plcs could be used, the llletllod 1dgllt bc extended t o the determination of sulfur in the parts per lnillion inight be accomplished by increasing the length of the catalyst zone or by lengthcning the chromatographic column. LITERATURE CITED
(1) Am. Soc. Testing Materials, “Tenta-
tive Method of Test for Sulfur in Petroleum Products and Liquefied Petroleum Gases, by C02-02 Lamp Method,” D 1266-55T.
( 2 ) Schluter, E. C., Jr., l’itrry, IC. l’., Matsuyama, G., ANAL. CHEX 32, 413 (1960). (3) Scott, W. W., “Scott’s Stmdard Methods of Chemical Arialysis,” 5th ed., p. 2509, Van Nostrand, New York, 1939. (4) Thoin son, C. J., Colemaii, 11. J., Ward, C., Rall, H. T., A N ~ LC~rehz. . 32, 424 (1960).
8.
RECEIVED for review June 18, 1962. Accepted August 6, 1962. Division of An:tl tical Chemistry, 140th Meeting, ACS, C&cago, Ill., September 1961. Invcstiga-
tion performed as part of the Ainerican Petroleum Institute Research Project 48A on the Synthesis, Properties, and Isolation of Sulfur Compounds in Petroleum, carried out by the Bureau of Mines a t Laramie, Wyo., and Bartlesville, Okla. Work was done under cooperative agreements among the Bureau of Mines, U. S. Department of the Interior, the American Petroleum Institute, and the University of Wyoming, Reference to specific commercial materials or models of equipment is made to facilitate understanding and does not imply endorsement by the Bureau of Mines.
Quantitative Reproducibility of a Programmed Temperature Gas Chromatographic System with Constant Pressure Drop Using Packed and Golay Columns L. S. ETTRE and F. J. KABOT The Perkin-Elmer Corp., Norwalk, Conn.
b The advantages of a programmed temperature gas chromatographic system with flame ionization detector utilizing constant inlet pressure during operation were discussed by Golay et a / . (7) for qualitative analysis. This paper proves its accuracy and reproducibility for quantitative analysis.
I
N MOST prograninied teml)er:tturc gas
chromatographs, the carrier gas flow is kept constant during operation. The outlet pressure of thc gas is coiistant (equal to atmospheric pressure) ; because the gas viscosity changes with temperature, n coiitinuous adjustment of thc carrier gas inlet pressure (using :tutom:ttic flon coiitrollers) is necess iry to maintain a constant flow during programmed temperature operation. ITon evcr, 1)). continuously changing the inlet pi essurc, thc pressure drop along the column 11 ill nlso change continuously. ‘This tcclinique \ins chosen because thc thermal conductivity detectors used in such units nere sensitive to the carrim gas flow and any change in the flow rate would result in n drift of the base line. The result of this mode of operation is that both the gas vclocity and the pressure drop along the column are changing and this complicates the mathematical treatment of retention times. On the other hand, the quantitatibe accuracy of such systems was ~ r o v e t l(4) and by using thermal conductir ity detectors, neither the relative peak are:t values nor the relatile re-
sponse factors \wrc affected by the different program rates relnt,i\,e to isothermal opera.tion. I n certain cascs--e.g., using flow sensitive detect,ors or dual columns where the flow rates through the respective coluiniis have to be mntchedflow regulation cannot he avoided. From a theoretical point of view, however, a system where both the coluinn inlet and outlet pressures are kept constant is more desirable. Specifically, in this case, the pressure drop along the column is independent of temperature as long as the entire column is uniformly heated and only the average velocity is a function of column teniperature. This simplification permits a relatively simple mathematical treatment of retention times which is discussed in detail separately (7’j. A recent’lydesigned programmed t,emperature gas chromatograph incorporates a flame ionization detector with both packed and Golay columns (6). Because the flame ionization detector is relatively insensitive to changes in the carrier gas flow rate (1, 2, 5 , a),the inlet pressure rat,her than the flow rate at column outlet is maintained constant during temperature programming; thus, the utilization of t,he quoted mathematical treatment for practical work became possible. While this treatment is related only to qualitative analysis, n gas chromatograph must also perform properly for quantitative analysis, with sufficient accuracy and reproducibility. Therefore, the quantitative results obtained with programmed temperature
