rather than with the tnaximum, as would be expected if the true second derivative of the thermometric titration curve were obtained. The required filtering, however, prevents presentation of the true second derivative. This does not diminish the usefulness Qf the procedure, for the end point may be selected precisely and accurately if the origin, rather than the maximum, of the deflection is chosen. The results of several titrations are presented in Table I. End point was selected, in each case, a t the origin of the deflection in the second derivative curve. Titrant delivery rates were 1 ml. per minute.
Table 1. Titration of Sodium Hydroxide with Hydrochloric Acid
Base Taken, Mmoles
Acid Concn., N
1.25 2.50
0.25 0.50
Relative Mean Error,
%
No. of Detns.
0.4 0.2
16 6
port of a part of this work by a Cottrell Grant of the Research Corp. LITERATURE CITED
( 1 ) Brown Instruments Division, Engi-
neering Dept., Minneapolis-Honeywell Regulator Co., Philadelphia, “Adaptability of the Measuring
ACKNOWLEDGMENT
The authors wish to acknowledge sup-
Circuit, Input Circuit, and Amplifier of the Brown Electronik Potentiometer,” Tech. Bull. B15-10
(1950). (2) Greenwood, I. A., Holdam, J., Jr., MacRae, D. Jr., “Electronic Instru~
ments,” Radiation Laboratories Series, MIT, Vol. 21, pp. 64-78, New York, McGraw-Hill, 1948. (3) Jordan, J., Alleman, T. G., ANAL. CHEM.29, 9 (1957). (4) Linde, H. W., Rogers, L. B., Hume, D. N., Ibid., 24, 1348 (1952). (5) . , Malmstadt. H. V.. Roberts. C. B..
Zbid., 28,‘ 1408 (1956).. (6) Zenchelsky, S. T., Penale, J., Cobb, J. C., Ibid., 28, 67 (1956).
RECEIVED for review June 6, 1957. Accepted August 8, 1957. Division of Analytical Chemistry, Beckman Award Symposium Honoring Ralph H. Muller, 131st Meeting ACS, Miami, Fla., April 1957.
Distribution of n-Paraffins and Separation of Saturated Hydrocarbons from Recent Marine Sediments E. D. EVANS, G. S. KENNY, W. G. MEINSCHEIN, and E. E. BRAY Field Research laboratory, Magnolia Petroleum
The hydrocarbons in recent marine sediments are of interest because of their possible relationship to petroleum. Certain differences in sediment and crude oil hydrocarbons have been observed. In the investigation of these differences alumina and silica gel chromatographic separations were used and significant variations were found in the fractions obtained from these adsorbents a t high gel-sample ratios. To facilitate the study of the separation on the alumina and silica gel columns, cholestane and n-hexacosane were added to a saturated hydrocarbon fraction of a recent marine sediment. Mass spectrometric analyses of the chromatographic fractions established the efficiency of the separation of naphthenic and paraffinic hydrocarbons on alumina columns a t different adsorbent-sample ratios, and showed that n-paraffin mixtures are separated on a molecular weight basis on alumina a t certain adsorbent to sample ratios. Silica gel was not so selective as alumina for the described separations. The n-paraffins in recent marine sediment extracts, unlike petroleum paraffins, contain higher concentrations of odd-carbon-number molecules than of their even-carbon homologs.
T
HE COMPOSITION of the organic extracts of recent marine sediments is of interest because most crude oils
1858
ANALYTICAL CHEMISTRY
Co., Dallas,
rex.
