Capillary GC on 50 Micrometer I.D. Columns Coated with Thick Films. Theory and Selected Practical Results Dirk Steenackers and Pat Sandra* Department of Organic Chemistry, University of Gent, Krijgslaan 281, S4, B-9000 Gent, Belgium

Key Words:

Capillary gas chromatography Film thickness Pressure drop Speed of analysis Applications

Summary

The chromatographic performance of 50 pm internal diameter (i.d.) fused silica columns coated with up to 2 p n films of immobilized SE-54 (methyl phenyl (5%) silicone) is evaluated under gas chro- matographic conditions. The influence of pressure drop on the plate height is discussed. In comparison to thick film 250-530 Fm i.d. columns, much higher efficiencies and faster analyses are obtained. Practical examples, performed on a standard GC instrument, illus- trate the features of thick film 50 ,tun i.d. columns.

1 Introduction

resolution. Fast temperature programming requires stable films and the stationary phase must be properly immobilized and washed before use.

As shown in 1962 by Desty et al. [6], speed of analysis in gas chromatography can be increased drastically by reduction of the internal diameter of the column. The effect of internal column diameter on the plate height can be deduced from the Golay-Gid- dings equation [7-91, which describes the band broadening in capillary columns, taking decompression effects into account:

(1)

2D,,o (I + 6 k + l l k 2 ) The analysis of highly volatile solutes on silicone type stationary H = [ - + phases at superambient temperatures requires very thick film un 96 ( k + 1)' D,,,, columns. Wide bore columns coated with 5 Fm PS-255 were introduced by Grob and Grob in 1983 [ 11. The efficiency behavior of apolar thick film columns was investigated by Ettre [2,3] and Sandra et al. [4]. David et al. [5] introduced and discussed thick film wide bore columns coated with polar phenyl methyl sili- cones.

2k d :

3 ( k + 1)2 Ds + x - x u o x f ~

Dm,o is the diffusion coefficient in the gas phase at column outlet uressure

The main advantage of thick film columns is the strongly in- creased retention, allowing analysis of volatile compounds at normal capillary GC temperatures. Another important feature is that the higher elution temperatures reduce the interactions be- tween polar solutes and residual activity on the fused silica surface, resulting in improved peak shapes for polar compounds. Additional advantages are the increased sample capacity and the possibility for direct water injection.

On the other hand, the main disadvantage of thick film columns is the low attainable efficiency, which is due to the important contribution of resistance to mass transfer in the stationary phase to the total band broadening. The dependence of the plate height on the nature of the carrier gas becomes larger than for thin film columns and nitrogen gives 50% more plates compared to hy- drogen but at a very low optimal speed (ca. 7-8 cm/s) [4]. Minimal plate height at low optimal speed results in very long analysis times.

To compensate for the low speed of analysis under isothermal conditions, in practice fast temperature programming is often applied to reduce the overall analysis time. Thick film columns indeed have the advantage over thin film columns, that fast temperature programming can be used without severe loss of

DS k dc

df

fl fi

uo

P

is the diffusion coefficient in the stationary phase is the capacity factor is the internal column diameter is the film thickness of the stationary phase is the mobile phase velocity at the column outlet

= [3(P' - l)] I [2(P3 - l)] , where is the ratio of column inlet over column outlet pressure

= [9(< - l)($ - l)] / [S(P3 - 1)7

As the column diameter decreases, the inlet pressure needed to retain the same mobile phase velocity as well as the pressure drop over the column increase. For high P values,fi approaches 918 andfi approaches 3/(2P).

The influence of the column characteristics on plate height, speed of analysis [ 10,111 and on minimum analyte concentration and minimum detectable amount [ 12,131 have been extensively stud- ied. Since long 50 pm i.d. columns were used in this study, for which high inlet pressures are required, the influence of the pressure gradient upon column efficiency [ 141 is especially im- portant, besides the effect of column diameter.

