- Email: [email protected]
Monitoring the mobile phase composition in supercritical fluid chromatography
103
Analytica Chimica Acta, 280 (1993) 103-109 Elsevier Science Publishers B.V., Amsterdam
Monitoring the mobile phase composition in supercritical fluid chromatography Dongjin Pyo and Hoon Hwang Department of &em&y,
kkgweon National University, Kangweon-do, Chuncheon 200-701 (South Korea)
(Received 15th November 1992; resubmitted 22nd January 1993)
Abstract Adding additional components to supercritical CO, in supercritical fluid chromatography can extend or significantly alter the fluid solvating properties. Polar samples that are diffkult to analyse using pure supercritical CO, because of their high polarity can be separated by adding polar modifiers to the supercritical CO,. A method for monitoring the mobile phase composition in modified supercritical fluid chromatography is introduced. The amount of water or methanol dissolved in supercritical CO, was measured using an amperometric microsensor made of a thin film of perfluorosulphonate ionomer. Keywords: Supercritical fluid chromatography;
Mobile phase modification
The use of compressed (dense) gases and supercritical fluids as chromatographic mobile phases in conjunction with liquid chromatographic (LC)-type packed columns was first reported by Klesper et al. in 1962 [l]. During its relatively short history, supercritical fluid chromatography (SFC) has become an attractive alternative to GC and LC in certain industrially important applications. SFC gives the advantage of high efficiency and allows the analysis of nonvolatile or thermally labile mixtures. The density of the mobile phase in SFC is about 200-500 times that in gas chromatography. The effect of shorter intermolecular distances and the resulting increase in molecular interactions is an enhanced solubilizing capability of the solvent towards various solutes. Compounds with much higher molecular weights than gas chromatography normally allows can therefore be Correspondence to: D. Pyo, Department of Chemistry, Kangweon National University, Kangweon-do, Cbuncheon 200-701 (South-Korea). 0003-2670/93/$06.00
chromatographed. However, the most commonly used mobile phases in SFC are all relatively nonpolar fluids. Carbon dioxide, the most widely used fluid, is no more polar than hexane [2], even at high densities. Solute polarity should be between that of the stationary phase and the mobile phase in order to have a well behaved separation. Few real samples contain only non-polar solutes, so a major objective of research in SFC has been directed toward increasing the range of solute polarity that can be handled by the technique. To bring the SFC technique into routine use, mobile phases that are more polar than the commonly used carbon dioxide are necessary. The solvent strength of supercritical CO,, even at high density, is not sufficient for the elution of polar solutes. Polar mobile phases such as NH, [41 exhibit useful properties, but a more practical way to extend the range of compounds separable by SFC is to use a mixed mobile phase. The solubility of the solute in the supercritical phase can be influenced considerably by adding modifiers to the mobile phase. The use of modifiers
0 1993 - Elsevier Science Publishers B.V. All rights reserved
104
has been reported by Jentoft and Gouw [3] and by Novotny et al. [4]. The latter group showed that adding 0.1% 2-propanol to pentane as the mobile phase decreases the observed partition coefficient (K) values for many polynuclear aromatic hydrocarbons by 20-35%. Thus, the addition of modifiers (generally organic solvents) to a supercritical mobile phase changes the polarity of the mobile phase and also leads to deactivation of the column packing material. In capillary SFC, most separations are made with pure CO, because of its compatibility with an flame ionization detection (FID); except for formic acid and water, the addition of any common modifier precludes the use of FID [5]. Modifiers are essential in packed-column SFC for the elution of polar compounds [6] and are extensively used. Several workers [6-81 have reported the influence of modifiers on peak shape, selectivity and retention time in capillary and packed-column SFC. A simple and effective way to add modifiers to a supercritical fluid mobile phase is to use a saturator column [9,103, which is usually a silica column saturated with polar modifiers. The column is inserted between the pump outlet and the injection valve. During the passage of the supercritical fluid mobile phase through the silica column, polar modifiers can be dissolved in the pressurized supercritical fluid. A disadvantage with this system is that the amount of modifier dissolved in the mobile phase varies as the mobile phase passes through the saturator column as the modifier holding capacity of the silica column is limited. Therefore, the monitoring of the amount of polar modifiers dissolved in a supercritical fluid is essential. In this paper, a method for the accurate determination of modifiers in the mobile phase in modified SFC is introduced.
