Evaporative light scattering detection aerosol

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. 

In the past, aerosol-based detection methods such as evaporative light scattering detection (ELSD), condensation nucleation light scattering detection... 

For analytes with no UV chromophore or weak UV chromophore, detection methods such as Corona charged aerosol detection (CAD), evaporative light scattering detection (ELSD), or MS can be used. These detectors require that analytes be nonvolatile and mobile phase be compatible with detectors. 

Evaporative light scattering is gaining popularity due to its ability to detect analytes on a nonse-lective basis. Basically, this detector works by nebulizing the column effluent, forming an aerosol that is further converted into a droplet cloud for detection by light scattering. This type of detector has been applied to studies of small molecule combinatorial libraries [13,14], carbohydrates [15], and lipids [16,17]. 

FIGURE 7.13 Schematic of an evaporative light scattering detector. The three stages are nebulization, in which column effluent is aerosolized evaporation, in which the mobile phase is vaporized and optical detection, in which the light scattering of the residual solute particles is recorded. Some detectors also include an obstacle in the flow path for droplet discrimination, which leads to a more homogenous distribution of droplet sizes. 

The advent of the use of mass spectrometers as detectors and new mass detectors such as the charged aerosol detectors (CAD) and evaporative light scattering detectors (ELSD) should provide high-sensitivity detection of compounds that do not absorb UV light. The only problem with most of these is that they are expensive and, therefore, not readily available. When prices come down, they should finally eliminate the use of derivatives in HPLC analysis. 

Although tocopherols and tocotrienols can be detected by UV absorbance at 280 nm, fluorescence detection (excitation 294 nm and emission 326 nm), as shown in Figure 11.3, has proven to be a much more sensitive method. Electrochemical detection such as pulsed amperometric and coulometric (Uspitasari-Nienaber, 2002) has also proven to be sensitive and potentially valuable for the quantitative analysis of tocopherols and Tocotrienols (Abidi, 2000), especially for tocol analysis in blood and serum samples. HPLC mass detectors such as flame-ionization detectors, evaporative light-scattering detectors, and charged aerosol detectors have proven to be valuable for the quantitative analysis of many types of lipids, but because tocols have... 

Additional detection techniques that can be employed to help solve mass balance issues with RP-HPLC are MS [30], chemiluminescent nitrogen-specihc detector [31], evaporative light-scattering detector, ELSD [32], and corona charged aerosol detection [CAD] [33],... 

In the introductory Section 2.1.3, it was discussed that an important aspect of optimization can be to improve a method for its applicability in trace analysis. The nature of the mode of detection is very relevant in this case whether the applied detector is concentration proportional like the very common UV detector or mass proportional hke nebulizer-based detectors, for example, evaporative light scattering detector (ELSD) or charged aerosol detector (CAD). This textbook contains dedicated chapters on nebulizer-based or aerosol detectors (Chapter 10 on trends in detection), as well as for the coupling of LC with mass spectrometry (Chapter 1). Here, the focus is on concentration proportional detectors UV detectors (VWD, DAD), fluorescence detectors (FLD), electrochemical detectors (ECD), and refractive index (RI) detectors. 

Detection methods applied in ion chromatography are divided into electrochemical, spectrometric, nebulization, and others. Conductometric, amperometric, and charge detection are electrochemical methods, while the spectrometric methods include UV/Vis, fluorescence, and refractive index detection. In addition, there are various application forms of these detection methods. Nebulization methods include evaporative light scattering (ELS) and charged aerosol detection (CAD). All of these methods are described in detail in this chapter. 

Some APIs and most counterions laclc or have only a weak chromophore, so that absorbance detectors cannot be used. At present, such analytes are typically detected by aerosol-based detection techniques such as evaporative light scattering and charged aerosol detection. Both detection techniques have already been described in Section 8.3. CAD has been shown to overcome many of the issues found with ELSD. CAD is more sensitive, has a wider dynamic range and better precision, and, although response is nonlinear, calibration is less complex than with ELSD. 

The unique detection principle of evaporative lightscattering detectors involves nebulization of the column effluent to form an aerosol, followed by solvent vaporization in the drift tube to produce a cloud of solute droplets (or particles), and then detection of the solute droplets (or particles) in the light-scattering cell. 

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