Coupling with Multiple Concentration Detectors

In the analysis of binary copolymers or polymer blends two different concentration detectors can be used. Typically, a combination of UV and RI detection is used, but other detector combinations have also been described. If the components of the copolymer have different UV-spectra, a diode-array detector will be the instrument of choice. One has, however, to keep in mind that non-linear detector response may also occur with UV detection. 

The principle of dual detection is rather simple when a mass mi of a copolymer which contains the weight fractions wk and wK (=l-wA) of the monomers A 

Once the weight fractions of the monomers are known, the correct mass of polymer in the slice can be calculated using  

It is clear that the interpolation between the calibration lines cannot be applied to mixtures of polymers (polymer blends). If the calibration lines are different, different molar masses of the homopolymers will elute at the same volume. The universal calibration is also not capable of eliminating the errors which originate from the simultaneous elution of two polymer fractions with the same hydrodynamic volume, but different composition and molar mass. Ogawa [33] has shown by a simulation technique that the molar masses of polymers eluting at the elution volume Ve are given by the corresponding coefficients K and a in the Mark-Houwink equation. 

In SEC of a polymer blend, molar masses of the homopolymers eluting in the same interval can be calculated using  

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To overcome the problems related to SEC of complex polymers, multidetector systems have been developed over the years. One approach is the combination of SEC with multiple concentration detectors. If the response factors of the detectors for the components of the polymer are sufficiently different, the chemical composition of each slice of the elution curve can be determined from the detector signals. Typically, a combination of UV and RI detection is used another possibility is the use of a diode array detector. In the case of non-UV-absorbing polymers, a combination of RI and density detection yields information on chemical composition. Similar information can be obtained by coupling SEC with spectroscopic detectors like FTIR, NMR, or mass spectrometry (MS). This approach is addressed in Section 2.03.5. Such detector combinations, however, are normally not able to differentiate between copolymers and polymer blends. In this case, it might be more suitable to carry out a separation according to chemical composition in a first step followed by a molar mass analysis (see for more details. Section 2.03.6). 

As shown in the discussion above, there are a multiplicity of desirable and undesirable features of OID s that impact their general application as detectors in analytical atomic emission spectrometry. It is therefore appropriate to compare, in a critical and objective sense, the experimental figures of merit of these devices vis-a-vis the classical polychromator photomultiplier approach. These comparisons should be performed virtually on a continuing basis because of advances in performances, not only of the array detectors themselves but also in the associated spectroscopic excitation sources. An evaluation of the overall performance figures of merit of OID s when they are employed in conjunction with induction-coupled plasma excitation is of particular current interest because of the popularity that this source is attaining for the simultaneous determination of the elements at all concentration levels. In this paper we present such an evaluation for self-scanned, photodiode array detectors... 

TIMS, depending on the application [3]. These developments with magnetic sector instruments include multiple Faraday cup detectors (multicollectors) and multiple ion counters. The Faraday cup multicollectors have enabled the determination of isotope ratios with accuracy and precision rivaling TIMS [4], Due to lower sensitivity and differences in instrument design, quadrupole instruments (quadrupole inductively coupled plasma mass spectrometry [Q-ICP-MS]) have not demonstrated the same accuracy and precision as magnetic sector instruments, especially at low concentrations [5], Nevertheless, improvements in the sensitivity of quadrupole instrumentation have also increased their utility for many isotope ratio applications [6]. 

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