Evaporation of the Liquid Stationary Phase in Gas Chromatography V. G. Berezkin/V. S. Gavrichev A. V. Topchiev Institute for Petrochemical Synthesis, Academy of Sciences of the U.S.S.R., Leninskii prospekt 29, Moscow V-71 (U.S.S.R.)

Key Words Gas chromatography Liquid stationary phase Liquid phase bleeding

Summary The theory of the evaporation of the liquid stationary phase is elaborated and experimentally verified. On the basis of this theory the role played by the losses in the amount of liquid phase present is quantitatively determined. General techniques are examined which minimize the losses; these techniques are based on saturating the incoming carrier gas with liquid phase vapours and raising the pressure of the carrier gas in the column, e.g. by connecting a capillary to the column outlet in order to offer resistance to the gas flow. The application of these techniques ensures stable performance of the gas chromatographic columns using a volatile liquid phase.

Introduction

In gas Chromatography, the evaporation ("bleeding") of the liquid stationary phase generally impairs the characteristics of the columns [1-4]. Maintaining the operating characteristics of the columns at a constant level is a major problem, particularly in the case of process gas chromatographs, in which the columns operate continuously for a prolonged period of time. Investigations on the evaporation of the liquid phases are essential not only in order to account for this phenomenon, but also to be able to control the evaporation. A solution to this problem would substantially lengthen the useful life of a column, extend the temperature range of the liquid phases, and also broaden the range of available liquid phases. It should be noted that the losses associated with liquid phase decomposition are not discussed in the present paper. Such losses, which are generally caused by the catalytic effect of the solid support, have been examined in a monograph [5]. One of the principal factors responsible for liquid phases losses is believed to be the evaporation of the liquid phase into the carrier gas entering the column [1, 6]. In order to compensate for these losses, it was suggested either to periodically introduce a liquid phase solution into the 472 0009 5893/83/9 0472-05 ~ 02.00/0

column [2, 4] or to employ a carrier gas enriched with liquid phase vapours [7-9]. In compliance with the techniques described in the literature [2, 4, 7, 8], carrier gas saturation with liquid phase vapours occurs in or before the sample injector and may result in contaminating this. The gas chromatograph described in a Russian patent [9] is designed so as to exclude the passage of liquid phase vap0ur through the injector. There may be another reason for the evaporation of the liquid phase in the column and this is the pressure gradient along the column [10]. Increasing the carrier gas flow rate should result in saturation with liquid phase vapours over the entire column length in addition to saturation at the column inlet. Nesvadba et al. have suggested a method resulting in stable column characteristics [10]. According to this method saturation of the carrier gas is combined with the application of a temperature gradient along the column. However, this method seems to be too complicated because the temperature gradient must precisely cortes. pond to the nonlinear function of the decompression of the carrier gas. It should be noted that there is a lack of investigations h the literature wtdch could serve as the foundation for the quantitative evaluation of tile aforesaid causes for the evaporation of the liquid phase in the column. Such an evaluation, however, is necessary particulary in order to suggest methods which would help in minimizing losses of the liquid phase in a gas chromatograph.

Theory The ~otal mass of liquid pbase losses, m, from the column during time t under isothermal conditions can be expres. sed as re=m2 -ml = c2F2t-clFat

(1)

where rn, and rn2 denote the mass of liquid phase vapours flowing into and emerging from the column, respectively; c~ is the concentration of liquid phase vapours at column inlet; c2 is the concentration of the saturated vapour at column outlet; and F, and F2 represent the flow rate of the carrier gas at the column inlet and outlet, respectively. The values of F~ and F2 are the actual volumetric flow rates that are not reduced to normal conditions. We may now introduce two additional terms, Ac and zXF: AC=C2

--C 1

A F = F2 - F1 Chromatographia Vol. 17 No. 9, September 1983

9 1983 Friedr: Vieweg & Sohn Verlagsgesellschalrt mbH

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The term Ac expresses tile difference in the concentration of the liquid phase vapour in the carrier gas at column outlet and inlet. In the case of pure carrier gas entering the colunm, c~ = 0 and Ac = c2 ; on the other hand if the carrier gas is presaturated with the vapour of the liquid phase then c~ = ca and Ac = 0. The term zXF' represents the difference in the carrier gas flow rate at column outlet and inlet. Substituting these two expressions into eqn. (1) we can write that m = A c F l t + AFcat

(2)

