Anal. Chem. 1990,62,1876-1883
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aid of R. Walsh in obtaining the scanning electron micrograph is gratefully acknowledged. We also thank Beckman Instruments for their gift of System Gold hardware and software as well as Polymicro Technologies for their gift of 2 pm i.d. capillary.
LITERATURE CITED (1) Wallingford, R. A.; Ewing, A. G. Anal. Chem. 1988, 6 0 , 1972-1975. (2) Ewing, A. G.;Wailingford, R . A.; Olefirowicz. T. M. Anal. Chem. 1989, 61, 292A-303A. (3) Chien, J. B.: Wallingford, R. A.; Ewing, A. G. J. Neurochem. 1990, 5 4 , 633-638. (4) Chien, J. B.; Saraceno, R. A.; Ewing, A. G. Redox Chemistty and Interfacial Behavfor of Biological Molecuk; Plenum Press: New York, 1988; pp 417-424. (5) Periman, R. L.; Sheard, B. E. Biochim. Biophys. Acta 1982, 779, 334-340. (6) Barber, A.; Kempter, 6. Comp. Biochem. Physiol. 1986, 64C, 17 1- 174. (7) Kennedy, R. T.; St. Claire, R. L., 111; White, J. G.:Jorgenson, J. W. Mkrochlm. Acta (Weln) 1987, I! 37-45. (8) Kennedy, R. T.; Jorgenson, J. W. Anal. Chem. 1989, 67, 436-441. (9) Kennedy, R. T.; Oates, M. D.; Cooper, B.; Nickerson, B.; Jorgenson, J. W. Science 1989, 246, 57-63. I
(10) Wallingford, R. A.; Ewlng. A. 0.Anal. Chem. 1987. 59. 678-681. (11) Budavarl, S., O'Neii, M. J.. Smith, A., Eds. Merdc Index; Merck 8. Co., Inc.: Rahway. NJ, 1969; p 758. (12) Waillngford, R. A.; Ewing, A. G. Anal. Chem. 1987, 59, 1762-1766. (13) Waiiingford, R. A.; Ewing, A. G. Anal. Chem. 1988, 6 0 , 258-263. (14) Abe, T.; Itaya, K.; Uchida, I.Chem. Lett. 1988, 399-402. (15) Kepley, L. J.; Bard, A. J. Anal. Chem. 1988, 60, 1459-1467. (16) Xiaohua, H.; Coleman, W. F.; Zare, R. N. J. Chromatogr. 1989, 480, 95-1 10. (17) Marsden, C. A.; Kerkut, G. A. Comp. Gen. Pharmacol. 1970, 7 , 101-1 16. (18) Pentreath, V. W.; Berry, M. S.; Cottrell, G. A. Cell Tissue Res. 1974, 757, 369-384.
RECEIVEDfor review February 15, 1990. Accepted June 4, 1990. This research was supported, in part, by grants from the National Institutes of Health and the National Science Foundation. Financial support of research related to the project has been provided by Beckman Instruments and Shell Development. A.G.E. is a National Science Foundation Presidential Young Investigator, an Alfred P. Sloan Fellow, and a Camille and Henry Dreyfus Teacher Scholar.
Gas Chromatographic Determination of Acetonitrile in Air Using a Thermionic Detector Stephan Hamm and Peter Warneck* Max-Planck-Institut fur Chemie (Otto-Hahn-Institut),6500 Mainz, Federal Republic of Germany
A gas chromatographic procedure for the determination of acetonitrile (CH,CN) in air is described, based a thermionic nitrogen+eiective detector. The bwer llmH of detection is 15 pg of CH,CN for a blank value of 5.0 f 3.2 pg. The method involves the collection of air In 2 - d d vessels (glass or stainless steel) and preconcentration of acetonitrile by cryogenic trapping. Preconcentration by sorption on modHied Chromosorb 102 was found unsuitable due to undesirable memory effects. Air samples stored In containers for periods longer than 3 days generally show a rise In CH,CN concentration, so that alr samples must be processed within a few days. Precautions agalnst contamination in the gas handling system or by intrusion of laboratory air also are important. Calibration tests based on gaseous and liquid dilution series lead to comparable accuracies of about 4 % in each case but a better reproducibility for liquid calibration mixtures. Air samples taken over the Atlantic Ocean, In the Bay of Helgoland, during akcrafl ascents over Europe, and in the urban air of Mainz indicate the range of CH,CN mixing ratios In the troposphere from 52.6 f 13.1 pptr in background air to 731 f 82 pptr in polluted city air.
