(3) Alexander, B. H., Barthel, W., J. Org. Chem. 21,1102 (1956). (4) Allen, P. T., J . Aar. Food Chem. 10, ' 249 (1962). ' (5) Alien, Allen, P' P.. T., 7 Beckman, H. F., Fudge, J. F., Ibid., p. 248. (6) Bentley, R., Sweeley, C. C., Makita,
M., Wells, W. W., Biochem. Biophys. Res. Commun. 11. 11, 14 (1963). (1QfiR) (7) ANAL. CHEW.26. 1970 . , Beroza. M.. ANAL. I
,
(1954). (8) Ibid., 28, 1550 (1956). (9) Beroza, M.,J . Agr. Food Chem. 4, 49 (1956). (10) Ibid., p. 53. (11) Ibid., 11, 51 (1963). (12) Beroza, M., Barthel, W., Ibid., 5, 855 (1957). (13) Beroza, M., Jones, W. A., Anal. Chem. 34, 1029 (1952). (14) Blum, M. S.,J . Agr. Food Chem. 3, 122 (1955). (15) Briggs, L. H., Colebrook, L. D., Fales, H. RZ., Wildman, W. C., ANAL. CHEM.29,904 (1957). (16) Eegriwe, E., 2. Anal. Chem. 110, 22 (1937). (17) Fales, H. &I., Bodenstein, 0. F., Beroza, M., J . Econ. Entomol. 49, 419 (1956).
(18) Gender, W. J., Samour, C. M., J . Org. Chem. 18,9 (1953). (19) Gersdorff, W. A., Piquett, P. G., Beroza, M.; J . Agr. Food Chem. 4 , 858 (1956). (20) Haller, H. L., LaForge, F. B., Sullivan, W. N., J . Econ. Entomol. 35, 247 (1942). (21) Haller, H. L., LaForge, F. B., Sullivan, W. N., J . Org. Chem. 7, 185 (1942). (22) Hansen. 0. R.. Acta Chem. Scand. 7; 1125 (1953). ' (23) Jones, H. A., Ackerman, H. J., Webster, M. E., J . Assoc. Ojic. Agr. Chemists 32,684 (1949). (24) Klouwen, M. H., Terheide, R., Kok, J. G. T., Fette, Seifen, Anstricmittel 65, 414 (1963). (25) Langejan, M., Pharm. Weekblad. 92,693 (1957). (26) Lichtenstein, E. P., Schultz, K. R., Cowley, G. T., J. Econ. Entomol. 56, 485 (1963). (27) Lorette, N. B., Brown, J. H., Jr., J . Oro. Chem. 24.261 (1959). (28) Mhson, L. H., Puschett, E. R., Wildman, W. C., J . Am. Chem. SOC. 77, 1253 (1955). (29) Moore, B. P., Hewlett, P. S., J . Sci. Food Agr. 9, 666 (1958). ~
(30) Pavolini, T., Malatesta, A,, Ann. Chim. Applicata 37, 495 (1947). (31) Philleo, W. W., Schonbrod, R. D.. Terriere, L. C., J . Agr. Food Chem. 13; 113 (1965). (32) Prabuiki, A. L., Lenz, F., Helv. Chim. Acta 45,2012 (1962). (33) Sawicki, E., Hauser, T. R., McPherson, S., ANAL. CHEM.34. 1461 (1962). ' (34) Schmidt, C. H., Dahm, P. A., J. Econ. Entomol. 49.729 (1956 ). (35) Sun, Y. P., Johnson, E.-R., J . Agr. Food Chem. 8,261 (1960). (36) Terriere, L. C., Boose, R. B., Roubal, W. T., Biochem. J . 79, 620 (1961). (37) West, P. W., Sen, B., 2.Anal. Chem. 153,477 (1956). (38) Williams, H. L., Dale, W. E., '
Sweeney, J. P., J . Assoc. Ofic. Agr. Chemists 39,872 (1956). (39) Zielinski, W. L., Jr., Fishbein, L., J . Gas Chromatog.3,260 (1965).
RECEIVEDfor review July 19, 1965. Accepted November 8, 1965. Study supported by Research Contract PH 43-64-57, National Cancer Institute, National Institutes of Health, Public Health Service.