EXPERIMENTAL
A CCS (Computer Chemical Systems, Avondale, PA) Model 5000 supercritical fluid chromatograph was used with a 100 mm X 2 mm i.d. packed column (Nucleosil diol). This system was equipped with a C14W loop injector (Valco) and a flame ionization detector. SFC-grade carbon
D. pVo and H. Hwang/Anal. Chim. Acta 280 (1993) 103-109
dioxide (Scott Specialty Gases) was used as a basic mobile phase. Experimental conditions for SFC separations are as follows: supercritical CO, at 15o”C, pressure programmed from 27.58 to 34.47 MPa at 0.276 MPa min-‘, detector at 300°C and 10 ml min-’ restrictor flow-rate at 10.34 MPa. For the addition of modifiers to supercritical CO,, a FPorasil column (250 mm x 4.6 mm i.d.1 which is manufactured for normal-phase HPLC by Waters was used. Its functional group is a silanol (SiOH) group. The PPorasil column was saturated with modifiers using a Model 600 syringe pump (Lee Scientific) and placed between the pump and injector. To measure the amount of modifier dissolved in the supercritical fluid, an amperometric microsensor was designed and made of perfluorosulphonate ionomer (PFSI) [ll]. A constant-current power supply (0.1 PA) (Sungeun, Seoul, South Korea) was used to measure the voltage drop across the sensor. The sensor output was recorded on a strip-chart recorder (Knauer). Figure 1 shows the cross-section of the modifier sensor used. Platinum wire was wrapped with PFSI thin film and another platinum wire was wound in a coil over the assembly. The sensor made in this way was placed in a plastic tube and the entire modifier measuring device was assembled as shown in Fig. 2.
RESULTS AND DISCUSSION
When modifiers are used with supercritical CO, in order to chromatograph more polar substances, the binary mixture of eluents can contaminate the instrument. Especially when water or formic acid is used as a modifier, the modifier remaining in a pump may cause corrosion of the pump, and when methanol is used as a modifier, methanol remaining in the pump can be eluted slowly during the next run. This may affect the time required to achieve chemical equilibrium for the subsequent separations, and also many modifiers can evaporate and contaminate the air in the laboratory. A good way to overcome these problems is to use a saturator column [9,10,12] to add polar modifiers to supercritical CO,. For the first experiment, water was used as a
D. pVo and H. Hwang/Anal.
PFSI Pt-wire Fig. 1. Diagram of the modifier sensor.
modifier and a PPorasil column was used as a saturator cohunn with system design similar to that described by Engelhardt et al. [9]. With this design, a polar modifier (water) can be added to pressurized CO, after the pump, and thus no modifier remains in the pump. Supercritical CO, is delivered from the pump to the PPorasil column which is saturated with water. When supercritical CO, passes through the PPorasil column, water molecules held on the OH groups of the ~Porasil by hydrogen bonding can dissolve in the
I
A-
ch
105
Chim. Acta 280 (1993) 103-109
pressurized supercritical fluid. Thus non-polar supercritical CO2 can have the characteristics of a polar mobile phase because it can absorb a polar solvent, i.e., water. Therefore, after passing through the PPorasil column, supercritical CO, is changed into a new mobile phase with different polarity, and it is possible to separate polar samples using this modified mobile phase. Mixtures of insecticides and fungicides were analysed with this modified mobile phase (supercritical carbon dioxide-water phase) using a PPorasil column as saturator column. The chromatograms are shown in Figs. 3 and 4. As expected, good separations were obtained. When only CO, was used as a mobile phase with these samples, unseparated and very broad peaks were observed in about 25 min. The addition of a small amount of water to supercritical CO, decreased the retention time and improved the peak shapes. These results agree well with those reported by Engelhardt et al. [9], Schwartz et al. [lo] and Blilie and Greibrokk [7]. The molecular struc-
Sample Gas in
Si
A
-i--
f Air in
P DC II PPC
1
out Fig. 2. Schematic diagram of the water content measuring device: P = pump; V = solenoid valve; S = sensor; FM = flow meter; MP = magnesium perchlorate; R = recorder; M = multimeter; DC = 12-V power supply; PPC = programmable process controller; Ch = charcoal; Si = silica gel; CS = current source.
