Use of Laboratory-Supplied Natural Gas in Breakthrough G. A. Eiceman and B. Davani New Mexico State University, Las Cruces, NM 88003 J. Gardea-Torresdeyl Autonomous University of Chihuahua. Chihuahua, Mexico Adsorption traps for atmospheric monitoring of human exposure to toxic, volatile, organic compounds have become widely accepted in the practice of industrial hygiene (1-3). Atmospheric sample is drawn through a trap containing an adsorbent such as charcoal, silica gel, alumina or a synthetic macroporous resin, and organic compounds are retained in the trap while common atmospheric gases are largely unretained. Advanced designs on this principle have been developed for specialized environmental projects including analyses of incinerator stack emissions ( 4 ) , water analyses (5), and blood analvsis for xenobiotics (6). .. One major characteristic of adsorption traps is finite capacity for retention of particular components during collection of sample. Movement of a component completely through an adsorbent trap is called breakthrough and occurs for components when sufficient volume of gaseous sample (the breakthrough volume) is drawn through the trap. Different components will have different hreakthrough volumes if retention characteristics on the adsorhent are simificantlv different. Failure to account for breakthrough in adsorption traps, regardless of accuracy of final instrumental measurements oftrap contents, wililead to serious quantitative errors. However, preparation of gaseous samples with sufficiently realistic complex composition is tedious and expensive for demonstration of characteristics of hreakthrough . pro. cesses. In-house supplied natural gas from lahorntory gas j1:~5 has been fuund satisfactor). for such experimenu as describtd below. Natural gas which is recovered from petroleum deposits is not a uure substance but a comulex mixture of CTto C Aalkanes, nitrogen, carbon dioxide, water, helium, and hydrogen sulfide (7). In addition. minor comuonents includine,. Cs.. to CI,+ hgdn)o~rbonsand aromatic hvdr(*.ilrbuns have been L'RS is exten~ivel\' found in naturnl -aas .tR-.91. . Althoueh na~urnl " treated and refined to remove these components which might liauifv in transmission lines. presence of such comnounds in effluent from transmission iiies is evidence that freatment is incom~lete(10). Thus, the uresence of Ca and lareer aromatic a i d aliphatic hydrocarbons at low c o n c e n t r a h s in natural aas should not he sururisina. If well characterized. natural gas as supplied directiy to laboratories could he & inexpensive and convenient source of gaseous samples for measurement or drmunstration in hrenkthrough studies. Breakthrough processes on adsorl~ent11ds in sampling traps have been treated extensively in gas chromatogriph; as frontal analysis in which sample is introduced continuously into the column (11). Numerous adsorbent materials are available for breakthrough studies but Tenax-GC (poly 2,6diphenyl-p-phenylene oxide) war chosen due tu several favorable properties. These include: I ) n.l,ntivtr low hydroscopic character. 21 wdl-defined and laree tmnkthruueh " \,ulumes for hydrocarbons (121, and 3) wide use in actual sampling procedures (13-16). Since frontal analysis is not widely
practiced or studied, a brief description of theory has been included here.
Theory Breakthrough phenomenon in adsorbent traps where concentrations of components in sample are dilute and constant can be described as linear frontal analysis. Frontal analysis was introduced by Tiselius (17), refined by others (18-21) and discussed relative to conventional gas chromatography . . . bv . Reillev (22). Only a summarv of frontal analvsis for very simple systems is &n here. In adsorbent trap sampling, solute diluted at fixed concentration in a mobile phase is introduced continuously into a trap or column bed. under these conditions linear isotherms may also be assumed (22). The shape of an effluent profile from breakthrough as described in frontal analysis is given as an integral of a step input function (22). A urofile for a multicomuonent mixture containing three compounds is shown in Figure 1. As in regular elution chromatography, retention (or breakthrough) is based on the partition coefficient. For compounds with large values of K, hreakthrough measured as retention time or volume will also be large. Moreover, area under the peak can be related to total eluting mass, and the first derivative of the profde shown in Figure 1may be used to determine individual concentrations of compounds. Results from highly complex mixtures are too complex to interpret using direct analyses of the effluent. However, collection of effluent using auxiliary traps with subsequent analyses of these traps using gas chromatography will provide detailed information on hreakthrouah ofiaiious components in complex mixtures. Experimental Instrumentation A Hewlett-Packard model 5721A gas chromatograph (GC)was equipped with flame ionization detecton WID), 3-m long X 2-mm
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' Present address: New Mexico State University. Las Cruces, NM 88003
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Figure 1. Efflwm profile In frontal analysis using Wee-component mixture.