The first expression on the R. H. S. of eqn. (2) defines the carrier gas saturation at column inlet while the second expression on the R . H . S . of eqn. (2) describes the stationary phase losses due to carrier gas expansion (expansion losses). According to Boyle's law Fa/FI = Pi/Po = n where Pi and Po are the pressures at column inlet and outlet respectively. We can write the share o f expansion losses relative to overall losses as 0-

AFca t m

(3)

For the case of c~ = 0, m can be expressed from eqn. (1) as m --- ca F a t

The value of c2 selected corresponds to the limiting operating temperature (see e.g. [12]). The result of this calculation will give a range of 0 . 0 5 - 0 . 4 8 for/a. As a conclusion we can state that the share of expansion losses, 0, and the magnitude of /1, tile relative loss, are significant and justify the efforts to minimize this type of loss. It is essential to emphasize tile fact that the expansion losses are not distributed uniformly over the column length. Indeed, pressure variation along the columns obeys a nonlinear law [13], pressure variation uniformity being likely only in tile case of rr ~< 1.5. At rr > 1.5, both the carrier gas pressure and flow rate vary most drastically toward the column end over a section equalling to about 1 0 - 2 5 % o f the column length (cf. Fig. la). Expansion losses over each section of the column are proportional to the carrier gas flow rate increment for the particular section. Hence, the value of/a varies only slightly provided the carrier gas flow rate changes almost monotonously (curve 1 in Fig. lb). In the case when the flow rate rises sharply toward the column end, the value of # would experience an even more drastic increase (curve 2 in Fig. lb), thereby causing a pronounced loss in the liquid phase not only at tile column inlet, but also over the entire column length and particularly toward the column outlet. Let us now consider possible approaches to the problem of minimizing the expansion losses of the liquid phase. Eqn. (5) can be transformed as follows:

and substituting this into eqn. (3) we obtain: /10=

F2 - F t Fa

= 1 --

1 n

(4)

c2tgSu2 (1 1 )

(6)

M

wherein ~r expresses the free cross-sectional part and S is the cross-sectional area of the column; ua represents the

We can also express the expansion losses relative to the mass of the liquid phase (M) in the column: AFcat #=

M

30

Substituting F 2 - F t for zXF and multiplying both sides by Fa we can write this expression as caF2t 1 # = T (1 - ~ )

~ 20--, - ~ - ~ - - ~ - - ~

(5)

us

In the practice the value o f rr varies between 1.1 and 5.0. Consecutively, according to eqn. (4), 0 will vary between 0.09 and 0.80. In the case of capillary columns the permeability of which is higher than that of packed columns [11 ], the effect of expansion losses will be less pronounced.

105v

0

Fig. 1

-

o

O,EL

V a r i a t i o n of the conditions along the columns:

Fa = 30cm a/min t = 1000hours M =3g n = 1.1-5.0 c2 = 10 -6 g/cm 3

Chromatographia Vol. 17 No. 9, September 1983

2

-

1E-

If a saturator is used upstream of the column then c~ = ca and m = k F c 2 t. Accordingly, 0 = 1 for any values of n. In order to assess the value of tt let us substitute into eqn. (5) the following variables characteristic of gas chromatography with packed columns:

10-

/~

(a)

f l o w rate variation; F 2 = 3 0 c m 3 / m i n ;

(b)

distribution o f the expansion losses of the liquid phase calculated f o r the following conditions: c 2 = 10 - 6 g/cm 3, t = 100 hours, M = 3 g, and F 2 = 3 0 c m 3 / m i n ;

1 n= 1.5;2n=5.0

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473

carrier gas velocity at column outlet. The mass of liquid phase contained, for example, in a packed column equals LSplI M= 10----O

(7)

where L is a column length, p is the bulk density of the solid support, and ll denotes the amount of liquid phase coated on the solid support, expressed as wt.-%. From eqns. (6) and (7) it is evident that the value of p can be decreased by minimizing the parameters c2, K, u2 and n, or by increasing the parameters L, p and II. It should be stressed, however, that in the GC practice, the parameters c2, K, U2, L, p, and II are optimized, with respect to the separation problem and not with a view to minimizing the liquid phase losses. The most effective and expedient way to minimize liquid phase losses consists of reducing the value of m For example, changing from n = 5.0 to n = 1.1 would result in a decrease of the value of # from 0.48 to 0.05, i.e. by a factor of about 10 (calculated using eqn. (5) for the characteristic values of c2, t, /'2, and M), and in simultaneously distributing the liquid phase losses more uniformly over the entire column length. Lowering the values of n can be achieved by using columns that offer low resistance to the carrier gas flow or as a result of increasing the overall pressure. In the latter case, a flow restrictor, e.g. a capillary tube may be placed between the column and the detector. This capillary resistance may control the mean gas flow rate F = F 2 ] , where ] is the compressibility correction factor.

