April, 1984
METHOD FOR THE DETERMINATION OF VOLATILE ORGANIC COMPOUNDSIN AMBIENT AIR USING CRYOGENIC PRECONCENTRATION TECHNIQUES
AND GAS CHROMATOGRAPHY WITH FLAME IONIZATION AND
ELECTRON CAPTURE DETECTION1.
Scope1.1
This document describes a method for the determination ofhighly volatile compounds having boiling points in therange of -10 to 200EC.
The methodology detailed in this document is currentlyemployed by numerous laboratories (1-4;8-11).Modifications to this methodology should be accompaniedby appropriate documentation of the validity andreliability of these changes.
1.2
2.Applicable Documents2.1
ASTM Standards
D1356 Definition of Terms Related to Atmospheric Samplingand Analysis
E 355 Recommended Practice for Gas Chromatography Termsand Relationships2.2
Other Documents
Ambient Air Studies (1-4).
U. S. EPA Technical Assistance Document (5).
3.Summary of Method3.1
Ambient air analyses are performed as follows. Acollection trap, as illustrated in Figure 1, is submergedin either liquid oxygen or argon. Liquid argon is highlyrecommended for use because of the safety hazardassociated with liquid oxygen. With the sampling valvein the fill position an air sample is then admitted intothe trap by a volume measuring apparatus. In themeantime, the column oven is cooled to a sub-ambienttemperature (-50EC). Once sample collection iscompleted, the valve is switched so that the carrier gassweeps the contents of the trap onto the head of thecooled GC column. Simultaneously, the liquid cryogen is
removed and the trap is heated to assist the sampletransfer process. The GC column is temperatureprogrammed and the component peaks eluting from thecolumns are identified and quantified using flameionization and/or electron capture detection. Alternatedetectors (e.g., photoionization) can be used asappropriate. An automated system incorporating thesevarious operations as well as the data processingfunction has been described in the literature (8,9).3.2
Due to the complexity of ambient air samples, highresolution (capillary column) GC techniques arerecommended. However, when highly selective detectors(such as the electron capture detector) are employed,packed column technology without cryogenic temperatureprogramming can be effectively utilized in some cases.
4.Significance4.1
Volatile 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.
Because these organic species are present at ppb levelsor below, some means of sample preconcentration isnecessary in order to acquire sufficient material foridentification and quantification. The two primarypreconcentration techniques are cryogenic collection andthe use of solid adsorbents. The method described hereininvolves the former technique.
4.2
5.Definitions
Definitions used in this document and any user prepared SOPsshould be consistent with ASTM D1356(6). All abbreviationsand symbols are defined within this document at the point ofuse.
6.Interferences/Limitations6.1
Compounds having similar GC retention times willinterfere in the method. Replacing the flame ionizationdetector with more selective detection systems will helpto minimize these interferences. Chlorinated species, inparticular, should be determined using the electroncapture detector to avoid interference from volatilehydrocarbons.
6.2
An important limitation of the technique is thecondensation of moisture in the collection trap. Thepossibility of ice plugging the trap and stopping theflow is of concern, and water subsequently transferred tothe capillary column may also result in flow stoppage andcause deleterious effects to certain column materials. Use of permaselective Nafion® tubing in-line before thecryogenic trap avoids this problem; however, the materialmust be used with caution because of possible losses ofcertain compounds. Another potential problem iscontamination from the Nafion® tubing. The user shouldconsult the literature (7-12) for details on the use ofpermeation-type driers.
7.Apparatus7.1
Gas chromatograph/Flame Ionization/Electron CaptureDetection System - must be capable of subambienttemperature programming. A recent publication (8)describes an automated GC system in which the cryogenicsampling and analysis features are combined. This systemallows simultaneous flame ionization and electron capturedetection.
Six-port sampling valve - modified to accept a samplecollection trap (Figure 1).
Collection trap - 20 cm x 0.2 cm I.D. stainless steeltubing packed with 60/80 mesh silanized glass beads andsealed with glass wool. For the manual system (Section9.2) the trap is externally wrapped with 28 gauge (duplexand fiberglass insulated) type \"K\" thermocouple wire.This wire, beaded at one end, is connected to a powerstatduring the heating cycle. A thermocouple is alsoattached to the trap as shown in Figure 1.Powerstat - for heating trap.