The possibility of using columns with reduced internal diameters has been convincingly demonstrated for thin film columns [ 15- 181. The lowest internal diameter used to any extent in practice is, however, 100 pm. Besides injection and detection problems,

I. High Resol. Chromatogr. VOL. 18, FEBRUARY 1995 77

Capillary GC on 50 Micrometer I.D. Columns Coated with Thick Films

the main reasons for this are the reduced volume loadability and sample capacity. The volume loadability for open tubular col- umns is proportional to dc , and (k+l) [19]. A 50 pm id. column thus will have at least 25 times lower volume load- ability compared to a 250 pm i.d. column. The maximum allow- able injection volume can be increased by using solvent venting or solute focusing methods. The sample capacity strongly de- pends upon the amount of stationary phase in the column. A 50 pm i.d. column only needs a 0.05 pm thick film to yield the same capacity factors as on a 250 pm i.d. column with a 0.25 pm thick film; however, the latter column contains 25 times more stationary phase. By increasing the film thickness, the sample capacity of 50 pm id. columns can be increased. 50 pm i.d. columns coated with 1 pm thick films of apolar silicone stationary phases have been prepared for supercritical fluid chromatogra- phy 1201 and open tubular liquid chromatography [21]. The lower diffusion coefficients in a supercritical and in a liquid mobile phase allow the use of higher film thicknesses. In the past, gas Chromatography on such columns was only used to evaluate the quality of these thick film columns for SFC and LC applications.

In the present study 50 pm i.d. columns with up to 2 pm thick films of SE-54 were evaluated under gas chromatographic con- ditions. The sample capacity for those columns is comparable to the sample capacity for thin film 250 pm i.d. columns. Despite the large film thicknesses, highly efficient separations could be performed in short analysis times.

2 L112 112 , h

2 Experimental

Fused silica tubing (50 pm i.d.) was obtained from Scientific Glass Engineering (Australia). After persilylation the columns were coated with SE-54. Stationary phase solutions were made in trichlorofluoromethane or pentane as solvent. Dicumyl perox- ide (2%) was added as free radical agent to the stationary phase solution. The filling and static coating procedure for the prepa- ration of thick film 50 pm i.d. columns is describcd elsewhere [22]. All experiments were performed on a Hewlett Packard 5890 series I1 CGC equipped with splithplitless injection and flame ionization detection. To enable the use of high inlet pressures, a high pressure regulator was directly installed in the carrier gas line.

3 Results and Discussion

The H-u curves for a 12.8 m long, 50 pm id . column, coated with 1 pm chemically bonded SE-54, using helium and hydrogen as mobile phases, are shown in Figure 1. The capacity factor of n-undecane, which was used as solute at 175 "C, is 6.1. The full lines are theoretical curves based on equation (l), and thus pres- sure drop corrected. As in the work of Leclercq et al. [23], the values for viscosity of hydrogen and helium were calculated according to Ettre [24] and values for the gas phase diffusion coefficients of n-alkanes in hydrogen and helium were calculated according to Fuller et ul. [25]. The values for the diffusion coefficients in the stationary phase were obtained by curve fitting of experimental plate height data to the Golay-Giddings equation. Experimental values for the diffusion coefficients of n-alkanes in OV-1 and CP-Sil5 have been measured by Cramers et al. [26]. Extrapolation to 175 "C of their Ds values for n-undecane at 75, 100, and 125 "C, measured on a 3 14 pm id. column coated with

O p s He

o , o o o . 2 0 ' " 40 " 60 " 80 ' 100 I mobile phase velocity (cmls)

Figure 1. Theoretical and experimental (+) H-u curves for a 12.8 m X 50 pm i d . column coated with 1 wrn immobilized SE-54. Conditions: n-undecane at 175 "C; capacity factor 6.1; He and Hz as carrier gases.

