the flow through the DMS column was stopped, the pressure equilibrated over the entire column. In restarting the DMS column, this causes a flow surge which lasts for 3.5 min. The first Cs product peak, propane-t, emerges and starts being counted 4 min after restarting. Consequently, the flow surge does not affect the analysis. Storage for a little over 2 hr has affected the peak shape. For example, the FWHM of a trans-2-butene mass peak increased by 10% because of peak storage. At 380 min, flow is similarly restarted in the DBTCP column. Again the flow surge does not affect the analysis. The FWHM of a cyclohexene mass peak increased by 15% because of peak storage in the DBTCP column for nearly 6 hr. Stepwise Pressure Programming. A constant helium flow rate through the detector was maintained when the DBTCP column and the DMS column were removed from the flow stream by decreasing the manifold pressure a t 30 and 38 min, respectively. Similar use of the pressure “presettings” is made a t later times in the analysis when columns are added to or removed from the flow stream. Stepwise Temperature Programming. At 98 min the operating temperature of the PCA column is changed from -78 to -8 “C. This shortens the elution time of the ethylene-t peak by 400 min. The stepwise temperature change causes a perturbation in the helium flow rate. This perturbation does not affect the analysis because no peaks are being counted. At 162 min, the Cz peaks in the PCA column have emerged and been counted. The temperature of the PCA column is then returned to -78 “C to minimize the number of pressure regulators required for analysis. Center Cut. It is known from calibration data that the unresolved butadiene-t and butadiene-dst peaks would have emerged from the DMS column and been counted a t 275 to 290 min. The center cut of these peaks is made by placing the AgN03 column down-stream from the DMS column during that interval. Recycle. The inherent recycle capability of this system is displayed in the permutations of the column order at 265, 300, and 375 min. A careful analysis of Figure 1 will reveal a nested series of recycle loops. The recycle capability is used here to allow separation of the butadiene-t and butadiene-dbt peaks to proceed simultaneously with
the counting of peaks emerging from the DMS and DBTCP columns. The recycling of the butadiene-t and butadiene-dst peaks through the DBTCP column is unnecessary for the sake of resolution. However, this recycling is advantageous because after 380 min the analysis is automatic. CONCLUSIONS
A general radio-gas chromatographic analysis system has been developed for hydrogen and C1 to C, alkanes and alkenes. Although all peaks had to be monitored at a constant flow rate in the same detector and the injection volume was large, more than 20 peaks have been analyzed, with good resolution of most peaks, in a total time of 1000 min. We conclude that (1) a recycle system of four-way valves and columns allows permutation to be made in the order of columns in a series. These permutations may be useful by themselves in addition to allowing peaks to be recycled and center cuts to be made. In addition, this system of four-way valves may shorten the time required for a particular analysis. (The long time required for the analysis shown here was due to the large injection volume.) (2) Stop-flow chromatography is a useful technique if the accompaning increase in FWHM can be tolerated. (3) Stepwise inlet pressure programming can be used to maintain a constant flow rate through the detector when a column is removed from the series in stop-flow chromatography. Stepwise pressure programming is additionally advantageous because it allows utilization of powerful stepwise temperature programming techniques. Therefore, we propose a new gas chromatographic system that has broad application. ACKNOWLEDGMENT
We would like to acknowledge a series of discussions on chromatography with F. S. Rowland, G. P. Miller, J . K. Garland, and D. H. Froemsdorf. We would like to thank Walt Niemi for assistance with the regulators and Fred Vogelsberg for handling the electronics. Received for review February 14, 1973. Accepted April 18, 1973. Work performed under the auspices of the U.S. Atomic Energy Commission.
Film Thickness of Dynamically Coated Open-Tubular Glass Columns for Gas Chromatography K. D. Bartle School of Chemistry, University of Leeds, Leeds, LS2 9JT, England
The dependence of the thickness, d ~ of, the film of stationary phase (silicone oil SF-96, hydrocarbon grease Apiezon L, silicone elastomer SE-30, and dinonyl phthalate) in glass open-tubular columns on column radius, properties of the coating solution, and the velocity of the coating plug have been studied. Values of d~ were determined from column capacity ratios for benzene and the specific retention volumes of benzene in the stationary phases determined on packed columns. The influence on d~ of column configuration during coating, and film-rear-
rangement processes immediately after coating, is also discussed. Surface chemistry measurements and the properties of the columns are used to show that chemical pretreatment of the column wall is necessary so that the stationary phase has both zero contact angle and “compatibility’’ with the surface.
