Gas-Solid Chromatography of Hydrocarbons on Activated Alumina Effect of Carrier Gas on Retention Time and Column Efficiency R. L.
HOFFMANN and C.
D. EVANS
Northern Regional Research Laboratory, Peoria, 111.
6 7 604
r-
b The quantitative accuracy of gassolid chromatography (GSC) of hydrocarbons on activated alumina depends on instrumental parameters. Retention times, peak width, peak height, and column efficiency vary with certain molecular characteristics of the carrier gases used to elute hydrocarbons from activated alumina. These characteristics include mass, atomic cross section, and Composition. Experimental GSC data have been correlated with these molecular characteristics for eight pure carrier gases. The molecular weight and structural types of hydrocarbons amenable to alumina GSC are strongly influenced b y the particular carrier gas selected. Hydrogen and carbon dioxide elute saturated hydrocarbons up to CIZ, whereas at the same theoretical plate efficiency helium can elute members up to Cg only. For unsaturated hydrocarbons, degree of unsaturation and molecular weight set the elution limit. At elevated temperatures column efficiencies for the different carrier gases became similar.
l4
t
~
Krypton,
Column Temperature, "C. Figure 1. ture
Retention time of n-butane vs. column tempera-
Sample size each carrier
lO-~i.n-butane
(gas) a t STP.
EXPERIMENTAL
A
of complex hydrocarbon mixtures have taken on new importance because of the presence of hydrocarbons in polluted air, biological preparations, and autoxidized lipids. An analytical technique that is both rapid and specific for hydrocarbons has been developed ( 5 , 6 ) . Refinement of the technique has now extended its use and quantitative accuracy. Retention characteristics of hydrocarbons on activated alumina and the efficiency with which hydrocarbons are separated are strongly influenced by the carrier gas employed for their chromatography. Greene and Roy (3) investigated carrier gas effects in the gas-solid chromatography (GSC) of hydrocarbons on activated charcoal. The present investigation includes eight carrier gases, three uncommon to ordinary GSC, and explains carrier effects on retention time and column efficiency relative to alumina GSC. NALYSIS AND QUANTITATIOX
Gaseous samples of n-butane were injected by a gas syringe into a 4-foot x '/*-inch 0.d. aluminum column of 60- to 80-mesh Microtek activated alumina. Resulting chromatograms were obtained from an isothermally operated F & M Model 1609 chromatograph equipped with a flame ionization detector. Various carrier gases [hydrogen (Hz), deuterium (DZ), helium (He), neon , carbon dioxide (Ne), nitrogen (Nz) (COZ),argon (Ar), and krypton (Kr)] were delivered to the analytical column through an eight-valve manifold. After data were gathered for a given carrier, the column was purged at 350' C. for an hour with the next gas to minimize effects contributed by irreversible carrier absorption on the alumina surface. Since COz is strongly adsorbed on activated alumina (B), it was necessary to purge for 2 hours with Hz to return the column to its original retention characteristics. Use of Nz reduced this purge time to 0.5 hour. Retention data were compared with those ob-
Flow rate: 65 ml./min.
for
tained from an identical alumina column through which only one carrier gas had been passed. Because there was essentially no difference between resulting chromatograms, one column, which was thoroughly purged between different gases, nras used for all carriers. Samples were repetitively injected until the peak heights and areas agreed within 1%. With this injection te;hnique retention times were identical within experimental error. In these studies only n-butane was chromatographed on the alumina column t o minimize any effects caused by partlal adsorption of other hydrocarbons. To obtain van Deemter plots, flow rates and column temperatures were varied for each of the carrier gases studied and were carefully stabilized between chromatograms. When either D Bor Hz was used as a carrier gas, NZwas fed in at the flame ionization detector (FID) hydrogen inlet to dilute the Hz carrier to the optimum concentration for good flame stability and high sensitivity. A diluent gas was necessary since Dz or HP burning alone produced a voluminous VOL. 38, NO. 10, SEPTEMBER 1966
1309
400'
I I 150 200 Column Temperature, "C.
1:o
Column Temperature, "C.
