Anal. Chem. 1889, 65, 3091-3700
90@1
Liquid Chromatography Based on a Low-Temperature Method of Producing Glassy Carbon Tina M. Engel: Susan V. Olesik,' Matthew R. Callstrom, and Mark Diener Department of Chemistry, The Ohio State University, 120 West 18th Avenue, Columbus, Ohio 43210
The chromatographic application of a new lowtemperature method for the generation of glassy carbon is demonstrated. The glassy carbon is generated by heating a diethynyl aromatic oligomeric precursor to temperatures .-
Table IV. Chemical Composition of LTGC-Coated Silicas as Determined by ESCA.
1
packing
;1
o.ool r;
V
-0.20
element
: .-
L
I
0 : : .-
0
Figure 8. Changes In LTGC conductivltyandretentivity (a*) wlth curing temperature. Lines are included as guldes to the eye.
h 9
-0
m
U
1
/
I
I
L
751
4:
I
e
25 0 ' 0.0
17 58 25
0.43
LTGC 7
7b 67 26
0.38
Calculated ae atom percentages; 95% confidence limit less than 4 atom %. b Calculated using s u m of two C1, peaks.
-1
200 400 500 600 800 LTGC Curing Temperature, "C
150
carbon oxygen silicon Si10 ratio
LTGC 9
:,
0.5
I
1.0
t
.
1.5
2.0
2.5
3.0
I
3.5
Capacity Factor, k '
Figure 9. Dependence of column efficiency on LTGC curing tern perature: (+) Hyperarb, (0)LTGC 8, 200 O C ; m) LTGC 9, 400 "C; (e) LTGC 10, 500 O C ; (A)LTGC 7, 600 O C ; and (V)LTOC 11, 800 "C. Llnes are included only as guldes to the eye.
indicate that the structure of the material is changing. As the curing temperature is increased, the material forms a more condensed ring structure, and the LTGC is more able to participate in electron-sharing type interactions. It is therefore not particularly surprising that dipolar interactions become increasingly important in the observed retention mechanism as the LTGC curing temperature is increased. It might be argued that the increased importance of dipolar interactions in the overall retention behavior of the LTGC might be a sign of other changes in the LTGC surface. For example, if increased curing temperature leads to active sites on the carbon surface, these active sites might preferentially retain solutes that have high a* values. However, the dependence of retention on LTGC phases on solute dipolarity is likely a characteristic of the glassy carbon and not due solelyto surface active sites. Hypercarb glassy carbon is also highly dependent on solute dipolarity, even though Hypercarb has been shown to have a uniform surface with few active sites.8 In summary, the models generated using multivariable linear regression techniques indicate that the LTGC is "tunable". LTGC behaves much like a reversed-phase material when cured to temperatures not exceeding 400 "C and displays behavior similar to that demonstrated by Hypercarb when cured at temperatures exceeding 500 OC. Effect of CuringTemperatureon Efficiency of LTGCCoated Silica Beads. Evaluation of chromatograms generated from columns packed with the LTGC-coated deactivated silica packings reveals a dependence of column efficiency on the LTGC curing temperature. This trend is obvious when measured reduced plate heights are plotted versus associated solute capacity factors are shown in Figure 9. These reduced plate heights were measured using solutes selected from Table I; specific solute identifies are not
important since the efficiencies seemed independent of solute chemical characteristics and dependent only on capacity fador. All data were collected at 0.15-0.2 cm/s mobile phase linear velocities. As shown in Figure 9, the reduced plate heights of the columns become larger as the curing temperature of the material is increased. This efficiency deterioration is evidencednot by increasinglywide peaks, but instead by increasingly tailing peaks. In addition, the slope of the lines shown in Figure 9 also become larger as the curing temperature is increased, indicating that the dependence of column efficiency on the LTGC curing temperature escalates. This increased dependence of efficiency on solute capacity factor will be more thoroughly discussed later. The reason for these observed trends has not been established. However,severalpossibilitiesexist. The heating process may lead to changes in the LTGC surface chemistry that lead to deterioration of chromatographic performance. It is doubtful that the graphitization process leads to such marked decreases in column efficiencies since other glassy carbon stationary phases such as Hypercarb