liters/gram of Tenax are providedin Table 1. These retention volume data aresupplied only as rough guidance and aresubject to considerable variability, dependingon cartridge design as well as samplingparameters and atmospheric conditions.10.1.2To calculate the maximum total volume of airwhich can be sampled use the followingequation:whereV is the calculated maximum total volume inMAXliters.V is the breakthrough volume for the leastbretained compound of interest (Table 1)in liters per gram of Tenax.W is the weight of Tenax in the cartridge,in grams.Q MAX 'VMAXtx1000 B'QMAXB r21.5 is a dimensionless safety factor to allowfor variability in atmospheric conditions.This factor is appropriate for temperatures inthe range of 25-30E C. If higher temperaturesare encountered the factor should be increased(i.e. maximum total volume decreased).10.1.3To calculate maximum flow rate use thefollowing equation:whereQ is the calculated maximum flow rate inMAXmilliliters per minute.t is the desired sampling time in minutes.Times greater than 24 hours (1440minutes) generally are unsuitable becausethe flow rate required is too low to beaccurately maintained.10.1.4The maximum flow rate Q should yield aMAXlinear flow velocity of 50-500 cm/minute.Calculate the linear velocity corresponding tothe maximum flow rate using the followingequation:whereB is the calculated linear flow velocity incentimeters per minute.r is the internal radius of the cartridgein centimeters.If B is greater then 500 centimeters perminute either the total sample flow rate (V)MAXshould be reduced or the sample flow rate(Q) should be reduced by increasing theMAXcollection time. If B is less then 50centimeters per minute the sampling rate (Q)MAXshould be increased by reducing the samplingtime. The total sample value (V) cannot beMAXincreased due to component breakthrough.10.1.5The flow rate calculated as described abovedefines the maximum flow rate allowed. Ingeneral, one should collect additional samplesin parallel, for the same time period but atlower flow rates. This practice yields ameasure of quality control and is furtherdiscussed in the literature (5). In general,flow rates 2 to 4 fold lower than the maximumflow rate should be employed for the parallelsamples. In all cases a constant flow rateshould be achieved for each cartridge sinceaccurate integration of the analyteconcentration requires that the flow beconstant over the sampling period.10.2Sample Collection10.2.1Collection of an accurately known volume ofair is critical to the accuracy of theresults. For this reason the use of mass flowcontrollers, rather than conventional needlevalves or orifices is highly recommended,especially at low flow velocities (e.g., lessthan 100 milliliters/minute). Figure 3aillustrates a sampling system utilizing massflow controllers. This system readily allowsfor collection of parallel samples. Figure 3bshows a commercially available system based onneedle valve flow controllers.10.2.2Prior to sample collection insure that thesampling flow rate has been calibrated over arange including the rate to be used forsampling, with a "dummy" Tenax cartridge inplace. Generally calibration is accomplishedusing a soap bubble flow meter or calibratedwet test meter. The flow calibration deviceis connected to the flow exit, assuming theentire flow system is sealed. ASTM MethodD3686 describes an appropriate calibrationscheme, not requiring a sealed flow systemdownstream of the pump.10.2.3The flow rate should be checked before andafter each sample collection. If the samplinginterval exceeds four hours the flow rateshould be checked at an intermediate pointduring sampling as well. In general, arotameter should be included, as shown inFigure 3b, to allow observation of thesampling flow rate without disrupting thesampling process.10.2.4To collect an air sample the cartridges areremoved from the sealed container just priorto initiation of the collection process. Ifglass cartridges (Figure 1a) are employed theymust be handled only with polyester gloves andshould not contact any other surfaces.10.2.5 A particulate filter and holder are placed onthe inlet to the cartridges and the exit endof the cartridge is connected to the samplingapparatus. In many sampling situations theuse of a filter is not necessary if only thetotal concentration of a component is desired.Glass cartridges of the type shown in Figure1a are connected using teflon ferrules andSwagelok (stainless steel or teflon) fittings.Start the pump and record the followingparameters on an appropriate data sheet(Figure 4): data, sampling location, time,ambient temperature, barometric pressure,relative humidity, dry gas