Chapter 7 Methodology Advanced Packed Columns

Chapter 7 Methodology Advanced Packed Columns

211 CHAPTER 7 METHODOLOGY Advanced Packed Columns TABLE OF CONTENTS Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...

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211

CHAPTER 7

METHODOLOGY Advanced Packed Columns TABLE OF CONTENTS Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .......... I. Modified Gas-Solid Chromatography . . . . . . . ............................ 1. SilicaGels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Physical Chemistry of Silica Gels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Selection of the Silica Gel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. General Procedures for Chromatographic Applications ........................ 4. Applications to Fast Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Specific Advantages for Industrial Analysis . . . . . . . . . . . . . . . . . . . . a. Column Stability . . . . . . . . . . . . . ................................. b. Sample Volume ............................. ................. c. Use of Steam in the Carrier Gas . . . . . . . . . . . . . . . . . d. Accuracy in Quantitative Analysis .................... 6. Procedure for the Preparation of Modified Silica Gels . . . . . . . . a. Preparation of the Adsorbent . . . . . . . . . . . . . . . . . . . . . . . b. Drying the Adsorbent . . . . . . . . . .......................... c. Coating of the Adsorbent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . d. Thermal Treatment . . . . .................................. 2. Graphitized Carbon Black . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Porous Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. Steam as Carrier Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Production of a Suitable Camer Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. General Procedure for the Use of Steam in the Carrier Gas ....................... 3. Optimization of the Experimental Conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Selection of the Adsorbent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Influence of the Specific Surface Area of the Adsorbent ....................... 3. Influence of the Water Content of the Carrier Gas ........................... 4. Influence of the Column Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Application to the Analysis of Aqueous Solutions . . . . . . . . . . . . . . . . . . . . Literature Cited .............................................

211 213 214 215 217 218 219 225 226

227 228 230 233 234 236 237 237 238 240 241 244

INTRODUCTION During the early 'eighties, following the surge of investments made in this area by many well-established companies and the appearance of several successful, dedicated new ones which was triggered by the expiry of the original patent on open tubular columns (l),the development by Hewlett-Packard of the use of quartz tubing for the preparation of these columns (1) and the publication of various successful recipes for the coating of the open tubular column walls (l), it seemed that gas chromatography was going to become carried out entirely with OTC. We are of the opinion, however, that in spite of the considerable advantages (1) offered References on p. 244.

212

by OTC's over conventional packed columns (CPC), the latter will continue to be used in routine and process control analysis for a long time. In the introduction to this chapter we present a summary of the case for advanced packed columns; in the introduction to Chapter 8, a more detailed discussion of the relative advantages and drawbacks of OTC's and CPC's is given. There is a number of important analyses and analytical applications which can be solved only with the use of packed columns. The analysis of permanent gases, and the analysis of many samples originating in the production of industrial chemicals either cannot be performed with open tubular columns or just do not require their use. Neither the resolution power of very efficient columns, nor the rapidity of OTC are usually required for these separations. Furthermore, for the analysis of many industrial products, the analyst rightly prefers the use of rather large samples, which are more difficult to pollute and easier to handle, and which have a larger probability of being representative of the composition of the feedstocks or products of the plant. The industrial analyst always looks for accurate analyses. These require the injection of the whole sample on the column; sample splitting has been proven to be the source of serious bias. Until quite recently, on-column injection was impossible with OTC's. The situation is now changing, but the new techniques are not yet fully developed, let alone accepted (1).Finally a very large number of these analyses has been performed satisfactorily for 20 years or more with packed columns and few objections have yet been made to the performance of this analytical tool. So, for many practical applications in the industrial laboratory, there is little incentive to change and adopt the OTC, as there is little reason to fix what is not broken. There is already a number of instances of on-line analyses, as well as off-line laboratory analyses, where it has become difficult to perform the required separations, and where still the use of OTC's appears to be unacceptable. Serious doubts are still voiced in a large segment of this rather conservative world regarding the long-term stability of OTC's, a problem which has only recently become seriously addressed by the manufacturers. The solution of many difficult analytical problems of practical importance in industry can be achieved with CPC's by using some approaches which have not been discussed much in the scientific literature but have proven useful during these last ten to fifteen years. They permit the progressive adjustment of column selectivity and the successful elution of the most polar compounds. Both methods discussed here are derived from gas-solid chromatography. The first method is gas-adsorption layer chromatography, where the stationary phase is a regular adsorbent, the surface of which has been modified by adsorption of a certain amount of a non-volatile product, whose presence greatly modifies the properties of the surface, and hence the adsorption energy and the selectivity. The second method uses steam as carrier gas or as a major component of the mobile phase. The presence of a large partial vapor pressure of water in the mobile phase has several important effects. It permits the injection of large samples of water, i.e., the chromatographic system is ideal for the analysis of small amounts of organic pollutants in water or water-rich products. It also permits the suppression of tailing,

21 3

with considerable improvement of band profiles and quantitative results in the case of the analysis of polar or very polar materials. These techniques have permitted the development of extremely selective and rapid methods of analysis using packed columns, resulting in analyses which are often as fast and more accurate and reliable than those based on the use of open tubular columns.

I. MODIFIED GAS-SOLID CHROMATOGRAPHY Modified gas-solid chromatography, also called gas-adsorption layer chromatography by Halasz (2), is a retention mechanism which uses an adsorbent covered by a stable layer of an organic or inorganic compound, whose presence modifies the chemical composition of the adsorbent surface, hence the nature and intensity of the molecular interactions taking place with the different analytes contained in the studied mixture. As a result the adsorption energy of each component of the mixture is usually much decreased, permitting the use of the sorbent material at a much lower temperature. At the same time, the selectivity of the stationary phase is much greater than that of any solvent and especially that of the solvent used to prepare the layer covering the adsorbent surface. Finally, by changing the nature of the modifying solvent and its surface concentration, it is possible to adjust absolute and relative retentions over a wide range. The advantages of this method over classical gas-solid or gas-liquid chromatography are of major importance. It permits a considerable reduction in the analysis times, a much larger flexibility in the adjustment of the selectivity of the stationary phase to the requirements of a new problem and the achievement of much greater column efficiencies at high carrier gas velocities. Modified gas-solid chromatography (mGSC) has been studied by numerous scientists, especially Halasz and Heine (2), Halasz and Horvath (3), C.G. Scott (4), Vidal-Madjar and Guiochon (5,6), Kiselev et al. (7), Di Corcia and Liberti (8). In fact, as will become obvious in the following, mGSC is the systematic use of a phenomenon which very often plagues the uninformed analyst: when a non-volatile solvent is spread on a solid of significant specific surface area, the properties of the stationary phase obtained are rarely those which could be predicted from the thermodynamical properties of the bulk solvent and the relationships derived in Chapter 3 from solution thermodynamics. The reason is due to the very small thickness of the liquid film formed at the surface of the solid and to the modifications of the properties of the solvent by the presence of a high-energy underlying solid, which modifies the structure of the liquid in the thin film (9). We have shown in the previous chapter (Chapter 6, Section III.3.a, Selection of Phase Ratio) that when the relationship between retention volume and coating ratio (in theory a strictly proportional relationship) is studied. experimentally, it is observed in most cases that the specific retention volume is a minimum for a certain value of the coating ratio (cf Figures 6.17a and 6.17b). Modified gas-solid chromatography is carried out with a solid of specific surface area greater than that of a References on p. 244.

