Factors Effecting Film Thickness in Glass Capillary Chromatographic Columns Coated by the "Mercury Plug" Method

G. A lexander* /S. R. Lipsky

Yale University Medical School, New Haven, Conn. 06510, USA

Summary

The "'Mercury Plug'" Dynamic (MPD) method, recently introduced for the coating of glass capillary columns, has several advantages over other methods. However, while general rules governing the film thickness in both the "static" and the conventional dynamic coating methods have been intensively studied, this in not the case for the MPD method. In this paper, the effect of several para- meters determining the film thickness were studied. The volumetric method applied to measure the film thickness was found to be faster and more accurate than the chromatographic methods used by other authors. The effect of viscosity of the coating solution and the velocity of the plug were studied in detail and as a result i t was found that the same type of equation as the one describing the film thickness in the dynamic method, was applicable also to the MPD technique. A model is suggested to explain this result.

Introduction Capillary (open tubular) columns are becoming more and more popular among gas chromatographers. This is especially true for columns made of glass, mainly because of the lower price of the capillary itself and therefore larger scope for experimentation.

Coating of the internal surface by the stationary phase is one of the areas where different methods compete. In the "static method" as introduced by Bouche and Verzele [1 ] the coating is realized without any movement of the solution of the stationary phase during the coating process. Because of the instant evaporation of the solvent in vacuo, the solution practically never exists in film form on the capillary wall. The result is a remarkably uniform film, and the method has been considered superior to other ways of coating [2, 3]. However, it has the inherent

Present address: Hungarian Acedemy of Sciences, Research Laboratory for Inorganic Chemistry, P. O. Box 132, H-1502 Budapest 112, Hungary

disadvantage of low speed, and therefore, for the preparation of columns longer than 20-30 m this method has been proved to be impractical.

Because of its apparent simplicity, the "dynamic" method of coating [4] became rather widespread for capillary co- lumn preparation. The method, in its original version, applies a plug of the stationary phase dissolved in a volatile solvent, and this plug is pushed through the column by gas. The plug leaves a layer of the solution on the colunn wall; later the solvent can be evaporated again by a stream of gas, leaving a dry film of the stationary phase behind.

By this method, the production of capillary columns is much faster; there are however, disadvantages: (i) the amount of the stationary phase remaining on the column wall after the coating procedure cannot be pre- dicted by the similar direct way as in the static method;

(ii) before removal of the solvent, the liquid layer on the internal surface of the cylindrical capillary is rather mobile. It easily happens therefore that the gas stream carries the liquid away from the original place of deposition. There- fore it is often difficult to get a uniform film of the stationary phase [5].

To overcome problem (i) the theory and mechanism of the liquid deposition onto the column wall have been intensively studied [6-10, 24] ; a good review of the field has been given by Bartle [ 11 ].

Several equations have been suggested to describe the thickness of the solution remaining on the column wall (ds) as a function of the coating parameters. Two of them have proved adequate (ref. [12] and [13] resp.):

rc ( u " r/s ~o-s

( U ' ~ s ~0.67 d s=1 .34 r e \ 7s } (2)

where r e is the internal radius of the capillary, u is the linear velocity of the solution plug in the capillary, r/s and 7s are the viscosity and the surface tension of the solution, respectively. As it has been determined [ 10], the two equations are quite similar, and usually both of them fit the published experimental data.

The result of a dynamic coating procedure can be pre- dicted; however the accuracy will certainly be less than in the static method.

Chromatographia, Vot. I0, No. 8, August 1977 487

As for improving the uniformity o f the liquid layer, a breakthrough has been achieved by the introduction of the "Mercury Plug Dynamic" (MPD) method of Schom- burg [14, 15]. This method applies a mercury plug between the solvent plug and the driving gas to "sweep" out a part of the deposited solution t-dm. The remaining layer is considerably thinner and therefore its mobility decreased. As a consequence it will be tess sensitive to subsequent disturbing effects.

Since its first introduction, the MPD method has been successfully used by a number o f authors [ 1 6 - 1 9 ] ; however, no systematic treatment of the effects of the coating parameters has appeared in the literature.

In the present work we examined the influence of the following coating parameters on d s: concentration of the coating solution, velocity o f the coating solution, velocity of the coating plug, length of the mercury plug. Our inten- tion was to find a relationship between the mechanisms of the dynamic and MPD coating processes.

Experimental

CapiUafies

The capillary tubings were drawn from soft glass (KIMBLE No. 46475) using the Shimadzu drawing machine. Internal diameters were measured by a microscope. Etching when applied was performed by gaseous HC1, as described earlier [3, 20].

