residuum. All show small absolute amounts of one-ring thiophenes in fractions 1 and 2 , but with South Houston and Wilniington crudes, these constitute a significant proportion of the total sulfur in the fraction. I n all the crudes, two- and three-ring thiophenes aceount for a much higher proportion of the thiophenic sulfur than do one- and four-ring compounds; compounds with five or more rings, however, dominate in the residuum fractions. There are small differences among crudes in the relative proportions of two- and three-ring thiophenes. For example, some crudes (Wasson) have more two-ring compounds, while others (South Houston) have more three-ring compounds; however, ratios of the two types usually do not vary among crudes by more than 2 to 1. Wlmington crude shows the most atypical distribution of sulfur types. For example, one-ring thiophenes account for about 6% of its total sulfur, as compared to only 1 to 374 for the other crudes, and nonthiophenic sulfur accounts for 54% of the sulfur, as compared to only 28 to 4lyG for the others. These differences probably are real; however, the high proportions of oxygen (0.44%) and nitrogen (0.65%) in
Kilmington possibly could cause errors. For example, saturated rings containing oxygen or nitrogen could open during catalytic decomposition, and some onering thiophenes would be produced. Also, aromatic rings containing sulfur along with oxygen or nitrogen might be more susceptible to opening than rings containing sulfur only. This problem would not exist with the other crudes, because they contain much smaller proportions of oxygen and nitrogen. LITERATURE CITED
(1) Am. Soc. Testing Materials, “ASThI
Standards, 1964,” )\lethod D-129-62, part 17, p. 63. (2) Ball, J. S., Rall, H. T., Proc. Am. Petrol. Inst. 42 (111).128 11962). (3) Coulson, D. M.,’ Cavanagh, L. A., “llicrocoulometric Detection in Gas Chromatography,” Pittsburgh Conference on Analytical Chemistry and A4ppliedSpectroscopy, March 1961. (4) Drushel, H. J’., AIiller, J. F., ANAL. CHEM.27. 495 11955). ( 5 ) Eggertsen, F. T.; Groennings, S., Holst, J. J., Ibid., 32, 904 (1960). (6) preen, L. E., Schmauch, L. J., Worman, J. C., Zbid., 36, 1512 (1964). ( 7 ) Hammar, C. G., Svensk Kem. T i d s k r . 63, 135 (1951). (8) Hastings, S. H., ANAL.CHEM. 25, 420 (1953). (9) Hastings, S. H., “Percolation-Mass
Spectrometric Method for Determining Thiophenes,” 10th Southwest Regional ACS Meeting, Fort Worth, Texas, December 1954. (10) Hastings, Y. H., Johnson, B. H., Lumpkin, H. E., ANAL.CHEM.2 8 , 1243 (1956). (11) Hubbard, R. L., Haines, W. E., Ball, J. S., I b i d . , 30, 91 (1958). (12) Klaas, P. J., Ibid., 33, 1851 (1961) (13) Lumpkin, H. E., Johnson, B. H.. Zbid., 26, i7i9 (1954);. (14) Martin, R. L., Grant, J. A , , Zbid., 37, 644 (1965). (15) Martin, R. L., Winters, J. C., Williams, J. A,, “Composition of Crude Oils by Gas Chromatography,” Proceedings Sixth World Petroleum Congress, Section \., p. 231, 1963. (16) l\IcCoy, 11. N., Weiss, F. T., ANAL. CHEM.26, 1928 (1954). (17) Snyder, L. R., Ibid., 33, 1538 (1961). (18) Snyder, L. R.,J . Chromatog. 6, 22 (1961). (19) Snyder, L. R., Union Oil Co. of Calif., Union Research Center, Brea, Calif., private communication. (20) Tamele, 11. W., Ryland, L. B., hIcCoy, R. N., AXAL.CHEM.32, 1007 (1960j. (21) Thompson, C. J., Coleman, H. J., Hookins. It. L.. J . Chem. Eno. Data 9. 293. (1964). (22) Yao, T. C., Porsche, F. W., ANAL. CHEM.31, 2010 (1959). RECEIVED for review December 14, 1964. Accepted February 4, 1965. Division of Petroleum Chemistry, 149th Meeting, ACS, Detroit, Mich., April 1965.
