1、Designation: D 2908 91 (Reapproved 2005)Standard Practice forMeasuring Volatile Organic Matter in Water by Aqueous-Injection Gas Chromatography1This standard is issued under the fixed designation D 2908; the number immediately following the designation indicates the year oforiginal adoption or, in t
2、he case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers general guidance applicable tocertain test methods for the qual
3、itative and quantitative deter-mination of specific organic compounds, or classes of com-pounds, in water by direct aqueous injection gas chromatogra-phy (1, 2, 3, 4).21.2 Volatile organic compounds at aqueous concentrationsgreater than about 1 mg/L can generally be determined bydirect aqueous injec
4、tion gas chromatography.1.3 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prio
5、r to use.2. Referenced Documents2.1 ASTM Standards:3D 1129 Terminology Relating to WaterD 1192 Guide for Equipment for Sampling Water andSteam in Closed Conduits4D 1193 Specification for Reagent WaterD 3370 Practices for Sampling Water from Closed ConduitsE 260 Practice for Packed Column Gas Chromat
6、ographyE 355 Practice for Gas Chromatography Terms and Rela-tionships3. Terminology3.1 DefinitionsThe following terms in this practice aredefined in accordance with Terminology D 1129.3.1.1 “ghosting” or memory peaksan interference, show-ing as a peak, which appears at the same elution time as theor
7、ganic component of previous analysis.3.1.2 internal standarda material present in or added tosamples in known amount to serve as a reference measurement.3.1.3 noisean extraneous electronic signal which affectsbaseline stability.3.1.4 relative retention ratiothe retention time of theunknown component
8、 divided by the retention time of theinternal standard.3.1.5 retention timethe time that elapses from the intro-duction of the sample until the peak maximum is reached.3.2 For definitions of other chromatographic terms used inthis practice, refer to Practice E 355.4. Summary of Practice4.1 This prac
9、tice defines the applicability of various col-umns and conditions for the separation of naturally occurringor synthetic organics or both, in an aqueous medium forsubsequent detection with a flame ionization detector. Aftervaporization, the aqueous sample is carried through the columnby an inert carr
10、ier gas. The sample components are partitionedbetween the carrier gas and a stationary liquid phase on an inertsolid support. The column effluent is burned in an air-hydrogenflame. The ions released from combustion of the organiccomponents induce an increase in standing current which ismeasured. Alt
11、hough this method is written for hydrogen flamedetection, the basic technology is applicable to other detectorsif water does not interfere.1This practice is under the jurisdiction of ASTM Committee D19 on Water andis the direct responsibility of Subcommittee D19.06 on Methods for Analysis forOrganic
12、 Substances in Water.Current edition approved Dec. 1, 2005. Published January 2006. Originallyapproved in 1970. Last previous edition approved in 2001 as D 2908 91 (2001).2The boldface numbers in parentheses refer to the list of references at the end ofthis practice.3For referenced ASTM standards, v
13、isit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.4Withdrawn.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocke
14、n, PA 19428-2959, United States.4.2 The elution times are characteristic of the variousorganic components present in the sample, while the peak areasare proportional to the quantities of the components. A discus-sion of gas chromatography is presented in Practice E 260.5. Significance and Use5.1 Thi
15、s practice is useful in identifying the major organicconstituents in wastewater for support of effective in-plant orpollution control programs. Currently, the most practical meansfor tentatively identifying and measuring a range of volatileorganic compounds is gas-liquid chromatography. Positiveiden
16、tification requires supplemental testing (for example, mul-tiple columns, speciality detectors, spectroscopy, or a combi-nation of these techniques).6. Interferences6.1 Particulate MatterParticulate or suspended mattershould be removed by centrifugation or membrane filtration ifcomponents of interes
17、t are not altered. This pretreatment willprevent both plugging of syringes and formation of condensa-tion nuclei. Acidification will often facilitate the dissolving ofparticulate matter, but the operator must determine that pHadjustment does not alter the components to be determined.6.2 Identical Re
18、tention TimesWith any given column andoperating conditions, one or more components may elute atidentical retention times. Thus a chromatographic peak is onlypresumptive evidence of a single component. Confirmationrequires analyses with other columns with varying physical andchemical properties, or s