214

regular support for GLC, at coating ratios lower than the one corresponding to that minimum, but not much smaller (see Figure 6.17b). Mixed retention mechanisms may take place in this range, but it has been recognized that, because the liquid film is very thin, most often close to a monomolecular layer or even less, the phenomenon is essentially adsorption on a modified surface. Modified gas-solid chromatography requires the combination of a solid adsorbent and a liquid solvent. Three types of adsorbents have been used extensively in the applications of the method: - inorganic, heterogeneous, polar adsorbents, such as silicagels (especially Spherosils), - non-polar, homogeneous adsorbents such as graphitized carbon blacks (especially Carbopacks). - organic adsorbents, such as porous polymers (especially Porapaks and Chromosorb 100). Solvents are mostly the very polar stationary phases of GLC, polyglycols, 3,3’-oxydipropionitrile, polyimines, etc. The properties of the stationary phases obtained by coating an adsorbent with small amounts of different solvents may be very different from the classical chromatographic properties of the pure materials. For example, the association of a very polar adsorbent such as silica and a very polar solvent, such as 3,3’-oxydipropionitrilemay be surprisingly non-polar (2). The properties of the new stationary phase depend on the nature of the reagents involved, on the surface energy of the adsorbent, on its specific surface area, etc. (10). We present here the conclusions of systematic studies carried out essentially with one kind of adsorbent, Spherosil, a silica gel available with a wide range of characteristics. The extension of these results to other adsorbents is also discussed. 1. Silica Gels

Silica gel is a generic name for an adsorbent made of silicon oxide. There is a huge number of different silica gels, about as many as there are different batches of the product prepared. They come in families, of which there are as many as there are commercial brands. They are characterized by their specific surface area (from 5 to 800 m2/g), their pore volume or porosity (from 0.3 to 1 mL/g), the average pore diameter (10 to 200 nm) and pore size distribution, the particle shape (spherical, ovoid or irregular), size (from about 1 pm to 500 pm) and size distribution and the surface chemistry (11-13). As an example, Figure 7.1 shows a plot of the pore diameter versus the specific surface area for Spherosil silica gels. Silica gels have been the basis for many stationary phases in gas and liquid chromatography, because of their excellent mechanical properties. They can be packed with minimum attrition and can withstand considerable pressures. They resist all chemicals used in gas chromatography, and especially oxygen and easily withstand temperatures up to 600 O C without change in their structure. The surface chemistry changes however, by heating under a stream of dry gas. Most of the work reported here was done in Guillemin’s laboratory, using Spherosil (Prolabo, Paris, France), a brand of silica gel manufactured by Rhone-

21 5

Spherosil

Figure 7.1. Plot of the average pore diameter of Spherosil silica gels (Angstrom) versus the specific

surface area (rn*/g).

Poulenc. It comes in a variety of specific surface areas and pore size distributions. Results obtained have shown that this product is reproducible and stable. I . Physical Chemistry of Silica Gels

In Chapter 3 we have shown that in gas-liquid chromatography, the corrected retention time is proportional to the amount of liquid phase (solvent) contained in the column, while in gas-solid chromatography it is proportional to the total surface area of the solid surface of adsorbent contained in the column, i.e. to its mass and its specific surface area. In gas-adsorption layer chromatography, the retention time, t,, is given by (lOJ4):

where L is the column length, ii the average carrier gas velocity, K , the equilibrium constant between the two phases, S the specific surface area of the adsorbent, p the weight of stationary phase contained in the column, d , the film thickness, and V, the volume available to the gas phase. If a series of adsorbents of similar chemical nature is prepared without special care, however, the true retention time is not going to be strictly proportional to the specific surface area of these products (10,15,16). The retention time depends rather on the adsorption (Henry’s) constant, which is also a function of the chemical nature of the surface and may vary greatly with the method of preparation and even References on p. 244.

216

H

I

"\Si /O

/\ Geminal

I

H I

0"

#*H\

I

Si

H

I

0

0

Si

m

A m

Free

Siloxane

I

A A Bound reactive

I

/O\

Si

Si

Si

Figure 7.2. Structure of the surface of silica gel (after Snyder, ref. 17).

with some minor changes in the experimental conditions of a specific method (15). The energy of adsorption depends on the number of silanol groups per unit surface area, on the activity of these groups, and hence on the nature of the chemical treatments applied to the product during its manufacture. The density of silanol groups on the surface of silica gels after careful drying under partial vacuum at 150-2Oo0C is approximately 5 groups per 100 A', i.e., an average of about 1 free silanol per silicon atom on the surface (12,15). There are four main categories of silanol groups, coexisting in various proportions on any silica surface (cf Figure 7.2): free silanols, geminal silanols, bound reactive silanols and siloxane groups (11,12,15,17). If the column capacity factor is determined for

-

1

GC

-I LC - GPC

Figure 7.3. Plot of the column capacity factor for an arbitrary compound versus the specific surface area of Spherosil silica gels. The range of useful applications for GC is from 10 to 400 m2/g. For liquid chromatography and size exclusion chromatography it ranges from 50 to 800 m2/g.

217

materials of the same origin, with increasing specific surface area, up to 400 m2/g in gas-solid chromatography, 800 m2/g in LC, two changes of the surface energy are observed. One takes place between 30 and 50 m2/g, in agreement with Kiselev’s data (15), the other between 600 and 800 m2/g (see Figure 7.3). This latter change has no application in gas-solid chromatography. Lower specific surface area Spherosil particles are prepared by a hydrothermal treatment of the original 400 m2/g product. Progressive dehydration takes place, first reversibly, then irreversibly. It causes parallel changes in the surface chemical composition and adsorption energy. The nature and density of the silanol groups change. The geminal and free groups disappear, and the density of siloxane bridges increases at the same time. Since the solid surface is highly heterogeneous, so is the liquid phase layer sorbed on the solid surface. The structure of the liquid film, usually a fraction of a monolayer thick, is very different whether it is close to highly active silanols or over the siloxane groups; it is very different from the structure of the bulk liquid, due to the orientation of the solvent molecules by the strong electrical forces near the silica surface. Selective adsorption of the solvent molecules is compounded by the Kelvin effect which changes the density of the liquid distribution, especially inside small pores, a capillary condensed liquid coexisting with the partial monolayer film. This complex structure has been discussed by Giddings (18) and Serpinet (19). The nature of the silica surface is further discussed by Kiselev and Yashin (15), Snyder (17), Unger (11) and Serpinet (19). 2. Selection of the Silica Gel The rules to be followed for the selection of the adsorbent, the adsorbate (or modifier) and the coating ratio depend on the conclusions of the previous discussion. As the problem is extremely complex, a few simplifying assumptions are made. These assumptions lead to approximate results. When the parameters describing a separation are critical, it may be useful to check whether some modifications of the experimental conditions thus derived would result in improved performance. The assumptions made are as follows: - We consider only adsorbents with specific surface areas not exceeding 200 m2/g. - The surface will be considered to be homogeneous, i.e. the retention time is assumed to be proportional to the specific surface area. - The layer of adsorbate will also be considered to be homogeneous; its thickness is supposed to be constant. The selection of the adsorbent is based on the properties of the compounds to be separated. The following rules are applied: - The higher the boiling point of the components of the mixture, the smaller the specific surface area of the support adsorbent used. - The higher the polarity of the analytes, the smaller the specific surface area of the adsorbent selected. - Depending on the position of the analytes in the Kiselev (12,15) classification (cf Chapter 6, Section I.l), molecules of the groups A and B are separated on an References on p. 244.

218

adsorbent which has a large or moderate specific surface area (depending on the two parameters just discussed); molecules of the groups C and D are analyzed on a small or moderate specific surface area adsorbent. 3. General Procedures for Chromatographic Applications (20)

Retention times in modified gas-solid chromatography depend on the specific surface area of the adsorbent used, at constant film thickness of the modifying solvent, and on the column length. Accordingly, analyses are first carried out on a series of columns packed with an intermediate grade of silica gel coated with the same solvent at different film thicknesses. The retention times of a few compounds selected for their importance or their relevance to the analysis under study are determined and the plots of the corrected retention times, ti, versus the film thickness, d , , are drawn. The film thickness at which the retention times are a minimum is usually selected (cf Figure 6.16). If different grades of Spherosils are used *, we have observed that when the coating ratio is adjusted to keep the film thickness constant, the selectivities of the stationary phases obtained are the same. Only the corrected retention times change, not their ratio. These corrected retention times are proportional to the product of the specific surface area of the adsorbent, S, by the column length, L (the column diameter is constant, so the product SL is itself proportional to the total surface area of adsorbent in the column). In practice, if the film thickness were kept constant, the retention time would depend only on the product SL and the column temperature. A 2 m long column packed with a 400 m2/g adsorbent gives the same corrected retention time as a 4 m long column packed with a 200 m2/g adsorbent. This is illustrated in Figure 7.4, which shows the chromatograms obtained for a mixture of aromatic hydrocarbons at the same temperature, with three columns packed with particles of three different grades of silica gels, coated so as to keep the film thickness constant (cu 9 Angstrom). The column length is inversely proportional to the specific surface area of the adsorbent. The carrier gas flow velocities have been adjusted to achieve the same corrected retention times for each compound on the three columns, which is possible only because the relative retentions of the different components of the test mixture are the same on all columns. Another advantage of the columns packed with modified adsorbents is the very rapid mass transfer between the mobile and the stationary phase. This results in a very flat Van Deemter curve (cf Figure 7.5). The thinner the film of liquid phase on the adsorbent surface, the better the efficiency, at least so long as all active sites of the adsorbent surface are deactivated by the liquid phase. As a consequence, the optimum velocity is larger and there is a smaller loss of efficiency associated with the selection of a gas velocity larger than the optimum value. Combined with the The same should be true for other brands of silica gels, but does not apply to products of different brands.

