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Performance and use of wall-coated open tubular fused-silica columns with liquid-modified graphitized carbon black
Journal of Chromatography, Science Publishers CHROMSYMP.
399 (1987) 87-97
Amsterdam
-
in The
1158
PERFORMANCE AND USE OF WALL-COATED SILICA COLUMNS WITH LIQUID-MODIFIED BLACK
FABRIZIO BRUNER*, GIANCARLO PALMA and MIN XIANG* Istituto
di Scienze
Chimiche,
CRESCENTINI,
OPEN TUBULAR FUSEDGRAPHITIZED CARBON
FILIPPO
Universitci di C’rhino, Piazza Rinascimento
MANGANI,
PIERANGELA
6. 61029 Urhino (Italy)
SUMMARY
The kinetic and thermodynamic changes induced in capillary columns with the internal area modified by a thin layer of graphitized carbon blacks are discussed and were tested experimentally. The selectivity due to the mechanism of gas-liquid-solid chromatography is greatly increased and the efficiency in terms of HETP is much improved at high linear gas velocities. This results in the possibility of obtaining faster analyses without the usual increase in operating temperature with respect to the gas-liquid technique. Examples of applications in practical analysis are given. ____ ____~_ ___. INTRODUCTION
In a recent paper], we presented the first experimental results obtained by coating the inner surface of fused-silica capillary columns with a thin layer of a low-surface-area graphitized carbon black (6.5 m*/g), impregnated with small amounts of a liquid phase. The use of graphitized carbon to make selective glass capillary columns was introduced several years ago by Guiochon and co-workers2.3 and Liberti and coworkers4.j. In spite of the good results obtained as far as selectivity is concerned, the efficiency in terms of height equivalent to a theoretical plate (HETP) was not entirely satisfactory in either set of experiments. In this paper, we show that it is possible to obtain high-performance fused-silica capillary columns that are highly competitive with gas-liquid chromatographic (GLC) fused-silica capillary columns but with higher selectivity and better HETP at high linear gas velocities. THEORY
Fused-silica GLC capillary columns have achieved very high performances, and columns showing HETP of 0.3-0.4 mm are commonly commercially available.
l
On leave of absence
0021-9673/87/%03.50
0
from
Yanshan
1987 Elsevier
Petrochemical Science Publishers
Corp..
B.V.
Research
Institute,
Beijing,
China
F. BRUNER
88
et al.
However, few liquid phases can be used to coat capillary columns, and the selectivity is usually not very high. Further, the C term of the Van Deemter equation is usually high, so that even with a good value of Hmi” a noticeable loss of efficiency is observed when high linear gas velocities have to be used to obtain a fast analysis. The chromatographic resolution, R, is related to the separation factor, a, the capacity ratio, k’, and the number of theoretical plates, N, by the well known equation 1
cc-1
k’
~~
R=4.
(1)
From the analytical examination of this equation. it seems clear that an increase in CIhas a greater effect on resolution than increasing the number of theoretical plates. Therefore, liquid-modified packed adsorption columns have been exploited by our of gas-liquid-solid chrogroup in the past few years6, as the working mechanism matography (GLSC) on graphitized carbon black is now sufficiently well understood7,8 and packed columns for GLSC are widely used and commercially available (e.g., ref. 9). Eqn. 1 can be written in the form
Provided one works in the linear part of the Van Deemter curve, where it may be assumed that H = C U, and considering that N is given by the column length, L, divided by HETP, H, the following expression is obtained: R2 _$_(F+>‘(,::)
(3)
L and as u=-_=
L
L(k’
to finally
+ 1) t
the expression10 A’2 (k’ + 1)”
(4)
is obtained. The term R2,/t should be maximized to obtain a certain separation with the highest resolution within the minimal analysis time. By comparing two columns performing the same analysis, the influence of the k’ terms can be neglected, whereas z and C play a very important role. As is well known, the separation factors, 2, are usually much higher for gassolid chromatography (GSC) than for GLC so that, provided that the adsorption isotherm is linear, GSC is preferable to GLC for faster and more effective analysis.
WCOT COLUMNS WITH LIQUID-MODIFIED
89
GCB
However, the drawback of GSC is that the elution temperature is usually much higher than in GLC and for this reason the use of GSC has been confined to the analysis of permanent gases or low-boiling vapours. The introduction of low-surface-area carbon blacks, such as Carbopack F or analogous materials, with suitable amounts of liquid phase added, have made liquid-modified GSC competitive with GLC, even with regard to elution temperature6. One could raise the question of whether liquid-modified adsorption chromatography can still be considered to be GSC. In the practical application of this technique we have obtained evidence that, if the coating of the surface is limited to one or two monolayers, the underlying mechanism of the GC process is still adsorption, as shown by the high values of the heat involved in the chromatographic process and by the good separation factors observed for isomers. Giddingsl l treated the resistance to mass transfer in the condensed phase and modified the classical Van Deemter equation for GSC, taking into account that the adsorptiondesorption process is much faster than the solution evaporation process, and obtained the following expression for a capillary column:
where C,, the kinetic mass transfer term, replaces the liquid-phase mass transfer term, Cr. The terms related to the diffusion in the gas phase (II) and to the resistance to mass transfer in the gas phase (C,) refer to the usual expressions, namely (e.g., ref. L2), B = 2 D,
and
C, =
1 + 6k’ + Ilk’* 3(1
+ k’)Z
I$ SD,
(6)
where r. is the radius of the capillary and D, is the diffusion coefficient in the gas phase. A comparison of the efficiency of two capillary columns used in GLC and GSC can be made, provided that the geometry of the columns is the same and the same compound is eluted at the same temperature and shows analogous values of k’. Then, both the C, and B terms of the Van Deemter equation can be considered to make the same contribution to the magnitude of H. Under these conditions, the differences between the overall C terms can be ascribed to the difference between C, and C,. According to Giddings,
(7) where to is the dead time (elution time of an unretained peak), t is the retention time of a given peak and & is the mean desorption~ time of an equilibrium population of sorbed molecules. If the adsorbent is sufficiently homogeneous for the energy of the adsorption sites concerned, which is the case in linear elution from liquid-modified graphitized carbon black, then
c, =
2.
