Detection aerosol charge

Dixon RW, Development and testing of a detection method for liquid chromatography based on aerosol charging , Analytical Chemistry 2002 74 2930-2937. 

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. 

In addition to ELS, charged aerosol (CA) or corona detector has more recently been introduced as a very promising HPLC detection system [105] while the sensitivity of the two systems is quite close, CA detector offers the advantage of a nearly linear response factor, particularly crucial for the assessment of enantiomeric purities, whereas ELS provides a nonlinear response at very low or high levels of analytes, resulting from several light scattering mechanisms and particle size distribution. 

A promising detection principle utilizes similar nebulization procedure as applied in ELS, namely the corona-charged aerosol detectors, CAD. In CAD, the aerosol particles interact with an ionized gas (usually nitrogen). The particles become charged and electrically detected [294]. It has been shown that the response of CAD does not depend on the nature of analyte. On the other hand, the size of the aerosol depends on the mobile phase composition and it has to be calibrated. 

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. 

CAD (Charged Aerosol Detector)—A universal, mass detector that evaporates column effluent using a gas nebulizer in the presence of a caronal discharge needle that ionizes compound droplets so they can be detected on an electrometer. 

Figure 15.5 Instrument schematic of the CAD (charged aerosol detection). (Reproduced...

Tadeusz G, Frederic L, Roman S, Pat S. 2006. Universal response in liquid chromatography using charged aerosol detection. Anal. Chem. 78 3186-3192. 

The physical quality of samples is assessed via analytical methods. Liquid chromatography with mass spectrometry (LC-MS) is the most common instrumentation used to determine sample presence and purity (Kerns et al., 2005). Chemiluminescent nitrogen detection (CLND), charged aerosol detection (CAD) and other techniques have also been used to determine concentration (Popa-Burke et al., 2004 Gamache et al., 2003). 

Gamache, P.H. et al. 2003. HPLC analysis of nonvolatile analytes using charged aerosol detection. LC/GC N. Am. 23, 150-155. 

Figure 11.3 Normal-phase HPLC separation of tocopherols (Ts) and tocotrienols (T3s) from barley kernels. A. Detection via fluorescence, ex 294 nm, em 326 nm, B. Detection via a charged aerosol detector. For FIPLC parameters see Moreau et al. (2006).

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],... 

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