219

I

GC

1

0 5: 28 rn2/g

L : 5rn

+ Corbowax 2 0 M

B)

L: 1.50m t

t

5 9 6 rn2/ g

Carbowox 2 0 M

L: 0.70 m

2gllOOg

6.7Og/lOOg

5:200 rn2/g

Carbowox 2 0 M

14g/lOOg

Figure 7.4. Chromatograms of a given mixture on three different columns, in gas-adsorption layer chromatography, with constant product SL (L, column length, S, specific surface area of the Spherosil) (10). 1, Methane; 2, benzene; 3, toluene; 4, ethylbenzene; 5, styrene. Columns: 1 nun id., packed with Spherosil coated with Carbowax 20M. Temperature 130 C. Carrier gas nitrogen. (A) Column length: 5 m. Specific surface area: 28 m2/g. Particle size: 160-180 pm. Flow rate: 0.85 L/h. (B) Column length: 1.5 m. Specific surface area: 96 m2/g. Particle size: 80-100 pm. Flow rate: 0.58 L/h. (C) Column length: 0.70 m. Specific surface area: 200 m2/g. Particle size: 80-90 pm. Flow rate: 0.26 L/h. Flow rates have been adjusted to show that if the corrected retention times of one compound on the three columns are equal, this is valid for all other compounds. Reprinted with permission of Journal of Chromarography, 158, 21 (1978).

other characteristics of modified silica gels, this makes the achievement of very rapid analyses possible, as we show in the next section. 4. Applications to Fast Analysis

As illustrated in this section, the systematic use of simple rules makes it possible References on p. 244.

220

5

0

10

-

U cm/sec

Figure 7.5. Plot of column plate height versus the flow velocity (Van Deernter plot). (A) Partition chromatography. Fluidized Chromosorb P (see Section III.1.3), specific surface area 4 m2/g, particle size 125-150 pg, coated with 20%(w/w) Carbowax 2OM. (B) Modified gas-solid chromatography. Spherosil, specific surface area 200 m2/g, particle size 100-106 pm, coated with 14%(w/w) Carbowax 20M.

to achieve a considerable reduction of the analysis time. Compared to the performance usually obtained with conventional gas-liquid chromatography, a reduction of the analysis time by a factor of about 30, while keeping the resolution constant, is typical (21). We have developed a four-step procedure to optimize the experimental conditions of an industrial analysis. These steps are described here, with the application to the development of the separation of a mixture of chlorohydrocarbons (vinylidene chloride, CH =CCl , methylene chloride, CH ,C1 ,, carbon tetrachloride, CC14 r 1,2-dichloroethane, CH,Cl-CH,Cl, 1,1,2-trichloroethane, CHCl ,-CH,Cl, vinyl chloride, CH,=CHCl) and benzene, as an example (cf Figures 7.6A to 7.6E). Figure 7.6A shows the chromatogram obtained with a conventional gas-liquid chromatography column (20%Carbowax 20M on Chromosorb P, 145-175 pm). The film thickness of Carbowax 20M is about 300 Angstrom, which is conventional. The column efficiency is good: H = 0.5 mm, reduced plate height (cf Chapter 4) cu 3, but the flow velocity, 6.6 cm/sec, is rather slow and the analysis time long, about 23 min, which may be too long for an on-line analyzer placed in a closed loop control in a chemical plant. To accelerate the analysis, the first step is to replace the support by a silica gel adsorbent. The use of Spherosil 28 m2/g, combined with a decreased liquid phase loading (24%only, corresponding to a film thickness reduced from 300 to about 17 Angstrom), permits the achievement of a greater efficiency and of a very large reduction in the analysis time, which is now only 5 minutes (see Figure 7.6B). The resolution of all components is excellent, since the recorder trace returns to base line between the peaks. The relative retentions have changed, however, due to the difference in selectivity observed between gas-liquid and gas-adsorption layer

,

,

'

,

221 7

I,. 0

7

1

! J

Figure 7.6. Optimization of the experimental conditions for an analysis using modified gas-solid chromatography. 1, Vinyl chloride; 2, vinylidene chloride; 3, methylene chloride; 4, carbon tetrachloride; 5, benzene; 6, 1.2-dichloroethane; 7, 1.1.2-trichloroethane. Temperature: 130 C. Carrier gas: nitrogen. (A) Conventional GLC. Column: 4 mm id., 4 rn long. Support: Chromosorb P (particle size: 145-175 pm). 20% Carbowax 20M. Inlet pressure: 1.8 atm. Flow rate: 3 L/h. Sample size: 1 pL. (B) Modified GSC. Column: 4 mm i.d.. 4 m long. Support: Spherosil 28 m2/g (particle size: 125-200 pm). 2% Carbowax 20M. Inlet pressure: 2.5 atm. Flow rate: 3 L/h. Sample size: 1 pL. (C) Modified GSC. Column: 1 mm i.d.. 5 m long. Support: Spherosil 28 m2/g (particle size: 125-200 pm). 2% Carbowax 20M. Inlet pressure: 5.0 atm. Flow rate: 0.9 L/h. Sample size: 0.1 pL. (D) Modified GSC. Column: 1 mm i.d.. 0.70 m long. Support: Spherosil 200 m2/g (particle size: 100-110 am). 14% Carbowax 2OM. Inlet pressure: 2.7 atm. Flow rate: 0.63 L/h. Sample size: 0.05 pL. (E) Modified GSC. Column: 1 mm i.d.. 0.70 m long. Support: Spherosil 200 m2/g (particle size: 100-110 pm). 14% Carbowax 20M. Inlet pressure: 5.0 atm. Flow rate: 1.6 L/h. Sample size: 0.1 pL. References on p. 244.

222

chromatography. A similar reduction in analysis time would be observed with the use of 5% Carbowax 20M on Chromosorb P, but the resolution of the early eluted peaks would have been very poor. Since the resistance to mass transfer in the stationary phase is considerably reduced and the plot of column plate height versus carrier gas velocity is very flat, the column can be operated at a much greater velocity than for Figure 7.6B, without experiencing a serious loss of efficiency. Because most detectors do not operate well at high carrier gas flow rates, this requires the use of a narrower column. On Figure 7.6C the result of the second step can be seen. The flow velocity has been multiplied by 4.8 (the flow rate has been multiplied by 0.3, but the column diameter has been divided by 4 and its cross section by 16) and it is now 32 cm/sec. Because the column is 1 m longer and the pressure drop is greater than on the previous column (cf Chapter 2, Compressibility Factor), the retention time is divided only by 2. In the third step, we increase the specific surface area of the support used, which permits a proportional reduction of the column length, and keep the corrected retention times constant (cf Figure 7.6D). This is done at constant film thickness, to keep the selectivity of the stationary phase and the relative retentions constant. A reduction of the analysis time by a factor of 2 results from the considerable reduction in the pressure drop and the consequent increase of the value of the compressibility factor j . The gas hold-up time is also reduced. A reduction of the particle size, from about 160 pm to 105 pm permits the achievement of a greater efficiency, with a plate height of 0.5 mm in spite of the rather high flow velocity. As a consequence, a base-line resolution is still observed in Figure 7.6D. Finally, the carrier gas flow velocity can still be increased, by raising the inlet pressure to the maximum we can afford within the constraints of our equipment and those of the working conditions in an industrial laboratory carrying out routine analysis, i.e., 5 atm. This is the final or fourth step. The chromatogram is shown on Figure 7.6E. The flow velocity has been raised to 57 cm/sec, and the retention time has been reduced to 36 sec, admittedly with quite a significant loss of resolution this time. A better result could probably have been achieved by a reduction of the column length rather than by an increase in the carrier gas flow rate. This approach is illustrated by the chromatograms on Figure 7.7. Two very short columns, 4 and 8 cm long, respectively, packed with 25-40 pm particles have been used. They provide excellent separations of the mixture components except the first two compounds (CH,=CHCl and CH,=CC12), because the column capacity factor for the second of them is too small (cf. Chapter 1, equation 35). The carrier gas velocities are relatively close to the optimum values. Velocities of 7.8 and 23.4 cm/sec, respectively, have been used with these two columns, corresponding to reduced velocities of approximately 3.2 and 10. The second chromatogram (Figure 7.7B) shows an HETP of about 0.1 mm for the last peak, a reduced efficiency of about 3, similar to the value achieved with most columns we have operated in GC. Huber et al. have investigated the use of very narrow particles in gas chromatography and have shown that very small HETP can be achieved (22). The results shown on Figure 7.7 are in agreement with the conclusions of their work. Unless very short columns can be used, however, the advantages of these columns are offset

