3 Fixed Wavelength High Pressure Mercury Lamp Mercury vapor lamps are probably themost common and emit intense lightat nm (and certain otherwavelengths). Because of the limitedemission spectra, of the lamp wavelengthsare not adjustable.Because of the intensityof the radiation, fixed wavelengthdetectors can be up to 20 times moresensitive than variable wavelengthdetectors.
4 Variable WavelengthThe variable types use a deuterium or similar lamp thatproduces a broad spectrum of wavelengths that are separatedby a defraction grating. The grating works like a prism butgenerally the resolving power is higher for gratingsthan for prisms.
5 Diode Array - Peak Purity - Quantify a peak with an interfering peak The photo diode array detector passes a wide spectrum of light throughthe sample. The spectrum of light is directed to an array of photosensitivediodes. Each diode can measure a different wavelength which allows forthe monitoring of many wavelengths at once.- Peak Purity- Quantify a peak with aninterfering peakCompound IdentificationMonitor compounds withdifferent UV max.
7 Lamp Life As the lamps age, the light intensity drops significantly. Therefore it is important to monitor lamp usage.Arc lamps have an average lifetime of about 200hours and most detectors are equipped with a timer thatkeeps track of lamp usage.
8 Fluorescence Detectors Fluorescence detectors are among the most sensitive ofHPLC detectors, and depending on the compound,can be from times more sensitive than UV detectorsanalyzing strong UV absorbing compounds.The fluorescence detector is based on the principle that somecompounds fluoresce when bombarded with UV light.If the compound of interest fluoresces this is a very sensitive detector.The analyte is excited by light commonly at nm from a lowpressure mercury lamp. The light is absorbed and the moleculeis excited and it gives off light of a different wavelength.This wavelength is monitored by a detector which sits at rightangles to the UV light source used to excite the analyte.
9 Electrical Conductivity The electrical conductivity detector was one of the first in-linedetectors developed and is still in use for detecting electricallycharged ions. It is based on the principle that the electricalconductivity of a liquid is proportional to the ionic concentrationin the mobile phase. The design is simple, and the detector canbe made small enough for an effective sensing volume of a fewnanoliters, making this detector very useful for capillary electrophesis.
10 Refractive Index Detectors In contrast to the fluorescence detector, the refractive indexdetector is one of the least sensitive (about 1000 times lesssensitive than UV detectors).Refractive index detectors are often used for the analysis of carbohydrates.Carbohydrates are generally not ionic do not fluoresce, and do not absorbUV or visible light to any great extent, so the refractive index is one of thefew detectors that can detect them.
11 Light Scattering Detectors The flow from the column isconverted into a fine spray or mist.The mobile phase is evaporated,leaving tiny particles of the analytesThe particles are passed through alaser beam and they scatter thelaser light. The scattered lightis measured at right angles tothe laser beam by a photodiodedetector.
12 Mass Spectrometric Detectors For all mass spectrometer techniques, the analytes are firstionized since the detector only measures charged particles.The charged particles then enter the separator that separatesions on the basis of mass using a magnetic field.The masses of different fragments hit the detector wherethey give up their charge creating an electric signalwhich is recorded.It is difficult to use a mass spectrometer with liquid chromatography becausethe mobile phase must be removed before the analytes can enter the detector.A number of interfaces have been developed to solve this problem(ie., electrospray, ion spray, and thermospray) and LC-MS is becomingmore common.