Detectors - Advanced Analytical Chemistry - Lecture Slides, Slides of Analytical Chemistry

Download Detectors - Advanced Analytical Chemistry - Lecture Slides and more Slides Analytical Chemistry in PDF only on Docsity! DETECTORS Important characteristics: 1) Wavelength response 2) Quantum response – how light is detected 3) Sensitivity 4) Frequency of response (response time) 5) Stability 6) Cost 7) convenience docsity.com Photoelectric detectors – main detectors in visible and UV Based on the photoelectric effect photon Eelectron = hν - w e- energy of electron impinging radiation energy work function of surface, lower w
easier to eject electron electron docsity.com Photomultiplier Tube or multiplier phototube (PMT)
essentially a phototube with built in amplifier 90 – 100 volts between photocathode & 1st dynode & between each successive dynode 1 photon
bunch of electrons Each dynode increases the number of electrons Typically 10-20 dynodes docsity.com Photomultiplier Tubes (PMTs) Standard PMT Normal device for UV-vis absorption End-On PMT Typically used where required by space or geometry constraints docsity.com Characteristic Parameters of PMTs: (typically specified by manufacturers) a) Quantum efficiency = f(λ) photoelectrons ejected = ------------------------------------------ photons striking photocathode b) Cathode sensitivity = µA/lumen or µA/watt have to specify λ and use a standard source at known temperature c) Gain = f (g δ)n Typical gain 106 electrons/photon in g δ = 4.5 collectorefficiency transfer efficiency dynode to dynode number of dynodes # of electrons emitted--------------------------------electron striking dynode docsity.com If photocathode is exposed to bright daylight without power, it traps energy and it takes 24 – 48 hrs in the dark with high voltage on in order for dark current to go back to equilibrium value Long term exposures to bright light leads to sensitivity loss particularly at longer λ Noise – due to random fluctuations in: 1) Electron current (shot noise) 2) Thermal motion of conducting electrons in the load resistor (Johnson noise) 3) Incident photon flux (quantum noise) – flux of photons varies statistically docsity.com Shot noise inoise = (2 e i ∆f)½ Shot noise is proportional to the square root of the signal Except at very low currents, shot noise predominates bandwidth of detection system charge on electronnoise current signal current docsity.com Advantages of PMTs 1) Stable except after exposure to high light levels 2) Sensitive 3) Linear over several orders of magnitude 4) Reasonable cost 1) Simple PMT for visible region = $100 2) Quartz jacketed PMT for UV & red sensitive tubes for near IR can be more expensive 5) Long lifetime 6) Rapid response (on the order of nanoseconds) IR detectors not nearly as good as PMTs docsity.com Dead Time – after each pulse, electronics need some time to recover = dead time. Any pulse arriving during the dead time interval will not be counted (typically 0.1 to 0.01 µsec) Dead Time Loss – decrease in signal because of uncounted pulses arriving during the dead time. This becomes significant at count rates somewhere between 105 & 106 counts/sec = upper limit to intensities measured by photon counting docsity.com Plot of number of photon counting pulses observed vs pulse heights for 2 conditions, constant light (top) & no light (lower) If discriminator is set too high
get too few pulses counted (see upper curve) If set too low
get too many pulses counted (lower curve) discriminator setting docsity.com Plot of number of photon counting pulses observed vs pulse heights for 2 conditions, constant light (top) & no light (lower) If discriminator is set too high
get too few pulses counted (see upper curve) If set too low
get too many pulses counted (lower curve) discriminator setting docsity.com