Gas Chromatographic Analysis of C, to C, Hydrocarbons in the Parts per Million Range in Air and in Vaporized Liquid Oxygen C.
J. KULEY
Analyfical laborafory, Dow Chemicul of Cunada, Limited, Sarnia, Ontario, Canada
b A suitable column has been developed for use with a @-rayionization detector for the determination of CSto Cq hydrocarbons and for use with a flame ionization detector for the determination of C1 to C4 hydrocarbons in air and vaporized liquid oxygen. Detectable limits for the @-ray ionization detector are in the order of 0.2 to 0.08 p.p.rn. for CS to C4 hydrocarbons using u 1 -cc. or 1 0-cc. gas sumple. Concentrations as low as 0.004 to 0.07 pap.m. for C1 to Cd hydrocarbons can be detected with a flame ionization detector, using a 10-cc. gas sample. Preparation of a modified activated alumina column for the analysis is described.
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of trace hydrocarbons in air or liquid oxygen is of particular importance to operators of air plants. Since acetylenic and unsaturated hydrocarbons can be tolerated only at very low levels (?'), any analytical method must not only be extremely sensitive to low p.p.m. concentrations but must also differentiate between the individual species present. High sensitivity ionization detectors permit direct analysis of the samples and t h e r e by eliminate any concentration steps (l-S,6). Co!umns packed with silica gel or activated alumina for the analysis of hydrocarbons with temperature programming from 5' to 150' C. (4) do not yield optimum separation. Also, the sample composition may change b e cause of polymerization and isomerization effects at higher temperatures. Isothermal operation of these columns produces unsymmetrical peaks with long retention times. Active alumina modified with some of the commonly known polar liquids (9) can not be utilized because of their relatively high vapor pressures which cause an extremely high background noise signal with ionization detectors and short column life. Other reported techniques used in air pollution study (6, 8) could not be applied because of the nature of the samples. HE DETERMINATION
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ANALYTICAL CHEMISTRY
Figure 1. Apparatus for preparation of standard synthetic blends of hydrocarbons in nitrogen in p.p.m. range EXPERIMENTAL
Equipment. The @-ray ionization instrument used was a Pye Argon Chromatograph, manufactured by W. G . Pye and Co., Cambridge, England. The ionization source is a 0.25-c. tritium detector. The instrument was modified by replacing the glass column with a length of '/rinch stainless steel tubing connected to the ionization cell, the inlet of which was drilled and tapped to '/,-inch NPT and equipped with a Swagelok male connector. A stainless steel '/,-inch 0.d. column is mounted externally and connected to the tubing with Swagelok fittings. The column inlet wm modified by addition of a Swagelok Tee. A Leeds and Northrup Model G 10-mv. recorder w&s used for all measurements. For flame ionization, a Model 1609 F&M Scientific Co. flame ionization detector attachment and a L & N Model H 10 or 5mv. recorder was used.
The samples were injected into both chromatographs with a Hamilton Teflon-t.ippedgas-tight syringe. The activated alumina is Burrell grade Cat. No. 341-35. The 20 M Carbowax waa obtained from F & M Scientific Corp. The gases used for calibration blends were Matheson C.P. grade and Phillips research grade. Column Preparation. The activated alumina was screened and the 60-100 mesh size used. Thirty grams of this alumina was treated for approximately 15 minutes with 2.5 grams of 50% aqueous sodium hydroxide dissolved in 60 ml. of methanol. The methanol-caustic solution was then decanted from the alumina, the alumina wm washed with three 60-ml. portions of methanol and dried at room temperature until the methanol had evaporated. This pretreated alumina was then heated at 350' C. with occasional stirring for a period
of 4 hours. As soon 11s the material had cooled to about 40' C.,2.3 grams of 20 M Carbowax dissolved in 60 ml. of methylene chloride was mixed with the alumina. The meth,ylene chloride waa then evaporated at room temperature with frequent mixing to ensure uniform distribution of the liquid phase. When the methylene chloride had evaporated, the packing was heated a t 160' C. in a porcelain dish for 1.0 hour, with occasional stirring. While this material was still warm, 24 grams was packed into a 54-inch long, '/,-inch o.d., stainless steel column. The column was maintained a t 110' C:. and purged with dry nitrogen for a period of 3 hours before use. This column packing is quite hygroscopic and must be protected from moisture when not in use. The retention times for components with columns prepcaed as described above is reproducide within 15%. Under normal operatting conditions a column showed no apparent deterioration after being in uzie for 5 months a t ambient or 40' C. temperatures. Operating Conditions. PYEARGON CHROMATOGRAPH: solumn temperature, ambient; detector temperature, 7 5 O C.; detector voltage, 2000 volts; argon carrier gas flow, 60 CC. per minute; inlet pressure, 10 p.s.i.g. The argon carrier gas is dried through Linde type 4A Molecular Sieve. FLAME IONIZATION DETECTION: Column temperature, 40" C.; carrier gas, nitrogen; carrier gari flow, 60 cc. per minute; hydrogen Bow, 50 cc. per minute; air flow, 350 cc. per minute. These flows are wllthin the range of maximum sensitivity. All three gases are dried with Molecular Sieve (as above) before use. A 10-cc. sample 0;'air or vaporized liquid oxygen tends to extinguish the flame; however, the ignitor is energized automatically, the Bame is reignited, and a constant base Lie is re-established before the methane peak emerges. CALIBRATION AND CALCULATION
