since both charge yield and resonant energy photon flux are proportional to the number of sodium atoms. It follows that ion pulses due to sodium-containing particles can be subtracted from the total charge delivered in pulse form to yield data on other particulate matter entering the flame. Work on an instrument using this theory of operation is in progress. It is of interest to calculate the sensitivity limit of the flame ionization method for sodium-containing aerosol detection. Assuming a charge detection limit of coulomb ( I ) , one calculates that this method can detect approximately g of sodium, corresponding to a sodium chloride particle with a diameter of about 0.1 p. This figure compares favorably with that of Crider and Strong (.?), who found a sensitivity limit of 0.25 p for sodium chloride aerosols using the flame ionization method. One further aspect of the flame ionization method deserves investigation. It is observed that the volatilization rates of inorganic particles appear to be at least three orders of
magnitude less than those of organic particles. For instance, Crider and Strong found that sodium chloride particles with a diameter of 1 p volatilized in about 2 msec. This compares with a volatilization time of 1 psec for an organic particle with the same diameter ( I ) . It follows that the use of a high-pass filter would allow the use of the flame method to determine information on organic aerosols in the presence of a large background of inorganic particles provided the respective particle diameters were not too different. RECEIVED for review September 23, 1968. Accepted November 6, 1968. Financial support from the National Science Foundation (NSF GP-5472X) is gratefully acknowledged. Work was taken in part from a thesis by E. Thall, submitted in partial fulfillment for the M.S. degree in chemistry at New Mexico Institute of Mining and Technology; and is based on a paper presented at the 155th National Meeting, ACS, San Francisco, Calif., April 1968.
Gas Chromatographic Characterization by Equivalent Degree of Polymerization and Incremental Equivalent Chain Length Constants Application to Poly(Ethy1ene Glycol) and Ethylene Glycol Derivatives Thomas K . Miwa Northern Regional Research Laboratory, Peoria, Ill. 61604 Every atom or functional unit in an entire molecule can be assigned an incremental Equivalent Chain Length (ECL) that contributes to the totalized ECL of the molecule. For example, in ethylene glycol each terminal hydrogen atom, oxygen atom, and methylene unit has an incremental ECL of -0.30, +2.30, and +O.lo, respectively. Such an assignment of characteristic constants is based on the linear relationship that exists between the degree of polymerization of poly(ethy1ene glycol) and the logarithm of its isothermal gas chromatographic retention time. Terminal alkyl substituents affect the retention of their parent glycols by characteristic constant values, observed as deviations in retention from that of the parent glycols and expressed either as Equivalent Degree of Polymerization (EDP) or as ECL. The equation for conversion is ECL = 2.5EDP 1.7, when Apiezon L is the liquid phase at 230-250 OC. Application of the incremental ECL method is illustrated for (a) ethylene glycol and its oligomers; (b) commercially available monoalkyl ethers of ethylene glycol and di(ethylene glycol); (c) the dimethyl ethers, diglyme and triglyme; (d) the cyclic ethers, p-dioxane and tetrahydrofuran; (e) the structural derivative by dehydroxylation of di(ethylene glycol), ethyl ether; and (f) the hydroxymethylated derivative of ethylene glycol, glycerol.
+
CORRELATIONS BETWEEN gas chromatographic retention and (chemical structure were observed recently for free poly(ethylene glycol) (PEG) when the distribution in size of PEG in surface-active brassylic acid-thylene oxide adducts was estimated by gas-liquid chromatography (GLC) ( I ) . Retention times of PEG were related, either semilogarithmically or linearly, to the degree of polymerization (DP) of ethylene oxide in the PEG.