Thermal Peaks Accompanying Solute Peaks in Preparative Scale Gas Chromatography JAMES PETERS and C. B. EUSTON’
F & M Scientific Corp., Avondale, Pa. Temperature variations have been measured radially across a 1-inch diameter preparative column, and it is suggested that these variations contribute to the decrease in efficiency observed as column diameter is increased. The high sample loadings frequently used in preparative scale gas chromatography increase these temperature differences and further reduce column efficiency. Programmed temperature preparative columns were considered, and the gradients existing were recorded. Gradients frequently reach several degrees centigrade and are related directly to program rate. Similar measurements on temperatureprogrammed columns show that the temperature at the column center lags the wall by about 1 minute for a 1 -inch diameter column. The temperature difference thus is proportional to heating rate, and may reach several degrees a t high programming rates.
P
of large samples through preparative scale columns may create a significant local disturbance in column temperature because of the evolution and absorption of the heat of solution of the sample. These temperature disturbances will alter the velocity of the solute zone and, of particular importance, this change in velocity will not be equal over the cross-section of the column. The creation of such a distorted velocity profile will seriously affect column performance. With this consideration in mind, the radial variations in column temperature were measured in 1-inch columns as component peaks passed through. The sample size was varied over a 50-fold range. Because of the considerable interest shown recently in the combination of programmed temperature and preparative scale gas chromatography, several temperature measurements were also made in 1-inch programmed columns. ASSAGE
An earlier paper by Scott (a)discussed the temperature changes associated with solute passage in analytical columns, but because of the small diameter of the columns, the radial variations so important in preparative scale work were not measured. EXPERIMENTAL
Apparatus.
The columns used for this study were 1-inch copper tube, 3 feet long, and contained 20% SE-30 on 60- to 80-mesh Chromosorb P. They were split a t their mid-sections t o allow accurate placement of three 5000-ohm thermistor beads radially across, the column a t distances of and 1 / 2 inch from the column wall. The column was then assembled and filled in the usual manner. Electrical connections to each bead kvere made via two thin magnet wire lead$ enclosed in l/l+j-inch stainless steel Present address, F&M Scientific Europa N.T’., Basisweg (Sloterdijk), Amsterdam, The Netherlands. VOL. 37, NO. 6, MAY 1965
657
F THERMISTOR
FROM COLUMN WALL
4.
THERMISTOR
5
3. TIME IN MIN FROM COLUMN WALL
2. I.
I
OJ
5-
'1
L
0
1
9
3 4 5 6 TIME (MINUTES)
1
0
9
4 4.
Figure 1. Concentration and temperature changes with time for thermistor positioned inch from column wall So\id line: Time/temperature relationship, 1 pa. through thermistor Broken line: Time/component relationship, 3 0 ma. through thermistor Conditions: 1.5 mi. o f n-hexane in 1 -inch. i.d. column o f 20% SE-30 on 60/80 Chromosorb P; temp,, 80' C.; flow, 1 OOO mI./min.; sensitivity, X 1 for 1 -pa. curve, X 51 2 for 30-ma. curve
3 THERMISTOR
5;
FROM COLUMN WALL
2I-
L
6
s
Figure 2. sample
3 POSITIVE
4
OJ
2
I
0 I DEGREES CENTIGRADE
2
RESULTS
The type of temperature change observed is given in Figure 1, which shows the time/temperature relationship for a thermistor in the column 658
ANALYTICAL CHEMISTRY
5
6
Temperature changes at three points across column during passage of
Conditions: 1.5 ml. o f n-hexane in 1 -inch 1.d. column of 20% SE-30 on 80' C.; flow, 1000 ml./mln.; sensitivity, X 1 for 1
[email protected] thermistor
hypodermic tubing and sealed with epoxy resin. These tubes were positioned downstream from the beads (with respect to carrier gas flow) to prevent flow disturbances from causing side effects. The thermistor beads were each incorporated into a bridge network and could be used in two ways. Under low current conditions, they operated as temperature sensing elements. Under high current conditions (10 ma. and above), the thermistors functioned as thermal conductivity component sensors. The transition between these modes of operation was gradual and was essentially complete at currents of 10 ma. and above. This was checked by injecting air samples, which would not be expected to cause temperature changes. S o signal was obtained a t low currents, hut a large response was observed a t the higher current level. The column was coupled to an F & 31 Model 500 gas chromatograph with a Model 770 (preparative scale) injection port fitted t o accommodate large sample sizes. The column was on occasion deliberately overloaded to intensify resultant temperature changes.
3 4 NEGATIVE-
center. Superimposed on this is a time/concentration curve which correlates the temperature change with sample passage. Although the exact shape of the thermal peak is changed considerably by varying operating conditions, the main characteristics of the curve remain the same. Differences in Area between Positive (Heat of Solution) and Negative (Heat of Vaporization) Peaks. The considerable difference between the two areas noted in Figure 1 will be considered. If a true adiabatic system could be constructed, we would observe a time/temperature relationship which would rise to a maximum positive temperature (considerably higher than that in Figure 1) and then fall off to the base line, but not below it. This would be so, as heat of solution equals heat of vaporization. This system is far from an adiabatic system, and thus a certain amount of heat is lost during the 1-minute temperature rise cycle to the surroundings. The heat lost a t this stage is later observed as a n excursion below the base line. At the lowest point reached, little further solution or vaporization occurs and temperature equilibrates back to ambient. Now consider the other extreme.
