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B. Composition Detectors
Many copolymers are compositionally heterogeneous in their molecular weights; that is, the composition can vary or drift with molecular weight. By adding a “composition-sensitive” detector in series with a concentration-sensitive detector, the composition drift or heterogeneity can be determined. This approach also is useful for the analysis of unknown polymeric ma-terials or blends, or for determining the composition of block or graft copolymers. It is im-portant to realize, however, that copolymers may be heterogeneous for both composition and molecular weight; that is, for a given molecular weight, there can be a range of compositions, and for a given copolymer composition, there can be a range of molecular weights. In this situation, a two-dimensional separation or cross-fractionation is required, such as HPLC fol-lowed by SEC or vice versa. Polymer cross-fractionation is beyond the scope of this chapter, and the reader is referred to References [6,41].
1. Ultraviolet Spectrophotometry
On-line UV diode-array detectors (DAD) are quite useful for measuring compositional heterogeneity and have been used in several SEC applications. The limitation of this detector is that comonomers or components must have characteristic UV spectra.
2. FT-IR Spectrophotometry
On-line Fourier-transform-infrared (FT-IR) detection has been reported, mainly for the determination of compositional heterogeneity of polyethylene copolymers, as well as for deter-mining short-chain branching in polyethylene [42,43]. The major limitations of this detector are solvent interference and sensitivity; milligram quantities are typically required. To over-come these difficulties, a commercially available FT-IR detector “interface” has been introduced in which column effluent is sprayed onto an aluminum-backed, rotating germanium disk . The mobile phase is evaporated, and the disk is placed onto a scanning turntable mounted within an FT-IR spectrophotometer in the reflection mode. Infrared spectra are taken along the polymer track, and FT-IR spectra recorded, essentially as a function of elution time.
This detector is most useful for qualitative analysis, especially for SEC analysis of unknown polymeric materials. With the availability of new instrumentation that focuses the effluent spray into a more narrow track, detection limits are in the submilligram range. Fur-« thermore, FT-IR microscopy can also be employed to analyze the deposited polymer. For SEC analysis of polymers, however, a possible concern is spectral interference caused by the deposition of nonuniform films , although it may be possible to smooth out the deposit by solvent or thermal annealing.
3. Mass Spectrometry
Mass spectrometry (MS) is the ideal detector for chromatography because of its high sensitivity and its ability to provide structural information. MS is especially useful for analyzing the molecular weight and composition of oligomers, for identifying chain-end groups, and for determining the composition and molecular weight of unknown or multicomponent polymeric materials. With electrospray ionization, the most useful on-line ionization mode, analysis of polymers of molecular weight less than 1 X 104 g òîÃ1 is possible . There are, however, several major concerns. Multiple charged species, insufficient resolution, and the presence of fragment ions can complicate spectral interpretation. Additionally, the total ion current may not reflect actual concentration, although a concentration-sensitive detector in series would solve this problem. Lastly, special mobile-phase conditions may be necessary to help induce ionization.
Matrix-assisted laser-desorption ionization-time-of-flight (MALDI-TOF) MS, an alter-native “soft”-ionization mode, which has recently been introduced, holds great promise as an SEC detector. With this technique, molecular weights as high as 105 g mol-1 are possible. The formation of fragment ions and multiple charged molecules are minimized, which greatly simplifies spectral interpretation. In practice, SEC fractions are collected, mixed with a given matrix, and analyzed off-line. (In a typical experiment, less than 0.5 mg of polymer is fractionated and above 5-/xg fractions taken for MALDI-TOF MS .) Two versions of automated microfraction collectors are now available commercially specifically for MALDI-TOF analysis (see Appendix C). Recently, the design and evaluation of an on-line SEC-MALDI-TOF system has been reported .
4. Nuclear Magnetic Resonance
For unambiguous structural determination, as well as for chain microstructural studies, on-line nuclear magnetic resonance (NMR) is required. Furthermore, if polymer end groups are well defined, NMR can also be used for determining molecular weights of less than 104 g mol-1 . Flowthrough microprobes (60-250 /xL) are now available [50,51]. When used in conjunction with 500-MHz instrumentation, with advanced shimming and solvent suppression features, sensitivity is in the microgram range for 'H-NMR, and the milligram level for 13C-