Module 7: Spectrophotometers Flashcards
6 parts of a spec
light source wavelength selector (monochromator) sample holder photodetector signal processor display
types of light source
Tungsten bulb
Tungsten-halogen (for visible light range)
Hydrogen lamp (for UV range)
Mercury and xenon Arc (very high intensity of UV light)
LED (for very specific wavelengths)
Laser (high intensity, nearly monochromatic)
wavelength selector used to
isolate a characteristic wavelength that the analyte to be measured absorbs or emits
desired wavelength
nominal wavelenth
represent peak intensity of the light leaving the wavelength selector
Band pass or spectral bandwidth
the range of wavelengths on either side of the nominal wavelength that exit the monochromator
ex. nominal wl = 450
bandwidth of 10
actual wavelength would be 445-455
the better the ability of device to select narrow wl
the better the spectral resolution
wavelength selector in a photometer
called a filter
wavelength selector in a spec
called a monochromator
2 filters used in photometry
glass absorption (green filter allows green through but no other wavelengths) Interference
Monochromator used in spectrophotometers
prisms (refraction of white light, nonlinear spectrum) Diffraction grating (diffraction of white light, linear spectrum)
3 parts of a monochromator
entrance slit
dispersing device
exit slit
2 types of gratings used
transmission (spectrum made as light passes through grating)
Reflection (spectrum made when light reflects off mirror surface with etched lines; more efficient; most common type)
Sample holder (Cuvet)
usually a set width
affects beers law
types of cuvets
borosilicate glass (visible range) Quartz or silica (UV range) Plastic (for visible and uv, but not good for repeated use)
photoelectric effect
some metals when the surface atoms are hit by radiant energy become excited and cause a current to flow
spectral response of a photodetector
the range of wavelengths over which it operates
Dark current
current that flows from a photodetector even though no light is hitting it
compensated for setting 0%T
Types of photodetectors
vacuum and gas photodiodes
photomultiplier tubes (PMTs)
Solid state photodiodes
Linear diode arrays
advantages to diode array spectrophotometers
speed
measurements for multiple wl at once
fewer mechanical parts
signal processor
amplifies and cleans electrical signal from photodetector and converts it into usable data relating to measurement
3 error signals in photometry measurements
drift
noise
artifact
Display
displays measurement data to operator
types of output displays
panel peter (analog) LED (digital) Video display terminals (VDT) Chart recorders Printers
Single beam photometer
single light path from light source
prone to variations of light intensity, drift in dark current
Double beam photometer
beam of light from source is split using beam splitters
2 types of beam splitters
dichroic mirror
photochopper
wavelength accuracy
check that the wl of light striking the sample is the one selected on the wl dial of the instrument
photometric linearity
check ensures that for a given change in concentration, there will be a proportional change in absorbance
linearity is affected by (5)
the detector samples response to beers law wide band pass stray light molecular fluorescence of the sample
Stray light
all radiant energy that does not pass through the sample but still reaches the photodetector
sources of stray light
light scatter by dust
smudges/scratches on optica surfaces
scattering and diffraction within monochromator
fluorescence of the solution
reflected light from the light source
room light leaking into the sample compartment
Stray light produces
a decrease in sensitivity and an apparent deviation from beers law by falsely decreasing the A
Photometric accuracy
check to verify that the A displayed by instrument is the true A of the sample
sensitivity vs slope
steeper slope = greater sensitivity
Instrument calibration
setting 0%T and 100%t on photometer
Calibrate and slope on pH meter
