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Effect of pressure in a chromatographic column on retention characteristics of hydrocarbons
EFFECT OF P R E S S U R E IN A CHROMATOGRAPHIC COLUMN ON RETENTION CHARACTERISTICS OF H Y D R O C A R B O N S * ~V[. S. VIGDERGAUZand V. I. SEMKII~ All-Union Scientific Research Institute of Hydrocarbon Raw Material A. Ye. Arbuzov Institute of Organic and Physical Chemistry, U.S.S.R. Academy of Sciences (Received 19 July 1968) CHROMATOGRAPHICseparation is normally effected at pressures close to atmospheric; it is therefore considered that retention characteristics are independent of the nature of the carrier gas. However, on using capillary columns, in which pressure is several atmospheres, deviation from the ideal state in the gaseous phase alters the retention parameters of substances analysed. It was pointed out in a previous paper [1] that the pattern of separation of certain hydrocarbons in a capillary column is altered on changing frQm hydrogen to nitrogen (pressure at the inlet in the column being approximately 10 atm) and a relation was established between the distribution coefficient of the component between the phases and the value of the second virial coefficient in the gas state equation. In recent studies Giddings [2-4] examined the advantages of chromatographic analysis carried out at increased pressure in a column and pointed (particularly in paper [5]) to the posibflity of changing the selectivity of separation of two substances b y increasing pressure. It is quite obvious that the effect of pressure an chromatographic characteristics should be considered when selecting the operating conditions of the column and when identifying the components analysed. We therefore investigated the effect of the nature of carrier gas and pressure on retention parameters of gasoline hydrocarbons. A "Tsvet" chromatographic with a flame-ionization detector was used. The device was equipped for investigation under pressure (fitted with corresponding reducing valves, pressure gauges and filter, and the hermetic closure of joints increased). The copper capillary column was about 120 m in length, 0.25 m in internal diameter. Between the evaporator and column a flow divider was situated and work was carried out with a discharge ratio of the carrier gas into the atmosphere (according to pressure in the column) of up to 25 : 1. To produce increased pressure, the end of t h e column, joined to the flame-ionization detector, was clamped with clips. The octadec-l-ene used as stationary phase had first been used b y Martin and Winters [6] to analyse a gasoline fraction boiling below 111 ° and recog* Neftekhimiya 9, No. 3, 470-475, 1969. 129
130
M.S. VIGDERGAUZand V. I. SEMKIN
nized as optimum phase [7, 8] for the analysis of fractions boiling under 125 °. Octadec-l-ene was applied on the internal walls of the capillary as an 8% solution in n-pentane. Helium, argon and carbon dioxide were used as carrier gases and to obtain comparable results, all experiments were made at practically the same average carrier gas velocity (11 cm/sec for helium, 9 em/sec for argon, 11 cm/sec for carbon dioxide). The average gas velocity was determined as the ratio of column length to retention time of a virtually unabsorbed gas (methane). Average pressure was determined from the Martin formula [9] assuming a constant pressure drop through the column (for a given carrier gas). Inlet pressure of the column varied from 3 to 27 arm. Column temperature was maintained at 25 °. Gasoline fractions boiling below 90, 100 and 120 ° were the mixtures analysed. Retention indices were determined from the hydrocarbon retention times measured [10]: /=100
