Evaluation of Support Materials for Use in Gas Chromatography DONALD T. SAWYER and JAMES K. BARR Department of Chemistry, University of California, Riverside, Calif.
b The characteristics of a number of materials have been investigated with a view to their possible use as column supports for gas chromatography. Chromosorb W, glass beads, Nichrome beads, Carborundum, and Fluoropak show the least adsorption; this is further reduced b y treating the materials with hexamethyl disilazane. The surface areas of the materials have been evaluated to permit comparisons of columns with equal thicknesses of liquid phase. Comparison of the chromatograms obtained by these columns for a nine-ketone mixture leads to the conclusion that silazaned Chromosorb W is the most satisfactory general support material, particularly for low-loaded columns. Some criteria for an ideal support material are discussed.
A
LTHOUGH THE SUPPORT MATERIAL
in gas liquid chromatography is commonly thought of as being inert and nonactive in the separation process, numerous workers have shown this t o be untrue (2-5, 8, 9, 11, 12, 14, 16). Interaction between the solute and the support material becomes increasingly evident as the solutes become more polar, as the amount of liquid phase is decreased, and as the sample size is decreased (14). I n general, efforts are made to decrease or eliminate adsorption effects to ensure that separation will be a pure partition process; however, a t least tivo groups (4, 11) have advocated that adsorption by the support be combined with liquid partitioning to give a greater degree of selectivity. K o r k in our laboratories using low-loaded columns ( I S ) has indicated again that adsorption by the support becomes an increasing problem as the amount of liquid phase is decreased. Because the extent to which the loading of a column can be reduced usefully is dependent upon the level t o which adsorption can be reduced, an investigation of support materials and the effect of deactivating agents upon their adsorption has been undertaken. The results of this study are the basis for the present discussion. EXPERIMENTAL
Apparatus and Materials. Chromatographic measurements were made
15 18
ANALYTICAL CHEMISTRY
with an Aerograph Model 600B gas chromatograph equipped with a flame ionization detector (Wilkens Instrum e n t and Research, Inc.). Columns were prepared from stainless steel inch in tubing, 3 feet long and diameter (0.085inch i.d.), and were packed in the conventional manner using a mechanical vibrator. The flow rates for the column studies were measured a t the outlet with a soapfilm flowmeter. Surface area measurements were made using an Aerograph Model A90 gas chromatograph equipped with a thermal conductivity detector. The modified B.E.T. method described by Ettre and Brenner was used for these determinations ('7). The various support materials mere obtained from a number of sources: Chromosorb W and Firebrick P, J17ilkens Instrument and Research, Inc.; Carborundum, Carborundum Corp. ; Fluoropak, Fluorocarbon Corp., hnaheim, Calif.; glass beads, Microbeads, Inc., Jackson, Miss.; Xichrome beads and stainless steel beads, Linde Division, Union Carbide Corp. The Fluoropak, glass beads, Nichrome beads, and stainless steel beads were sieved using standard grade brass sieves; the Chromosorb W, Firebrick P, and Carborundum were used as received. The various liquids used as solutes were obtained from either Eastman or Matheson Coleman & Bell and were used without further purification. The mixture of nine ketones (Figure 1) was pre-
E 2-methyl4-penlonone F cyclopenlonone G 3-heplanone H 2-heplonone
llS 131 I50
I cyclohexanone
156
152 #
G
1
f
L, 5
IO
0
T I M E , Min
Figure 1. Chromatogram illustrating separation of a nine-ketone mixture by a 1.4% loaded column Sample size, 0.1 MI.