Rapid Gas Chromatographic Determination of C, to C,, Hydrocarbons in Automotive Exhaust Gas EMMETT S. JACOBS Research and Development Division, Organic Chemicals Department, Jackson laboratory, E. 1. du font de Nemours and Co., Wilmington, Del.
b A gas chromatographic procedure is described for the rapid determination of C1 to Clo hydrocarbons in automotive exhaust gas. The method features the application of a programmed temperature run from -55" C. to $140" C. with a single open tubular column and flame ionization detector. As many as 85 C1 to Cl0 paraffins, olefins, and aromatic hydrocarbons may be determined within 13 minutes. Automatic integration of the flame detector signal is used to provide instantaneous quantitative analysis for as little as 1 p.p.m. (v./v.) of each hydrocarbon. The equipment and operating conditions of the chromatographic method are discussed and an analysis of exhaust gas is illustrated.
C
automotive exhaust gas is needed to ascertain its relation to air pollution problems. Altshuller ( I ) has stated in a review that the largest effort in the application of gas chromatography to air pollution has been directed toward investigations of auto exhaust gas composition. However, there is still a need for refinement of the analytical methods to provide a more rapid determination OXPREHENSIVE ANALYSIS Of
of the composition of autc -haust, namely, hydrocarbons, oxidized fragments, nitrogen oxides, etc. Most of the gas chromatographic procedures applied to auto exhaust gas have employed a number of different packed columns connected in series to provide a full range hydrocarbon analysis. Such procedures may require as long as two hours to obtain a complete analysis of a single exhaust gas sample. Recent advances in gas chromatographic instrumentation and technique such as flame and electron capture detection, temperature programming, and open tubular columns should afford a means for a more complete exhaust gas analysis in a shorter time. McEwen (6, 6) used one packed and one open tubular column to effect a two-stage separation of hydrocarbon gas mixture. The gas sample was injected into the cold (dry ice, -78" C.) packed column which was warmed with water to elute the C1 to Ca hydrocarbons. The higher (C, to C,) hydrocarbons were then backflushed off the packed column onto the open tubular column which was temperature programmed to 80' C. to provide rapid elution of these higher boiling hydrocarbons. The method proposed in this paper uses only a single open tubular column
with an initial subambient temperature of -55" C. followed by a temperature program to a final temperature of 140' C. This procedure yields a fast and efficient separation of the full range (C1 to GO)of hydrocarbons found in auto exhaust. The use of a large sample and a large split ratio provides a hydrocarbon sensitivity of 1 p.p.m. EXPERIMENTAL
Apparatus. A Perkin-Elmer Model 800 gas chromatograph with an adapter for open tubular columns was used for this study. All coated open tubular columns were used as purchased from Perkin-Elmer. The detector heaters were rewired by connecting directly to 110 volt power line to maintain the detector a t or near 100' C. during subambient temperature periods. This was necessary to prevent water from flame combustion from condensing and corroding electrical points in the base of the detector. A Leeds & Northrup Type G Speedomax recorder with a 2-mv. range, second full scale pen response was used to record all chromatograms. The recorder was also equipped with an Insco multispeed chart drive which provided of the variable speeds from 1 to basic chart speed of 12 inches/minute. An Infotronics Model CRS-11HS extended range digital integrator with VOL. 38, NO. 1, JANUARY 1966
43
Table I. Chromatographic Conditions for Determination of C1 to Cl0 Hydrocarbons
A. Column data
Length, ft. I.D., inch O.D., inch Liquid phase
150 0,010 0.062 DC-200 Silicone
B. Instrument conditions Sample size, ml. 5.20 Sample split ratio 150: 1 Needle vent NO. 29, 0.007inch i.d. Column temp., "C. Start - 55 End 140 Prz ram rate, t./min. 10 Injector port t e y p "C. 60 Detector temp., C: 90 Helium, ml./min. 4.0 Air, ml./min. 280 Hydrogen, ml./min. 40
+
C. Integrator settings Filter frequency, CPS 3 Sensitivity 4 8 Trip level Threshold, 70 0.1