log (tB~--t0)--log (tnz--to) . . . . . . . . . q-±vvz, log (t~(z+l)--to)--log (tR(z)--to)
(1)
where tp~, tlu and t~(~+'a)~re the retention times of the component analysed and of n-paraffins with z and z-k I carbon atoms in the molecules; t o is the retention time of methane. Figure 1 illustrates the dependence of retention index on average pressure in the column for various carrier gases. The H e n r y coefficient (ratio of volumes of stationary and mobile phases) [11] was also determined: H I - ~ H Kl~---t/~--1, ~¢ to
(2)
where H is the true H e n r y coefficient, K1 and ~ are the proportions of column volume, occupied b y liquid and gaseous phases, respectively. H 1 values for some hydrocarbons are tabulated. Some chromatograms of gasoline fractions are shown in Fig. 2. A study of the dependence of retention index on pressure indicates that in the interval studied the indices vary somewhat with pressure, this variation becoming more significant and dependent on hydrocarbon structure when using argon a n d carbon dioxide than with helium. This is due to the increase in the absolute value of the second virial coefficient. I t is essential here to point out a definite analogy with the tendency of variation of the retention index on changing column temperature. As pointed out previously [8, 12, 13] irrespective of the non-linear relation between retention index and temperature, a linear correlation can be assumed in the temperature range determined and the gradient of lines OI/OT is a characteristic value for a group of compounds of a certain structure (slightly branched
131
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FIQ. 1. Dependence of the retention index of hydrocarbons (I) on average pressure (P) in the column. Carrier gases: helium (a), argon (b), carbon dioxide (c). 1--Isopentane; 2 - - n - p e n t a n e ; 3--2,2-dimethylbutane; 4--cyclopenta~e; 5--2,3-dimethylbutane; 6-2-methylpentane; 7--3-methylpentane; 8--n-hexane; 9--methylcyclopentane; 1 0 - - 2 , 2 dimethylpentane; 1 1 - - 2,4-dimethylpentane; 1 2 - - 2,2,3-trimethylbutane; 1 3 - - benzene; 14--cyclohexane; 15 -- 3,3-dimethylpentane; 16--1,1-dimethylcyclopentane; 17--2-methylhexane; 18 - - 2,3-dimethylpentane; 1 9 - - 3-methylhexane; 20 - - 1,3-dimethylcyclopentane, cis; 2 1 - - 1 , 3 - d i m e t h y l e y c l o p e n t a n e , tr ans ; 2 2 - - 1 , 2 - d i m e t h y l c y c l o p e n t a n e , trans; 2 3 - - 3-ethylpentane; 2 4 - - 2,2,4-trimethylpentane; 2 5 - - n - h e p t a n e ; 2 6 - - 1,2-dimethylcyclopentane, cis; 27--methylcyclohexane; 2 8 - - 1 , 1 , 3 - t r i m e t h y l e y e l o p e n t a n e ; 29--2,2-dimethylhexane; 30--ethyleyclopentane; 31--2,5-dimethylhexane; 32--2,4-dimethylhexane; 3 3 - - 2,2,3-trimethylpentane; 3 4 - - 1,2,4-trimethylcyclopentane, t r a n s , cis; 3 5 - - 3,3-dimethylhexane; 3 6 - - 1,2,3-trimethyleyclopentane,t r a n s , cis; 37--toluene (same as for Fig. 2).
paraffins, strongly branched paraffins, naphthenes, aromatic hydrocarbons) In the event of a pressure change the H e n r y coefficient can be determined from the correlation [1]: 2B/~ In H(~) ~ l n H - } - R---T-' (3) where Hv corresponds to P average pressure in the column, H is the limiting value, which corresponds to zero pressure; B is the second virial coefficient for a mixture of the carrier gas and the substance to be analysed; R is the uni-
