Evaporative light scattering detector selection

Current IPC detectors are on-stream monitors. HPLC detectors range from (1) non selective or universal (bulk property detectors such as the refractive index (RI) detector), characterized by limited sensitivity, (2) selective (discriminating solute property detectors such as UV-Vis detectors) to (3) specific (specific solute property detectors such as fluorescence detectors). Traditional detection techniques are based on analyte architecture that gives rise to high absorbance, fluorescence, or electrochemical activity. Mass spectrometry (MS) and evaporative light scattering detectors (ELSDs), can be considered universal types in their own right... 

If no derivatization takes place, detection is preferably accomplished by UV at a low wavelength (200-210 nm) in order to enhance detection sensitivity. However, detection selectivity is sacrificed at such low wavelengths. Electrochemical detection, when applied to the analysis of free amino acids, offers higher selectivity but suffers from a small linearity range. Furthermore, most amino acids (with the exception of tryptophan, tyrosine, and cysteine) are not intrinsically electrochemically active within the current useful potential range [5]. Lately, the development of the evaporative light-scattering detector (ELSD) offers an attractive alternative for the determination of nonderivatized amino acids (see Fig. 1). 

The evaporative light scattering detector (ELSD) is an instrument for the non-selective detection of non-volatile analytes. The column eluate is nebulized in a stream of inert gas. The liquid droplets are then evaporated, thus producing solid particles which are passed through a laser beam. The resulting scattered light is registered by a photodiode (Fig. 6.12). 

The evaporative light scattering detector works by measuring the light scattered from the particles remaining after the eluent solvent has been nebuhzed and evaporated. The eluent solvent enters the detector and is evaporated in a heated device. As chromatographic peaks are eluted from the column and enter the detector the solvent is evaporated away. Conditions are selected so that the sample peak molecules become particulate. These particles enter a chamber into which a... 

One scheme for separating polymer mixtures by HPLC is based on a selective precipitation redissolution model [11]. It has been used with the evaporative light-scattering detector to monitor the separation of a three-polymer mixture (Fig. 20). Solvent programming was used. The author suggests that this technique can be used to determine chemical composition of copolymers, but no examples are shown. 

The evaporative light-scattering detector (ELSD), sometimes wrongly called a mass detector, is influenced by many variables. Schulz and Engelhardt [28] showed that its specific response in addition to the analyte s molecular size also depends on the linear flow rate of the mobile phase consequently, this detector is mass sensitive. Furthermore, they found that the ELSD behaves in certain situations more like a selective and not a universal detector. For example, it can detect only those solutes that are nonvolatile under... 

Any of the methods of detection used in liquid chromatography can be used in IC, though some are more useful than others. If the eluent does not affect the detector the need for a suppressor disappears. Common means of detection in IC are ultraviolet (UV) absorption, including indirect absorption electrochemical, especially amperometric and pulsed amperometric and postcolumn derivatization. Detectors atomic absorption spectrometry, chemiluminescence, fluorescence, atomic spectroscopic, refractive index, electrochemical (besides conductivity) including amperometric, coulometric, potentiometric, polaro-graphic, pulsed amperometric, inductively coupled plasma emission spectrometry, ion-selective electrode, inductively coupled plasma mass spectrometry, bulk acoustic wave sensor, and evaporative light-scattering detection. 

In LC, a means of detection is employed for identification and quantification. While detection is covered elsewhere in this volume, mobile phase selection can play an important role in the detectability of the compounds of interest, and vice versa. A solvents lowest usable (cutoff) wavelength is important for UV detectors, solvent refractive index (RI) effects the sensitivity of RI detection, and solvent volatility is an important consideration for evaporative light scattering and mass spectrophotometric based detectors. Table 4 lists some common LC mobile phase spectral data. 

Other widely used detectors for HPLC include refractive index (RI), fluorescence and evaporative light-scattering (ELS). The use of Rl and ELS detectors for pantothenic acid analysis in multivitamin dietary supplements has not been reported. The main reason is that the two detectors are not selective and thus cannot resolve pantothenic acid from other components existing in a multivitamin dietary supplement. Although fluorescence detection can be highly selective depending on the application, pantothenic acid does not have fluorescence excitation and emission and so fluorescence detection cannot be used for pantothenic acid analysis unless derivatization methods are applied (Pakin et al. 2004 Takahashi et al. 2009). Derivatization adds more complexity to analytical method and should not be used unless neeessary. For deteetion and quantitation of pantothenic add in multivitamin dietary supplements with HPLC/UHPLC, a highly selective detector such as MS should be the instrument of choice. 

---