INTRODUCTION Acetonitrile, CH3CN, is emitted into the atmosphere as a byproduct of combustion processes. The main sources according to recent estimates (I,2) are automobile exhaust gases and the burning of biomass (living and dead plants). Thus, acetonitrile will be a useful indicator for emissions from such processes. The presence of CH3CN in the atmosphere was originally inferred from the composition of positive water-cluster ions, studied in the stratosphere by means of air-borne mass 0003-2700/90/0362-1676$02.50/0
spectrometers (3,4). Since then, two mass spectrometric (3-6) and two gas chromatographic (7, 8) techniques have been utilized to determine the CH3CN content of air and its distribution in the atmosphere. In this paper we describe a third gas chromatographic technique, which we consider particularly suitable for measurements of CH3CN in tropospheric air, and we shall present results from several applications. Mass spectrometry requires either a passive sampling of ambient ions or the artificial ionization of an air sample. Ionization is then followed by a series of ion-neutral reactions leading to the formation of protonated acetonitrile hydrates. Both methods currently are restricted to altitudes above 10 km. A knowledge of the ion chemistry is necessary to interpret the data (5, 6 ) . Gas chromatography (GC) requires a preconcentration of acetonitrile from ambient air samples, because detectors currently in use are not sufficiently sensitive for a direct analysis of air containing less than about 50 ppb of CH&N (1 ppb = parts by volume). The mixing ratio of acetonitrile in the atmosphere is 2 to 3 orders of magnitude lower. Becker and Ionescu (7) collected air in stainless steel containers. They preconcentrated acetonitrile from air by passing it through a Chromosorb trap kept at -90 "C. The trap was then heated to 270 "C to transfer the material into the GC sampling loop, which was cooled with liquid nitrogen. Finally, the sample was flushed onto the GC column by heating the loop. A packed column was used, and the detector was either a mass spectrometer operated in the single ion mode at e l m = 38 daltons or an electron capture detector (ECD). The detection limit was given as 0.01 ppb. Snider and Dawson (8,9) used a vertical metal plate cooled to a temperature below the ambient dew point to collect acetonitrile from the air by co-condensationwith water vapor. The aqueous condensate drained from the collector plate was further concentrated by microdistillation and adsorption in 0 1990 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 62, NO. 17, SEPTEMBER 1, 1990
+n 1 ' 1
Ib-bb*
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Nitrogen
Flow Metal Belows Buffer Pump
Make up
Volume Integrator
4 Purge Gas Pressure
I 6-Port Valve"
I
r
Air
-L=l II I - - -
Hydrogen
Container
n
.
I
x
I I
I I I
r
I I
I
I Ad sorption Tube yyyyyl
,*
I I I GC Oven
I I I
Ports connected in position 1 o . . . o Ports connected in position 2
Flgure 1. Experimental setup for the transfer, preconcentration, and analysis of acetonitrile.
a polymer trap, from where the material was transferred into the GC sampling loop by heating the trap to 125 "C. The analysis was carried out with a packed column and a flame ionization detector (FID). The detection limit for CH3CH in the condensate was given as 0.1 pg/L, typical concentrations found were 0.35 pg/L. The analytical technique described in this paper makes use of a thermionic nitrogen-specific detector (TID). Under suitable operating conditions the device is highly sensitive for reduced N compounds, whereas the sensitivity for hydrocarbons is 103-l@ times lower. This allows an unambiguous identification and detection of acetonitrile in the presence of hydrocarbons. However, a preconcentration step is still necessary, and we have found cryofocusing suitable for this purpose. We have also explored a sorption technique for the preconcentration of acetonitrile to be used in conjunction with an FID. Tests with an ECD showed this device to be less sensitive for CH3CN in comparison with the other detectors. The main purpose of the present paper is to describe our experience with sampling and preconcentration procedures and conditions best suited for the analysis of acetonitrile in air.
EXPERIMENTAL TECHNIQUES Apparatus. Figure 1 shows the experimental arrangement wed for the transfer, preconcentration, and analysis of acetonitrile in ambient air samples. A U-shaped stainless steel tube (2 mm i.d., 25 cm in length), filled with silanized glass wool, served as cryotrap and GC sampling loop. An air sample was preconcentrated by passing it through the trap, which was cooled to a temperature of about -185 OC. The gas flow was maintained with a stainless steel bellows pump, the flow rate was held constant with a mass flow controller, and the totalgas volume was measured with an associated electronic volume integrator. Pump and integrating flow controller were positioned downstream of the trap. Flow rates from 50 to 100 cm3/min were employed, and volumes of air drawn through the trap ranged from 50 to 1.800 cm3. The precision of volume determination was 13.3%. The cryotrap was connected to the six-port inlet valve of the gas chromatograph. Trace components were transferred onto the GC column by first removing the coolant, then switching the GC carrier gas stream