R E T E N T I O N TIME ( M I N I
Flaure 2. Chmmatwram hom W I D analysis of condensae in natural ws supplisd to l a b d a y . Identification at components is referenced to lhe table using numbers near peaks. Column was a 10-m OV-1 capillary column. Identlty of Compounds In Figure 2 i.d. glass column containing 3% OV-101 on Chromosorh W, and H P mode1 3390A integrating recorder. Conditions for GC analysis were: injector temperature, 250°C; detector temperature, 275%; initial temperature, 60°C; temperature program rate, 4'CImin.; final temoerature. 260°C. A H P model 5995A eas chromatoeranh/mass snec& n e t & (GCIMS) . ~. was used in charnderization nftrao extracts for ccnain naturnl gas samples and was equ~pprdwith p t icparator. rhromatograph~rcolumn as ahovr, and tlprtron impact imuation. Chromatographic conditions for GCIMS analyses were the same as used in GC analyses. Mass spectrometry conditions for scanning analysis were: upper m a s limrt. GO0 am"; lower masi limit, 4.'1 amu: wan rare 6611 m u 9 ; and rlerrron multiplier volmgr, 121X1 V. Sample wlumrs in (X!and C.C/MS analws were 2.0 to 2.5 ul.delivered urinr ~
~~
~~~
~
~~~~~
~
~
~
~~
Procedures Adsorbent traps containing 250 mg of 100-120 mesh Tenan-GC (Applied Science, Bellefonte, PA) were prepared using 8-cm long X l-cm i.d. horosilicate tubes. Packina was retained using glass wwl pluga. Prior to every experiment, each trap was washed using 100 mL of distilled-in-glass-gradeacetone (Burduck and Jackson Laboratories. Muskeeon. .. . MI\ and dried a t 150°C for 1to 2 h. Simpling was accmnplichrd by plnr~ngrhree traps in series (runnrckd hy L.mm lengths id polyethylene tubing exhaustively washed in nL.etone nnd own dnedl. Traps uere hutrrd tightly toune another to reduce exposure uf gns to the tulmg. The trap assembly was .it. tachrd magas jet in a fume hood w d gas wa- ndjusted 10375 ml. min. and samplrs wcrecollt.ctrd continuc,usly for prriudr of 1. :3,6.12, 18. and 24 h:After the sampling step, traps were separated and extracted individuallv,usine.. 50 mL of acetone which was delivered to the trao using2 hlllton Hoy mmipump IRrrr I.auderdalt, PI.,. Kxtracb were condensed tu I ml. ,wing a m t q waporalor @trch~~ Hrillckmann Co., New York) and further evaporated using Nz gas to 0.1 mL. Each extract was analyzed separately by GC. A procedure blank was used to insure no contamination from solvents or glassware. Additionally, gas was pmsed through a glass condenser trap cooled to dry icelacetone temperatures and extraded for GC analysis to measure possible contamination or reactions from Tenax-GC adsorbent material or from tubing usrd t t r ronnrrt trap5 No ron~aminnrmndue tu thew or orher intsrt'crences war, drtcctrd. ~
~
Retention Time Peak No. (in Fig. 2) (mi")
Vapor Pressure
at 2 5 T (mmHg)
1 2
0.286
3
...
7.62 8.08 9.18 9.46 10.58 11.98 12.40 12.74 13.12 13.70 15.16
10 11
15.76 16.36 16.76 17.86 18.32 18.70 19.78 22.66 25.42 28.08
12 13 14 15 16 17 18 19 20 21
4 5 6 7 8 9
.. .
*C13~28 biphenyl ethyl-naphthalene dimelhyl-naphthalene ethyl-naphthalene *C~rHso (methylbiphenyl+ alkylbenzena) isopropyi-naphthalene dimethyl-biphenyl isomer *Ct~H32
dimethyl-biphenyl isomer dimethyl-biphenyl isomer buiyl-naphthalene Kdh
*CnHza *C18Ha8
*ClsHm
Results and Discussion Organic Compounds in Natural Gas Results from GC analysis of condensed washings of the cold t r a p a r e shown in Figure 2 a n d included 80 t o 100 organic compounds. These components were present originally in laboratory-supplied natural gas a n d were condensed during passage of natural gas through t h e cold trap. Solvent a n d procedure blanks were free of detectable levels of such compounds. Total concentration of this mixture in natural gas a t standard temperature a n d pressure was estimated a t 100 mg/m3 n o t corrected for efficiency of collection a n d sample preparation. T h u s this value may b e slightly lower than actual
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Fiqure 3. Bar plats of GC analyses of adswbent traps in breakthrouqhstudies. Letters are for hap systems in which sampling times were: A, 24 h; B, 18 h; C. 12 h: ~ , - 6h: and € , 3 h, at gas flowmte of 375 mL1min.
