Anal. Chem. 2006, 78, 2051-2054
Technical Notes
Group-Type Analysis of Oxygenated Compounds with a Silica Gel Porous Layer Open Tubular Column and Comprehensive Two-Dimensional Supercritical Fluid and Gas Chromatography Andre Venter,† Peter R. Makgwane, and Egmont R. Rohwer*
Chemistry Department, University of Pretoria, Pretoria, South Africa
A porous layer open tubular (PLOT) silica gel column was used together with subcritical CO2 as the mobile phase to effect the group separation of polar oxygenated compounds. Aliphatic and aromatic compounds were shown to elute together. This group was followed by ethers and aldehydes, which were separated from compounds containing an alcohol functional group. Compounds with a carboxylic acid moiety could also be eluted from the silica gel. The group separation obtained when a silica gel PLOT column is used together with subcritical CO2 was also demonstrated to be valuable as the first dimension of a comprehensive two-dimensional SFC×GC analysis where the GC analysis in the second dimension is performed with a fast and independently heated temperature programmed gas chromatograph. With this combination of SFC and GC, many of the oxygenated compounds, routinely found in petroleum samples, could successfully be separated and identified. Although one-dimensional chromatographic methods have been proposed for oxygenate analysis, multidimensional methods are better suited for the complexity of petroleum and natural samples.1 In some instances, multidimensional chromatographic systems were used. Oxygenates are selectively retained in a polar column before being back-flushed into a nonpolar column for alcohol and ether component separation.2 Selective detection is sometimes used to aid the analysis of oxygenated molecules, some examples of which are oxygen-selective flame ionization,3 atomic emission detection,4 and Fourier transform infrared.5 The use of * To whom correspondence should be addressed. E-mail: erohwer@ postino.up.ac.za. † Current address: Chemistry Department, Purdue University, 560 Oval Dr., West Lafayette, IN 47907. (1) Pauls, R. E. Adv. Chromatogr. 1995, 35, 259. (2) Annual Book of ASTM Standards; ASTM D4815-94a; ASTM International: West Conshohocken, PA, 1997; Vol. 05.02. (3) Annual Book of ASTM Standards; ASTM D5599-95; ASTM International: West Conshohocken, PA,1997; Vol. 05.02. (4) Quimby, D. B.; Giarrocco, V. HRC 1995, 15, 108. (5) Annual Book of ASTM Standards; ASTM D5986-96; ASTM International: West Conshohocken, PA, 1997; Vol. 05.02. 10.1021/ac051693a CCC: $33.50 Published on Web 02/04/2006
© 2006 American Chemical Society
comprehensive two-dimensional gas chromatography (GC×GC) has been demonstrated for the separation of benzene, toluene, ethylbenzene, and the xylenes, alcohols, and ethers.6 Methyl tertbutyl ether (MTBE) and several alcohols were successfully separated from a petrol sample and quantified; however, in this separation other important ethers, such as diisopropyl ether (DIPE), tert-amyl methyl ether (TAME), and ethyl tert-butyl ether (ETBE), coeluted with nonpolar components. Silica gel together with supercritical fluids is often used for group-type analysis of aliphatic and aromatic compounds. However, with highly polar molecules such as alcohols and carboxylic acids, strong hydrogen bonding is present with polar stationary phases such as silica gel. Other oxygenated compounds, for example, ethers and carbonyls, are subjected to strong dipoledipole and proton donor and acceptor interactions. The strong interactions with the stationary phase drive the equilibrium constant to favor the adsorbed state of the analytes. Thus, these compounds have very high retention factors on packed column silica gel columns that also have high surface area where the ratio of stationary phase to mobile phase, the phase ratio (β), is large. The strength of the interaction with the stationary phase can be reduced by competitive interaction of the mobile phase with the stationary phase, for example, when methanol is added as a modifier to the CO2 mobile phase. But this action increases instrumental complexity and precludes the use of the flame ionization detector (FID). Furthermore, it is not useful when alcohols are to be measured. It has been suggested7 that the use of smaller diameter columns will aid the elution of very polar analytes from polar stationary phases. Reducing the column surface area decreases the adsorptive activity of the packed column. Open tubular supercritical fluid chromatography is preferred for separations of complex mixtures, isomers, and eluting polar solutes with neat carbon dioxide.7 The low-pressure drop allows for longer open tubular columns and the corresponding higher plate numbers whereas the well-deactivated small surface area ensures few (6) Frysinger, G. S.; Gaines, R. B. HRC 2000, 23, 197-201. (7) Blomberg, L. G.; Demirbuker, M.; Hagglund, I.; Andersson, P. E. Trends Anal. Chem. 1994, 13, 126-137.