The present investigation shows that alkyl substituents at the terminal positions affect the retention of ethylene glycol and PEG by characteristic constant values, observed as deviations in retention from that of the parent glycols. These deviations are expressed either in Equivalent Degree of Polymerization (EDP) or Equivalent Chain Length (ECL) (2, 3), which is a chromatographic concept applicable to derivatives of a series of homopolymers or of homologs. This paper reports the first application of the ECL principle to the characterization of a series of compounds by the assignment of incremental ECL constants to individual atoms and functional units throughout each molecule. Summation of these incremental ECL gives a calculated value for each molecule that agrees with the experimentally determined ECL. The method allows rapid identification or prediction of structure of unknown but related components, which are represented as peaks in gas chromatograms or listed in printost; from the computer. EXPERIMENTAL
GLC. A Burrell Kromo-Tog K-5 gas chromatograph with a thermal conductivity detector was used with a U-shaped, glass column (0.6 cm i d . , 275 cm long), packed with 10% Apiezon L grease on acid-washed, silylated Chromosorb W (60-80 mesh). Helium flow was held at a constant 40 ml/ minute, 40 psig. The temperature of the column bath was 250 OC for analyses of PEG and 230 "C for analyses of the more volatile ethylene glycol derivatives. The combination of 10 ml/minute with 250 "C was also used for the volatile derivatives. Retention times were measured from the air peak. All components, except those in the mixture of PEG, ~~~~~~
(1) T. K. Miwa, R. V. Madrigal, W. H. Tallent, and I. A. Wolff, J . Amer. Oil Chem. SOC.,45, 159 (1968).
(2) T. K. Miwa, K. L. Mikolajczak, F. R. Earle, and I. A. Wolff, ANAL.CHEM., 32, 1739 (1960). (3) T. K. Miwa, J . Amer. Oil Chem. SOC.,40,309 (1963). VOL. 41, NO. 2, FEBRUARY 1969
307
t I
A
A
A
0
1
2
3
0-
A
A
A
4 5 6 Degree of Polymerization
7
8
A
A
Figure 1. Linear relationship between logarithm of retention time and degree of polymerization of poly(ethylene glycol) Isothermal analyses on 10 2 Apiezon L grease at ( A ) 250 "C,40 ml/minute; ( B ) 250 "C, 10 mI/minute; and (C) 230 "C, 40 mI/minute
were identified individually and in combinations and were compared with a standard mixture of mono-, di-, and tri(ethylene glycols) and with an eleven-component, standard mixture of C&Z n-alkanoic acid methyl esters. ECL was determined as reported earlier ( 2 , 3). EDP was determined similarly by plotting EDP in place of ECL or by solving the Kovats-type (4) equation:
where SI and SZare known EDP values of standard glycols and t s l , tS2, and tz are retention times of the two knowns and the one unknown. EDP is thus defined as a retention value that is equivalent to the retention of a fraction or a multiple of the monomeric unit in the polymer. Materials. Mono-, di-, and tri(ethy1ene glycols), their alkyl derivatives, and the methyl n-alkanoates were purchased from commercial sources and used as received. Mixtures of PEG were prepared by the HC1-catalyzed methanolysis of brassylic acid-ethylene oxide adducts and the ensuing extraction of the water soluble PEG ( I ) . RESULTS AND DISCUSSION
EDP. A linear relationship between logarithm of retention time and D P of PEG was observed for isothermal analyses of ethylene glycol oligomers, as shown in Figure 1. For Curve A , the initial three components were identified by comparison with the standard mono-, di-, and tri(ethy1ene glycols). The remaining five were identified by their EDP alone ( I ) . When conditions were adjusted to increase the separation of the more volatile components--i.e., increasing the retention times by lowering either the carrier (4) E. Kovats, Helc. Chim. Acta, 41, 1915 (1958).