106
D. pVo and H. Hwang/AnaL Chim. Acta 280 (1993) 103-109
time 0
5.0
t
I 4200
4000
(min) 1qo 1
I
I 4400 pressure(
1s;o I
I 4600
psi)
Fig. 3. Chromatogram of a mixture of insecticides and fungicides. Peaks: A = Thiolii; B = Kitaxine; C = Captan.
tures corresponding to the peaks are shown in Table 1. However, when dealing with the use of a saturator column, a serious problem always arises, namely the bad reproducibility of the amotmt of modifier dissolved in the supercritical CO,. When the same experiments were repeated several times, it was very difficult to obtain reproducible results for the chromatograms as the amount of modifier in the mobile phase does not remain constant with time. For these reasons, it was decided to monitor the exact amount of modifier in the supercritical CO, mobile phase as a function of time. To measure the amount of water dissolved in supercritical CO, in the water-modified system, the PFSI polymer, film [91, which has a high affinity for water, was used. When the PFSI film was in contact with two electrodes and a constant current flowed through the film, the water that partitioned into the film from the surrounding environment was electrolytically de-
TABLE 1 Identification of peaks in the chromatograms Chromatogram
Peak
Commercial name
Chemical name
Fig. 3
A
Thiolix (insecticide)
1,4,5,6,7,7-Hexachloronorbom-5-ene2,3-dimethanol sulphide
B
Kitaxine (fungicide)
S-Benxyl-O,O-diisopropyl
C
Captan (fungicide)
N-(Trichloromethylthiojcyclohex-4-ene1,Zdicarboximide
A
Hinosan (fungicide)
0-Ethyl-S,S-diphenyl
B
Parathion (insecticide)
Structure
0
Fig. 4
phosphorothioate
dithiophosphate
ip*-i-s %d ?
EtO-P,-S-Ph S-Ph
O,O-Diethyl-O-4-nitrophenyl thioate
phosphoro-
! EtO-T-0 Et0
C
DDVP (insecticide)
2,2-Dichlorovinyldimethyl phosphate
:: H,C-O-,P-O*cy H,C-0
D. Pyo and H. Hwang/Anal.
C
time
(min)
0
50
t
I
4000
4200
107
Chim Acta 280 (1993) 103-109
lO0 4400
pressure(psi)
Fig. 4. Chromatogram of a mixture of insecticides and fungicides. Peaks: A = Hinosan; B = Parathion; C = DDVF.
composed. The change in voltage across the two electrodes was used as a measure of the water content of the environment surrounding the sensor. The resistance of the PFSI film changed according to the water content of the supercriti-
cal CO, fluid and the voltage difference between the two platinum wires was recorded. As a control, air saturated with water was injected into the sensor at different time intervals. Air saturated with water was generated by bubbling air through water in two sequential bottles and was injected directly into the sensor through a three-way solenoid valve (Radio Shack). The results are shown in Fig. 5 and demonstrate that there is a good correlation between the voltage difference across the two electrodes of the sensor and the amount of water exposed to the sensor. Another parameter that affects the sensor response is the temperature of the environment surrounding the sensor. To consider the influence of temperature, Fig. 6 was obtained by measuring the sensor outputs with six different standard relative humidity streams at various temperatures. H,SO, solutions of known composition [13] were used to generate the standard relative humidity streams. Using the correlation of peak height and the percentage relative humidities at different temperatures (Fig. 61, the water content in supercritical CO, after passing through the PPorasil column saturated with water could be measured as a function of time (Fig. 7). Figure 7 shows that the amount of water in the mobile phase remained constant for about 35 min, but then decreased
Fig. 5. Relative peak heights at different time intervals for air saturated with water. Flow-rate, 0.5 1 min-‘; ordinate scale, 200 mV cm-‘.