are found in marine environments and are believed by many authorities to have originated in marine sediments. Evidence in support of this view was who showed the provided by Smith (I), presence of liquid hydrocarbons in the extracts of recent marine sediments, Similar extracts have been studied in the authors’ laboratory, and as reported by Stevens, Bray, and Evans (6) the hydrocarbons in shallow, recent marine sediments in the Gulf of Mexico differ in certain respects from those found in crude oils. Mass spectra indicate, for example, that the n-paraffins in crude oils are evenly distributed between molecules of even and odd carbon number while those from the recent marine muds have higher concentrations of molecules of odd carbon number. The greater concentration of odd-carbon n-paraffins suggests that these hydrocarbons in sediment extracts more closely resemble plant and animal paraffins than petroleum paraffins. This paper presents conclusive proof that the n-paraffins in some recent marine sediments have a preference for odd-carbon-number molecules. Silicagel and alumina chromatography were used to fractionate the sediment extracts and differences were observed in the fractions obtained from these adsorbents. I n particular, the mass spectra of the n-heptane eluates from alumina did not show the odd-carbon preference that was apparent in these
fractions from silica gel. Because of the general need for an efficient means to separate saturated hydrocarbons, the separations of these hydrocarbons on alumina were investigated. It was found that n-paraffis, cycloalkanes, and n-paraffins of different molecular weights can be separated on alumina a t high adsorbent-gel ratios. REAGENTS
Silica gel, commercial grade, 100 to 200 mesh, Davison Co. Activated prior t o use a t 42.5” C. for 6 hours. Alumina, activated powdered catalyst grade AI-OlOlP, Harshaw Scientific Co. Activated prior to use a t 343” C. for 15 hours. Extraction and chromatographic solvents. All solvents were carefully distilled and checked to contain less than 0.3 y per ml. of impurities which were not volatile under the conditions employed for sample recovery. Urea, C.P. or equivalent, recrystallized from ethyl alcohol. n-Hexacosane and cholestane, obtained from Pennsylvania State University. PROCEDURES
Silica G e l Chromatography. The marine sediment extracts were fractionated on two sizes of columns. Large samples, 0.3 to 1 gram, were separated on 120-gram gel columns 24 cm. in length and 28 mm. in diameter. Small samples, 20 mg. to 0.3 gram, were fractionated on 9-gram gel columns 18 cm. in length and 9 mm. in diameter.
---ADDED
used by Clerc, Hood, and O'Sm.1 (1). All peak heights plotted in the figures are corrected and normalized in this manner. The operating conditions of the mass spectrometer were: Ionization current, pa. 45 Magnet current, amperes 1 . 1 5 Temperature, " C. Ionization chamber 250 Inlet system 340 Scan rate, volts 3600 to 250 in 35 minutes
n-HEXACOSANE
IC -
Figure 1 . Urea separation of hydrocarbon mixture
500--
5 1
:il 0
+ DISCUSSION A N D RESULTS 0
1
20
25
30
/ I35\
NUMBER O F C A R B O N A T O M S I N n - P A R A F F I N
The gel-total extract ratio in all chromatographic columns was approximately 300 to 1, and the gel-saturated hydrocarbon ratio was roughly 6000 to 1. The extracts were placed on the silica gel columns, which were prewet with nheptane. Fifteen milliliters of n-heptane were passed through the smaller columns and 165 ml. through the larger columns. The eluates were recovered by blowing away the eluent with filtered air a t 40" C. Alumina Chromatography. The fractionations of the total extract on alumina were made in the same manner as for silica gel. T h e saturated hydrocarbons were separated on 90gram alumina columns 18 cm. in length and 28 mm. in diameter, with the exception of the separation made a t a gel-sample ratio of 1000 to 1. The latter sample was run on a 36gram column 18 cm. in length and 17 mm. in diameter. The gel-sample weight ratios were varied. n-Heptane followed by carbon tetrachloride was used as eluent. The volume of each eluent was 150 ml. on the 90-gram columns and 60 ml. on the 36-gram column. The n-heptane and carbon tetrachloride eluates were recovered by blowing off the solvents. Preparation of Hydrocarbon Mixture. A mixture of hydrocarbons was prepared by adding measured volumes of standard solutions of n-hexacosane and cholestane in carbon tetrachloride to a weighed amount of the saturated hydrocarbon fraction of recent marine sediment extracts obtained by chromatographic separation on silica gel.