a 2 pm thick film chemically bonded CP-Sil 5, using equation (2): In Ds = K - E/RT

in which R is the universal gas constant, T the absolute tempera- ture, E the energy of activation, and I( a fitting constant [27], gives a Ds value of 1.2 x cm2/s. This value is only slightly higher than our experimental value of 1 .O x cm2/s. Millen et al. [28] also reported diffusion coefficients for n-alkanes in a methyl silicone stationary phase (SE-30). The values, obtained from plate height data measured on columns packed with 0.29 1 % SE-30 coated on 60-80 mesh solid glass beads, are however systematically higher than the values reported by Cramers et aZ. and our experimental values. Extrapolation to 175 "C resulted in a Ds value of 2.9 x

Using hydrogen as carrier gas, a minimum plate height of 0.085 mm is observed at a mobile phase speed of about 40 c d s . The maximum efficiency for the 12.8 m long column is 150 000 plates. Calculation of the optimum conditions for a corresponding 25 m long, 250 pm i.d. column with a 5 pm thick film of stationary phase using the same values fork, D m , and Ds as for the 50 pm i.d. column gives a minimum plate height of 0.85 mm at an optimum speed of 20 cm/s. This means that under these condi- tions the 50 pm i.d. column gives 20 times more plates per second (660/s) than the corresponding 250 pn i.d. column with the same P-value (35/s).

The 10 times lower minimum plate height for the 50 pm id . column, compared to the 250 pm i.d. column, can not be ex- plained solely by the 5 times lower column diameter but is also due to the large pressure drop over the 50 pm i.d. column. The outlet pressure is in both cases atmospheric but for the 50 pm 1.D column inlet pressures up to 30 bar are used. The effect of pressure drop on the minimum plate height can be illustrated by comparing H-u curves for different column lengths. In Figure 2, calculated H-u curves are shown for a 3 meter and a 15 meter long column, respectively. The longer the column, the lower the minimum plate height will be. This is summarized in Figure 3 showing the minimum plate height as a function of column length. The pres- sure drop effect, being negligible for 250 pm i.d. columns at

(2)

cm2/s for n-undecane.

78 VOL. 18, FEBRUARY 1995 J . High Resol. Chromatogr.

Capillary GC on 50 Micrometer LD. Columns Coated with Thick Films

0 3 I\

mubile phase velocity (cmls)

Figure2.CalculatedH-ucurvesfora3anda 15mlong,S0pmi.d.columncoated with 1 pm immobilized SE-54. Conditions: n-undecane at 175 "C; capacity factor 6.1; H2 as canier gas.

0 5 10 15 20 2 5 30 0906

column length (m)

Figure 3. Dependence of minimum plate height on column length. Column: 50 p i.d., film thickness 1 pm. Conditions as in Figure 2.

atmospheric column outlet pressure, reduces the minimum plate height for a 10 m long thick film 50 pm id. column by a factor of 2 compared to the minimum plate height at 0 m column length (no pressure drop). This effect is even more pronounced when more viscous mobile phases like helium or nitrogen are used.

Besides the positive effect on attainable efficiency, the pressure drop however has a negative effect on the optimum linear gas velocity (Figure 4). The result of both counteracting effects is that the number of plates generated per second under optimum conditions, reduces upon increasing column length. This means that in practice long columns should only be used for those analyses requiring high efficiencies. In other words, if for sepa- ration of two compounds a certain plate number is required, a column should be selected of which the length is only slightly longer than necessary. As example, in Figure 5 the analysis time is plotted as function of the column length for a required plate number of 50,000 using the same conditions as above. The analy- sis time increases rapidly for long columns, due to the steep slope

~

*V 0 10 20 30

column length (m)

Figure 4. Dependence of optimum linear gas velocity on column length. Column: 50 pm id., film thickness 1 pm. Conditions as in Figure 2.

40' I 0 10 20 30 40

Column length (m) Figure 5. Dependence of analysis time on column length. Column: 50 pm i.d., film thickness 1 pm; efficiency: 50,000 theoretical plates. Conditions as in Figure 2.

of the H-u curves above the optimum mobile phase velocity. The same analysis on a 250 pm i.d. column with a 5 Fm thick film however will take about 20 times longer as compared to the 50 pm id. column.

