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Toluene solvent system for the headspace gas chromatographic determination of styrene monomer in polystyrene
Analytica Chimica Acta, 218 (1989) 345-350 Elsevier Science Publishers B.V., Amsterdam -
345 Printed in The Netherlands
Short Communication
TOLUENE SOLVENT SYSTEM FOR THE HEADSPACE GAS CHROMATOGRAPHIC DETERMINATION OF STYRENE MONOMER IN POLYSTYRENE
M.K. WONG*, L.M. GAN, L.L. KOH and K.H. NG Department of Chemistry, National University of Singapore, Singapore (Singapore) (Received 18th August 1988)
Summary. The use of toluene as a solvent to dissolve polystyrene for the determination of styrene monomers is studied. Compared with the modified solution headspace method using dimethylacetamide as solvent, with toluene the system is slightly less sensitive. However, there appeared to be minimal matrix effect when toluene is used, so a simpler external calibration method can be applied.
Several methods are available for the gas chromatographic determination of trace styrene monomers in polystyrene products. The IS0 method [ 11, involving dissolution of the polymer sample in an appropriate solvent, precipitation of the polymer by addition of methanol and injection of the supernatant liquid into the gas chromatograph, is widely used. The detection limit is about 10 pg ml-‘. In recent years, headspace gas chromatographic methods have gained wide acceptance for the determination of styrene and other volatile monomers. The modified solution approach by Steichen [2] increased the quantitation limit for styrene many-fold over that of the normal solution headspace method [ 31. In Steichen’s method, a polystyrene sample is first dissolved in dimethylacetamide (DMA) and water is injected into the solution to precipitate polystyrene and to separate styrene monomer in the liquid phase. The matrix effect due to polystyrene precipitation is appreciable, so the tedious standard addition method must be used. In most of the studies on the determination of styrene monomer packed columns were used [ 2-81 and there have been fewer reports on the application of headspace gas chromatography using capillary columns [g-11]. Compared with packed columns, capillary columns are more sensitive and, having a superior separation efficiency, tailing of the solvent peak is expected to be much less of a problem. This investigation was initiated to study the use of toluene as a solvent in the determination of styrene monomer in polystyrene samples.
0003-2670/89/$03.50
0 1989 Elsevier Science Publishers B.V.
A comparative study was also made of the relative sensitivities and DMA as solvents in the modified solution approach.
with toluene
Experimental Apparatus and materials. A Hewlett-Packard Model 5890A gas chromatograph equipped with a capillary inlet system and flame ionization detector was used and headspace sampling was done with a Hewlett-Packard Model 19395A headspace sampler. The headspace chromatographic conditions are given in Table 1. The column used was a 30 m x 0.32 mm i.d. fused-silica capillary containing polar Supelcowax-10 (Supelco). A Hewlett-Packard HP-l non-polar column was used for confirmation. A Hewlett-Packard Model 3393A computing integrator was used for recording the chromatograms. Liquid chromatographic-grade (99.9% ) N,N-dimethylacetamide (Aldrich) was used without purification. Technical-grade toluene and ethanol purchased from a local refinery were distilled twice over a 1.2-m column packed with Raschig rings. Styrene monomer ( > 99%, as analysed by liquid chromatography) (Fluka) was used for the preparation of 2000 pg ml-l stock solutions in the respective solvents. The working standards were prepared by serial dilution with the solvent. Sample preparation and determination. Polystyrene samples were cut into small pieces with a pair of stainless-steel scissors and about 0.25 g was weighed into a lo-ml headspace vial. A 4-ml volume of solvent (DMA or toluene) was added to dissolve the sample. The vial was capped with a teflon-coated septum and finally sealed with a crimped cap. With DMA as solvent, 3 ml of water were injected with a syringe into the solution and the mixture was shaken; polymer material precipitated immediately. The vial was then placed in the headspace sampler and equilibrated for 2 h prior to headspace sampling. With toluene as solvent, the use of ethanol to precipitate polystyrene from the solution was investigated at an equilibration temperature of 70’ C. Two methods of measurement in the two solvents were studied. In the direct TABLE 1 Headepace gas chromatographic conditions Carrier gas (nitrogen) flow-rate (column) Splitting ratio Oven temperature: Temperature 1 (time) Rate 1 (50-120°C) Rate 2 (120-200°C) Temperature 2 (time )