Experimental

The experimental investigations covered two aspects: (1) determination of the retention characteristics of chromatographic colunms and evaluation of the reproducibility of the measurements, and (2) measurement of the amount of liquid phase presenl in the columns before and after purging the columns with the carrier gas. Hexadecane was used as the stationary liquid phase. Its upper temperature limit is given as 50 ~ [14]. Two systems were used for the investigations; their schem, atics are shown in Figs. 2 and 3. System A

The system shown in Fig. 2 was used to study tile evaporation of tile liquid phase at room temperature. It contains three columns in parallel: reference column 6, column 6' with presaturator 4 and column 6" having the capillary restrictor 7 connected to its outlet. The columns were of stainless steel and packed with Chromaton N-AWDMCS (0.1-0.125 ram) coated with 5 wt-% hexadecane. The glass saturator contained a mixture of hexadecane and the solid support in a ratio of 1 : 2 (by volume). The pressure of the carrier gas (helium) upstream of columns 6 and 6' was equal to 2.13 and 2.30 atm, respectively. Upstream of column 6" the pressure was 3.92 atm and at its outlet 2.40 atm, the carrier gas flow rate at the outlet of capillary ? being 55.6cm3/min. Under these conditions the retention

Table I presents the values for the liquid phase losses when columns offering high or low resistance to the carrier ~as flow flow are used. It was assumed in the calculations that f = 10 cma/min and the already specified characteristic values were used for c2, t, and M. The following conclusions can be drawn from the data of Table 1: (1) employing a capillary restrictor, together with the use of a saturator represents an effective way to minimize liquid phase losses; (2) employing only a saturator upstream of high-resistance columns decreases only slightly the overall losses, and (3) simultaneous use of a saturator and a capillary restrictor is advantageous not only in the case of columns having a high hydrodynamic resistance but also for low flow-resistance columns.

3 F~><

' l

n*

4,

6 i

5;,

;-(2)

~--~E"

i

6'

..

C--q 5"

7

tI l

~

I

.....

1

Fig. 2 Schematic of System A used at room temperature. 1 line from the helium cylinder; 2 and 2' pressure gauges (measurement range: 6 atm); 3 and 3' fine adjustment valves;4 saturator;5, 5', and 5" sample injectors; 6, 6' and 6 " chromatographic columns (1 m X 2 mm i.d.), 7 capillary (stainless steel, 1.5 m X 0.25mm i.d.); 8 thermostated thermal conductivity cell.

Table I. Total Liquid Phase Losses in Relation to the Mass o f the Liquid Phase in the Columns (re~M). Gas Chromatographic System

5 []

2

~,

--5

re~M,%

3

/+ r

6

7

- L--~-~

81 L___~

Systems w i t h low-resistance columns Column Saturator + column Saturator + column + capillary

1.5 1.5 1.1

25.4 8,5 0.9

* n

474

Pi/Po

5,0 5.0 1.5 1.5

'

6'

I

I

L

d

Fig. 3

Systems w i t h high-resistance columns Column Saturator + column Column + capillary Saturator + column + capillary

-

69,0 55,2 25.4 8,5

Schematic of System B used at 95 ~ 1 line from the nitrogen cylinder; 2 and 2' flow saturator; 4 and 4' sample injectors; 5 pressure gauge range: 6 atm); 6 and 6' chromatographic columns i. d.), 7 copper capillary (3m X 0.25 mm i.d.); 8 thermostated thermal conductivity cell.

C h r o m a t o g r a p h i a V o l . 17 No. 9, S e p t e m b e r 1983

controllers;3 (measurement (3m X 4rnm thermostat;9

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time of an unretained gas (nitrogen) was nearly identical on all the columns and varied from 8.5 to 9.0 seconds. In order to measure the pressure upstreana of columns 6 and 6' pressure gauge 2' was alternately connected to the sample injection ports 5 and 5'. Columns 6' and 6" were alternately connected to a thermal conductivity detector. The retention time of n-hexane was determined before and after column purging for a period of 13.5 days. Its capacity factor (kh) was calculated for both cases. System B