temperature,
20°C;
D. Pyo and H. Hwang/Anal. Chim. Acta 280 (1993) 103-109 Peak r
Hcight,mm(Voltage,5Omm=1000mV) 1
Cont. of MeOH (lo-4 M)
30
36
40
25 c3
30
20
* 0
0
15 -
20
0 0 *
10 / 10
t
0
m
5, -
*
0
0
10
20
30
40
Percent
50
60
Relatve
70
80
90
100
O-
Humidity
Fig. 6. Sensor response with various relative humidity levels (18.8, 37.1, 47.2, 58.3, 70.4 and 100%) at various temperatures: + = 10; * = 15; 0 = 20; X = 25T.
exponentially. This is, of course, because only limited amounts of water were held on the OH groups of the PPorasil. As supercritical CO, was passing through PPorasil column saturated with water, the fluid stripped water from the PPorasil and thus water held within the pores of the column was desorbed with time. Relative 70
50
Humidity(%)
**
1 0
*
0
40 *
30
01
0
*
1
I
I
I
1
12
24
36
48
60
P @
I
J 72
Time(min) Fig. 7. Measurements of the water content in supercritical CO, as a function of time at (* 1 100 and (01200 atm.
0
10
20
30
40
50
60
70
80
90
Time (mid Fig. 8. Measurements of the methanol content in supercritical CO, as a function of time at (*) 100 and (0) 200 atm.
Another experiment was made using methanol as a modifier, in which the same ~Porasil column and the same sensor were employed. The PFSI sensor is known to respond to alcohols in a way similar to water [ll]. In this methanol-modified system using a saturator, the mobile phase composition could be monitored successfully in the same way. The results are shown in Fig. 8. With methanol, the time period during which a constant concentration of methanol was maintained in supercritical CO, was much shorter than that for water. The amount of methanol in the mobile phase remained constant for only 10 min and then decreased rapidly, probably because the solubility of methanol in supercritical CO, is greater than that of water [14]. Therefore, when a saturator column is used for addition of a modifier to carbon dioxide, it must be resaturated with modifier after a certain period of time. With the saturator column is used in this study, and using water as a modifier, the column must be resaturated with water every 35 min, and when methanol is used, it must be resaturated every 10 min to obtain reproducible separations. When methanol is used as a modifier, it is essential to maintain a constant level of
D. Pyo and H. Hwang/Anal.
Chim. Acta 280 (1993) 103-109
modifier in the mobile phase, otherwise it is almost impossible to obtain reproducible separations. A mixing device that can maintain the amount of modifier in the mobile phase constant for a much longer period is currently being developed. This investigation was supported by a grant from the Korea Science and Engineering Foundation.
REFERENCES E. Klesper, AH. Convin and D.A. Turner, J. Org. Chem., 27 (1962) 700. CR. Yonker, S.L. Frye, D.R. Lalkwarf and R.D. Smith, J. Phys. Chem., 90 (1986) 90, 3022. R.E. Jentofi and T.H. Gouw, J. Chromatogr. Sci., 8 (1970) 138.
