Applications of Linear Elution Adsorption Chromatography to the Separation and Analysis of Petroleum II.
Compound Class Separations by a Routine Micro Procedure. Determination of Gasoline Polya romatics L. R. SNYDER Union Research Center, Union Oil Co. of California, Brea, Calif.
b A general experimental procedure is presented for the use of micro linear adsorption chromatography in the routine separation of compounds and compound classes. The technique is quick, convenient, and reproducible. The determinations of total naphthalenes and of phenanthrene in gasoline are described as examples of its application to the analysis of petroleum components.
P
applications (6,4, 6) of elution adsorption chromatography to the custom analysis of complex mixtures largely depend on the collection and subsequent, characterization of numerous small fractions. Where linearity in the adsorption isotherm exists, however, much simpler routine procedures are possible. -4 preceding communication (7) has discussed several of the theoretical and experimental factors which govern such linear elution adsorption chromatographic (LEAC) separations. Three problems exist in the development of routine LEAC methods for the determination of specific compounds or compound classes. First, th_e separability of the various compound types it is required to separate must be established, w well as the c u t points in nn optimum chromatographic system. Previous papers have discussed this in some detail ( 7 , 9 ) . Second, the experimental procedure and equipment must be optimized with respect to convenience and reliability in routine operation. This is largely the subject of the present paper. Finally, the measurement of sample components in separated fractions is required. The low sampIe concentrations normally required in LEAC separations restrict applicable detection techniques to those sensitive to milligram per liter concentrations. Ultraviolet absorption is particularly suited to the measurement of aromatic compound classes because REVIOUS
of its sensitivity and the relative constancy of molar absorptivities. In addition to the discussion of experimental procedure and equipment which follows, the principles involved in the design of routine LEAC analytical methods are illustrated in the case of a simple model problem. EXPERIMENTAL
The experimental procedure and equipment to be described are adaptable to a wide range of routine LEAC separations. Detachable glass-Teflon columns as shown in Figure 1 are used in conjunction with a glass manifold connected to a pressurized eluent reservoir. The eluent vessel and manifold are permanently fastened to a wooden frame equipped with brackets for holding the Equipment.
2mrn ID
columns and collecting flasks. I t is assumed that a multiple set of samples (1 through 8) are simultaneously analyzed for the same components. Each column is first packed with adsorbent of the required activity directly from an air-tight. storage bottle. Net adsorbent weights can be obtained, but it is less time consuming to pack each column to approximately maximum adsorbent density and constant weight, by tapping the filled column along the sides and dropping it lightly against a hard surface. Different operators can easily duplicate adsorbent weights within =t50j, for identical columns; this is sufficiently precise for most applications. Atmospheric water uptake by the adsorbent must be avoided, and is minimal for the rapid direct transfer of adsorbent from storage vessel to column. A filled column, capped with cotton plugs a t each end of the adsorbent packing, is snapped into its connecting brackets, and the various ball and socket joints secured with spring-loaded clamps. The :oading and assembling of a single column takes about a minute. After the assembly af all columns, each column stopcock is set In the position shown, and vacuum is applied (by means of a hose connecting the open end of a column to a vacuum source) to a column to be charged. The sample is added to the inlet bulb by a micropipet, and then sucked onto the dry column by opening the 3-way stopcock (Lab-Crest 2-mm., Teflon plug). The inlet bulb is rinsed with the first eluent to be used, as is the pipet, and the rinsings also sucked onto the column. To avoid poor separation, a. minimum volume of total eluent rinse should be used. With care, it is possible to confine the sample and rinse within the bottom 2 inches (1 ml.) of the 8-mm. 0.d. column. The sample charging of each column is completed in the same manner, following which simultaneous elution is begun through all columns into the receiving flasks. An eluent flow rate of 5 ml. per minute or less is maintained. Any number of fractions may be obtained for each column, with any eluent for each fraction; the eluent can be changed a t the beginning of each new fraction if desired.
-PTYNG
GROUND 1GLASS 2/5 -rc
ADSORBENT
COTTON
-
-
3-WAY STOPCOCK
6WJ
AL Figure
1.