operation must not differ from those determined under isothermal conditions. Because a flame ionization detector had not previously been used with continuously changing carrier gas flow rate, the quantitative reproducibility of the data obtained nith temperature programming under such conditions relative to the isothermal values from constant flow operation was as yet unproved. I n this respect, the first question is related to detector response. It is known that the response of the flame ionization detector is dependent upon the amount of sample component arriving in unit time at the detector. As the flow rate changes during a run, this value nil1 not be constant; i t is, therefore, necessary to demonstrate that the integral oC the resp0nse-i.e. the response for the total amount of sample component-1s ill still remain uiichanged. Because the carrier gas flow rate changes during operation, the ratio of hydrogen flow rate (which is constant) to carrier gas flow rate also varies during temperature programming; therefore, it is important to determine whether this will affect the quantitatisre results. Finally, a third problem arises if Golay columns are utilized in a gas chromatographic system: the question of split linwrity. I n a previous paper ( 3 ) this question was discussed in detail; because in the present system, the construction of the splitting device was similar to that described pre\ iously and the flow rate during splitting v a s VOL. 34, NO. 1 1 , OCTOBER 1962
1431
kept constant] i t is not necessary to prove again the linearity of the splitting. The split is a part of the whole gas chromatographic system; thus, if the whole system produces quantitative data, i t also demonstrates the linearity of splitting. The evident advantages of the liew programmed temperature system with constant pressure drop in qualitative analysis as compared to a system with constant flow rate a t column outlet induced us to investigate the accuracy and reproducibility of the new construction for quantitative analysis. EXPERIMENTAL
Apparatus. A Perkin-Elmer Model 226 gas chromatograph was used for our investigations. This instrument is equipped with a flame ionization detector (i.d. of the detector jet: 0.020 inch) and can either be used isothermally or i t can b e temperature programmed with both packed a n d Golay columns; under programmed temperature operation, t h e pressure drop through the column is kept constant. Both packed and Golay columns are constructed in a special sandwichlike configuration to allow minimum temperature lag betneen heater and column which is indispensable for the utilization of the instrument to the prediction of retention times ( 7 ) . The split system of the instrument has the construction of two concentric tubes with different diameters and a restrictor built into the larger diameter tube, the smaller tube being connected to the column. This construction is similar to that described previously (3) except that in the present instrument] the sample injection point, the duct to the split point, the two concentric split tubes, and the split restrictor are built into one metal block which is maintained at a uniform temperature. At the same time, the carrier gas is also preheated in the same block. ~~
~
Table I.
Column length Outside diameter Internal diameter Ratio liquid phase-support Temperature of Injection block Detector block Helium inlet pressure Split ratio during sample injection Sample size Inlet pressure, hydrogen Flow rate, hydrogen Inlet pressure, air Flow rate, air Isothermal column tempera-
1432
Analytical Conditions
OS-138 Dimensions Golay Packed 150 10 feet inch 0.020 0.125 0.010 0.085 inch v;./w.
c. C: p.s.1.g. P1.
p.s.i.g. ml./min. p.s.1.g. ml./min.
c.
ture
Carrier gas flow rate at isothermal operation Temperature programming Initial temperature Initial time a Or as given.