Figure 2. Column efficiency (theoretical plates) vs. column temperature for various carrier gases at 65 mi. per minute fbw
Figure 3. Column efficiency (theoretical plates) vs. column temperature for hydrogen at various flow rates Sample size: loo-$. n-butane (gar) at STP
Somple dzs: 1 0 - 4 n-butane (gad at STP
flame envelope that e n c o m p d the detector ion-collection electrode. Such flame-geometry distortion created considerable thermionic noise, which could be overcome only through the use of a diluent gas. Column efficiencies were determined by a simple theoretical plate (TP) calculation : n=
Is(zy
Some experiments were repeated with n-pentane and n-hexane as samples to check the applicability of the data to other hydrocarbons. No discrepancies or anomalies were observed. RESULTS AND DISCUSSION
Figure 1 shows the relationship between the retention time of n-butane on alumina to the column temperature
=1000
for eight ditTerent carriers. Even though Cot is strongly adsorbed on alumina and should be an effective competitor for active sites on the adsorbent, both DI and Hz provide shorter retention times a t all temperatures. When comparing DZand Hs a t 125" and B5" C. (Figure 2), an inverse relationship exists between column efficiency and operating temperature. As column temperature was increased, D2 became more efficient, whereas a t about 150" to 175' C. the efficiency, of Hzdecressed rapidly (Figure 2). The He provided a median efficiency, between the extremes exhibited by Dt and Hzr which was fairly constant over a broad temperature range. At low column temperatures (Figure 2) Cot exhibited the greatest TP efficiency and, except for the uncommon carriers DZand H,, CO, provided the
shortest retention times of any of the gases studied. Chromatograms with the most symmetrical appearance were produced by COz at flows and temperatures somewhat higher than those providing maximum T P number. Resolution at low flows and low temperatures was good, but the peaks tended to be broad and %at. Figure 3 is a TP us. column temperature plot for He a t various flow rates showing how efficiency is maximal at low flow rates (30ml. per minute) and degrades as flow and temperature are increased. Figure 4 is a similar plot for He but shows that its maximum e5ciency is at higher flow rates. Such plots were also made for N t , Cot, and Ar. Optimum flow conditions for each carrier were determined from the plots and are summarized in Figure 5. All the carriers, except He, exhibited their maximum efficiency a t low flow rates. Butane chromatographs most efficiently with these carrier gases a t approximately 150" C. A comparison
-
v)
01
-
0
800-
.-%a 6000
nl./min. WitregeII -30 Helium....__._.__-.__ 75
.0 01
c
4002001
........30
30 (p' 100
I
I
I
I
I
150 200 250 300 Column Temperature, "C.
Figure 4. Column efficiency (theoretical plates) vt. column temperature for helium at various Row rates Sample size: loo-$. n-butane (gas) at STP
1310
ANALYTICAL CHEMISTRY
Column Tenpcrrturc,
OC.
Figure 5. Column efficiency (theoretical plates) V.I column temperature for various carrier gases each at their optimum flow rates Sample size: 1 00-/A. n-butane (gas) at STP
of Figures 1 and 5 points out that, generally] the best column efficiency is provided by carriers yielding the longest retention times. Pronounced differences between efficiencies of carrier gases plotted in Figures 2 and 5 arise from differences in flow rate and the 10-fold difference in sample size. Because retention times were not significantly altered by sample size, Figure 1 holds over the sample range specified. Efficiency, however, is dramatically influenced by sample size and will be the subject of a future publication. Examination of Table I will show that reducing sample size 10fold has almost the same effect as optimizing flow rates. Butane retention times observed for the series Dz, Hzl and He may possibly be explained on the basis that DZ is dimensionally smaller and more polarizable than He but heavier than Hz. These differences should make Dz the most efficient competitor with butane for active sites on the alumina surface. In comparison to Hzl the increased mass of D, should account for its ability to elute butane more quickly. Shorter retention times observed during the use of COS as a carrier undoubtedly result from its high affinity for active sites on the alumina. The reluctance of COz to be displaced by HS probably rests in the ineffectively low mass of Hz. Since Nz is both heavier and more polarizable than Hz, Nz is a more effective displacer than hydrogen. Flow rate affects column efficiency and, with the exception of HS, all gases tested showed maximum efficiency at low flow rates. Apparently long retention time, and its destructive effect on column efficiency] could only be overcome with He by high flow rates. Argon showed a concurrent tendency toward maximum efficiency at higher flow rates although this effect was not so pronounced as that observed for He.