display good efficiencies (see Figure 9). If anything, increased graphitization should lead to improved surface homogeneity which is usually evidenced by improved chromatographic performance. The increased hydrophobicity of the LTGC with higher curing temperatures may also lead to problems in packing the columns. If the packing material is not well distributed (closely packed) in the column, the column may appear less efficient. However, this again is not likely since efficient Hypercarb columns are prepared with relative ease. A more likely possibility is that active sites are formed on the LTGC surface during the heating process. Although these materials are heated under vacuum, the presence of relatively little oxygen might lead to oxidation of the carbon surface. This hypothesis was investigated using electron spectroscopy for chemical analysis (ESCA). ESCA results for two LTGC phases are shown in Table IV. The underlying silica support was detected by ESCA. Therefore, comparison of carbonto-silicon or carbon-to-oxygenpercentages is not particularly useful. Evaluation of silicon-to-oxygenratios is more helpful. When silica is heated, oxygen atoms are lost as two Si-OH bonds condense which produces water and a Si-0-Si bond on the silica surface.23 Therefore if this reaction were monitored as a function of temperature the Si/O ratio would increase with increasing temperature. In the LTGC cured at 400 OC (LTGC 9) the silicon-to-oxygenratio is 0.43 which is lower than the value expected for a silicon dioxide support matrix. The Si/O ratio also decreases to 0.38 when the LTGC is cured at 600 "C (LTGC 7). An increase in oxygen on the support is therefore indicated as the temperature is increased. The oxidation of the carbon surface is further indicated by the observed ESCA bands. Chemical shifta in the 01,band were not apparent in either sample. However, a single C1, band at a binding energy of 287 eV binding energy was observed in the LTGC 9 sample. Alternatively,two C1, bands, a moderate intensity band at 285 eV and a larger band at (23)KBhler, J.; Chase,D. B.; Farlee, R.D.;Vega, A. J.; Kirkland, J. J. J. Chromatogr. 1986,352,276-305.
ANALYTICAL CHEMISTRY. VOL. 65. NO. 24, DECEMBER 15, 1993
287.5 eV binding energies, were observed for the sample of LTGC 7. The absolute values of measured binding energies for the oxygen, silicon, and silver bands were shifted +1 eV due to variations in sample charging. The two carbon bands observed in the LTGC 7 sample may indicate the presence of oxidized and nonoxidized carbon in the packing that was expOsedtothe higbercuringtemperature. This interpretation in verified by published studies which measured C,,, binding energy shifts of 2 and 4 eV for C-O and C=O in carhon fibers, respectively.'0 The presence of oxygen on the carbon surface is not a desirable attribute. In fact, poor chromatographic performance and severe peak tailing observed in some "bad" Hypercarb samples may be attributed to oxidation of the glassy carbon surface introduced during the beating process.8 If surface oxidation is truly causing the deterioration of chromatographic performance with higher LTGC curing temperatures, significantchanges in the heating processused to create these LTGC stationary phases may he required. Effect of Solute Capacity Factor on Efficiency on LTGC-Coated Silica Beads. As noted earlier, reduced plate heights observed using some of these LTGC stationary phases rise sharply with increased solute capacity factor. Solute polarity had no apparent impact on the observed tailing. The principal attribute controlling whether peak tailing occurred was the magnitude of the capacity factor. Previously published work with pyrocarhon-coated silica stationary phases suggested that carbon packings are prone to efficiency losses and that these losseaareespeciallyapparentatcapacityfactors between 0 and 2.1,24 Some researchers suggest that this inefficiencyisinherent to thegraphiticsurface. They suggest that the carbon surface is prone to slow adsorption kinetics which lead to efficiency losses that become more apparent withmorestrongly-retainedsolu~.Additionalinvestigations have indicated that efficiency losses may be predominantly due to adsorption energies associated with the graphitized surface.