109 M. Novotny, W. Bertsch and A. Zlatkis, J. Chromatogr., 61 (1971) 17. B.W. Wright and R.D. Smith, J. Chromatogr., 355 (1986) 367. S. Schmidt, L.G. Blomberg and E.R. Campbell, Chromatographia, 25 (1988) 775. A.L. Blilie and T. Greibrokk, Anal. Chem., 57 (1985) 2239. C.R. Yonker and R.D. Smith, J. Chromatogr., 361 (1986) 25. 9 H. Engelhardt, A. Gross, R. Mertens and M. Petersen, J. Chromatogr., 477 (1989) 169. 10 H.E. Schwartz, P.J. Barthel, S.E. Moring, T.L. Yates and H.H. Lauer, Fresenius’ Z. Anal. Chem., 330 (1988) 204. 11 H. Huang and P.K. Dasgupta, Anal. Chem., 62 (1990) 1935. 12 B.L. Karger, R.C. Castells, P.A. Sewell and A. Hartkopf, J. Phys. Chem., 75 (1971) 3870. 13 R.C. Weast (Ed.), CRC Handbook of Chemistry and Physics, CRC Boca Raton, FL, 70th edn., 1989, p. E-43. 14 B.A. Charpentier, in M.R. Sevenants (Ed.), Supercritical Fluid Extraction and Chromatography (ACS Symposium Series, No. 3661, American Chemical Society, Washington, DC, 1987, p. 16.
Analytica Chimica Acta, 280 (1993) 103-109 Elsevier Science Publishers B.V., Amsterdam
Monitoring the mobile phase composition in supercritical fluid chromatography Dongjin Pyo and Hoon Hwang Department of &em&y,
kkgweon National University, Kangweon-do, Chuncheon 200-701 (South Korea)
(Received 15th November 1992; resubmitted 22nd January 1993)
Abstract Adding additional components to supercritical CO, in supercritical fluid chromatography can extend or significantly alter the fluid solvating properties. Polar samples that are diffkult to analyse using pure supercritical CO, because of their high polarity can be separated by adding polar modifiers to the supercritical CO,. A method for monitoring the mobile phase composition in modified supercritical fluid chromatography is introduced. The amount of water or methanol dissolved in supercritical CO, was measured using an amperometric microsensor made of a thin film of perfluorosulphonate ionomer. Keywords: Supercritical fluid chromatography;
Mobile phase modification
The use of compressed (dense) gases and supercritical fluids as chromatographic mobile phases in conjunction with liquid chromatographic (LC)-type packed columns was first reported by Klesper et al. in 1962 [l]. During its relatively short history, supercritical fluid chromatography (SFC) has become an attractive alternative to GC and LC in certain industrially important applications. SFC gives the advantage of high efficiency and allows the analysis of nonvolatile or thermally labile mixtures. The density of the mobile phase in SFC is about 200-500 times that in gas chromatography. The effect of shorter intermolecular distances and the resulting increase in molecular interactions is an enhanced solubilizing capability of the solvent towards various solutes. Compounds with much higher molecular weights than gas chromatography normally allows can therefore be Correspondence to: D. Pyo, Department of Chemistry, Kangweon National University, Kangweon-do, Cbuncheon 200-701 (South-Korea). 0003-2670/93/$06.00
chromatographed. However, the most commonly used mobile phases in SFC are all relatively nonpolar fluids. Carbon dioxide, the most widely used fluid, is no more polar than hexane [2], even at high densities. Solute polarity should be between that of the stationary phase and the mobile phase in order to have a well behaved separation. Few real samples contain only non-polar solutes, so a major objective of research in SFC has been directed toward increasing the range of solute polarity that can be handled by the technique. To bring the SFC technique into routine use, mobile phases that are more polar than the commonly used carbon dioxide are necessary. The solvent strength of supercritical CO,, even at high density, is not sufficient for the elution of polar solutes. Polar mobile phases such as NH, [41 exhibit useful properties, but a more practical way to extend the range of compounds separable by SFC is to use a mixed mobile phase. The solubility of the solute in the supercritical phase can be influenced considerably by adding modifiers to the mobile phase. The use of modifiers
0 1993 - Elsevier Science Publishers B.V. All rights reserved
104