Relationship of Sample Size, Flow Rate, and Temperature to Efficiency of n-Butane Chromatographed on Activated Alumina"
Table 1.
10 rl. (gas)
Arbitrary Temp.oof flow, n max., C. ml./min. 175 100 150 200 a
Data
Carrier
n ma. 65 1450 65 1150 65 1250 65 1050 from Figures 2 and 5.
gas
Hz cot Nz He
I.
I
0.6
0.8
100 PI. (g-1 Optimum
flow, Temp. of ml./min. n m a . , O C.
n ma. 920 1050 1020 1100
30 30 30 75
125 150 125 150
I
1 .o
1.2
=[l/Atomic Radiusl* Figure 6. Retention time vs. the reciprocal of the atomic radii squared for the inert carrier gases Sample size; 100-pl. n-butane (gas) at STP. Column temperature: 1 50°C.
As a group the behavior of the rare gases is unusual because the heavier gases provided the longer retention times. A linear relationship exists between hydrocarbon retention times on alumina and the reciprocal of the square of atomic radius of the carrier gas (Figure 6). This behavior is contrary to expectations when purely kinetic energies are considered. Since these gases are monatomic] they would be expected to have a zero dipole moment and be completely
Flow r a t e 8 0 ml. per minute.
nonpolarizable. With a stable octet of electrons, they should show no affinity for active sites on the alumina and would therefore be poor competitors with a hydrocarbon for these sites. The only apparent means for these gases to displace an adsorbate would be through their energetic collisions with the adsorbed species. Within the series Kr, Ar, Ne, and He the heavier Kr atom would possess the greater kinetic energy for any given linear gas velocity. On this basis, Kr would be expected to yield the shortest retention times for butane since its collisions would be the most energetic of the series. This was not true. Retention times for n-butane were longer with the rare gases, when compared with other gases, and increased from He to Kr.
0
n E 0
4
SL
u n
=
Figure 7. Alumina GSC of maturated and n-unsaturated hydrocarbons (CI to CIO) showing retention and efficiency effects produced by two carriers
a-
Y
z 0 V
u
s ( 1 ) Melhane (2) Ethane (3)Ethene 14) Propane ( 5 ) Propene
(6)Butane
Retention Time
(7) 1-Butene (8) Pentane (9) 1 -Pentena
(10) Hexane (1 1 ) 1 -Hexene (1 2) Heptane (1 3) 1 -Heptene (1 4) Octane (1 5) 1 -0ctene (1 6) Nonane (1 7) Decane
VOL. 38, NO. 10, SEPTEMBER 1 9 6 6
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The correlation existing between retention time and atomic cross section of the rare gases (Figure 6) indicates the possibility of a desorption hindrance effect. Once the adsorbate leaves an active site on the alumina surface it collides with carrier atoms in the moving gas phase. Depending on the carrier gas atomic cross section, the adsorbate may be swept along to another site or may be returned to the original site from which it desorbed by elastic collision with a nearby large inert gas atom. The larger the carrier atom the more desorption hindrance it can offer the adsorbate. Within the adsorbate series n-butane, n-pentane, and n-hexane, this linear relationship between their retention times and the carrier gas atomic cross section was valid, although the magnitude of the effect on retention time decreased with increases in adsorbate molecular weight. Other unpublished work from this laboratory (I) indicates that the rare gases have certain atomic properties which could give rise to the effects observed in this study. I n a GSC column packed with activated alumina, sample size of any hydrocarbon must be kept small for maximum column efficiency. When an F I D is used, this limitation is not particularly restrictive because this detector exhibits maximum sensitivity for hydrocarbons. Best results with n-butane were obtained with sample sizes between 10 and 100 pl. (0.26 to 2.6 X lo-‘ gram) whereas in ordinary