*' Evaluation of the data shown in Figure 9 does not confirm this hypothesis. As the LTGC curing temperature is increased from 200 'C to 800 "C, the material becomes more graphitized as evidenced hy its observed retention behavior. As shown in Figure 9, the dependence of column efficiency on solute capacity factor also becomes stronger with increased LTGC curing temperature. However, the commercially-availableHyperearb glassy carbon column doea not show severedeterioration of efficiency with capacityfactor, indicating that this trend is not intrinsic to glassy carbon surfaces. The loss inefficiencyobaervedwithmorestronglyretained solutes may instead be a function of the increased presence of oxygen o b r v e d on the LTGC surfacenby ESCA. As shown in Table IV, the process we used to heat the LTGC also leads to oxidation of the carbon surface. The presence of oxygencontaining active sites on the LTGC surface might well lead to peak tailing and the observed efficiencylosses. Evaluation of alternative processing methods is again suggested. Effect of Mobile Phase Linear Velocity on Efficiency of LTGC-Coated Silica Beads. A common method for evaluating liquid chromatographic stationary phases is to measure reduced plate heights as a function of the mobile phase linear velocity; examples of these measurements for twoLTGCcolumns, LTGC 9 and LTGC 7,areshown inFigure 10. In both cases, the minimum of the curve showing the maximum efficiency achievable at slow mobile phase linear velocities was not identified. This is not unusual for microcolumn HPLC applications. However, reduced plate heightsofapproximately 5-8 weremeasured for both columns at low flow rates (< 0.1 cm/s). Reduced plate heights equal to approximately 5 are generally considered good for chro~
~
(24) Colin. H.:Guiochon, G . J. Chmmotogr. 1976,126 43-62.
I
0' 0.0
SEW
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Linear Veiocily. cmlsec Flour IO. oepenama, of r e d d plate h e m (*I moblle p h w nnear.veloclly fa a LTQC 9 column using 30% H20 In acemnitrlk mobile phase and solutes wW (0)k'= 0.88 and (+) k'= 1.50 and a LTQC 7 column using 10% H a in acetonimlemobile phase wW (A) k'= 0.56andmk'= 1.13. Linesareinciudedasguidesto~eye.
Flgure 11. Scanning electron micrograph of a cross secibn of LTGG
coated fused-silicatubing. The dark line is the interface btween the fused sillca (lefl of dark line) and Me carbon layer (right of the dark line). matographic applications. LTGC 9 provided much lower plate heights than LTGC 7. This again shows that the glassy carbon with the higher fmal curing temperature has poorer efficiency (LTGC 7,600 "C versus LTGC 9,400 OC). Also for both LTGC samples significant increases in plate height with increasing linear velocity was observed. Initial Demonstration of a LTGC Wall-Coated Column. To demonstrate the potential applications of LTGC stationary phases, initial efforta were made to coat the inside surface of a 1-m length of fused silica tube with LTGC. SEM evaluation of a cross section of the coated tube revealed the presenceofa LTGC filmapproximatelv0.5um thickasshown in Figure 11. The chromatographic performance of a 1-m-longpiece of LTGC-coated fused silica was evaluated using SFC. Sample chromatograms of mixtures containing straighhchain bydrocarbonsandpolynucleararomatichydrocarbonsareshown in Figure 12, parts A and B, respectively. The mobile phase used was supercritical COz; the mobile phase pressure was held at 102 atm for 5 min, programmed from 102 to 306 atm at 10.2 atm/min, and then held at 306 atm for 5 min. The mobile phase temperatures were 80 "C and 200 "C for the chromatograms shown in Figure 12, parts A and B, respectively. Thehigher columntemperature (200°C)wasnecessary to elute the polynuclear aromatic hydrocarbons from the column, presumably because these solutes are highly retained on the carbon surface. Theoretical plates measured using nonprogrammed conditions (102 atm Cot and 100 OC, for h' = 0 at linear velocity of 0.45 cm/s) ranged from 4000to 6000
3700
ANALYTICAL CHEMISTRY, VOL. 65, NO. 24, DECEMBER 15, 1993
B
A
/ Naphthalene Biphenyl
Phenanthrety
r^" 1
0
I
I
I
2
I
I
4
I
i
6
I
T
0
I
2
I
I
4
I
I
6
I
1
i
8
Retention Time, min
Retention Time, mln
Figuro 12. Supercritical fluid chromatogram of (A) alkane test mixture obtained using a LTGC walkoated open tubular column maintained at 80 O C and (E) polynuclear aromatlc hydrocarbon test mixture obtained using a LTGC walkoated open tubular column maintained at 200 O C .