Analytical Chemistry, Vol. 78, No. 6, March 15, 2006 2051
adsorptive sites, often obviating the use of a modifier when polar compounds are analyzed. The approach followed in this study to decrease the elution time of polar compounds on adsorbents is to use a porous layer open tubular (PLOT) column.8 This is achieved with the same silica gel stationary phase normally used in packed columns, by a decrease in phase ratio through increasing the volume of mobile phase relative to the stationary phase. In this paper, the novel application of a silica gel PLOT column (designed for gas chromatography) together with subcritical CO2 is demonstrated to facilitate oxygenate analysis. With this method, compounds can be separated into various subgroups depending on the oxygenated functional group. Oxygenates are analyzed on silica gel and the flame ionization detector, without the need to use modifiers or back-flushing techniques, purely by altering the phase ratio (β) of the chromatographic column. Individual compound separation can be obtained when the coeluting members of a group are further separated by volatility. This paper also demonstrates the use of a silica gel PLOT column as the first dimension of a comprehensive two-dimensional SFC×GC instrument with fast temperature programming as the second separation axis.9 Small fractions of an initial group-type separation on the PLOT column are refocused and analyzed sequentially by a very fast temperature-programmed GC. The resulting GC chromatograms are arranged to form a threedimensional chromatographic plane where the polarity axis is orthogonal to the volatility axis.
EXPERIMENTAL SECTION SFC Instrumentation. A supercritical fluid chromatograph (Lee Scientific 501SFC) was used to deliver the subcritical CO2 (SFC grade, Air Products), without helium head pressure, to a 30-m, 0.32-mm-i.d. Chrompack CPsilica PLOT column. The PLOT column was connected to a carrier gas filter (Valco ZUFR1) by means of a two-piece removable fused-silica adapter (Valco FS1.85). This prevented particles of the PLOT column from damaging the stop-flow valve used in the SFC×GC experiments. Integral restrictors were used at the column exit to maintain supercritical pressure conditions. These were manufactured according to the process described by Guthrie and Swartz.10 Two restrictors were coupled to the column exit by means of a T-junction (Valco PN, ZT1C, Valco) to improve FID flame stability for group quantitation and to allow for connection to the modulation interface for SFC×GC experiments. The isothermal column conditions were maintained by a PYEUnicam GCD gas chromatograph. The FID was maintained at 300 °C. Chromperfect software (Justice Innovations) was used for data acquisition. An electrically actuated internal loop injector (Vici C14W, Valco) with an 0.2-µL internal loop was used for sample injection. All connections were made of 1/16-in.-o.d., 120-µm-i.d. stainless steel (SS) tubing with electropolished ends and connected with SS ferrules and connectors. Experiments were performed with the SFC mobile phase at a pressure of 150 atm and a temperature of 28 °C.11 (8) (9) (10) (11)
Ji, Z.; Majors, R. E.; Guthrie, E. J.; J. Chromatogr., A 1999, 842, 115-142. Venter, A.; Rohwer, E. R. Anal. Chem. 2004, 76, 3699-3706. Guthrie, E. J. Swartz, H. E. J. Chromatogr. Sci. 1986, 24, 236-241. Klee, M. S.; Wang, M. Z.; Giarrocco, V. HP Appl. Note 1993, 228 (6), 1-7.