meter reading (ifapplicable), flow rate, rotameter reading (ifapplicable), cartridge number and dry gasmeter serial number.10.2.6Allow the sampler to operate for the desiredtime, periodically recording the variableslisted above. Check flow rate at the midpointof the sampling interval if longer than fourhours. At the end of the sampling periodrecord the parameters listed in 10.2.5 andcheck the flow rate and record the value. Ifthe flows at the beginning and end of thesampling period differ by more than 10% thecartridge should be marked as suspect.10.2.7Remove the cartridges (one at a time) andplace in the original container (use glovesfor glass cartridges). Seal the cartridges orculture tubes in the friction-top cancontaining a layer of charcoal and package forimmediate shipment to the laboratory foranalysis. Store cartridges at reducedtemperature (e.g., -20E C) before analysis ifpossible to maximize storage stability.Q A 'Q1%Q2%...QNNVm'T×QAV s 'Vm×PA760×298273%tA10.2.8Calculate and record the average sample ratefor each cartridge according to the followingequation:whereQ = Average flow rate in ml/minute.AQ, Q, ....Q = Flow rates determined at 12Nbeginning, end, and intermediate points duringsampling.N = Number of points averaged.10.2.9Calculate and record the total volumetric flowfor each cartridge using the followingequation:whereV =Total volume sampled in liters atmmeasured temperature and pressure.T =Stop time.2T =Start time.1T =Sampling time = T = T, minutes2110.2.10The total volume (V) at standard conditions,s25E C and 760 mmHg, is calculated from thefollowing equation:whereP = Average barometric pressure, mmHgAt = Average ambient temperature, E C.A11.GC/MS Analysis11.1Instrument Set-up11.1.1Considerable variation from one laboratory toanother is expected in terms of instrumentconfiguration. Therefore each laboratory mustbe responsible for verifying that theirparticular system yields satisfactory results.Section 14 discusses specific performancecriteria which should be met.11.1.2 A block diagram of the typical GC/MS systemrequired for analysis of Tenax cartridges isdepicted in Figure 5. The operation of suchdevices is described in 11.2.4. The thermaldesorption module must be designed toaccommodate the particular cartridgeconfiguration. Exposure of the sample tometal surfaces should be minimized and onlystainless steel, or nickel metal surfacesshould be employed. The volume of tubing andfittings leading from the cartridge to the GCcolumn must be minimized and all areas must bewell-swept by helium carrier gas.11.1.3The GC column inlet should be capable of beingcooled to -70E C and subsequently increasedrapidly to approximately 30E C. This can bemost readily accomplished using a GC equippedwith subambient cooling capability (liquidnitrogen) although other approaches such asmanually cooling the inlet of the column inliquid nitrogen may be acceptable.11.1.4The specific GC column and temperature programemployed will be dependent on the specificcompounds of interest. Appropriate conditionsare described in the literature (1-3). Ingeneral a nonpolar stationary phase (e.g., SE-30, OV-1) temperature programmed from 30E C to200E C at 8E/minute will be suitable. Fusedsilica bonded phase columns are preferable toglass columns since they are more rugged andcan be inserted directly into the MS ionsource, thereby eliminating the need for aGC/MS transfer line.11.1.5Capillary column dimensions of 0.3 mm ID and50 meters long are generally appropriatealthough shorter lengths may be sufficient inmany cases.11.1.6Prior to instrument calibration or sampleanalysis the GC/MS system is assembled asshown in Figure 5. Helium purge flows(through the cartridge) and carrier flow areset at approximately 10 ml/minute and 1-2ml/minute respectively. If applicable, theinjector sweep flow is set at 2-4 ml/minute.11.1.7Once the column and other system componentsare assembled and the various flowsestablished the column temperature isincreased to 250E C for approximately fourhours (or overnight if desired) to conditionthe column.11.1.8The MS and data system are set according tothe manufacturer's instructions. Electronimpact ionization (70eV) and an electronmultiplier gain of approximately 5 x 10 should4be employed. Once the entire GC/MS system hasbeen setup the system is calibrated asdescribed in Section 11.2. The user shouldprepare a detailed standard operatingprocedure (SOP) describing this process forthe particular instrument being used.11.2Instrument Calibration11.2.1Tuning and mass standardization of the MSsystem is performed according tomanufacturer's instructions and relevantinformation from the user prepared