The system shown in Fig. 3 was used to study the evaporation of the liquid phase at elevated temperatures. Column 6 had saturator 3 upstream of it while its outler was connected to capillary restrictor 7. Reference column 6' is parallel to rite main column. The temperature was maintained at 95 ~ i.e. 45 ~ higher than the specified upper temperature limit of hexadecane. Columns 6 and 6' were packed with Chromaton N-AW-DMCS (0.1-0.125 ram) coatcd with i0 wt-% hexadecane, while saturator 3 contained a mixture of hexadecane (4 cm 3) and the solid support (12cm3).

flow rate from tile column is equal to 23.2cm 3/min, lower thau the flow rate from the column without a capillary. By comparison with the reference column, it is reasonable to expect that the hexadecane losses would decrease by a factor of 29.6 : 19.3 = 1.53 in the column with the saturator and by a factor of 29.6 : 23.2 = 1.28 in the column with the capillary. The capacity ratio of hexane diminished by 22.0, 15.4, and 16.5 rel. % in the reference column, in the column with the saturator, and in the column furnished with the capillary, respectively. Hence, in this instance, the losses experience a 1.43-fold decrease when a saturator is used, and a 1.33fold decrease when a capillary is employed. These values are close to those calculated earlier on the basis of the carrier gas flow rate. It should be noted that when using the capillary, the mass flow rate of the carrier gas increased while the liquid phase losses decreased.

The pressure upstream of columns 6 and 6' was equal to 4.38 and 2.02 arm, respectively. The carrier gas (nitrogen) flow rate at the end of column 6' was 36.8 cm3/min while at the end of capillary 7 it was 109.0 cm3/min. The retention times of an unretained gas (helium) in both lines were close to each other equalling 67.5 and 70.5 seconds, respectively. The columns were purged with the carrier gas for a period of 26 days and the retention time of methylene chloride was determined at room temperature both before and after this purging. Its capacity factor (km) was calculated for both cases. The amount of hexadecane in the original column packing was determined by measuring the weight loss of an aliquot in a muffle furnace, at 550 ~ and also, on the basis of organic elementary analysis. The remaining amount of hexadecane in the column packing after purging was determined by taring aliquots from the columns at 1, 2 and 3 m distances from the column inlet, and carrying out the measurements in a similar way. From these measurements, the mean value was calculated.

Results obtained in System B

As seen from tile data in Table lII, the liquid phase losses in the experimental column decreased due to two factors: (a) tile carrier gas entering the column is saturated with tile liquid phase vapours (Fl = 24.9 cm 3/min); and (b) the value of z2ut; diminished from 18.6 to 6.0cm 3/rain. As compared to the reference column, the liquid phase losses can be expected to decrease by 36.8:6.0 = 6.1 times. After flushing, the value of k m in the reference colunm fell by 71.2 rel.% (0.82vs. 2.85) while in the experimental column, k m decreased by only 11.5 rel.% (2.61vs. 2.95) so

Table II. Characteristics of the Columns Employed in System A

Characteristics

Column with saturator

Column with capillary

Reference column

F 2, cm3/min AF, cm3/min F1, cm3/min

34.1 19.3 14.8

23.2 9,0 14.2

29.6 15.7 13,9

k h prior to purging k h after purging

13.0 1 1.0

12.7 10.6

12,7 9.9

Results and D i s c u s s i o n The results of tile experiments carried out using the two systems are presented in Tables II and III. Results obtained in System A The measured flow rate values were used for the calculation of d:e exprected liquid phase losses. The values of AF are proportional to the overall expansion losses. The real losses are determined from the decreasing values of kh. When the column is furnished with a presaturator, liquid phase losses are exclusively due to the expansion of the carrier gas (ZXF= 19.3 cIn 3/min). Adding a capillary restrictot, the value of z2utr decreased to 9.0 cm 3/min, diminishing liquid phase losses, resulting from the fact the carrier gas Chromatographia Vol. 17 No. 9, September 1983

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Table III. Characteristics of the Columns Employed in System B

Characteristics

Experimental column with saturator and capillary

Reference column

F 2, cm3/min AF, cm3/min F1, cm3/min

30.9 6.0 24.9

36.8 18.6 18.2

k m prior to purging k m after purging I] 1, wt-% 1-12,wt-% I13, wt-% II, wt-%

2.95 2.61 10.4 8.5 6.8 8,6

2.85 0.82 1.0 7.2 0.6 2.9

475

that in the latter, the losses were lower by a factor of 71.2:11.5 = 6 . 2 .