concentrations. Retention indices for these components calibrated using Clz to Cg4 n-hydrocarhons showed most compounds with carbon numbers of Clo to C19. While large molecular weight compounds have been reported in natural gas, C I ~ compounds + have never been reported in natural gas in consumer distribution lines. Identities of major compounds shown in Figure 2 are given in the table. Of the 25 major components detected, most were aliphatic hydrocarbons with a few aromatic hydrocarhons. However, detailed analysis not described here also showed very complexmixture of aromatic and polycyclic aromatic hydrocarbons present as trace constituents. Althoueh - results are expected to varv in interlaboratorv comparisons as a function of gas composition, the presence of such compounds in aas obtained from 15 ~rocessineplants throughoutthe U.S. (8) was sufficient evidence to propose further study of natural gas as a source of complex mixtures for undergraduate laboratories a t advanced levels. For example, natural gas might he used for breakthrough studies (as shown below), preparation of samples for analytical chemistry in instrumental analysis, biochemistry testing for mutagenic activity, experiments in distillation phenomenon, and other experiments where an inexpensive, easily available, and realistic complex mixture may be used to supplement simple mixtures of pure standards. Breakthrough Studies Using Natural Gas Results from breakthroueh studies in which lahoratorvsupplied natural gas was through three solid adsorb& tram in series are shown in Fipure 3 as bar olots. In a bar d .o t ., chromatograms are reduced in form to allow convenient comparison of several complex chromatograms (23). Since trap 1was the first trap in series, the presence of a compound in trap 2 a t anv period of time (3 to 24 h) is evidence of breakthrough f i r t h a t compound on trap l.'similarly, presence of com~oundson trap 3 means breakthroueh (of onlv those compounds) on t r a p 2. In Figure 3, sever2 trends ih breakthrough can be observed and employed to explain principles of breakthrough. For example, in a gas-solid ad-
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Journal of Chemical Education
sorhent system for molecules of similar chemical characteristics, hreakthrough is inversely proportional to vapor pressure as a first approximation. Since a similar statement also can he made regarding gas chromatography (for nonpolar solutes with nonpolar liquid phases), first components to hreakthrough traps should also be early eluting components (i.e., compounds with high vapor pressure) in GC analysis. This pattern can he seen in Figure 3 if the composition of trap 2 is inspected as a function of time (or volume) of gas passed through the trap. At 3 h (E) only a few early-eluting components are seen in GC plots. However, larger molecular weight compounds (i.e., lower vapor pressure, and thus later-eluting GC oeaks) are seen at increasindv lareer concentrations with increasing volume (D to A) whi& L e a i s hreakthrough on trap 1.Similar patterns mav also be seen in trao 3 and regarded as breakthrobgh on trap%. Although hreakthrough processes are reasonably apparent in GC plots which were normalized to reduce influence of absolute concentration on visual recognition of patterns, auother method to present results for illustration of hreakthrough is a plot of concentration of a component on trap 2 versus volume of gas passed as shown in Figure 4. Several compounds with a ranee of molecular weiahts (or vapor pressure) were selected from the GC results f i r presentation. Results follow profiles for classical frontal analvsis within experimental error and demonstrate clearly the relationship between hreakthrough volume and vapor pressure. No attempts were made to use results here for precise measurements and none are possible using these data. However, principles for use of natural gas have been established and natural gas has been satisfactory for these studies. A better kxperimental design for collection of precise hreakthrough data would involve continuous monitoring of rfflt~rntcompwltion using detectors. While such des~gnshave heen successiul in frontal nnalyiis ( ; C whrrr components arc to use FID and Dresent in > I r b tv/\.l c~~ncenrrations.attemnts electron capturkdktectors (for ardmatic compounds) connected directly to traps were unsuccessful. With each detector, large amounts of methane were thought responsible for low