Mixtures containug various hydrocarbons were p r e p a d using the blending system shown in Figure 1. All tubing is '/(-inch 0.d. stainless steel and valves are Hoke type PY 271 K stainless steel. The pressure gauge is a master Test Type 210, Zlb. subdivided, manufactured by Marsh Instrument Corp., Skokie, Ill. The vacuum gaugc? is a Model No. FA 160 Serial No. FF 09761, manufactured by Wallace and Tiernan, Belleville, N. J. The container is a U. S. government suqdus low pressure oxygen cylinder type (2-1, Spec. MIL-G 5890 with an interna,l volume of 2100 cubic inches (34.4L). The calibration standards for the gaseous hydrocarbons were prepared by injecting a known vcllume of pure hydrocarbon from a gas-tight Hamilton syringe into the evacuated cylinder. Hydrocarbon-free, Matheson purified
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Chromatogram of standard blend of p.p.rn. hydrocarbons in nitrogen Pye argon 8-ray detector
grade nitrogen was then added to a pressure to give a desired hydrocarbon concentration, calculated as follows: cc. of Hydrocarbon X lo8 p.p.m. vol. H.C. = 34,400 X Gauge Pressure (p.8.i.) 14.7
The blend was allowed to cool to room temperature before the final reading of the gauge and the blend was used for calibration after 2 hours from the time of preparation, Homogeneity of the mixture was checked by analyzing the blend in a 48-hour period, venting a part of the blend between each analysis and until the pressure of the container was a t atmospheric. The results of the checks were in excellent agreement indicating a homogeneous state. Calibration of Fye Argon Chromatograph. Samples of 10.0, 1.0, and 0.5 cc. were injected. Peak heights, in millimeters, were used to calculate the response in p.p.m. vol. for each hydrocarbon. Detectable limits were deter-
mined relating p.p.m. concentration to peak height in millimeters, allowing 2 mm. for background noise level. It is assumed that the smallest peak that can be detected with assurance is 2 mm. -411 measurements were done a t maximum detector voltage (2000 volts) and attenuation (XI). Table I ilTable l. Detectable Limits of Hydrocarbons by P-Ray Ionization Detector
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Sample cc. cc. 10 cc. Ethane 0.45 0.20" Not detectable Ethylene 0.23 0.10" Not detectable 0.36 0.18 0.02 Propane Propylene 0.44 0.22 0.02 Acetylene 0.52 0.30 0.03 Isobutane 1.05 0.50 0.05 n-Butane 1.25 0.60 0.06 1.28 . . . Butenes 0.06 1,3-Butadiene 2.06 ... 0.04 a Applies t o air samples only or samples containing not more than 2001, oxygen. VOL. 35, NO. 10, SEPTEMBER 1963
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Table
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Sample Ethane Ethylene Propane Propylene Acetylene Isobutane +Butane
Variation of Calibration Factors
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lustrates the detectable limits using various sample sizes. The detector response was linear with concentration for a wide concentration range, but calibration for individual components was necessary. The variation of the calibration factors (p.p.m./mm.) was determined over a period of 2 weeks using four different blends (see Table 11). Calibration Using the Flame Ionization Detector. Calibration blends similar to those above were used except that methane was also added. Figures 2 and 3 show chromatograms of synthetic blends using both instruments. The limits of detection with the flame ionization detector (Table 111) were determined as above. Peak heights were used and were more convenient that area measurement because of the small half width of some of the peaks. The plots of peak heights (in mm.) us. p.p.m. have good linearity for a given sample size and a deviation of about 0.8%. All tests were done using the highest electrometer sensitivity range (XI),andin therangeof 0.1 to200p.p.m. Because of the time required to inject a large sample (5 or 10 cc.) with a syringe, broader and lower height peaks were obtained, than when using a 1-cc. sample. The peak height is about 50% lower for the earlier eluting peaks (CH4, C4H4,C2Hs)and about 15% lower for the later eluting peaks (acetylene, and Cis). Using a 5-mv. recorder, the detectable limits can be increased without loss of base line stability. Table 111. Detectable Limits of Hydrocarbons b y Flame Ionization Detector p.p.m. /l-mm. peak height Sample 0.5 cc. 10 cc. Methane 0,041 0.004
Ethane Ethylene Propane Propylene Acetylene Isobutane n-Butane Butene-I 1,3-Butadiene
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0.038 0,037 0.072 0.11 0.22 0.17 0.20
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ANALYTICAL CHEMISTRY
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