to pass through the loop, and finally raising its temperature to 175 "C by means of an aluminum heating block. The connection between valve and gas chromatograph was heated to about 100 "C in order to prevent the condensation of water in the inlet line. Materials that were found suitable in constructing the flow system included stainless steel, glass, and Teflon. Viton and other types of rubber had to be avoided because they showed large memory effects with regard to acetonitrile and led to high blank values. In addition, it was necessary to prevent contamination with laboratory air. For this purpose the system was continuously purged with nitrogen of high purity except when a sample was processed. After the connection of a sample container to inlet A (see Figure l),the volume between container valve and inlet was briefly flushed with a small portion of sample air. The purge gas flow was resumed for several minutes in order to free the line before the gas flow was shut off, the trap was cooled, and sample transfer was executed. A similar procedure was used for the transfer of samples from sorption tubes. Columns and Detectors. The FID used in conjunction with capillary column separation was a standard model with a sensitivity of 0.015 g of C/s. Separation of acetonitrile from other compounds was achieved with a combination of two 50 m (0.32 mm i.d.) columns in series. The first was coated with Carbowax (Chrompak CP-Wax 57 CB, 0.43-pm film thickness), whereas the second column was coated with polysiloxane (Chrompak CP-Si1 76, 0.2-pm film thickness). The helium carrier gas flow was adjusted to 1.5 cm3/min. The temperature was programmed to remain at 50 "C for 8 min followed by a 3 OC/min increase. These conditions led to a retention time for acetonitrile of about 20 min. Patterson (IO) has recently summarized the current state of development of thermoionic ionization detectors for use in gas chromatography;Marano et al. ( I I ) , Ramstad and Nichelson (12), and Cooper et al. (13) have reported on the application of such detectors in the analysis of nitriles. The flameless TID employed in the present study was a Dani Model 68/20, which works with a RbpSiOl glass bead afixed to a heated platinum wire. Since sensitivity, specificity, and stability of the detector depended greatly on the mode of operation, the optimum operating conditions for the present application were sought by varying both bead temperature and hydrogen flow. The results are shown in Figure 2. An optimum signal to noise ratio was obtained with a bead temperature of 640-650 O C and a hydrogen flow of 6.7 cm3/min. The other parameters are given in the legend to Figure
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H 2 FLOW RATE 67Cm3/min 0 4.2cm3/min
2.0 cm3/min 11 cm'/min
640
ALKALI
BEAD
680
720
TEMPERATURE 1°C)
Flgure 2. Signal to noise ratio for the Dani 68/20 thermionic detector as a function of alkali bead temperature: left, 1.5 ng of CH,CN, H, flow 3 cm3/min:right, 0.5 ng of CH3CN; Other conditions unchanged for both were as follows: N, carrier gas flow 25 cm3/min, air flow 130 cm3/min, make-up gas (N,) flow 8 cm3/min.
2. The sensitivity for acetonitrile under these conditions was 0.2-1.2 A/g of N s depending on the age of the bead. Day to day variations of the detector response for the same settings of gas flow and temperature were less than 20% for the longer term average. The specificity of the detector allowed the use of a packed column. Good separation of acetonitrile from other compounds, specifically water, was achieved with a glass column of 2 m length and 2 mm i.d., packed with Porapak QS (100-/20 mesh). The column was kept at a constant temperature of 150 "C. A nitrogen carrier gas flow of 25 cm3/min resulted in a retention time for acetonitrile of 6.2 h 0.1 min. Calibration. Standard mixtures of CH3CN in synthetic air or in pure nitrogen were prepared in a glass mixing bulb by the static dilution technique. The gas handling system was similar to one described previously (14). Pressures were measured with calibrated capacitance manometers. Initially, acetonitrile was degassed under vacuum and a calibrated volume of 10.5 0.15 cm3 was filled with CH3CN at a pressure of about 3 kPa. The contents of this volume were then flushed into the mixing vessel (volume = loo06 40 cm3)with carrier gas until the total pressure reached 100 kPa. The resulting mixture was subsequently further diluted in three steps by repeated pumping to achieve a 10-fold reduction in pressure, and refilling the mixing vessel to 100 kPa by adding more carrier gas. The final mixing ratio was calculated by application of the ideal gas law. A gas-tight syringe was used to withdraw samples from the 10-dm3 bulb through a silicone rubber septum. In addition to gas mixtures, liquid mixtures of acetonitrile in methanol were used for calibration. They were prepared volumetrically by successive dilution, starting with a mixture of 0.005 cm3CH3CN in 50 cm3of methanol. The densities for acetonitrile at the prevailing temperatures were taken from the tables of Landolt-Bornstein (15). The solutions were calculated to contain 35-230 pg of CH3CN/mm3. Sorption Tubes. The collection of acetonitrile from ambient air by thermally reversible sorption requires sorbents that combine a low retention of water vapor with a high breakthrough volume for CH3CN. In this regard, the frequently used hydrophobic polymers Tenax and Chromosorb 102 were found unsuitable. At ambient temperatures the breakthrough volumes are too small, and if one lowers the temperature of the sorption trap to increase the breakthrough volume, the retention of water increases as well. Williams and Sievers (16) recently described a modification of Chromosorb 102, which they called Eu-Sorb, and whereby they have achieved a selective increase of the retention volume for nucleophilic compounds such as aldehydes, ketones, and nitriles. The modification involved bonding of fluorinated P-diketone moieties onto the styrene-divinylbenzene copolymer, followed by the incorporation of europium(III), which then acts as a complexation site for the analytes. Since Eu-Sorb appeared well suited for the collection of acetonitrile from air,we have prepared it according to the prescription of Williams and Sievers (16). The final product was washed (with water and ethanol) and dried at elevated temperatures in an inert gas stream. A number of U-shaped glass tubes (2 or 5 mm i.d., filling length 10 cm) were packed with 250-800 mg of Eu-sorb