1.2 ml min-’
Inlet temperature
250°C
1O:l
Detector temperature Equilibrium time Volume of headspace vapour injected Bath temperature Volume of vial
300°C 2h 1 ml
50°C (1 min) 10°C min-’ 20°C min-’ 200°C’ (5 min)
60-90°C 10 ml
347
calibration method, a calibration graph was constructed from a series of standard styrene solutions in the respective solvents. The concentration of styrene in a polystyrene sample was then read from the calibration graph. In the standard addition method, known amounts of styrene monomer standard were added to the sample and the headspace styrene concentrations measured. A standard addition graph was constructed and the styrene monomer concentration in a polystyrene sample was obtained by extrapolation. Four equilibration temperatures (60,70,80 and 90” C ) were studied. Results and discussion Two polystyrene samples were used for detailed methodology studies, a semifinished polystyrene sheet (SF) and a finished polystyrene tray used as a fastfood container (MPS). Every measurement was repeated at least four times. The results are given in Table 2 and indicate that in DMA direct calibration gave lower concentration readings at all the equilibration temperatures studied. With one exception, consistently higher values were obtained from the standard addition method. The difference in values between the direct calibration and standard addition methods appears to be dependent on the equilibration temperature and on the type of sample analysed. At 60°C the difference in values is appreciable, but decreases progressively as the equilibration temperature is raised. For the semi-finished polystyrene sheet, similar styrene concentrations were obtained by both methods at 90’ C. For the polystyrene tray, the difference in concentrations between the two methods was significant for all the equilibration temperatures. A few studies were carried out at. 100” C equilibration temperature, but the vapour pressure of toluene was too high, TABLE 2 Concentrations (pg g-’ )” of styrene monomers in polystyrene products Solvent
Equilibration temperature (“C)
SF
MPS
Direct calibration
Standard addition
Direct calibration
Standard addition
DMA
60 70 80 90
236 260 259 305
f 15 + 14 f 24 I!z13
295+4 293 f 22 291f3 301 f 27
508 k 35 565 + 54 583f5 595+23
626f51 643 f 26 690 f 20 699f41
Toluene
60 70 80 so
3Olk 19 304 f 19 323 + 38 305f9
316+18 318f22 311flO 313 f 27
634 + 13 642 + 19 685b 666 + 36
637+4 632 I!I32 698b 667 + 10
*Expressed as mean k standard determinations.
deviation
(4-8
determinations).
bAverage of duplicate
348
resulting in a broader solvent peak, and bulging of the sample vial septum. Styrene also appeared to polymerize at an appreciable rate at this temperature. With toluene as solvent, addition of 3 ml of ethanol at 70’ C caused polystyrene to precipitate and left styrene monomer in the solution, because ethanol is fairly miscible with toluene but is a bad solvent for polystyrene. This addition of ethanol increased the sensitivity of detection appreciably. However, even with this increase in sensitivity, it is still much lower than that at 90°C without the addition of ethanol. It was observed that in DMA the polymer precipitated immediately and coalesced into a lump on addition of water. Thus the matrix became very different from that of pure styrene monomer in DMA, where no precipitate was formed when water was added. The matrix effect might be further influenced by other components and additives used in producing the polystyrene and products. This was seen on comparing the chromatograms of the SF and MPS samples. The MPS sample gave a few more peaks than the SF sample, indicating the presence of other components or additives, some of which produced volatile vapours. The results indicate that with DMA as solvent the standard addition method must be used for the determination of styrene monomer in polystyrene. With toluene as solvent, both the direct calibration and standard addition methods gave similar results, indicating a minimal matrix effect. Toluene has a higher vapour pressure than DMA but with the capillary column, even at 9O”C, the toluene solvent peak does not interfere with the styrene peak. The headspace chromatograms of styrene in toluene and in DMA at 90” C equilibration temperature are given in Fig. 1. The styrene monomer contents in four samples from different manufacturers were determined using toluene as solvent (direct calibration) and the DMA
Li)
iii)
0)
-
Fig. 1. Headspace chromatograms of styrene in (A) toluene and (B) DMA. (i) Toluene; (ii) styrene; (iii) DMA. Retention time in minutes. Equilibrium temperature 90°C.