pared to contain equal volumes of each. The hesamethyl disilazane used for treating some of the support materials was obtained from the Peninsula Chemical Co., Gainesville, Fla. ; the method of treatment was the same as described by Bohemen and coworkers ( 3 ) . RESULTS AND DISCUSSION
Columns of the different uncoated support materials have been prepared so that the adsorption of each material could be established and quantitatively compared. Each column has been studied a t 100' C. by injecting 50-pl. samples of the vapor (at room temperature) for each of five different organic compounds. The results of this comparison are tabulated in Table I. Data are included for the untreated support materials and for the support materials after being treated n-ith hesamethyl disilazane ( 3 ) ; the performance of acid washed Carborundum is included also. Comparison of the retention times indicates clearly that silazaning the supports gives a significant decrease in the amount of adsorption, particularly for polar compounds. The degree of improvement is significantly greater than Bohemen and coworkers noted (3, 12) because of the much smaller sample sizes used here. The effects of adsorption are more pronounced as the size of the sample is decreased ( 1 4 ) ; thus there is a much greater chance for observing significant improvements. In the case of Fluoropak, silazaning has little or no effect because of the almost total absence of any adsorption by this support; the low adsorption by glass beads leads to a similar small improvement from silazaning. The performance of a support with a liquid phase on it is the real criterion for judging its quality and lack of adsorption. Thus, on the basis of the data in Table I, the five support materials exhibiting the least adsorption were selected for further study with liquid coatings. To obtain a meaningful comparison of the adsorption for coated support materials, equal thicknesses of a liquid phase must be placed on each support. This can be done, assuming the material coats uniformly, by determining the surface areas of the support materials; after which the amount of
liquid (with a known density) can be calculated to give the desired thickness. Using the method described b y Ettre and Brenner ( 7 ) , the surface areas for five support materials (plain and silazaned) have been determined and are tabulated in Table 11. At the bottom of this table three literature values are given for Chromosorb JJ7 and Fluoropak. The agreement is reasonably good considering that the mesh sizes are different and that the materials are from different lots. Table I1 also gives the surface area per cubic centimeter of bulk volume for each support material, which is a more useful value if the surface area per packed column is desired. The performance of five silazaned support materials, coated with sufficient Carbowax 400 t o give equal thicknesses of liquid phase, has been compared using a nine-ketone mixture (Figure 1). As a first step, the height equivalent to a theoretical plate for cyclohexanone has been determined for each column as a function of carrier gas velocity and is shown in Figure 2. Following this the columns have been compared using the velocity for each column which gives a minimum plate-height for cycloheuanone. The results of this comparison are tabulated in Table 111, and represent the data for 3-feet b y 1/8-inch columns operated a t 24' C. This tabulation includes, for each column, the weight of liquid phase to give 0.020 gram of Carbowas 400 per square meter of surface (a 170-A. thick layer for a liquid density of 1.13); the corrected retention volume, V R ,and the specific retention volume, V,, for cyclohexanone; the minimum height equivalent to a theoretical plate for the cyclohexanone peak; the flow rate and velocity at the minimum plate height; and the number of ketones resolved of the nine-ketone mixture-e.g., see Figure 1. The specific retention volumes, V,, given in Table I11 have been determined b y dividing the corrected retention volume, V R , by the weight of liquid phase and multiplying b y the temperature factor 273,297 ( I O ) . The Ti, values were calculated at the velocity of minimum plate height for each column as indicated in Table I11 and Figure 2. Experimental precision of V , is within 10% of the average given in the table. Duplicate columns were prepared in each case t o verify the results obtained. The validity of this calculation is based on the assumption that all of the liquid phase is rapidly and completely contacted b y the solute. and that a state of equilibrium is reached. Previous Ivork b y the authors (13) has shown that Carbowax 400 on silazaned Chromosorb W gives the same specific retention volume, V,, for cyclohexanone for loadings from
Table 1.
Support Materials
Retention in secondsa Column Blank column Firebrick P, 60-80 mesh Firebrick P, sil., 60-80 mesh Chromosorb W, 100-120 mesh Chromosorb W, sil., 100-120 mesh Glass beads, 70-100 mesh Glass beads, sil., 70-100 mesh Carborundum, 100 mesh Carborundum, sil., 100 mesh Carborundum, acid washed, 100 mesh Stainless steel. 170-325 mesh Stainless steel; sil., 170-325 mesh Nichrome, 170-325 mesh Nichrome, sil., 170-325 mesh Fluoropak, 50-60 mesh Fluoropak, sil., 50-60 mesh a
174 Dioxane
MIBKb
15
15 >3200
>2600 2400 1680 22 20
>4700
25
>4600 62 25 10 >3700 150
1320 360 50 165 25 15 17
>3800 3150 .~ 270 1380 120 20 20
10 840
%-Butanol Cumene o-Xylene 15 15 15 >2800 970 955 >3800 190 150 >5000 90 80 360 20 16 120 12 12 110 12 10 >3700 68 50 >3000 20 12 >2000 >3400 >2100 >2300 >3700 20 15
210 290 55 90 55 25 25
120
300
35 90
40 30 30
Carrier gas; N,, 15.0 ml./min., with each column at 100' C. ketone.
