Document odzEwadw2MMO3zdkO098QqGD

Am. Ind Hyg Assoc J 43(8):S69-574 (1982) A simple method for the determination of desorption efficiencies of organic compounds collected on the 3M 3500 Organic Vapor Monitor has been devel ped. The technique involves the introduction of the organic compound, in the liquid state, onto a piece of filter paper placed between the elutriation cap and the diffusion plate of the organic vapor monitor. This is accomplished by direct injection of the organic compound through the center elutriation port of the monitor cap. The port is closed and the organic compound is given sufficient time to vaporize and consequently be adsorbed by the charcoal sorbent. The filter paper is removed and analyzed as a separate sample to assure complete transfer of the organic compound. All samples are desorbed by a suitable s lv ntand analyzed by gas chromatography. Desorption efficiencies for methyl chloroform, benzene, heptane, acetone, dioxane, isobutanol and ethyl acetate are determined. A comparison with results by the phase equilibrium method is made. Determination of desorption efficiencies in the 3M 3500 Organic Vapor Monitor S.T. RODRIGUEZ, M S , D W. GOSSEUNK, Ph D, and H E MULLINS Occupational Health & Safety Products Division, 3M Company, St Paul, Minnesota 55144 introduction For a quantitative determination of the amount of contami The ratio in which the different classes of pores are present nant collected on any sorbent, it is necessary to know how in a specific type of activated charcoal dictates its suitability much of the adsorbate is eluted out. The ratio of the mea for specific purposes. Micropomus activated carbons are sured weight recovered by the desorbing solvent to the most effective lor adsorption of small concentrations of known weight of contaminant added to the adsorbent is defined as the desorption efficiency or recovery coefficient One procedure for determining recovery coefficients from charcoal tubes involves direct injection of a known amount of the liquid compound into the activated charcoal using a microlitcr syringe After overnight standing, the sample is eluted with an appropriate solvent and analyzed by gas chromatography. Especially for polar compounds, the results from this method are often inconsistent.,J| In addition, direct spiking of liquid does not correspond to actual field sam pling of vapor from the atmosphere. Liquid spiking may be a source of error, since the compound is localized in a small area of the sorbent rather than being uniformly distributed throughout the sorbent. gases and vapors On the other hand, mesoporous carbons are better used to recover industrial solvent vapors. The pore size distribution determines the adsorption desorption pro cesses involved and also the resulting efficiencies Micropores may be suitable lor efficient adsorption ol vapors but consequent desorption from them is dif f icult since there may be no room for a displacing agent to enter Consider a model picture ol the microcrystallites of acti vated charcoal 111 During a v apor spike, schematically shown in Figure la. we can imagine the molecules first entering the macropores, then the mesopores and finally the micropores where some interactions with the walls tend to increase retention and decrease recovery by a solvent. However, a liquid spike would be concentrated in spots and tend to wet the walls of the mesopores. Imagine now that we have In activated charcoal sorbent, the spaces, called pores, menisci as in Figure I b which exhibit some surface tension between the individual microcrystallites may be further clas It has been verified experimentally that the vapor pressure of sified as. solvents in capillaries of a few microns radius arc lowered 10-80 times more than what the Kclv m Equation predicts/6' a) micropores, with effective radii of less than 18-20 The Kelvin Equation relates the normal vapor pressure of a Angstroms, comprising a minimum of 959f of the liquid (assuming a flat surface) to that observed over the total surface area; curved surface of the liquid given its spherical surface radius. b) mesopores, also called transitional pores, where capillary condensation takes place; with effective radii of roughly 20-1000 Angstroms which make up not more than 5% of the total surface area; and A decrease