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organic compounds in industrial hygiene. Both concentration of individual components and complexity of condensate were acceptable for resolution with packed columns and detection with flame ionization detectors. Breakthroueh volumes for individual components were governed in large part by vapor pressures of these components as expected with a nonpolar solid adsorbent and nonpolar solutes. Acknowledgment
The research on which this report is based was financed in part by the US. Department of the Interior, as authorized by the Water Research and Develo~mentact of 1978 (P.L. 95-
Figore 4. Plots of peak area v e r s s sampling tlme (h) for components found in trap 2 Peak areas were taken from GC analyses of lrap extracts as shown in Flgure 3. Components were: A, low molecular weight compound passlbly dcdecana; 8, hexadecane; C, octadeeane: and D, nonadecane,
sensitivity and poor performance generally. However, use of flow-through cells in certain optical spectroscopic methods may be promising for continuous detection of trap effluent. Summary
Natural eas which is ~ r o v i d e dthrough laboratorv aaslines was found & contain organic compounds with carbon &nbers between 10 and 19 in addition to the commonly known methane-to-heptane mixture. These compounds were present a t total concentrations of about 100 mg/m3 and were easily obtained by passing natural gas directly through glass cold traps a t dry icelacetone temperatures. Components were resolved using gas chromatography and identified using gas cbromatography/mass spectrometry. Major components included n-alkanes, branched alkanes, alkylated benzenes, and several atomic hydrocarbons such as substituted naphthalenes and biphenyls. Natural gas was found to be a satisfactory source of small amounts of these compounds, diluted in methane, for demonstration of breakthrough characteristics of solid adsorbent (Tenax-GC) traps. Such traps are presently in use widely for monitoring atmospheric exposure to toxic
Contents of this publication do not necessarily reflect the views and policies of the U S . Department of the Interior nor does mention of trade names or commercial products constitute their endorsement or recommendation for use by the US. government. Literature Cited (11 Marehello, J. M., and Kelley, J. J., "Gas Cleaning for Air Quality Contml, lndustrisl and Environmental Health and Sahty Requirement: Chemical Pmcpsning and Engineering. An International Series, 1915. vol. 2, pp. 4 M 3 . (2) Nanhebel, G.. "Praeoaes for Air Pollution ContmL"CRCPresn.The ChemicalRubber co.. Ohio, 1972, p. 77. (3) Cherrmisinoff, P-N., and Eliubusch, F., "Carbon Adborption HandbooY Ann Arbor Science Publisher, Inc., Ann Arbar, MI, 1918, p. 131. (4) Parson,J. S., and Mitzncr,S., Enuiron. Sci. Tech.. 9.1053 (1915). (5) Dovty, R. J., Carlisle, D. R., and L-ter, J. L.,Enuiron. Sei. Tech., 9,762 (1975). (6) Zletkis, A., Lichtenstein, H. A , and Tishhee,A., Chromofogrophia, 6.67 (1913). (7) Kstz, D. L..Cornell. D., Kobaymhi, R.,Poeftmann, F. H..vsry, J. A..Elenbaaa, J. R.,and Weinaug. C. F.,"Handhkaf NsLural Gas Engineering? McCrsw-Hill Bmk Co.. New York, 1959, pp. 46H2.L. (8) Katz, D.L., and Bergman, D. F., "Use of Extended Gm Analyaea in Daaign of Proeem Association, Piants? Proceding8 of the 55th Annual Convention of Gas P-rs 1916.pp. 1&26. (9) Huber. L., J C h m m t o g r S c i , 21.519 (1983). (10) Eiceman, G. A,. Baker, B. D., and Lemure, C. S.,Internot. J. Emuiron. Anol. Chem,
(15) (16) (17) (18) (19) (201
Renberg, L., Anol. Chem, 50,1836 (1978). Elim, L., MeCweyc, M.,and Gardner, G., Geophys. Re8 L P ~ L . .17 ~ (1976) , Tissli%A.,Arkiv Kern;, Minerd. Gea,l4B.5(1940l. Clse-n, S.,Akn'u. Kemi.Minoro1 G e o , 23A, 133 (1946). Glueckauf. E., Nature (Landon). 156.146 (1954). 11965) Kridee. - .G. J.. and Pretorius. V...Anol. Chsm... 37.1186 , . . P.. S. A . ~ i o d ~J chromotogr. ~ i ~ ~ 210,313 , (1961). (22) Reilly,C.N.,Hilderando,G.P.,andAshley,J. W., Jr., Anol. Cham.,34,1198U862).
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