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A detector can be compared to a gate watchman who verifies the visitors before permitting them entry inside a building. The chromatographic detector is capable of establishing both the identity and concentration of eluting components in the mobile phase stream. A broad range of detectors is available to meet different sample requirements. Specific detectors respond to a particular compound only and the response is independent of mobile phase composition. On the other hand the response of bulk property detectors is dependent on collective changes in composition of sample and mobile phase.
The desirable features of a detector are:
- Sensitivity towards solute over mobile phase
- Low cell volumes to minimize memory effects
- Low detector noise
- Low detection limits
- Large linear dynamic range
UV-VIS Detector is the most commonly used detector. Its response is specific to a particular compound or class of compounds depending on the presence of light absorbing functional groups of eluting molecules. Some compounds which do not have such light absorbing groups can give suitable response after post column derivatization to introduce light absorbing entities.
Photo Diode Array Detector
Incorporation of large number of diodes which serve as detector elements makes possible simultaneous monitoring of more than one absorbing component at different wavelengths. This provides benefit of time saving and cost reduction on expensive solvents.
Fluorescence detection offers greater sensitivity than a UV-VIS detector. However, the number of naturally fluorescent compounds is smaller in comparison to light absorbing compounds. This limitation is overcome by post column derivatization.
Mass Spectroscopic Detector
Mass spectroscopy offers very high sensitivity and selectivity. Detection is based on fragmentation of molecules by electric fields and separation on basis of mass to charge ratios of fragmented molecules. LC –MS technique has opened up new application areas due to advantages of resolution and sensitivity.
Bulk Property Detectors
Refractive Index Detector
The response is dependent on changes in refractive index of eluting compounds in the mobile phase. The mobile phase itself should have refractive index different from the sample. Gradient programming is not possible due to resulting changes in refractive index of mobile phase. The detector is less sensitive than UV-VIS detector. Temperature control is necessary as it has high temperature sensitivity. Typical applications are in Size Exclusion Chromatography.
Based on electrochemical oxidation or reduction of sample on electrode surface. It is, however, sensitive to changes in composition or flow rate of mobile phase.
Light Scattering Detectors
Light scattering detectors are useful for detection of high molecular weight molecules. After removal of mobile phase by passing through a heated zone the solute molecules are detected by light scattering depending on molecular sizes.
Tip of the day
Never exceed pressure abruptly or exceed prescribed pressure limits as it can damage detector cell walls.
The last three modules covered the stationary phase – solute interactions. mobile phase requirements, and detectors commonly used in HPLC. You will find an elaborate treatment on these and other related topics in the certificate programme.
Please revisit the link http://lab-training.com/product/join-our-certificate-course-on-hplc/. You’re sure to get answers to most of your questions without going through voluminous texts and references.
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