2052 Analytical Chemistry, Vol. 78, No. 6, March 15, 2006
Figure 1. Schematic diagram showing instrumentation for SFC×GC. The stop-flow SFC is coupled inline to a resistively heated GC column housed in a GC oven at subambient temperature. (1) SFC syringe pump, (2) injector valve, (3) GC oven containing PLOT column, (4) stop-flow valve, (5) GC oven containing resistively heated GC column, and (6, 7) flame ionization detectors.
Figure 2. Elution order of some general groups with the silica gel PLOT column and CO2 at a pressure of 150 atm and a temperature of 28 °C.
SFC×GC Instrumentation. The details of the resistively heated GC and the SFC×GC instrumentation (Figure 1) have been reported previously.10 Flow modulation of the SFC eluent was achieved with an electrically actuated six-port valve. Valve switching was computer controlled. The valve was placed between the splitter/restrictor assembly and the SFC column outlet. The eluent was transferred to the split/splitless injector on a Varian 3300 gas chromatograph through one of the SFC restrictors. The injector splitter was opened and closed with a solenoid valve controlled from the PC. The stainless steel GC column was cooled to the ramp starting temperature with CO2 using the subambient temperature control facility of the Varian 3300. External control over the GC oven temperatures was achieved through a modification on the GC temperature control board. The SFC fractions were concentrated on the head of the cold column due to the loss of solvation strength of the supercritical mobile phase when it expands at the restrictor exit combined with cryogenic focusing. A 1-m, 0.25-mm-i.d., SE-30 stainless steel column (Restek, Ultra alloy) was resistively heated at a rate of 450 °C/min from -50 to +300 °C by applying a controlled current ramp directly to the stainless steel analytical column. A very small thermocouple (made from two 25-µm chromel and alumel wires), glued to the outside of the steel capillary, was used for accurate temperature measurement. The column was connected to the split/splitless injector and FID detector on a Varian 3300. A flexible heater tape was coiled around the detector and injector column inserts to ensure that their temperature was maintained at the maximum temperature of the ramp.
Figure 3. Separation relevant to the petrochemical industry. A 30m, 0.35-mm-i.d. silica gel PLOT column was used with CO2 at a pressure of 150 atm at a temperature of 28 °C with a linear flow rate of 7.7 cm/s.
RESULTS AND DISCUSSION The use of a silica gel PLOT column was initially investigated for petrochemical group separation of aliphatic and polyaromatic compounds. However, nonpolar compounds showed almost no retention on this polar silica gel stationary phase. Even anthracene, which contains three benzene rings, was only marginally separated from dodecane. However, the low retention of the PLOT column effected the elution of very polar oxygenated compounds. In Figure 2, it can be seen that the silica gel PLOT column successfully separates the oxygenated compounds from nonpolar compounds such as alkanes as well as polyaromatics. The oxygenate class is further separated into various groups, e.g., aldehydes and ketones, followed by the alcohol group. Carboxylic acids also elute from the PLOT column. Figure 3 shows the retention of compounds relevant to the petrochemical industry. Ethers elute in the same retention time window as the carbonyl compounds showed in Figure 2. The alcohols are very well separated from the ethers. Diisopropyl ether (DIPE) is separated from MTBE and TAME, but unfortunately, TAME and MTBE are not well resolved under these conditions. Likewise, alcohols with near carbon numbers will also coelute. A second separation, to analyze for the individual components of each group, may be needed if compound specific as opposed to group analysis is required. Nonpolar compounds are however unlikely to coelute with ethers, which is a major advantage over the previously described GC×GC method.6 Further pressure, temperature, and flow rate optimization may improve resolution of the coeluting compounds. However, for application in two-dimensional SFC×GC, the resolution that is obtained here is already more than adequate. When The PLOT column is used as the first dimension in a comprehensive two-dimensional SFC×GC system, alkanes, alcohols, ethers, and other groups can be separated into their individual components. Figure 4 shows the analysis of a sample composed of n-alkanes from octane to hexadecane, DIPE, MTBE, TAME, diisoamyl ether, and various alcohols. All components in the mixture were well separated. Coelution of nonoxygenated compounds with oxygenates is precluded as all nonpolar compounds elute together as the first group in the chromatogram.