*
and preconditioned for several days at 210 "C under a flow of pure nitrogen. Despite extensive purification of the starting material by extraction with dichloromethane and thermal preconditioning of the final product, it was found that heating EU-Sorb to 175 "C released a great number of impurities, which were identified by mas spectrometric analysis as initiator, emulgator, and monomeric fragments originating from the polymer. Attempts to achieve a better prepurification of Chromosorb 102 by treating it with different solvents were unsuccessful. However, good results were obtained by subjecting EU-Sorb to repeated cycles of heating to 180 "C and cooling to liquid nitrogen temperature over a period of several days under inert gas flow. This procedure released most of the impurities by mechanical stress. Thermogravimetric tests showed the material to be stable at temperature up to 230 "C. Significant decomposition was observed only when the temperature had reached 260 OC. The breakthrough volume for acetonitrile was determined by the indirect method (17). A tube fied with Eu-Sorb was used as GC column and the CH3CN retention time was measured as a function of temperature. Extrapolation of the data toward 20 "C and correction for 99% sampling efficiency (17)indicated a breakthrough volume of 145 dm3/g. This may be compared with 55 dm3/g reported by Williams and Sieven (16). For the collection of acetonitrile from air a sorption trap was connected to a metal-bellows pump and about 5 dm3 of air, as measured with an integrating flow controller, was drawn through the trap. Subsequently, the sorption tube was disconnected and the open ends were closed with tightly fitting Teflon-sealed caps. Sampling Containers. Due to the high sensitivity of the TID, the volume of air required for analysis of acetonitrilewas less than 2 dm3. This amount was conveniently collected in containers. Two types of containers with volumes of 1-2 dm3 were used: (i) glass cylinders (8.5 cm i.d., 25 cm long) fitted at both ends with Teflon-sealed stopcocks; (ii) nearly spherical stainless steel cans 16 cm in diameter). The metal containers were constructed from half-spheres and a pair of inlet tubes provided with metal-bellows shut-off valves. The tubes were mounted side by side on the top half with one long enough to reach down to the bottom of the container. The interior surfaces were electropolished before assembly, and the individual parts were joined in vacuo by electron-beam welding. Preparation of the weld seams required great care. In a few cases, due to mechanical imperfections, the electron beam appeared to have penetrated into the container and damaged its interior surface. Although these containers were airtiiht as shown by helium leak tests, they suffered from a strong absorption of acetonitrile and had be scrapped. Glass and metal containers were cleaned before use by heating them to 120 "C and flushing with pure nitrogen. Helium leak and blank tests completed the preparation. Teflon-sealed metal-bellows pumps were used for air sampling. Prior to filling, each container was first flushed with ambient air for at least 5 min at flow rates of 15-20 dm3/min. The container was then pressurized with air to 2-3 bar and the inlet valve clmed. Drying Tubes. Because moist air, primarily in marine air samples, sometimes caused interferences in the gas chromatograms due to excess water vapor, the utility of potassium carbonate as a drying agent was explored. For this purpose, 1 g of K2C03was filled into glass tubes of 5 mm i.e. and 7 cm length, and the filling was secured with glass wool. The tubes were placed between sample inlet and cryotrap (see Figure l),and they were conditioned for at least 30 min by heating to 120 "C in a purge gas flow.
-
RESULTS AND DISCUSSION Cryogenic Preconcentration. Both liquid nitrogen and liquid argon were suitable as coolants of the cryotrap. Argon was more convenient as its boiling temperature is high enough to prevent the condensation of oxygen in the air. When nitrogen was used as coolant the trap was provided with a copper mantle of sufficient length so that the copper was immersed in the coolant whereas the trap itself was not. With the coolant kept at an appropriate level, the lower part of the trap could be adjusted to a temperature of -185 "C as measured with a platinum resistance thermometer. The efficiency of cryogenic preconcentration was evaluated in the following way. A glass injector with low dead volume
ANALYTICAL CHEMISTRY, VOL. 62, NO. 17, SEPTEMBER 1, 1990
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Table I. Recovery of Acetonitrile following Cryogenic Collection from Synthetic Air, Averages from Five Runs
gas flow rate, cm3/min
CH&N injected, Pg
cm3
ratio, pptr
70 recovery
95.5 95.5 95.5
61.9 154.6 433.0
487 476 489
74 190 519
97.5 9.7 100.4 f 3.2 106.1 f 2.2
gas volume, calcd mixing
