Molecular weight/mass detectors

Like the UV detector, the IR detector is relatively insensitive to fluctuations around a set temperature and therefore particularly suitable for SEC at temperatures far above ambient conditions. One of the main uses for this type of detector is in the detection of polyolefins which can only be chromatographed at temperatures in excess of 135°C. Infrared detection is also useful in measuring polymer stoichiometry as a function of molecular weight/mass. Here the detector is used to monitor specific functional groups in a copolymer. 

Molecular weightjmass detectors include light scattering detectors and viscosity detectors. When SEC is used in the characterisation of polymer systems, its main aim will be the production of a molecular mass/weight distribution and where possible absolute molecular weights. Mass calibration is a complicated matter (section 9.3.5.1) in that calibration curves differ for different polymer types, and for many commercial polymers, direct molar mass calibration is not possible because of the lack of suitable, known molecular weight standards. 

An alternative is to determine the polymers molecular weight/mass in the SEC eluent in situ, by use of on-line molecular mass sensitive detectors. Two such detectors are commercially available, the light scattering detector and the viscosity detector. These detectors are usually used in series with a mass concentration detector and require specialised data handling/software to compute the outputs from the twin detectors and to produce molecular weight/masses and distributions. 

GPC/SEC is a controlled separation technique in which molecules are separated on the basis of their hydrodynamic molecular volume or size [26], With proper column calibration or using molecular weight-sensitive detectors, such as light scattering, viscosimetry, or mass spectrometry, the MWD and the statistical molecular weight averages can be readily obtained. In their review, Barth et al. [26] mentioned that the GPC/SEC is the premier technique to evaluate these properties for both synthetic polymers and biopolymers. 

When the secondary effects are eliminated, a SEC system may be calibrated using standards of known molecular weight. After cahbration the system may be used to very aeeurately determine the moleeular weight and moleeular weight distribution of a polymer sample. Calibration is always required unless a molecular weight sensitive detector is used to convert measured elution volumes to a molecular weight or mass. 

In its simplest form, a mass spectrometer is an instmment that measures the mass-to-charge ratios ml of ions formed when a sample is ionized by one of a number of different ionization methods (1). If some of the sample molecules are singly ionized and reach the ion detector without fragmenting, then the ml ratio of these ions gives a direct measurement of the molecular weight. The first instmment for positive ray analysis was built by Thompson (2) in 1913 to show the existence of isotopic forms of the stable elements. Later, mass spectrometers were used for precision measurements of ionic mass and abundances (3,4). 

The use of separation techniques, such as gel permeation and high pressure Hquid chromatography interfaced with sensitive, silicon-specific aas or ICP detectors, has been particularly advantageous for the analysis of siUcones in environmental extracts (469,483—486). Supercritical fluid chromatography coupled with various detection devices is effective for the separation of siUcone oligomers that have molecular weights less than 3000 Da. Time-of-flight secondary ion mass spectrometry (TOF-sims) is appHcable up to 10,000 Da (487). 

Mass spectrometry (see Chapter 3) is capable of providing molecular weight and structural information from picogram amounts of material and to provide selectivity by allowing the monitoring of ions or ion decompositions characteristic of a single analyte of interest. These are the ideal characteristics of both a qualitative and a quantitative detector. 

The mass spectrometer provides the most definitive identification of aU of the HPLC detectors. It allows the molecular weight of the analyte to be determined - this is the single most discriminating piece of information that may be obtained - which, together with the structural information that may be generated, often allows an unequivocal identification to be made. 

The application areas for LC-MS, as will be illustrated later, are diverse, encompassing both qualitative and quantitative determinations of both high-and low-molecular-weight materials, including synthetic polymers, biopolymers, environmental pollutants, pharmaceutical compounds (drugs and their metabolites) and natural products. In essence, it is used for any compounds which are found in complex matrices for which HPLC is the separation method of choice and where the mass spectrometer provides the necessary selectivity and sensitivity to provide quantitative information and/or it provides structural information that cannot be obtained by using other detectors. 

Mjj and My or [q] for the broad MWD standard are taken as known quantities. Fy(v) is the normalized chromatogram for the broad MWD standard obtained with a mass detector. D2 is the slope of the molecular weight calibration curve at the peak position of the chromatogram (the equation of the tangent is given by M(v) = Dj exp(-D2v). is the variance of the single-species chromatogram... 

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