Anal. Chem. 2008, 80, 783-792
APCI Interface for LC- and SEC-MS Analysis of Synthetic Polymers: Advantages and Limits Bernard Desmazie`res, William Buchmann,* Peran Terrier, and Jeanine Tortajada
Laboratoire Analyse et Mode´ lisation pour la Biologie et l’Environnement, Universite´ d’Evry-Val d’Essonne, CNRS UMR 8587, Baˆ t. Maupertuis, Bd. F. Mitterrand, 91025 Evry Cedex, France
The main advantage of the APCI interface for the LC-MS analysis of synthetic polymers resides in its compatibility with the main chromatographic modes: reversed-phase liquid chromatography, normal-phase liquid chromatography, and size exclusion chromatography in organic phase, with the usual flow rates. Moreover, APCI can be used in positive or negative modes. Representative applications are described to highlight benefits and limitations of the LC-APCI-MS technique with the analysis of industrial polymers up to molecular masses of 5 kDa: polyethers; polysiloxanes; and copolymers of siloxanes. Results are discussed in regard to those obtained by more classical techniques: SEC and MALDI-MS. The use of an APCI interface in LC-MS and SEC-MS coupling applied to synthetic polymers is efficient up to 2000-4500 Da. The main drawback of the APCI interface is the in-source decomposition that is observed above m/z ) 2000-3000 and can induce an underestimation of average molecular weights. However, APCI allows detection on a wide range of polarity of sample/solvent and appears to be complementary to ESI. Mass spectrometry (MS), with the introduction and the development of soft ionization methods such as electrospray ionization (ESI), by Fenn et al.,1 and matrix-assisted laser desorption/ionization (MALDI), by Tanaka et al.2 and by Karas and Hillenkamp,3 has been shown to be a very powerful tool for polymer analysis.4-6 MALDI-MS allows the checking of the nature of the repeat units and the end groups, estimating the average molecular weights, the polymerization degrees, and the polydispersity indices when polymers are few disperse. ESI-MS has also been shown to be a very useful ionization technique also, but ESIMS has been seldom used comparatively to MALDI-MS for polymer analysis due to the presence of several charge states. Charge-state distribution overlaps with oligomer chain length distribution; thus, very complex mass spectra can be obtained with * To whom correspondence should addressed. E-mail: william.buchmann@ univ-evry.fr. (1) Fenn, J. B.; Mann, M.; Meng, C. K.; Wong, S. F.; Whitehouse, C. M. Science 1989, 246, 64-71. (2) Tanaka, K. Angew. Chem., Int.l Ed. 2003, 42, 3860-3870. (3) Karas, M.; Hillenkamp, F. Anal. Chem. 1988, 60, 2299-2301. (4) Pasch, H.; Shrepp, W. In MALDI-TOF Mass Spectrometry of Synthetic Polymers; Barth, H. G., Pash, H., Eds.; Springer-Verlag: Berlin, 2003 (5) Montaudo, M. S. Mass Spectrom. Rev. 2002, 21, 108-144. (6) Montaudo, G.; Lattimer, R. P. In Mass Spectrometry of Polymers; Montaudo, G., Lattimer, R, P., Eds.; CRC Press: Boca Raton, FA, 2002. 10.1021/ac0715367 CCC: $40.75 Published on Web 12/28/2007
© 2008 American Chemical Society
higher masses. Amazingly, the use of atmospheric pressure chemical ionization (APCI) as an ionization method for synthetic polymer analysis has been scarce whereas singly charged pseudomolecular ions are generally obtained in contrast to ESI. APCI was initially introduced by Horning et al.7 in the mid-1970s as an interface between high-performance liquid chromatography (HPLC) and MS. In APCI, ions are generated at atmospheric pressure, from the LC effluent through a heated pneumatic nebulizer via corona discharge ionization. Usually, APCI is preferred over ESI for the analysis of compounds of low polarity and of low molecular weights. During the APCI process, the mobile phase acts as a reactant gas to ensure ionization. APCI is known to be somewhat less mild than ESI due to the fact that the evaporation of mobile phase is supported by a heated nebulizer (350-400 °C). To our knowledge, only a few papers deal with the use of APCI for LCMS polymer analysis. Huang and Rood8 showed the advantages of APCI-MS in the infusion mode over GC/CH4C-MS with various silylated and unsilylated ethoxylates and carboxylates. Jandera and co-workers9,10 studied the chromatographic behavior of ethoxylated alcohols and ethylene oxide/propylene oxide copolymers in normal- and reversed-phase HPLC-MS using APCI-MS as a detector. Cyclic oligomers of poly(ethylene terephtalate)11-13 and poly(butylene terephtalate)14 were characterized by reversed-phase HPLC-APCI MS. In these reports, the use of APCI was always described in the positive ion mode only and over a limited mass range (usually 600 Da were taken into account in order to avoid the contribution of low-mass ions coming from in-source decompositions. The estimation of the average polymerization degrees gave for each sort of repeat unit: DPn (DMS) ) 14.9, DPn (MS) ) 5.4. 790 Analytical Chemistry, Vol. 80, No. 3, February 1, 2008
Comparison of LC-APCI MS Results with SEC and MALDI MS Data. Finally, average molecular weights deduced from the LC-APCI MS experiments were compared with those obtained by SEC and MALDI MS (see Table 1). First, MALDI MS and
Figure 8. RPLC/APCI MS (positive mode) separation of a methylsiloxane/dimethylsiloxane triblock copolymer.