Table II. Hydrocarbon Retention Data
No. 1 2
3 4 5
6 7
8
9 10 11
12
13
14 15 16 17 18
19 20 21 22 23 24 25 26 27 28 29
30 31 32
heavy duty printer and automatic base line drift corrector was used for all quantitative peak area measurements. The integrator, when connected t o the 80-mv. output of the chromatograph, provides a maximum count rate of 100,000 counts per second. Because of the flow requirements of the splitter and open tubular column, the flow controllers in the instrument were not used. The column flow sensing valve was kept full open while the column flow reference valve was closed and the carrier flow was operated at a constant inlet pressure with a n accessory pressure regulator. Accessory pressure regulators were also used to control Hz and air flow to the detector. ,411 gases were passed through silica gel filter driers prior to pressure regulation. The carrier gas flow rate was measured a t 25" C. at the exit of the open tubular column. Hydrogen and air flow rates were adjusted to provide maximum detector response as measured by the peak area for a sample of 1200 p.p.m (v./v.) propane in nitrogen. A Perkin-Elmer gas sample valve or disposable hypodermic syringes equipped with 2.5-inch needles were used to introduce gas samples. When required, the valve has heated with electrical heating tape and the syringe and needle were heated in an oven. All apparatus including the cylinders for fuel and detector gas supply were mounted on a mobile cart. Reagents. Gaseous hydrocarbons were obtained from Matheson Co., and liquid hydrocarbons were obtained from Phillips Petroleum Co. Impure hydrocarbon samples were analyzed by gas chromatography and time-of-flight mass spectrometry to identify all components in each hydrocarbon sample. 44
ANALYTICAL CHEMISTRY
33 34 35 36 37 38 39
40
41 42 43 44 45 46 47 48
49
50 51 52 53
54
55 56 57 58 59 60
61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82
83
84 85
Component Methane Ethylene Ethane Acetylene Propylene Propane Isobutane Isobutylene 1-Butene 1,&Butadiene n-But ane 2,2-Dimethylpropane trans-2-Butene cis-2-Butene 3-Methyl-1-butene Isopentane 1-Pentene 2-llethyl-1-butene n-Pent ane 2-Methyl-1,3-butadiene trans-2-Pentene cis-2-Pent ene 2-illethyl-2-butene 2,2-Dimethylbutane Cyclopentene 4Methyl-1-pentene Cyclopentane 2,3-Dimethyl-l-butene 2,3-Dimethylbutane 4 M e thyl-czs-2-pent ene 4-3Iethyl-trans-2-pent ene 2-Nethylpentane 3-Methylpent ane 2-hIet hyl-1-pent ene 1-Hexene 2-Ethyl-1-butene n-Hexane trans-3-Hexene cis-3-Hexene trans-2-Hexene cis-2-Hexene Net hylcyclopentane 2,3-Dimethyl-2-bu t ene 2,PDimethylpentane 2,2,3-Trimethylbutane Benzene Cyclohexane 2,3-Dimethylpentane Cyclohexene 3-?rIethylhexane 2,2,4Trimethylpentane trans-3-Heptene cis-3-Heptene n-Heptane trans-2-Hep tene 2,4,4-Trimethyl-l-pentene cis-2-Hept ene hIethylcyclohexane 2,4,4Trimethyl-2-pentene
4RIet hylcyclohexene 2,2,3-Trimethylpentane Toluene 2,3,4Trimethylpentane 2,3-Dimethylhexane 1-Octene 2,2,5-Trimethylhexane 1-trans-2-Dimethylcyclohexane n-Octane 1-cis-2-Dimethylcyclohexane cis-2-Oct ene Ethylcyclohexane Ethylbenzene p-Xylene m-Xylene o-Xy 1ene n-Nonane Isopropylbenzene n-Propylbenzene 1,3,5-Trimethylbenzene tert-But ylbenzene 1,2,PTrimethylbenzene n-Decane sec-Butylbenzene p-Cymene n-But ylbenzene
B.P., ' C. -161.49 -103.71 -88.63 -83.6 -47.70 -42.07 -11.73 -6.90 -6.26) -4.33 -0.50 $9.50 -0.88
+3.72 20.06 27.85 29.97 31.16 36.07 34.10 36.35 36.94 38.57 49.74 44.24 53.87 49.26 55.62 57.99 56.39 58.61) 60.27 63.28 62.11 63.49 64.70 68.74 67.09 66.45) 67.88 68.89 71.81 73.21 80.50
80.87 80.1 80.74
89,78
82,98 91,85 99.24 95.67 95.75) 98.43 97.95 101.4 98.41 100.9 104.9 102.2 109.8 110.6 113.47 115,61 121.3
124.08
123.42 125.66 129.73 125.6 131.8 136.1
Retention time, min.:sec. 1:18 1:20 1:22 1:24 1:37 1:39 2:09
2:23 2:25
2:28 2:32 2:35 2:41 2:55 3:08 3: 17 3:21 3:24 3:28 3:31 3:34 3:38 3:48 3:58
4:01 4: 06 4: 08
4:10 4: 12 4:15 4: 26
4:30 4:34 4:40 4:42 4:45 4:49
4:54
5:02 5:07
5:10
5: 13 5:25 5:33 5:47 5:49
5:55 6:09 6: 19 6:22 6:27 6:30 6:35 6:40 6:51 6:54 7 :13 7:20 7:28
7:35
7:50 7:54
8:03 8:15 8:36
8:40
8:44
9:08
9: 19 144.4 l50,4 152.4
159.2 164.7 169.1 169.4 174.0 173.3 176.0 183.3
9:44 10:11
10:23 10:57
11:18
11:44 11:47
12:08
12: 12
12 :26 13:04
1
-
5-6ilfl Column Temp. t25OC.
