Absolute mass detector

Simple homopolymers, where monodisperse standards and suitable solvents are available, are easily characterized by SEC. Homopolymers for which no monodisperse standards are available additionally require some more elaborate detection system for transformation of the retention time into molecular weight. This can be done by, e.g., universal calibration. Alternatively, an absolute molar mass detector, like an on-line light scattering detector or mass spectrometer, can be used. 

The concentration of the polymer molecules eluting from SEC columns is continuously monitored by a detector. The most widely used detector in SEC is the differential reftactometer (DRI), which measures the difference in refractive index between solvent and solute. Other detectors commonly used for SEC are functional group detectors ultraviolet (UV) and infrared (IR), and absolute molecular weight detectors low angle laser light scattering (LALLS) and in-line continuous viscometers. Applications of these detectors to SEC analysis will be discussed later in the Multiple Detectors Section. Other detectors also being used are the densimeter (11-19) and the mass detector (20-23). 

Direction of the gas chromatographic effluent into a vessel containing activated carbon attached to an automatic recording electromicrobalance is the basis for the device known as the Brunei mass detector (47). This is an absolute analytical method and requires no calibration, and in fact, can be used to calibrate other detectors which have unpredictable responses. The sensitivity of the detector is in the same range as the thermal conductivity detector. 

Finally, several attempts have been made to develop an absolute molar mass detector based on osmotic pressure measurements. Commercially available membrane osmometers are designed for static measurements, and the cell design with a flat membrane is not suited for continuous flow operation. Different from the conventional design, Yau developed a detector which measures the flow resistance of a column caused by osmotic swelling and deswelling of soft gel particles used for the packing (see Fig. 12) [65,78]. With a microbore gel column, a... 

From the Th-FFF retention data, it is possible to obtain a molar mass distribution after a suitable calibration for the determination of the Mark-Houwink constants (straight-line plot of log(D/DT) vs. log M [15]). Another possibility is to couple an absolute molar mass detector like MALLS (see Sect. 4.3.2) or a suitable detector combination such as an on-line viscometer coupled with a refractive index detector. This possibility does not require prior knowledge of DT... 

The level portions between the peaks are unambiguous so reasonable quantitative accuracy could be expected. Although a highly unlikely candidate for general analytical use, the mass detector represents one of the few chromatography detectors that has an absolute response. 

Usually r is kept fixed and, by varying // (or V), ions of differing M/e are allowed to reach the detector. The signal recorded always refers to a mass to charge ratio and not to an absolute mass. 

With concentration proportional detectors in particular, the difference between the concentration-related and the absolute (mass- or amount-related) LOD and LOQ must be identified. This was already briefly discussed in Section 2.1.3, with the following sections diving deeper into the details together with mathematical calculations. This will guide the practitioner to the optimization parameters and should also take away some common misconceptions. The typical acronyms LOD and LOQ will be used. 

With the exception of the Fourier transform mass spectrometer described above, ion detection in other mass spectrometers is the destructive event in the sequence. Each individual ion carries such a small charge and is of such low absolute mass that direct detection is difficult (but not impossible). Considerable amplification of the ion charge or the ion mass is necessary to make mass spectrometry practicable. Electron multiplier detectors, known since the 1950s, provide the requisite amplification of the charge into an easily manipulated current. 

Analytical standards are prepared for two purposes for fortifying control matrices to determine the analytical accuracy and for calibrating the response of the analyte in the mass spectrometer detector. The purity of all standards must be verified before preparation of the stock solutions. All standards should be refrigerated (2-10 °C) in clean amber-glass bottles with foil/Tefion-lined screw-caps. The absolute volume of the standard solutions may be varied at the discretion of the analyst, as long as the correct proportions of the solute and solvent are maintained. Calibrate the analytical balance before weighing any analytical standard material for this method. 

Electron Density. Continuing the preceding considerations, calibration to absolute intensity means normalization to the scattering of a single electron , Ie that can be expressed in electron units, [e.u.]. Inevitably a calibration to absolute units involves also a normalization with respect to the irradiated volume V. Thus, for the field of materials science a suitable dimension of the absolute intensity is [I/V] = e.u./nm3 - The intensity measured in the detector is originating from a material with an average electron density of 400 electrons per nanometers cubed . The electron density itself is easily computed from mass density and chemical composition of the material (cf. Sect. 2.2.1). 