modified solution system (standard addition). Duplicate determinations were performed. The results are given in Table 3. The concentrations of styrene in the samples range from about 140 pugg-’ for the polystyrene foam cups to about 1150 fig g-l for the transparent polystyrene cups. Similar results were obtained by both methods. However, the time required to perform the analysis using the DMA modified solution system (where the standard addition method must be used) is over four times longer than that required with toluene as solvent. As shown in Table 4, with DMA as solvent the measurement of styrene is more sensitive than with toluene. With toluene the sensitivity increases considerably when the equilibration temperature is increased from 60 to 90’ C. At 90” C the sensitivity with DMA as solvent system is about twice that with toluene. At this equilibration temperature, no secondary displacing solvent such as ethanol is needed for the toluene system, and the simple direct calibration method can be used. Therefore, unless maximum sensitivity is needed, toluene seems to be a practicable alternative solvent for the headspace gas chromatographic determination of styrene monomer in polystyrene materials using a capillary column. TABLE 3 Styrene contents of polystyrene products determined using toluene and DMA solvents at 90 ’ C equilibration temperature Product
Styrene concentration (K g-‘) Toluene
Polystyrene foam tray A Polystyrene foam tray B Polystyrene foam cup C Transparent polystyrene cup D
183,191 800,818 141,148 1170,1208
DMA 198,217 828,852 132,138 -1130,1144
TABLE 4 Sensitivity expressed as area counts per pg ml-’ of styrene in solution Temperature (“C) 60 70 80 90
Sensitivity” DMA
Toluene
558 f 30 665 f 39 1003f 125 1045 k 27
40+6 122 f 14 238+25 503 + 33
“Concentration range of styrene monomer in solutions, lo-70 mean k standard deviation (20-30 determinations).
M ml-‘;
results expressed as
350 REFERENCES
7 8 9 10 11
International Standard 2561-1974, International Organization for Standardization, 1974. R.J. Steichen, Anal. Chem., 48 (1976) 1398. L. Rohrschneider, Fresenius’ Z. Anal. Chem., 255 (1971) 345. J.R. Withey, Environ. Health Perspect., 17 (1976) 125. J.R. Withey and P.G. Collins, Bull. Environ. Contam. Toxicol., 19 (1978) 86. H.C. Hollifield, C.V. Breder, J.L. Dennison, J.A.G. Roach and W.S. Adams, J. Assoc. Off. Anal. Chem., 63 (1980) 173. W.K. Miller and E.L. Harper, J. Appl. Polym. Sci., 28 (1983) 3585. I. Santa Maria, J.D. Carmi and A.G. Ober, Bull. Environ. Contam. Toxicol., 37 (1986) 207. K.J. Rygle, J. Coating Technol., 52 (1980) 47. A.J. van Straten, J. High Resolut. Chromatogr., Chromatogr. Commun., 8 (1985) 521. S.V. Dubiel, Characterization of Volatiles in Polymers by Headspace Gas Chromatography, Technical Communications, Allied Bendix Aerospace, 1986.
345 Printed in The Netherlands
Short Communication
TOLUENE SOLVENT SYSTEM FOR THE HEADSPACE GAS CHROMATOGRAPHIC DETERMINATION OF STYRENE MONOMER IN POLYSTYRENE
M.K. WONG*, L.M. GAN, L.L. KOH and K.H. NG Department of Chemistry, National University of Singapore, Singapore (Singapore) (Received 18th August 1988)
Summary. The use of toluene as a solvent to dissolve polystyrene for the determination of styrene monomers is studied. Compared with the modified solution headspace method using dimethylacetamide as solvent, with toluene the system is slightly less sensitive. However, there appeared to be minimal matrix effect when toluene is used, so a simpler external calibration method can be applied.
Several methods are available for the gas chromatographic determination of trace styrene monomers in polystyrene products. The IS0 method [ 11, involving dissolution of the polymer sample in an appropriate solvent, precipitation of the polymer by addition of methanol and injection of the supernatant liquid into the gas chromatograph, is widely used. The detection limit is about 10 pg ml-‘. In recent years, headspace gas chromatographic methods have gained wide acceptance for the determination of styrene and other volatile monomers. The modified solution approach by Steichen [2] increased the quantitation limit for styrene many-fold over that of the normal solution headspace method [ 31. In Steichen’s method, a polystyrene sample is first dissolved in dimethylacetamide (DMA) and water is injected into the solution to precipitate polystyrene and to separate styrene monomer in the liquid phase. The matrix effect due to polystyrene precipitation is appreciable, so the tedious standard addition method must be used. In most of the studies on the determination of styrene monomer packed columns were used [ 2-81 and there have been fewer reports on the application of headspace gas chromatography using capillary columns [g-11]. Compared with packed columns, capillary columns are more sensitive and, having a superior separation efficiency, tailing of the solvent peak is expected to be much less of a problem. This investigation was initiated to study the use of toluene as a solvent in the determination of styrene monomer in polystyrene samples.
0003-2670/89/$03.50
0 1989 Elsevier Science Publishers B.V.