II
1i
~Ikl
; I
'-3
Column Temp. -55OC.
111
I
I I I I
150:l SPLIT
1.0 ML. SAMPLE
I I
I
I20
I
60
SECONDS
Figure 1. Effect of sample size and split ratio on component resolution See Table I for instrument conditions. See Table II for peak identification A. No split; 0.1 ml.-sample 6. 1 5 1 split; 0.1 -ml. sample C. 150:l split; 1 .O-ml. sample
Procedure. Table I lists the instrumental conditions that were developed for gas chromatographic analysis of C1to Cl0 hydrocarbons. The column was cooled with powdered dry ice held in two aluminum trays (8 X 3 X 1.5 inches) placed inside the column oven. Two fillings of the trays were usually sufficient to lower the column temperature from ambient to -55" C. within 8 minutes. The column temperature was measured with a thermocouple attached directly to the column. A gas sample was injected when a constant temperature of -55" C. was obtained. Ten seconds after the sample had been injected the ice trays were removed from the column oven, the column temperature control was set for +30" C., and full heat was applied to the oven. Two minutes and 15 seconds after sample injection the column temDerature was nroerammed from 30" c: to 140' c. 'atvloo C./ minute. Calibration Mixtures. Gas mixtures of hydrocarbons and air were prepared to determine retention times
Seconds
Figure 2. Effect of column temperature on separation of light hydrocarbons
as well as chromatographic response factors for quantitative analysis. The mixtures used for quantitative calibration were prepared by injecting, a t 25" C., a measured volume of pure gas or a weighed amount of pure liquid hydrocarbon into an evacuated 12-liter glass flask. As many as 10 components were mixed in a flask to give a standard containing 10 to 500 p.p.m. (v./v.) of each component. The total hydrocarbon concentration was never allowed to exceed l/20 of the saturation concentration of the least volatile component. After all components were added, the flask was pressurized with pure air or Nz to one atmosphere. The flask was then heated, stirred, and cooled and allowed t o stand overnight a t 25' C. before the content was analyzed. RESULTS AND DISCUSSION
Column Selection. The choice of liquid phase in the open tubular column was dictated by the need for a nonvolatile material which would not bleed a t high temperatures and would provide separation for the full range
See Table II for peak identification A. Column temp. f25'C. B. Column temp. -55'C.