Glass open-tubular columns are commonly used in gas chromatography when inertness coupled with very high resolution is required, as in the analysis of tobacco smoke
A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 11, SEPTEMBER 1973
1831
(1-4) and biochemical materials ( 2 ) . Knowledge of the thickness (d,) of the film of stationary phase on the wall of the column is then necessary, so that the capacity ratio (k), efficiency, and durability of the column can be controlled. There is confusion, however, about the correct relation between dF, the properties of the coating solution, the conditions used in the dynamic coating method ( 5 ) , and the nature of surface of the column. Thus, Kaiser (6) related dF to the v/v concentration ( c ) of the solution of stationary phase in hexane, the inner capillary radius ( r ) , and the velocity of the coating plug ( u ) via the empirical equation C
+
d, = m ( 0 . 2 6 5 ~
0.25)
For silicone oil SF-96 in toluene, however, we found (7, 8) that dF depends directly on r, and on ~ 1 ' 2 , in agreement with an empirical relation found by Fairbrother and Stubbs (9) for the thickness ( h ) of a film of liquid on the wall of a tube
where 9 and y are respectively, the viscosity and surface tension of the liquid. Tesa'rik and NeEasovl confirmed (10) the dependence of d~ on r and also found that an increase in coating temperature gave a thinner film, presumably because of a decrease in 7. Equation 2 has been verified experimentally in nonchromatographic work by a number of authors (9, 11-13) for values of the dimensionwhile Taylor less group u a / y ( = R ) larger than 3 x has confirmed its validity until R reaches 9 x (12). No proof of Equation 2 has apparently yet been advanced. On the other hand, Guiochon has suggested that Equation 3 should be applicable to the coating of open-tubular columns ( 1 4 , pointing out that this was derived by Concus (15) from an analysis given by Levich (16) for the thickness of a film that remains on the wall of a right circular cylindrical vessel as it is being drained of a wetting liquid. (3)
Equation 3, the limiting case for a hemispherical meniscus, was in fact derived by Bretherton ( 1 1 ) in 1961 by an approach which used a substitution similar to that of Levich (16), but with the more accurate numerical integration also later reported by Concus (15). A condition of for R x howthe derivation was that R < 3 x ever, experimental verification was unsatisfactory (11).
K. D. Bartle, L. Bergstedt. M. Novotny, and G . Widmark, J. Chromatogr., 45, 256 (1969) M. Novotny and A. Zlatkis, Chromafogr. Rev.. 14, 1 ( 1 9 7 1 ) . K. Grob, Beitr. Jabakforsch.. 3, 403 (1966). K . Groband J . A. Vollmin, Beitr. Jabakforsch.. 5 , 52 (1969). G . Dijkstra and J. de Goey, in "Gas Chromatography 1958," D. H . Desty, Ed., Butterworths. London, 1958, p 56. R. Kaiser, "Gas Phase Chromatography," Vol. II, Butterworths, London, 1963, p 45. M . Novotny, K . D. Bartle, and L. Blomberg, J. Chromatogr., 45, 469 (1969). M . Novotny, L. Blomberg. and K . D. Bartle. J. Chromatogr. Sci., 8, 390 (1970). F. Fairbrother and A. E. Stubbs, J. Chem. SOC.,1935, 527. K. Tesaiik and M. Netasova, J. Chromatogr.. 65, 39 (1972). F. P. Bretherton, J. FluidMech.. 10, 166 (1961). G . I . Taylor, J. FluidMech., 10, 161 (1961). H . L. Goldsmith and S. G . Mason, J. Colloid Sci., 18, 237 (1963) G . Guiochon. J. Chromatogr. Sci., 9, 512 (1971) P. Concus, J. Phys. Chem., 74, 1819 (1970). V.-G. Levich, "Physicochemical Hydrodynamics," Prentice-Hall, Englewood Cliffs, N . J. 1832
Table I. Specific Retention Volumes, V,, for Benzene in Stationary Phases with Nitrogen Carrier Gas Tempera102Vg, ture, "C m3kg-1 Methyl silicone oil, SF-96 28 30.5 f 0.5
Hydrocarbon grease, Apiezon L Methyl silicone elastomer, SE-30 Dinonyl phthalatea
59 59 28
f 0.5 7.8 f 0.1
16.2 69.2
Measured by Adlard, Khan, and Whitham (25).