in vapor pressure makes it harder for the liquid to vaporize and occupy the rmcropores The liquid would tend to stay in the mesopores where wall effects are small and the energy of adsorption is less. Consequent elution would then result in higher recoveries c) macropores with effective radii of more than 1000 Angstroms which are too big to be filled by capillary condensation/3' With these possible deficiencies of liquid spiking in mind, the vapor-state spiking technique for the 3M 3500 Organic Vapor Monitor was developed CopytiQhi 1982 American Industrial Hygiene Allocation American Industrial Hygiene Association JOURNAL (43)8/82 3H 101491 569 Figure la -- Concept of moleculesin activated charcoal pores Figure It. Membo formation ov molecules i it activated c ha i coal pore; experimental gas chromatography A Hewlett-Packard 5K40A gas chromatograph equipped with a I la me mm/at ion detect ot and an automat te samp let, Model "6~2A win used I lie columns wcie I 5', Caibowax 20M v'n Ml 100 mesh Supeleoport. I H m ' h 4 ntni o d, class (6 It 14 in ) 15'i Carbowux 20M on HO 100 mesh Chromosorh W. 2 3 m X 3.2 mm o d. stainless steel (1 I 2 It XI Sin I 10', SP-2 100 on HO 100 mesh Supeleoport. 3 0 m v t,2miiiod sta in less steel ( 10 11. X I H in |. 0 I', S I'-1000 on HO 100 mesh C urhopaek C. I H m ' 3 2 mm o.d stainless steel (6 It s | sin) and Chromosorh 101. 100 120 mesh. 0 4 m v h 4 mm o d glass (3 It, X I 4 in I Samples to he analv/ed were contained in I ml glass \ nils sealed with leflon-lincd septum caps procedure hoi the vapor-state spiking technique, Whatman 4 lillet paper. 1.6 cm in diameter, was placed between the eltitnution cap and the diffusion plate in each 3M 3500 Organic Vapor Monitor The elutriation caps were snapped on and a known quantity ot the organic solvent under study was iniected onto the paper through the center elutriation port 1 he port was dosed and the system was allowed to stand tit room temperature tor 16-24 hours betore elution to give sufficient time I or total transfer of the organic compound trom the liltct paper to the sorbent. 570 (jaseoiis compound', i < , ctItv I i hloi nk wen studled bv nisei i mg a t uhhei septum ini o t he i. cut c cl n mu Mon poi: a ml miect mg t he k now n quantitv ol uus i hi ouch i lie sept tint v\ it h a g.istiglv wiiiiis 111 till' cusi tin liltei par ; wu' elmn ii.it ed 1 lie s\ 'tem w as a llnw ed t o si a nd .u i oom l emivi a 1111 e ov el night pi mi lo elution \nothei a ll el mil i \ e loi gas spik.' w.n lo s.o.1 the 'N1 1501) f )t lmiiic 3 a poi Momtoi with a e v Imd i n a 1 g la ss dome cq m pped with an mice t ion poi t and a I et Ion-I med scpi um, as in I mill'. 2 lo insine ae.misl leakae I'aiatilili" was wiapperi atoiiml the class at contact iViii1- with the illonitoi I he k now n a mount ol gas w as t hen miei t ed t hi ough i lie septum nno the an above ilie momtoi Mtei standing 16-24 ho in s t he dome ha 11 lei t dm and t he ba 11 ie: 111m holdet ol the momioi weie tcnioved 1 lie elmi iai ion L.ip was then snapped into pl.ue and both polls doM d A ml'.is111ed v olume ( 1 ' ml ) ol cun m was added in each momtoi through the centet pint using an automatic dis pense! 1 he port was icscu led and t he su tuple vv as a I low ed t o clutiialc toi one-hall horn with oli.isioii.iI ucntle agita tion Altet decanting the vluatc liom the momtoi into glass vials, anaivsis bv gas ditomntogtaphv was peiloimul on cadi sample I he liltei pupci til each sample was sepa rate! v analvsetl Standards vveie ptcpaicd bv spiking an evpiiv alem amount of the organic compound into sealed glass vials containing I 3 nil ol tile uppi uptime eluent using the same miection 3M 101492 v... riugc The punctured seals wete replaced bv neve ones , :k, the standaidx prepared were kept in the refrigerator at 5 ~r Before analysis, thev were allowed to come to rn-.rr temperature ' parallel expel imeni using the Phase bqudihrium method tvr. simuliatieousiv Working solutions ol organic compounds diluted with the appropnate solvent were pre pared T ne charcoal sorbent in some monitors was removed i I c. mi ol the working solution were added through the "r:ei p.