A Leeds & Northrup "Speedomax" 5-mv. recorder was used with the instrument. A Perkin-Elmer Model 194 printing integrator was connected directly t o the detector amplifier output for peak area computation. Four different columns were used for the investigations. The hydrocarbon sample was investigated on a Golay and a packed column, both prepared by using OS-138 polyphenyl ether (Monsanto Chemical Co.) as liquid phase. The fatty acid methyl ester mixture was analyzed on a Golay column with Apiezon L and a packed column with SE-30 silicone gum rubber liquid phase. The parameters of the individual columns and the fixed analytical conditions are given in Table I for all four columns. Helium was used as carrier gas. I n the case of the packed column, the total carrier gas flow entered the column; when using the Golay columns, the flow (plus sample vapor) was split and the larger part vented through the split restrictor. The samples were introduced with Hamilton microsyringes. A 1-p1. syringe was used for the investigations with constant sample volume and a 10pl. syringe when the sample volume was varied. The air and hydrogen flows to the detector were held constant during the whole investigations. The given flows represent an air/H2 ratio of 24.6 and a n 02/H2 ratio of 4.9. Samples.-Two sample mixtures were used for t h e studies. T h e first test mixture was similar t o t h a t used previously for split investigations (S) and had the following nominal (liquid) concentration (in volume yo): n-hexane, 35.71; n-octane 25.00; ndecane 21.43; n-dodecane 17.86. This mixture represents a boiling point range between 69' and 214.5' C. and a molecular weight range between 86.17 and 170.33. The reason this hydrocarbon sample was used for the detailed studies is that it allowed not only the comparison of the pro-
ml./min.
c.
min.
ANALYTICAL CHEMISTRY
...
240 175 20
15/85
240 175 20
ilpiezon, Golay 150 0,020 0.010
SE-30, Golay 15 0.125 0.085 1.5/98.5
300
330 200 20
...
180 20
1/398 0.2" 12 16.7 35 411
...
1/256
12 16.7 35 411
12 16.7 35 41 1
... 0.1 12 16.7 35 411
140
150
240
180
1.17
0.2=
15.8
80
40
1
1
0.5
0.52 200
1
15.7 ...
...
grammed temperature results with those obtained under isothermal conditions but also the comparison of all results with values calculated using the method described previously (3). A second test mixture with known qualitative but unknown quantitative composition was also used in our investigations, for two reasons. First, we also wanted t o check the operation of the system when analyzing a high boiling sample. Secondly, we felt it necessary t o have proofs on quantitative accuracy (and a t the same time, split linearity) with a nonhydrocarbon sample. This sample was a mixture of the methyl esters of caproic (C8)] caprylic (C8), capric (GO), lauric (C12), myristic (C14), and palmitic (C18)acids with a molecular weight range between 130.18 and 270.45. The boiling point of methyl caproate is 149.5' C. and of methyl myristate 295.8' C.; the highest member of the sample, methyl palmitate has a boiling point of 202' C. a t 20 mm. of mercury pressure. Because response factors for fatty acid methyl esters on the flame ionization detector were not yet reported, we compared the results obtained under different experimental conditions and on different columns with each other. RESULTS
Hydrocarbon Mixture. First, a 0.2-pI. sample was analyzed under isothermal conditions on both packed and Golay columns prepared with polyphenyl ether liquid phase. Table I1 lists the analytical results and also the theoretically calculated values. The next series was carried out under programmed temperature operation with different programming rates but with identical injected sample volumes. First, we measured the respective carrier gas flow rates a t different temperatures on both columns during temperature programming; the pressure drop through the system was kept constant at 20 p.s.i.g. Figures 1 and 2 plot the flow rates against column temperatures for both packed and Golay columns measured at column exit with a soap bubble flow meter. Then, a 0.2pl. sample was analyzed using different programming rates; the programming was always started one minute after
