The actual separation of each component in the sample is carried inside a column; however this separation needs to be “collected” for us to be able to see it. The detectors are used for this purpose. The separated components are monitored and expressed electronically. There is no universal detector that can monitor all compounds and there are many detectors used for LC analysis. Some are listed below.
|Evaporative Light Scattering||
|Multi Angle Laser Light Scattering||
|Photo Diode Array||
The UV, VIS, and PDA detectors are categorized as absorbance detectors. They provide good sensitivity for light-absorbing compounds at ~pg level. They are easy to operate and provide good stability. UV detector is a very commonly used detector for HPLC analysis. During the analysis, sample goes through a clear color-less glass cell, called flow cell. When UV light is irradiated on the flow cell, sample absorbs a part of UV light. Thus, the intensity of UV light observed for the mobile phase (without sample) and the eluent containing sample will differ. By measuring this difference, the amount of sample can be determined. Since the UV absorbance also differs depend on what wavelength is used, it is important to choose an appropriate wavelength based on the type of analyte. A standard UV detector allows user to choose wavelength between 195 to 370 nm. Most commonly used is 254 nm. Compared to a UV detector, a VIS detector uses longer wavelength (400~700 nm). There are detectors that provide wider wavelength selection, covering both UV and VIS ranges (195~700 nm) called UV/VIS detector.
PDA detects an entire spectrum simultaneously. UV and VIS detectors visualize the obtained result in two dimensions (light intensity and time), but PDA adds the third dimension (wavelength). This is convenient to determine the most suitable wavelength without repeating analyses.
RI detector has lower sensitivity compared to UV detector, and that’s the main reason why RI is not as commonly used as UV. However there are some advantages over UV detector.
- It is suitable for detecting all components. For an example, samples which do not have UV absorption, such as sugar, alcohol, or inorganic ions obviously cannot be measured by a UV detector. In contrast, change in reflective index occurs for all analyte, thus a RI detector can be used to measure all analyte.
- It is applicable for the use with solvent that has UV absorbance. A UV detector cannot be used with solvent which has UV absorbance. Sometimes the organic solvent used for GPC analysis absorbs UV, and thus UV detector cannot be used.
- It provides a direct relationship between the intensity and analyte concentration.
The amount of UV absorbed depends on each analyte, thus the intensity of UV detector peak does not provide information on the analyte concentration. While intensity observed by a RI detector is comparable to the concentration of analyte.
Because of those advantages, RI is often used for the detection of sugars and for SEC analysis.
The analytes are detected based on their MW. The obtained information is especially useful for compound structure identification. However, its use is not limited to structure identification and can be used to quantify very low detection limit of elemental and molecular components.
There are several different types of ECs. The detection is based on amperometry, polarography, coulometry, and conductrometry. They offer high sensitivity, simplicity, convenience, and wide-spread applicability. It is especially suitable for the use with semi-micro or capillary type system.
Analytical Chemistry 7th (Seventh) Edition by Skoog 1999
Ekikurono Kotsu Detector (in Japanese) by Hiroshi Nakamura 2006