1007
Anal. Chem. 1907, 5 9 , 1007-1015
Theoretical Aspects and Practical Potentials of Rapid Gas Analysis in Capillary Gas Chromatography Robert Tijssen,* Nico van den Hoed, and M. Emile van Kreveld
KoninklijkelShell-Laboratorium,Amsterdam (Shell Research B. V.),Badhuisweg 3, 1031 CM Amsterdam, The Netherlands
This work describes some theoretical aspects and practlcai potentials of very rapid gas analysis In open tubular, capillary columns for gas chromatography (GC). I n the theoretical part the close relationship between small column diameters and high separatlon speed is establlshed. The Important role of the extracolumn peak broadenlng effect Is stressed. Notably manual lnjectlon of gas samples turns out to yield tailing peaks In practlce far wider than those predicted from the theory for peak broadenlng in the column Itself. To Improve this situatlon, whlch is common in caplllary GC, we have developed a new and very rapid Injector, capable of producing sample pulses wlth variances less than 1 ms'. The remainlng part of the extracolumn broadening effect could be attributed to nonoptimai behavlor of the flame Ionization detector; upon optimization deviations between theoretlcai and experimental column efficiencies were smaller than 10 %. With these modifications, wail-coated caplliarles allow the separation of hydrocarbons, present in natural gases, withln seconds. The beneflciai effect of tightly colling the columns Is shown.
Although there has been an upsurge of interest in the use of capillary columns for chromatographic gas analysis in the past few years, it is surprising to note how little has been done to achieve the potentially attainable speed of analysis. Apart from the pioneering work by Desty et al. ( I ) , it has hardly been realized how the speed of analysis (apart from the detection limits, gas flow rate, and column efficiency) is connected with the column diameter. Guiochon (2) stressed the important role of a small column diameter, whereas Desty as far back as 20 years ago proved experimentally that the analysis times could be greatly reduced by using short and narrow-bore capillary columns. Yet capillary gas chromatography (GC) is still mainly being carried out in relatively wide (0.1-0.5 mm i.d.) and long (10-25 m) columns, despite the fact that glass-drawing machines are very flexible, allowing the manufacture of columns of any length and any (optimal) diameter. Especially, the advent of flexible fused silica columns ( 3 , 4 ) with diameters down to 10 Fm, in principle, opens new perspectives. Fused silica columns with diameters down to 30 Fm have been used only recently (5). Although the trend toward smaller diameters slowly sets in, an obvious cause for the still common use of relatively wide columns is the lack of adequate equipment necessary for successful application of microcapillary high-speed chromatography. Equipment such as injectors and detectors must be accurately designed in order to be compatible with the high efficiency and short analysis times associated with small-bore columns. Therefore we have developed a new sample injector capable of generating very short inlet pulses of gas. Equally important factors are the need for sufficiently short response times of the electronic equipment (amplifiers and recorder) and the phenomena of peak broadening within the detector. It will become clear from the present work how important the total extracolumn peak
broadening is for successful application of the potential column performance. The same applies, of course, to rapid gas analysis in highly efficient packed columns, as discussed by Jonker and Poppe (6). Although in this study no special attention has been paid yet to the experimental realization of very small column diameters (say O), we did some preliminary
ANALYTICAL
CHEMISTRY, VOL. 59.
NO. 7, APRIL 1. 1987
1
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loo0 Re Sc
Figure 3. Experimental plate height as a function of gas velochy In straight capillary columns wim different lengms: R = 0.008 28 cm: solute. CH,: canler gas. N,: manual injection.
measurements with this apparatus to explore the potential of capillary GC for rapid gas analysis. T o this end we used the common dynamic coating method for obtaining a retentive layer of a stationary liquid phase onto the column wall (7). We produced layers with an average thickness less than 1pm of, e.g., squalane onto stainless steel, copper, and silver capillaries, stable up to velocities of lo4cm/s (outlet conditions). More stable coatings could be obtained by using more viscous stationary phases (OVI, Apiezon, SE-30). Figure 4 shows a separation obtained by (adsorption) gas-lid chromatography on a capillary coated with submicrometer porous silica particles (Cab-0-Sil, see later). If shows clearly that retained peaks are also slightly nonideal (tailing), probably due partly to the nonlinear adsorption isotherm but also partly to the same additional extracolumn peak broadening effects which cause e* > 0 for nonretained peaks, albeit to a lesser extent. Although not being able, in this phase of this study, to avoid the effects that produce e* > 0, we thought it of interest to have an impression of the reliability of the Golay equation, eq 1. for retained solutes. T o this end we corrected for the e* influences by obtaining c from nonretained solutes and using this t* value also for retained solutes as far as peak broadening from mobile phase effects was concerned. In that case the
~ . j F. r n h
Flpm 5. Axialdsperslon h staaonaryphase h a straw capllaty column: nwblla phase, N,: staMnary phase. squalane: R = 0.0125 cm. L = 10 m; solutes (0)npentane (k = 0.18). (0)nhexane (k = 0.33).( 0 )nheptane (k = 0.58).