223

2

2

@ L

4 5 scc

Figure 7.7. Fast analysis by gas-adsorption layer chromatography, using very short columns. 1, Vinyl chloride; 2, vinylidene chloride; 3, carbon tetrachloride; 4, benzene; 5, 1,2-&chloroethane; 6, 1,1,2-trichloroethane. Spherosil 200 m2/g, coated with 14.5% 3,3'-oxydipropionitrile. Particle size: 25-40 pm. Column temperature: 85 C. Carrier gas: nitrogen. Column diameter: 1 mm. (A) Column length: 8 cm. Flow rate: 0.66 L/h. Inlet pressure: 1.9 atm. Sample sue: 0.05 pL. (B) Column length: 4 cm. Flow rate: 0.22 L/h. Inlet pressure: 0.75 atm. Sample size: 0.02 pL. Reproduced with permission of the Journal of Chromatography, 139, 259 (1977).

by the requirement of a very large inlet pressure, often prohibitively large in practice. This illustrates the theoretical findings that, in chromatography generally and especially in GC, easy separations can be carried out very rapidly, using very small particles or very narrow open tubular columns (23). Because of excessive pressure requirements, however, difficult separations have to be carried out with much coarser packing material, at the cost of a considerable increase in analysis time. In the analysis of mixtures of light chlorohydrocarbons discussed here, as in the case of many important analyses in the heavy chemical industry, separations are usually not very difficult and very fine particles can be used to achieve very fast analysis when needed. As shown on Figures 7.7A and 7.7B, the pressure requirement to achieve a reasonable reduced velocity of 10 is well within the range of capability of current commercial equipment (1.9 atm). The quality of the separation would be improved, however, if an apparatus designed to be used with capillary columns were to have been used: the dead volumes and the response time of the chromatograph used to carry out the chromatograms shown on Figure 7.6 are,too large and contribute quite significantly to the band width.

224

With these very short, efficient columns, extremely small sample sizes must be used. This also pushes the capability of the instrument to the limit. Figure 17.27 shows an application of fast GC analysis to the control of pollutants in the atmosphere of a workshop. The chromatogram has been obtained with a process control gas chromatograph. Other similar analyses have been described in the literature (21,24). Such short columns, operated at room temperature, are also used in the portable chromatographs utilized for air pollution monitoring. 5. Specific Advantages for Industrial Analysis

In the previous sections we have explained the meaning of modified gas-solid chromatography, how it is carried out with silica gels and what kind of results can be obtained with some of the most classical and useful adsorption layers prepared by coating silica gels with polar solvents. In this section we discuss the major m2/g

+ 5%

2Omln

PEG 400

15

10

5

0

Figure 7.8. Quantitative analysis of trace impurities in 1,2-dichloroethane.

1. Methylene chloride; 2, 1,l-dichloroethane; 3, benzene; 4, trichloroethylene. Column: 4 mm id., 4 m long. Spherosil 83 m*/g, coated with 5 % Polyethyleneglycol 400. Carrier gas, nitrogen. Flow rate 3 L/hour. Quantitative analysis (response factors have been determined using the gas density balance, as explained in Chapter 14): results are given in Table 7.1.

TABLE 7.1 Quantitative Analysis in Modified GSC

CH2CI2 CH 3 -CHCI2 C6H6

CHCl= CCl, Cf. Figure 7.8.

Concentration (ppm) Standard

Found

(%I

Difference

66 102 41 94

71 101 int. std. 89

+7 -1 -5

225

advantages of the technique over classical gas-liquid chromatography. For a discussion on the weaknesses and difficulties of application of gas-solid chromatography in the analysis of all mixtures, except gases, see Chapter 3, Section B.VI. The main advantages we have found are the following: (i) the long term stability of the columns, (ii) the flexibility in the adjustment of relative retention times by changing the coating ratio (film thickness), (iii) the large sample volumes which can be injected without overloading or flooding the column and (iv) the possibility of using steam as a component of the mobile phase. Finally, we show that this technique permits the achievement of quantitative analyses which are as accurate as those obtained by other retention mechanisms (see Figure 7.8 and Table 7.1). a. Column Stability We have observed over the years that the stability of the performance of modified gas-solid chromatography columns is excellent. It much exceeds that of conventional columns prepared with the same stationary phase and operated at the

1.5 pi

i I

J

Figure 7.9. Application of modified GSC to process control analysis. Analysis of trace impurities in

1.2-dichloroethane. 1, Trichloroethylene; 2, benzene; 3, 1,2-dichloroethane; 4, 1,1,2-trichloroethane; 5, 1,1,2,2-tetrachloroethane. Column 1 mm i.d., 2.50 m long. Spherosil 55 m*/g, coated with 9% hexakiscyanoethoxyhexane. Carrier gas: nitrogen, 0.36 L/h. Temperature: 108OC. Sample sizes: 1 pL (right) and 1.5 pL (left). Since the last component ( # 5 ) would have a very long retention time, a column switching valve permits the elution of components 1-4 on the entire column,while component 5 is eluted only on a short (ca 50 cm) section (cutting procedure). The valve is actuated at B (see Figures). For details on column switching see Chapters 9 and 17. References on p. 244.

226

same temperature. The organic solvent sorbed on the silica gel surface has a vapor pressure at a given temperature which is much less than that of the same solvent in bulk. This is due to the strong adsorption energy of a polar solvent on a polar adsorbent. As a result the upper temperature limit of the column is quite a lot higher in gas-adsorption layer chromatography than it is in gas-liquid chromatography. For example, 3,3'-oxydipropionitrile coated on Spherosil can be used routinely at 70°C, without noticeable base-line shift, but with a column lifetime of several months. b. Sample Volume The sample volume depends on the curvature of the equilibrium isotherm (cf Chapter 5) and the total surface area of adsorbed layer contained in the column (in modified GSC) or the total volume of solvent (in GLC). This total surface area or volume is of course proportional to the surface area of the column cross-section. Silica gels are available in a large range of specific surface areas. Packing material used can be selected to afford the required loading capacity. The sample size is sometimes determined by the detector's detection limits, but often by the fact that automatic sample valves used in process control analyzers are unable to properly deliver sample volumes smaller than 0.5 pL. This might be one of the most stringent reasons why open tubular columns are not used in process control chromatographs (stream splitting is not compatible with the achievement of precise quantitative analyses). The chromatograms on Figure 7.9 show that 1 mm i.d. conventional columns packed with coated Spherosil perform well with samples of 1 and 1.5 pL. This would correspond to a 16 or 24 pL sample, respectively, on a 4 mm i.d. packed column. The column is somewhat overloaded for the major component, but the resolution of the trace impurities (100 ppm level) is still very good, provided the first compound eluted after the major component is well resolved from it. c. Use of Steam in the Carrier Gas We have already mentioned the increased stability of the modified GSC columns compared to conventional columns where the same solvent is coated on a low specific surface area support. We have observed this phenomenon to be especially important when steam is incorporated in the carrier gas (cf Section I1 of this chapter), so much so that we consider the use of steam to be nearly impossible with most conventional gas-liquid chromatography stationary phases.

d. Accuracy in Quantitative Analysis Modified GSC gives quantitative results as precise as those of GLC. As an example, the data in Table 7.1, corresponding to the chromatogram shown on Figure 7.8, show that the accuracy of the quantitative analysis carried out with a modified GSC column is the same as that obtained by conventional GLC. The response factors used for the analysis have been determined using a conventional GLC column and the gas density detector (cf Chapters 10 and 14). The results obtained show that there is no loss of sample component by strong adsorption.