has been reported by Jentoft and Gouw [3] and by Novotny et al. [4]. The latter group showed that adding 0.1% 2-propanol to pentane as the mobile phase decreases the observed partition coefficient (K) values for many polynuclear aromatic hydrocarbons by 20-35%. Thus, the addition of modifiers (generally organic solvents) to a supercritical mobile phase changes the polarity of the mobile phase and also leads to deactivation of the column packing material. In capillary SFC, most separations are made with pure CO, because of its compatibility with an flame ionization detection (FID); except for formic acid and water, the addition of any common modifier precludes the use of FID [5]. Modifiers are essential in packed-column SFC for the elution of polar compounds [6] and are extensively used. Several workers [6-81 have reported the influence of modifiers on peak shape, selectivity and retention time in capillary and packed-column SFC. A simple and effective way to add modifiers to a supercritical fluid mobile phase is to use a saturator column [9,103, which is usually a silica column saturated with polar modifiers. The column is inserted between the pump outlet and the injection valve. During the passage of the supercritical fluid mobile phase through the silica column, polar modifiers can be dissolved in the pressurized supercritical fluid. A disadvantage with this system is that the amount of modifier dissolved in the mobile phase varies as the mobile phase passes through the saturator column as the modifier holding capacity of the silica column is limited. Therefore, the monitoring of the amount of polar modifiers dissolved in a supercritical fluid is essential. In this paper, a method for the accurate determination of modifiers in the mobile phase in modified SFC is introduced.
EXPERIMENTAL
A CCS (Computer Chemical Systems, Avondale, PA) Model 5000 supercritical fluid chromatograph was used with a 100 mm X 2 mm i.d. packed column (Nucleosil diol). This system was equipped with a C14W loop injector (Valco) and a flame ionization detector. SFC-grade carbon
D. pVo and H. Hwang/Anal. Chim. Acta 280 (1993) 103-109
dioxide (Scott Specialty Gases) was used as a basic mobile phase. Experimental conditions for SFC separations are as follows: supercritical CO, at 15o”C, pressure programmed from 27.58 to 34.47 MPa at 0.276 MPa min-‘, detector at 300°C and 10 ml min-’ restrictor flow-rate at 10.34 MPa. For the addition of modifiers to supercritical CO,, a FPorasil column (250 mm x 4.6 mm i.d.1 which is manufactured for normal-phase HPLC by Waters was used. Its functional group is a silanol (SiOH) group. The PPorasil column was saturated with modifiers using a Model 600 syringe pump (Lee Scientific) and placed between the pump and injector. To measure the amount of modifier dissolved in the supercritical fluid, an amperometric microsensor was designed and made of perfluorosulphonate ionomer (PFSI) [ll]. A constant-current power supply (0.1 PA) (Sungeun, Seoul, South Korea) was used to measure the voltage drop across the sensor. The sensor output was recorded on a strip-chart recorder (Knauer). Figure 1 shows the cross-section of the modifier sensor used. Platinum wire was wrapped with PFSI thin film and another platinum wire was wound in a coil over the assembly. The sensor made in this way was placed in a plastic tube and the entire modifier measuring device was assembled as shown in Fig. 2.
RESULTS AND DISCUSSION
When modifiers are used with supercritical CO, in order to chromatograph more polar substances, the binary mixture of eluents can contaminate the instrument. Especially when water or formic acid is used as a modifier, the modifier remaining in a pump may cause corrosion of the pump, and when methanol is used as a modifier, methanol remaining in the pump can be eluted slowly during the next run. This may affect the time required to achieve chemical equilibrium for the subsequent separations, and also many modifiers can evaporate and contaminate the air in the laboratory. A good way to overcome these problems is to use a saturator column [9,10,12] to add polar modifiers to supercritical CO,. For the first experiment, water was used as a
D. pVo and H. Hwang/Anal.
PFSI Pt-wire Fig. 1. Diagram of the modifier sensor.