of hydrocarbons in auto exhaust. No column was found which would provide complete separation of all the hydrocarbon components. Therefore, because of the importance of unsaturated hydrocarbons in air pollution studies, when a question of hydrocarbon separation arose, more emphasis was placed on separating the unsaturates from paraffins than separating paraffins from each other. Squalane and Dow Corning 200 silicone oil proved to be the best liquids of those tested when compared by the above criteria. These two liquids are about equal in that both are sufficiently nonvolatile to be used a t high temperatures without serious bleed and both liquids gave good separation of CS to Clo hydrocarbons. Yeither liquid phase, when used in an open tubular column, gave adequate separation of the C1 to C4 hydrocarbons a t room temperature. Hbwever, the DC-200 silicone oil was selected primarily because it gave a superior separation of the C1 to C4 hydrocarbons a t subambient temperature. Although the instrument VOL. 38, NO. 1, JANUARY 1 9 6 6
45
was designed for dual columns and differential flame detection the bleed from the silicone column was so low that only a single column and flame were used. The length and diameter of the open tubular column chosen for this method (Table I) represent a compromise between sensitivity and resolution. A larger diameter column (0.02-inch i.d.) has been used for some work because it permits the analysis of smaller concentrations. Sample Size and Split Ratio. Samples injected in the instrument are split by a linear input splitter so that a minor portion of the sample goes on the column and the major portion is vented through a hypodermic needle inserted in the reference port. The difference in resistance of the needle to that of the column determines the size of the split. In practice the split ratio was determined by the ratio of the carrier gas flow out the vent to that out the exit of the column. The chromatograms shown in Figure 1 illustrate the advantage in resolution of hydrocarbons obtained by using a large split ratio. Although the split ratio was varied by using different size needles an optimum ratio might have been obtained if a continuously variable splitter had been used. Temperature Conditions. Although Averill and Ettre (6)have reported the ambient temperature separation of C1 to Ca hydrocarbons with an open tubular column, a faster and more efficient separation may be obtained a t subambient temperatures. The chromatograms in Figure 2 show the effect of temperature on the separation of a dilute mixture of C1 to C4hydrocarbons. The separation of a small, more concentrated sample of these light hydrocarbons was complete a t -45" C. but the diluent effect of the air in the large sample of the dilute mixture caused some spreading of the peaks. Additional cooling to -55' C. was necessary to obtain adequate separation. This diluent effect of large samples was also seen by McEwen (5).
Table 111.
46
ANALYTICAL CHEMISTRY
p.p.m. v./v.
317 126 101 120
86 74
92 101 85
106
b F
T
P
Minutes Figure 3.
Calibration chromatogram
See Table II for peak identification
Figure 3 illustrates the separation of C1 to CIOhydrocarbons, which is obtained using the conditions listed in Table I. This chromatogram is an example of the fast and efficient analysis which can be obtained on a temperature-programmed open tubular column. The over-all temperature sequence was chosen from several investigated, because it afforded the best resolution and most rapid elution of the components in this mixture. The use of the subambient temperature with temperature program provides adequate separation of the full range of hydrocarbons in auto exhaust on a single column. This eliminates the multiple columns and backflushing
Quantitative Calibration
Concn., Component Ethane Propylene n-Butane 1-Butene n-Pent ane 1-Pentene 2-Methylbutane n-Hexane Methylcyclopentane 2,2,4Trimethylpent ane
h
Integrator counts 7420 4172 4960 5236 5590 4387 6161
7900 6529 10670 Av.
Response, counts/p.p.m. 23.4 33.1 49.1 43.6 65.0 59.3
Deviation of response, yo 2.4
2.8 2.3 2.8
2.6
3.2 3.3 2.8 78.2 76.8 3.0 2.8 100.6 dev. of Response, yo = k 2 . 8 67.0
techniques used in other procedures and gives a more rapid and simpler analysis. Generally a heated injector port is not needed with gas samples. However, the use of a subambient temperature a t the start made it necessary to employ a heated port to prevent condensation of high boiling components. A temperature of 60" C. was chosen because it was high enough to prevent condensation without affecting the resolution of light hydrocarbons. Retention Time of Hydrocarbons. Because it was not known exactly what or how many hydrocarbons would be in exhaust gas, retention data were accumulated for as many available components as possible. The mixture illustrated in Figure 3 plus several others, containing 39 olefinic, 14 aromatic, and 32 cyclic, branched, and normal paraffins for a total of 85 C1 to Cl0 hydrocarbons, were used to catalog the retention data in Table 11. These data are used to identify components in exhaust. More rapid identification can be made by direct comparison of the unknown chromatogram with a calibration chromatogram such as Figure 3. Generally the absolute retention times of the C1 to Cs components can be reproduced to within 1 to 2 seconds while the times of the C6 to Clo hydrocarbons can be reproduced to 3 to 4 seconds. Quantitative Calibration. The gas