132
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Pressure in a chromatographic column
133
EI~FECT OF PRESSURE O1~ THE I-IE~-RYCOEFFICIENT (H 1) FOR He, Ar, CO2 CARRIER GASES Helium
Argon
Carbon dioxide
pressure, arm
Component
I
2.5
9.1 15.8 16.4 2.3 i 5.6 9.1 18.3, 2.1
i
n-Heptane n-I-Iexane Methylcyclopentane Benzene Cyclohexane n-Heptane
6'3 i 12"3
i
17.66
i
0.681 0.69 I 0"701 0"68 0"66 2.16 2"30 2"261 2"231 2"14 2.68 2.85 2'821 2'761 2'68 3.32 3.60 3"541 3'441 3"35 3.77 4-05 4"01 3'901 3'84 7"26
0"63 0.60 2'01 1-89 2'53 2.38 3'17 3.04 3"63 3.39
0.51 0-68 1.561 2-18 1.981 2-73 2-511 3"35 2"801 3"90 4-681 6-89
I
0"60:0'51 1"88 1"54 2-341 1"93 2"85 2'31 3-30! 2"71 5-701 4'40
0"42 1 "23 1"56 1'85 2"14 3'37
versal gas constant, T is absolute c o l u m n t e m p e r a t u r e . H e n c e the r e t e n t i o n index In H x - - l n g z + 2 ~ /5
I(~)= 100
In H(z+l)--ln H + 2 ~
ABx, z + 100z,
(4)
AB(z+I),
where ABe, ~ a n d AB(z+I), z r e p r e s e n t the differences between the second virial coefficients. Thus, the d e p e n d e n c e of r e t e n t i o n i n d e x on pressure is non-linear in general. H o w e v e r , as shown b y Fig. 1, in the pressure i n t e r v a l studied it m a y be a s s u m e d t h a t :
I(~)~,I+AP
(5)
(OI~
w h e r e c o n s t a n t A ----\~]~ is sensitive to the s t r u c t u r e of the substance to be a n a l y s e d (a certain d i s t o r t i o n o f the u p p e r lines in Fig. lb is due to the absence f r o m the c h r o m a t o g r a m of a p e a k of heptane, indices were t h e r e f o r e calculated b y e x t r a p o l a t i o n f r o m the r e t e n t i o n characteristics of p e n t a n e a n d hexane). Calculations show t h a t in c a r b o n dioxide the value of OI/OP for slightly branched paraffins is less t h a n 0.1 a r m -1, for 2 , 2 , 3 - t r i m e t h y l b u t a n e 0.17, for fivem e m b e r e d n a p h t h e n e s 0-2-0.3, for cyclohexane 0.3 a n d for benzene 0-2 a t m -1. Ill argon the OI/OP value for f i v e - m e m b e r e 4 n a p h t h e n e s is 0.13-0.15 a t m -1, for cyclohexane 0.16 a t m -1 a n d for benzene 0.28 a t m -1. I t is quite obvious t h a t these relations can be an additional m e a n s for identifying t h e s u b s t a n c e s analysed, all the m o r e so as in m a n y eases on changing pressure the order of succession o f peaks can also change (Figs. 1 a n d 2). I n c r e a s e d pressure in the column a n d the use of a heavier carrier gas increases the non-ideal state of the gaseous phase, which is expressed in increased
134
M. S. VIGDERGA~YZ a n d V. I. S E ~ r ~
solubility of the substance analysed and a corresponding reduction in retention time. As shown by Giddings [5], the use of pressures of the order of hundreds and more atmospheres facilitates gas-chromatographic elution of nonvolatile high- molecular weight compounds. Thus, work at increased pressures not only increases column efficiency (see e.g. [14]) and somewhat alters selectivity but also reduces adsorption capacity, lowering analysis time and increasing the rapidity coefficient [15]: H
7i
61 5
!
2 3
4 3 2
0.8
a6: o.41 5
6
7
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F I e . 3. Curve showing the d e p e n d e n c e of t h e H e n r y coefficient on the n u m b e r of C a t o m s in a n-paraffin molecule for various a v e r a g e pressures ~ (atm). 1--2.1 arm; 2 - - 6 . 3 a t m ; 3-- 12.3 a t m ; 4 - - 1 7 . 6 arm.