227

6. Procedure for the Preparation of Modified Silica Gels The preparation of a good packing material for modified gas-solid chromatography requires that several steps - washing, drying, coating and thermal treatment of the material - be thoroughly performed. Strict adherence to the following procedure should result in a satisfactory stationary phase. Although most of the results discussed above have been obtained with Spherosil, we are of the opinion that similar results could be obtained with other silica gels, provided they are chosen among those which have similar physical and physicochemical properties. This statement is supported by the results obtained by Lin, Pfaffenberger and Horning (25), who have prepared open tubular columns with a wall coated by a layer of Silanox, impregnated with various amounts of non-polar solvents, such as silicone oils. Excellent analytical results were obtained. The use of polar phases was less successful, however (26). Further discussion of results obtained with modified GSC open tubular columns is presented in Chapter 8. a. Preparation of the Adsorbent The adsorbent must first be washed very carefully to eliminate traces of inorganic materials, especially sodium, contained in the silica and of organics adsorbed on its surface. The adsorbent is washed with concentrated (68%) nitric acid for 8 hours, in a rotary evaporator, at room temperature (slow rotation on, no vacuum or gas stream). The product is then washed with distilled, sodium free water until a p H of 7 is achieved. It is then dried in an oven. The comparison between the two chromatograms on Figure 7.10 illustrates the extreme importance of a thorough nitric acid wash. On Figure 7.10A the alcohols give terribly tailing peaks and the material is certainly unsuitable for any analysis involving alcohols. After careful washing the same material gives an excellent chromatogram for the same mixture. b. Drying the Adsorbent T h s is another step of major importance. Failure to use a carefully dried silica gel for the coating step invariably results in a stationary phase which exhibits poor performance, low efficiency and tailing peaks. It seems that the reason for this behavior is related to the explosive vaporization of the microdroplets of water condensed in the bottom of pores in the silica gel by the Kelvin effect. This brutal phenomenon which takes place at a temperature somewhat higher than 100 O C (Kelvin effect) results in the bursting of the film of solvent coated on the adsorbent surface, releasing unprotected active sites on the surface of the silica. The result is a stationary phase offering mixed retention mechanisms, with adsorption on a free silica surface, which invariably leads to unsymmetrical, strongly tailing peaks (cf Chapter 3, Section A.IX and Figure 7.10). The drying aims at eliminating all the water which is physically sorbed. It does not change the nature and density of the silanol groups on the silica surface. It is carried out at 15OoC, for 2 hours, under vacuum. For this operation, the silica gel References on p. 244.

228

Figure 7.10. Influence of the nitric acid washing of Spherosil on the performance of a packing material. 1, Ethanol; 2, 2-propanol; 3, I-propanol; 4, 2-butanol; 5, 2-methyl-1-propanol; 6, I-butanol; 7, 2-methyl-1-butanol;8, 3-methyl-1-butanol. Columns: 1 mm i.d., 2 m long. Spherosil18 m2/g coated with 1.3%Carbowax 20M. Carrier gas: nitrogen, flow rate: 0.25 L/h. Temperature 100 C. (A) Unwashed Spherosil. (B) Spherosil washed as described in text.

should be placed in the same vessel used to coat it in the next step. After the adsorbent is dried, it is brought to atmospheric pressure by introduction of dry nitrogen, and then cooled to room temperature. c. Coating of the Adsorbent

Any classical liquid phase used for gas-liquid chromatography may be coated on Spherosil. The coating of a non-polar phase is much more difficult, however, than the coating of a polar solvent. It is rarely attempted and the operation is tricky. The washing and drying of the adsorbent must be performed with care. Preferred liquid phases are: - Carbowax 20M, - 3,3'-oxydipropionitrile, - Squalane. The coating ratio can be chosen using the nomogram in Figure 7.11. This graph permits the calculation of the coating ratio (amount of stationary phase for 100 g of adsorbent), as a function of the specific surface area of the support and the desired film thickness. It is necessary to know the density of the solvent used.

229

S

m2/g

1

100

90 80

70 60

50 40 30 20 10

2

*

df 8,

*

'.5

Figure 7.11. Nomogram for the calculation of the coating ratio. Abscissa: film thickness (Angstrom). Ordinates: right, specific surface area, left, coating ratio. p : density of coated liquid. w = 100 dJp To obtain a 15 Angstrom thick film on a 30 m2/g silica, with a liquid of density 1.5, the analyst draws the line 00 on the upper part of the graph, then the vertical from the intersection point between the horizontal at p = 1.5 and the slanted line corresponding to df = 15. The intersection of this vertical and the line 00 is at c, which corresponds to the coating ratio 6.8%.

The dried adsorbent is mixed with pure methylene chloride in a rotary evaporator. The solvent is gradually drawn into the slowly rotating flask, until the silica gel is covered by a solvent layer of about 1 cm. The flask is kept rotating for another 30 minutes. After that a solution of the desired amount of stationary liquid in an excess of methylene chloride is slowly added to the flask. The mixture is slowly rotated for one hour before vacuum and/or heat is applied to vaporize the solvent. References on p. 244,

230

It is very important, again, to use extremely dry methylene chloride and stationary liquid. The methylene chloride is dried as follows. Molecular Sieve 13X is dried at 300 O C for 3 hours under a stream of dry nitrogen, then cooled under dry nitrogen. The methylene chloride is kept for 48 hours in a carefully closed glass bottle in contact with a large amount of this Molecular Sieve. The rotating flask is slowly heated to vaporize the methylene chloride. This operation should proceed slowly and take at least three hours. d. Thermal Treatment This treatment improves the thermal stability of the stationary phase. Several phenomena are involved and their relative importance is still unknown. - mechanical effect: the viscosity of the liquid phase decreases considerably at high temperature. This favorizes an even spreading and the filling of the smallest pores. - physico-chemical effect: the liquid phase molecules migrate on the surface to the sites of highest adsorption energy. - chemical effects. It seems that certain phases, such as Carbowax may react with chemical groups on the surface, silanols or siloxane bridges, and become bound to it. The thermal treatment is carried out at the following temperatures: - 3,3’-oxydipropionitrile at 90 O C for 3 hours, - squalane at 150O C for 10 hours, - Carbowax 20M at 200 O C for 3 hours. The thermal treatment can be carried out either on the bulk material prepared, in an oven, or on the packed column, under a stream of nitrogen, immediately before use. 2. Graphitized Carbon Black

Graphitized carbon black (GCB) is one of the most reproducible adsorbents known (12). Its surface is highly homogeneous. It is prepared by the treatment of thermal carbon blacks at 2,700-3,000 O C, under an inert atmosphere (graphitization). In spite of this treatment active groups exist on the surface. Also the surface is reactive enough to capture oxygen above 300 O C, resulting in the appearance of a variety of selective adsorption sites: free radicals, phenols, ketones, quinones, and carboxylic groups. When the GCB is freshly prepared and has been exposed only to low temperatures, the free radicals, which seem to be the most abundant active sites on the surface of this adsorbent, can be reacted by soaking the powder for a week in a concentrated solution of styrene. Oligostyrene chains grow on the surface, permitting the easy dispersion of the powder in organic solvents, by reducing the strong interactions between the 001 graphite facets of different polyhedron particles which constitute graphitized thermal carbon black. Although impressive results have been obtained with this material used in pure GSC (8,12,15, 27-37), the practical applications have been limited. The very high surface energy of graphite results in high adsorption energies and very large

231

1.3

r

0.0-

-c-

- -- - -- I . l I 1

I

- - - - - - - - - - - - _- -_ - _ -

I

,

I

, I , /

I

I , I , I , I , I ,

Figure 7.12. Relative retention of some free acids on FFAP modified graphitized carbon black Sterling FT-G (60-80 mesh). (After Di Corcia, ref. 38). Squares: p/m-methylbenzoic acids. Open triangles: 3/2-methylbutyric acids. Solid triangles: m /o-chlorobenzoic acids. Reverse triangles: m /o-chlorobenzoic acids. The dashed lines indicate the values obtained with FFAP in pure gas-liquid chromatography. A, p/m-methylbenzoic acids; B, p/m-chlorobenzoic acids; C, m /o-chlorobenzoic acids. Reproduced with permission of Analytical Chemistry, 45, 492 (1973).

retention volumes for most solutes, except those (like pinenes, borneol, adamantane) whch have a structure preventing their molecules from lying on the flat 001 surfaces (28). Most other compounds cannot stand the temperatures which would permit their elution without experiencing some thermal degradation, in spite of the inertness of the graphitized carbon black (35). The few selective adsorption sites may also result in tailing peaks for polar compounds. Because of their stability, inertness, large surface energy and relative surface homogeneity, graphitized thermal carbon blacks are very suitable adsorbents for modified GSC. Carbopack B and C have been studied extensively by Liberti, Di Corcia and Bruner (33,34). Their specific surface areas are 100 and 10 m2/g, respectively. Carbopack products have been treated under hydrogen at 1,000 to 1,20OoC to eliminate a large fraction of the heteroatoms still present in the graphitized carbon black. Total hydrogenation is not possible, unfortunately. The general behavior of graphitized carbon blacks in modified GSC is similar to the one observed with silicagels as previously described. For example, the retention times of several organic acids on Sterling FT (specific surface area: 12 m2/g), coated with FFAP, decreases constantly with increasing film thickness, or coating ratio, in the range studied (0.2 to 2.5%), as shown on Figure 7.12. A monomolecular layer is not yet reached for a 2.5% coating ratio, which explains why in this case a minimum is not observed. The decrease in retention time is ascribed first to the decrease in the References on p. 244.