chromatographic procedure for determining hydrocarbons was calibrated by analyzing gas standards using the conditions given in Table I. The results of chromatographic analysis for representative hydrocarbons given in Table I11 are average values obtained from replicate analyses of several standards. Each component was also analyzed a t two different concentrations. I n general, the response of the hydrocarbons is proportional to the number of carbon atoms. However, the difference in response values between components with the same carbon number was, in some cases, greater than the standard deviation for determining the response of a single component. Therefore, it was necessary to calibrate the procedure for each hydrocarbon. Ettre (3) has demonstrated that there is a linear relationship between molar response of hydrocarbons and carbon number for normal paraffins. Isomers with the same carbon number will have a different relative molar response. The sensitivity, expressed in terms of coulombs/mole was determined from data given in Table IV. Based on nbutane, this value is 0.037 coulombs/ mole, with a noise level of 2 pv. Using a 5.2-ml. sample the minimum detectable quantity of n-butane is estimated to be 1 p.p.m. Kormally the area of a peak for 1 p.p.m. of n-butane was too small to be measured precisely by triangulation or with a planimeter. However, the integrator used for quantitative measurement was able to provide a reproducible count for such small peaks. The detector signal is fed directly to the integrator and errors resulting from slight nonlinearity or excessive time constants in the recorder have no effect on the integrator count. In addition, the large Av/At (mv./sec.) signal for peaks from the open tubular column allowed the use of integrator peak sensor settings which eliminated any noise or baseline drift from actuating the counter. This high discrimination still afforded sensitivity to detect peaks due to 1 p.p.m. (v./v.) of butane. An average value of 7 counts/mm.* was obtained for all peaks with a chart speed of 6 inches/minute. APPLICATION
This procedure was developed for and is now being used for the analysis of automobile exhaust gas. A chromatogram of a typical exhaust sample taken during high speed cruise engine operation is illustrated in Figure 4. In general, the hydrocarbons seen in the exhaust gas are composed of the C1 to C4 saturated and unsaturated hydrocarbons, as well as the components of the original gasoline fuel.
n
Minutes
Figure 4.
Chromatogram of cruise exhaust gas
See Table II for peak identification
CONCLUSION
Extension of the analysis from CIOto higher carbon numbers is possible by means of a more rapid temperature program and a higher final temperature. However, the method would be more complicated because resolution of the Cg to CS saturates and unsaturates would be incomplete. In addition the life of the silicone liquid phase would be considerably shortened. The utility of the present method could be improved by increasing the sensitivity. This could be accomplished by sacrificing some resolution and using a larger diameter column. A disadvantage to the use of open tubular columns is the difficulty in obtaining positive identification of the components in a complex sample such as auto exhaust gas. Retention data are insufficient for positive identification, because other materials such as oxygenated products of incomplete combustion are known to be present in
auto exhaust (4). Some work in th!s laboratory with larger diameter open tubular columns (0.02-inch i.d.) and small split ratios (20:1) has demonstrated the utility of time-of-flight mass
Table IV.
Sample : Integrator: Recorder:
Sensitivity Determination
5.2 ml., 150:l split, 760 mm. Hg., 25' C. 4.4 X lo-* mv. see./
count
2 mv. full scale; 1 mv. = 12.7 em. Chart speed: 6 inches per minute Carrier flow: 4.0 ml./min. Amplifier gain: 2.4 X 10-12 amps./mv.
Component n-But ane
Sensitivity
Concn., p.p.m. 1010
= 0.037
Integrator counts
48,920 50,660 49,110 Av. = 49,563
coulombs/mole
VOL. 38, NO. 1, JANUARY 1966
47
spectrometry for identification of components resolved on a capillary column a t a concentration of 100 p.p.m. Further refinement in this technique should provide adequate sensitivity for identifying exhaust hydrocarbon Ponents resolved On an Open column.
LITERATURE CITED
(1) Altshuller, A. P., J . Gas Chrom. 1, No. 7, 6 (1963). (2) Averill, W., Ettre, L. S., Nature 196,
1198 (1962). (3) Bryner, N., Callen, J. E., Weiss, ?*I. D., Gas Chromatography,” pp. 30727, Academic Press, New York, 1962. (4) Hum, R. W., Hughes, K. J., J . Air
Pollution Control Assoc. 10,367 (1960). (5) McEwen, D. J., AXAL. CHEM.35, 1636 (1963). (6) Ibid., 36, 279 (1964). RECEIVEDfor review March 8, 1965. Accepted November 3, 1965. Division of Water, Air, and Waste Chemistry, 149th Meeting, ACS, Detroit, hlich., April 1965.