1215 GROUND
GLASS MANIFOLD SECTION
Diagram of
LEAC columns
micro
VOL. 33, NO. 1 1 , OCTOBER 1961
1535
The various ball and socket connections are never greased, because of the possibility of eluent contamination. If eluent leakage occurs at the bottom ball and socket connection, separation is unaffected, and no sample is lost. Properly tightened, the Teflon plug stopcock will not leak, and with normal seating of the top connection, no leakage occurs there because of the small pressure drop across the joint. Leakage can be eliminated by externally coating the ball and socket joints with a suitable paint, but this requires additional time and in our experience is unnecessary. Since each column has its individual sample inlet system, i t is impossible to contaminate neighboring columns with small amounts of sample during multiple analyses. This precaution is important n-hen the samples analyzed show widely varying concentrations of the compound or class being measured. Routine Analysis of Naphthalenes and Phenanthrenes in Gasoline. A 400 X 8 mm. column containing 10 =t grams of 4.0% water deactivated Alcoa F-20 alumina is set u p as described above. Fifty microliters of sample are charged and washed on with pentane (Phillips' research grade n-pentane which has been purified over silica gel). Elution is continued with purified n-pentane, collecting a first fraction (A) of 8 ml. (conveniently measured by using a 10-nil. volumetric flask to which 2 ml. of liquid has been added previously). This fraction is discarded and a second 25-ml. pentane fraction ( B ) collected. Eluent is changed to 10% ether-pentane (reagent grade diethyl ether), a 5-ml. fraction (C) collected and discarded, and a final 25-ml. fraction ( D ) collected. The corrected absorbances of the B fraction at 285 mp ( A B ) and of the D fraction at 253 mp ( A I ) are obtained in 2.5-em. cells. The concentrations of sample naphthalenes and phenanthrene are given by
c/o v. naphihalenes
=
% v. phenanthrenes
0.714
A B
(1)
= 0.060 A D (2)
Khere the absorbance of the eluents is appreciable a t 253 and 285 mp, it is necessary to correct A B and A Drelative to a blank run (charging no sample). The analysis time per sample is 20 to 30 minutes for sets of 4 to 8 samples. The reproducibility of the method is better than =t5%, as suggested by the data of Table I The method is
Table 1.
Comparative Analyses
_7 -0Wt. Naphthalenes
Sample A B C
D E F
1536
hTormal charge (0.050 ml.)
Double charge (0.100 ml.)
0.320 1.08 0.242 0.48 0.82 1.06
0.320 1.06 0.236 0.48 0.82 1.06
ANALYTICAL CHEMISTRY
applicable to all normal gasoline range samples. Accuracy, which is further discussed in the following section, is decreased in the analysis of narrow fractions from gasoline distillation residue samples. Materials. As previously described ($), adsorbent activity can be controlled by a preliminary calcining of the manufacturer's product, followed by water deactivation to lower activity levels. The activity of the final adsorbent is tested by measuring the retention volume R" of a standard solute, eluted by a standard eluent (naphthalene eluted by pentane in the case of Alcoa F-20 Grade alumina) and further corrections in activity effected by adding either calcined adsorbent or water until the desired activity as measured by this standard activity test is obtained. Since E o values are a function both of adsorbent surface area Ti, and activity CY (7, 9 ) , such a test is not completely definitive; that is, compensating variation in both V, and LY can orcur with resultant constancy in R". T h e r e the surface area of the starting adsorbent is reasonably constant, as in the case of Xlcoa F-20 alumina, no ambiguity exists in this sense. If a single cut point is made in a separation using an adsorbent where both Ti, and CY vary, Ro - for a solute eluted close to the cut point can be used to control adqorbent actiiity a i t h good results. I n the general case, where both V , and CY vary, and where more than one cut point is involved, it is necessary to consider their effect on separation cut points in terms of Equation 1 of a preceding paper ( 7 ) . Variations in V , can be exactly compensated by proportionate change in the weight of adsorbent used in the separation, since compound retention volumes are proportional to V a ,all other factors being equal. Eluent purity has been discussed (0) with respect to the possibility of adsorbent deactivation by polar eluent contaminants. Usually, purification of commercial eluents by passage through silica gel is required. Less polar eluent impurities which are not removed by this procedure and M hich give an appreciable blank relative to pure eluent can lead to erroneous results if part or all of the interfering contaminant is adsorhed during a micro LEAC separation where the original eluent is used as blank. This effect can be verified by use of blank separations and analysis, with correction of results using the impure eluent then being possible. DISCUSSION
The tendency of individual hydrocarbons in motor fuel to produce cylinder deposits increases with compound boiling point and aromaticity (6).