19、pectrometric confirmation of theisolated peak, or both.6.3 AcidificationDetection of certain groups of compo-nents will be enhanced if the sample is made neutral or slightlyacidic. This may minimize the formation of nonvolatile salts incases such as the analysis of volatile organic acids and basesan
20、d certain chlorophenols.6.4 GhostingGhosting is evidenced by an interferencepeak that occurs at the same time as that for a component froma previous analysis but usually with less intensity. Ghostingoccurs because of organic holdup in the injection port. Re-peated Type I water washing with 5-L injec
21、tions betweensample runs will usually eliminate ghosting problems. Thebaseline is checked at maximum sensitivity to assure that theinterference has been eliminated. In addition to water injec-tions, increasing the injection port temperature for a period oftime will often facilitate the elimination o
22、f ghosting problems.6.4.1 Delayed ElutionHighly polar or high boiling com-ponents may unpredictably elute several chromatograms laterand therefore act as an interference. This is particularly truewith complex industrial waste samples. A combination ofrepeated water injections and elevated column tem
23、perature willeliminate this problem. Back flush valves should be used if thisproblem is encountered often.7. Apparatus7.1 Gas System:7.1.1 Gas RegulatorsHigh-quality pressure regulatorsshould be used to ensure a steady flow of gas to the instrument.If temperature programming is used, differential fl
24、ow control-lers should be installed in the carrier gas line to prevent adecrease in flow as the pressure drop across the columnincreases due to the increasing temperature. An unsteady flowwill create an unstable baseline.7.1.2 Gas Transport TubingNew tubing should be washedwith a detergent solution,
25、 rinsed with Type I cold water, andsolvent rinsed to remove residual organic preservatives orlubricants. Ethanol is an effective solvent. The tubing is thendried by flushing with nitrogen. Drying can be accelerated byinstalling the tubing in a gas chromatograph (GC) oven andflowing nitrogen or other
26、 inert gas through it, while heating theoven to 50C.7.1.3 Gas LeaksThe gas system should be pressurechecked daily for leaks.To check for leaks, shut off the detectorand pressurize the gas system to approximately 103 kPa (15psi) above the normal operating pressure. Then shut off thetank valve and obs
27、erve the level of the pressure gauge. If thepreset pressure holds for 10 min, the system can be consideredleak-free. If the pressure drops, a leak is indicated and shouldbe located and eliminated before proceeding further. A soapsolution may be used for determining the source of leaks, butcare must
28、be exercised to avoid getting the solution inside thetubing or instrument since it will cause a long lasting, serioussource of interference. Leaks may also occur between theinstrument gas inlet valve and flame tip. This may be checkedby removing the flame tip, replacing it with a closed fitting andr
29、echecking for pressure stability as previously noted.7.1.4 Gas FlowThe gas flow can be determined with abubble flow meter. A micro-rotameter in the gas inlet line isalso helpful. It should be recalibrated after each readjustmentof the gas operating pressure.7.2 Injection PortThe injection port usual
30、ly is insulatedfrom the chromatographic oven and equipped with a separateheater that will maintain a constant temperature. The tempera-ture of the injection port should be adjusted to approximately50C above the highest boiling sample component. This willhelp minimize the elution time, as well as red
31、uce peak tailing.Should thermal decomposition of components be a problem,the injection port temperature should be reduced appropriately.Cleanliness of the injection port in some cases can be main-tained at a tolerable level by periodically raising the tempera-ture 25C above the normal operating leve
32、l. Use of disposableglass inserts or periodic cleaning with chromic acid can bepracticed with some designs. When using samples larger than5 L, blowback into the carrier gas supply should be preventedthrough use of a preheated capillary or other special design.When using 3.175-mm (0.125-in.) columns,
33、 samples largerthan 5 L may extinguish the flame depending on columnlength, carrier gas flow, and injection temperature.7.2.1 SeptumOrganics eluting from the septum in theinjection port have been found to be a source of an unsteadybaseline when operating at high sensitivity. Septa should beprecondit
34、ioned. Insertion of a new septum in the injection portat the end of the day for heating overnight will usuallyeliminate these residuals. A separate oven operating at atemperature similar to that of the injection port can also be usedto process the septa. The septa should be changed at least oncea da