T
( 4> 1
_
&
90
F. BRUNER
where Kd is the first-order desorption constant. From further considerations and acceptable approximations, Giddings to the conclusion that the term Ck can be well expressed by the equation
et al.
came
where
& is the gas phase volume of the column, P is the fraction of the molecules a unit area in unit time that actually are adsorbed and A is the overall area of the inner walls coated with the adsorbent. Giddings forecasted a value of the ratio for a highly porous and homogeneous adsorbent of about 10p3, while /? should range between 0.1 and 1.0. In this instance, a theoretical value for Ck of lO-7-1O-s can be assigned if the surface area of the adsorbent is very high (lo3 m”/g>. Unfortunately, it is impossible to obtain an adsorbent that is highly porous and at the same time homogeneous in the energy distribution of the adsorption sites. Therefore, the best compromise can be reached by using a non-porous, homogeneous adsorbent, such as graphitized carbon black, which has the advantage of exhibiting a linear adsorption isotherm. Working with this kind of adsorbent, one can assume a value for C, of about 10p4. This value, also forecast by Giddings, is hardly reached for C, in chromatographic practice. The kinetic approach to faster analysis brings one to the conclusion that capillary columns operating on the basis of the GLSC mechanism should allow a better resolution within a shorter analysis time. This is essentially due to the lower values of the C term in the Van Deemter equation. From the thermodynamic point of view, it is well known that GSC yields higher separation factors than GLC for the separation of isomers. This characteristic behaviour is also shown by liquid-modified adsorption columns. It has been demonstrated7 that by coating a non-porous, non-specific adsorbent, such as graphitized carbon black, with a polar liquid phase, the analytical potential of GSC is preserved to a greater extent if non-polar compounds are eluted. This is due to the fact that if the liquid phase and the eluate molecules are scarcely soluble in each other, then adsorption on the two-dimensional liquid layer takes place when a monolayer of liquid phase is placed on the adsorbent, The overall effect of the liquid phase is to deactivate the surface for the non-specific active sites and to reduce the heat of adsorption and, as a consequence, the retention parameters. However, as the basic mechanism is adsorption, higher separation factors are still obtained for molecules of similar structure. Hence the thermodynamic approach to fast a&ysis leads to the conclusion that a polar liquid phase, such as Carbowax and SP1000, should be the coating of choice for the separation of non-polar compounds in Iiquid-modified adsorption GC. striking
EXPERIMENTAL
All GC measurements were performed on dedicated capillary gas chromatographs, such as the Carlo Erba (Milan, Italy) Model HRGC 5160 “Mega” and Dani (Monza, Italy) Model 6500. Empty and coated fused-silica capillary columns were obtained from Supelco (Bellefonte, PA. U.S.A.). Hydrogen was used as the carrier
WCOT COLUMNS
WITH LIQUID-MODIFIED
GCB
91
gas. A Branson Model B15 cell disruptor was used to generate ultrasonic waves in the preparation of the carbon black liquid-phase slurry. Carbopack F was obtained from Supelco . “Bleeding” measurements were obtained with the total ion current (m/e 50450) obtained at 70 eV by directly coupling the two columns to a VG 70-70H mass spectrometer under the same flow-rate conditions. The flow-rate was measured at the vent exit of the rotary pump of the ion source. The method used for the column preparation is a substantial modification of that suggested by Xu and Vermeulen i3 for coating GLC capillary columns. This is essentially static coating, in which the evaporation of the solvent is carried out while the column is kept above its boiling point at atmospheric pressure. This method is similar to that described recently1 but with the difference that the damping column is replaced with a fused-silica capillary (7-10 cm long, 50 pm I.D.), attached to the column with an epoxy adhesive. The slurry was prepared by sonication of dichloromethane-pentane (1:l) to which was added 0.35-0.50% (w,/v) of graphitized carbon black, previously sieved to particles smaller than 120 mesh, and about 10% of liquid modifier (SPlOOO, Carbowax 20M). After sonification for 30 min, the average size of the graphite particles was reduced to about 0.2 pm. The capillary was filled with the slurry and a short 50-pm fused-silica capillary was fixed with epoxy adhesive at one end of the column. The outlet of the capillary was closed with a flame. Particular care must be taken to avoid the formation of air bubbles along the column and at its back end. The column was immersed in a water-bath kept at a temperature higher than the boiling point of the mixture. Slow evaporation of the solvent from the open end, restricted by the 50-pm capillary, took place. After complete evaporation of the solvent, the column was kept under a flow of dry nitrogen for 2 h and conditioned overnight at the maximal operating temperature of the liquid phase. RESULTS
AND DISCUSSION
The first problem encountered was to establish the actual coverage of carbon black by the liquid phase in a capillary column operating in GLSC, prepared according to the procedure previously described. In fact. the accepted surface area of Carbopack F ranges between 6.0 and 6.5 m2/g. However, this value was measured for particle sizes of the order of 200-1000 pm. When the slurry has been prepared by sonication, the average particle size becomes much smaller (ea. 0.2 pm) and the surface area of the material increases. The actual value is different from that given above and cannot be easily forecast. In order to solve this problem, the following experiment was performed. A series of conventional packed columns, made with Carbopack F (60-80 mesh) coated with different amounts of liquid phase, were prepared and the isosteric heat of adsorption (QsJ were measured for n-dodecane using the classical chromatographic method7. The results are shown in Fig. 1. The isosteric heat of adsorption was also measured for the same compound within the same temperature range using the capillary column, and this value was compared with those obtained for the packed columns. The value measured for the capillary column was 14.1 kcal/mol, which corresponds to a surface coverage of about 1.20-1.75 monolayers in Fig. 1. In fact,
F. BRUNER
92
et al.