expected plate height, H,predicted by the 200, a velocity range not covered by the earlier experiments in Figure 3). It is imposaible to be sure whether this behavior stems from the flow Characteristics of the new injector or from the behavior of the column itself. In the latter case one could think of incipient turbulence phenomena induced, e.g., by the relatively large wall roughness of these smalldiameter metal columns. Although Re numbers are only 300 where the curvature begins, literature reports show that turbulence affects can indeed play a role even at levels so far
2 -
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6~1100 2
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Flgure 8.
Experimental plate height as a function of gas velocity In
two straight (slber)capillaries with dflerent lengths (100and 30.4 cm)t R = 0,008234cm: solute. CH,; carrier gas. N,; 0 , ' = 0.23 cm21s: Sc = 0.685;0.L = 100.0 cm: 0 , L = 30.4 cm.
below the (ideal) critical Re number of 2300 (la-20). Another possible explanation is that, although the columns used were carefully straightened, the end section contains one short 90' bend in order to enter the base of the FID. It has been shown (16,17) that such bends may influence flow and dispersion. In the next section this effect is exploited in columns that are coiled throughout their full lengths. Coiled Columns. Another way to induce a reduction in peak broadening by flow phenomena is through application of strongly coiled columns, as has been extensively investigated by us (7,8) and other authors (J5-J7). Centrifigual forces create a secondary flowin the column cross section even at velocities far below ReSc = 300. This can he seen from measurements with silver capillaries having the same diameter as before, hut now tightly coiled around thin metal bars (syringe needles) with diameters of 0.2 and 0.1 cm. Figure 9 shows the experimental results obtained with several column lengths. For Re% < 30 it is clear that the columns behave as if they were straight, the results being nearly identical with those of Figure 8. For the 1-m columns in particular we observe a smooth change from straight-column behavior (dotted line taken from Figure 8) to coiled-column behavior at ReSc > 30. This is in accordance with theoretical expectations (7.13, 15-17), For all columns it appears that above this velocity secondary flow strongly reduces peak broadening, which is of utmost importance for the development of very rapid gas analysis in capillary columns. These new data fit very well with the compilation of all available data from experiments in coiled open tubular columns earlier prepared hy us (8). which shows that at the highest velocities the gain in plate height as compared with straight column behavior may he even 2 orders of magnitude. We will not elaborate on the theoretical treatment of coiled columns here, hut stipulate their great potential for rapid gas analysis, shown later on with a typical example, Returning to the present results in Figure 9, we observed that at low velocities (Re Sc < 30) the 132-cm column has a dispersion curve that lies above that for the 100-cm columns, which is in contradiction with eq 14. This phenomenon can
ANALYTICAL CHEMISTRY, VOL. 59, NO. 7, APRIL 1, 1987
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8 6 -
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Re Sc Flgure 9. Experimental plate height as a function of gas velocity in straight (A) and coiled (B) capillary columns: nonretalned solute, CH,; carrier gas, N,. (A) L = 100 cm straight column from Figure 8. (B) Coiled silver capillaries R = 0.008 234 cm; D," = 0.23 cm2/s; Sc = 0.685; (0)L = 299 cm, A = 0.081; (0) L = 132.4 cm, A = 0.19; (V) L = 100.0 cm, A = 0.082.