was mounted upstream of the throttle valve (at point A in Figure l),the flow system was purged with acetonitrile-free synthetic air (containing 330 ppmv of COJ, and a blank test was performed. After the trap was cooled, a fixed volume of a CH3CN/N2 gas mixture was slowly injected into the purge gas flow, and the total volume that had passed through the trap was measured. Subsequently, the trap was switched into the carrier gas flow, it was heated, and a chromatogram of the released substance was taken. The CH3CN peak area was then compared with that derived from the injection of 1cm3 of the same gas mixture directly into the carrier gas flow. Results for three different doses of CH3CN are shown in Table 1. The percentage of recovery was close to 100 in every case. The excess value of 106% for the highest concentration is probably due to a systematic error in extrapolating the much smaller peak area obtained with the comparison standard. The precision for the higher concentrations is better than 4%. This agrees with reproducibilities of 2-7% obtained for repeated analyses of real air samples. The larger standard deviation observed for the lowest concentration in Table I presumably arises from the decreasing signal to noise ratio of detection. To achieve optimal conditions in the analysis of ambient air samples containing less than 100 pptrv (parts per trillion by volume) of acetonitrile thus made it necessary to process air volumes larger than 500 cm3. Results similar to those in Table I were also obtained when the purge gas flow rate was lowered to 35 cm3/min. It should be noted that the cryotrap required replacement after several months, because aging led to an irreversible adsorption of acetonitrile and loss of reliability. For routine tests of the recovery efficiency a standard gas mixture was introduced via the carrier gas injector (see Figure 1). Depending on the position of the six-port valve, the sample either reached the column directly with the carrier gas flow or was diverted through the trap where it could be collected and then released again for analysis. Sorption Tubes. The efficiency of preconcentration by adsorption on Eu-Sorb was tested in a similar way. In this case again, an auxiliary injector was used to load the purge gas with acetonitrile. The substance was collected in the sorption tube, then released at a temperature of 175 "C and re-collected in the cryotrap for subsequent analysis. A quantitative desorption of CH3CN from Eu-Sorb was achieved by purging the sorption tube for 10 min with helium at a flow rate of 14 cm3/min. Comparison of results for samples treated in this manner and samples injected directly onto the GC column indicated a recovery efficiency of 99.7 f 2.5%. Capillary column separation and FID were used in the analysis of air samples preconcentrated on Eu-Sorb. Figure 3 shows chromatograms obtained under various conditions. The upper trace was derived from a 5-dm3ambient air sample collected at a rate of 500 cm3/min onboard aircraft 600 m above ground. The middle trace is a blank from a sorption tube obtained right after preconditioning a t 210 "C under a flow of nitrogen. The lower trace resulted from the same sorption tube after it had been capped and stored for 3 days. These chromatograms show that acetonitrile is well separated from other compounds. On the combination of columns used, CHBCNelutes between the Cl0 and C1, alkanes and between
o
5
20
is
0
5 10 15 20 Retention Time ( min )
25
Yo
is
Flgure 3. Gas chromatograms obtained with sorption tubes and ca-
pillary columns after cryofocusing, He carrier gas flow 1.5 cm3/min, 8 min isotherm, then increasing at 3 "Clmin: upper trace, ambient air sample collected from aircraft 600 m above ground; middle trace, desorption of Eu-Sorb at 175 "C for 12 min after preconditioning; lower trace, desorption of Eu-Sorb after preconditioning and storage for 3 days in capped tube. benzenes and toluenes, i.e. later than the short-chained hydrocarbons that are present in the air with much higher concentrations. The CH3CN signal in the chromatogram of the air sample corresponds to a mass of 3.2 ng, which is equivalent to a mixing ratio of 354 pptrv. This amount still includes the (unknown) blank value. The middle trace in Figure 3 shows that a low blank value can be obtained by the cleaning procedure described above. Unfortunately, as the bottom trace indicates, the blank value of preconditioned sorption tubes increased after storage for a number of days. This behavior precluded a quantitative assessment of acetonitrile collected by adsorption. During the storage period, several other peaks grew as well, but they were fewer in number than those present in the air sample. This feature suggests that the deterioration is not caused by the intrusion of outside air but from the release of impurities by diffusion from the interior of the polymer. Although it may perhaps be possible to reduce the interference by a more extensive cleaning procedure, we have not continued work in this direction, because the analysis of whole air samples with the TID was more convenient. Packed Column and TID. Figure 4 shows chromatograms for several air samples obtained with the TID after separation on the Porapak QS column. These chromatograms are very simple when compared with those of Figure 3. Peak identification presented no problems. It was accomplished both by the addition of acetonitrile to an authentic sample and by the retention time. Acetonitrile is well separated from other compounds. Table I1 summarizes retention times for a number of nitrogen-containing compounds to which the TID is sensitive. None of these compounds causes any interference. Methane, ethane, and other hydrocarbons with high concentrations in the air elute much earlier than does acetonitrile. Specificity tests were performed for mixtures of CH3CN in nitrogen containing in addition one of the following compounds: ethene, methanol, acetone, dichlormethane, or n-
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I -
I
0 5 10 Time [ m i n i
1
:
:
:
0
5 10 15 Time ( m i n i
-
0 2 4 6 8 1012
4
20
Time I m i n l
-
0 2 4 6 8 1012 Time lminl
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P
0 2 4 6 8 1 0 Time ( m i n i
0
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8
1
2
Time ( m i n i
Figure 4. Gas chromatograms obtained with the packed column after cryofocusing with liquid N,: (a) marine air sample collected on North Sea research platform; (b) slmiiar sample with CH3CN added; (c) air sample from roof of the air chemistry building in Mainz; (d) air sample from aircraft flight, 4.1 km altitude (dry air).