Temperature readout device - for measuring traptemperature during heating cycle.
Glass dewar flask - for holding cryogen.
Sample volume measuring apparatus - capable of accuratelyand precisely measuring a total sample volume up to 500cc at sampling rates between 10 and 200 cc/minute. SeeSection 9.Stopwatch.
7.27.3
7.47.57.67.7
7.8
7.9
Dilution container for standards preparation - glassflasks or Teflon (Tedlar) bags, .002 inch film thickness(see Figure 2).
7.10Liquid microliter syringes - 5-50 Fl for injecting liquid
standards into dilution container.7.11Volumetric flasks - various sizes, 1-10 mL.
7.12GC column - Hewlett Packard 50 meter methyl silicone
cross-linked fused silica column (.3 mm I.D., thick film)or equivalent.7.13Mass flow controller - 10-200 mL/minute flow control
range.7.14Permeation drier - PermaPure® - Model MD-125F, or
equivalent. Alternate designs described in theliterature (7-12) may also be acceptable.8.
Reagents and Materials8.18.28.38.48.58.68.78.8
Glass beads - 60/80 mesh, silanized.Glasswool - silanized.
Helium - zero grade compressed gas, 99.9999%.Hydrogen - zero grade compressed gas, 99.9999%.Air - zero grade compressed gas.Liquid argon (or liquid oxygen).Liquid nitrogen.
SRM 1805 - benzene in nitrogen standard. Available fromthe National Bureau of Standards. Additional suchstandards will become available in the future.
Chemical standards - neat compounds of interest, highestpurity available.
8.99.
Sampling and Analysis Apparatus
Two systems are described below which allow collection of anaccurately known volume of air (100-1000 mL) onto acryogenically cooled trap. The first system (Section 9.1) isan automated device described in the literature (8,9). Thesecond system (Section 9.2) is a manual device, also describedin the literature(2).
9.1
The automated sampling and analysis system is shown inFigure 3. This system is composed of an automated GCsystem (Hewlett Packard Model 5880A, Level 4, orequivalent) and a sample collection system (Nutech Model320-01, or equivalent). The overall system is describedin the literature (8).9.1.1
The electronic console of the sampling unitcontrols the mechanical operation of the six-port valve and cryogenic trapping componentsas well as the temperatures in each of thethree zones (sample trap, transfer line, andvalve).
The valve (six-port air activated, SeiscorModel 8 or equivalent) and transfer line areconstantly maintained at 120EC. During samplecollection the trap temperature is maintainedat -160 + 5EC by a flow of liquid nitrogencontrolled by a solenoid valve. A cylindrical250 with heater, held in direct contact withthe trap, is used to heat the trap to 120EC in60 seconds or less during the sampledesorption step. The construction of thesample trap is described in Section 7.3.The sample flow is controlled by a pump/massflow controller assembly, as shown in Figure3. A sample flow of 10-100 mL/minute isgenerally employed, depending on the desiredsampling period. A total volume of 100-1000mL is commonly collected.
In many situations a permaselective drier(e.g., Nafion®) may be required to removemoisture from the sample. Such a device isinstalled at the sample inlet. Twoconfigurations for such devices are available.The first configuration is the tube and shelltype in which the sample flow tube issurrounded by an outer shell through which acountercurrent flow of clean, dry air ismaintained. The dry air stream must be freefrom contaminants and its flow rate should be3-4 times greater than the sample flow toachieve effective drying. A secondconfiguration (7) involves placing a dryingagent, e.g., magnesium carbonate, on theoutside of the sample flow tube. Thisapproach eliminates the need for a source ofclean air in the field. However, contaminationfrom the drying agent can be a problem.