The effect is particularly pronounced for aromatics boiling above 400" F., most of nhich occur in present gasolines as polynuclear aromatics. The determination of the concentrations of polyaromatics in finished gasolines and refinery blending stocks is therefore of interest. Compositional analysis studies (by mass, ultraviolet spectrometry) of polyaromatic concrntratrs isolated from representative gasoline blending stocks and finished gasolines show the presence of the Cloto CI2alkyl naphthalenes in major amounts, mith trace quantities of alkyl benzothiophenes, biphenyls, and/or cycloalkyl naphthalenes. Phenanthrene and pyrene also occur. Two previously reported analytical techniques suggest themsleves for the determination of gasoline polyaromatics. An XSTAI procedure for the measurement of total naphthalenes in jet fuPls (10) uses the distinctive ultraviolet absorption of naphthalenes at 285 mp, while low ionizing voltage mass spectral techniques have been described for the determination of aromatic components in gasolines (1) and gas oils (3. The ilSThI procedure for jet fuels is inapplicable to gasoline samples, because of serious interference of sample monoaromatics in some cases. I n addition, phenanthrene cannot be determined from the ultraviolet absorption of the total sample because of masking absorption of the more concentrated naphthalenes. The heaviest polyaromatics in gasoline samples do not vaporize completely in the room temperature mass spectrometer sample inlet system normally employed for the analysis of gasolines. The high temperature inlet system used in the analysis of gas oils solves this problem, but accurate sample charging is a n additional problem that requires use of an internal standard or other time-consuming technique. Futhermore, sensitivity and freedom from interference of other sample components are not potentially as great as is attainable in the procedure described above. Figure 2 shows the elution bands which result from the LEXC separation of a gasoline by pentane elution from a 10-gram column (400 X 10 mm.) of 4.0y0 H20-A120a. Ultraviolet absorption is used to follow the elution of monoaromatics (270 mp), diaromatics (285 mp), and phenanthrene (253 mp). The latter gasoline component is stripped from the column with 10% ether-pentane. The fractions indicated as A , B, and D in Figure 2 show the characteristic ultraviolet absorption spectra of the alkyl benzenes plus thiophenes, naphthalenes, and phenanthrene, respectively. The separation of the three compound types in Figure 2 is essentially complete. The monoaromatics possess a small but signifi-
Table II. Contribution of Gasoline Monoaromatics to Naphthalene Absorption at 285 mp, Expressed as Apparent Per Cent Naphthalenes
Apparent Naphthalenes from True MonoNaphtha- aromatic lene Absorption Content," at 285 mp,O Sample % V . %V. A 0.008 0.006 B 1.52 0.04
W
3
0.8-
c
D 0.4 -
e
\
_ _ _ - - - ----
\
I
I
IO
ELUENT VOLUME, rnl
Figure 2.
-
-%-
I
*
Elution of gasoline aromatics from a ten-gram column of 4% HzO-AI,Os Sample charged to dry column Eluate absorbance at 2 7 0 mp A t 2 0 5 rnp At 2 5 3 mp
---
...
E F G H I J K L ~~
M N 0 P Q
0.014 0.018 5.00 0.64 0.326 1.08 0.242 0.48 0.82 1.06 0.222 0.42 0 64 0.94 1.06
0.018 0.124 0.38 0.12 0.020 0.08 0.030 0.12 0.06 0.06 0.114 0 24 0.06 0.06 0.06
Relative Error in Direct Ultraviolet Analysis,
% 75 3 129 689 8 19 6 7 12 25 7 6 51 57 9 6 6
Determined from ultraviolet measurement of separated monoaromatic fraction. cant absorption a t 285 mp, thus interfering with the determination of sample naphthalenes by direct ultraviolet measurement of the total sample a t 285 mp (as in the ilST11 procedure), 'The extent of this interference is shown in Table 11, \\here the true naphthalene contents (measured by the present LEA4C-ultraviolet) of several refinery blending streams and finished gasolines are compared with the apparent naphthalene content which results from the absorption of the sample monoaromatics a t 285 mp (measured from d fraction of separation as in Figure 2). As a result of the variability of the ultraviolet absorption spectra of gasoline monoaromatics, it does not appear feasible t o correct naphthalene absorption a t 285 mp for monoaromatic contribution. This precludes the direct ultraviolet analysis of naphthalenes in gasolines. The X S T N procedure assumes a n absorptivity for the naphthalenes at 285 mfi of 33.7 sq. cm. per gram. This is a weightcd average of the Clo through C12 naphthalcnes. The naphthalenes by carbon number show absorptivities of 28.5 for Clo, 25.9 for CI1 (assuming a ratio of 1:2 for alpha:beta), and 36 for C12 (IO). While a n absorptivity of 33.7 may be representative of jet fuel naphthalenes, '7570 or more of gasoline naphthalenes normally consist of CloH, and CIIH,,, and hcnce should have lox-er nb3orptivities. Four representati\.-e gasoline stocks were chromatographically separated to provide diaromatic concentrates; the average absorptivity of these concentrates a t 285 mfi nas 27.5 =k 0.7 sq. cm. per gram in agreement with pure compound ab-