Table II. Comparison of the Isothermal Analyses with Calculated Data
Relative peak
area values,
Sample components n-Hexane n-Octane n-Decane n-Dodecane
Calculated 33.38 25.02 22.35 19.25 100.00
Found Golay Packed column column split no split 32.39 32.30 25.33 25.17 22.75 22.80 19.53 19.73 100.00 100.00
I 5 0 FT 0010 IN I D COLUMNS 2 50
u
-
z 200 I
-
-
a a : Y
n
-
I
-
w
150-
z
-
Z
L
ical range (0.7-19.0) has a more than twenty fold variation. However, as shown in Table V, this variation in the carrier gas flow rates and in the He/H2 flow ratios did not influence the analytical results. Not only did the relative peak area values remain unchanged but even the absolute peak area values were fairly constant; this fact demonstrates the good reproducibility of the injection with the Hamilton microsyringes. For the previous investigations, the sample volume was kept constant. We, however, also investigated whether
quite significantly. I n the case of the Golay column, i t was reduced from 1.46 ml. per minute (at 84.7" C.) to 0.88 ml. per minute (at 212' C.); with the packed columns, the respective values were 22.4 ml. per minute (44.6' C.) and 17.3 ml. per minute (123.1' (2.). Because the hydrogen flow was always 16.7 ml. per minute, this means that the ratio of the hydrogen and carrier gas flow rates varied between 11.4 and 19.0 (Golay column) or between 0.7 and 1.0 (packed column), respectively. Thus, the hydrogen to carrier gas ratio over the whole analyt-
IO0
\ 10 C A R R l E R G A S FLOW R A T E
05
I5 ml/m,n
Figure 1. Relationship between column temperature and carrier gas flow rate, Golay columns
sample injection. The column temperature during this isothermal initial period was 40' C. when using the packed column and 80' C. with the Golay column. The column temperatures at which the individual sample components emerged and the corresponding flow rates (interpolated from the figures) are given in Tables I11 and IV; the analytical results are summarized in Table V. The third experimental series was carried out using only the Golay column; here, the program rate was always ' 4 C. per minute with a 1-minute initial period at 80' C., and the sample volume was varied between 0.4 and 9.0 fil. Table VI lists the results obtained. Fatty Acid Esters. This sample was first analyzed on t h e SE-30 packed column under isothermal conditions and then on t h e Apiezon L Golay column under both isothermal a n d programmed temperature conditions. Table VI1 gives t h e analytical results. Finally, Table VI11 lists t h e column temperatures at which t h e individual sample components emerged during temperature programming a n d t h e corresponding flow rates which were taken b y interpolation from Figure 1.
Table Ill.
Temperature of Emergence and Carrier Gas Flow Rate-Packed
Column
n-Hexane n-Octane n-Decane n-Dodecane Pro- Temp. Temp. Temp. Temp. gram of Corresp. of Corresp. of Corresp. of Corresp. flow emerflow emerflow emerflow rate,' emer' C./ g;nce, rate,* gence, rate,b gence, rate,* gence, rate,b C. ml./min. O C. ml./min. nun. C. ml./min. C. ml./min. 4 44.6 22.4 59.4 21.4 81.3 20.0 101.6 18.7 10 46.9 22.2 72.5 20.5 98.1 18.9 123.1 17.3 a Programming starts after a 1-minute isothermal period, a t 40" C. Interpolated values from graph.
Temperature of Emergence and Carrier Gas Flow Rate-Golay Column
Table IV.
n-Decane n-Dodecane n-Octane n-Hexane Temp. Corresp. Temp. Corresp. Temp. Corresp. Temp. Corresp. of flow of flow of flow of flow emerrateJ5 rate,b emeremerrate,* Program emerrate,b rate," mL/ gence, ml./ m!,/ gence, gence, m!,/ goence, C. min. " C. min. C. min. ' C./min. ' C. min. 113.7 1.29 1.38 97.7 4 84.7 87.9 1.44 1.46 137.3 1.18 1.28 116.4 10 91.7 1.42 99.1 1.37 212,s 0.88 1.06 163.6 12.5 1.33 125.3 1.23 108.9 a Programming starts after a 1-minute isothermal period, at 80" C. * Interpolated values from graph. O
Table
V.