Qs
-I kcal
mole
1
Fig. 1. lsosteric Carbopack F.
2
heat of adsorption
3
as a function
4
of the surface
SP loco~%,wlw)
coverage
*
or percentage
of liquid phase on
formation is indicated it has been shown by several workers 7.14.15 that the monolayer by a sharp decrease in the graph of heat of adsorption vc~rsus percentage of modifier. With graphitized carbon black coated with SPlOOO a decrease of about 5 kcal/mol is high enough to make the hypothesis of homogeneous monolayer formation reasonable. From this value the 8 (surface coverage) axis was constructed by a simple proportion to the percentage of modifier axis. From the ratio of the retention parameters (k’) for the packed and the capillary columns, a more precise value for the actual surface coverage can be found, at about 1.4 monolayers. For this hypothesis, an increase in the surface area of about 25-fold because of the decrease in the particle dimensions is assumed, which seems reasonable. Fig. 2 shows the experimental Van Deemter plots yielded by two fused-silica capillary columns of the same geometrical characteristics. and comparative data
WCOT
HET
COLUMNS
WITH
LIQUID-MODIFIED
93
GCB
d
:j
5
,
t
,
,
10
15
20
25
,
30
35
40
1
45
50
t
55 60 ii (cmisec)
Fig. 2. Van Deemter plots obtained with two fused-silica capillary columns of the same geometrical characteristics (20 m x 0.25 mm I.D.). Coatings: n = graphitized carbon black, Carbopack F + SPIOOO (for details see text): l = SPIOOO: film thickness 0.25 pm. Sample. n-hexadecane.
are reported in Table I. The two Van Deemter plots coincide in the left part, as expected. In fact, having used the same sample and carrier gas at the same temperature in the two columns with the same geometry. the B term of the Van Deemter equation should have the same value. The right parts of the curves differ substantially, and calculation of the C term yields values of 1.3 lop3 for the GLC column and 2.3 . lop4 for the adsorption (GLSC) column. The latter value is close to that assumed according to the Giddings theory for a non-porous adsorbent with a relatively low surface area (ca. 150 m’jg) and it is lower by a factor of 6 with respect to that of the GLC column. The value of the separation number (Trennzahl, TZ), which takes into account both resolution and column efficiency, is higher for the GLSC column, and the same is true for the term that also takes the analysis time into account. The latter value was calculated from the separation of two consecutive n-alkanes at the same temperature and at k’ values close enough to minimize the effect of k’ in eqn. 2. The “Trennzahl”, TZ, was calculated using compounds with similar k’ values for the more retained one. It is interesting that, in spite of the higher value found for the heat of adsorption, the overall retention of the GLSC column for the same compound is lower. This implies a very important entropy effect in the chromatographic process and confirms the hypothesis, supported by the kinetic data, in particular the C value, that the mechanism of the chromatographic process in GLSC is adsorption. This is further confirmed by comparison of the values obtained for the “bleeding” of the stationary
I
.~.
I (GLC) 2 (GLSC)
-
Column
Column
~~
1 (GM?,
0.30 0.30
1.3 2.3
(s)
(mm)
.~
c
Hmnin
32
34
IO-4
(cm/s)
Urni”
column
lo-’
SPIOOO,0.25 pm (Supelco);
COMPARISON OF THE CHROMATOGRAPHIC SIONS (20 m x 0.X mm l.D.)
TABLE
.-..
0.54 5.20
RZjt (s-1)
2 (GLSC).
21.4 31.5 IS
1.5
1%)
Coating efficiency
_ __.--
x.9 5.7
K-c,, (IOYC)
carbon
CAPILLARY
on a layer of’graphitized
OF TWO FIJSED-SILICA
SPIOOO, supported
CHARACTERISTIS
9.4 14.1
Qst (n-cd
fkcallmol)
DIMEN-
17 5
Bleeding at 240°C (u/0 full-scale)
F)
OF THE SAME
black (Carbopack
(‘OLIJMNS
P
WCOT
COLUMNS
WITH
LIQUID-MODIFIED
95
GCB
a 9
‘1
b 2
1
6 7 5
4 3
4
6
0
-L
6
- ILL
-11 L-
6
1
4
:
8
12
16
20
mm
Fig. 3. Separation of some herbicides on GLC and GLSC capillary columns. I = Diclobenil; 2 = trifluralin; 3 = 2,4-DME; 4 = propazine; 5 = atrarine: 6 = simazine: 7 = silvex ME; 8 = 2,4STME; 9 = DCPA. (a) Fused-silicacapillary column (20 m x 0.25 mm I.D.) coated with Carbopack F + SPIOOO. Temperature programme, 140°C for 2 min, then increased at 18”C.‘min to 230°C. (b) SPB 5 fused-silica capillary column (30 m x 0.25 mm I.D.). Temperature programme. 130°C for 2 min, then increased at 4”Cmin to 230°C.