Figure 9. Contour plot deduced from a RPLC/APCI MS (positive mode) separation of a methylsiloxane/dimethylsiloxane triblock copolymer (ion intensities reported as a function of the number of methylsiloxane and dimethylsiloxane units).
LC-APCI MS provide much more information than SEC. For example, SEC gives only one average molecular weight value for the mixture C16/C18EOn (samples I, IV) whereas MALDI MS and LC-APCI MS can give one value of Mn and DPn for each polymer as a function of the fatty chain length. In the case of sample VII, SEC cannot determine the composition of the copolymer. Second, the results concerning the molecular weights depend on the polymer. With poly(ethylene oxides) (samples I-IV), lower molecular weight values were obtained from LC-APCI MS than from SEC or MALDI MS analyses (except for C12EOn). Average molecular weights deduced from LC-APCI-MS for sample IV seem strongly underestimated. Fragmentations of the heavier chains can explain the strong differences in the estimation of polymerization degrees. For the poly(tetrahydrofuran) (V), the PDMS (VI),
the values deduced from LC-APCI MS were higher than those from MALDI-MS but slightly lower than SEC values. This can be partially explained by a lack of desorption-ionization efficiency in MALDI-MS with the heaviest oligomers. The copolymer PDMSb-PMS-b-PDMS (VII) leads to similar results by both MALDI-MS and LC-APCI-MS. As already mentioned, SEC does not seem to be the most appropriate method in this case because this technique is not able to distinguish the different repeat units of the copolymer. Concerning the polydispersity indexes, except for sample I, they roughly decrease in the following order: SEC > LC-APCI MS > MALDI-MS probably because the detection of the heaviest chains is limited in both MALDI-MS and APCI-MS analyses. It must be emphasized that even if LC-APCI MS can lead to an underestimation of the average molecular weights; the accuracy of the quantitative results depends on the polymer. Owing to the separation step before mass analysis, LC-APCI MS coupling provides the best qualitative data. To conclude, the use of APCI interface in LC-MS and SECMS coupling applied to synthetic polymers is efficient up to 20004500 Da, pseudomolecular ions are easily detected both in normaland in reversed-phase LC conditions, as well as in organic-phase SEC (whatever the ion mode: positive or negative). The main results are that the analysis of polyethers is possible in RPLC, NPLC, or SEC APCI-MS coupling up to m/z ) 2500 at least. The analysis of polysiloxanes is easy up to at least m/z ) 4500 in RPLC or SEC/APCI MS, in the positive or negative ion modes. The main drawback of the APCI interface is the in-source decomposition, Analytical Chemistry, Vol. 80, No. 3, February 1, 2008
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which is observed above m/z ) 2000-3000 and can result in underestimation of molecular weights. However, APCI allows detection on a wide range of polarity of sample/solvent and appears to be complementary to ESI. Fundamental studies of the chemical ionization process and a comparison with the newly developed atmospheric pressure photoionization33,34 could permit
the extension of the range of detectable molecular weights by optimizing source parameters.
(33) Raffaelli, A.; Saba, A. Mass Spectrom. Rev. 2003, 22, 318-331.
(34) Robb, D. B.; Covey, T. R.; Bruins, A. P. Anal. Chem. 2000, 72, 3653-3659.
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Analytical Chemistry, Vol. 80, No. 3, February 1, 2008
Received for review July 20, 2007. Accepted October 29, 2007. AC0715367
