Off-line SEC-NMR

Off-line SEC-NMR (which consists of collecting the SEC fractions, evaporating the solvent and then adding a deuterated solvent) does not suffer the above drawbacks. The signal-to-noise ratio is good when medium-field magnets (200 MHz up to 500 MHz) are employed thus, the cost of such experimental set-up is attractive, i.e., on the same order of magnitude of double-detector SEC assemblies. 

Off-line SEC-NMR is certainly more labour-intensive and time-consuming than on-line SEC-NMR. Nevertheless, this difference can be minimized by reducing the number of fractions. Clearly, the reduction cannot go beyond a certain limit, otherwise it will cause a loss of accuracy in the measurement of copolymer properties. The hypothesis that it is possible to reduce the time for copolymer analysis without the cited loss of accuracy was put forward by Murphy et al. (24) in another context (2D-chromatography). It would be interesting to check whether the hypothesis is valid also in SEC-NMR and to develop a methodology which allows the determination of the optimal conditions, viz., those where the cited loss of accuracy is still negligible. 

Montaudo [11, 13, 14] also described a new method for fully characterising copolymers, which is based on off-line SEC-NMR and SEC-MALDI. It was applied to the analysis of random copolymers reacted at high conversions. The method involves fractionation of the copolymers by size exclusion chromatography, analysis of the fractions by NMR and MALDI mass spectroscopy and derivation of bivariate distribution of composition of the fractions. These copolymers include copolymers containing units of methyl methacrylate, butyl acrylate, styrene and maleic anhydride. Perspectives and limitations of the technique are also considered. 

FTIR in multiply hyphenated systems may be either off-line (with on-line collection of peaks) [666,667] or directly on-line [668,669]. Off-line techniques may be essential for minor components in a mixture, where long analysis times are required for FT-based techniques (NMR, IR), or where careful optimisation of the response is needed. In an early study a prototype configuration comprised SEC, a triple quadrupole mass spectrometer, off-line evaporative FTIR with splitting after UV detection see Scheme 7.12c [667]. Off-line IR spectroscopy (LC Transform ) provides good-quality spectra with no interferences from the mobile phase and the potential for very high sensitivity. Advanced approaches consist of an HPLC system incorporating a UV diode array, FTIR (using an ATR flow-cell to obtain on-flow IR spectra), NMR and ToF-MS. 

Where the use of multiple spectroscopic analysis on a single HPLC separation is an advantage, the benefit of using the simplest possible mobile phase for separations is manifest. While selecting compatible solvent systems for NMR and MS is sometimes complex, addition of IR (even off-line) places even more constraints on solvent composition. For SEC-NMR-IR CDCR is a suitable eluent [666] for RPLC-FTIR-UV-NMR-MS D2O-CD3CN is recommended. Superheated D20 has been proposed as the mobile phase [670],... 

Applications Ideally, multiply hyphenated systems should be assembled rapidly in response to real need. Access to these means is restricted to a few laboratories only. Multiple LC hyphenations have been used to analyse test mixtures of polymer additives see Table 7.74. The relative ease with which SEC-UV using CDCI3 as a solvent can be coupled to on-line 1II NMR and an in series off-line FUR (Scheme 7.12b), has been shown for a mixture of polymer additives (BHT, Irganox 1076, DIOP) [666]. Figure 7.35 shows representative spectra for on-flow NMR and MS and off-line FTIR of 2,6-di-f-butyl-4-methoxyphenol. 

Copolymer sanq)le MB41 is a high conversion (100%) random copolymer of methyl methacrylate (MMA) and butyl acrylate (BA) produced by radical initiation. The san le was injected into the SEC apparatus, and about 40 fractions were collected. Several SEC fractions were then subjected to off-line MALDI and NMR analysis, respectively. 

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