be attributed, however, to the fact that this particular column has been coiled so tightly around a very small bar (diameter 0.1 cm) as to cause deformation of the circular cross section. The column cross section is reduced by this deformation, which is also apparent from the relatively high pressure needed to obtain the same velocities as in the other columns (correcting for the differences in length). This decrease in effective radius is of course convenient for practice, as it reduces the absolute peak width further, but the resulting t is somewhat larger according to eq 14, which explains the higher position in Figure 9. Straight Columns with Larger Diameter. In wider capillaries,we may approach the turbulent flow region at relatively low pressure needed. Figure 10 shows our results with silver capillaries with an internal radius of R = 0.015 46 cm. The outside diameter of these columns was 0.1 cm, which cannot be accommodated by the FID, nor can the end sections of these soft silver columns be filed to obtain the required dimensions. Thus we had to connect a smaller-diameter, short stainless steel capillary (0.035 cm i.d., 0.6 mm 0.d.) to the column outlet by means of a small piece of soft plastic tubing. The steel capillary, with filed end section, protrudes into the FID. This nonideal connection will undoubtedly contribute to peak broadening. This can indeed be seen from our measurements presented in Figure 10 in the low-velocity region (Re Sc < 1000). The values here are higher than expected and of the same magnitude as those for the smaller sized columns. Nonetheless it is clearly observed how above Re Sc = 1200 (Le., Re = 1700) a reduction in peak broadening occurs (even a maximum in plate height is found), either as a result of the aforementioned incipient turbulence phenomenon or as the result of the one 90" bend present. As expected, these phenomena will only become observable at higher Re numbers (Re > 1700) in these wider capillaries than in the smaller bore columns described above. Although of only limited practical importance (wide columns, very high velocities) these measurements turned out
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l l 68100
I
I
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6810002
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6 8
Re Sc Flgure 10. Dependence of h , on column length as a function of Re Sc: straight silver caplllaries R = 0.015 46 cm; Dm0= 0.23 cm2/s; Sc = 0.685; (0)L = 155.8 cm, (0)L = 53.0 cm.
FI
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4l 2
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200
400
€00
800
loo0
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SIGNAL, mV P, ms
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Flgure 11. Examples of the influence of detector parameter setting on peak width: L = 155.8 cm; R = 0.01546 cm; capillary exit 1 mm downstream of burner tip; carrier gas, 60 mL/min; air, 400 mL/mln; hydrogen A. 50 mL/min; B, 80 mL/mln; make-up gas a, 60 mL/min; b, 0 mL/min.
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 7, APRIL 1, 1987 h
/ 2 L
i;1
THEORETICAL
Od
L 1.77 s
2
4
6 810
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8100 2
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ReSc
Figure 12. Experimental plate height as a function of gas velocity in a straight capillary column for different magnitudes of the signal (peak height) with improved detector performance: straight silver capillary; R = 0.01546 cm; L = 155.8 cm; Dm0= 0.23 cm2/s;Sc = 0.685; (0)250 mV, (W) 60 mV, (0)extrapolated to 0 mV.
to be very useful in recognizing another form of extracolumn peak broadening. This additional broadening presented itself in the observation that a relatively large scatter o f t data was found to exist in repeated measurements at one fixed gas velocity. (The data in Figure 10,therefore, are average values of several experiments.) After some time we found out that this scatter was not random but clearly correlated with several detector properties, to be described in the next paragraph. Detector Developments and Properties. The flame ionization detedor used in this work was so far operated under conditions that were nearly optimal in several respects. For instance, the column outlet was always positioned at the very position of the flame base to avoid any dead volumes (e.g., of the flame burner). The dead volume of the detector, an important quantity in determining the peak broadening, is thus limited to the volume of the flame itself, being on the order of 10-20 kL. As the flow amounts to 120-200 mL/min gas (burner gas, H,,plus make-up gas, N,, plus carrier gas, N,), this results in a time constant (volume/flow) of several milliseconds (say 3-10), under normal circumstances sufficiently low to be negligible. We observe, however, from these f i i e s that, the variances of injection pulses being kept under 1ms2, a distinct possibility of peak broadening in the detector occurs. The above reasoning is based on the idea that the flame volume is an ideally mixed volume, which in fact can be only approximated to a certain extent by the turbulent structure of the flow. It is to be expected that the actual flow pattern will depend to a certain extent on the velocities of the gases supplied. This is, apart from constant detector sensitivity and higher stability, a further reason for adding make-up gas to the flame such that changes in the carrier gas flow will be compensated for. It appeared that at every carrier gas flow, optimum burner gas flows could be found (H,and air) with respect to peak broadening. Even under these optimum conditions for every gas velocity, two more aspects should be considered for minimizing peak broadening. The first and more or less obvious one is the position of the column outlet with respect to the flame base. In all earlier measurements we had intuitively chosen the flame base on the same level as the column exit. I t appears, however, as a direct consequence of the
1.45 s
!
