Salvatore R. DlNardl Division of Public Heaith University of Massachusetts Amherst, 01002 and Elaine S. Brlggs General Dynamics Corporation New London. Connecticut
Hydrocarbons in Ambient Air A laboratory experiment
The Federal Ambient Air Quality standard for nonmethane hydrocarhons is 0.2 parts per million (pprn); a 3hour average from 6:OO-900 a.m. which shall not he exceeded more than once per year (I). Non-methane hydrocarbons a t this level pose no health problems; however, as particles or in connection with particulates, they can he important air contaminants and may contain potentially carcinogenic agents ( 2 4 ) . These hydrocarbons are a part of the photochemical air pollution problem because they react with excited oxygen and ozone molecules, and with free radicals generated by the reaction of sunlight on other compounds (5). Photochemical reactions of hydrocarhons are imnortant in the ambient atmosnhere because thev eive rise to secondary air pollutants and reaction intermediates which cause nearly all the harmful effects of hydrocarbon pollution ( 2 4 ) . Some products of photochemical smog reactions are ozone, nitrogen dioxide, formaldehyde, acrolein, and peroxyacetyl nitrate (PAN) (5). I t has been observed that when non-methane hydrocarbon levels are within Federal Ambient Air Quality standards the resulting secondary pollutants will not exceed those levels which are intended to protect the public's health (6). The day of concern for environmental deterioration and the need t o incorporate relevant real world problems into lahoratory courses is here. In this experiment, aspects of air sampling and analysis are applied to the study of amhient hydrocarhon levels which will enhance the student's trace gas analysis laboratory skill and increase hislher understanding of an environmental problem. The student needs onlv a brief introduction to the nrinciples of operation of a-gas chromatograph and the response characteristics of the flame ionization detector (7). The students will perform the calibration of the instrument and then analyze an amhient air sample. The entire experiment is designed for completion in a 4-hour lahoratory period. Students can then perform alternate experiments to assess the effects of various experimental procedures that are critical in trace gas analvses. This ex~erimentcould he successfully performed in chemistry pro&ams or within related disciplines (e.g. Environmental Health, Environmental Engineering, etc.) from the community college level through the first year of graduate school.
Gas Chromatographic Conditions for
Ambient Air
Column:
Hydrocarbons in
1.5 ft X 118 in. 0.d. Stainless steel, packed with 0.5% SE-30 o n Cnromororb W-HP. 801100 mesh (Johns-
Manrvllle Corporation)
Temoerature: Column oven: 220°c Gar Sampling Valve: 150 Detector: 250 30 c m v m i n Air: 450 H ~. ~ d r w e n : 35
Gar Flow: Helium:
Attenuation varies with cancentratton of sample.
Sample S i z e 5 or 10 mi.
--
Apparatus Gas chromatograph equipped with a hydrogen-air flame ionization detector (FID) and a gas sampling valve with a 5- or 10-ml gas sampling loop. One-hundred-microliter gas tight syringe (Hamilton 1710 or equivalent). Vacuum source (Little Giant Pressure/vaeuum pump, Gelman 13152 or house vacuum). Planimeter. Gas sampling bags made of Saran or Mylar (ANSPEC Company, Ann Arbor, Michigan). Calibrated flowmeter 5-20 Liters per minute (lpm) (Dwyer Instruments, Philadelphia, Pennsylvania). Reagents Methane gas (C.P. grade Matheson Scientific Corp.) Zero air (maximum hydrocarbon 0.1 ppm) to prepare standard solutions of methane in air. Helium carrier gas may also be used as the diluent providing appropriate flow correction is applied to the flowmeter readings (8).
Procedure Because of the number of hydrocarhons in amhient air no attempt is made to separate the components, instead, the total hydrocarhon concentration is reported against methane as the datum to which all results are compared ( I ) . Methane solutions of concentrations ranging from 0.5-5 ppm are used to calihrate the gas chromatograph (50 fi1 pure methane in 50 1 of zero air is 1ppm methane in air). A calibration curve is plotted as FID response (peak area) versus concentration of methane in the standard and is linear over a wide range of methane concentrations. Calihratiou standards should he prepared in bans similar to those used for sampling. A linear-regression analysis gives the line of best fit for the final calibration curve 19). The calibration and analysis of ambient samples should he performed under identical conditions (see the table). The bag is flushed with zero air and evacuated to remove any hydrocarbon residue. I t is then filled with zero air t o the desired volume as determined hy the flowmeter and a rubber septum is placed over the inlet port of the hag. The 100-pl gas tight syringe is flushed three times and filled by inserting it into a surgical rubber tube connected t o the tank of methane and vented throneh a water filled 250-ml Erlenmeyer flask in a hood. The syringe should be filled with a t least 20 ul inore than the auantitv desired and the plunger lowered to the desired vo1;me. ~ i methane e is injected into the bag immediately to minimize sample loss from the syringe. After injection, the hag should he rotated and kneaded for a t least 1 min to assure complete mixing. Type 301 stainless steel tubing, 118 in. o.d., which has been flushed with acetone and benzene and haked out a t 2B0°C overnight under a nitrogen flow, can he used t o transfer the mixture to the gas sampling valve. A vacuum pump is used to pull the sample through the gas sampling valve; as most pumps give off hydrooarhous from their lubricants, the sample cannot he pushed into the valve. The gas sampling valve should be flushed with sample for several minutes hefore injection. Each standard solution is injected a t least three times and an average of the peak response used. After analysis of a solution, the hag is evacuated, flushed several times with zero-air, and evacuated again. The hag may he filled with zero-air blank and checked for residues. If the hag has been flushed adequately, there should be no methane residue in the hag. Do not let a solution of methane stay in the hag any longer than necessary in order t o minimize absorption of methane on the hag. Solutions should he made starting with the lowest concentration of methane. The peak area is measured using a planimeter.
Volume 52, Number 12, December 1975 / 811
Sampling is done using hags similar to those used for calibration. Each hag should he filled with zero-air blank and checked for hydrocarhons. Mechanical pumps cannot he used to fill the hags with sample due to possihle hydrocarbon contamination. Pumps for sampling are fabricated from 2-in. wide X ?$-in. thick strips of press hoard and taped to the outside surface of the hag in a letter "H" configuration. This allows the sides of the hag to he pulled apart and cause air to enter. The hags mav be filled for UD to 2 min. denendine on their size,capped with a septui, and brought i o the gboratory for analysis. After analvsis. the bags are evacuated. flushed with zero-air, and evacuated again. Analysis consists of running a sample through the gas chromatograph which yields the total hydrocarbon in the sample. Next, a drying tube, packed with Silica Gel, is inserted between the bag and the sampling valve and an analysis made. The Silica Gel will retain all the non-methane hydrocarhons in the sample and the peak produced will he due to methane type hydrocarhons only. This peak can then be subtracted from the total hydrocarbon peak to give the concentration of total non-methane hydrocarhons in the sample. Discussion
This experiment, using the techniques ahove, has proven quite successful in the hands of students with average lahoratory ability. I t has been performed by students in the Division of Public Health and School of Engineering in an Air Sampling and Analysis course. Students' backgrounds in chemistry vary over a broad spectrum from those with only freshman chemistry to others with three or more years of chemical lahoratory experiences. A list of suggested readings has been useful to supplement the classroom presentation and helped to smooth out the variations in students' backgrounds. This is an important experiment because it teaches the student how to chromatograph a gaseous sample while providing exposure to the technique of grah sampling. Grab samples are important to industrial hveiene proerams or any program where portable equipment& no