phase at the same temperature for the two columns, which is lower by a factor of 3.4 for the GLSC column. The vapour pressure of the stationary phase is lower in this instance, as the molecules are strongly adsorbed and their volatility is lower than in the bulk liquid. In Figs. 3 and 4 two examples of the effectiveness of the GLSC column relative to the GLC columns are reported. The GLC columns used contained the most selective liquid phases for the type of compounds selected in both instances. However, a cluster of compounds is obtained, in spite of the slower temperature programme used. The total analysis time is reduced by a factor of 3 when the GLC columns are used for the separation of herbicides. The very good separation obtained with the GLSC column for propazine, atrazine and simazine, which have very similar structures and polarities, is a further proof of the high selectivity obtained with GLSC. Fig. 5 shows the fast and effective separation of some phthalates, compounds of environmental interest, obtained with the same column. In Fig. 6, the separation of some polar compounds, such as alcohols and phenols, on a column prepared with Carbowax 20M as the liquid modifier is reported. Although the capillary column was only 9 m long, it had cu. 20 000 theoretical plates. Owing to the selectivity of the stationary phase and the column efficiency, short capillary columns may be used for fast analysis.
F. BKUNkK
et (II.
96
a
b 5
L Jl_! 4
10
'2
i 4
'2
6
10
6
12
min
Fig. 4. Separation of some pesticides on GLC and GLSC capillary columns. 1 = 3 = aldrin; 4 = lindane; 5 = heptachlor epoxidc: 6 = &BHC; 7 = dieldrin; 10 = p,p’-DDT. (a) SPB 608 fused-silica capillary column (20 m x 0.25 mm gramme, 150°C for 4 min. then increased at 8”C.min to 290°C. (b) Fused-silica x 0.25 mm I.D.), coated with Carbopack F i- SPlOOO. Temperature programme, increased at 13”Cimin to 240°C
I/
11
0
2
s 4
!
3
6
1,.
.I
1
8
IO
I-BHC; 2 = heptachlor; 8 = DDE; 9 = endrin; I.D.). Temperature procapillary column (20 m iOO”C for 2 min, then
I.
12
min
Fig. 5. Separation ofphthalates. I = Dimethyl phthalate; 4 = benzylbutyl phthalate: 5 = ethylhexyl phthalate; column (20 m x 0.25 mm I.D.) coated with Carbopack for 1 min, then increased at lS”C.‘min to 240°C.
2 = diethyl phthalate: 3 = di-lz-butyl phthalate: 6 = di-n-oct),l phthalate. Fused-silica capillary F + SPlOOO. Temperature programme. 125°C
WCOT
COLUMNS
WITH
LIQUID-MODIFIED
GCB
97
12 IO 11
li.!-
0
2
4
6
8
IO min
Fig. 6. Separation of alcohols and phenols. 1 = Hexanol; 2 = 2-octanol; 3 = 1-heptanol; 4 = I-octanol; 5 = l-decdnol; 6 = 2nitrophenol; 7 = 2chlorophenol; 8 = phenol: 9 = 2,4-dimethylphenol; 10 = 2,4-dichlorophenol; 11 = 2,4,6-trichlorophenol; I2 = 4-chloro-3-mcthylphenyl. Fused-silica capillary column (9 m x 0.25 mm I.D.) coated with Carbopack F + Carbowax 20M. Temperature programme, 90°C for 1 min, then increased at IZ”C/min to 140°C. followed by 25”Cmin to 220°C.
Both columns have been used extensively over a period of 8 months and no significant changes in retention and selectivity have been observed. The results reported here confirm the Giddings theory of GSC. Many interesting applications may be foreseen of various types of carbon blacks, differing in surface area, and of different cross-linked liquid phases. REFERENCES F. Mangani and G. Furlani, paper presented at the 7th International SJW1 F. Bruner, G. Crescentini. posium on Capillary Chromatography. Gifu, MaJ 11-14, 1986. 2 C. Vidal Madjar, G. Ganansia and G. Guiochon, in R. Stock (Editor), Gas Chromatography 1970, Institute of Petroleum, London, 1971. p. 20. P. Arpino and G. Guiochon, Anal. Chem., 49 (1977) 3 C. Vidal Madjar. S. Bekassy, M. F. Gonnord. 768. 4 G. Goretti, A. Liberti and G. Notd. Chromatc~graphia, 8 (1975) 486. 5 G. Goretti, A. Liberti and G. Pili, J. High R~soluf. Chromatogr. Chromatogr. Commun.. 9 (1978) 143. 6 A. Di Corcia and A. Liberti, Adv. Chromatogr.. 14 (1976) 305. 7 F. Bruner, P. Ciccioli, G. Crescentini and M. T. Pistolesi, And. Chrm.. 45 (1973) 1851. 8 F. Mangani and F. Bruner, J. Chromutogr.. 289 (1984) 85. 9 Supeko Catalog, No. 25> Supelco, Bellefonte. PA. 1986. 10 F. Bruner and G. P. Cartoni, An&. Cizrnt.. 36 (1964) 1522. I1 J. Giddings, Anal. Chem.. 36 (1964) I 170. 12 L. S. Ettre. in F. Bruner (Editor). The Scicncr of Chromatography. Elsevier, Amsterdam, 1985, pp. 87-106. 13 B. Xu and N. P. E. Vermeulen, Chromalogruphiu, 18 (1984) 520. 14 A. V. Kiselev and Y. I. Yashin. Gas Adsorption Chromatogrupi~~.. Plenum Press, New York, 1969, p. 128. 15 D. M. Young and A. D. Crowell, Ph~xicai Adsorption of Gase.v. Butterworths, London, 1962, p. 54.