35、y to minimize gas leaks and sample blowback. Septa withD 2908 91 (2005)2TFE-fluorocarbon backings minimize organic bleeding and canbe used safely for longer periods.7.2.2 On-Column InjectionWhile injection into the heatedchamber for flash vaporization is the most common injectionset-up, some analyse
36、s (for example, organic acids) are betterperformed with on-column injection to reduce ghosting andpeak tailing and to prevent decomposition of thermally degrad-able compounds. This capability should be built into theinjection system. When using on-column injection a shortercolumn life may occur due
37、to solid build up in the injection endof the column.7.3 Column OvenThe column ovens usually are insulatedseparately from the injection port and the detector. The ovenshould be equipped with a proportional heater and a squirrel-cage blower to assure maximum temperature reproducibilityand uniformity t
38、hroughout the oven. Reproducibility of oventemperature should be within 0.5C.7.3.1 Temperature ProgrammingTemperature program-ming is desirable when the analysis involves the resolution oforganics with widely varying boiling points. The column ovenshould be equipped with temperature programming be-t
39、ween 15 and 350C (or range of the method) with select-ability of several programming rates between 1 and 20/minprovided. The actual column temperature will lag somewhatbehind the oven temperature at the faster programming rates.Baseline drift will often occur because of increased highertemperatures
40、experienced during temperature programming.This depends on the stability of the substrate and operatingtemperature range. Temperatures that approach the maximumlimit of the liquid phase limit the operating range. Utilizationof dual matching columns and a differential electrometer canminimize the eff
41、ect of drift; however, the drift is reproducibleand does not interfere with the analysis in most cases.7.4 DetectorThe combination of high sensitivity and awide linear range makes the flame ionization detector (FID) theusual choice in trace aqueous analysis. The flame ionizationdetector is relativel
42、y insensitive to water vapor and to moderatetemperature changes if other operating parameters remainunchanged. If temperature programming is used, the detectorshould be isolated from the oven and heated separately toensure uniform detector temperature. The detector temperatureshould be set near the
43、upper limit of the programmed tempera-ture to prevent condensation. The detector should also beshielded from air currents which could affect the burningcharacteristics of the flame. Sporadic spiking in the baselineindicates detector contamination; cleaning, preferably withdiluted hydrochloric acid (
44、HCl, 5 + 95), and an ultrasonic washwith water is necessary. Chromic acid also can be used ifextreme care is taken to keep exposure times short and iffollowed by thorough rinsing. Baseline noise may also becaused by dirty or corroded electrical contacts at switches dueto high impedance feedback.7.5
45、RecorderA strip-chart recorder is recommended toobtain a permanent chromatogram. Chart speeds should beadjustable between 15 and 90 in./h.7.6 Power SupplyA 105 to 125-V, a-c source of 60-Hzfrequency supplying 20-A service is required as a main powersupply for most gas chromatographic systems. If vol
46、tagefluctuations affect baseline stability, a voltage regulating trans-former may be required in addition to the one incorporatedwithin the chromatographic instrument.8. Reagents and Materials8.1 Purity of ReagentsReagent grade chemicals shall beused in all instances for gas purification, sample sta
47、bilization,and other applications. Unless otherwise indicated, it is in-tended that all reagents shall conform to the specifications ofthe Committee onAnalytical Reagents of theAmerican Chemi-cal Society, where such specifications are available.5Othergrades may be used, provided it is first ascertai
48、ned that thereagent is of sufficiently high purity to permit its use withoutlessening the accuracy of the determination.8.1.1 All chemicals used for internal standards shall be ofhighest known purity.8.2 Purity of WaterUnless otherwise indicated, referencesto water shall be understood to mean reagen
49、t water conformingto Type I of Specification D 1193.8.3 Carrier Gas SystemOnly gases of the highest purityobtainable should be used in a chromatographic system desig-nated for trace-organic monitoring in water. The commoncarrier gases used with a flame ionization detector (FID) arehelium and nitrogen. Trace contaminants in even the highestpurity gases can often affect baseline stability and introducenoise. Absorption columns of molecular sieves (14 by 30-mesh) and anhydrous calcium sulfate (CaSO4, 8 mesh) inseries between the gas supply tank and t