Effect of Program Rate on Peak Area-Hydrocarbon
Sample
Relative peak area values, % Program rate,
c./ n-Hexane min. Packed Golay 4 31.74 32.64 10 31.92 31.83 . . . 32.55 12.5
0
n-Octane Packed Golay 25.52 25.37 25.25 25.38 ... 25.17
%-Decane Packed Golay 22.80 22.29 23.18 23.03 ... 22.59
n-Dodecane Packed Golay 19.94 19.70 19.62 19.76 ,.. 19.69
Effect of Sample Size on Peak Area-Golay Column n-Hexane n-Oc tane n-Decane n-Dodecay 'NormalNormalNormalNormalized ized ized ized ueak Relative uenk Relative Deak Relative peak Relative peak peak area, peak area, peak &ea, Sample 'area, 1 pl. = area, area, 1 ~ 1= , area, area, 1 pl. = size, lpl. = rl. 100 % 100 % 100 % 100 7% 26.80 41 22.76 41 19.28 32.16 42 0.4 42 0 6 67 32.26 65 25.29 65 22.94 66 19.51 i . 0 loo 32.79 io0 25.46 100 23.13 100 19.62 317 19.41 314 31.22 324 326 23.58 3.0 25.79 19.57 23.52 918 923 31.88 905 25.03 936 9 0 Table VI.
DISCUSSION
The isothermal results (Table 11) show a very good reproducibility of the data obtained on both the packed column (without split) and the Golay column (with split). The agreement with the calculated values is also quite good. When analyzing the hydrocarbon sample with different program rates, t h e flow rate of the carrier gas varied
~~
VOL. 34, NO. 1 1 , OCTOBER 1962
e
1433
Table VII.
Column Packed Golay
F
Compatison of the Analyses of the Fatty Acid Methyl Ester Mixture
{
Program rate, O C./niin. Isothermal Isothermal 4
10
Relative peak area values, yo CapryMyrisCaproate late Caprate Laurak tate 0.74 8.79 6.70 45.96 18.58 0.76 18.07 8.70 6.90 46.40 0.78 8.66 6.66 46.01 18.78 0.73 8.72 7.19 46.27 18.40
Palmitate 19.23 19.17 19.11 18.69
o V
1501
W
rr
3
i
w
significant changes in sample sizes (which is equal to the analysis of samples wit.h wide concentration ranges) have any influence on the analytical results. During these investigations (Table VI) not only the relative but also the absolut'e peak area values were calculat'ed. T o simplify the comparison of the latter data: we normalized the \-dues obtained, assuming a value of 100 to bhe respective areas from the analysis of a 1-pl. sample. If the system reproduces itself correctly, the normalized area values of each coinl)onent, corresponding to the mine sample injection must be close to each other and their ratio with different sample injections (different sample volumes) should be close t o bhe nominal sample volumes. The differencc of the values as given in Table VI is within the esperimental error. Although, in our opinion, the invcstigations with t'he hydrocarbon sample adcquat,ely proved the quantitative reproducibility of the programmed temperature gas chromst~ographic syst'em with coiistant pressure drop, we also checked the systeni with another sample. The mist'ure of the even number C6-CI6 nornial saturated fatty acid met,hyl csters was selected for two reasons: First, the sample has a wide boiling point range and its last components boil a t or well above 300" C.; sccond, the difficulty of lincarly splitting a fatty acid ester niisture has been espresslg mentioned during informal discussions by several workers. The investigation of this sample \\.as bawd on comparison of thc rrlative p c d area ohtaincd on a packed column without split, under isothermal conditions, and on a Golay column with split, undrr both isothermal and progranimed
Table VIII.
Program
Caproate Trn>p. gence,
rate,b ml./min.
gence, C.
207
0.65
210
4
01
C.