9.1.2
9.1.3
9.1.4
9.2
The manual sampling consists of the sample volumemeasuring apparatus shown in Figure 4 connected to thecryogenic trap/GC assembly shown in Figure 1. Theoperation of this assembly is described below.9.2.1
Pump-Down Position
The purpose of the pump-down mode of operationis to evacuate the ballast tank in preparationfor collecting a sample as illustrated inFigure 4. (While in this position, helium canalso be utilized to backflush the sample line,trap, etc. However, this cleaning procedureis not normally needed during most samplingoperations). The pump used for evacuating thesystem should be capable of attaining 200 torrpressure.
9.2.2
Volume Measuring Position
Once the system has been sufficientlyevacuated, the 4-way ball valve is switched toprepare for sample collection. The 3-positionvalve is used to initiate sample flow whilethe needle valve controls the rate of flow.
9.2.3
Sample Volume Calculation
The volume of air that has passed through thecollection trap corresponds to a known changein pressure within the ballast tank (asmeasure by the Wallace Tiernan gauge).Knowing the volume, pressure change, andtemperature of the system, the ideal gas lawcan be used to calculate the number of molesof air sampled. On a volume basis, thisconverts to the following equation:
Vs'
where
Vs =Volume sampled at 760 mm Hg pressure and
25EC.
)P =Change in pressure within the ballast
tank, mm of Hg.
V =Volume of ballast tank and gauge.TA =Temperature of ballast tank, EC.
The internal volume of the ballast tank and gauge can bedetermined either by H2O displacement or by injectingcalibrated volumes of air into the system using largevolume syringes, etc.
)P298
×
760TA%273
10.Sampling and Analysis Procedure - Manual Device
10.1This procedure assumes the use of the manual sampling
system described in Section 9.2.10.2Prior to sample collection, the entire assembly should be
leak-checked. This task is accomplished by sealing thesampling inlet line, pumping the unit down and placingthe unit in the flow measuring mode of operation. Aninitial reading on the absolute pressure gauge is takenand rechecked after 10 minutes. No apparent changeshould be detected.10.3Preparation for sample collection is carried out by
switching the 6-port valve to the \"fill\" position andconnecting the heated sample line to the sample source.Meanwhile the collection trap is heated to 150EC (orother appropriate temperature). The volume measuringapparatus is pumped-down and switched to the flowmeasuring mode. The 3-position valve is opened and aknown volume of sample is then passed through the heatedsample line and trap to purge the system.10.4After the system purge is completed, the 3-position valve
is closed and the corresponding gauge pressure isrecorded. The collection trap is then immersed into adewar of liquid argon (or liquid oxygen) and the 3-position valve is temporarily opened to draw in a knownvolume of air, i.e. a change in pressure corresponds toa specific volume of air (see Section 9). Liquidnitrogen cannot be used as the cryogen since it will alsocondense oxygen from the air. Liquid oxygen representsa potential fire hazard and should not be employed unlessabsolutely necessary.10.5After sample collection is completed, the 6-port valve is
switched to the inject position, the dewar is removed andthe trap is heated to 150EC to transfer the samplecomponents to the head of the GC column which isinitially maintained at -50EC. Temperature programmingis initiated to elute the compounds of interest.10.6A GC integrator (or data system if available) is
activated during the injection cycle to provide componentidentification and quantification.
11.Sampling and Analysis Procedure - Automated Device
11.1This procedure assumes the use of the automated system
shown in Figure 3. The components of this system arediscussed in Section 9.1.11.2Prior to initial sample collection the entire assembly
should be leak-checked. This task is completed bysealing the sample inlet line and noting that the flowindication or the mass flow controller drops to zero(less than 1 mL/minute).11.3The sample trap, valve, and transfer line are heated to
120EC and ambient air is drawn through the apparatus(-60mL/minute) for a period of time 5-10 minutes to flushthe system, with the sample valve in the inject position.During this time the GC column is maintained at 150EC tocondition the column.11.4The sample trap is then cooled to -160 + 5EC using a
controlled flow of liquid nitrogen. Once the traptemperature has stabilized, sample flow through the trapis initiated by placing the valve in the inject positionand the desired volume of air is collected.11.5During the sample collection period the GC column is
stabilized at -50EC to allow for immediate injection ofthe sample after collection.11.6At the end of the collection period the valve is
immediately placed in the inject position, and thecryogenic trap is rapidly heated to 120EC to desorb thecomponents onto GC column. The GC temperature programand data acquisition are initiated at this time.11.7At the desired time the cryogenic trap is cooled to
-160EC, the valve is returned to the collect position andthe next sample collection is initiated (to coincide withthe completion of the GC analysis of the previoussample).