* Methyl isobutyl
0.4% to 207, by weight. The constancy of VO with changing amounts of liquid phase indicates that chromatographic theory is obeyed, that there is no significant adsorption b y the s u m o r t , and that the column allows complete intimate contact and rapid equilibration between the vapor and the liquid phase. Thus, the value of 11,200 ml. is thought to be the true specific retention volume for cyclohexanone in Carbowax 400. The quality of the Chromosorb W column is further supported b y its low plate
1
"
'
I
"
'
I
"
'
I
'
I
Table II.
Surface Areas of Supports
Column Chromosorb W, 100120 mesh, sil. Chromosorb W, 100120 mesh Carborundum, 100 mesh, sil. Carborundum, 100 mesh Fluoropak, 50-60 mesh. sil. Fluoropak, 50-60 mesh Glass beads, 70-100 mesh, sil. Glass beads, 70-100 mesh Nichrome, 170-325 mesh. sil. Nichrome, 170-325 mesh
sq.
sq.
0.71
0.23
1.00
0.32
0.39
0.72
0.41
0.74
1.3 1.4*
1.0 1.2
0.19
0.28
0.36
0.53
0.031
0.16
0.026
0.13
meters/ meters/ gram cc.
a Ettre's value ( 6 ) , 1.41: Baker, Lee, and Wall's value ( I ) , 1.2. b Fluorocarbon Corp. value, 0.64. 4
i
-1
4
1
L
o ~ " " ' " " " " ' " ' ~ 4 a 12 6,cm /sec
16
Figure 2. Height equivalent to a theoretical plate for cyclohexanone as a function of the average carrier gas velocity for five different support materials Each column coated with equal thickness of Carbowax 400 and operated a t 24' C. Sample size was 0.1 MI. of nine-ketone mixture.
height value and its ability to resolve eight of the nine ketones. Figure 1 shows a chromatogram obtained with this column for the ketone mixture, and illustrates the sharpness of the peaks and the degree of separation. According t o gas chromatographic theory (IO)the specific retention volume for a given solute-solvent pair is only dependent upon the amount of liquid phase and the temperature of the column. Thus, within experimental error all of the V, values in Table I11 should be equal t o the value for the Chromosorb W column, which is assumed to be the correct value (13). VOL. 34, NO. 12, NOVEMBER 1962
1519
Table 111.
Comparison of Coated Columns
Cyclohexanone Column A. 0.020 g. of Carbowax 400/m. VR(m1.) Chromosorb W, 100-120 mesh 200 0.017 g. (1.4%) Glass beads, 70-100 mesh 220 0.021 g. (0.387c) Nichrome, 170-325 mesh 160 0.012 g. (0.064%) Carborundum. 100 mesh 500 0.52 g. (0.77%) Fluoropak, 50-60 mesh 760 0.078 g. (2.670) B. Other loadings Carborundum, 100 mesh 230 0.017 g. (0.2570) Fluoropak, 50-60 mesh 280 0.030 g. ( l . 0 7 0 )
V,(ml./g.)
F(m1.l min.)
1,200
0.09
9.0
2.7
8
9,900
0.14
2.2
1.2
7
2,200
0.15
6.2
3.5
8
8,900
0.13
2.6
1.7
7
9,000
0.79
4.3
1.5
5
12,400
CONCLUSIONS
The shortcomings of many possible support materials become more evident with low loadings of liquid phase and small sample sizes. For such columns selection of an inert support material which ensures intimate, complete, and rapid contact between solute and liquid phase is particularly important for
0
ANALYTICAL CHEMISTRY
LITERATURE CITED
8,700
The low V, values for the Carborundum and Fluoropak columns are a t first surprising. Apparently these values result from inefficient and incomplete contact between the solute and the liquid phase. Hence the effective amount of liquid phase is in reality less than the amount applied to the support. To test this conclusion two additional columns (Carborundum and Fluoropak) with different loadings have been studied; the data are tabulated a t the bottom of Table 111. I n the case of Carborundum, decreasing the loading b y a factor of three caused the specific retention volume to increase to 12,400 ml. This indicates that a larger fraction of the liquid phase is participating in the separation process and that the liquid-phase coating can be highly nonuniform. By decreasing the loading on Fluoropak, the specific retention volume is essentially unchanged. Thus, for this support, decreasing the amount of Carbowax 400 has little effect on the efficiency of t h e separation process.