Table 1. Solution film thickness as function of coating solution concentration for SE-30 stationary phase. Solvent: Methylene chloride. Coating method: MPD

Solution concentration Density Viscosity • 102 d s • l0 s (%, weight/volume) (g/cm 3 ) (poise) (cm) (g/100 cm a) non-etched etched

0 1.336 0.43 0.52 1.9 0.75 3.312 0.99 1.10 2.4 1.50 1.309 1.89 2.03 3.6 3.0 1.305 6.02 5.02 6.2 4.5 1.299 16.66 10.75 11.9 5.5 1.296 36.22 16.56 17.3 6.0 1.295 54.97 16.21 19.0

rc: non-etched columns: 0.0127 cm etched columns: 0.0119 cm

u: 0.72 cm/s

Table II. Solution film thickness as function of coating solution concentration for Carbowax 20 M stationary phase. Column: ctched. Solvent: Methylene chloride. Coating method: MPD

Solution concentration Density Viscosity • 10: d s • l0 s (%, weight/volume) (g/cm ~ ) (poise) (cm)

0 1.336 0.43 1.91 3 1.315 1.57 2.94 6 1.311 3.75 5.95

12 1.305 12.26 9.40 20 1.293 42.42 19.47 30 1.279 126.62 31.26

rc: 0.0118 cm u: 0.72 cm/s

Physical properties of the coating solutions

Two stationary phases were selected for the measure- ments, SE-30, and Carbowax 20M fAnalabs, North Haven, Conn., USA). Both o f the phases form very viscous solutions, therefore a careful selection o f concentrations made it possible to cover the viscosity range useful for column coating purposes with most o f the stationary phases.

Viscosities were measured in a Hoeppler-viscosimeter; densities were determined by a pycnometer. All the measure ments were made at room temperature, between 22 ~ and 24 ~

Concentrations (weight/volume %), viscosities and densities o f all the solutions used are given in Tables I and II.

The surface tensions of the solutions were compared to the surface tension o f the pure solvent, methylene-chloride, by the capillary rise method.

For all the solutions used by us, we found only a few percent change in the surface tensions compared to the pure solvent.

Measurement of film thicknesses

Capillaries of exactly 30 m lengths were used for coating; with and without etching. The MPD method was applied as suggested by Schomburg [14, 15] with the slight modifications required by the technique used for film thickness measurements.

To determine the film thicknesses left behind by the solution plug (ds), previous authors investigating the dynamic coating procedure have used indirect methods [8, 9, 11, 24]. In the coated columns,capacity ratios (k') for certain standard compounds have been determined at standard temperatures. Based on this measured k' values, literature data enabled the estimation of the stationary phase film thicknesses (dp) and through this (with the concentrations) values of d s.

We considered this indirect approach to be time consum- ing and of limited accuracy because of the necessity of conditioning and testing each column, and because of the well known difficulties in comparing retention volumes measured in different laboratories on different instruments. Instead, we used a direct, "volumetric tube"-method, suggested by Kaiser as early as 1961 [21] (Fig. 1).

Both ends o f the column to be coated (4) were coupled to a pair o f straight glass capillary tubes (3) o f about 1000mm length; I.D.: 0.51 ram. The tubes were equipped with a millimeter scale, and were used to measure the volume of the coating solution before entering and after leaving the column. The velocity of the coating plug was varied by the inlet pressure of the driving gas and measured directly by stopwatch. Tile amount of solution used to coat the column was about 10 % of its volume; it was found that this amount was more than enough to provide an acceptably constant plug velocity, The dead volumes between the volumetric tubes and the column were minimized, and the volume of

488 Chromatographia, Vol. 10, No. 8, August 1977

J I

l 2 L Lrn

Fig. 1 1 Pressure regulator; 2 Pressure gauge;

3 Volumetr ic tube; 4 Co lumn

the solution consumed for coating was corrected by the small amount needed to coat the volumetric tube itself. A certain skill was needed to " t rap" the liquid into the outlet volumetric tube, but it caused no serious problems. Calculation of cl s, the thickness of the adhered solution layer is straightforward from the equation

D 2 ds = 8 r c 1 ( L i n - L~ (3)

where re and l are the internal radius and length of the column, respectively, D is the (common) internal diameter o f the volumetric tubes, Lin and Lout are the lengths o f the coating solution plugs before and after the coating process, respectively.