399 (1987) 87-97
Amsterdam
-
in The
1158
PERFORMANCE AND USE OF WALL-COATED SILICA COLUMNS WITH LIQUID-MODIFIED BLACK
FABRIZIO BRUNER*, GIANCARLO PALMA and MIN XIANG* Istituto
di Scienze
Chimiche,
CRESCENTINI,
OPEN TUBULAR FUSEDGRAPHITIZED CARBON
FILIPPO
Universitci di C’rhino, Piazza Rinascimento
MANGANI,
PIERANGELA
6. 61029 Urhino (Italy)
SUMMARY
The kinetic and thermodynamic changes induced in capillary columns with the internal area modified by a thin layer of graphitized carbon blacks are discussed and were tested experimentally. The selectivity due to the mechanism of gas-liquid-solid chromatography is greatly increased and the efficiency in terms of HETP is much improved at high linear gas velocities. This results in the possibility of obtaining faster analyses without the usual increase in operating temperature with respect to the gas-liquid technique. Examples of applications in practical analysis are given. ____ ____~_ ___. INTRODUCTION
In a recent paper], we presented the first experimental results obtained by coating the inner surface of fused-silica capillary columns with a thin layer of a low-surface-area graphitized carbon black (6.5 m*/g), impregnated with small amounts of a liquid phase. The use of graphitized carbon to make selective glass capillary columns was introduced several years ago by Guiochon and co-workers2.3 and Liberti and coworkers4.j. In spite of the good results obtained as far as selectivity is concerned, the efficiency in terms of height equivalent to a theoretical plate (HETP) was not entirely satisfactory in either set of experiments. In this paper, we show that it is possible to obtain high-performance fused-silica capillary columns that are highly competitive with gas-liquid chromatographic (GLC) fused-silica capillary columns but with higher selectivity and better HETP at high linear gas velocities. THEORY
Fused-silica GLC capillary columns have achieved very high performances, and columns showing HETP of 0.3-0.4 mm are commonly commercially available.
l
On leave of absence
0021-9673/87/%03.50
0
from
Yanshan
1987 Elsevier
Petrochemical Science Publishers
Corp..
B.V.
Research
Institute,
Beijing,
China
F. BRUNER
88
et al.
However, few liquid phases can be used to coat capillary columns, and the selectivity is usually not very high. Further, the C term of the Van Deemter equation is usually high, so that even with a good value of Hmi” a noticeable loss of efficiency is observed when high linear gas velocities have to be used to obtain a fast analysis. The chromatographic resolution, R, is related to the separation factor, a, the capacity ratio, k’, and the number of theoretical plates, N, by the well known equation 1
cc-1
k’
~~
R=4.
(1)
From the analytical examination of this equation. it seems clear that an increase in CIhas a greater effect on resolution than increasing the number of theoretical plates. Therefore, liquid-modified packed adsorption columns have been exploited by our of gas-liquid-solid chrogroup in the past few years6, as the working mechanism matography (GLSC) on graphitized carbon black is now sufficiently well understood7,8 and packed columns for GLSC are widely used and commercially available (e.g., ref. 9). Eqn. 1 can be written in the form
Provided one works in the linear part of the Van Deemter curve, where it may be assumed that H = C U, and considering that N is given by the column length, L, divided by HETP, H, the following expression is obtained: R2 _$_(F+>‘(,::)
(3)
L and as u=-_=
L
L(k’
to finally
+ 1) t
the expression10 A’2 (k’ + 1)”
(4)
is obtained. The term R2,/t should be maximized to obtain a certain separation with the highest resolution within the minimal analysis time. By comparing two columns performing the same analysis, the influence of the k’ terms can be neglected, whereas z and C play a very important role. As is well known, the separation factors, 2, are usually much higher for gassolid chromatography (GSC) than for GLC so that, provided that the adsorption isotherm is linear, GSC is preferable to GLC for faster and more effective analysis.
WCOT COLUMNS WITH LIQUID-MODIFIED
89
GCB
However, the drawback of GSC is that the elution temperature is usually much higher than in GLC and for this reason the use of GSC has been confined to the analysis of permanent gases or low-boiling vapours. The introduction of low-surface-area carbon blacks, such as Carbopack F or analogous materials, with suitable amounts of liquid phase added, have made liquid-modified GSC competitive with GLC, even with regard to elution temperature6. One could raise the question of whether liquid-modified adsorption chromatography can still be considered to be GSC. In the practical application of this technique we have obtained evidence that, if the coating of the surface is limited to one or two monolayers, the underlying mechanism of the GC process is still adsorption, as shown by the high values of the heat involved in the chromatographic process and by the good separation factors observed for isomers. Giddingsl l treated the resistance to mass transfer in the condensed phase and modified the classical Van Deemter equation for GSC, taking into account that the adsorptiondesorption process is much faster than the solution evaporation process, and obtained the following expression for a capillary column:
where C,, the kinetic mass transfer term, replaces the liquid-phase mass transfer term, Cr. The terms related to the diffusion in the gas phase (II) and to the resistance to mass transfer in the gas phase (C,) refer to the usual expressions, namely (e.g., ref. L2), B = 2 D,
and
C, =
1 + 6k’ + Ilk’* 3(1
+ k’)Z
I$ SD,
(6)
where r. is the radius of the capillary and D, is the diffusion coefficient in the gas phase. A comparison of the efficiency of two capillary columns used in GLC and GSC can be made, provided that the geometry of the columns is the same and the same compound is eluted at the same temperature and shows analogous values of k’. Then, both the C, and B terms of the Van Deemter equation can be considered to make the same contribution to the magnitude of H. Under these conditions, the differences between the overall C terms can be ascribed to the difference between C, and C,. According to Giddings,
(7) where to is the dead time (elution time of an unretained peak), t is the retention time of a given peak and & is the mean desorption~ time of an equilibrium population of sorbed molecules. If the adsorbent is sufficiently homogeneous for the energy of the adsorption sites concerned, which is the case in linear elution from liquid-modified graphitized carbon black, then
c, =
2.