Table 11. Retention Times of Several Nitrogen-Containing Compounds (Column, 2 m X 2 mm i.d. Porapak) substance
dicyanogen hydrogen cyanide acetonitrile acrylonitrile
propionitrile acetonitrile monomethylamine formamide
column temp, 150 150 150 150 150 190 190 190
OC
retention time, min 1.1 1.7 6.2 9.2 14.2 3.3
5.7 14.1
pentane. The detector was a t least a factor of 1000 more sensitive toward acetonitrile than for the other compounds, except CH2C12. This compound eluted later than CH3CN, however. Water was the only compound whose presence required attention, because high concentrations in the air led to an overloading of the column and peak broadening. This is evident in several of the chromatograms in Figure 4. When the cryotrap was cooled with liquid nitrogen, water eluted before acetonitrile, whereas with liquid argon, water appeared later. The different behavior must have resulted from the difference in trap temperatures. When argon was used, the
Flgure 5. Gas chromatograms of 380 cm3 air samples collected in Mainz, after cryofocusing with liquid N,: (a) direct treatment, CH3CN peak area 24.6 X lo3 units; (b) after drying the sample with K,C03, CH3CN peak area 25.3 X lo3 units.
whole trap was immersed in the coolant; with nitrogen, in order to prevent the condensation of 02,a temperature gradient was set up such that the lower part of the trap was colder than the upper. In order to minimize possible interferences by water vapor, the utility of K2C03as a drying agent was tested. Figure 5 shows results for two air samples; one was collected directly in the cryotrap, the other after having passed through the drying tube. While the results show that the removal of water was incomplete-we estimate 80-9070 removal-the use of K2C03did reduce the interference effect. Under these conditions losses of acetonitrile by absorption in the drying tube are negligible. The difference of the two peak areas in Figure 5 is 3.9% and this is within the error limits. Accuracy and Reproducibility of Calibration. Figure 6 shows a calibration curve to demonstrate the linear response of the TID. The calibration points were obtained from two dilution series each with both liquid and gaseous standard mixtures. Table I11 presents relative errors calculated for the dilution processes from Gauss' law of error propagation. In the preparation of gaseous mixtures the main sources of errors were uncertainties in the measurement of pressures (f100 Pa), in the calibration volume (10.5 0.5 cm3),and in the volume of the gas-tight syringe used for sample injection (1f 0.005 cm3). Errors in the preparation of liquid sample mixtures resulted mainly from uncertainties in the volumes of syringes used to prepare the first mixture and then dilute it further. The two syringes used had volumes of 10 f 0.03 mm3 and 0.5 f 0.005 cm3. The syringe employed for injection onto the GC column had a volume of 1 0.03 mm3. Table I11 makes evident that the accuracy obtained for the concentrations of liquid calibration mixtures is higher than that of the gas
*
*
- 200 .c v)
0
c
.n
150
I
I
I
I
I
-
liquid standard mixtures CH3CN /CH30H dilution sequence A liquid standard mixtures CH3CNI CH30H dilution sequence B
3 L
I
I
A
gaseous standard mixtures CH3CN I N t dilution sequence C
A
gaseous standard mixtures CH3CN I N 2 dilution sequence D
L
d
d
d
.-El
100
v, L
c 0 U
c (u
0" c .-0 E
-
50
W
e o
50
100
150 200 250 Mass of Acetonitrile ( p g )
300
350
Flgure 6. Calibration plot for acetonitrile using gaseous and liquid standard mixtures and two dilution sequences in each case.
Table 111. Relative Errors for Concentrations of Calibration Mixtures and Masses of Acetonitrile Injected onto the GC Column % relative
mixtures
concn, pg/cm3 for g.m. pg/mm3 for 1.m.
%
re1 error
of concn
Gas Mixture (gem.)
standard 1. dilution 2. dilution 3. dilution
58843 7363 920 117
error of injected mass'
f3.5 f3.6 f3.7 f3.8
Liquid Mixtures (1.m.) standard 77920 f0.6 1. dilution 3710 fl.1 2. dilution 223 f1.3 3. dilution 108 f1.8 4. dilution 37 f2.3 Vi, volume injected.
vi = vi = 0.5 cm3 0.3 cm3 f3.7 f3.9 f3.8 f4.0 f3.9 f4.1 f4.0 f4.2 Vi = 1 mm3 f3.1 *3.2 f3.3 f3.5 f3.8 ___
mixtures. This advantage is lost, however, by the error incurred in applying the smaller liquid injection volume. The contribution of relative error in this case is 3% versus 1 % for the gas-tight syringe. In order to determine the reproducibility of the calibration procedure, the preparation of a standard calibration mixture was repeated 5 times and the signals recorded. For this purpose an FID was used, because its stability was superior to that of the TID. The results indicate a standard deviation of the peak areas obtained of f4% for the gas mixtures and of 0.7% for the liquid mixtures. This comparison shows again the advantage of liquid calibration. Blank Value and Detection Limit. Since the cryotrap is purged with zero gas prior to sample collection, any impurity entering during the purge period will add to the blank value. Such impurities may result from the intrusion of laboratory air via leaks in the flow system or from memory effects. The magnitude of the blank value was determined by performing a preconcentration procedure with 0.5-1.5 dm3 of synthetic air. Values in the range of 2-9 pg of acetonitrile were obtained. Similar values were observed when the purge gas was first led
through a trap cooled with liquid nitrogen. This shows that impurities in the purge gas contribute little to the total blank. From a series of 28 determinations on different days the average blank value was found to be 5.0 f 3.2 pg. If we defiie the detection limit as the blank value plus 3 times its standard deviation (I@, we obtain a detection limit of 15 pg for about 0.5 dm3of air sample. In the analysis of ambient air, therefore, we have always used sample volumes, which allowed the collection of CH3CN in amounts several times larger than that given by the detection limit of 15 pg. Behavior of Containers. Glass and stainless steel containers were subjected to various tests. The general procedure outlined earlier was to flush each container with zero gas, establish an overpressure, and then close the inlet valve. Blank tests after storage periods of 13-100 days showed no detectable amounts of CH&N in practically all tests. In the collection of synthetic or ambient CH3CN/air mixtures, a container was flushed with 10-25 times its volume of sample air before an overpressure was applied and the valves were closed. Recovery tests were initially made with CH3CN/air mixtures of 106 and 147 pptr prepared in the