To get dp, the thickness of the stationary phase layer itself, d s should be multiplied by the concentration, c %:

C

d p = d s" 100 (4)

Results and Discussion

Eqs. (1) and (2) can be written in generalized form, as

d s = f [re, u, */s (c), ~'s (c)] (5)

where c is the concentration of the coating solution. Of these parameters, 3's was proved to be only slightly dependent on concentration, and to study thoroughly the effect of rc seemed to be of minor interest: its range in column preparation practice is usually small.

Therefore in our experiments u, the coating velocity and r/s, the viscosity (through concentration c) were varied and their effects on d s examined.

In the MPD method,d s is possibly iiffluenced also by the length of the mercury plug; this effect was also studied. Experimental results describing the solution concentration (i.e. viscosity) vs. film thickness relationship are given in Tables I and II, the same for the coating velocity is given in Table III.

Statistical evaluation of the experimental data

Eqs. (1) and (2) are power functions for both of variables 7/s and u. Both equations have the general form:

y = ax b. (6)

Table IlL Solution film thickness as function of coating velocity. Coating solution: 3 % SE-30 in methylene chloride. Coating method: MPD

Etched Non-etched Velocity d s • lO s Velocity d s X lO s (era/s) (dm) (em/s) (cm)

0.33 3.96 0.32 6.09 0.34 2.95 0.44 6.88 0.46 3.78 0.74 7.85 0.47 5.03 0.98 9.19 0.71 4.94 1.33 9.99 1.31 8.40 1.45 10.10 1.88 10.33 2.36 12.72

rc: non-etched columns: 0.0127 cm etched columns: 0.0119 cm

It seemed logical to evaluate the experimental data by a power curve fitting calculation. By this way, a number of a and b values and correlation coefficients, r 2 of (6)-type equations, each best fitting a data series, were calculated. All the results are summarized in Table IV.

The following conclusions can be drawn from these data:

(i) The remarkably good correlations show, that an equation for the power function type (like Eqs. (1) and (2)) is valid for the MPD method the same way as has been proved valid earlier for the dynamic coating method.

(ii) The exponents of the individual power curve do not correspond uniformly to either of 0.5 Eq. (1), or 0.67 Eq. (2); however, all o f them are near to that figure. It proves that the mechanism governing the MPD coating process should be similar but maybe more complicated than the ones resulting in Eqs. (1) and (2).

(iii) There are only minor differences between the behaviour of the very different stationary phases SE-30 and Carbowax 20M. This fact maybe permits the generalization that the chemical nature of the stationary phase has no direct influence on the layer thickness; the governing factor is the viscosity of the solution. However, more data with more stationary phases are needed to prove this point.

(iv) There are characteristic differences between etched and non-etched columns: (a) the correlation coefficients

Chromatographia, Vol. 10, No. 8, August 1977 489

Table IV. Parameters of the best fitting power curves

Type of data Coefficient (a) Exponent (b) Correlation coefficient (r 2)

d s vs. ns SE-30 non-etched 3.33 X 10 -4 0.733 0.984

etched 2.74 X 10 -4 0.507 0.993

Carbowax 20M etched 2.84 X 10 -4 0.508 0.992

d s vs. u SE-30 non-etched 6.85 X 10 -~ 0.635 0.915

etched 7.96 X 10 -s 0.599 0.985

Equation type: y = ax b, [ds]= cm, ['0sl = poise

for the latter columns are significantly lower; (b) the exponents of the non-etched columns are significantly higher, i.e. the ds-values at lower viscosities and velocities are smaller for non-etched columns. The last two facts reflect some disturbing effects prevent- ing the formation of a layer of reproducible thickness on the smooth glass walt, if this layer is thin. This is in agreement with the observation ofSchomburg et at. [22] who found a decreased reliability of the MPD method when the columns were non-etched.

Observing the mercury plug through the column wall during the coating process we found that the plug had a special "wavy" character if the column was non-etched; i.e. the layer between the mercury plug and the wall seemed to form "waves" or droplets, which disappeared after the passing of the plug. This phenomenon was more pronounced at lower plug velocities, and never happened on etched columns. It may be a relationship between this observation and the experimental data; it requires, however, a deeper study.

The mechanism of the MPD coating method

The figures in Table IV and especially the fact that the experimental values of the exponents are in the range o f 0 .5 -0 .7 i.e. similar to the exponents of Eqs. (1) and (2) suggests that the MPD coating should be closely related in mechanism to the conventional dynamic metlaod. Based on this supposition, the following qualitative theory was made for the mechanism of the MPD coating. The thickness of the liquid film remaining on the capillary wall is the result of two counteracting factors: the viscosity and the surface tension of the coating solution; the former tends to retain, the latter tends to remove the liquid.