T
( 4> 1
_
&
90
F. BRUNER
where Kd is the first-order desorption constant. From further considerations and acceptable approximations, Giddings to the conclusion that the term Ck can be well expressed by the equation
et al.
came
where
& is the gas phase volume of the column, P is the fraction of the molecules a unit area in unit time that actually are adsorbed and A is the overall area of the inner walls coated with the adsorbent. Giddings forecasted a value of the ratio for a highly porous and homogeneous adsorbent of about 10p3, while /? should range between 0.1 and 1.0. In this instance, a theoretical value for Ck of lO-7-1O-s can be assigned if the surface area of the adsorbent is very high (lo3 m”/g>. Unfortunately, it is impossible to obtain an adsorbent that is highly porous and at the same time homogeneous in the energy distribution of the adsorption sites. Therefore, the best compromise can be reached by using a non-porous, homogeneous adsorbent, such as graphitized carbon black, which has the advantage of exhibiting a linear adsorption isotherm. Working with this kind of adsorbent, one can assume a value for C, of about 10p4. This value, also forecast by Giddings, is hardly reached for C, in chromatographic practice. The kinetic approach to faster analysis brings one to the conclusion that capillary columns operating on the basis of the GLSC mechanism should allow a better resolution within a shorter analysis time. This is essentially due to the lower values of the C term in the Van Deemter equation. From the thermodynamic point of view, it is well known that GSC yields higher separation factors than GLC for the separation of isomers. This characteristic behaviour is also shown by liquid-modified adsorption columns. It has been demonstrated7 that by coating a non-porous, non-specific adsorbent, such as graphitized carbon black, with a polar liquid phase, the analytical potential of GSC is preserved to a greater extent if non-polar compounds are eluted. This is due to the fact that if the liquid phase and the eluate molecules are scarcely soluble in each other, then adsorption on the two-dimensional liquid layer takes place when a monolayer of liquid phase is placed on the adsorbent, The overall effect of the liquid phase is to deactivate the surface for the non-specific active sites and to reduce the heat of adsorption and, as a consequence, the retention parameters. However, as the basic mechanism is adsorption, higher separation factors are still obtained for molecules of similar structure. Hence the thermodynamic approach to fast a&ysis leads to the conclusion that a polar liquid phase, such as Carbowax and SP1000, should be the coating of choice for the separation of non-polar compounds in Iiquid-modified adsorption GC. striking
EXPERIMENTAL
All GC measurements were performed on dedicated capillary gas chromatographs, such as the Carlo Erba (Milan, Italy) Model HRGC 5160 “Mega” and Dani (Monza, Italy) Model 6500. Empty and coated fused-silica capillary columns were obtained from Supelco (Bellefonte, PA. U.S.A.). Hydrogen was used as the carrier
WCOT COLUMNS
WITH LIQUID-MODIFIED
GCB
91
gas. A Branson Model B15 cell disruptor was used to generate ultrasonic waves in the preparation of the carbon black liquid-phase slurry. Carbopack F was obtained from Supelco . “Bleeding” measurements were obtained with the total ion current (m/e 50450) obtained at 70 eV by directly coupling the two columns to a VG 70-70H mass spectrometer under the same flow-rate conditions. The flow-rate was measured at the vent exit of the rotary pump of the ion source. The method used for the column preparation is a substantial modification of that suggested by Xu and Vermeulen i3 for coating GLC capillary columns. This is essentially static coating, in which the evaporation of the solvent is carried out while the column is kept above its boiling point at atmospheric pressure. This method is similar to that described recently1 but with the difference that the damping column is replaced with a fused-silica capillary (7-10 cm long, 50 pm I.D.), attached to the column with an epoxy adhesive. The slurry was prepared by sonication of dichloromethane-pentane (1:l) to which was added 0.35-0.50% (w,/v) of graphitized carbon black, previously sieved to particles smaller than 120 mesh, and about 10% of liquid modifier (SPlOOO, Carbowax 20M). After sonification for 30 min, the average size of the graphite particles was reduced to about 0.2 pm. The capillary was filled with the slurry and a short 50-pm fused-silica capillary was fixed with epoxy adhesive at one end of the column. The outlet of the capillary was closed with a flame. Particular care must be taken to avoid the formation of air bubbles along the column and at its back end. The column was immersed in a water-bath kept at a temperature higher than the boiling point of the mixture. Slow evaporation of the solvent from the open end, restricted by the 50-pm capillary, took place. After complete evaporation of the solvent, the column was kept under a flow of dry nitrogen for 2 h and conditioned overnight at the maximal operating temperature of the liquid phase. RESULTS
AND DISCUSSION
The first problem encountered was to establish the actual coverage of carbon black by the liquid phase in a capillary column operating in GLSC, prepared according to the procedure previously described. In fact. the accepted surface area of Carbopack F ranges between 6.0 and 6.5 m2/g. However, this value was measured for particle sizes of the order of 200-1000 pm. When the slurry has been prepared by sonication, the average particle size becomes much smaller (ea. 0.2 pm) and the surface area of the material increases. The actual value is different from that given above and cannot be easily forecast. In order to solve this problem, the following experiment was performed. A series of conventional packed columns, made with Carbopack F (60-80 mesh) coated with different amounts of liquid phase, were prepared and the isosteric heat of adsorption (QsJ were measured for n-dodecane using the classical chromatographic method7. The results are shown in Fig. 1. The isosteric heat of adsorption was also measured for the same compound within the same temperature range using the capillary column, and this value was compared with those obtained for the packed columns. The value measured for the capillary column was 14.1 kcal/mol, which corresponds to a surface coverage of about 1.20-1.75 monolayers in Fig. 1. In fact,
F. BRUNER
92
et al.