I 38
IONS
The composition of the hydrocarbon mixture was 4% cholestane, 21% nhexacosane, and 75% saturated hydrocarbons from recent marine extracts. Aliquots of the carbon tetrachloride solution of the hydrocarbons were transferred to vials and the carbon tetrachloride was removed by blowing the vials to constant weight with air a t 40" C. These aliquots of the hydrocarbon mixture were used for the analyses and separation described. Urea Adduction. A urea adduct was prepared by adding 1 ml. of methanol saturated with urea t o a solution of 22.8 mg. of the prepared hydrocarbon mixture in 3 ml. of benzene and 1 ml. of methanol. After 24 hours, the adduct was transferred t o a sinteredglass filter and washed with benzene. The filtrate and washings were extracted with distilled water, and the hydrocarbons which did not form adducts were recovered from the benzene layer. The adduct and sintered-glass filter were placed in a 50-ml. beaker and heated with 5 ml. of distilled water. The adducted hydrocarbons were extracted from the water phase with benzene and recovered. Mass Spectrometry. Mass spectra were obtained with a Consolidated Electrodynamics Corp., Model 21-103, mass spectrometer with modifications similar t o those described in the literature ( 3 ) . The mass spectral data m-ere corrected t o a monoisotopic basis for carbon and hydrogen, normalized to the major peak which was given the value of 100,000, and arranged in the grid form
I n the arrangement of mass spectral data the peak heights, which indicate the number of ions of a particular mass, are placed in 14 columns that correspond to 14 values of z in the general hydrocarbon formula CnHpn+r.The peak heights of ions having thc same masses as n-paraffins appear in the z = +2 column. I n hydrocarbon mixtures of high niolecular weight, the peaks a t large carbon numbers in even z columns are produced in part by the positive ions of the unfragmented molecules and are commonly referred to as parent peaks. The parent peaks of a homologous series are recorded in a single z column. However, in complex hydrocarbon mixtures more than one homologous Feries may be placed in the same columnfor example, heptacyclic naphthenes (z = -12) have the same masses as n-paraffins ( z = +2) with one less carbon atom. The parent peaks of these compounds would ail appear in the z = +2 column. The parent peaks of branched-chain paraffins are also recorded in the z = +2 column, but these peaks are so small that their contributions to the parent peak sizes may be ignored. The mass spectra of the saturated hydrocarbon fractions of organic extracts of recent marine sedimentsi.e., sediment-saturate fractions-have larger peaks a t odd than a t even carbon numbers in the 23- to 35-carbon number range of the z = +2 column. These larger peaks indicate that oddcarbon-number n-paraffins are predominant. However, as the peaks are arranged on the basis of mass, it is possible that the odd-carbon preference could be caused by even-cnrbonnumbered C,H2.-12 hydrocarbons. Elemental, infrared, ultraviolet, and mass spectrometric analyses of the authors' silica gel chromatographic fractions have shown that the n-heptane eluate is composed primarily of saturated hydrocarbons which are free of aromatics and nonhydrocarbons. The only compounds, other than n-paraffins, that could produce the maxima in the z = +2 (or -12) column in the mass spectra of these n-heptane eluates are heptacyclic, olefinic hexacyclic, diolefinic, pentacyclic, or other compounds having a total of rings plus double bonds VOL. 29, NO. 12, DECEMBER 1957
1859
$
500
:I-
29
0 430 V
k-
5
50-
A L U M I N A G E L ( n - HEPTANE E LIJATh,
\ ]
LL
equal to seven. Actually, the concentrations of molecules containing two or more olefinic bonds n-ould be very low in this chromatographic fraction. However, the argument that follows assumes that olefins are possible contributors to the z = +2 (or -12) peaks. I n order to determine which of the possible compound types cause the oddcarbon preference, n-hexacosane, a Cz0 n-paraffin. and cholestane, a C27 tetraCyclic naphthene, were added as tracers to a sediment-saturate fraction. This hydrocarbon mixture n as separated into two fractions by urea adduction. Mass spectrometric analyses were obtained of the two fractions. The n-liesacosane and cholestane v,-ere almost quantitatively separated into the urea adducted-Le.. adduct-and the nonadducted-i.e., nonadduct-fractions, respectively. The plots of the z = + 2 peak heights us. carbon numbers for the adduct and nonadduct are shown in Figure 1. The maxima a t odd carbon numbers and a t 26 carbons in the adduct prove that the compounds in the sedinient-saturate fraction n hich cause the odd-carbon preference are compounds that form urea adducts as does n-hexacosane. As urea forms adducts only with cyclic compounds having long n-alkyl substituents (3) it is impossible that heptacyclic, hexacyclic, pentacyclic, or even bicyclic molecules containing as IOK as 23 carbon atoms could form stable adducts with urea. I n fact, l(1-naphthyl) pentadecane, a 25-carbon bicyclic C,HZ,-I~ hydrocarbon with a 15-carbon n-alkyl substituent,. is nonadductable ( 2 ) . When it is considered that the conipounds which cause the odd-carbon preference have the same mass spectra and masses as n-paraffins, are saturated hydrocarbons, and are adducted by 1860