AF

The determined amount of hexadecane in the original column packing equalled 10.0 wt-%. In the reference column, the liquid phase content dropped sharply in the initial part of the column (111 = 1.0 wt-%) due to liquid phase evaporation into the incoming carrier gas and also in the terminal part of the column (Ii3 = 0.6wt-%), as a result of carrier gas expansion. In the experimental column, the liquid phase content having experienced practically no change over the initial part of the column, gradually decreases toward the column end (from 10.4 wt-% to 6.8 wt-%) because of expansion losses. Calculations show the mean amount of the liquid phase (I1) in the reference column decreased by 7.1 wt-% (from 10.0 to 2.9 wt-%) and in the experimental columns, by 1.4wt-% (from 10.0 to 8.6wt-%), the losses in the latter case being reduced by a factor of 7.1:1.4 = 5.1. According to the analysis of the column packings discharged from the columns, the amount o f liquid phase in the experimental column is 6.3 times greater thin: that in the reference column.

F P~ Po t S / Uz L rr

The experimental data, therefore, agree with the values calculated according to the suggested theory which accounts not only for liquid phase evaporation into the incoming cartier gas, but also for losses due to cartier gas expantion.

Acknowledgement The authors are highly obliged to Dr. L. S. Ettre for his valuable remarks. The CH-analysis was performed in the Analytical Laboratory o f the Institute for Petrochemical Synthesis, Academy o f Sciences of the USSR.

Nomenclature rn ml rn2 M c2

= = = = =

cl

=

2xc

=

Ft F2

= =

476

total mass of liquid phase losses from the column mass of liquid phase vapours entering the column mass of liquid phase vapours leaving the column mass o f liquid phase in the column concentration of saturated liquid phase vapour in the carrier gas concentration o f liquid phase vapours in the carrier gas entering the column difference between the concentrations of liquid phase vapours in the carrier gas at column outlet and inlet (c2 - c l ) actual carrier gas flow rate at column inlet actual carrier gas flow rate at column outlet

0

17

= difference between the carrier gas flow rates at column outlet and inlet (F~ --Fl ) = mean carrier gas flow rate = carrier gas pressure at column inlet = carrier gas pressure at column outlet = duration of column purging by the carrier gas = cross-sectional area of the column = compressibility correction factor = carrier gas velocity at column outlet = column length = ratio o f cartier gas pressures at column inlet and outlet (Pi/Po ) = share of expansion losses in relation to the total liquid phase losses = share of expansion losses in relation to the liquid phase mass in the column = free part of the column cross-section = liquid phase content of the column packing, expressed as wt-%

References [1] R. .4. Keller, R. Bate, B. Costa, P. Forman, J. Chromatogt., 8, 157 (1962). 121 D. S. Payn, W. D. Reardon, L. J. Harvey, Nature (London), 200, 467 (1963). 131 Ir Gerrard, S. J. Hawkes, E. F. Mooney, in R. P. W. Scott (Editor), Gas Chromatography 1960,(Edinburgh Symposium), Butterworths, London, 1960, p. 250. [41 L Braddock le Roy, N. Marec, J. Gas Chromatogr., 5,588 (1967). 151 V. G. Berezkin, V. P. Pakhomov, K. L Sakodinskff, Solid Supports in Gas Chromatography, Supelco Inc., BeUefonte, Pennsylvania, 1981. 161 R. F. Kruppa, R. S. Henly, J. Chromatogr. Sci. 12, 127 (1974). 171 O. Grubner, L. Duskova, Collect. Czechosl. Chem. Communs., 26, 3109 (1961). 181 L R. Hunter, M. K. WaMen, Anal. Chem. 35, 1765 (1963). 191 V. G. Berezkin, M. N. Budantseva, V. S. Gavrichev, A. K. Davydenkov, V. N. Lipavskii, Gazovyi khromatograf (Gas Chromatograph), USSR Specification No. 721,752, 1960. I101 K. Nesvadba, J. Matena, L. Obstrcil, M. Slavik, Chem. prum. 16, 392 (1966). [lll G. Guiochon, in J. C. Giddings and R. A. Keller (Editors), Advances in Chromatography, Vol. 8, Marcel Dekker, Inc., New York, 1969,p. 179. [12] H. M. McNair, /:2 J. Bonelly, Basic Gas Chromatography, Consolidated Printers, Oakland, California, 1967. [13] A. L M. Keulernans, Gas Chromatography, Reinhold, New York, Chapman and Hall, Ltd., London, 1959. 114} iV. Kotsev, Spravochnik po gazovoi khromatografii (Handbook of Gas Chromatography), "Mir", Moscow, 1976. Received: April 25, 1983 Revised manuscript received: July 1, 1983 Accepted: July 5, 1983 A

Chromatographia Vol. 17 No. 9, September 1983

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