Qs
-I kcal
mole
1
Fig. 1. lsosteric Carbopack F.
2
heat of adsorption
3
as a function
4
of the surface
SP loco~%,wlw)
coverage
*
or percentage
of liquid phase on
formation is indicated it has been shown by several workers 7.14.15 that the monolayer by a sharp decrease in the graph of heat of adsorption vc~rsus percentage of modifier. With graphitized carbon black coated with SPlOOO a decrease of about 5 kcal/mol is high enough to make the hypothesis of homogeneous monolayer formation reasonable. From this value the 8 (surface coverage) axis was constructed by a simple proportion to the percentage of modifier axis. From the ratio of the retention parameters (k’) for the packed and the capillary columns, a more precise value for the actual surface coverage can be found, at about 1.4 monolayers. For this hypothesis, an increase in the surface area of about 25-fold because of the decrease in the particle dimensions is assumed, which seems reasonable. Fig. 2 shows the experimental Van Deemter plots yielded by two fused-silica capillary columns of the same geometrical characteristics. and comparative data
WCOT
HET
COLUMNS
WITH
LIQUID-MODIFIED
93
GCB
d
:j
5
,
t
,
,
10
15
20
25
,
30
35
40
1
45
50
t
55 60 ii (cmisec)
Fig. 2. Van Deemter plots obtained with two fused-silica capillary columns of the same geometrical characteristics (20 m x 0.25 mm I.D.). Coatings: n = graphitized carbon black, Carbopack F + SPIOOO (for details see text): l = SPIOOO: film thickness 0.25 pm. Sample. n-hexadecane.
are reported in Table I. The two Van Deemter plots coincide in the left part, as expected. In fact, having used the same sample and carrier gas at the same temperature in the two columns with the same geometry. the B term of the Van Deemter equation should have the same value. The right parts of the curves differ substantially, and calculation of the C term yields values of 1.3 lop3 for the GLC column and 2.3 . lop4 for the adsorption (GLSC) column. The latter value is close to that assumed according to the Giddings theory for a non-porous adsorbent with a relatively low surface area (ca. 150 m’jg) and it is lower by a factor of 6 with respect to that of the GLC column. The value of the separation number (Trennzahl, TZ), which takes into account both resolution and column efficiency, is higher for the GLSC column, and the same is true for the term that also takes the analysis time into account. The latter value was calculated from the separation of two consecutive n-alkanes at the same temperature and at k’ values close enough to minimize the effect of k’ in eqn. 2. The “Trennzahl”, TZ, was calculated using compounds with similar k’ values for the more retained one. It is interesting that, in spite of the higher value found for the heat of adsorption, the overall retention of the GLSC column for the same compound is lower. This implies a very important entropy effect in the chromatographic process and confirms the hypothesis, supported by the kinetic data, in particular the C value, that the mechanism of the chromatographic process in GLSC is adsorption. This is further confirmed by comparison of the values obtained for the “bleeding” of the stationary
I
.~.
I (GLC) 2 (GLSC)
-
Column
Column
~~
1 (GM?,
0.30 0.30
1.3 2.3
(s)
(mm)
.~
c
Hmnin
32
34
IO-4
(cm/s)
Urni”
column
lo-’
SPIOOO,0.25 pm (Supelco);
COMPARISON OF THE CHROMATOGRAPHIC SIONS (20 m x 0.X mm l.D.)
TABLE
.-..
0.54 5.20
RZjt (s-1)
2 (GLSC).
21.4 31.5 IS
1.5
1%)
Coating efficiency
_ __.--
x.9 5.7
K-c,, (IOYC)
carbon
CAPILLARY
on a layer of’graphitized
OF TWO FIJSED-SILICA
SPIOOO, supported
CHARACTERISTIS
9.4 14.1
Qst (n-cd
fkcallmol)
DIMEN-
17 5
Bleeding at 240°C (u/0 full-scale)
F)
OF THE SAME
black (Carbopack
(‘OLIJMNS
P
WCOT
COLUMNS
WITH
LIQUID-MODIFIED
95
GCB
a 9
‘1
b 2
1
6 7 5
4 3
4
6
0
-L
6
- ILL
-11 L-
6
1
4
:
8
12
16
20
mm
Fig. 3. Separation of some herbicides on GLC and GLSC capillary columns. I = Diclobenil; 2 = trifluralin; 3 = 2,4-DME; 4 = propazine; 5 = atrarine: 6 = simazine: 7 = silvex ME; 8 = 2,4STME; 9 = DCPA. (a) Fused-silicacapillary column (20 m x 0.25 mm I.D.) coated with Carbopack F + SPIOOO. Temperature programme, 140°C for 2 min, then increased at 18”C.‘min to 230°C. (b) SPB 5 fused-silica capillary column (30 m x 0.25 mm I.D.). Temperature programme. 130°C for 2 min, then increased at 4”Cmin to 230°C.
