Detectors theoretical considerations

A simphstic view of a FIA analytical system is outfined in Fig. 2.19. The liquid flow can be obtained in a number of ways, most commonly by using a peristaltic pump. In addition, gravity-feed systems, overpressure systems on liquid vessels and simple and double piston pumps normally associated with HPLC systems, are often used. The interrelation of the pumping system and the bore size of the transport tubing to a great extent modify the theoretical considerations involved and the operation of the FIA regime. In common with CFA, the minimization of dead volumes—in detector cells and between T pieces for example—is particularly important. Injections of the sample into... 

The ELS detector was previously also referred to as a mass detector, pointing to the fact that the response is (mainly) determined by the mass of the sample rather than by its chemical structure. Van der Meeren et al., though, demonstrated that the ELSD calibration curves of phospholipid classes were also dependent on the fatty acid composition (52). The dependence on the fatty acid composition is, however, completely different in nature and much less pronounced than for UV detection. The reason for this behavior is to be found in the partial resolution of molecular species, even during normal-phase chromatography. Thus, the peak shape depends not only on the chromatographic system but also on the fatty acid composition and molecular species distribution of the PL sample (47). Because it was shown before, based on both theoretical considerations and practical experiments, that the ELS detector response is generally inversely proportional to peak width (62,104), it follows that the molecular species distribution of the PL standards used should be similar to the sample components to be quantified. It was shown that up to 20% error may be induced if an inappropriate standard is used (52). 

H.B. Hanekamp and H.J. van Nieuwkerk, Theoretical considerations on the performance of electrochemical flow-through detectors. Anal Chim. Acta, 1980, 121, 13-22. 

At higher column oven temperatures with increased linear velocity of carrier gas, capillary separations can be achieved that mimic those on a packed column but with a shortened time of analysis. The rednced amonnt of stationary phase in a capillary column imparts another advantage to the chromatographer, namely, one observes less bleed of stationary phase from the colnmn at elevated temperatures and this means less detector contamination. Theoretical considerations of the capillary column are discussed in Section 3.10. 

H. B. Hanekamp and H. J. van Nieuwkerk (1980) Theoretical Considerations on the Performance of Electrochemical Flow Through Detectors , Analytica Chimica Acta, 121, 13. 

Arora and Kulkarni [1] describe a theory of how the combination of detectors and correctors lead to fault-tolerant systems. Roughly, our approach makes their theoretic considerations practically applicable, as using our approach, engineers combine and nest detection and handling requirements in similar way and our tools provide argumentation on certain fault-tolerance aspects. 

Golay (1947a) Theoretical Consideration in Heat and Infra-Red Detection, with Particular Reference to the Pneumatic Detector by M. J. E. Golay, Rev. Sci. Instrum. 18, 347-356. 

Probably the simplest mass spectrometer is the time-of-fiight (TOP) instrument [36]. Aside from magnetic deflection instruments, these were among the first mass spectrometers developed. The mass range is theoretically infinite, though in practice there are upper limits that are governed by electronics and ion source considerations. In chemical physics and physical chemistry, TOP instniments often are operated at lower resolving power than analytical instniments. Because of their simplicity, they have been used in many spectroscopic apparatus as detectors for electrons and ions. Many of these teclmiques are included as chapters unto themselves in this book, and they will only be briefly described here. 

To circumvent this need for calibration as well as to better understand the separation process itself, considerable effort has been directed toward developing the theoretical basis for the separation of molecules in terms of their size. Although partially successful, there are enough complications in the theoretical approach that calibration is still the safest procedure. If a calibration plot such as Fig. 9.14 is available and a detector output indicates a polymer emerging from the column at a particular value of Vj, then the molecular weight of that polymer is readily determined from the calibration, as indicated in Fig. 9.14. 

Despite the attraction of high-speed separations, the choice of whether to use standard-bore HPLC or small-bore HPLC is mostly governed by practical considerations (the need for a detector capable of providing positive identifications), environmental considerations (low solvent consumption), sample size considerations (small injection volumes), or price considerations (the need to use exotic solvents and expensive samples, such as DNA). Simple separations are best carried out in conventional short, wide columns, whereas more difficult separations can be achieved with the use of longer, narrower columns.15 However, difficulties arise when downscaling HPLC. The difficulties are divided into theoretical and practical considerations. 

Theoretically, CCD s offered the most attractive features of the remaining choices, photodiodes and CCD s. However, at the time when the decision had to be made, CCD technology was, and still is, too much in flux, for their use in a mass spectrometer system and too higji in cost to be a reasonable choice. The cost factor would be amplified even further when one considers the increased requirements on the data acquisition system due to the 60-fold increase in data rate ( 860,00 vs 14,300 ) data points per spectrum. These and other considerations led to the decision to implement the second generation detector with a photodiode (Reticon) based camera. This system is now in operation producing excellent data and will be described in detail in the following section. 

Separation efficiency in terms of the number of theoretical plates per meter of column length varies inversely with column radius better separation is achieved on smaller diameter columns. Columns whose inner diameters are less than 100 um, however, are extremely difficult to Interface with normal inlets and detectors. In addition, their capacities are very limited, they are easily overloaded, and their behaviour with inlet splitters (which at the present time is the most practical means of introducing a sample on these very small bore columns) can be capricious. Even the 100 tun ID column suffers from these limitations skilled chromatographers have used them to good advantage, but at our present state-of-the-art, many will experience considerable frustration with these columns. 

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