Detector Considerations

Choice of the proper detection scheme is dependent on the properties of the analyte. Different types of detectors are available such as ultraviolet (UV), fluorescence, electrochemical, hght scattering, refractive index (RI), flame ionization detection (FID), evaporative light scattering detection (ELSD), corona aerosol detection (CAD), mass spectrometric (MS), NMR, and others. However, the majority of reversed-phase and normal-phase HPLC method development in the pharmaceutical industry is carried out with UV detection. In this section the practical use of UV detection will be discussed. 

---

The dynamic range of LC detectors is usually considerably less than their GC counterparts which evinces more care in determining sample size in quantitative analysis. A GC detector may have a linear response over a concentration range of five or six orders of magnitude, for example, the flame ionization detector, whereas an LC detector is more likely to have a dynamic range of only three orders of magnitude and some detectors considerably less. 

In 1972, Day Zimmermann engineers started a test program. One of the first steps was to select the type of sensor or detector. Consideration was given to infra-red, ultraviolet, and audio signals. After careful study of the alternatives, the ultraviolet was chosen, primarily because of available components and that it would not be affected by sunlight. Audio was discarded because of frequent thunderstorms during the rainy season. 

Biina, R., Pons, S. and Fleischmann, M. 0988) Ultramicroelectrode sensors and detectors. Considerations of the stability, sensitivity, reproducibility and mechanism of ion transport in gas-phase chromatography and in high-performance liquid-phase chromatography. Journal of Electroanalytical Chemistry, 244, 244. 

For locating detectors, consideration must be given to natural calamities such as flood, rain, storm, etc. Outdoor detectors should use weatherproof enclosures as applicable. It is preferable to use sunshade and avoidance of direct sunlight. 

Trace analysis of explosives in postblast debris samples requires a detector considerably more sensitive than an FID. ECD, nitric oxide detection (TEA), and GCMS have all been nsed for the chromatographic detection of explosives (209). Gas chromatographic analysis with specific detection is normally used for screening purposes. Other methods, such as TLC, microscopy, X-ray diffraction (XRD), and SEM, are often used to confirm the presence of trace... 

Since any DAC is defined by its coordinates P (Ai,S ) and the instrument sensitivity Gg f (reference gain) during DAC recording, any recalculation of the curve including the consideration of individual corrections (transfer loss, sound attenuation, etc.) is an easy task for modern PC based flaw detectors and does no longer burden the operator. 

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. 

The basic instrumentation for capillary electrophoresis is shown in Figure 12.41 and includes a power supply for applying the electric field, anode and cathode compartments containing reservoirs of the buffer solution, a sample vial containing the sample, the capillary tube, and a detector. Each part of the instrument receives further consideration in this section. 

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. 

Further, peak overlap results in nonlinear detector response vs concentration. Therefore, some other detection method must be used in conjunction with either of these types of detection. Nevertheless, as can be seen from Figure Ilf, chiroptical detection can be advantageous if there is considerable overlap of the two peaks. In this case, chiroptical detection may reveal that the lea ding and tailing edges of the peak are enantiomerically enriched which may not be apparent from the chromatogram obtained with nonchiroptical detection (Fig. He). 

---