Several kinds of detection systems have been applied to CE [1,2,43]. Based on their specificity, they can be divided into bulk property and specific property detectors [43]. Bulk-property detectors measure the difference in a physical property of a solute relative to the background. Examples of such detectors are conductivity, refractive index, indirect methods, etc. The specific-property detectors measure a physico-chemical property, which is inherent to the solutes, e.g. UV absorption, fluorescence emission, mass spectrum, electrochemical, etc. These detectors usually minimize background signals, have wider linear ranges and are more sensitive. In Table 17.3, a general overview is given of the detection methods that are employed in CE with their detection limits (absolute and relative). 

The most intense peak of a mass spectrum is called base peak. In most representations of mass spectral data the intensity of the base peak is normalized to 100 % relative intensity. This largely helps to make mass spectra more easily comparable. The normalization can be done because the relative intensities are independent from the absolute ion abundances registered by the detector. However, there is an upper limit for the number of ions and neutrals per volume inside the ion source where the appearance of spectra will significantly change due to autoprotonation (Chap. 7). In the older literature, spectra were sometimes normalized relative to the sum of all intensities measured, e.g., denoted as % Lions, or the intensities were reported normalized to the sum of all intensities above a certain m/z, e.g., above m/z 40 (% L 4o)-... 

A universal calibration is therefore possible for SEC by plotting log ([q] M) vs. Vg when a viscosity detector is used. Absolute molar masses can be obtained using a light-scattering detector. 

The significant intrinsic limitation of SEC is the dependence of retention volumes of polymer species on their molecular sizes in solution and thus only indirectly on their molar masses. As known (Sections 16.2.2 and 16.3.2), the size of macromolecnles dissolved in certain solvent depends not only on their molar masses but also on their chemical structure and physical architecture. Consequently, the Vr values of polymer species directly reflect their molar masses only for linear homopolymers and this holds only in absence of side effects within SEC column (Sections 16.4.1 and 16.4.2). In other words, macromolecnles of different molar masses, compositions and architectures may co-elute and in that case the molar mass values directly calculated from the SEC chromatograms would be wrong. This is schematically depicted in Figure 16.10. The problem of simultaneous effects of two or more molecular characteristics on the retention volumes of complex polymer systems is further amplifled by the detection problems (Section 16.9.1) the detector response may not reflect the actual sample concentration. This is the reason why the molar masses of complex polymers directly determined by SEC are only semi-quantitative, reflecting the tendencies rather than the absolute values. To obtain the quantitative molar mass data of complex polymer systems, the coupled (Section 16.5) and two (or multi-) dimensional (Section 16.7) polymer HPLC techniques must be engaged. 

Other important pitfalls lie again in the low selectivity of SEC, which does not allow identifying small amounts of the macromolecular admixtures that is the minor components of polymer blends. The bell-shaped chromatograms with a broad base and a slim upper part are often erroneously proclaimed to signalize the narrow molar mass distribution of sample. On the other hand, the accumulation peaks due to presence of macromolecules excluded from the packing pores (Section 16.8.1) are interpreted as the sign of sample bimodality. The absolute detectors may also contribute to erroneous conclusions concerning sample polydispersity (Section 16.8.1). 

The detection of a test gas using mass spectrometers is far and away the most sensitive leak detection method and the one most widely used in industry. The MS leak detectors developed for this purpose make possible quantitative measurement of leak rates in a range extending aaoss many powers of ten (see Section 5.2) whereby the lower limit = 10 mbar l/s, thus making it possible to demonstrate the inherent gas permeability of solids where helium is used as the test gas. It is actually possible in principle to detect all gases using mass spectrometry. Of all the available options, the use of helium as a tracer gas has proved to be especially practical. The detection of helium using the mass spectrometer is absolutely ( ) unequivocal. Helium is chemically inert, non-explosive, non-toxic, is present in normal air in a concentration of only 5 ppm and is quite economical. Two types of mass spectrometer are used in commercially available MSLD s ... 

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