Marchessault and Mason proposed (17) a relation for R between 7 x 10-6 and 2 x
where a, a constant, has value -0.0053 m1/2 sec-1/2, but their results were not confirmed in Bretherton's careful experiments (11). Recent work has shown that porous layer open-tubular columns may also be coated by the dynamic procedure using a slurry of solid support in a solution of the liquid phase in a volatile solvent (18); liquid phase loading also increases with u. In this paper, experiments aimed a t discerning the applicability, or otherwise, of Equations 1-4 to the preparation of glass open-tubular columns are described, along with an investigation of other factors influencing dF.
EXPERIMENTAL Physical Properties. Surface tensions of stationary phase solutions were determined by t h e maximum bubble pressure (19) and differential capillary rise (20) methods in a room a t 22 f 1 "C. The apparatus was first washed with chromic acid and then distilled water before drying in an oven. Calibrating liquids were analytical grade benzene, chloroform, and toluene. Checks were made for zero contact angle in the capillary rise method by allowing t h e meniscus to approach an equilibrium position in turn from below and above. T h e drop-weight method (21) was unsuitable for surface-tension determination because evaporation of solvent left a coating of stationary phase on the capillary tip. Further capillary rise experiments were carried out with tubes of 5 X l o - * m internal diameter previously either: ( a ) washed with acetone; ( b ) washed with chromic acid; (c) silanized by filling with the mixed vapors of hexamethyldisilazane and trimethylchlorosilane ( 5 : l ratio) and heating a t 150 "C for 48 hr (8); or ( d ) silanized as above, b u t with allyltrichlorosilane, and then heated in a current of oxygen a t 150 "C for 2 hr ( 2 2 ) . Contact angle, 0, measurements with a simple adaptation of the Adams and Jessop tilting-plate apparatus (23) were made for SF-96 silicone oil and dinonyl phthalate using Pyrex glass slides treated by the above cleaning methods or by t h e same silanization procedures in a closed vessel. Viscosities of stationary phase solutions were determined in a n Ubbelohde suspended-level viscometer ( 2 4 ) in a water bath a t 22.0 0.1 "C. The viscosity-average molecular weight of the SF-96 was determined for solutions in toluene in this viscometer. Calibrating liquids were analytical grade toluene and n-propyl alcohol. Coating of Open-Tubular Columns. Pyrex glass columns (20 m ) were treated by the above silanization procedures and then
*
(17) R. N . Marchessault and S. G. Mason, Ind. Eng. Chem., 52, 79 (1960). (18) J. G .Nikeily,Anal. Chem., 44, 623 (1972). (19) S. Sugden. "The Parachor and Valency," Knopf, New York. N . Y . , 1930. (20) S. Sugden, J. Chem. SOC.,1921, 1483. ( 2 1 ) W. D. Harkins and F. E. Brown, J. Amer. Chem. Soc.. 41, 499 (1919). (22) K . D. Bartle and M . Novotny, Chrornatographia. 3, 272 (1970). (23) N . K . Adams and G . Jessop, J. Chem. SOC.,1925, 1965. (24) A. M . Kragh, "A Laboratory Manual of Analytical Methods in Protein Chemistry, Including Polypeptides," Vol. 3, P. Alexander and R. J. Block, Ed., Pergamon Press, Oxford, 1961, p 173.
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 11, SEPTEMBER 1973
Table II. Properties of Stationary Phase Solutions at 22 f 1 "C Concentration Stationary phase
Solvent
Methyl silicone oil, SF-96 Hydrocarbon grease, Apiezon L Methyl silicone elastomer, SE-30
Toluene Cyclohexane Chloroform
Phenyl silicone oil, OV-17 Polyethylene glycol, 20 M Dinonyl phthalate Di-n-decyl pht$alatea Measured by TesaXk and NeEasova (10)
% w/v
%v/v
10 5.4 3.3
10 12.8 5.9 3.6 10
5
5
Toluene Chloroform Toluene o-Xylene
Density, kg m - 3
10 15
io3 Viscosity
l o 2 Surface tension, Nm-'
Max. bubble Capillary pressure rise
I
kgm-'sec-
876 788 1451 1455 888 1460 874
2.60 1.81 42.6 11.7 0.86 5.29 0.71 1.26
2.49 2.51
2.56 2.56 2.57
2.87 2.69 2.82 2.38
2.77
Table 111. Gradients of Graphs of Film Thickness, dF, against Column Radius, r, and Coating Plug Velocity, u Apiezon L
SF-96
dF graphed against Number of columns Units of gradient
Experimental gradient Theoretical gradient Equation 2 Equation 3 Equation 4 Ratio, experiment: theory Equation 2 Equation 3 Equation 4
r
u1 I 2
u2/3
u1/2
6
19
16
6
... 7.01
f 0.56 X
11.4 x 1 0 - 4 8.6 X 18.8 x 1 0 - 4 0.61 f 0.05 0.81 f 0.06 0.37 f 0.03
ml/2
seclI2
12.7 f 0.5 X
x
m1/3 ~ 8 ~ 2 1 3
20.6
f 1.0 X
m1/2
13.3
sec1/2
f 1.6
X
~ 2 1 3
6 m1/3
sec2/3
20.8 & 2 . 4 X
... x 10-7
17.2
...