In fact, as shown by the Table, the particular H e n r y coefficient on increasing average pressure by 14-16 arm drops by 30-40% for argon; helium does not produce a corresponding effect. On using carbon dioxide adsorption capacity decreases 1.5-2 times in this pressure range, and for heavy components it decreases more considerably than for light components. This can be seen in particular in Fig. 3. I t m a y be assumed that, on further increasing pressure, the relation between the retention characteristic and the number of C atoms in a n-paraffin molecule is transformed from logarithmic to linear, which is typical of gas chromatography with temperature programming, or with exponential programming of carrier gas velocity. Hence it is evident that gas chromatography at increased pressure can ensure the same advantages as the above methods. The free energy variation of solution of n-paraffins in octadec-l-ene, calculated for the CH~ group in the case of argon, when i6=2-3 atm, is 700 cal
Pressure in a chromatographic column
135
and when P-----18.3 atm, 658 cal (for a hexane-pentane pair). Corresponding values in the case of carbon dioxide are 692 and 621 cal (hexane-pentane), 689 and 596 eal (heptane-hexane). Finally, another very important advantage of carbon dioxide as carrier gas in the analysis of gasoline fractions, and evidently also in the analysis of various polar compounds, should be pointed out. I t is well known that, in analysis of gasoline in capillary columns (particularly copper columns) benzene and toluene often appear ~s strongly asymmetrical peaks, which is due to adsorption by the column wall. I t is natural t h a t this effect considerably reduces the accuracy of quantitative determinations since part of the component zone is beyond the sensitivity limits of the detector. At the same time the use of carbon dioxide enables narrower and more symmetrical peaks of aromatic hydrocarbons to be obtained, which m a y be due to displacement by molecules adsorbed by the inner wall of the column (Fig. 2). Calculations show t h a t on changing from argon to carbon dioxide, the product of the height of benzene peak and the half-width of the peak increases approximately by 35% and the height (in relative units) almost two-fold. An increase in carbon dioxide pressure causes a further increase in the relative height and the peak area of benzene. The authors are grateful to V. V. Pomazanov for his help in preparing the capillary column. SUMMARY
1. On changing the carrier gas and pressure in a chromatographic column the selectivity and duration of separation are modified. 2. The variation of retention index with pressure depends on the structure of the substance analysed, which m a y be an additional means of chromatographic identification. 3. The use of carbon dioxide as carrier gas in the analysis of gasoline fractions considerably increases the accuracy of determining aromatic hydrocarbons. REFERENCES
1. A. GOLDUP, G. R. LOCKHURST and W. T. SWANTON, Nature 193, No. 4813, 333, 1962 2. M. N. MYERS and J. C. GIDDINGS, Analyt. Chem. 37, No. ]2, 1453, 1965; 38, No. 2, 294, 1966 3. J. C. GIDDINGS, g. Chromatogr. 18, 221, 1965 4. J. C. GIDDINGS, Separation Science 1, I~'o. 6, 761, 1966 5. J. C. GIDDINGS, Separation Science 1, 1~o. 1, 73, 1966 6. R. L. MARTIN and J. C. WINTERS, Analyt. Chem. 35, 1930, 1963 7. M. S. WIGDERGAUS, Gas-Chromatographic, 1968, Berlin 8. M. S. WIGDERGAUS and V. V. POMAZANOV, Sb. Uspekhi gazovoi khromatografii, Kazan', 1, 61, 1969 9. A. T. JAMES and A. J. MARTIN, Biochem. g. 50, 679, 1952 10. E. KOVATS, W. SIMONS and E. HEILBRONNER, Helv. ehim. aeta 41, 275, 1958
136
M . S . VIGDERGAUZ a n d V. I. SEMKIN
11. K. A. GOL'BERT a n d M. S. WIGDERGAUS, Kurs gazovoi khromatografii (Cours on Gas Chromatography). Khimiya, Moscow, 1967 12. L. S. ETTRE and K. B. BILLEB, J. Chromatogr. 30, No. l, 1, 1967 13. R. I. SIDOROV and M. P. IVANOVA, Neftekhimiya 7, 640, 1967 14. M. S. WIGDERGAUS, Neftekhimiya 5, No. 3, 425, 1965 15. M. S. WIGDERGAUS, L. V. ANDREYEV and O. G. CHABROVA, Zh. analit, khimii 22, No. 2, 265, 1967
130