308
ANALYTICAL CHEMISTRY
-
0
2
4
6 8 EDP
1
0
Figure 2. Linear relationship between logarithm of retention time and degree of polymerization of poly(ethy1ene glycol) and its direct convertibility to equivalent chain length Isothermal analysis on 10% Apiezon L grease at 250 "C, 40 ml/minute
gas flow rate to 10 ml/minute (Curve B ) or the column bath temperature to 230 "C (Curve CFethylene glycol showed a linear relationship with di- and tri(ethy1ene glycols) on the semilogarithmic plot. This linearity was utilized in the assignment of identification constants to ethylene glycol derivatives. Each substituent affected the retention of its parent glycol by a characteristic constant value, observed as a deviation in retention from that of the parent glycol and expressed as EDP. For example, when the glycols were converted to methyl, ethyl, or butyI ethers, each substituent affected the parent glycols by an EDP of -0.1, +0.1, or +0.8, respectively (Table I). The value for diglyme and triglyme were 0.2 unit lower than those of their parent glycols and also differed from each other by 1.0 EDP. The consistencies in the effects of ethyl and butyl substitutions were manifested by their monoethers of ethylene glycol and di(ethylene glycol) as seen in Table I. ECL by Increments. In the isothermal analyses of PEG, the logarithm of retention time was also linearly related to the ECL of the PEG. This relationship enabled the interchange of EDP with ECL as illustrated in Figure 2. Variations in carrier gas flow or column temperature will change the retention time scale but will not change either the slope or the intercept. The equation for conversion of retention expressed as EDP to the same retention expressed as ECL, when Apiezon L grease is the stationary liquid phase at 230250 "C, is as follows: ECL
=
2.5 EPD
+ 1.7
Consequently, a retention of 1 EDP unit is equal to 2.5 ECL units when long chain, methyl n-alkanoates are used as standards for ECL units. The intercept at 1.7 ECL is due to the contribution of the terminal moieties H O - H , which structurally complement ethylene oxide and its polymers in the formation of diols, Starting with the two values: (CHzCHzO-) = 2.5 ECL, and HO-(. a)-H = 1.7 ECL, it was possible to assign ECL constants to every constituent in the molecule for a number of ethylene glycol derivatives (Tables 11, 111). ECL was chosen over EDP for incremental assignments because of the precedent of ECL assignment to functional units that are attached to a linear polymethylene backbone (2, 3), and because of the clear contrast between the -CHzin a polymethylene backbone and the X H 2 - in an ethylene glycol derivative. By definition, the -CHin a polymethylene backbone has an ECL of 1.O. Conversion of a glycol to a methyl ether, which is equal to a methylene insertion at the terminal position, gave a loss of o.25 ECL per insertion. The twofold deviation-Le., -o.& ECL x 2-observed for diglyme and triglyme proved the applicability of this technique to derivatives that no longer possessed the basic functional group; namely, hydroxyl. This observation led to the further resolution of the intercept in the equation into components representing specific parts of the molecule. Monoethyl ethers of ethylene glycol and di(ethy1ene glycol) both gave +0.20 ECL for ethylene insertion, and their butyl analogs gave +2.00 for tetramethylene. The value +2.30 for oxygen in hydroxyl was determined by contrasting both di(ethy1ene glycol) with ethylene glycol monoethyl ether and tri(ethy1ene glycol) with di(ethy1ene glycol) monoethyl ether. Knowing the values +2.30 for oxygen, +2.50 for oxyethylene, and 1.70 for H O - ( . .)-H, the ethylene moiety in ethylene glycol then was assigned f0.20 and the terminal hydrogens were assigned -o.30 ECL. Because both -CH*groups in this ethylene moiety have identical structural environment, this methylene has an ECL of +O.lo. The terminal ethylene, which does not have such symmetrical environment, most probably does not have the same electron distribution as the internal ethylene, even though both have an identical +0.20 ECL. An incremental assignment of ECL to atoms and units in di(ethy1ene glycol) monoethyl ether is given below as an illustration of the technique:
-
+
CH2
CH2
0
H -0.30
0 4-2.30
so.10 +o.lo f 2 . 3 0
CHz fO.10
CHz +o.lo
0 $2.30
CH2 f0.45
CHz -0.26
H -0.30