60/80 Chromosorb P. Temp.,
If a sample could be absorbed on a section of column packing to raise it to a certain maximum temperature above ambient, and if this section could then be cooled to ambient without any vaporization of sample, we could observe a negative peak exactly equal in size to the positive peak upon vaporization of this sample. To achieve this we would require a n isothermal system in which we would see no temperature effects whatsoever. This situation could never occur, because obviously we cannot prevent sample from vaporizing, nor can we achieve an isothermal system. In actual practice, the system is between these two extremes of adiabatic and isothermal operation and varies according to the peak width of the sample. Samples with a narro1v peak approach the adiabatic system (little negative deflection), while wider peaks will approach an isothermal system with large negative deflections. The amplitude of the temperature changes will depend on several variables, three of which are discussed belo\\-. R.kD1.4L TEhlPERATURE DIFFERENCES.
With the passage of a given sample through the column, the thermal peak varies considerably from point t o point across the column and is largest in the
1/2" FROM WALL
4.01
INC *C
t
114" FROM WALL
80. 20
7010
AMBIENT
1
90
30
TEMP
100 TEMPERATURE ABOVE START (6OOC)
60-
50-
0
1184~)
( V 2 " FROM W A L L )
4010
L 30-
20 112"
F R O M WALL
2010 IO-
0 0.
3.01
40L
:
01
: 0.2
:
:
:
:
0.3 0.4 0.5 0.6 SAMPLE SIZE ML
:
0.7
'
:
6.8 0.9
4
1.0
C6 1
Figure 3. Variation in maxima and minima temperature observed with change in sample size Conditions: 1 -inch i.d. column; sample, Cg; temp., 80" C.; 10 Ma. through thermistor; flow, 800 ml./min.; column packing as in Figure 1
0
S
IO MINUTES
20
25
Figure 4. Tirne/temperature relationship for a 1 -inch i.d. column programmed from 60" C. a t 4" C./minute (33% of each); flow, 1000 ml./
Sample, 10 ml. of N-CG; N-Cs; N-CN min.; column packing as in Figure 2
center. The thermistor a t the column edge indicates a temperature change approximately one half that of the center bead, which clearly illustrates better heat transfer in the column packing adjacent to the column wall. Examples of these temperature changes at three points across the column are given in Figure 2. The difference in temperature between center and edge of the column would be greater with increases in column diameter and may be a t least partly responsible for the large drop in efficiency noted on increasing diameter. A recent paper by Wright ( 3 ) has shown that a significant increase in column efficiency was obtained by using a column with large internal fins to conduct heat more readily to and from the center of the column. Wright's findings would support the evidence reported in this paper; also, column packing or carrier gas with a higher thermal conductivity would help in heat transfer.
IS
SAMPLESIZE. Changes in sample size also affect the magnitude of the temperature maxima and minima observed, as shown in Figure 3. The deviation from a straight line is caused by increasing sample size raising the column temperature higher, and thus heat losses to the column wall, etc., are also higher (heat losses being directly proportional to temperature difference). RETENTION TIME. This affects the maxima and minima of a peak in the same way as in a normal chromatographic peak, an increase in retention time being seen as a decrease in maxima and minima with a n increase in peak widths. The difference in temperature change between the three thermistors is still essentially the same, the greatest temperature change still occurring at the column center. Thus the reduction in efficiency due to the thermal changes will be greater with earlier peaks than with later peaks.
The experimental set-up was ideal for measuring a factor often discussed theoretically-temperature lag in programmed temperature preparative scale column, which is fully discussed by Giddings ( 1 ) . As expected, lag is proportional to heating rate, and proved the column center lagged the outside by approximately 1 minute-e.g., 10" C. lower a t a linear programmed rate of 10" per minute. The effects of a sample passing through a programmed column are illustrated in Figure 4. LITERATURE CITED
(1) Giddings, J. C., Gas Chromatog. 1, 12 (1963). (2) Scott, R. P. W., ANAL. CHEM. 35, 480 (1963). (3) Wright, J. L., Gas Chromatog. 1, 10 ( 1963).
RECEIVED for review December 17, 1964. Accepted March 8, 1965.
VOL. 37,
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