sorptivities and the average composition of gasoline polyaromatics. For the phenanthrene fraction, only two components occur in quantity, phenanthrene and pyrene. The large concentration of the former relative to the latter, as well as the fourfold increased absorptivity of phenanthrene a t 253 mp relative to pyrene, permits the measurement of phenanthrene in fraction D of Figure 2 from the absorbance of the fraction a t 263 mp and the absorptivity of phenanthrene a t the same wave length (330 sq. em. per gram). The long wave length absorption of pyrene a t 334 mp permits its separate determination in gasoline by direct ultraviolet measurement and a sketched base line calculation. The choice of adsorbent (4.0% Hz0-.&03) in the present procedure was suggested by the superior characteristics of alumina for the separation of aromatic classes, and the high linear capacity of alumina 1%hich has been deactivated by 2 to 4% added water ( 8 ) . An adsorbent actitity level was also required vvhich nould provide complete separation of the three compound classes of Figure 2, yet give reasonably small retention volumes for the elution of mono- and diaromatics by pentane as eluent (for convenience and increased sensitivity). While i t would have been possible to use a more active adsorbent (e.g., 27, HZ0-Al2O8) with comparable linear capacity and good separability of aromatic classes, this would have resulted in larger eluate fractions for each sample component with resulting lowered sensitivity for the detection of each component and greater experimental incon-
venience from the increased elution volume and time. Smaller columns of 27, H20-A1203 could be used to maintain eluate volumes comparable to those for larger columns of 47, H20A1203,but this would mean decreased column linear capacity, and so decreased sensitivity for the method. I n the present separation over 4.0y0 H2O-hl2O3, the eluent volume required to reach the cut point between monoaromatics and diaromatics is too small for maximum separation efficiency, but large enough to permit essentially complete separation in gasoline samples. As sample average molecular weight increases, the separation of monoaromatics from diaromatics becomes increasingly difficult (7'). The extension of the present analysis to heavier samples would therefore require a more active aluminn as adsorbent for comparable separation of these two compound classes. Sample recovery is critical in the use of adsorbent chromatography for quantitative work a t low column loadings. Quantitative recoveries are a general rule using alumina. As a test of this, solutions of naphthalene (0.222 7, v.) and of phenanthrene (0.016 % v.) in pentane were separated by the method described, 13-ith 1007, recovery resulting in each case. Maintenance of separation linearity is essential for techniques of the general class under discussion. The surest test of linearity in normal operation is provided by the analysis of a number of representative samples using both norVOL. 33, NO. 11, OCTOBER 1961
* 1537
mal sample aim and one substantially larger than normal. If the same results are obtained for each sample size, nonlinearity is unlikely t o occur for the analysis when using the normal sample size. Table I shows comparative analyses for naphthalenes in a series of representative gasolines, using both the normal (0.050 ml.) sample size, and a double charge (0,100 ml.), ACKNOWLEDGMENT
The author acknowledges the experimental assistance of F. 0. Wood
and A. E. Youngman, both of whom contributed substantially to the design of the routine equipment and procedure, as well as helpful discussions with J. K. Fogo. LITERATURE CITED
(1) Field, F. H., Hastings, S. H., ANAL. CHEM.28, 1248 (1956).
(2) Gordon, R. J., Moore, R. J., Muller, C. E.. Ibid.. 30. 1221 (1958). (3) Kearns, G. L., Maranowski, N. C., Crable, G. F., Ibid., 31, 1646 (1959). (4) Kenyon, W. C., McCarley, J. E., Boucher, E. G., Robinson, A. E., Wiebe, A. E., Ibid., 27, 1888 (1955).