laboratory. Experiments with seven different stainless steel containers, which were flushed with 10-25 times their volume of test gas, showed a recovery efficiency of 97.4 f 9.5% after 2-3 h of storage time. For four glass containers the recovery rate was only 78.7 f 10.7% despite flushing them with 25 times their volume. It appears that in this case the amount of flushing was insufficient. Indeed, recovery tests with ambient air in field work showed no differences for the two types of containers when the glass containers were flushed with 50-100 times their volume of ambient air. Synthetic acetonitrile/air mixtures stored in the containers for 2-3 days showed a decrease in the mixing ratio by 7-15% in stainless steel containers and by 1-36% in glass containers. Ambient air samples, in contrast, exhibited a reasonable short-term stability over a period of 2-3 days, but thereafter the CH3CN mixing ratios increased, usually at a rate of 1-3 pptr per day. This is shown in Figure 7. All containers, from which these data were obtained, had zero blanks after 35-100 days of storage. The upper part of Figure 7 presents data for marine air samples to show that the increase of the CH3CN mixing ratio occurs in both types of containers. The lower part of Figure 7 contains data for air samples collected during two aircraft flights over Europe. In some of these samples
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 17, SEPTEMBER 1, 1990
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Figure 7. Changes of CH,CN mixing ratio in ambient air stored in containers (container number shown in each case): upper part, shipborad air samples collected in March-April 1987 over the Atlantic Ocean; lower part, aircraft samples over Europe for various altitudes.
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Change of CH,CN mixing ratio in four stratospheric air samples, collected by aircarft in stainless steel containers.
Flgure 8.
the growth of CH&N mixing ratios is quite substantial, leading to a doubling of the original values after storage for 80-100 days. These results make clear that sample integrity is not maintained in either glass or stainless steel containers during longer periods of storage time. Samples must be processed as soon as possible during the first 1-2 days after collection. Even then, however, problems may arise as data for several stratospheric air samples demonstrated. They are shown in Figure 8. In two of the samples the mixing ratios rose dramatically to about 7 times the original valve after 80-100 days. We have no clues regarding the origin of the increase of acetonitrile in the containers. Leaks can be ruled out, since the blank tests were negative. We can only speculate on a production of acetonitrile by reactions of HCN or (CN),
50
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Flgure 9. Frequency distributions of atmaspheric acetonitrile mixing ratios (a) in marine air of the North Atlantic Ocean (23-50' N, 30' W-1' E) 22 m above the sea surface, (b) in the Bay of Helgoland (54.7' N, 7.2' E) 22 m above the sea surface, (c) in the free troposphere (49-73' N, 5-19" E) 900-9500 m above ground, and (d) in the urban air of Mainz (50' N, 8.2' E) 10 m above ground.
a t the container walls, or the release of acetonitrile from carbonaceous particles inadvertently collected together with the air samples. As noted previously (19),the stratospheric air samples were obtained in the vicinity of Spitsbergen a t a time when a northern province of China was plagued by devastating forest fires. Air mass back trajectory calculations made it likely that air from the location of the forest fires was advected toward the Spitsbergen area, so that excess carbonaceous particles may have been present in the air samples a t that time. Atmospheric Measurements. Analytical results for air samples from various locations are summarized in Figure 9. The samples were taken on the flat roof of the air chemistry building in Mainz, during aircraft ascents in the troposphere over Europe, on a research platform located in the Bay of Helgoland (North Sea), and over the North Atlantic Ocean onboard the research vessel Polarstern during her cruise in March-April 1987. Altogether 99 samples were processed. Surface air samples were analyzed either directly or within a few hours after collection, whereas the aircraft samples were processed 11-48 h later. The aircraft data have been discussed in some detail previously (19). The marine data are part of larger series of measurements at various latitudes, which will be treated elsewhere (2). Here, we mainly wish to indicate the range of acetonitrile/air mixing ratios in the troposphere, and for this purpose the results are shown in Figure 9 in the form of frequency distributions. The lowest mixing ratios were observed over the Atlantic Ocean with a fairly narrow, nearly Gaussian distribution around an average value of 52.6 f 13.1 pptr. This may be considered the tropospheric background in the northern hemisphere. Higher values were found over the European continent. The aircraft samples indicated no significant variation of the mixing ratios with altitude for each flight. The average value from three flights was 160 f 32 pptr. The true average presumably is somewhat lower, because 13 data points from two ascents gave an average of 144 f 27 pptr, whereas the remaining six samples from the third flight gave 194 f 7 pptr in the troposphere. As a result, the overall frequency distribution looks slightly skewed. The distribution of mixing ratios for samples taken in the Bay of Helgoland is again nearly Gaussian though broader than the two data sets described so far. The average mixing ratio of 117 f 37 pptr lies between the tropospheric background value and that for continential air. This is expected for a site influenced by continental air most of the time, except when air masses are advected from the north. The lowest mixing ratios with an average value of 96 f 11 pptr (n = 7) were indeed observed with winds coming from the north. Finally, the data obtained in Mainz contain the highest CH&N mixing ratios among all air samples. The frequency distribution shown in Figure 9 is bimodal, and this is a clear indication for the vicinity of sources of acetonitrile. The major source in the city presumably is automobile traffic. The lowest mixing ratios from six samples collected on one day in July 1986 gave an average of 154 f 22 pptr, which is close to the average continental mixing
ANALYTICAL CHEMISTRY, VOL. 62, NO. 17, SEPTEMBER 1, 1990
ratio derived from the aircraft data.