The breakdown of the terminal ethylene unit was achieved by systematically calculating the additive effect of each methylene as it was being inserted between the terminal unit and the oxygen. A similar breakdown of the tetramethylene unit in the monobutyl ether of ethylene glycol or di(ethy1ene glycol) allowed the prediction that the propyl derivatives, which are not commercially available, would have respective ECL of 5.20and 7.70under similar experimental conditions. Cyclic derivatives of ethylene glycol require, in addition to the sum of the incremental ECL, a value that corresponds to the two terminal hydrogen atoms that were deleted. For example, p-dioxane is equivalent in structure to the cyclic dimer of ethylene oxide, the anhydro derivative of di(ethylene glycol), or the dehydrocyclization product of ethylene glycol monoethyl ether. The totalized ECL from the increments
______~
Table I. Equivalent Degree of Polymerization (EDP) of Ethylene Glycol Derivatives (Apiezon L, 230-250 "C) Compound Structure EDP Ethylene glycol HO -CHZCHZO- H 1.o Di(ethy1ene glycol) HO (CHzCH20)zH 2.0 Tri(ethylene glycol) HO (CHICHzO)3H 3.0 Ethylene glycol monomethyl ether HO -CHzCHzO- CHa 0.9 Dig1yme H E 0 (CHzCHz0)zCHa 1.8 Triglyme Hac0 (CHzCHz0)sCHs 2.8 Ethylene glycol monoethyl ether HO -CHzCHzO- CHzCHJ 1.1 Di(ethy1ene glycol) monoethyl ether HO (CH2CH20)zCHzCH3 2.1 Ethylene glycol monobutyl ether HO -CHzCHzO- CH2CH2CHzCH3 1.8 Di(ethy1ene glycol) monobutyl ether HO (CHzCHz0)2CHzCHzCH2CH3 2 . 8 Table 11. Incremental Equivalent Chain Length (ECL) of Atoms and Units in Ethylene Glycol Derivatives (Apiezon L, 230-250 "C) Incremental Atom or unit (in italics) ECL (each) H 0 CHz CHz 0 CHz CHz 0 H -0.30 H 0 CHz CHz 0 CHz CHz 0 H 2.30 H 0 CHz CHz 0 CHt CHz 0 H 0. lo H 0 CHz CHz 0 CHz CHz 0 H 0 . lo H 0 CHz CHz 0 CHz H -0.26 H 0 CHz CHz 0 (CHzCHz) H 0.20 H 0 CHz CHz 0 (CHzCHzCHz) H 1.W H 0 CHz CHz 0 (CHZCH~CHZCHZ) H 2.00 H CHz 0 CHz CHz 0 CHz CHs 0 CHz H -0.25 H 0 CH2 CH(0H) CHz 0 H -0.30 H 0 CHz CH(0H) C H z 0 H 0.70 H 0 CHz CH(0H) CHz 0 H 2.30 Predicted. Q
Table 111. Calculated and Experimental Equivalent Chain Length (ECL) of Ethylene Glycol Derivatives (Apiezon L, 230-250 "C) Compound ECL, calculated ECL, found Ethylene glycol 4.20 4.20 Di(ethylene glycol) 6.70 6.70 Tri(ethy1ene glycol) 9.20 9.20 Ethylene glycol monomethyl ether 3.95 3.95 Diglyme 6.20 6.17 Triglyme 8.70 8.68 Ethylene glycol monoethyl ether 4.40 4.39 Di(ethy1ene glycol) monoethyl ether 6.90 6.88 Ethylene glycol monobutyl ether 6.20 6. 20 Di(ethylene glycol) monobutyl ether 8.70 8.70 p-Dioxane 4.40 4.33 Tetrahydrofuran 3.70 3.82 Ethyl ether 1.40 1.40 Glycerol 6.60 6.62
was expected to be 5.00,but experimentally was found to be 4.33. Evidently, under the prescribed GLC conditions, the formation of a bond between two incomplete ends of a single chain to yield a nonstrained cyclic structure is equivalent to the addition of two hydrogen atoms at the terminal positions to comp!ete the chain.
VOL. 41, NO. 2, FEBRUARY 1969
309
Glycerol can be considered a derivative of ethylene glycol where hydroxymethyl (ECL = 4.20 i2) has replaced a hydrogen (ECL N -0.30) in one of the methylenes. The experimental ECL was identical to the calculated, and this agreement allows the assumption that all hydrogens in ethylene glycol have the same incremental ECL and that each of the two carbon atoms has an incremental ECL of +0.7,,. The foregoing results provide the basis for rationalizing the observed ECL of two ethers that could be considered the cyclic and noncyclic dehydroxylated derivatives of di(ethy1ene glycol). Tetrahydrofuran, also named tetramethylene oxide, has an experimental ECL that corresponds to the sum of an internal oxygen, a tetramethylene unit, and a ring closure (or hydrogen termination). The higher volatility of its noncyclic equivalent, ethyl ether, is probably due to the difference in structure between the tetramethylene chain and the two short ethylenes. Furthermore, ethyl ether is exactly equal to a combination of two terminal methyl groups (as in diglyme) with an internal ethylene oxide unit (ECL = 2S0), if this unit is viewed as -CHz-O-CHzas found in the middle of the di(ethy1ene glycol) chain. This view is justified because the conversion of ethylene glycol to di(ethy1ene glycol) is equal, structurally, to the insertion of the symmetrical -CH2-O-CH2unit into the middle of the ethylene glycol
chain, although, mechanistically, 4H2-CH2--Ois inserted at the terminal position. These observations reveal the potentialities of the ECL method in alerting the experimenter to the differences that exist in electronic environments for a structural unit, such as for ethylene in ethylene glycol, ethylene glycol monoethyl ether, and ethyl ether. The calculated and experimentally determined ECL of ethylene glycol and its derivatives are listed in Table 111. Experimental values were constant over the range 230-250 “C. ACKNOWLEDGMENT
Dr. W. H. Tallent contributed helpful suggestions in the preparation of this manuscript.