Figure 4. SFC×GC analysis of a petrochemical standard containing alkanes, ethers, and alcohols. CO2 at a pressure of 150 atm and a temperature of 28 °C was used as mobile phase in the SFC analysis. The flow through the PLOT column was collected for intervals of 5 s. The GC was repeatedly (1 cycle/min) temperature programmed from -50 to +250 °C at 450 °C/min and then cooled again to -50 °C while hydrogen was supplied as carrier gas to obtain a linear flow rate of 1 m/s.
The high degree of order in the 2D chromatogram allows for easy identification of separated compounds. Methyl ethers are more retained than symmetrical ethers. This is due to effective shielding of the polar oxygen atom by the bulky side chains. The volatility analysis of the second dimension separates these two groups further into their individual components. TAME, MTBE and isopropyl ether are also well separated from each other due to their different volatilities. The effective polarity of longer alcohols is reduced due to the effect of the long aliphatic chain. Thus, it is observed that dodecanol elutes slightly earlier from the SFC column than do shorter alcohols such as ethanol. This “molecular weight” effect causes correlation between the two separation dimensions of polarity and volatility and slightly reduces the effective peak capacity of the combined technique. This may be improved by increasing the solvent strength of the mobile phase. A commercial gasoline sample was obtained from a petrol station. This sample was analyzed using the SFC×GC (Figure 5). The presence of a methyl ether corresponding to TAME as well as small amounts of diisopropyl methyl ether and diisoamyl ether by volatility axis displacement could be confirmed. Analytical Chemistry, Vol. 78, No. 6, March 15, 2006
2053
Figure 5. SFC×GC analysis of a commercial lead-free gasoline sample showing the presence of TAME, diisopropyl ether, and diisoamyl ether. CO2 at a pressure of 150 atm and a temperature of 28 °C was used as mobile phase in the SFC analysis. The flow through the PLOT column was collected for intervals of 5 s. The GC was repeatedly temperature programmed from -50 to +250 °C at 450 °C/min while hydrogen was supplied as carrier gas to obtain a linear flow rate of 1 m/s.
CONCLUSIONS A silica gel PLOT column with a subcritical fluid mobile phase was shown to separate complex mixtures containing oxygenated compounds into different groups of oxygenates. Nonpolar com-
2054 Analytical Chemistry, Vol. 78, No. 6, March 15, 2006
pounds such as alkanes and polyaromatic hydrocarbons were poorly retained under these conditions and could easily be separated from oxygenated compounds. No coelution of nonpolar and oxygenated compounds is likely to occur. The oxygenated compounds were further separated into different groups of increasing polarity. Methyl and symmetrical ethers appear as two distinct groups. The ethers, ketones, and aldehydes were separated from alcohols and alcohols could be separated from a carboxylic acid. This reduction in retention of oxygenated compounds is believed to occur due to the reduction of the phase ratio (β term of the retention factor). The analysis of a mixture of compounds pertaining to the petrochemical industry was demonstrated. Here the separation of the ethers from the methyl ethers and alcohols without any interference from the nonpolar molecules promises to provide a fast and accurate group-type analysis of oxygenates in gasoline, diesel, and light oil samples. The unique separation power obtained when a silica gel PLOT column was used in the first separation in a comprehensive twodimensional SFC×GC instrument was demonstrated. Initial group separation of the oxygenated compounds, followed by a fast, repetitive gas chromatographic analysis, distributed the components of complex mixtures over a chromatographic plane. Through this technique, group separation was improved and separation of individual compounds was achieved. As an example, ethers and alcohols likely to be found in gasoline samples were all separated as individual peaks. This test sample included MTBE, ETBE, IPE, ethanol, and isopropyl alcohol. The presence of TAME in a commercial lead-free gasoline sample was also demonstrated. With future refinement of the technology, we believe that the SFC×GC with a silica gel PLOT column will be ideally suited for the analysis of complex mixtures containing oxygenated compounds. Received for review September 21, 2005. Accepted December 22, 2005. AC051693A