A comparative study was also made of the relative sensitivities and DMA as solvents in the modified solution approach.
with toluene
Experimental Apparatus and materials. A Hewlett-Packard Model 5890A gas chromatograph equipped with a capillary inlet system and flame ionization detector was used and headspace sampling was done with a Hewlett-Packard Model 19395A headspace sampler. The headspace chromatographic conditions are given in Table 1. The column used was a 30 m x 0.32 mm i.d. fused-silica capillary containing polar Supelcowax-10 (Supelco). A Hewlett-Packard HP-l non-polar column was used for confirmation. A Hewlett-Packard Model 3393A computing integrator was used for recording the chromatograms. Liquid chromatographic-grade (99.9% ) N,N-dimethylacetamide (Aldrich) was used without purification. Technical-grade toluene and ethanol purchased from a local refinery were distilled twice over a 1.2-m column packed with Raschig rings. Styrene monomer ( > 99%, as analysed by liquid chromatography) (Fluka) was used for the preparation of 2000 pg ml-l stock solutions in the respective solvents. The working standards were prepared by serial dilution with the solvent. Sample preparation and determination. Polystyrene samples were cut into small pieces with a pair of stainless-steel scissors and about 0.25 g was weighed into a lo-ml headspace vial. A 4-ml volume of solvent (DMA or toluene) was added to dissolve the sample. The vial was capped with a teflon-coated septum and finally sealed with a crimped cap. With DMA as solvent, 3 ml of water were injected with a syringe into the solution and the mixture was shaken; polymer material precipitated immediately. The vial was then placed in the headspace sampler and equilibrated for 2 h prior to headspace sampling. With toluene as solvent, the use of ethanol to precipitate polystyrene from the solution was investigated at an equilibration temperature of 70’ C. Two methods of measurement in the two solvents were studied. In the direct TABLE 1 Headepace gas chromatographic conditions Carrier gas (nitrogen) flow-rate (column) Splitting ratio Oven temperature: Temperature 1 (time) Rate 1 (50-120°C) Rate 2 (120-200°C) Temperature 2 (time )
1.2 ml min-’
Inlet temperature
250°C
1O:l
Detector temperature Equilibrium time Volume of headspace vapour injected Bath temperature Volume of vial
300°C 2h 1 ml
50°C (1 min) 10°C min-’ 20°C min-’ 200°C’ (5 min)
60-90°C 10 ml
347
calibration method, a calibration graph was constructed from a series of standard styrene solutions in the respective solvents. The concentration of styrene in a polystyrene sample was then read from the calibration graph. In the standard addition method, known amounts of styrene monomer standard were added to the sample and the headspace styrene concentrations measured. A standard addition graph was constructed and the styrene monomer concentration in a polystyrene sample was obtained by extrapolation. Four equilibration temperatures (60,70,80 and 90” C ) were studied. Results and discussion Two polystyrene samples were used for detailed methodology studies, a semifinished polystyrene sheet (SF) and a finished polystyrene tray used as a fastfood container (MPS). Every measurement was repeated at least four times. The results are given in Table 2 and indicate that in DMA direct calibration gave lower concentration readings at all the equilibration temperatures studied. With one exception, consistently higher values were obtained from the standard addition method. The difference in values between the direct calibration and standard addition methods appears to be dependent on the equilibration temperature and on the type of sample analysed. At 60°C the difference in values is appreciable, but decreases progressively as the equilibration temperature is raised. For the semi-finished polystyrene sheet, similar styrene concentrations were obtained by both methods at 90’ C. For the polystyrene tray, the difference in concentrations between the two methods was significant for all the equilibration temperatures. A few studies were carried out at. 100” C equilibration temperature, but the vapour pressure of toluene was too high, TABLE 2 Concentrations (pg g-’ )” of styrene monomers in polystyrene products Solvent
Equilibration temperature (“C)
SF
MPS
Direct calibration
Standard addition
Direct calibration
Standard addition
DMA
60 70 80 90
236 260 259 305
f 15 + 14 f 24 I!z13
295+4 293 f 22 291f3 301 f 27
508 k 35 565 + 54 583f5 595+23
626f51 643 f 26 690 f 20 699f41
Toluene
60 70 80 so
3Olk 19 304 f 19 323 + 38 305f9
316+18 318f22 311flO 313 f 27
634 + 13 642 + 19 685b 666 + 36
637+4 632 I!I32 698b 667 + 10
*Expressed as mean k standard determinations.
deviation
(4-8
determinations).