>r ol the eltnriatton cap These were used as the incut,d' An equal aliquot was added to another set ol "'opiti a s inch still contained the charcoal sorbent o-un sets ol monitors lot each organic compound under 1 r' -riLvtmn were allowed to elutriate tor one-half hour at room temperature, decanted into glass vials, sealed, and analyzed by gas chromatograph;. results and discussion The sampling rates of the 3M 3500 Organic Vapor Monitor for acetone, benzene, dtoxane. cthv I acetate, ethyl chloride, heptane, isobutanol, methvl chlorotorm and methvl ethvl ketone are listed in Table 1 The Threshold Limn \alue Time-Weighted-Average (TT V-1 W At concentration lor each compound was taken trom the AC(ilH handbook' T he amount spiked was the mimbei ol milligrams expected in a monitor which samples a concentration ot one-hall II V-T W A lor lout hotiis except when n was bevond the capacitv of the monitot Pot those cases '5', o I the captisitv was used Considering ben/ene w till a sampling rats ot is 5 cc mm and a TI V-TWA ol 30 mg m'. we expect 0 I mg adsothed bv the monitor at one-half T 1 V -1 \V A I ot lour hours (mg 35,5 cc mm >1 60 min hi V 4 his 1(1 mg m' ' I 3 | m' 10h ec) A comparison ol desorption efficiencies obtained hv the vapor-state spike and bv the Phase 1 quilibimm method i-shown in Table II The icsult' ol live dcternimations pci experiment ate given with standard deviations Theic p close agreement between the two methods except loi tlie results ol tsobutanol and elhv I chloride Both ol these were eluted with methvlene chloride while the rest wcic desorbed with carbon disulfide T able 111 lists some phvsteal constants toi niethxlenc chlo ride. isobntanol and ethvl chlotwk When twii atoms with different elccttonegaiiv ittes ate bonded togethei. the honding elections spend a greatet time neat the atom ol litghei elecironegativ itv. so that there is a net positive charge at the othei atom and the molecule takes on a point ehatactei T lie dielectric constant and dipole moment ate measures of polarity. the highet the value, the mote point is the com pound The measured diffusion coclficients lor methvlene chloride and tsobutanol are listed while that tot ethvl clilotide is a theoretical value horn the Htrschlelder Fquation TABLE I Compound Guide for Determination of Desorption Efficiencies in the 3M 3500 Organic Vapor Monitor Compound OVM Sampling____________ _____ Rate cc/min PPM mg/m Spiked Amount* mg Acetone Benzene Dioxane Ethyl Acetate Ethyl Chloride Heptane tsobutanol Methyl Chloroform Methyl Ethyl Ketone 40 1 35 5 34 5 34 5 38 8 28 9 35 9 30 9 36 3 (1000) 10 (50) 400 1000 400 50 350 200 (2400) 30 (180) 1400 2600 1600 150 1900 590 22 01 08 58 02 56 06 70 26 Amount sampled by the monitor at a Concentration of 1 /2 TLV-TWA tor four hours except when it is beyond the capacity ot the monitor For those cases, the amount spiked is 75% of the capacity A^uesn industrial Hygiene Association JOURNAt m S'B2 3M 101493 571 The molecular radius of the compounds were predicted from atomic volumes.'10' From our concept of the activated char coal surface with the different pore structures, we can imagine that with the presence of both methylene chloride and isobutanol in the Phase Equilibrium method, there results a competition between the two compounds as they trasel into the pores. The greater mobility of the smaller molecule, in this case, methylene chloride, also results in faster diffusion rmo the microporcs Another contributing tact or is the higher concentration ot methylene chloride, present as the solvent, which allows for more collision and interaction with the surface than the isobutanol, permitting deep penetration into the finer pores 1 here exists, therefore, a higher concentration of isobutanol m the mesopores which hate lower energies of adsorption L pon decantation and analysis of the resulting solution, high isobutanol recoseries are obtained I'sing sapor-state spiking, there is no competitor with isobutanol for the active sites on the charcoal surface, so the molecules are free to seek even the fine pores which hate higher energies of adsorption We hate obserted that a 30 minute interval was sufficient to transfer all the spiked isobutanol from the filler paper to the charcoal for an experiment, seteral monitors were spiked with isobutano! and allowed to stand for 0.5. 1.0. 