In our opinion, our in\-estigations tlemonstratcd convincingly that a programmed temperature gas chroniatographic system. with a flame ionization detector where the pressure drop through the column (an1 not the flow rate a t column outlet) is held constant during operation, givcs good reproducible and accurate quantitstive results and the changci in the rat.io of the carrier gas and hydrogen flow rates during analysis had in a \vide range no effect on the rcsults. The results w-ere equally good using both packed and Golay columns. The linearity of the split systcm used was also pro\.ed.
Caprylate Temp. of Corresp.
Corresp. flow
inin.
CONCLUSION
Temperature of Emergence and Carrier Gas Flow Rate-Fatty
r:iteJ0 einei-
C./
temperature operations with different program rates. However, we went one step further: We used diferent liquid phases in preparing the packed and Golay columns. Because the composition of t,he sample was identical and the same type of det,ector was used, the relative peak area \ alues could be espected to be identical. In this way, we also wanted to demonstrate the reproducibility of the quantitative analytical values obtained on a Golay column relative to a packed column with different liquid phases. Table VI1 demonstrates that even with this sample and under the described circumstances, the agreement of the individual results is good. Table VI1 I gives the teinperatures of (mergence for the individual peaks and the corresponding carrier gas flow rates. The relative change in the ratio of the hydrogen and carrier gas flow rates was here even greater than in the case of the hydrocarbon sample: I t varied between 25.7 (at 207' C J and 45.1 (at 280' C.)i.e. more than 70y0.
emer-
flow
rate,b nd./min. 0.64 0.59
0.61 222 10 218 Programming starts after a I-nunute isothermal period at 200" C. Interpolated from graph.
0
1434
ANALYTICAL CHEMISTRY
%
-
l 0 O L
d
r
50k
t I
I
I
I
1
I
1
1
1
1
15 CARRIER
1
(
'
1
'
20
GAS FLOW R A T E
rnL/min
Figure 2. Relationship between column temperature and carrier gas flow rate, packed column ACKNOWLEDGMENT
The authors are indebted to Warren Averill for many valuable discussions and particularly for his advice in the instrumental aspects of this work. LITERATURE CITED
(1) Condon, R. D., Scholly, P. H., Averill, W., Ibid., p. 30. (2) Desty, D. H., Geach, C. J., Goldup, A., in "Gas Chromatography 1960," R. P. W. Scott. ed.. D. 46, Butterworths. London, 1960. 13'1 Ettre. L. S.. dverill. Vi.. A s . 4 ~ . CHEW33, 680 (1961). (4) Ettre, I,. S., Cieplinski, E. M', Coates, V. J., ACHEMA-European Convention of Chemical Engineering, Frankfurt am Main, West Germany, June 12, 1961; to be published in I
\ - ,
.
~
Dechema Monograph.
(5) Ettre, I,. S., Claudy, S . L., Chem Can. 12 (Y), 34 (1960). (6) Gill H. A, Averill, X., 13th Pittsburgh Conference on Analytical Cheniistry and Applied Spectroscopy, Pittsburgh, Pa., March 5, 1962. (7) Golay, M. J. E., Ettre, L. S., Norem, S. R., 4th International Gas Chromatography Symposium, Preprints, XI. van Swaa,v, ed., Butterworths, London, 1962, p. Z1. (8) Ongkiehong, L., in "Gas Chromatog raphy 1960," R. P. W. Scott, ed., p. 7, Butterworths, London, 1960. RECEIVEDfor review April 19, 1962 .4ccepted July 31, 1962.
Acid Methyl Ester Sample on Golay Column
Lsurate Caprate Temp. Temp. of Corresa of Corrwp. flowerneremerflow, rate,b gence, gence, rate,b ' C. ml./min. C. ml./min. 0.57 226 0.62 215 0.51 0.56 242 230 O
I O F T l/B IN O D COLUMN O S - 138 ON S l L A N l Z E D CHROMOSORB W
Mylistate Temp. . of Correap. flowemerrate,b gence, O C. nil./min. 0.51 242 0 45 258
Palmitate Temp. of Corresp flowemerrate,* gence, C. inl./min. 263 0 43 0 37 280