1520
No. il(cm./ ketones sec.) resolved
HETP (em.)
constant V, a t all loadings, high resolution of the ketone mixture. and low adsorption. For particular conditions and liquid phases Fluoropak may be preferable. Furthermore, supports more nearly ideal than silazaned Chromosorb W are possible and should be sought. However, supports that exhibit low adsorption tend to be difficult to wet or coat uniformly. ilctive sites on the support will tend to be deactivated by the liquid if it is more polar than the constituents of the sample mixture. Thus, silazaned Chromosorb W coated with a liquid phase slightly more polar than the sample mixture may be close to the ideal system.
high column efficiencies. Sumerous potentially useful support materials become unsatisfactory for low-loaded columns because of their adsorption of the solute. In the present study all of the uncoated supports tested exhibit some adsorption when no liquid phase is present. Treatment of the support nith hexamethyl disilazane decreases the amount of adsorption, but with the exception of Fluoropak, does not completely eliminate it for polar compounds. Investigation of the five least adsorbing silazaned supports has shown that although adsorption is essentially absent when these supports are coated with liquid phase, four of the five materials are unsatisfactory for efficient chromatographic purposes. The fissured surface of Carborundum causes incomplete contact and equilibration between the solute and the liquid phase at low loadings, and leads to column inefficiencies. All columns tested except Chromosorb W require unnecessarily slow velocities a t minimum plate height and therefore require a longer analysis time. Teflon exhibits very poor plate heights here and preliminary work in this laboratory indicates that the higher surface area Teflons are even worse. However, one of the five materials, silazaned Chromosorb If7, s h o w little if any adsorption and appears to be coated uniformly by Carbowax 400. Thus, of the materials studied, it is the only one to give low theoretical plate heights,
(1) Baker, W. J., Lee, E. H., Wall, R. F., “Gas Chromatography,” H. J. Nobels, R. F. Wall, K. Brenner, Eds., p. 21, Academic Press, Sew York, 1961. (2) Bens, E. RZ., ANAL. CHEM.33, 178 (1961). (3) Bohemen, J., Langer, S. H., Perrett, R. H., Purnell, J. H., J . Chem. SOC.1960, 2444. (4) Craig, B. M.,Intern. S y m p . on Gas Chrom., S o . 5, Instrument Society of America, Michigan State University, preprints, p. 27, June 1961. (5) Eggertsen, F. T., Knight, H. S., ANAL.CHEM.30, 15 (1958). (6) Ettre, L. S.,J . Chronmtog. 4, 166 (1960). (7) Ettre, L. S.,Brenner, S . , “Instrumental Achievements for Investigation of Catalysts and Catalytic Reactions,” Perkin-Elmer Corp., Norwalk, Conn., 1960. (8) Hista, C., hlesserly, J. P., Reschke, R. F., AXAL. CHEM.32, 1730 (1960). (9) Hornstein, I., Crowe, P. F., Ibid., 33, 310 (.1961), (10) ,Littlewood, A . B., Phillips, C. S. G., Pnce, D. T., J . Chenl. SOC.1955, 1480. (11) Martin, R. L , -$SAL. CHEM. 33, 347 (1961). (12) Perrett, R. H., Purnell, J H., J . Chromatog. 7 , 455 (1962). (13) Sawyer, D. T., Barr, J. K., h . 4 ~ . CHEM.34, 1052 (1962). (14) Scholz, R. G., Brandt, W.K., Intern. Symp. on Gas Chrom., IVO 3,Instrument Society of America, Michigan State University, preprints, p 9, June 1961. (15) Smith, E. D.. Radford. R. D., A N ~ L . CHEM.33, 1160 (196lj. RECEIVED for review March 28, 1962. Accepted August 10, 1962. Presented before the Second Annual Research Conference on Gas Chromatography, University of California, Los Angeles, January 1962. \T70rk supported bv the Research Corp. and the Bell and Howell Research Center. One of the authors (J. K. B.) has been the recipient of a Graduate Fellon-ship from the Research Corp.