4 0 and 24 hours prior to elution and analysis Eten at 30 minutes, no trace of isohutanol was found on the filter paper. Table IV shows that a longer adsorption time cat cause lower recoseries for isohutanol More molecules are adsorbed in the microporcs. thus desorption is difficult. Since analysis of samples does mu immediately follow the field sampling, it is therefore reeom mended that the method used for determination of desoip non efficiencies be designed to duplicate actual use situa tions as closely as possible The effect of time was also studied for the Phase Equili brium method using ethyl chloride Aliquots of a prepared solution ot ethyl chloride in methylene chloride were added to several organic sapor monitors and were analyzed at different time intervals, the results in Table V show a change in the adsorption desorption process within 24 hours, giving an aserage desorption efficiency of 1.02 0.01 TABLE II Comparison of Dasorpti n Efficiencies Desorption Efficiencies Compound Vapor Spike Phase Equilibrium Acetone Benzene Dioxane Ethyl Acetate Ethyl Chloride Heptane Isohutanol Methyl Chloroform 0 93 * 0,03 0 99 =: 0.01 0 92 r 0 03 098 : 003 0 81 - 0 03 1 04 : 0 Ofo 90 z c 02 1 02 z 0 02 0 94 * 0 01 0 99 - 001 0 88 r 0 02 0 98 - 0 O' 1 01 z 0 01 1 03 - 0 O' 1 04 - 0 01 1 02 - 0 01 These data aiso demonstrated that the 'M 3500 Organic \ apor Monitor does not leak and that the monitor materials are inert both chemically and physically T he ellicient seal of the organic \apot monuoi was fmthei sentied by a test wherein monitors with the elutriation caps on were exposed to a chamber saturated with x\ lene sapors tor tsso hours A control sample without a cap ssas used and this collected about 59 mg ot xylene while the capped ones onls showed trace amounts The results Irom an insesngation of concentration effects on desorption etticieneies bs both sapoi-siatc spiking and Phase Equilibrium methods are summarized in Tahte VI I he concentration' chosen sseie all less than the suggested spiked amount ol 2 h mg MEK 1 5 nil carbondisiillidefsee I able I) I he unpaired t statistical esaluation ot thedata shosved no m enitleant dit let cnees bet ween the desorption ct I iciencies ol . oneentrntion 1 cs els II and 111 lor both methods"1' At a onlidenee lesel ol 95r,. the test statistic t salucs for the ..ipor-state spiking method and loi the Phase Equilibrium method were 0 24 and -- 0.X9. respcctisels. Desorption effi ciency salues at concentration lesel I were significantly ddleicnt tiom the pooled results ol concentration Levels II a nd 111 loi both methods. At loss concentrations, the walls of the poies in the sorbent strongly hold onto the molecules of the compound and the eluent cannot completely1 seset all bonds formed, thus creatinga case ol irresersible adsorption A eompaitson hetween a sapor-state spike and a directly inieeted liquid spike onto the chaieoal soibent ot the 3M TABLE III Physical Constants '" Methylene Chloride Isobutanol Molecular Formula Molecular Weight Boiling Point Dielectric Constant Dipole Moment Diffusion Coefficient Molecular Radius CHjCI, 84 94 39 75 C 1,0065 (gas) 9.08 (liq ) 1 60 debyes 0 1037 crrdzsec 4 754 l C,H,,,0 74 12 108 C 1 5 8 (liq ) 1 64 debyes 0 0680 em'/sec 5 637 Ethyl Chloride CjH .Cl 64 52 12 3 C 1 0132 (gas) 2 05 debyes 0 1134 cm'/sec 4 856 & 572 Ar^ tm1 Hyp 4ssoc J (43) August 1982 3M 101494 TABLE IV Effect of Time on Vapor State Spiking of Isobutanol Adsorption Time (hrs) Desorption Efficiency 05 10 40 24 0 0 97 0 99 0 98 091 TABLE V Effect of Time on Phase Equilibrium Method for Ethyl Chloride Adsorption Tima (hrs) Desorption Efficiency 05 10 20 30 50 60 70 24 0 Average 1 02 1 01 1 01 1 03 1 01 1 02 1 02 1 01 1 02 3 0 01 ^00 Oiganu Vapor Monitor was made and the results ,r listed in I able VII No difference in the desorption etfieie: eics o! Ml K h\ both methods was seen when overmg' advotption was utilized (test statistic t was --0 971 at <t 0 Os) I he ellect ot ditlercnt adsorption time intervals studied Results are given in Table VIII It was noted th.r complete transler ol the compound from the filter papi to the sorbent was accomplished within 30 minutes pno' to elution \o significant change was observed even after 24 hours adsorption time by the vapor-state spike method On the other hand, a slight decrease in desorption efficiency, was observed using the direct liquid spike method summary T he elltciency of elution recovery