modifier and a PPorasil column was used as a saturator cohunn with system design similar to that described by Engelhardt et al. [9]. With this design, a polar modifier (water) can be added to pressurized CO, after the pump, and thus no modifier remains in the pump. Supercritical CO, is delivered from the pump to the PPorasil column which is saturated with water. When supercritical CO, passes through the PPorasil column, water molecules held on the OH groups of the ~Porasil by hydrogen bonding can dissolve in the
I
A-
ch
105
Chim. Acta 280 (1993) 103-109
pressurized supercritical fluid. Thus non-polar supercritical CO2 can have the characteristics of a polar mobile phase because it can absorb a polar solvent, i.e., water. Therefore, after passing through the PPorasil column, supercritical CO, is changed into a new mobile phase with different polarity, and it is possible to separate polar samples using this modified mobile phase. Mixtures of insecticides and fungicides were analysed with this modified mobile phase (supercritical carbon dioxide-water phase) using a PPorasil column as saturator column. The chromatograms are shown in Figs. 3 and 4. As expected, good separations were obtained. When only CO, was used as a mobile phase with these samples, unseparated and very broad peaks were observed in about 25 min. The addition of a small amount of water to supercritical CO, decreased the retention time and improved the peak shapes. These results agree well with those reported by Engelhardt et al. [9], Schwartz et al. [lo] and Blilie and Greibrokk [7]. The molecular struc-
Sample Gas in
Si
A
-i--
f Air in
P DC II PPC
1
out Fig. 2. Schematic diagram of the water content measuring device: P = pump; V = solenoid valve; S = sensor; FM = flow meter; MP = magnesium perchlorate; R = recorder; M = multimeter; DC = 12-V power supply; PPC = programmable process controller; Ch = charcoal; Si = silica gel; CS = current source.
106
D. pVo and H. Hwang/AnaL Chim. Acta 280 (1993) 103-109
time 0
5.0
t
I 4200
4000
(min) 1qo 1
I
I 4400 pressure(
1s;o I
I 4600
psi)
Fig. 3. Chromatogram of a mixture of insecticides and fungicides. Peaks: A = Thiolii; B = Kitaxine; C = Captan.
tures corresponding to the peaks are shown in Table 1. However, when dealing with the use of a saturator column, a serious problem always arises, namely the bad reproducibility of the amotmt of modifier dissolved in the supercritical CO,. When the same experiments were repeated several times, it was very difficult to obtain reproducible results for the chromatograms as the amount of modifier in the mobile phase does not remain constant with time. For these reasons, it was decided to monitor the exact amount of modifier in the supercritical CO, mobile phase as a function of time. To measure the amount of water dissolved in supercritical CO, in the water-modified system, the PFSI polymer, film [91, which has a high affinity for water, was used. When the PFSI film was in contact with two electrodes and a constant current flowed through the film, the water that partitioned into the film from the surrounding environment was electrolytically de-
TABLE 1 Identification of peaks in the chromatograms Chromatogram
Peak
Commercial name
Chemical name
Fig. 3
A
Thiolix (insecticide)
1,4,5,6,7,7-Hexachloronorbom-5-ene2,3-dimethanol sulphide
B
Kitaxine (fungicide)
S-Benxyl-O,O-diisopropyl
C
Captan (fungicide)
N-(Trichloromethylthiojcyclohex-4-ene1,Zdicarboximide
A
Hinosan (fungicide)
0-Ethyl-S,S-diphenyl
B
Parathion (insecticide)
Structure
0
Fig. 4
phosphorothioate
dithiophosphate
ip*-i-s %d ?