plates/m which is respectable efficiency for open-tubular SFC applications. Although preparation of these LTGC-coated open tubular columns is still under development, the excellent efficiency displayed by the short open-tubular columns prepared thus far is encouraging. Durability of Columns. One commonweakness of carbon columns is the surfaces are often so adsorptive that they are easily corrupted with permanently adsorbed species and therefore the measured capacity factors of solutes vary with time. Several of the LTGC packings were used continuously for at least 2 weeks with 10-h-long run times without any noticeable deterioration of chromatographic performance. The capacity factors remained constant for the studied test solutes for all the LTGC packings evaluated. However, the LTGC packings were not stressed by use of strong acid or base mobile phase systems, and highly purified chromatographic grade solvents and high purity test solutes were also always used. The fused-silica open-tubular columns were difficult to maintain (i.e. they would sometimes explode when initially pressurized); presumably heating the columns to 400 "C for prolonged times to cure the LTGC weakened the fused silica columns. The fused silica wall coated columns were normally only used for a few days, again without noticeabledeterioration of chromatographic performance. We have recently replaced the fused silica tubing with glass-lined stainless steel capillary tubing and the tube stability problem was alleviated.
CONCLUSIONS Initial studies demonstrating the chromatographic application of a low-temperature method for the generation of glassy carbon films indicate the excellent potential for this technology. These oligomers can be dissolved in organic solvents and applied to various surfaces for the purpose of generating a glassy carbon film. The oligomer loses very little
mass during the heating process, making the generation of thin f i i possible. This minimal mass loss and simple coating procedure also allows the use of many different underlying conformations or packing materials. The glassy carbon is generated by heating to 200-800 OC;these conservativeheating requirements offer a significant improvement over classical methods for generation of glassy carbon which require heating polymer precursors to temperatures exceeding 10oOOC. Since the oligomer does not need to be heated to exceedingly high temperatures to generate glassy carbon, the choice of underlying supports is greatly expanded. Generation of LTGCcoated silica bead supports and wall-coated open-tubular fused-silica columns were demonstrated. This preparation technique also suggests the possibility of generating "tunable" stationary phases from a single oligomeric precursor. Retention mechanism studies revealed that LTGC demonstrates retention characteristics ranging from those of a classical octadecyl polysiloxane reversed-phase stationary phase to those similar to the commercially-available glassy carbon stationary phase simply by varying the curing temperature of the oligomer precursor.
ACKNOWLEDGMENT The authors wish to thank Dr. Mike Huston for many enlightening discussions and for help with the high-temperature oven and ESCA analyses. We also wish to acknowledge John Mitchell of the GeologicalSciencesDepartment for SEM and EDS analyses, Dr. Patrick Gallagher for use of the TGA equipment, and Keystone Scientific for donating chromatographic columns. This work was supported by the National Science Foundation under Grant CHE-9118913. RECEIVEDfor review December 28, 1992. Accepted September 20, 1993.' e Abstract
published in Advance ACS Abstracts, November 1,1993.