12.Calibration Procedure
Prior to sample analysis, and approximately every 4-6 hoursthereafter, a calibration standard must be analyzed, using theidentical procedure employed for ambient air samples (eitherSection 10 or 11). This section describes three alternativeapproaches for preparing suitable standards.
12.1Teflon® (or Tedlar®) Bags
12.1.1
The bag (nominal size; 20L) is filled withzero air and leak checked. This can be easilyaccomplished by placing a moderate weight(text book) on the inflated bag and leavingovernight. No visible change in bag volumeindicates a good seal. The bag should also beequipped with a quick-connect fitting forsample withdrawal and an insertion port forliquid injections (Figure 2).
Before preparing a standard mixture, the bagis sequentially filled and evacuated with zeroair (5 times). After the 5th filling, asample blank is obtained using the samplingprocedure outlined in Section 10.
In order to prepare a standard mixture, thebag is filled with a known volume of zero air.This flow should be measured via a calibratedmass flow controller or equivalent flowmeasuring device. A measured aliquot of eachanalyte of interest is injected into the bagthrough the insertion port using a microlitersyringe. For those compounds with vaporpressures lower than benzene or for stronglyadsorbed species, the bag should be heated(60E C oven) during the entire calibrationperiod.
To withdraw a sample for analysis, thesampling line is directly connected to thebag. Quick connect fittings allow this hook-up to be easily accomplished and alsominimizes bag contamination from laboratoryair. Sample collection is initiated asdescribed.
12.1.2
12.1.3
12.1.4
12.2Glass Flasks
12.2.1
If a glass flask is employed (Figure 2) theexact volume is determined by weighing theflask before and after filling with deionizedwater. The flask is dried by heating at200EC.
To prepare a standard, the dried flask isflushed with zero air until cleaned (i.e., ablank run is made). An appropriate aliquot of
12.2.2
each analyte is injected using the sameprocedures as described for preparing bagstandards.
12.2.3
To withdraw a standard for analysis, the GCsampling line is directly connected to theflask and a sample obtained. However, becausethe flask is a rigid container, it will notremain at atmospheric pressure after samplinghas commenced. In order to prevent room airleakage into the flask, it is recommended thatno more than 10% of the initial volume beexhausted during the calibration period (i.e.,200cc if a 2 liter flask is used).
12.3Pressurized Gas Cylinders
12.3.1
Pressurized gas cylinders containing selectedanalytes at ppb concentrations in air can beprepared or purchased. A limited number ofanalytes (e.g., benzene, propane) areavailable from NBS.
Specialty gas suppliers will prepare customgas mixtures, and will cross reference theanalyte concentrations to an NBS standard foran additional charge. In general, the usershould purchase such custom mixtures,ratherthan attempting to prepare them becauseof the special high pressure filling apparatusrequired. However, the concentrations shouldbe checked, either by the supplier or the userusing NBS reference materials.
Generally, aluminum cylinders are suitablesince most analytes of potential interest inthis method have been shown to be stable forat least several months in such cylinders.Regulators constructed of stainless steel andTeflon® (no silicon or neoprene rubber).Before use the tank regulator should beflushed by alternately pressuring with thetank mixture, closing the tank valve, andventing the regulator contents to theatmosphere several times.
For calibration, a continuous flow of the gasmixture should be maintained through a glassor Teflon® manifold from which the calibration
12.3.2
12.3.3
12.3.4
12.3.5
standard is drawn. To generate variouscalibration concentrations, the pressurizedgas mixture can be diluted, as desired, withzero grade air using a dynamic dilution system(e.g., CSI Model 1700).
13.