M.S. VIGDERGAUZand V. I. SEMKIN
nized as optimum phase [7, 8] for the analysis of fractions boiling under 125 °. Octadec-l-ene was applied on the internal walls of the capillary as an 8% solution in n-pentane. Helium, argon and carbon dioxide were used as carrier gases and to obtain comparable results, all experiments were made at practically the same average carrier gas velocity (11 cm/sec for helium, 9 em/sec for argon, 11 cm/sec for carbon dioxide). The average gas velocity was determined as the ratio of column length to retention time of a virtually unabsorbed gas (methane). Average pressure was determined from the Martin formula [9] assuming a constant pressure drop through the column (for a given carrier gas). Inlet pressure of the column varied from 3 to 27 arm. Column temperature was maintained at 25 °. Gasoline fractions boiling below 90, 100 and 120 ° were the mixtures analysed. Retention indices were determined from the hydrocarbon retention times measured [10]: /=100
log (tB~--t0)--log (tnz--to) . . . . . . . . . q-±vvz, log (t~(z+l)--to)--log (tR(z)--to)
(1)
where tp~, tlu and t~(~+'a)~re the retention times of the component analysed and of n-paraffins with z and z-k I carbon atoms in the molecules; t o is the retention time of methane. Figure 1 illustrates the dependence of retention index on average pressure in the column for various carrier gases. The H e n r y coefficient (ratio of volumes of stationary and mobile phases) [11] was also determined: H I - ~ H Kl~---t/~--1, ~¢ to
(2)
where H is the true H e n r y coefficient, K1 and ~ are the proportions of column volume, occupied b y liquid and gaseous phases, respectively. H 1 values for some hydrocarbons are tabulated. Some chromatograms of gasoline fractions are shown in Fig. 2. A study of the dependence of retention index on pressure indicates that in the interval studied the indices vary somewhat with pressure, this variation becoming more significant and dependent on hydrocarbon structure when using argon a n d carbon dioxide than with helium. This is due to the increase in the absolute value of the second virial coefficient. I t is essential here to point out a definite analogy with the tendency of variation of the retention index on changing column temperature. As pointed out previously [8, 12, 13] irrespective of the non-linear relation between retention index and temperature, a linear correlation can be assumed in the temperature range determined and the gradient of lines OI/OT is a characteristic value for a group of compounds of a certain structure (slightly branched
131
Pressure in a chromatographic column
a
b 22 ~
L
17 f8 5'50 P 15:
:_
=
13~
-.
|12 •
,
-_ :
[5 = f#
,
c
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550 &.
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.
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FIQ. 1. Dependence of the retention index of hydrocarbons (I) on average pressure (P) in the column. Carrier gases: helium (a), argon (b), carbon dioxide (c). 1--Isopentane; 2 - - n - p e n t a n e ; 3--2,2-dimethylbutane; 4--cyclopenta~e; 5--2,3-dimethylbutane; 6-2-methylpentane; 7--3-methylpentane; 8--n-hexane; 9--methylcyclopentane; 1 0 - - 2 , 2 dimethylpentane; 1 1 - - 2,4-dimethylpentane; 1 2 - - 2,2,3-trimethylbutane; 1 3 - - benzene; 14--cyclohexane; 15 -- 3,3-dimethylpentane; 16--1,1-dimethylcyclopentane; 17--2-methylhexane; 18 - - 2,3-dimethylpentane; 1 9 - - 3-methylhexane; 20 - - 1,3-dimethylcyclopentane, cis; 2 1 - - 1 , 3 - d i m e t h y l e y c l o p e n t a n e , tr ans ; 2 2 - - 1 , 2 - d i m e t h y l c y c l o p e n t a n e , trans; 2 3 - - 3-ethylpentane; 2 4 - - 2,2,4-trimethylpentane; 2 5 - - n - h e p t a n e ; 2 6 - - 1,2-dimethylcyclopentane, cis; 27--methylcyclohexane; 2 8 - - 1 , 1 , 3 - t r i m e t h y l e y e l o p e n t a n e ; 29--2,2-dimethylhexane; 30--ethyleyclopentane; 31--2,5-dimethylhexane; 32--2,4-dimethylhexane; 3 3 - - 2,2,3-trimethylpentane; 3 4 - - 1,2,4-trimethylcyclopentane, t r a n s , cis; 3 5 - - 3,3-dimethylhexane; 3 6 - - 1,2,3-trimethyleyclopentane,t r a n s , cis; 37--toluene (same as for Fig. 2).
