Introduction GPEC-IR

Characterization of poly(styrene-co-methylacrylate)s
using gradient polymer elution chromatography-infrared detection

S.J. Kok1, 2, P.J.C.H. Cools1, T. Hankemeier1 and P.J. Schoenmakers2
1 TNO Voeding, Analytical Sciences Division, Utrechtseweg 48, 3704 HE Zeist, The Netherlands
2 University of Amsterdam, Faculty of Chemistry, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands

click here for PDF-file.

 

Introduction

Copolymers can be characterized according to chemical composition by gradient polymer elution chromatography (GPEC). Reversed phase separation of random copolymers by GPEC is mainly based on the ratio of the monomers (mol fraction) [1]. Since chromatography is a relative method, selective detection and identification is necessary [2]. By using spectroscopic techniques e.g., infrared, the GPEC separation can be calibrated without the use of appropriate standards.
 

Objective

The aim was to study the applicability of FT infrared for calibration of the RP-GPEC separation, e.g. the determination of the (average) chemical composition of chromatographic fractions. This was evaluated by means with poly(styrene-co-methylacrylate) samples with known chemical composition.
 

Beer’s law

If Beer’s law is obeyed, the absorbance ratio should be linear to the ratios of the mol fractions if selective spectral bands are selected for each compound to be measured according to:

absorbance styrene / absorbance methylacrylate = k · (mol fraction styrene / mol fraction methylacrylate)

The molar extinction coefficients should be independent of the copolymer composition.

 

Liquid Chromatography - IR coupling

The HPLC effluent is directed to the spraying interface (LC Transform), see Figure 1a. After evaporation of the effluent and sample deposition on a germanium disc, the disc is transferred to the IR optics module and spectra of the deposited chromatogram are collected by stepwise moving the Ge-disc (Figure 1b)

Diagram of Labconnections Lc Transform 500 nebulizer

Figure 1a: Diagram of LC Transform 500 nebulizer. (With permission of LabConnections, Inc., Marlborough, MA, USA)

Diagram of Labconnections infrared optics scanning module.

Figure 1b: Diagram of infrared optics scanning module. (With permission of LabConnections, Inc., Marlborough, MA, USA)

 

Optimization of LC Transform deposition

Optimization of LC effluent deposition was performed by optimizing the nozzle temperature at different eluent compositions. During the gradient run, the nozzle temperature was programmed to maintain optimum evaporating conditions during the RP-GPEC separation.
 

Instrumentation

 

Results and Discussion

 

Chemical composition determination by RP-GPEC

A plot of mol fraction styrene vs. retention time (Figure 3) results in a linear calibration curve (r = 0.9867) for the known poly(styrene-co-methylacrylate) samples.

 

Figure 2: RP-GPEC ELSD and UV (215 nm) chromatograms: poly(styrene-co-methylacrylate)s (m=35 µg). Peak assignment: a, mol fraction styrene = 0.095, b=0.2, c=0.3, d=0.4, e=0.6, f=0.7, g=0.8, h=0.9, ? = impurity. Conditions: column, Novapak C18 (Waters), 150 * 3.9 mm I.D.; gradient, 50:50 % (v/v) H2O/AcN to 100 % (v/v) AcN to 100 % (v/v) THF (2 % (v/v)/min); flow, 0.5 mL/min.

Figure 3: Calibration plot of mol fraction styrene in poly(styrene-co-methylacrylate)s vs. ELSD retention time.

 

Chemical composition determination by IR

LC-IR coupling is possible, but appeared to be critical with respect to deposition, e.g., increasing deposit width on the Ge disc during separation, or nebulizer temperature.

Possible explanations of a non-linear relationship (Figure 7) between IR-peak ratio and mol fraction for the known poly(styrene-co-methylacrylate) samples are: (i) response and molar extinction coefficient both depend on molecular structure, i.e., chemical composition; (ii) the absence of selective IR absorbance peaks in the styrene spectrum, i.e., a region where methylacrylate is not absorbing IR radiation (Correction for these contributions was not sufficient, especially in the region where the styrene fraction is small or high); (iii) unequal contribution of transmittance and/or reflectance response due to inhomogeneous deposit.

Figure 4: Functional group RP-GPEC-IR chromatograms: black trace, IR of wavenumber region 1744 - 1724 cm-1, characteristic for C-O (MA); blue trace, IR of wavenumber region 688 - 708 cm-1, characteristic for phenyl (S). Peak assignment, see Figure 2.

Figure 5: RP-GPEC-IR spectra of copolymer with varying styrene mol fraction (black trace, f=0.095; red trace, f=0.6; blue trace, f=0.9).

Figure 6: IR contour plot of RP-GPEC separation of copolymers with mol fraction styrene f = 0.095 - 0.9. Third dimension: transmittance (%). For details, see Figure 2.

Figure 7: Calibration plot of peak height ratio vs. mol fraction f(S)/f(MA).

 

Conclusion

RP-GPEC-IR has been realized. In principle, an almost linear relationship was found between retention time and mol fraction styrene, however, influence of molar mass is not known, but a deviation was found for small and high mole fractions of styrene. This was mainly assigned to interference of the absorbance bands of styrene by the absorbance bands characteristic for methylacrylate. Therefore, in principle IR ratio-ing has to be improved to realize the goal of independent determination of the chemical composition by IR.

The next step is implementation of multi-variate data analysis. Multi-variate data analysis will be used for background correction in GPEC-IR and for calibration of peak height vs. molar ratio. Therefore, it will be investigated in the near future. The use of a larger range of the IR spectra from methylacrylate rather than a single IR absorbance peak will improve determination of the chemical composition distribution. This is even more true for polymer systems consisting of more than two monomers.

 

Acknowledgement

The authors thank Harold Schoonbrood from the Technical University of Eindhoven (The Netherlands) for the poly(styrene-co-methylacrylate) samples.
 

References

  1. Teramachi, S., Macromol. Symp., 110 (1996) 217-229
  2. Cools, P.J.C.H., Thesis, Technical University of Eindhoven, The Netherlands, 1999
 
last update: augustus 11, 2007