((5) 5 ) Lumpkin, Lumokin. H. E., E.. Johnson, Johnson. B. 9., 9.. Ibid., 26, 1719 (1954). (6) Shore, L. B , Ockert, K. F., S.A.E. Trans. 66,285 (1958). (7) . . Snyder, L. R.. ANAL. CEIEM.33, 1527
(mi).
(8) Snyder, L. R., J. Chromatog. 5 , 468
(1961). (9j I b G , 6, 22 (1961). (10) American Society for Testing Materials, ASTM Standards on Petroleum Products and Lubricants, “Proposed Method of Test for Naphthalene Hydrocarbons in Jet Fuel by Ultraviolet Spectrophotometry,” 1135, 1959. RECEIVEDfor review March 6, 1961. Accepted August 7, 1961.
Applications of Linear Elution Adsorption Chromatography to the Separation and Analysis of Petroleu m 111.
Routine Determination of Certain Sulfur Types
LLOYD R. SNYDER Union Research Center, Union Oil Co. o f California, Brea, Calif.
b linear elution adsorption chromatographic methods have been developed for the routine determination in heavy (400”F. plus) petroleum samples of sulfur present in each of three compound types. Per cent sulfur as (I) alkyl thiophenes, (11) alkyl monosulfides, (including saturated cyclic monosulfides) and (111) nonvicinal aromatic sulfides plus nonvicinal polysulfides is reported. Routine methods for these sulfur types have not been previously described. The Hastings method for aliphatic sulfide sulfur (I1 plus 111) gives high results for some sample types, due to interference from basic nitrogen compounds.
T
first detailed and relatively complete description of the sulfur types occurring in a petroleum heavy distillate WW reported by Lumpkin and Johnson (6). This study provided the basis for the subsequent routine mass spectral procedure of Hastings, Johnson, and Lumpkin (4) for the determination of condensed thiophenes in similar petroleum stocks. This method in conjunction with the iodinecomplex method of Hastings (2, 3) for determining aliphatic sulfides permits the routine breakdown of sulfur compounds in heavy samples into four classes. The mass spectral method determines b e n zo t h i o p h e n es, dibenzot h io p h e n es , a n d naphthobenzothioHE
1538
ANALYTICAL CHEMISTRY
phenes. The iodine-complex method reports aliphatic sulfide sulfur, which comprises acyclic monosulfides; saturated cyclic monosulfides; nonvicinal aromatic sulfides
where /
R-S-R,
the sulfur is not joined to an aromatic nuclei; and nonvicinal polysulfides, R-S-R-S-R. Vicinal aromatic sulfides,
@S-R,
where the sulfur is / joined to an aromatic nucleus and are not vicinal polysulfides, R-SxR, included in this aliphatic sulfide determination. The combination of the above two routine methods for the determination of sulfur type offers one of the most detailed and complete analyses of its kind. The only major sulfur type not represented are the alkyl thiophenes; their nonroutine detection by ultraviolet spectroscopy in narrow chromatographic fractions has been reported ( 4 ) . Further breakdown of the various sulfide types has not been previously accomplished. As will be discussed in a following section, we have recently found that the Hastings sulfide determination is subject to interference from petroleum basic nitrogen compounds. For most sample types the method possesses a t least semiquantitative accuracy, but in some c u c s the re-
ported sulfide sulfur value is as much as a n order of magnitude high. Routine methods for determining alkyl thiophene sulfur and sulfur present in two or more individual sulfide types, without interference from basic nitrogen, appear desirable. Combined with methods such as that of Hastings et al. for the determination of condensed thiophene sulfur types, these procedures would increase the completeness, detail, and accuracy possible in the routine determination of sulfur type in gas oil samples. EXPERIMENTAL
Determination of Alkyl Thiophene Sulfur by Nonlinear Separation Plus Elemental Analysis. T o a d r y 4 foot
by 3/4 inch column containing about 150 rams of Alcoa F-20 activated alumina calcined at 400’ C.) are charged approximately 2 grams of sample. Elution is begun with n-pentane and IO-ml. fractions collected. The ultraviolet absorbance of the fractions at 270 mp is measured. Absorbance first rises and then falls aa the monoaromatics are eluted. When the absorbance falls to one tenth of the maximum value, or when i t reaches a minimum and begins to rise B second time, elution is discontinued and the fractions collected to that point combined as fraction A. A second B fraction is obtained by stripping the column with methanol-benzene. These two fractions are freed from solvent and weighed to obtsin a weight
4