(C)
Figure 13. Fast separation of some lower alkanes present in natural gases in capillary columns coated with Cab-0-Sil: mobile phase, nitrogen; temperature, 23 O C ; recorded with X-Y recorder (T = 0.1 s).
Trace a: methane, n-hexane, and n-heptane in a straight column (0.027 cm radius, 501 cm length);pressure drop, 0.5 bar. Trace b: same separation in the same column as under (a)but now tightly coiled 0.4 cm (coil radius); pressure drop, 0.5 bar. trace c: methane, n-pentane, and n-hexane in a microcapillary column (0.0082 cm radius, 299 cm length) tightly coiled into a coil of radius 0.1 cm; pressure drop, 2 bar. Sample consists of methane (80% v/v), n-pentane vapor (8% v/v), and n-hexane vapor (12% v/v): split ratio, l/103; slow in injector, 31 cm3/s;estimated sample volume injected, 1.25 X loe9cm3. For the methane peak this corresponds to a sensitivity of 5 X g/s; lower limit of detection, 3.5 X mol of CH,/s; plate height cm (hex= 0.207; e = 0.19);plate methane peak, He, = 6.8 X number, n = 4.4 X lo4; separation speed, n / t = 3.0 X lo4 s-'. change of flow patterns, that a slight lowering of the column exit, somewhat below the flame base, produced more narrow peaks. Again, there is an optimum position to be found, which position differs for every carrier gas velocity. Flow patterns are also influenced by the burner gas and make-up gas, and so the optimum exit position also depends on the flow rates of these gases. Once the optimum position had been established, a second and more unexpected factor was found to influence the peak width, viz., the concentration of the sample gas or, equivalently, the signal height. It was found that lowering the sample size by using a smaller overpressure of the sample gas produced more narrow peaks. Figure 11 gives several examples of the peak height effect on peak broadening under different conditions of the burner gas flows at one constant carrier gas flow. Irrespective of the choice of either higher or lower burner gas flows than the optimum flow rates (which produce the highest signals), linear dependences are observed in all cases, having nearly the same limiting values when extrapolated to
ANALYTICAL CHEMISTRY, VOL. 59, NO. 7. APRIL 1, 1987
zero peak heights, as can be seen in Figure 11. This fact proves that the present observations are caused by the detector itself and not by some obscure mechanism in the column or a malfunctioning of the injector. As we have no insight into the mechanism behind this detedor behavior, we cannot minimize this effect and, for purposes of obtaining quantitative results on testing peak broadening theories, we have no choice but to extrapolate a series of several measurements to zero signal height. The results will be discussed in the next paragraph. Peak Broadening Using the New Injector and Optimized Detector Parameters. A final series of measurements was performed in the straight 155.8 cm long, 0.01546 cm radius silver capillary, already investigated. Optimizing the flow rates of the burner gases as well as the column outlet position at every carrier gas flow used, we obtained the results described in Figure 12 from our four experiments with different peak heights at each carrier gas velocity. Quite arbitrarily we have chosen 60, 125, 180, and 250 mV as standard levels for the peak height, extrapolating back to zero peak height, just as in Figure 11. Figure 12 clearly shows how drastically the deviation from theory decreases with peak height: E = 2.0 for 250 mV drops to E = 0.98 for 180 mV and t = 0.41 for 60 mV. Extrapolation to zero peak height leads to t = 0.19, quite near the theoretical prediction (E = 0). With these measurements we think to have indicated that deviations from theoretical dispersion behavior (at least for unretained peaks) can almost completely be attributed to extracolumn mechanisms. The injector being quite ideal and the detector having been optimized as much as possible, we may have opened up the pathway to future application of rapid gas analysis in very small bore, open capillary columns. By way of example the next section illustrates the rapid analysis of hydrocarbons present in natural gases in still wide bore capillaries. Rapid Gas Analysis in Coated Capillary Columns. Columns were coated with submicrometer silica particles (“Cab-0-Sil”)after a slightly revised method originally developed by Schwartz et al. (21-23) and Cramers et al. (24). Using the apparatus described, we have been able to perform separations of some lower alkanes, present in natural gases, within 2 s, examples of which are shown in Figure 4 (with manual injection) and Figure 13. Trace a in the latter figure was obtained with a virtually straight column, having a relatively wide internal diameter of 0.54 mm. Trace b shows that the same column, but now tightly coiled into a helical shape, performs much better. Thus the favorable coiling
1015
effects in high-speed GC are well-demonstrated. The same separation can be further improved by using a coiled column with a smaller diameter. Trace c shows the result obtained in a 0.16-mm (i.d.) coiled column, requiring a slightly higher inlet pressure (2 bar) than that applied for traces a and b (0.5 bar). Comparably rapid natural gas analyses using packed columns have been described in the literature (6, 25, 26).