paraffins, strongly branched paraffins, naphthenes, aromatic hydrocarbons) In the event of a pressure change the H e n r y coefficient can be determined from the correlation [1]: 2B/~ In H(~) ~ l n H - } - R---T-' (3) where Hv corresponds to P average pressure in the column, H is the limiting value, which corresponds to zero pressure; B is the second virial coefficient for a mixture of the carrier gas and the substance to be analysed; R is the uni-
132
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Pressure in a chromatographic column
133
EI~FECT OF PRESSURE O1~ THE I-IE~-RYCOEFFICIENT (H 1) FOR He, Ar, CO2 CARRIER GASES Helium
Argon
Carbon dioxide
pressure, arm
Component
I
2.5
9.1 15.8 16.4 2.3 i 5.6 9.1 18.3, 2.1
i
n-Heptane n-I-Iexane Methylcyclopentane Benzene Cyclohexane n-Heptane
6'3 i 12"3
i
17.66
i
0.681 0.69 I 0"701 0"68 0"66 2.16 2"30 2"261 2"231 2"14 2.68 2.85 2'821 2'761 2'68 3.32 3.60 3"541 3'441 3"35 3.77 4-05 4"01 3'901 3'84 7"26
0"63 0.60 2'01 1-89 2'53 2.38 3'17 3.04 3"63 3.39
0.51 0-68 1.561 2-18 1.981 2-73 2-511 3"35 2"801 3"90 4-681 6-89
I
0"60:0'51 1"88 1"54 2-341 1"93 2"85 2'31 3-30! 2"71 5-701 4'40
0"42 1 "23 1"56 1'85 2"14 3'37
versal gas constant, T is absolute c o l u m n t e m p e r a t u r e . H e n c e the r e t e n t i o n index In H x - - l n g z + 2 ~ /5
I(~)= 100
In H(z+l)--ln H + 2 ~
ABx, z + 100z,
(4)
AB(z+I),
where ABe, ~ a n d AB(z+I), z r e p r e s e n t the differences between the second virial coefficients. Thus, the d e p e n d e n c e of r e t e n t i o n i n d e x on pressure is non-linear in general. H o w e v e r , as shown b y Fig. 1, in the pressure i n t e r v a l studied it m a y be a s s u m e d t h a t :
I(~)~,I+AP
(5)
(OI~
w h e r e c o n s t a n t A ----\~]~ is sensitive to the s t r u c t u r e of the substance to be a n a l y s e d (a certain d i s t o r t i o n o f the u p p e r lines in Fig. lb is due to the absence f r o m the c h r o m a t o g r a m of a p e a k of heptane, indices were t h e r e f o r e calculated b y e x t r a p o l a t i o n f r o m the r e t e n t i o n characteristics of p e n t a n e a n d hexane). Calculations show t h a t in c a r b o n dioxide the value of OI/OP for slightly branched paraffins is less t h a n 0.1 a r m -1, for 2 , 2 , 3 - t r i m e t h y l b u t a n e 0.17, for fivem e m b e r e d n a p h t h e n e s 0-2-0.3, for cyclohexane 0.3 a n d for benzene 0-2 a t m -1. Ill argon the OI/OP value for f i v e - m e m b e r e 4 n a p h t h e n e s is 0.13-0.15 a t m -1, for cyclohexane 0.16 a t m -1 a n d for benzene 0.28 a t m -1. I t is quite obvious t h a t these relations can be an additional m e a n s for identifying t h e s u b s t a n c e s analysed, all the m o r e so as in m a n y eases on changing pressure the order of succession o f peaks can also change (Figs. 1 a n d 2). I n c r e a s e d pressure in the column a n d the use of a heavier carrier gas increases the non-ideal state of the gaseous phase, which is expressed in increased
134
M. S. VIGDERGA~YZ a n d V. I. S E ~ r ~
solubility of the substance analysed and a corresponding reduction in retention time. As shown by Giddings [5], the use of pressures of the order of hundreds and more atmospheres facilitates gas-chromatographic elution of nonvolatile high- molecular weight compounds. Thus, work at increased pressures not only increases column efficiency (see e.g. [14]) and somewhat alters selectivity but also reduces adsorption capacity, lowering analysis time and increasing the rapidity coefficient [15]: H
7i
61 5
!