gas-liquid chromatography, sample loads are of the order of 100 x 10-4 gram. F I D sensitivity is maximum with carriers of low thermal conductivity (4). Since cclumn efficiency is influenced by the carrier gay used in alumina GSC, carrier choice is largely dictated by limitations inherent in the analysis problem. When resolution is important, carriers providing long retention times seem best. Fortunately, most carriers exhibiting long retention times have low thermal conductivities and will enhance F I D sensitivity. If He has the best resolution for a particular analysis, the concomitant reduction in F I D sensitivity must be tolerated. Although CO, gives excellent F I D sensitivity, it is relatively inefficient as a carrier and valuable only when higher boiling hydrocarbons are to be analyzed. Under isothermal chromatographic conditions He, for example, reaches its practical elution limit at about Cs, whereas HI or COP easily extend this range to CIZ. Extension of this limit is accompanied by reduced retention time and column efficiency (Figure 7). Peaks are narrower and retention times are shorter when H2 is used and compared with the effects produced by He. This reduction in elution time results from the inefficiency of Hzand C02and also from the multiplicity of hydrocarbon isomers as the number of carbon atoms increases. Nitrogen appears to be a good compromise choice for
maximum resolution, elution limit, economy, and F I D response. Alumina GSC can be a valuable technique for separation of hydrocarbons if care is taken to select the proper carrier gas. The effort necessary to make the proper choice and to optimize flows for maximum column efficiency will be rewarded by fast reproducible analyses and chromatograms of maximum resolution. LITERATURE CITED
(1) Erlander, S., Northern Regional Research Laboratory, Peoria, Ill., personal communication, 1966. (2) Greene, S. A., Pust, H., ANAL.CHEM. 29, 1055 (1957). (3) Greene, S. A., Roy, H. E., Zbid., p. 569. (4) Hoffmann,R. L., Evans, C. D., Science 153, (3732) 172 (1966). (5) Hoffmann, R. L., List, G. R., Evans, C. D., Nulure 206(4986), 823 (1965). (6) List, G. R., Hoffmann, R. L., Evans, C. D., J. Am. Oil Chemisls’ SOC.42, 1058 (1965).
RECEIVEDfor review May 3, 1966. Accepted June 27, 1966. Presented before Division of Agricultural and Food Chemistry, Winter Meeting, ACS, Phoenix, A r k , January 1966. The Northern Laboratory is headquarters for the Northern Utilization Research and Development Division, Agricultural Research Service, U. S. Department of Agriculture. 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.
Automated Anion Exchange Chromatography of Some Carboxylic Acids RICHARD C. ZERFING Merck Sharp and Dohme Research Laboratories, Merck and Co., Inc., Rahway,
N. 1.
HANS VEENING Department o f Chemistry, Bucknell University, lewisburg, Pa.
A completely automated method was developed for the simultaneous separation and determination of pyruvic, glutaric, citric, 2-ketoglutaricI and trans-aconitic acids. Separation of the acids was achieved by an anion exchange column using a gradient sodium acetate solution as the eluent. The separated acids in the affluent stream were oxidized by dichromate, and the decrease in dichromate concentration of the stream was measured colorimetrically and recorded continuously. The instrument used to propagate and monitor the stream was a Technicon AufoAnalyzer. Standardization curves were determined for each acid and analytical results
1312
ANALYTICAL CHEMISTRY
were obtained for a synthetic mixture of the five acids. Variables such as background oxidation of acetate ion, completeness of oxidation of the acids, and the effect of column temperature were studied.
A
of methods describing the separation and determination of carboxylic acids have been cited in the literature. Alfredsson and Bergdahl (1) described a method for separating some hydroxy acids using a resin column packed with Dowex 1-X8 on acetate cycle. The column eluate was collected using a time-actuated fraction collector, and analyzed using a Technicon A* Analyzer. In an earlier paper, AlfredsNUMBER
son and Gedda (2) reported a similar method using a resin column packed with Dowex 1-X8 on borate cycle. Fractions were also collected and analyzed using a Technicon AutoAnalyzer. Goudie and Rieman (3) published a procedure for separating and determining some fruit acids. I n their method, a Dowex 1-X8 resin column on acetate cycle was used to separate the acids. The fractions were collected and their acid content was determined using a manual dichromate colorimetric technique. Shimomura and Walton (6) reported a method by which continuous monitoring of the column stream with a recording refractometer was achieved. Of these methods, few