Registry No. Methane, 74-82-8;n-hexane, 110-54-3;n-heptane, 142-82-5. LITERATURE CITED (1) Desty D. H. A&. Chromatogr. 1985, 1 , 199-227. (2) Guiochon, G. Anal. Chem. 1978,50, 1812. (3) Lipsky, S.R.; McMurray, W. J.; Hernandez, M.; Purcell, J. E.; Bilieb, K. A. J . Chromatogr. Sci. 1980, 18, 1. (4) Jennings, W. HRC CC, J . High Resoiut. Chromatogr. Chromatogr. Commun. 1980,3. 601. (5) Schutjes, C. P. M.; Vermeer, E. A.; Rijks, J. A,; Cramers, C. A. In Proceedings of the Fourth International Symposium on Caplllaty Chromatography, Hindelang 1981; Kaiser, R. E., Ed.; Institut fur Chromate graphie: Bad Durkheim, 1981; pp 687-702. (6) Jonker, R. J.; Poppe, H.; Huber, J. F. K. Anal. Chem. 1982,5 4 , 2447. (7) Tijssen. R. Sep. Sci. Techno/. 1978, 13, 681. (8) TlJssen,R. Ph.D. Thesis, Delft, The Netherlands, 1979. (9) Golay, M. J. E. In Oas Chromatography 1958; Desty, D. H., Ed.; Butterworths: London, 1958; p 36. (10) Ogan, K.; Scott, R. P. W. HRC CC, J . High Res. Chromatogr. Chromatcgr. Commun. 1984,7 , 382-388. (11) Sternberg, J. E. Adv. Chromatogr. 1968,2 , 205. (12) Gaspar, G.; Annino, R.; VidaCMadjar, C.; Guiochon, G. Anal. Chem. 1978,50. 1512. (13) Tijssen, R.; Bleumer, J. P. A,; Smit, A. L. C.; Van Kreveld, M. E. J . Chromatogr. 1981,218, 137. (14) Van Kreveld, M. E.; Van de Hoed, N. J . Chromatogr. 1978, 149, 71-91. (15) Golay, M. J. E. J . Chromatogr. 1980, 196, 349. (16) Atwood, J. G.; Golay, M. J. E. J . Chromatogr. 1981,218, 97-122. (17) Atwood, J. G.; Goidstein, J. W. J . Phys. Chem. 1984, 8 8 , 1875-1885. (18) DOUG,F.; Guiochon, 0. Sep. Sci. 1970,5 , 197. (19) Flint, L. F.; Eisenklam, P. Can. J . Chem. Eng. lS80,4 7 , 101. (20) Schieke, J. D.; Smuts, T. W.; Pretorius, V. Sep. Sci. 1068,3 , 27. (21) Schwartz, R. D.; Brasseaux D. J.; Mathews, R. G. Anal. Chem. 1968, 3 8 , 303. (22) Schwartz, R. D.; Brasseaux, D. J.; Shoemake, G. R. Anal. Chem. 1963,35, 496. (23) Schwartz, R. D.; Brasseaux, D. J.; Shoemake, G. R.; J . Chromatogr. 1979, 186, 183. (24) Cramers, C. A.; Vermeer, E. A.; Franken, J. J. Chromatographia1877, 10, 412. (25) Dandeneau, R.; Hawkes, S. Chromatographia 1980, 13, 686. (26) Poppe, H. Recl. Trav. Chim. Pays-Sas 1981, 100, 169. (27) DOUG,F.; Merie d’Aubigne, J.; Guiochon, G. Chim. Anal. (Paris) 1971, 5 3 , 363.
RECEIVED for review February 4,1986. Resubmitted October 20, 1986. Accepted October 20, 1986.