232

density of selective adsorption sites with increasing coating ratio (especially the rapid decrease in the range 0.3-0.9%, Figure 7.12), and then, to the increasing degree of microheterogeneity of the adsorbent surface brought about by the polymer chains occupying an increasing part of the carbon surface and preventing analyte molecules from lying flat on the carbon surface (28). The selectivity is a function of the coating ratio (38). Excellent analytical results have been reported by Vidal-Madjar et al., using Sterling FTG (Cabot, Boston, MA, U.S.A.), coated with various amounts of Carbowax 20M, Free Fatty Acid Phase (FFAP) and Polyphenyl ether sulfone (ASL). The material has the same ability as pure Sterling FTG to separate geometrical isomers, but carbazole and azaarenes are eluted with very symmetrical peaks (36). The large column loadability permits the identification of trace compounds by GC-MS at concentration levels much below that possible to achieve with conventional columns (37). Graphitized carbon blacks and especially Carbopacks are remarkably useful for the analysis of very polar analytes which exhibit differences in their geometrical structure and their polarizability. Cis-trans double bond isomers and positional isomers of multisubstituted aromatic (homo- or heteronuclear) compounds are often easily resolved. Analysis of free, underivatized carboxylic acids, alcohols, amines, nitrosamines, thiols can be carried out successfully on modified Carbopacks. A review contains many examples of separation (30). The coating of graphitized carbon blacks is relatively easy and carried out much like the coating of classical supports for GLC. Since graphitized carbon black does not absorb water, there is no need for a careful drying of the material. 3. Porous Polymers A number of polymers have been used in gas chromatography. It is possible to prepare small particles with reticulated polymers, in the size range required for successful gas chromatographic applications. The only popular materials at present are Porapaks and Chromosorb loo's (Johns Manville). Both commercial products come in a number of grades among which the analyst choses depending on the particular application. Most of these products are copolymers of styrene, ethylvinylbenzene and divinylbenzene. Some of them contain vinylpyrrolidone or other vinylic monomers. The specific surface areas of these products range from 15 m2/g for Chromosorb 103 to 600 m2/g for Porapak Q. The original and pioneering work of Hollis (39,40), who demonstrated the amazing properties of these non-polar materials, included separations of wet gases with the water eluting first. In most applications these adsorbents are used pure, as in classical GSC. It is always possible, however, to add some small amounts of liquid phases to adjust the selectivity of the stationary phase (39,40). The underlying polymer always seems to contribute to some extent to the retention, maybe because it would be very difficult with most products to achieve coating ratios large enough to really bury the surface. Furthermore, coating a reticulated polymer with a liquid layer is not like coating silica or graphitized carbon black. Some swelling or '

233

dissolution of the coating solvent in the porous polymer is likely to take place. Used at small coating ratios a number of additives, called modifiers or tailing reducers, permit a profound improvement of the band profile of some polar compounds (e.g. the use of polyimines results in symmetrical amine bands (40)). This phenomenon has been illustrated by Baumann and Gill (41). One of the most important applications of the porous polymers in routine analysis is the analysis of traces of water (42). Water is eluted very early with most materials. For example on Porapak Q, water elutes just after ethylene, totally resolved from this compound. With a good thermal conductivity detector (Chapter lo), the detection limit is about 10 ppm. On the other hand, columns packed with porous polymers can be used for the analysis of organic pollutants in water, since water is much less retained than light pollutants. Systematic investigations of this application have been made by Supina and Rose (43) and by Dave (44),among others. Compounds studied include: alcohols, glycols, ethers, aldehydes, ketones, acids, esters, chloroalkanes, amines, aromatic amines, diamines, nitriles, aromatic hydrocarbons, etc. As a general rule, all compounds belonging to any of the four classes distinguished by Kiselev (cf. Chapter 6, Section 1.1) can be separated on some porous polymer phase, provided their vapor pressure is rather large. High boiling compounds cannot be eluted in a reasonable time. For example chlorinated hydrocarbons with one or two carbon atoms are eluted in prohibitively long times when their boiling point exceeds 70-80 O C. The coating of porous polymers by a modifier is carried out like the coating of graphitized carbon black or of classical gas-liquid supports, without a careful drying prior to the coating procedure. 11. STEAM AS CARRIER GAS

Water can be used in gas chromatography either as a stationary or as a mobile phase. In 1957 Pollard and Hardy (45) demonstrated the potentiality of water as a stationary phase and used it for the separation of the chloromethanes. The vapor pressure of water proved too high, however, for a successful application in routine analysis. In the early 'sixties Wilkens (now Varian) offered a steam generator for use as a carrier gas or carrier gas component with any chromatograph (46,47). Used with conventional liquid phases in GLC, this system was not very successful either. Most stationary phases are steam distilled out of the column, which generates a variety of troubles: progressive decrease in the retention times and in the resolution, sometimes also large changes in the relative retention, base-line drift and excessive noise due to the response of the detector to the stationary phase, fouling of the detector, etc. The device was rapidly forgotten. More recently, several authors have reflected that a steam-rich gas stream could be an excellent carrier gas for gas-solid chromatography (2,16,48-50). A variety of References on p. 244.

234

applications have been described, including the analysis of heavy organics dissolved in water (51,52). Gradually, it has been demonstrated that the use of steam as a component of the carrier gas is much more than a fancy topic of academic interest (53). It may be developed into a highly reliable and very powerful method for routine analysis, in the laboratory or on-line, in combination with classical or modified gas-solid chromatography. In this case, water is both a component of the mobile phase and a constituent of the stationary phase, since a more or less important film of water is sorbed on the surface of the adsorbent. The situation is somewhat reminiscent of the retention mechanism in reversed phase liquid chromatography, where the exact composition of the adsorbent layer depends on the concentration of the organic solvent and organic modifier in the mobile phase (54). There is one important difference, however. The competition aspect between the analyte and the components of the mobile phase for adsorption on the surface of the adsorbent or within the chemically bonded groups there, so important in reversed phase HPLC,does not have any equivalent in normal gas chromatography, where the carrier gas is practically not adsorbed at all on the surface of the adsorbent or of the support, nor dissolved in the liquid stationary phase. In the present case on the contrary, the water molecules are sorbed on the adsorbent surface and interact with the surface modifier molecules. The analyte molecules can be either sorbed on the silica surface, at a gas-solid interface, on the silica surface at a liquid-solid interface or on the layer of sorbed water, at the gas-liquid interface. Multilayer adsorption is definitely a possibility in gas-solid adsorption equilibria (55). The interaction free energy is quite different in the three cases, however, and the retention volumes will depend on the nature and partial pressure of the carrier gas additive (see Chapter 3, Section V). Karger et al. have studied the adsorption of vapors at the gas-liquid water interface, using thin films of water sorbed on silica (60). Be that as it may, the use of a phase system composed of a steam-inert gas mixture as mobile phase and of an adsorbent covered by a film of a water-organic solvent mixture is a very powerful tool, offering the possibility of fine tuning the selectivity by adjusting the water content of both phases. 1. Production of a Suitable Camer

Gas

Several systems have been described in the literature, permitting either the production of steam as the only component of the mobile phase (45,46,51), or of mixtures of steam and a more conventional camer gas, with an adjustable composition (49,50,53). The latter permits a more flexible use of steam, and this is the method we choose. The principle is to bubble a stream of nitrogen or helium through a mass of water contained in a pressurized container and maintained at a carefully controlled temperature (cf Figure 7.13). The installation of such a system on a commercial gas chromatograph is easy. The camer gas line starts at the pressure controller, 1, on the inlet of the inert gas stream. The flow controller is eliminated and replaced by a pressure controller, for reasons discussed in Section 11.3.5 below, to prevent the effects on the detector

235

Figure 7.13. Schematic of the camer gas line of a gas chromatograph using steam as a carrier gas component. 1, Pressure controller on the inert gas. 2, Safeguard tank for the water in 4. 3, Stop valve. 4, Water tank. 5, Pressure gauge. 6, Sampling port. 7, Column. 8, Detector.

