SPECTROPHOTOMETER - Principle and working procedure
Ø A
spectrophotometer is an instrument that measures the amount of light absorbed
by a sample.
Ø It measures
the concentration of solutes in the solution by measuring the amount of light
that is absorbed by the solution in a cuvette placed in the spectrophotometer.
Ø The
photometer measures the amount of light that passes through the sample and
delivers a voltage signal to the display.
Principle
Ø Beer-Lambert Law states that there is a linear
relationship between the absorbance and the concentration of a sample.
Ø A=ϵlc
Where,
o
A is the measure of absorbance (no units),
o
ϵ is the molar extinction coefficient or molar absorptivity
(or absorption coefficient),
o
l is the path length, and
o
c is the concentration.
Basic parts or instrumentation
Ø The basic
spectrophotometer instrument consists of
o
a light source - Tungsten Halogen Lamp, Hydrogen / Deuterium
Lamps (uv spectrum), Xenon flash lamps – uv and visible spectrum
o
a digital display,
o
a monochromator (Prism) – isolate different wavelength, glass
prism (visible), the quartz prism can cover the ultraviolet spectrum.
o
lens or slit - Accepts polychromatic input light from a lamp and
outputs monochromatic light
o
a wavelength sector to transmit selected wavelength (slit),
o
a collimator for straight light beam transmission (lens),
o
photoelectric detector and
o
a cuvette to place a sample.
Working
procedure
Ø First a collimator (lens) transmits a
straight beam of light (photons) that passes through a monochromator (prism) to
split it into several component wavelengths (spectrum).
Ø Then a wavelength selector (slit)
transmits only the desired wavelengths
Ø After the desired range of wavelength of
light passes through the solution of a sample in cuvette.
Ø The photometer detects the amount of photons
that is absorbed and then sends a signal to a galvanometer or a digital
display.
Ø
A
spectrophotometer is made up of two instruments: a spectrometer and a
photometer.
Ø
The
spectrometer is to produce light of any wavelength, while the photometer is to
measure the intensity of light.
Ø
The
spectrophotometer is designed in a way that the liquid or a sample is placed
between spectrometer and photometer.
Types
1. Single
beamed spectrometer –
o
only one
sample can be placed at a time
o
first
invented, cheaper
o
time taking
procedure
2. Double
beamed spectrometer –
Two samples can be analysed simultaneously
Chopper - alternately transmits
and reflects the light beam so that it travels down the blank and the sample
optical paths to a single detector.
Applications
- Concentration
measurement
- Detection of impurities
- Molecular weight determination
Comments
Post a Comment