A Method for Deriving Metastable Ion Transitions in Hydrocarbon Mass Spectra R. E. RHODES, M. BARBER,’ and R. L. ANDERSON Mellon Insfifute, Pittsburgh, Pa. 7 52 7 3 A method is described in which a computer is used to derive the transigiving rise tions, m l + + m2+ to metastable ions in a mass spectrum. The metastable ions are measured to less than 0.1 m/e units from the output trace of a direct writing oscillograph and used as input data in conjunction with the parent and fragment peaks from the spectrum. From the relation m* = m22/m1, where m* is the m / e of the metastable ion, the computer derives all mathematically possible transitions within certain prescribed limits and thus provides a small and coherent list of transition possibilities from which the analyst may select the most probable transition. A discussion of the errors introduced into the method is presented. As an example, the metastable ions in n-decane have been analyzed by this technique and from the list of computer-derived transitions a fragmentation process has been proposed.
+
I
mass spectrometer it is possible for an ion that has received the full acceleration from the source to fragment during its flight between the electrostatic and magnetic analyzers. When fragmentation occurs in this region, the process is indicated by a metastable peak. This peak is characterized by having low intensity and a broad, almost Gaussian shape. The Nier-Johnson design of double focusing mass spectrometers has been shown to be particularly advantageous for detecting these metastable ions (1). It has been shown previously @ , 3 )that the appearance of a metastable ion gives almost certain evidence that a one-step decomposition process has taken place, and the fragments involved yield very useful information concerning N A DOUBLE FOCUSING
1 Visiting Fellow, permanent address, A.E.I. Ltd., Manchester, England.
48
ANALYTICAL CHEMISTRY
the structure of unknown compounds. When the number of metastable ions is large, as in compounds of high molecular weight such as n-hexadecane, with more than 80 metastables (4), and n-decane with more than 40, the assignment of transitions ml+ + m2+ mSo, by the relationship m* = m22/m1, where m* is the m/e of the metastable peak, is by no means routine. (Current investigations on the metastable ions appearing in the paraffin series have been tabulated in each of the compounds from CS to Cle. There is no doubt that at least 80 metastable ions exist in n-hexadecane. A rerun of the compound a t greater sensitivity and source pressure has yielded many more weak metastable ions not previously observed. At this time 108 metastable ions have been tabulated.) This is due to the many combinations of ml and mz which can yield approximately the same m*. In order to determine which particular transition actually produces the observed metastable ion, it is necessary to exhaust all combinations of ml and m2 that give a calculated m* within a prescribed distance of the recorded peak. The resulting family of transitions must then be considered in light of known physical and chemical constraints, which will normally rule out a large percentage of the theoretical transitions, leaving only a few remaining to be considered in determining the true decomposition process. To facilitate this type of analysis, a computer program has been written which will accept a list of observed metastable peaks and will calculate all mathematically possible transitions for this peak subject to certain constraints which appear as input data in the program. In this paper, we will describe the method, the required data for the computer program and the computerderived transitions from the metastable ions appearing in n-decane. From the
+
observed parent to daughter transitions, a fragmentation scheme is proposed which accounts for all the major fragment peaks in the spectrum. In addition, some of the errors of the technique are discussed. EXPERIMENTAL
The type of mass spectrometer used in this study is an A.E.I. hZS-9 doublefocusing instrument. n-Decane was introduced through the heated inlet system with resolution of the instrument 100. Source set for -1000 a t m/e focusing controls and ion repeller were set so that the ion beam was maximum and would pass through the center of the monitor slit which is positioned between the electrostatic and magnetic analyzers. In order to increase metastable ion intensity, sample pressure of -1 X 10-5 torr. was used. Scan rates and chart drive speeds were chosen arbitrarily so as to provide sharp normal mass peaks and well-defined metastable peaks. The separation between the normal mass peaks in the m/e 100region was then about 5 mm.
-
RESULTS AND DISCUSSION
Figure 1 shows a portion of the mass spectrum of n-tetradecane. This scan is typical of the metastable ion pattern in paraffins. Close lying metastable peaks, such as those between m/e 32 and 34 are easily resolved from each other. In this case, the m/e value for each metastable ion is interpolated over 2 mass units rather than 1. The metastable peaks which appear a t 32 and 43 are indicative of those whose measurements are complicated by the superimposed normal fragment peak. However, in these cases, by using the base width of the metastable peak as a guide, a metastable peak can be approximated. As can be seen from Table I, the error between m*obsd and m*calod.is still less than 0.1 m/e. Measurement of