30.6
0.79 f 0.03
...
0.77 f 0.08
,..
...
0.70 f 0.04
0.69 f 0.08
0.45 f 0.02
...
... 0.43 f 0 . 0 6
16.0
10-7
... 28.2
x
10-7
coated (2. 8 ) by passing through a plug of stationary phase solution at controlled u and a temperature of 22 f 1 " C . Gas Chromatographic Measurements. The specific retention volumes, V,, of benzene in a number of stationary phases were determined for nitrogen carrier gas by a method similar to that of Adlard et al. ( 2 5 ) . The apparatus was a Gas Chromatography Ltd. Oven Chromatograph Series S 3K, modified with a mercury manometer near the injection port, a soap-bubble flowmeter at the exit, and a thermocouple near the column. Capacity ratios, k, for open-tubular columns for benzene with nitrogen carrier gas were measured using a Perkin-Elmer 900 chromatograph. The split ratio was 1OOO:l for injections of 1 pl headspace.
RESULTS AND DISCUSSION Thickness of the Film. Values of d~ were calculated from V, (Table I) for open-tubular columns coated (8) after silanization with 5 : 1 hexamethyldisilazane and trimethylchlorosilane with (a) 10% v/v SF-96 in toluene, ( b ) 10% w/v Apiezon L in cyclohexane, and (c) 3.3% w/v SE-30 in chloroform, from the formula
where p~ is the density of the phase at column temperature T , a n d k refers to measurements for benzene at T. Substitution in Equation 5 of p 2 2 , the value of the density at 22 "C, then yields the value of d~ a t the column coating temperature. This procedure contrasts with the previous, more dubious, approach (7, 8) of measuring dF by comparison with parameters measured for open-tubular columns coated by the static method (26-29). Relationships between dF for SF-96 and coating parameters were investigated by varying (i) r, between 6.4 and (25) E. R . Adlard, M . A. Khan, and B. T. Whitham. "Gas Chromatograp h y 1960," R. P. W. Scott, Ed., Butterworths, London, 1960, p 251,
29.4
x
x
...
10-7
...
30.0 10-7
x
10-7
,..
...
18.6 X 10-5 m for a constant u of 5.0 X m sec-l, and (ii) u, between 2.0 and 40.0 X 10W3 m sec-1 for a constant r of 1.00 x 10-4 m. A graph of d~ against r is linear with correlation coefficient 0.988, confirming Equation 1 as inapplicable; the gradient of the least squares line is nearest to that predicted by Equation 3. Values of 7 and y (Table 11) for this coating solution yield a value of R = 5.1 x for u = 5.0 x 10-3 m sec-l, well below the R = 3 x 10-3 limit, so that Equation 3 might be expected to be most relevant here. The listed value of y (Table 11) was shown to be applicable to calculation of dF values by measuring y in a capillary previously trimethylsilanized, and finding the same value as for chromic acid washed apparatus. For the variation of dF with u , R is in the range u p to 4 X 10-3, just outside the limits of derivation of Equation 3. Graphs of d~ against both u 2 ' 3 (for values of R up to 3 X 10-3) and u1s2 (all values) are linear (correlation coefficients 0.986 and 0.988, respectively) as suggested by Guiochon ( 2 4 ) . On the other hand, the gradients of these graphs showed a slightly better agreement for Equation 2 (Table 111). A similar result was obtained by measuring ciF for columns with r = 1.00 X 10-4 m, previously trimethylsilanized, and then coated a t different values of u (2.2 to 33.0 X m sec-l) with a 10.0% w/v 112.8% v/v) solution of Apiezon L in cyclohexane. Similarly linear graphs against both u1I2 and u 2 , 3were observed, with rather better agree(26) J. Bouche a n d M . Verzele, J. Gas Chromatogr.. 6, 501 (1968) (27) T. Boogaerts, M . Verstappe, and M . Verzele. J. Chromatogr. Sci., 10, 217 (1972). (28) J. M . d'Aubigne. C. Landauit, and G . Guiochon, Chromatographia, 4, 309 (1971). (29) E . L. ilkova and E. A. M i s t r y u k o v , J. Chromatogr. So., 9, 569 (1971 ) .