base-line of the surge of vapor following the injection of a large water sample. The carrier gas bubbles into the water contained in the water tank, 4, and goes to the sampling port, 6 , and the column, 7 (cf Figure 7.13). The water tank 4, tested to 10 atm, is placed in a temperature-controlled oven, separated from the column oven, but immediately next to it. There should be no cold spot, i.e. no place where the gas stream temperature becomes lower than the temperature of the water in the tank. Otherwise water would condense in these parts, then, when the amount of condensed water is large enough to interfere with the gas stream, liquid droplets would burst and be projected into hot parts of the apparatus where they would vaporize very rapidly, creating flow rate instabilities, resulting in an unsteady base-line and a very noisy detector signal. Similarly, noise resulting from the formation of bubbles in the water tank 4 can be considerably reduced by use of a metal frit with a 10 to 20 pm porosity. The critical sections of the carrier gas line are the connecting tubes between the water tank 4 and the sampling port 6 and between the column exit and the detector, 8, as well as the detector itself. The sampling port, the column and the detector should always be operated at a temperature higher than that of the water tank. An empty tank, 2, and a valve, 3, immediately upstream the water tank 4, permit the protection of the inert gas line against backflow of water in case of pressure surges in the line when the chromatograph is being started or stopped. The setting of the inert gas and the steam flow rates is done by adjusting the inlet pressure of the inert gas and the temperature of the water tank. These operations are independent and do not interact. First, the water tank being at room temperature, the inert gas flow rate is set by adjusting the inlet pressure. This flow rate is measured downstream from the detector using a conventional soap bubble flowmeter. The inlet pressure is adjusted to provide a flow rate of e.g. 2 L/hour for a 4 mm i.d. column. The inlet pressure References on p. 244.

236

corresponding to the total flow rate desired (inert gas+ steam) should also be determined at that stage. Then the temperature of the water tank is raised, so that the inlet pressure gauge, 5 (Figure 7.13) reads the pressure required to achieve the total flow rate desired (usually equal to 3 L/h, the flow rate which typically corresponds to the maximum column efficiency with the columns we use). This assumes that the viscosity of the inert gas-steam mixture is the same as that of the inert gas. This is only an approximation since the viscosity of steam is low, intermediate between that of nitrogen and hydrogen (cf Chapter 2, Table 2.1). So the actual flow rate achieved is somewhat greater than calculated, but for analytical applications, the consequences are negligible, as long as the flow rate and the composition of the carrier gas are reproducible, which they are. As a first approximation, the partial pressure of steam at the column inlet is the difference between the initial inlet pressure (inert gas alone) and the final inlet pressure (inert gas plus steam). Both partial pressures, that of steam and that of the inert gas, decrease along the column, although their ratio remains constant. Accordingly, the amount of water adsorbed per unit surface area of the adsorbent decreases along the column. In most analyses performed by this method the detector is a flame ionization detector. The hydrogen flow rate to the detector must be slightly higher than for conventional analyses, to keep the flame hot enough, permit a good ionization yield of the analytes, and keep a satisfactory response factor. The hydrogen flow rate is typically 3 L/h in these applications, while the air flow rate is 15 L/h. A large excess of air is required to avoid condensation of water in the detector. 2. General Procedure for the Use of Steam in the Carrier Gas

These rules are similar to those used for the selection of the adsorbent in gas-adsorption layer chromatography (cf Section I above): - The higher the boiling point of the analytes, the lower the specific surface area of the adsorbent selected and the higher the steam concentration in the carrier gas. - The higher the polarity of the analytes, the lower the specific surface area of the adsorbent selected and the higher the steam concentration in the carrier gas. Furthermore, referring to the Kiselev classification of compounds (Chapter 6, Section 1.1): - For molecules of groups A and B: - the specific surface area will be moderate to large, - the carrier gas composition will be 25 to 50% steam. - For molecules of groups C and D: - the specific surface area will be small to moderate, - the carrier gas composition will be 50 to 75% steam. These figures are orders of magnitude, to be adjusted as required for each application. Water molecules from the mobile phase are adsorbed on the silica surface and form a film of variable thickness. Sites of the highest energy are saturated with water molecules, so the silica surface is transformed essentially into a water surface,

\

231

which is much more homogeneous than the original silica surface, but retains some of its properties. The thickness of the water layer essentially depends on the column temperature and the partial pressure of water in the mobile phase, or more exactly on the ratio of the partial pressure to the vapor pressure. Since the vapor pressure of water decreases continuously from the column inlet to the outlet, the average film thickness of the water layer also decreases regularly from the column inlet to the outlet. The retention or column capacity factor, as well as the selectivity of the stationary phase, increases continuously from the inlet to the outlet of the column. This may result in difficulties in the elution of some heavy, polar compounds, which may be strongly retained and whose profiles may acquire tailing on the end of the column. There are two ways to attempt to correct this effect: - either by increasing the water content of the mobile phase, and hence that of the stationary phase, i.e. by decreasing the column temperature or by increasing the water concentration of the mobile phase, or both, - or by decreasing the activity of the adsorbent and selecting a silicagel with a lower specific surface area. The use of steam as a component of the mobile phase may permit a moderate increase of the solubility of the analytes in the mobile phase, because of favorable molecular interactions in the vapor phase, which is strongly analogous with liquid chromatography.

3. Optimization of the Experimental Conditions The main parameters to adjust are the nature of the adsorbent and its specific surface area, the column temperature and the water content of the mobile phase. It should be emphasized at this point that other suitable polar vapors or gas can be added to the carrier gas, the only major requirement being that this compound has a very low response factor with the flame ionization detector. Ammonia, formaldehyde, formamide, formic acid, could make excellent choices for the solution of a variety of analytical problems. Major corrosion problems may be encountered in certain cases. 1. Selection of the Adsorbent

There is a large variety of adsorbents among which to choose. Silica gels, silica gels coated with pyrocarbon (56,57), aluminas, activated carbons or charcoals, porous polymers, etc. Since the retention properties of the stationary phase depend on the composition of the mobile phase, rather important changes in selectivity, including inversions in the elution order of some compounds will result from adjustments in this composition. They will be used for the solution of specific problems. Figure 7.14 illustrates the influence of the nature of the adsorbent. With approximately 45% water, the elution orders are: - on silica: (1) 3-methylpentane, (2) cyclohexane, (3) n-heptane, (4) 1,2-dichloroethane, (5) acetone and ( 6 ) methyl ethyl ketone, References on p. 244.

238

.3

-2 B

A 4,

6

n

5

6

7min

L I

I

I

Figure 7.14. Separation of a test mixture on silica and pyrocarbon-coated silica, with steam as a component of the carrier gas (59). 1, 3-Methylpentane; 2, cyclohexane; 3, n-heptane; 4, 1,2-dichloroethane; 5, acetone; 6, methyl ethyl ketone. (A) Column: 4 nun i.d., 2 m long. Spherosil32 mz/g, particle size: 150-200 pm. Temperature: 105" C. Carrier gas:nitrogen 5 2 1 , steam 48%.Flow rate: 2.9 L/h. (B)Column: 4 mm i.d., 1 m long. Spherosil50 m2/g, particle size: 150-200 pm, coated with pyrocarbon. Temperature: 150 O C. Carrier gas: nitrogen 56%, steam 44%. Flow rate: 3.06 L/h. Reprodud with permission of Journul of Chromurogruphy, 301, 11 (1984).

