Lab1-Spectrophotometry.ppt

Introduction to
Spectrophotometry

Spectroscopy
 Is the study of the
interaction of light & matter
 Spectrophotometer –
instrument that uses
electromagnetic radiation
from UV, visible or IR to
analyze the absorption or
transmission of a sample
 We will use visible in our
lab

Properties of Light
 Electromagnetic radiation moves in
waves

Colors & Wavelengths
COLOR WAVELENGTH (λ in nm)
Ultraviolet < 380
Violet 380 – 435
Blue 436 – 480
Greenish-blue 481 – 490
Bluish-green 491 – 500
Green 501 – 560
Yellowish-green 561 – 580
Yellow 581 – 595
Orange 596 – 650
Red 651 – 780
Near Infrared > 780

What is Colorimetry?
 The solutions of many compounds have
characteristic colors.
 The intensity of such a color is
proportional to the concentration of the
compound.

What are Spectroscopy and
Spectrophotometry??
 Light can either be transmitted or absorbed by
dissolved substances
 Presence & concentration of dissolved substances is
analyzed by passing light through the sample
 Spectroscopes measure electromagnetic emission
 Spectrophotometers measure electromagnetic
absorption

Instruments of Measurement
 Two most common:
1. Visible Spectrophotometer
 Spect 20, Spect 88
 Uses Xe or W lamps as light sources
 Glass cuvettes hold the sample
2. Atomic-Absorption Spectrophotometer

Instruments of Measurement
 What do visible spectrophotometers
measure?
– Amount of light absorbed by the dissolved
substance
– Qualitative – color gives info about the
solution composition
– Quantitative – provides numerical data for
the concentration

Absorption of Light
 White light
– All colors
– Polychromatic light

Absorption of Light
 Monochromatic light
– Light of one color
Red light is
absorbed
by the green
solution
Monochromator-spreads out light into its
component wavelength

Success of spectrophotometry…
 Requires sample to absorb light differently to the
other chemicals in the solution
 How is the correct wavelength selected?
– The amount of light absorbed depends on the
energy difference between 2 electron energy
levels
– Optimum wavelength for spectrophotometric
analysis is selected by measuring the visible
spectrum of the substance
– This is done by plotting absorbance (A) versus
wavelength (λ)

Food Dyes
 Only 7 dyes are
approved by the
FDA for use in
foods, drugs &
cosmetics
 All artificial food
colors are mixtures
of these 7 dyes
 We will be using
FD&C Blue in this
lab

FD&C Blue 1
 A solution containing this dye is
blue in white light
 The colors absorbed by
solution are complementary to
the transmitted color
 Blue solution absorbs yellow,
orange, & red light
 So expect dye solution to peak
at 580 – 650 nm
 Optimum wavelength is
determined from wavelength of
max. absorption λmax = 630 nm
for Blue 1
 This is given for the blue
solution but you will have to
claculate this for the red

Wavelength of light absorbed:
 Is related to electronic structure of substance
 Intensity of light absorbed depends on the
concentration of solution
 More concentrated, the more intense color & the
greater intensity of light absorbed
 When light is absorbed, the radiant power (P) of light
beam decreases

Transmittance (T)
 This is the fraction
of incident light
(P/Po) that passes
through the sample
 T = P
Po
 Po = intensity of
“blank”
 Blank – is solution
identical to sample
but without solute

Definitions & Symbols
 Intensity (I)
 Transmittance (T)
– It’s also referred to as %T or T x 100
– T = P/Po
• Where Po is the intensity of the blank
• Can also use I = Intensity instead of Power
• T = I / Io

Graphical Relationship
 % transmission and % absorption are
not linearly related to concentration
 For a graph to be useful, a straight line
is needed
 ABSORBANCE = log(1/T) = -log(T)

The amount of light absorbed
depends upon:
 Concentration (c)
 Path length of
sample cell (b) thru
which light passes
 Defined by Beer’s
Law
P0 P
b
c

Beer’s Law
 The intensity of a ray of
monochromatic light
decreases exponentially
as the concentration of
the absorbing medium
increases
 More dissolved
substance = more
absorption and less
transmittance
 ε = molar absorptivity
coefficient and is
constant for a
substance
 %T = Tx100 = P/P0x100%
 A = - log T
 A = ε b c

Spectral Transmission Curve
 Optimum wavelength

Standardization Graph
- Standards (solutions of known
concentration) of the compound of
interest are made, treated, and their
absorbances (ABS) and concentration
values are used to create a
Standardization Graph.

Standardization Graph
0.000
0.050
0.100
0.150
0.200
0.250
0.300
0.350
0.400
0.450
0.500
0.550
0 20 40 60 80 100 120
Absorbance
% Concentration
Absorbance vs. Concentration (%) of CoCl2

Lab1-Spectrophotometry.ppt

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Lab1-Spectrophotometry.ppt