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1 1 , SEPTEMBER 1973
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ment between the experimental gradient and that predicted by Equation 2 (Table 111). These results, summarized in Table 111, suggest that throughout the range of R required to give useful film thicknesses (1-2 X 10-7 m) for commonly used column radii (1-2 X m), Equation 2 is a t least as applicable as the theoretically based Equation 3. The theoretical limit on R in Equation 3 adds a difficulty in its application so that Equation 2 is recommended. For low R values, such as that used in the r dependence test, Equation 3 should be used, as recommended by Bretherton (11). Equation 4 predicts much too high film thicknesses. A stable column, prepared by coating a trimethylsilanized column ( r = 1.00 X 10-4 m ) with 10% v/v SF-96 in toluene a t 2.0 x m sec-l, had maximum efficiency ( 7 ) and a film thickness of 1.6 x 10-7 m. This result was confirmed in the preparation of a highly efficient (430,000 theoretical plates) 120-m column for practical use (1, 30). The viscosity average molecular weight of the SF-96 sample was 36,000 when determined from the Mark-Houwink equation with a value of the exponent appropriate to polydimethylsiloxanes (31). An only slightly thicker layer of SF-96 (dF = 2.3 X 10-7 m) was unstable after very limited use, and the H E T P of a similar column rose markedly (8). A similar result was obtained in an attempt to prepare a 90-m column with high k by coating with 10% SF-96 in toluene a t 6 X 10-2 m sec-1; the film thickness fell rapidly. In the same way, a column coated with 10% w/v Apiezon L in cyclohexane a t 5.3 x 1 0 - 2 m sec-1 had a film thickness of only 2.4 x 10-7 m (compared with a calculated value of 4.0 x 10-7 m), showed poor efficiency and (visible) poor distribution of stationary phase in globules. Stable and efficient columns prepared by Grob had d~ = 1 X m (32). Configuration of the Column during Coating. Verzele has observed that break up of the coating plug may occur during dynamic coating if the column is supported so that the coils hang vertically (27). Other authors have spread the column on a right cylinder so that the column is only inclined slightly to the horizontal (2, 8, 27, 33). Now, the Bond number, B, a dimensionless parameter which is the ratio of gravitational to capillary forces (34) is of the order of 4 x 10-3 for the stationary phase solutions listed in Table 11. For a vertical tube, dF is increased by a factor of 1 + (2/3)B for a rising plug (if terms in B2 are neglected) and by 1 - (2/3)B for a descending plug Variations of within only 1% in dF over a complete uertically suspended loop are expected from this source. In this work, the configuration of the column during coating was unimportant. Similar values of dF were found with both configurations, and similar stationary phase film break up (uide infra) was also observed. Stability of the Film of Stationary P h a s e Solution. While column configuration during coating seems relatively unimportant, films of SF-96 and Apiezon L are uniformly thinner by about 25% than those calculated, and some rearrangement of the film either before or after evaporation of the solvent must be considered. There have been two contrasting approaches to the stability of an annular coating of liquid on the inside of a small tube. Goren found (35) that there is a disturbance
of a certain wavelength, A, which grows more rapidly than disturbances of any other wavelength. For values of r(r d ~ ) - lcommonly met in capillary column work, plugs a t -10r intervals ( i e . , m for columns tested here) are expected eventually if the film is sufficiently thick. A simple calculation shows, however, that unless hr-1 > 3 x the film volume is insufficient to form plugs consisting even of touching opposed hemispherical menisci, so that this approach is not apparently applicable to the initial film of stationary phase solution for a stable column which has hr-1 z 2 x An alternative scheme is that of Haynes (36, 37) who considered the film on the inside of a narrow tube as an unduloid, a distorted cylindrical surface with constant mean curvature, and having a periodically varying diameter. Although the initial film of stationary phase solution may be insufficiently thick to exist as a plug, a group of unduloids combine to form the smallest possible number of maximum size. The driving force for coalescence arises from an increase of curvature with volume. Distillation takes place from smaller to larger unduloids, and if a connection exists through a liquid film, the hydrostatic pressure operates in the same direction. Our observations, reported both here and elsewhere ( 8 ) , during the coating of r = 10-4 m open-tubular columns, of the formation and growth of wave disturbances in the film of phase solution, followed by eventual short-plug formation, at intervals greater than 10-3 m, are entirely compatible with the Haynes model. It was further observed, however, that plug formation was reversible and, if the pressure of purging gas were increased, scarcely noticeable for dF values