Comparison with Other Gas Chromatographic Methods. The procedures of Becker and Ionescu (7)and of Snider and Dawson (8, 9) were described in the Introduction. The first group of authors collected 16 surface air samples in stainless steel containers at urban, rural, coastal, and mountainous locations in Europe and reported mixing ratios in the range of 2-7 ppb with little variation. Our sample handling and analysis techniques differed from those of Becker and Ionescu (7) in several ways: To minimize interference problems, we have used a thermionic nitrogen-selective detector, which suppressed signals due to hydrocarbons. We have used a cryotrap for the preconcentration of acetonitrile, whereas Becker and Ionescu (7) employed a sorption technique. Our experience with sorbents such as a modified Chromosorb 102 was unfavorable in that it gave rise to memory effects. We have also found that the construction of the gas handling system required a careful selection of materials and we have purged our system with zero gas in order to prevent contamination by the intrusion of laboratory air. Snider and Dawson reported CH,CN/air mixing ratios averaging to 56 f 20 pptr from 18 aqueous condensate samples collected at the crest of Mount Lemmon (elevation 2900 m) and in the foothills of the Santa Rita Mountains (elevation 1300 m) in Arizona. The first site is located 27 km to the north of Tucson, AZ, the second 52 km to the south of the city. Preconcentration from aqueous condensates appears to be less susceptible to contamination than the method described by Becker and Ionescu, although the technique pioneered by Snider and Dawson also involves adsorption in a polymer trap as a preconcentration step, which may give rise to memory effects. Somewhat perturbing is the fairly low efficiency of 20% reported for the sorption-desorption procedure when
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compared with the nearly complete recovery of acetonitrile from both cryogenic and sorption trapping described here.
ACKNOWLEDGMENT We are greatly indebted to the crews of the R.V. Polarstern and the research platform Nordsee, and to flight captains Gartner and Lechner of FVG Company for cooperation and support during the field measurements.
LITERATURE CITED Arijs, E.; Brasseur, G. J . Geophys. Res. 1986, 91, 4003-4018. Hamrn, S.; Warneck, P. J . Geophys. Res., in press. Arijs. E.; Ingels, J.; Nevejans, D. Nature 1978, 2 7 1 , 642-644. Arnold, F.;Bohringer, H.; Henschen, G. Geophys . Res. Lett. 1978, 5 , 653-656. Schlager, H.; Arnold, F. Planet. Space Sc/. 1985, 3 3 , 1363-1366. Knop, G.; Arnold, F. Planet. Space Sci. 1987, 3 5 , 259-266. Geophys. Res. Len. 1987, 14, 1262-1265. Becker, K. H.; Ionescu, A. Geophys. Res. Lett. 1982, 9 , 1349-1351. Snider, R. J.; Dawson, G. A. Geophys. Res. Len. 1984, 1 1 , 241-242. Snlder, R. J.; Dawson, 0. A. J . Geophys. Res. 1985, 90, 3797-3805. Patterson, P. L. J . Chromatogf. Sci. 1986, 2 4 . 41-52. Marano, M. S.; Levine, S. P.; Harvey, T. M. Anal. Chem. 1978, 5 0 , 1948-1950. Rarnstad, T.; Nicholson, L. W. Anal. Chem. 1982, 54. 1191-1196. Cooper, S. W.; Jayanty. R. K. M.; Knoll, I . E.; Mldgett. M. R. J . Chromtogr. SCi. 1986. 2 4 , 204-209. Moortgat, G. K.; Warneck, P. J . Chem. Phys. 1979, 70, 3639-3651. Borchers, H., Hausen, H., Hellwege, K. H., Schiifer, K., Schmidt. E., E&. Zahlenwerte und Funktlonen, 6th ed.;Landolt-Wnsteln; Springer Veriag: Berlin, 1971; Vol. 2, part 1, p 688. Williams, E. J.; Sievers, R. E. Anal. Chem. 1984, 56, 2523-2528. Senum, G. I.Environ. Sci. Technol. 1981, 15, 1073-1075. Miller, J. C.; Miller, J. N. Statistics for Ana/yt/cal Chemistry; Wiley 8 Sons: Chichester, 1984. Harnm, S.; Helas, G.; Warneck, P. Geophys. Res. Len. 1989, 16, 483-486.
RECEIVED for review February 28,1990. Accepted May 23, 1990. This work was supported by a grant to S.H.from the Deutsche Forschungsgemeinschaft.