2 3
4 3 2
0.8
a6: o.41 5
6
7
Numbep of C atoms
F I e . 3. Curve showing the d e p e n d e n c e of t h e H e n r y coefficient on the n u m b e r of C a t o m s in a n-paraffin molecule for various a v e r a g e pressures ~ (atm). 1--2.1 arm; 2 - - 6 . 3 a t m ; 3-- 12.3 a t m ; 4 - - 1 7 . 6 arm.
In fact, as shown by the Table, the particular H e n r y coefficient on increasing average pressure by 14-16 arm drops by 30-40% for argon; helium does not produce a corresponding effect. On using carbon dioxide adsorption capacity decreases 1.5-2 times in this pressure range, and for heavy components it decreases more considerably than for light components. This can be seen in particular in Fig. 3. I t m a y be assumed that, on further increasing pressure, the relation between the retention characteristic and the number of C atoms in a n-paraffin molecule is transformed from logarithmic to linear, which is typical of gas chromatography with temperature programming, or with exponential programming of carrier gas velocity. Hence it is evident that gas chromatography at increased pressure can ensure the same advantages as the above methods. The free energy variation of solution of n-paraffins in octadec-l-ene, calculated for the CH~ group in the case of argon, when i6=2-3 atm, is 700 cal
Pressure in a chromatographic column
135
and when P-----18.3 atm, 658 cal (for a hexane-pentane pair). Corresponding values in the case of carbon dioxide are 692 and 621 cal (hexane-pentane), 689 and 596 eal (heptane-hexane). Finally, another very important advantage of carbon dioxide as carrier gas in the analysis of gasoline fractions, and evidently also in the analysis of various polar compounds, should be pointed out. I t is well known that, in analysis of gasoline in capillary columns (particularly copper columns) benzene and toluene often appear ~s strongly asymmetrical peaks, which is due to adsorption by the column wall. I t is natural t h a t this effect considerably reduces the accuracy of quantitative determinations since part of the component zone is beyond the sensitivity limits of the detector. At the same time the use of carbon dioxide enables narrower and more symmetrical peaks of aromatic hydrocarbons to be obtained, which m a y be due to displacement by molecules adsorbed by the inner wall of the column (Fig. 2). Calculations show t h a t on changing from argon to carbon dioxide, the product of the height of benzene peak and the half-width of the peak increases approximately by 35% and the height (in relative units) almost two-fold. An increase in carbon dioxide pressure causes a further increase in the relative height and the peak area of benzene. The authors are grateful to V. V. Pomazanov for his help in preparing the capillary column. SUMMARY
1. On changing the carrier gas and pressure in a chromatographic column the selectivity and duration of separation are modified. 2. The variation of retention index with pressure depends on the structure of the substance analysed, which m a y be an additional means of chromatographic identification. 3. The use of carbon dioxide as carrier gas in the analysis of gasoline fractions considerably increases the accuracy of determining aromatic hydrocarbons. REFERENCES
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