EtO-P,-S-Ph S-Ph
O,O-Diethyl-O-4-nitrophenyl thioate
phosphoro-
! EtO-T-0 Et0
C
DDVP (insecticide)
2,2-Dichlorovinyldimethyl phosphate
:: H,C-O-,P-O*cy H,C-0
D. Pyo and H. Hwang/Anal.
C
time
(min)
0
50
t
I
4000
4200
107
Chim Acta 280 (1993) 103-109
lO0 4400
pressure(psi)
Fig. 4. Chromatogram of a mixture of insecticides and fungicides. Peaks: A = Hinosan; B = Parathion; C = DDVF.
composed. The change in voltage across the two electrodes was used as a measure of the water content of the environment surrounding the sensor. The resistance of the PFSI film changed according to the water content of the supercriti-
cal CO, fluid and the voltage difference between the two platinum wires was recorded. As a control, air saturated with water was injected into the sensor at different time intervals. Air saturated with water was generated by bubbling air through water in two sequential bottles and was injected directly into the sensor through a three-way solenoid valve (Radio Shack). The results are shown in Fig. 5 and demonstrate that there is a good correlation between the voltage difference across the two electrodes of the sensor and the amount of water exposed to the sensor. Another parameter that affects the sensor response is the temperature of the environment surrounding the sensor. To consider the influence of temperature, Fig. 6 was obtained by measuring the sensor outputs with six different standard relative humidity streams at various temperatures. H,SO, solutions of known composition [13] were used to generate the standard relative humidity streams. Using the correlation of peak height and the percentage relative humidities at different temperatures (Fig. 61, the water content in supercritical CO, after passing through the PPorasil column saturated with water could be measured as a function of time (Fig. 7). Figure 7 shows that the amount of water in the mobile phase remained constant for about 35 min, but then decreased
Fig. 5. Relative peak heights at different time intervals for air saturated with water. Flow-rate, 0.5 1 min-‘; ordinate scale, 200 mV cm-‘.
temperature,
20°C;
D. Pyo and H. Hwang/Anal. Chim. Acta 280 (1993) 103-109 Peak r
Hcight,mm(Voltage,5Omm=1000mV) 1
Cont. of MeOH (lo-4 M)
30
36
40
25 c3
30
20
* 0
0
15 -
20
0 0 *
10 / 10
t
0
m
5, -
*
0
0
10
20
30
40
Percent
50
60
Relatve
70
80
90
100
O-
Humidity
Fig. 6. Sensor response with various relative humidity levels (18.8, 37.1, 47.2, 58.3, 70.4 and 100%) at various temperatures: + = 10; * = 15; 0 = 20; X = 25T.
exponentially. This is, of course, because only limited amounts of water were held on the OH groups of the PPorasil. As supercritical CO, was passing through PPorasil column saturated with water, the fluid stripped water from the PPorasil and thus water held within the pores of the column was desorbed with time. Relative 70
50
Humidity(%)
**
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Time(min) Fig. 7. Measurements of the water content in supercritical CO, as a function of time at (* 1 100 and (01200 atm.
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Time (mid Fig. 8. Measurements of the methanol content in supercritical CO, as a function of time at (*) 100 and (0) 200 atm.
Another experiment was made using methanol as a modifier, in which the same ~Porasil column and the same sensor were employed. The PFSI sensor is known to respond to alcohols in a way similar to water [ll]. In this methanol-modified system using a saturator, the mobile phase composition could be monitored successfully in the same way. The results are shown in Fig. 8. With methanol, the time period during which a constant concentration of methanol was maintained in supercritical CO, was much shorter than that for water. The amount of methanol in the mobile phase remained constant for only 10 min and then decreased rapidly, probably because the solubility of methanol in supercritical CO, is greater than that of water [14]. Therefore, when a saturator column is used for addition of a modifier to carbon dioxide, it must be resaturated with modifier after a certain period of time. With the saturator column is used in this study, and using water as a modifier, the column must be resaturated with water every 35 min, and when methanol is used, it must be resaturated every 10 min to obtain reproducible separations. When methanol is used as a modifier, it is essential to maintain a constant level of
D. Pyo and H. Hwang/Anal.
Chim. Acta 280 (1993) 103-109
modifier in the mobile phase, otherwise it is almost impossible to obtain reproducible separations. A mixing device that can maintain the amount of modifier in the mobile phase constant for a much longer period is currently being developed. This investigation was supported by a grant from the Korea Science and Engineering Foundation.
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