In the case o f the MPD coating,there is a second liquid present, non-miscible with the solution and non-wetting for the glass wall. This plug is not just a rigid plunger removing a certain excess of the solution, but rather an elastic drop, displaying a certain force toward the column wall. It can be supposed, that this force is proportional to

the interracial surface tension of the mercury/solution boundary, and this will replace the surface tension of tile solution as a removing force, while the overall mechanism remains unchanged. It can be expected therefore, that putting the interfacial surface tension in the place of 7s into Eqs. (1) and/or (2), the equations will be quantitatively correct also for the MPD method. It can be expected further, that using the best fitting exponents from Table IV instead of the original exponents for u and r/s, the agreement will be even better.

Eqs. (1) and (2) were compared to the experimental results and checked for quantitative correctness the following way: as was proved, the experimental data series can be expressed as power curves the same way as Eqs. (1) and (2). The coefficient o f Eq. (6), a, can be ex- pressed from Eqs. (1) and (2) like as in Eq. (7)- (10) . Eqs.(7) and (8) were derived from Eq. (1); Eqs. (9) and (10) from Eq. (2). Substituting the values o f the para- meters which were constant througllout an experiment, values for the coefficient a can be calculated from Eqs. (7) - (10) and compared to experimental values of a; the latters are contained in Table IV.

In these calculations the best fitting exponents from Table IV were used in Eqs. (7 ) - (10) for u and r/s, instead of the original exponents; it can be expected, that the agreement will be better that way.

In Eqs. (7) - (10) Tit is the interfacial surface tension, mentioned above.

0.s a (r/) =-~- (7)

r c l 0.5

a ( u ) = 2 7if "77sb (8)

a @) = 1.34 r e �9 u u (9)

0.67 (') a ( u ) = 1.34 r e ~ �9 r/s b (10)

For the exponents of "/'if, the exponents, required by the corresponding equations were kept. As a value for ")'if, the interfacial surface tension between mercury and pure methylene chloride was applied; 7i~ ~176 = 341 dynes/cm [231.

It can be seen from Table V, that the agreement between calculated and experimental figures is excellent, showing, that the mechanism suggested above is most probably true. Besides, there is no difference between the accuracy of Eqs. (1) and (2) if the experimental exponents are used.

There is one pair o f data, where the difference is larger; however, the experimental figures are more uncertain in this case (see Table IV).

The difference between the exponents o f r/s and u in Eqs. (1) and (2) and in Table IV is not too large. It means that in most o f the practical cases Eqs. (1) and (2) can be used to predict ds (and ~ ) without the time consuming determination o f the exponents (e.g. for stationary phases

490 Chromatographia, Vol. 10, No. 8, August 1977

Table V. Comparison of experimental and calculated coefficients a of power curves

a value Exper./Calc. Type of data Equation calculated experimental

d s vs, r/s

SE-30

Carbowax 20 M

d S vs . u

SE-30

non-etched (7) 2.77 • 10 -4 3.33 X 10 -4 1.20 (9) 2.80 • 10 -4 1.19

etched (7) 2.69 • 10 -4 1.02 (9) 2.73• 10 -4 2.74• 10 -4 1.00

etched (7) 2.68 X 10 -4 1.06 (9) 2.72 X 10 -4 2.84 X 10 -4 1.04

non-etched (8) 4.38 • 10- 5 1.56 (10) 4.44 X 10- ~ 6.85 • 10- s 1.54

etched (8) 7.75 • 10- s 1.03 (10) 7.86• 10 -~ 7.95X 10 -s 1.01

o ther than SE-30 or Ca rbowax 20 M) wi th an acceptable

accuracy.

The e f fec t o f the me rcu ry p lug length

Being an independen t exper imenta l pa ramete r , the ef fec t

o f the mercu ry plug length on d s was also studied. The

results are given in Table VI . The statistical evaluat ion

showed a l inear relat ionship wi th a slope very near to 0

and a corre la t ion coef f ic ien t o f 0.703. The la t ter is a

sign o f a l imi ted reliabili ty, however , it can be stated,

that d s is pract ical ly independen t o f the mercu ry plug

length.

Table Vl. The effect of mercury plug length on d s. Column: etched. Coating solution: 3 % SE-30 in methylene chloride

Mercury plug length d s • l0 s (cm) (cm)

0.6 6.98 2.6 7.37 6.3 7.45

17.1 6.86 24.9 5.88

rc: 0.0119 cm; u: 0.74 cm/s

A c k n o w l e d g e m e n t

This work was sponsored by the U.S. Energy Research and Development Administration under Contract No. EY-76-S-02-2958.

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