For thin film 50 pm id . columns, for which the contribution of the stationary phase to the total band broadening is negligible, the influence of the pressure drop on the H-u curves is different than for thick film columns. The minimum plate height will not decrease with increasing pressure drop, but will remain nearly the same. The optimum velocity for short columns will be higher than for thick film columns, but for longer columns (> 5 m), the optimum velocity is almost uniquely controlled by the pressure drop and thus is similar as for thick film columns. Schutjes et al. for example [ 111 recorded H-u curves for four 6.5 to 8.6 m long 50 pm id . columns coated with 0.1 ym SE-30. The experimental optimum velocities with helium as carrier gas ranged between 28 and 36 cm per second. These values are similar to our experi- mental values (Figure 1) for a 1 pm thick film column.

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Capillan GC on SO Micrometer I.D. Columns Coated with Thick Films

I +

mobile phase velocity (cmls)

Figure 6. Theoretical and experimental (+) H-u curves for a 5.8 m long, 50 pm i.d. column coated with 2 mm immobilimd SE-54. Conditions: n-undecane at 175 "C; capacity factor 12; H2 as carrier gas.

In Figure 6 the theoretical and experimental H-u curves are shown for a 5.8 m long, 50 pm i.d. column with a 2 pm thick film of immobilized SE-54 using hydrogen as carrier gas. The curve was calculated using the same values for Dm and Ds as for the 1 pm thick film column. For this column, the minimum plate height is 0.15 mm (Nmx = 38500), which is higher than for the 1 pm thick film column (Figure 1). This is partially due to the two times thicker film, but also to the lower pressure drop, since the column is more than two times shorter. This lower pressure drop is also the reason for the higher optimum mobile phase velocity compared to the 1 pm thick film column.

3 1

i "

220 'C

I I I I 20 40 60 see

Figure 7. Separation of natural gas + isobutane, pentane, and hexane. Conditions: 12.8 x 50 pm id., I pn SE-54 column; 220 "C; 20 bar H2 inlet pressure. Peak identifications: (1) methane, (2) ethane, (3) propane, (4) isobutane, (5) butane, (6) pentane, (7) hexane.

To demonstrate the separation power of these columns for very volatile solutes, the analysis of natural gas (with added vapors of isobutane, pentane, and hexane) at 220 "C in less than 70 s is presented in Figure 7, and Figure 8 shows the analysis of 33 solvents from methanol to dibutyl ketone in 7 minutes.

The capacity factors on this 50 pm id . column with a 1 pm thick film are the same as on a 250 pm,i.d. column with a 5 pm thick film but the higher efficiencies at'higher optimum mobile phase velocities reduce the analysis time more than tenfold on the 50 pm i.d. column. The separation of some vinylic compounds on the same column is shown in Figure 9.

m I00 "C, 10"lrnin

1 I

I 2 3 4 5 6 7 min

Figure 8. Separation of solvents. Conditions: 12.8 X 50 pm id . , 1 pm SE-54 column; 100 "C, 10 "min-' to 180 "C; 14 bar H2inlet pressure. Peak identifications: (1) methanol, (2) ethanol , ( 3 ) acetone, (4) diethyl ether, (5 ) ethyl formate, (6) dichloromethane, (7) n-propanol, (8) carbon disulfide, (9) trans-1,2-dichloroetylene, (10) methyl ethyl ketone, (1 1) ethyl acetate, (12) ois-l,2-dichloroethylene, (13) chloroform, (14) tetrahydrofuran, (15) n-butanol, (16) 1,2-dichloroethane7 (17) benzene, (18) isooctane, (19) p-dioxane, (20) isobutyl methyl ketone, (21) toluene, (22) ethyl propyl ketone, (23) dimethylfommide, (24) butyl acetate, (25) n-butyl ether + m-xylene, (26)p-xylene, (27) cyclohexanol, (28) o-xylene, (29) dimethyl d o n a t e + cyclohexanone, (30) anisole, (31) fenetol, (32) benzyl alcohoL (33) dibutyl ketone.