RECEIVED for review August 19, 1968. Accepted November 15, 1968. Presented at the Great Lakes Regional Meeting of the American Chemical Society, Milwaukee, Wisconsin, June 13-14, 1968. The Northern Regional Research Laboratory is headquarters for the Northern Utilization Research and Development Division, Agricultural Research Service, U S . Department of Agriculture. The mention of firm names or trade products does not imply that they are endorsed or recommended by the Department of Agriculture over other firms or similar products not mentioned.
Identification of Aromatic Ketones in Cigarette Smoke Condensate J. H. Bell, Sue Ireland, and A. W. Spears Research Division, Lorillard Corp., Greensboro, N . C . Because of the complexity of cigarette smoke, extensive fractionation was necessary to isolate and identify minor components. The separation techniques involved solvent partition, column, paper, and gas chromatography. The gas chromatographic system allowed the collection of smoke constituents for subsequent analysis by ultraviolet and infrared spectroscopy and mass spectrometry. From the study of one subfraction which represents 0.08% of the original weight of the condensate, fluoren-9-one, the four methylfluoren-9-ones and seven other alkylated fluoren-9-ones were identified.
IN RECENT YEARS a number of papers have appeared dealing with the fractionation of cigarette smoke condensate into acidic, phenolic, basic, and neutral fractions. The ether soluble neutral fraction is a large and complex fraction and has received considerable attention. While the number of investigations dealing with the separation of the neutral fraction are too numerous to cite, bibliographies can be found in recent publications by Wynder and Hoffmann ( I ) and Stedman (2). The neutral fraction is usually separated by column chromatography into a series of subfractions of increasing polarity, and from this point the separation scheme is designed for the isolation and determination of either a single compound or a particular group of compounds. The intention of this work was to reproducibly fractionate the smoke condensate to the extent that even the minor smoke constituents -___
(1) E. L. Wynder and D. Hoffmann, “Tobacco and Tobacco
Smoke,” Academic Press, Inc., London and New York, 1967. (2) R. L. Stedman, Chern. Rev., 68,153 (1968). 310
ANALYTICAL CHEMISTRY
could be isolated in sufficient quantities to obtain reliable identification, and therefore more completely characterize the composition of cigarette smoke. EXPERIMENTAL
Smoking and Collecting of Condensate. Nonfilter cigarettes of 85 mm length were smoked on an automatic smoking machine with a capacity of 100 cigarettes, 20 of which were smoked simultaneously. The cigarettes were smoked to an approximate 20-mm butt with a puff frequency of 3 puffs/min and a puff duration of 2 sec. The smoke was drawn into a tubular shaped trap which was maintained at room temperature and then passed into a series of three 2-liter flasks submerged in a dry ice-methanol slurry. After the smoking of each 12,000 cigarettes, the condensate was collected from the traps. Because of the great number of cigarettes being smoked, a rather large volume of water was also condensed. By warming the collection vessels with hot water, the condensate could be poured from the flasks and collected without the use of organic solvents. The initial separation of the condensate followed the general procedure of extracting an ethereal solution of the condensate with HzSOa and NaOH solutions. The ether portion was dried over Na2SO4and the ether evaporated at a low temperature under reduced pressure. Approximately 90 grams of the neutral fraction thus obtained was chromatographed on a 135-cm X 6.5-cm column containing 1400 grams of florisil (60/100 mesh) which had previously been washed with methanol and reactivated at 165 “C for 3 hours. Fractionation of Condensate. As seen in the fractionation scheme (Figure l), the column was eluted with increasingly polar solvents. The NPfraction, which is similar to fraction
