Broad MMD calibration

An alternative approach to calibrating an SEC system has been to use a single broad-MMD calibrant. The most satisfactory approach for this form of calibration is to use a calibrant which has been particularly well characterized. This would require that the calibrant be fractionated and that each of the many fractions be carefully examined by absolute techniques in order to build up a detailed molecular weight profile of the whole distribution [4]. 

Clearly, the characterization of a broad-MMD calibrant requires a considerable amount of effort and expertise, and they have been mainly used where there are problems in producing narrow-MMD calibrants. In practice, the main applications of this approach have been the use of NBS Standard Sample 706 (polystyrene) and, in particular, NBS Standard Sample 1475 (linear polyethylene). SEC has been a very valuable technique for examining the MMDs of polyethylene, but as there has only been limited success in producing narrow-MMD polyethylene calibrants, NBS 1475 has been widely used for calibrating high-temperature SEC systems used for this application. Unfortunately, the molecular mass range of NBS 1475 is rather low and is not really appropriate for many samples. Barlow et al [5] used NBS 1475 and extended the calibration range with additional polymers (see Chapter 4, section 4.5.1.1). 

It is clear from the previous two sections that there is only a limited range of polymer types for which either narrow- or broad-MMD calibrants are available. Clearly it would be desirable to have a mathematical procedure which would allow for the difference in chemical type between the calibrants available and the samples of interest. 

In addition to replacing the universal calibration procedure when used with conventional calibration methods, viscosity detectors can also be used to enhance the calibration with broad-MMD calibrants. In section 3.3.2, the use of a broad-MMD calibrant with known weight and number averages was mentioned. If a viscosity detector is used, there is no prior requirement for... 

An example can best explain the procedure. A poly(bisphenolA carbonate) sample characterized by a broad-MMD was injected in an SEC apparatus, about 100 fractions were collected, and 24 of them were analyzed by MALDI [7]. Figure 15.2 reports the SEC trace of the PC sample. The trace covers a quite broad range of elution volumes and it is centered at about 30 ml. The polymer starts eluting at about 26 ml and ends at about 38 ml. The MALDI spectra yielded MP values (see above). Using this information, the SEC trace in Fig. 15.2 is calibrated and the average molar masses turn out to be Mw = 55,800, Mn = 23,600. 

Conventional SEC is the classical method for measuring the MMD of synthetic or natural polymers. Conventional SEC essentially means the use of an on-line single concentration detector - differential refractometer (DRI) or UV. In such SEC systems, the molar mass is calculated by means of a direct or universal calibration applying a set of appropriate narrow and/or broad MMD standards. Unfortunately, classical narrow MMD standards for aqueous SEC mobile phases such as pullulan, poly(ethylene oxide), poly (ethylene glycole), and dextran are not suitable for characterization of HA. The listed SEC standards are neutral molecules existing in solution as flexible random coils, and some of... 

The accuracy of the SEC/MALDI procedure has been demonstrated." A series of narrow distributed PMMA samples were injected into a SEC apparatus, and the elution volumes were recorded. A PMMA sample with a broad MMD (M = 13000 Ma, = 33000) obtained by free radical polymerization was injected in an SEC apparatus, and SEC fractions were collected." The MALDI-TOF mass spectra of the SEC fractions were recorded and the respective MM could be computed. Figure 10.14 reports the SEC calibration line obtained using the MALDI-TOF mass spectra of the SEC fractions and also the SEC calibration line obtained using the PMMA standards. " " The two lines are virtually superimposed, thus confirming the accuracy of this procedure, which makes use of MALDI-TOF mass spectra of SEC fractions. The M and... 

A dual-calibration method for the determination of molecular mass (M) and MMD of styrene-maleic anhydride copolymers by SEC has been reported [24]. This method was applicable without preparative fractionation for copolymers of broad composition and M ranges. The values of and M for... 

---