phase at the same temperature for the two columns, which is lower by a factor of 3.4 for the GLSC column. The vapour pressure of the stationary phase is lower in this instance, as the molecules are strongly adsorbed and their volatility is lower than in the bulk liquid. In Figs. 3 and 4 two examples of the effectiveness of the GLSC column relative to the GLC columns are reported. The GLC columns used contained the most selective liquid phases for the type of compounds selected in both instances. However, a cluster of compounds is obtained, in spite of the slower temperature programme used. The total analysis time is reduced by a factor of 3 when the GLC columns are used for the separation of herbicides. The very good separation obtained with the GLSC column for propazine, atrazine and simazine, which have very similar structures and polarities, is a further proof of the high selectivity obtained with GLSC. Fig. 5 shows the fast and effective separation of some phthalates, compounds of environmental interest, obtained with the same column. In Fig. 6, the separation of some polar compounds, such as alcohols and phenols, on a column prepared with Carbowax 20M as the liquid modifier is reported. Although the capillary column was only 9 m long, it had cu. 20 000 theoretical plates. Owing to the selectivity of the stationary phase and the column efficiency, short capillary columns may be used for fast analysis.
F. BKUNkK
et (II.
96
a
b 5
L Jl_! 4
10
'2
i 4
'2
6
10
6
12
min
Fig. 4. Separation of some pesticides on GLC and GLSC capillary columns. 1 = 3 = aldrin; 4 = lindane; 5 = heptachlor epoxidc: 6 = &BHC; 7 = dieldrin; 10 = p,p’-DDT. (a) SPB 608 fused-silica capillary column (20 m x 0.25 mm gramme, 150°C for 4 min. then increased at 8”C.min to 290°C. (b) Fused-silica x 0.25 mm I.D.), coated with Carbopack F i- SPlOOO. Temperature programme, increased at 13”Cimin to 240°C
I/
11
0
2
s 4
!
3
6
1,.
.I
1
8
IO
I-BHC; 2 = heptachlor; 8 = DDE; 9 = endrin; I.D.). Temperature procapillary column (20 m iOO”C for 2 min, then
I.
12
min
Fig. 5. Separation ofphthalates. I = Dimethyl phthalate; 4 = benzylbutyl phthalate: 5 = ethylhexyl phthalate; column (20 m x 0.25 mm I.D.) coated with Carbopack for 1 min, then increased at lS”C.‘min to 240°C.
2 = diethyl phthalate: 3 = di-lz-butyl phthalate: 6 = di-n-oct),l phthalate. Fused-silica capillary F + SPlOOO. Temperature programme. 125°C
WCOT
COLUMNS
WITH
LIQUID-MODIFIED
GCB
97
12 IO 11
li.!-
0
2
4
6
8
IO min
Fig. 6. Separation of alcohols and phenols. 1 = Hexanol; 2 = 2-octanol; 3 = 1-heptanol; 4 = I-octanol; 5 = l-decdnol; 6 = 2nitrophenol; 7 = 2chlorophenol; 8 = phenol: 9 = 2,4-dimethylphenol; 10 = 2,4-dichlorophenol; 11 = 2,4,6-trichlorophenol; I2 = 4-chloro-3-mcthylphenyl. Fused-silica capillary column (9 m x 0.25 mm I.D.) coated with Carbopack F + Carbowax 20M. Temperature programme, 90°C for 1 min, then increased at IZ”C/min to 140°C. followed by 25”Cmin to 220°C.
Both columns have been used extensively over a period of 8 months and no significant changes in retention and selectivity have been observed. The results reported here confirm the Giddings theory of GSC. Many interesting applications may be foreseen of various types of carbon blacks, differing in surface area, and of different cross-linked liquid phases. REFERENCES F. Mangani and G. Furlani, paper presented at the 7th International SJW1 F. Bruner, G. Crescentini. posium on Capillary Chromatography. Gifu, MaJ 11-14, 1986. 2 C. Vidal Madjar, G. Ganansia and G. Guiochon, in R. Stock (Editor), Gas Chromatography 1970, Institute of Petroleum, London, 1971. p. 20. P. Arpino and G. Guiochon, Anal. Chem., 49 (1977) 3 C. Vidal Madjar. S. Bekassy, M. F. Gonnord. 768. 4 G. Goretti, A. Liberti and G. Notd. Chromatc~graphia, 8 (1975) 486. 5 G. Goretti, A. Liberti and G. Pili, J. High R~soluf. Chromatogr. Chromatogr. Commun.. 9 (1978) 143. 6 A. Di Corcia and A. Liberti, Adv. Chromatogr.. 14 (1976) 305. 7 F. Bruner, P. Ciccioli, G. Crescentini and M. T. Pistolesi, And. Chrm.. 45 (1973) 1851. 8 F. Mangani and F. Bruner, J. Chromutogr.. 289 (1984) 85. 9 Supeko Catalog, No. 25> Supelco, Bellefonte. PA. 1986. 10 F. Bruner and G. P. Cartoni, An&. Cizrnt.. 36 (1964) 1522. I1 J. Giddings, Anal. Chem.. 36 (1964) I 170. 12 L. S. Ettre. in F. Bruner (Editor). The Scicncr of Chromatography. Elsevier, Amsterdam, 1985, pp. 87-106. 13 B. Xu and N. P. E. Vermeulen, Chromalogruphiu, 18 (1984) 520. 14 A. V. Kiselev and Y. I. Yashin. Gas Adsorption Chromatogrupi~~.. Plenum Press, New York, 1969, p. 128. 15 D. M. Young and A. D. Crowell, Ph~xicai Adsorption of Gase.v. Butterworths, London, 1962, p. 54.