- on pyrocarbon coated silica: (3) n-heptane, (2) cyclohexane, (1) 3-methylpentane, ( 5 ) acetone, (6) methyl ethyl ketone, (4) 1,2-dichloroethane. Whereas, on silica, the retention order is mainly controlled by the polarity of the molecule (first, hydrocarbons, then chlorinated hydrocarbons, ketones last), on pyrocarbon-coated silica the size of the molecule is much more important (57). This property has been used in the analysis of acetaldehyde in drinking water at the 10 ppb level: on pyrocarbon-coated silica this compound is eluted before acetone and well resolved from it. In the literature there are examples of the use of a large variety of adsorbents with steam as a mobile phase (52). 2. Influence of the Specific Surface Area of the Ahorbent (58) Retention volumes are a function of the specific surface area of the adsorbent used. Of course the specific surface area of a silica gel used with steam as a

239

Figure 7.15. Plot of the logarithm of the column capacity factor versus the specific surface area of the adsorbent used (59). 1, 3-Methylpentane; 2, cyclohexane; 3, n-heptane; 4, 1,2-dichloroethane; 5, acetone; 6, methyl ethyl ketone. Column: 4 m m i.d., 2 m long. Temperature 115OC. Flow rate: 3 L/h. Carrier gas: nitrogen 42%, steam 58%. Spherosils 32, 53, 108, 230 and 367 m2/g, particle size: 150-200 pm. Reproduced with permission of Journal of Chromatography, 301, 11 (1984).

component of the carrier gas will depend not only on the specific surface area of the initial (unmodified) silica gel, but also on its average pore size and pore distribution. When a pore is filled with water, condensed by capillarity or by the Kelvin effect, the surface of the water layer available for adsorption is independent of the total inner area of the pore, and is much smaller. This phenomenon is illustrated in Figure 7.15, a plot of the column capacity factors determined for the same mixture of test solutes as separated on the chromatograms in Figure 7.14, as a function of the specific surface area of the silica gel used. From this figure we can make the following observations: - the column capacity factors increase almost linearly with increasing specific surface area. - the resolution between compounds which have similar adsorption energies requires the use of large specific surface area adsorbents, e.g., the separation of 3-methylpentane and cyclohexane. - the retention of polar compounds on large surface area adsorbents may be prohibitively long. References on p. 244.

240

L

5-

-

4- 1:

a5 -

4-3 1*2

:

I

,

I

I

1

,

I

,

I

Figure 7.16. Plot of the logarithm of the column capacity factor versus the water content of the mobile phase (59). 1, 3-Methylpentane; 2, cyclohexane; 3, n-heptane; 4, 1,2-dichloroethane; 5, acetone; 6, methyl ethyl ketone. Column: 4 mm i.d., 2 m long. Spherosil 32 m2/g, particle size: 150-200 pm. Temperature 115 O C. Flow rate: 3 L/h. Carrier gas: nitrogen and steam 33, 52 and 66%. Reproduced with permission of Journal of Chromrography, 30I, 11 (1984).

These results also illustrate the rules given in the previous section regarding the selection of the adsorbent. Finally, we want to emphasize that the method is most useful to enhance the resolution between some of the important components of a sample, rather than to improve the separation of a complex mixture.

3. Znfruence of the Water Content of the Carrier Gas (58,59) The increase in water content of the carrier gas results in a decrease in the surface activity, since the surface becomes increasingly water-like and homogeneous. Experimental results are in agreement with this prediction, as is illustrated by the data in Figure 7.16. The column capacity factor decreases exponentially with increasing water content. The decay constant of the exponential increases with the polarity of the analytes (compare acetone and cyclohexane in Figure 7.16). This observation also confirms our analysis of the retention mechanism in GSC with steam as a component of the mobile phase since, in modified GS.C, the retention time also decreases with increasing surface coverage of the adsorbent, at least as long as the film of organic modifier is thin (cf. Chapter 6, Figure 6.17).

241

The use of plots such as the one shown in Figure 7.16 permits the selection of the optimum steam content of the mobile phase. In difficult cases it is conceivable to program the steam content, for example to permit the separation of light, weakly polar compounds on an active adsorbent, followed by the separation and elution with symmetrical peaks of the heavy, polar components of the mixture. This procedure would be identical to the gradient elution programming of liquid chromatography, an analytical procedure which as far as we know has not yet been carried out in gas chromatography. 4. Influence of the Column Temperature (59)

A change in the column temperature has several effects. First, the vapor pressure of water is changed. Accordingly, at constant water concentration the amount of water sorbed and the average film thickness increase with decreasing temperature. On the other hand, since adsorption is an exothermic process, the retention volume of analytes increases with decreasing temperature. The combination of the two effects may result in considerable variations of the relative retention of some pairs of analytes with changes in the column temperature.

4

50 -

\

10 -

5-

I

150

200

I

250 ‘C

,

Figure 7.17. Plot of the logarithm of the corrected retention time versus the reverse of the column temperature (59). 1, 3-Methylpentane; 2, cyclohexane; 3, n-heptane; 4, 1,2-dichloroethane; 5, acetone; 6, methyl ethyl ketone. Column: 4 mm i.d., 1 m long. Spherosil 50 m’/g, coated with pyrocarbon, particle size: 150-200 pm. Flow rate: 3.06 L/h. Carrier gas: nitrogen 56%, steam 44%.Temperatures: 150 C, 170 C, 216 O C and 260 O C. Reproduced with permission of Journal of Chromarogruphy, 301, 11 (1984).

References on p. 244.

242

Some conventional plots of the logarithm of the retention time versus the inverse of the absolute column temperature are shown in Figure 7.17. The same test compounds have been used as for the previous figures. Care should be taken to acquire data only at constant water concentration in the mobile phase, or at a constant value of the ratio of the water partial pressure to the vapor pressure. In this latter case most of the effect of changing the temperature on the amount of water sorbed and on the average film thickness is cancelled. The dramatic variation of the relative retentions of l,Zdichloroethane, acetone and methyl ethyl ketone in Figure 7.17 illustrates the consequences of the phenomena just described and the potentialities offered to the analyst for the optimization of a separation. 5. Application to the Analysis of Aqueous Solutions

The most important field of application of this technique is obviously in the analysis of organic pollutants in water samples. Such samples are difficult to analyze due to a number of problems associated with the injection of a large amount of water in the chromatograph, when operating under classical conditions, i.e., with a very dry carrier gas. A very large injection is required for trace analysis. The migration of a large water band creates drastic but temporary changes in the properties of the support (especially the degree of activation of the support, whether a silica gel or a diatomaceous material), and hence in retention and resolution, alters the working conditions of the detector (i.e. changes the response factors), and makes quantitative analysis less accurate and less reliable. The conventional alternative,

Figure 7.18. Application of the use of steam in the camer gas to the analysis of water pollutants. Detection of 20 ppb of vinyl chloride in water. Sample sue: 200 pL. Column: 4 mm i.d., 2 m long. Spherosil 360 mz/g, particle size 100-200 pm. Column temperature: 128OC. Carrier gas, nitrogen (728) and steam (288), flow rate 3.35 L/hour. FID, hydrogen flow rate 4 L/hour, air flow rate 9 L/hour. Reproduced by permission of Journal of Chromatographk Science, 17, 677 (1979).

243

extraction with a solvent or with a non-polar adsorbent (Tenax, Amberlite, chemically bonded C18 silica, etc.), is cumbersome and introduces a whole new set of problems and sources of error. The injection of large amounts of water samples (20, 50, 100, 200 pL) results in the production of a large volume of mobile phase (25, 62, 125, 250 mL NTP, respectively), an effect which takes place with all solvents, but is especially important with water, due to its low molecular weight. This is a major perturbation for a column and a detector operating at 3 L/h or 50 mL/min. To avoid spurious detector signals and base-line drifts which might hide the peaks of trace compo-

1 7min

Figure 7.19. Detection limits of organic compounds in water samples. Column: 4 mm i.d., 2 m long. Spherosil32 m2/g, particle size 150-200 pm. Column temperature 160O C. Carrier gas, nitrogen (35%) and steam (65%), flow rate 3 L/hour. FID, hydrogen flow rate 4 L/hour, air flow rate 20 L/hour. Sample size 25 pL. (After Guillemin et al., ref. 59). Reproduced with permission of Journal of Chromarogruphy, 301, 11 (1984).

References on p. 244.

244

nents, or erratic detector response which might lead to major quantitation errors, the flow-rate controller on the inert carrier gas line is replaced by a pressure controller, operating at the pressure required for maintaining the desired flow rate through the column. A one-way valve, placed between the sampling port and the pressure controller, prevents back flow of steam with subsequent flooding of the upstream gas line. Thus, when the large water sample is injected and vaporized abruptly, giving rise to a strong pressure surge in the sampling port, the inert gas flow rate is reduced and the total flow rate through the detector is kept nearly constant. The migration of the large band of steam-enriched carrier gas does not seem to create serious enough changes in the retention pattern to result in significant errors on either the determination of the retention times (qualitative analysis) or the peak area (quantitative analysis). The chromatogram obtained for the analysis of a 200 pL sample of polluted water with a Spherosil column and a flame ionization detector is shown on Figure 7.18. The base line is quite acceptable at that level of sensitivity, permitting the detection of 20 ppb of vinyl chloride, thus demonstrating the validity of the method. Figure 7.19 shows another example of the detection of trace amounts of organic compounds in aqueous samples.

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