Calibration Strategy
13.1Vapor phase standards can be prepared with either neat
liquids or diluted liquid mixtures depending upon theconcentration levels desired. It is recommended thatbenzene also be included in this preparation scheme sothat flame ionization detector response factors, relativeto benzene, can be determined for the other compounds.The benzene concentration generated in this fashionshould be cross-checked with an NBS (e.g., SRM 1805) foraccuracy determinations.13.2Under normal conditions, weekly multipoint calibrations
should be conducted. Each multipoint calibration shouldinclude a blank run and four concentration levels for thetarget species. The generated concentrations shouldbracket the expected concentration of ambient airsamples.13.3A plot of nanograms injected versus area using a linear
least squares fit of the calibration data will yield thefollowing equation:
Y'A%BX
where
Y = quantity of component, nanogramsA = intercept
B = slope (response factor)
If substantial nonlinearity is present in the calibrationcurve a quadratic fit of the data can be used:
Y'A%BX%CX2
where
C = constant
Alternatively, a stepwise multilevel calibration scheme may beused if more convenient for the data system in use.
14.Performance Criteria and Quality Assurance
This section summarizes the quality assurance (QA) measuresand provides guidance concerning performance criteria whichshould be achieved within each laboratory.14.1Standard Operating Procedures (SOPs)
14.1.1
Each user should generate SOPs describing thefollowing activities as accomplished in theirlaboratories:1)2)3)4)
14.1.2
assembly, calibration and operation ofthe sampling system.
preparation and handling of calibrationstandards.
assembly, calibration and operation ofthe GC/FID system and
all aspects of data recording andprocessing.
SOPs should provide specific stepwiseinstructions and should be readily availableto, and understood by, the laboratorypersonnel conducting the work.
14.2Method Sensitivity, Precision and Accuracy
14.2.1
System sensitivity (detection limit) for eachcomponent is calculated from the data obtainedfor calibration standards. The detectionlimit is defined as
DL'A%3.3S
where
DL =calculated detection limit in nanograms
injected.
A =intercept calculated in Section 13.
S =standard deviation of replicate
determination of the lowest levelstandard (at least three determinationsare required).For many compounds detection limits of 1 to 5nanograms are found using the flame ionizationdetection. Lower detection limits can beobtained for chlorinated hydrocarbons usingthe electron capture detector.
14.2.2
A precision of + 5% (relative standarddeviation) can be readily achieved atconcentrations 10 times the detection limit.Typical performance data are included in Table1.
Method accuracy is estimated to be within +10%, based on National Bureau of Standardcalibrated mixtures.
14.2.3
REFERENCES
1.
Holdren, M., Spicer, C., Sticksel, P., Nepsund, K., Ward, G.,and Smith, R., \"Implementation and Analysis of HydrocarbonGrab Samples from Cleveland and Cincinnati 1981 OzoneMonitoring Study\Protection Agency, Research Triangle Park, North Carolina,1982.
Westberg, H., Rasmussen, R., and Holdren, M., \"GasChromatographic Analysis of Ambient Air for Light HydrocarbonsUsing a Chemically Bonded Stationary Phase\1852-1854, 1974.
Lonneman, W. A., \"Ozone and Hydrocarbon Measurements in RecentOxidant Transport Studies\Oxidant Pollutant and Its Control Proceedings, EPA-600/3-77-001a, 1977.
Singh, H., \"Guidance for the Collection and Use of AmbientHydrocarbon Species Data in Development of Ozone ControlStrategies\Agency, Research Triangle Park, North Carolina, 1980.
Riggin, R. M., \"Technical Assistance Document for Sampling andAnalysis of Toxic Organic Compounds in Ambient Air\600/4-83-027. U.S. Environmental Protection Agency, ResearchTriangle Park, North Carolina, 1983.
Annual Book of ASTM Standards, Part 11.03, \"AtmosphericAnalysis\Philadelphia, Pennsylvania, 1983.
Foulger, B. E. and P. G. Sinamouds, \"Drier for Field Use inthe Determination of Trace Atmospheric Gases\51, 1089-1090, 1979.Pheil, J. D. and W. A. McClenney, \"Reduced TemperaturePreconcentration Gas Chromatographic Analysis of AmbientVapor-Phase Organic Compounds: System Automation\Chem., submitted, 1984.