ANALYTICAL CHEMISTRY
Figure 3. Separation of n-heptane eluate from silica gel on alumina
Alumina-sample ratio "4500
0,
+++
n-HEPTANE
1 a,--
@, - n - H E P T A N E
ELUATE,SiLICA ELUATE, A-UMIYA
GEL GEL
CARBON TETQACHLOSIDE ELUATE,
f t
urea, it is evident that they are nparaffins. The odd-carbon preference displayed by the n-paraffins in these fractions provides a n interesting means of observing the difference in separations obtained on alumina and silica gel columns. The z = + 2 plots of the nheptane eluates from an alumina and a silica gel column are presented in Figure 2. The chromatographic separations were made a t gel-total extract ratios of 300 to 1, or a t gel-saturated hydrocarbon ratios of 6000 to 1. Although both eluates are primarily saturated hydrocarbons, the absence of maxima at odd-carbon number in the alumina fraction means that this fraction does not contain n-paraffins. Kaphthenes can be separated from nparaffins on alumina a t high gel to sample ratios. The separations on alumina were investigated by using aliquots of the fraction used previously for the urea adduction. These were fractionated a t various adsorbent to sample ratios using n-heptane, followed by carbon tetrachloride, as eluents. The weight percentages and mass spectrometric analyses of the n-heptane and carbon tetrachloride eluates are given in Table I. The analyses of the adduct and nonadduct are also presented in Table I to
1
ALUMINA
permit comparison. The percentages of the added n-hesacosane and cholestane in the fractions were calculated from parent peak heights and should be accurate to lvithin 147,. The analpes for alkanes (ALK), noncondensed cycloalkanes (NCCA), condensed cycloalkanes (CCA), and aromatics (ARM) were determined by the Shell hydrocarbon analysis (I), using, as suggested, a correction curve determined for the authors' instrument. These analyses give only approximate data, as is indicated by the percentages of aromatics in Table I. These percentages are too large, as ultraviolet studies show that the fractions do not contain measurable amounts of aromatics. However, these analyses provide useful information about the separations. I n Table I, as the aluminasample ratios increase, both the nlieuacosane concentrations and the weight percentages in the carbon tetrachloride fractions increase. The best separation of the added nhexacosane and cholestane occurred a t an adsorbent to sample ratio of 10,000 to 1. However, as is suggested by the increase in percentage of cholestane in the carbon tetrachloride eluate a t ratios higher than 5000 to 1, it is possible that the separation of paraffins and naphthenes could have been im-
proved a t adsorbent-saniplc ratios of 10,000 and 20,000 to 1 by using more n-heptane as a n eluent. In the Shell analyses, the perccntage of alkanes in the n-heptane eluates dccrcases continuously as the alurniii;t-sample ratios increase, and a t Iiigher ratios the aluinnia separations approach the wparation accomplished by urea adduction. In addition to the separations of paraffins and naphthenes, molecular weight separations of n-paraffins also occur. The percentage of the individual nparaffins in the n-heptane and carbon tetrachloride eluates at various aluminasample ratios are given in Table 11. The contribution of ions produced from polycyclic naphthenes, particular11 a t loii er carbon numbers, introduces some error in the values shown. I n Table 11, the paraffins of higher molecular n eight are concentrated in the cai bon tetrachloride eluate a t all adsorbcntsample ratios, and as the alumina-sample ratios increase, the concentrations of the n-paraffins of lower molecular n eight in the carbon tetrachloride fiactions increase. The niolpcular n eight eeparation of n-paraffins is shonn by the z = f 2 plots in Figure 3. The silica gel fraction is the original sample that was separated on alumina. The nheptane eluate from aluiiiina contains the n-paraffins of lou-er-molecular weight as shown by the maxinia a t lair-er carbon numbers in the z = f 2 plot ( B ) . The maxinia a t higher carbon numbers in plot C show that the carbon tetrachloride fraction contains the n-paraffins of higher niolecular n-eight CONCLUSIONS
The n-paraffins in recent marine sediment extracts haye higher concentrations of odd- than of even-carbon-number molecules. The mass spectra of the saturated hydrocarbon fractions containing these n-paraffins provide a n excrllent means of studying the separations of thr n-paraffin. and cvcloalkanes in t h r v fraction