processes can be affected by many variables, including sample introduction, adsorpti on desorption time, chemical structures and concentration Much of the variability can be explained by differences in the mechanisms ol filling and emptying the pores of specific shape and size. Since the filling of pores of the sorbent in the 3M 3500 Organic Vapor Monitor in an actual field sampling of vapors from the atmosphere runs parallel to that in the vapor-state spiking method, more reliable desorption effi ciencies by this latter method is expected. Although it was shown that no differences were found between the vapor-state spiking method and the direct liquid spiking method for the determination of the MEK desorp tion efficiency, other factors like concentrations, the length of time of adsorption and chemical structure may make American Industrial Hygiene Association JOURNAL M3! 8/82 TABLE VI Effect of Concentration on Desorption Efficiency of MEK Concentration (pg/mU Vapor-State Spiking Technique Phase Equilibrium Technique Level 1 80 54 0 70 0 69 0 69 0 68 0 70 Average 0 69 r 0 01 0 94 087 0 92 0 86 0 85 0 89 : 0 04 Level II 805 4 0 90 0 89 0 90 Average 0 90 * 0 01 0 91 0 95 0 93 0 93 * 0 02 Level III 1610 8 0 87 0 90 0 92 0 89 0 89 Average 0 89 = 0 02 1 02 0 93 0 95 0 93 0 93 0 96 .: 0 04 G C Column Temperature Carrier Gas MEK Retention Time 1 5% Carbowax 20M on 80 100 mesh Supelcoport 18m v 64 mm o d glass (6 ft - 1 /4 in ) Isothermal 60 C Helium 30 3 mL min 1 25 minutes TABLE VII Comparison of Desorption Efficiencies of MEK Vapor Spike Liquid Spike 0 92 0 92 0 89 0 91 0 92 Average 0 91 0,01 Average 0 91 0 92 0 92 0 92 0 92 0 92 0.004 TABLE VIII Effect of Time on Desorption Efficiency of MEK Desorption Efficiency Adsorption Time Vapor State Liquid (hrs) Spike Method Spike Method 05 10 40 24,0 091 0 90 0 92 0.90 0.94 0 92 0,91 0.91 3M 101495 573 greater differences. In our experiment, we used the suggested spiked amount for MEK representing an exposure of onehalf TLV-TWA for four hours. Measuring lower concentra tions in the field may produce 5-29% differences depending on whether the determination of the desorption efficiency of MEK is doen by the vapor-state spike or the Phase Equili brium techniques. The differences seem to be more pro nounced with polar compounds. The results of the two techniques agreed when less polar to nonpolar compounds were studied I he time of adsorption to achieve equilibrium of the j,h.-'bed molecules with the structural atoms of the surface o! ih monent affects the vapor-state spiking method more iha- the Phase Equilibrium technique. II . s jpor-state spiking technique used to determine 3M puh oMcd ulues is therefore recommended for the determin,i:.i r or desorption efficiencies using the 3M 3500 Organic \ jpo Munitor. references ) MOSH Analytical Methods tor Set A Standards Completion Program National Technical Information Service. Springfield, VA H975) 2. Burnett, R.D.: Evaluation of Charcoal Sampling Tubes. Am Ind. Hyg Assoc. J. 37:37-45 (1976) 3. Ponec, V., Z. Knor and S. Corny: Adsorption on Solids p 578, CRC Press, Cleveland. OH (1974) 4 Haaalar. J.W.: Activated Carbon p 247, Chemical Publish ing Co . Inc., New York, N Y (1963) 5 Adsorption Handbook. Activated Carbon Division Catgon Corp,, Pittsburgh, PA 6 Adamson, A.W.: Physical Chemistry ol Surfaces. 2nd Ed p 59, Imerscience Publishers. N Y (1967) 7 Donner. R.A. and R.G- Melcher: Phase Equilibrium Method for Determination of Desorption Efficiencies Am Ind Hyg Assoc J 39 240-246 (1978) 8 Threshold Limit Values lor Chemical Substances and Physi cal Agents in the Workroom Environment with Intended Changes (or 1979. pp 9-31 American Conference of Govern mental Industrial Hygienists, Cincinnati. OH (1979) 9 Chemical Engineers'Handbook JH Perry, ed Fourth Edi tion Section 14-20 McGraw Hill Book Co . N Y (1963) 10 Le Bas: The Molecular Volume ol Liquid Chemical Com pounds Longmans. London (1915) 11 Ryan, T,A., Jr.. B.L. Joiner and 8 F Ryan MINITAB Stu dent Handbook pp 138-140 Duxbury Press. North Scduaie. MA (1976) 12 The Merck Index M Wmdhole, ed 9th Ed Merck and Co Inc . Rahway, N.J (1976) 13 Nelson. G.O.: Controlled Test Atmospheres Ann Arbor Science Publishers Inc . Ann Arbor, Ml (1971) 21 !anuar\, I9K2. Reined k Fcbrujn, Idk? 574 Am Ind Hyy Assoc J143) August 198? 3M 101496