Holdren, M. W., W. A. McClenney, and R. N. Smith \"ReducedTemperature Preconcentration and Gas Chromatographic Analysisof Ambient Vapor-Phase Organic Compounds: SystemPerformance\
Holdren, M., S. Rust, R. Smith, and J. Koetz, \"Evaluation ofCryogenic Trapping as a Means for Collecting Organic Compoundsin Ambient Air\3487, 1984.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
Cox, R. D. and R. E. Earp, \"Determination of Trace LevelOrganics in Ambient Air by High-Resolution Gas Chromatographywith Simultaneous Photoionization and Flame IonizationDetection\Burns, W. F., O. T. Tingy, R. C. Evans and E. H. Bates,\"Problems with a Nafion® Membrane Dryer for ChromatographicSamples\12.
Sample VolumeMeasuring ApparatusHeated Sample LineSample SourceHeated ValveG.C. CarrierGasG.C. ColumnSample Collection TrapVariacTemperatureControllerTemperatureReadouta. Fill PositionSample VolumeMeasuring ApparatusHeated Sample LineSample SourceHeated ValveG.C. CarrierGasG.C. ColumnSample Collection TrapVariacTemperatureControllerTemperatureReadouta. Injection PositionFigure 1. Schematic of Six-Port Valve Used for Sample Collection.
Septum SealGlass/Teflon Valve2 LiterGlass FlaskQuick ConnectSampling Port20 LiterTeflon BagPinhole InsertionPort or SeptumInjection PortFigure 2. Dilution Containers for Standard Mixtures
MassFlowControllerVoltage toCartridgeHeatersSampleInPumpVentCryogenicSamplingElectronicsConsoleZone 1Zone 3Zone 2Carrier Gas InVoltage to SolenoidDetectorAnalyticalLiquid NColumn2Six Port ValveSolenoidValveTrapGas Chromatographic SystemFigure 3. Automated Sampling and Analysis System for Cryogenic Trapping
PumpVentWallaceTiernanGauge4 Way BallValveShut OffValveHelium TankNeedle ValveBallast Tank3 Position Valve(1) Gas Chromatograph 6-Port Valve(2) (Optional 2nd GC System)(3) Off12(a) Volume Measuring PositionPumpVentWallaceTiernanGauge4 Way BallValveShut OffValveHelium TankNeedle ValveBallast Tank3 Position Valve(1) Gas Chromatograph 6-Port Valve(2) (Optional 2nd GC System)(3) Off12(b) Pump - Down PositionFigure 4. Sample Volume Measuring Apparatus
TABLE 1.VOLATILE ORGANIC COMPOUNDS FOR WHICH THE CRYOGENIC SAMPLING METHOD HAS BEEN EVALUATED(a)
4444444444444444444444444444444444444444444444444444444444444444444444444444444444444444Compound
Vinylidene ChlorideChloroform
1,2-DichloroethaneMethylchloroformBenzene
TrichloroethyleneTetrachloroethyleneChlorobenzene
Retention Time, Minutes(b)
9.2612.1612.8013.0013.4114.4817.3718.09
Test 1
(4 runs, 200cc samples) Mean (ppb)%RSD 144 84 44 63 93 84 69 46
4.43.83.74.54.03.73.73.3
Test 2
(8 runs, 200-cc samples) Mean (ppb)%RSD 6.1 3.5 1.9 2.7 3.9 3.5 2.9 1.9
3.9 5.8 5.1 4.9 5.1 4.1 4.3 3.2
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Recovery efficiencies were 100 + 5% as determined by comparing direct sample loop (5cc)injections with cryogenic collection techniques (using test 1 data). Data fromreference 10.
GC conditions as follows:
Column -Hewlett Packard, crosslinked methyl silicone, 0.32 m ID x 50 m long,thick film, fused silica.
(b)
Temperature Program - 50EC for 2 minutes, then increased at 8EC/minute to 150EC.
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