SOP.Perfluorotributylamine should generally beemployed for this purpose. The material isintroduced directly into the ion source thougha molecular leak. The instrumental parameters(e.g., lens voltages, resolution, etc.) shouldbe adjusted to give the relative ionabundances shown in Table 2 as well asacceptable resolution and peak shape. Ifthese approximate relative abundances cannotbe achieved, the ion source may requirecleaning according to manufacturer'sinstructions. In the event that the user'sinstrument cannot achieve these relative ionabundances, but is otherwise operatingproperly, the user may adopt another set ofrelative abundances as performance criteria.However, these alternate values must berepeatable on a day-to-day basis.11.2.2After the mass standardization and tuningprocess has been completed and the appropriatevalues entered into the data system the usershould then calibrate the entire system byintroducing known quantities of the standardcomponents of interest into the system. Threealternate procedures may be employed for thecalibration process including 1) directsyringe injection of dilute vapor phasestandards, prepared in a dilution bottle, ontothe GC column, 2) injection of dilute vaporphase standards into a carrier gas streamdirected through the Tenax cartridge, and 3)introduction of permeation or diffusion tubestandards onto a Tenax cartridge. Thestandards preparation procedures for each ofthese approaches are described in Section 13.The following paragraphs describe theinstrument calibration process for each ofthese approaches.11.2.3If the instrument is to be calibrated bydirect injection of a gaseous standard, astandard is prepared in a dilution bottle asdescribed in Section 13.1. The GC column iscooled to -70E C (or, alternately, a portion ofthe column inlet is manually cooled withliquid nitrogen). The MS and data system isset up for acquisition as described in therelevant user SOP. The ionization filamentshould be turned off during the initial 2-3minutes of the run to allow oxygen and otherhighly volatile components to elute. Anappropriate volume (less than 1 ml) of thegaseous standard is injected onto the GCsystem using an accurately calibrated gastight syringe. The system clock is startedand the column is maintained at -70E C (orliquid nitrogen inlet cooling) for 2 minutes.The column temperature is rapidly increased tothe desired initial temperature (e.g., 30E C).The temperature program is started at aconsistent time (e.g., four minutes) afterinjection. Simultaneously the ionizationfilament is turned on and data acquisition isinitiated. After the last component ofinterest has eluted acquisition is terminatedand the data is processed as described inSection 11.2.5. The standard injectionprocess is repeated using different standardvolumes as desired.11.2.4If the system is to be calibrated by analysisof spiked Tenax cartridges a set of cartridgesis prepared as described in Sections 13.2 or13.3. Prior to analysis the cartridges arestored as described in Section 9.3. If glasscartridges (Figure 1a) are employed care mustbe taken to avoid direct contact, as describedearlier. The GC column is cooled to -70E C,the collection loop is immersed in liquidnitrogen and the desorption module ismaintained at 250E C. The inlet valve isplaced in the desorb mode and the standardcartridge is placed in the desorption module,making certain that no leakage or purge gasoccurs. The cartridge is purged for 10minutes and then the inlet valve is placed inthe inject mode and the liquid nitrogen sourceremoved from the collection trap. The GCcolumn is maintained at -70E C for two minutesand subsequent steps are as described in11.2.3. After the process is complete thecartridge is removed from the desorptionmodule and stored for subsequent use asdescribed in Section 9.3.11.2.5Data processing for instrument calibrationinvolves determining retention times, andintegrated characteristic ion intensities foreach of the compounds of interest. Inaddition, for at least one chromatographicrun, the individual mass spectra should beinspected and compared to reference spectra toensure proper instrumental performance. Sincethe steps involved in data processing arehighly instrument specific, the user shouldprepare a SOP describing the process forindividual use. Overall performance criteriafor instrument calibration are provided inSection 14. If these criteria are notachieved the user should refine theinstrumental parameters and/or operatingprocedures to meet these criteria.11.3Sample Analysis11.3.1The sample analysis process is identical tothat described in Section 11.2.4 for theanalysis of standard Tenax cartridges.。