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Capillary GC on 50 Micrometer I.D. Columns Coated with Thick Films

1 I I 1 2 n i o

Figure 9. Separation of vinylic compounds. Conditions: 12.8 in x 50 pn id . , 1 pn SE-54 column; 150 "C, 30 "C min? to 240 "C; 15 bar H2 inlet pressure, after 1.5 min increased to 30 bar. Peak identifications: (1) ethyl vinyl ether, (2) methyl acrylate, (3) isobutyl vinyl ether, (4) n-butyl vinyl ether, ( 5 ) ethyl acrylate, (6) isomers of divinylbenzene and ethylvinylbenzene.

After 1.5 minutes, the inlet pressure was manually increased from 15 to 30 bar to elute the isomers of ethylvinylbenzene and divinyl- benzene in a short analysis time. As fast temperature program- ming is often used on these thick film columns, the reduction of the mobile phase speed during analysis is considerable. A pres- sure control system, capable of working at these high pressures would be very useful, not only to keep the mobile phase speed close to the optimum speed and to elute the less volatile solutes fast, but also to improve the reproducibility of injection. Split injection with a syringe could then be performed at lower pres- sure, after which the pressure could be increased to the desired

value. The speed of injection is not a major problem because the compounds are easily trapped at the beginning of the column by the stationary phase focusing effect. Only for very short thick film 50 pm i.d. columns, for which the optimum mobile phase speed is high and the residence time in the column is short, should pneumatic injection systems be considered [ 15,171. Because of the low volume loadability however, samples should be concen- trated. To reduce the minimum detectable concentration, pro- grammed temperature vaporization injection or solvent venting techniques with a precolumn, as used in open tubular supercritical fluid chromatography [29,30] to increase the allowable injection volume, should be considered. Because these thick film columns are especially used for volatile solutes it is, however, difficult to selectively vent the solvent without discriminating the com- pounds of interest. Nevertheless, thick film 50 pm id . columns will be most useful for the analysis of gaseous mixtures, for process control and in automated monitoring systems.

The analysis of lower alcohols from methanol to octanol on a 5.8 m long and 2 Lrn thick film column is shown in Figure 10 which illustrates the high inertness of thick film columns. The efficiency of this column is lower, but one should realize that, even if it would be possible to prepare very long (50 m) 250 pm i.d. columns with 10 pm thick films of stationary phase, the analysis time for the same separations would be considerably higher. Also, the physical stability and column bleeding of 10 pm thick films upon applying temperature programming rates of 15 "/min will be problematical.

4 Conclusions

Thick film 50 pmi.d. columns (SE-54) provide very fast analyses. The high efficiencies attainable for long columns are mainly due to the influence of pressure drop on the minimum plate height. This type of column has the potential to be applied in environ- mental and industrial process control.

60T, 5"lmin

I I 5 10 15 min

Figure 10. Separation of alcohols. Conditions: 5.8 m x 5 0 ~ i.d., 2 K r n SE-54 column; 60 "C, 5C mind; 6.5 barH2inlet pressure. Peak identifications: (1) methanol, (2) ethanol, (3) 2-propanol, (4) ten-butanol, (5) dichloromethane, (6) n-pmpanol, (7) 2-butano1, (8) n-butanol, (9) 2-pentano1, (10) n-pentanol, (1 1) 3-hexanol, (12) 2-bexanol, (13) n-hexanol, (14) 3-heptano1, (15) 2-beptanol, (16) n-heptanol, (17) 4-octano1, (18) 3-octano1, (19) 2-octanol, (20) n-octanol.

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Capillw GC on 50 Micrometer I.D. Columns Coated with Thick Films

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