3. An analysis involves several
steps and operations which
depend on:
•the particular problem
• your expertise
• the apparatus or
equipment available.
The analyst should be
involved in every step.
STEPS INVOLVED IN CHEMICAL ANALYSIS
6. A compound has a molar absorptivity of 2.17x103
Lcm-1 mol-1 . What concentration of the compound
would be required to produce a solution that has a
transmittance of 8.42% in a 2.5 cm cell?
PROBLEM -1
PROBLEM -2
15. • A Stable radiant energy source
• A monochromator, to break the polychromatic radiation into component wavelength (or)
bands of wavelengths.
• Cuvettes to hold the sample
• A Photosensitive detector and an associated readout system
16. Radiant Energy Sources for UV-Visible range
Materials that can be excited to high energy states by a high-voltage electric
discharge (or) by electrical heating serve as excellent radiant energy sources.
• Sources of Ultraviolet radiation: The most commonly used sources of UV
radiation are the hydrogen lamp and the deuterium lamp. Xenon lamp may also
be used for UV radiation, but the radiation produced is not as stable as the
hydrogen lamp.
• Sources of Visible radiation: “Tungsten filament” lamp is the most commonly
used source for visible radiation. It is inexpensive and emits continuous radiation
in the range between 350 and 2500nm. “Carbon arc” which provides more
intense visible radiation is used in a few commercially available instruments.
22. FILTERS
absorption filters, cut-off filters, interference filters and interference wedges.
Absorption filters absorb most
polychromatic radiation and
transmit only a specific band of
wavelengths. They are inexpensive
and can be as simple as colored
glasses or plastics. Only about 10-
20% of the incident radiation is
transmitted through an absorption
filter.
23. INTERFERENCE FILTERS
Interference filters are sometimes called Fabry-
Perot filters and are dependent upon the concept of
wave interference. These filters will reflect some
wavelengths of radiation while transmitting others.
An interference filter is composed of a transparent
dielectric sandwiched between two
semitransparent metallic films and then two glass
plates to protect the filter. The thickness of the
dielectric and the reflectivity of the metallic films
are carefully selected because these factors control
the transmitted wavelengths. The transmitted
radiation will have a very narrow bandwidth.
24. MONOCHROMATOR
Monochromators: A monochromator resolves polychromatic radiation into its
individual wavelengths and isolates these wavelengths into very narrow bands. The
essential components of a monochromator are.
• Entrance slit -admits polychromatic light from the source
• Collimating mirror - Collimates the polychromatic light onto the dispersion
device.
• Wavelength resolving device like a PRISM (or) a GRATING
• A focusing lens (or) a mirror
• An exit slit–allows the monochromatic beam to escape.
25. The kinds of resolving element are of primary importance
• PRISMS
• GRATINGS
PRISMS:
• A prism disperses polychromatic light from the source into its constituent
wavelengths by virtue of its ability to reflect different wavelengths to a different
extent;
• The degree of dispersion by the prism depends on upon
The optical angle of the Prism (usually 600)
The material of which it is made
• Two types of Prisms are usually employed in commercial instruments.
Namely, 600 cornu quartz prism and 300 Littrow Prism.
26. When considering a grating monochromator, think of the entrance slit as a
polychromatic radiation source. A collimating mirror reflects a parallel beam of
polychromatic radiation toward a grating. The grating will reflect and disperse
component wavelengths to a mirror that will focus a narrow band of wavelengths
on an exit slit.
Different wavelengths can be brought into focus at the exit slit by rotating the
grating appropriately. The slits of a monochromator are an important
consideration when determining the performance of a particular instrument. In a
standard monochromator design, the entrance and exit slits have equal width. The
slit widths should be kept as narrow as possible while allowing enough radiant
power to reach the detector. If the slits are too wide, multiple wavelengths will
pass through and cause poor resolution. If the slits are too narrow, the radiant
power that can reach the detector will be decreased and difficult to
detect. Therefore, the slit width is a compromise between resolution and
detectability. Although the width of the slit should be carefully adjusted, some
instruments have a fixed slit width optimized for general purpose applications.
GRATINGS:
Gratings are often used in the monochromators of spectrophotometers
operating ultraviolet, visible and infrared regions.
30. PHOTOTUBES(PHOTO ELECTRIC CELL)
• An ordinary phototube contains
only two electrodes:
photosensitive cathode
an anode.
• During operation, some voltage
(e.g. 15 V or 50 V) is applied to the
electrodes (the positive pole to
the anode),so that photoelectrons
are rapidly swept from the
cathode to the anode, and
a photocurrent can be measured.
31. where η is the quantum efficiency, e is the electron charge,
and hν the photon energy. (The quantity S is
called responsivity.)
For an evacuated phototube (vacuum phototube), the
photocurrent depends on the incident optical power according
to the equation
32. PHOTO MULTIPLIER TUBES
A photomultiplier contains various parts in a vacuum glass tube, in which the
following happens:
• Light is absorbed on a photocathode and generates free photoelectrons
(external photoelectric effect).
• The electrons are subsequently accelerated with a high voltage (hundreds of
volts) to a first dynode (an electrode), where they generate several secondary
electrons. Those are accelerated towards further dynodes, where the number of
electrons is getting several times larger each time.
• Finally, a strongly amplified photocurrent is collected with an anode near the last
dynode.
35. The centre of control is the microcomputer(CPU), which performs all controls of light sources,
switching of light sources, filter switching, wavelength scanning pulse motor, LCD motor display,
keyboard and printer, USB interface etc. After the sample side beam are picked up by detectors
and converted into the voltage by the pre-amplifier, the signal is then fed into an A/D converter
and finally read by the CPU. On energy-measurement mode (of spectrum mode) only the signal
from the sample side beam is read in this case, S/R switching status is “Normal”. If the status
“Reverse”, only the signal from the reference side beam is read.
ELECTRICAL DIAGRAM OF DOUBLE BEAM UV
VISIBLE SPECTROSCOPY
36. TECHNICAL SPECIFICATIONS
Wavelength range 190 to 1100 nm
Spectral
bandwidth
1nm (190 to 1100 nm)
Wavelength
display
0.1-nm increments
Wavelength setting 0.1-nm increments (1-nm increments when setting scanning
range )
Wavelength
accuracy
±0.1 nm at 656.1nm D2
±0.3 nm (190 to 1100 nm)
Wavelength
repeatability
±0.1 nm
Stray light less than 0.02% NaI at 220 nm, NaNO2 at 340 nm
less than 1.0% KCｌ at 198 nm
Photometric
system
Double Beam
Photometric range Absorbance: -4 to 4 Abs
Transmittance: 0% to 400%
Photometric
accuracy
±0.002 Abs (0.5 Abs)
±0.004 Abs (1.0 Abs)
±0.006 Abs (2.0 Abs)
Photometric
repeatability
less than ±0.001 Abs (0.5 Abs)
less than ±0.001 Abs (1 Abs)