bAverage of duplicate
348
resulting in a broader solvent peak, and bulging of the sample vial septum. Styrene also appeared to polymerize at an appreciable rate at this temperature. With toluene as solvent, addition of 3 ml of ethanol at 70’ C caused polystyrene to precipitate and left styrene monomer in the solution, because ethanol is fairly miscible with toluene but is a bad solvent for polystyrene. This addition of ethanol increased the sensitivity of detection appreciably. However, even with this increase in sensitivity, it is still much lower than that at 90°C without the addition of ethanol. It was observed that in DMA the polymer precipitated immediately and coalesced into a lump on addition of water. Thus the matrix became very different from that of pure styrene monomer in DMA, where no precipitate was formed when water was added. The matrix effect might be further influenced by other components and additives used in producing the polystyrene and products. This was seen on comparing the chromatograms of the SF and MPS samples. The MPS sample gave a few more peaks than the SF sample, indicating the presence of other components or additives, some of which produced volatile vapours. The results indicate that with DMA as solvent the standard addition method must be used for the determination of styrene monomer in polystyrene. With toluene as solvent, both the direct calibration and standard addition methods gave similar results, indicating a minimal matrix effect. Toluene has a higher vapour pressure than DMA but with the capillary column, even at 9O”C, the toluene solvent peak does not interfere with the styrene peak. The headspace chromatograms of styrene in toluene and in DMA at 90” C equilibration temperature are given in Fig. 1. The styrene monomer contents in four samples from different manufacturers were determined using toluene as solvent (direct calibration) and the DMA
Li)
iii)
0)
-
Fig. 1. Headspace chromatograms of styrene in (A) toluene and (B) DMA. (i) Toluene; (ii) styrene; (iii) DMA. Retention time in minutes. Equilibrium temperature 90°C.
modified solution system (standard addition). Duplicate determinations were performed. The results are given in Table 3. The concentrations of styrene in the samples range from about 140 pugg-’ for the polystyrene foam cups to about 1150 fig g-l for the transparent polystyrene cups. Similar results were obtained by both methods. However, the time required to perform the analysis using the DMA modified solution system (where the standard addition method must be used) is over four times longer than that required with toluene as solvent. As shown in Table 4, with DMA as solvent the measurement of styrene is more sensitive than with toluene. With toluene the sensitivity increases considerably when the equilibration temperature is increased from 60 to 90’ C. At 90” C the sensitivity with DMA as solvent system is about twice that with toluene. At this equilibration temperature, no secondary displacing solvent such as ethanol is needed for the toluene system, and the simple direct calibration method can be used. Therefore, unless maximum sensitivity is needed, toluene seems to be a practicable alternative solvent for the headspace gas chromatographic determination of styrene monomer in polystyrene materials using a capillary column. TABLE 3 Styrene contents of polystyrene products determined using toluene and DMA solvents at 90 ’ C equilibration temperature Product
Styrene concentration (K g-‘) Toluene
Polystyrene foam tray A Polystyrene foam tray B Polystyrene foam cup C Transparent polystyrene cup D
183,191 800,818 141,148 1170,1208
DMA 198,217 828,852 132,138 -1130,1144
TABLE 4 Sensitivity expressed as area counts per pg ml-’ of styrene in solution Temperature (“C) 60 70 80 90
Sensitivity” DMA
Toluene
558 f 30 665 f 39 1003f 125 1045 k 27
40+6 122 f 14 238+25 503 + 33
“Concentration range of styrene monomer in solutions, lo-70 mean k standard deviation (20-30 determinations).
M ml-‘;
results expressed as
350 REFERENCES
7 8 9 10 11
International Standard 2561-1974, International Organization for Standardization, 1974. R.J. Steichen, Anal. Chem., 48 (1976) 1398. L. Rohrschneider, Fresenius’ Z. Anal. Chem., 255 (1971) 345. J.R. Withey, Environ. Health Perspect., 17 (1976) 125. J.R. Withey and P.G. Collins, Bull. Environ. Contam. Toxicol., 19 (1978) 86. H.C. Hollifield, C.V. Breder, J.L. Dennison, J.A.G. Roach and W.S. Adams, J. Assoc. Off. Anal. Chem., 63 (1980) 173. W.K. Miller and E.L. Harper, J. Appl. Polym. Sci., 28 (1983) 3585. I. Santa Maria, J.D. Carmi and A.G. Ober, Bull. Environ. Contam. Toxicol., 37 (1986) 207. K.J. Rygle, J. Coating Technol., 52 (1980) 47. A.J. van Straten, J. High Resolut. Chromatogr., Chromatogr. Commun., 8 (1985) 521. S.V. Dubiel, Characterization of Volatiles in Polymers by Headspace Gas Chromatography, Technical Communications, Allied Bendix Aerospace, 1986.