SPECTROPHOTOMETER UV/VIS

Wavelength

Wavelength and Color of the Light

Colors and Complementary Colors

Ultra Violet /VIS Spectrum

Lambert – Beer’s Law

• At this time, I/Io is called transmittance (T) , I/Io x 100 is percent transmittance (T%) and 1/T = log (Io/I) is called absorbance (Abs).

T = I/Io = 10.k l cAbs = log (1/T) = log (Io/I) = k l c

log T = - ε l c or – log T = ε l c

• log T is also defined as absorbance (A) or optical density (OD), thus log T = A = ε l c.

• ε is defined as the molar absorbtivity and the concentration is given in mol/liter (also called the molar extinction coefficient).

• If the concentration is given in grams/liter, ε is replaced by a, the specific absorbtivity.

Qualitative and Quantitative Analysis

To analyze what this substance is and what substances are mixed are called qualitative analysis, while to analyze the amount of these substances is called quantitative analysis.

Qualitative Analysis

Spectrum and Chemical Structure

Colorimetric Analysis (Quantitative Analysis)

The methods for performing quantitative analysis by comparing color darkness of substance is called colorimetric analysis. When the substance is transparent, if absorbance exists in the visible and ultraviolet, it is measured.

The quantitative method to measure concentration of the sample with unknown concentration from absorption of the sample with known concentration is provided in two methods :

1. calibration curve method

2. standard additive method

Calibration Curve

The main steps to analyze with spectrophotometer uv/vis

1. Formation of molekuls which can absorp uv/vis light

2. Selection of wave length, λmax and Absmax

3. Preparation of calibration curve4. Measurement of samples absorbance (A)

Spectrophotometer types

Spectrophotometer can be categorized by optical system as follows :

Measurement optical System

Single beam configuration

Double beam configuration

Single Beam Configuration

Double Beam Configuration

ATOMIC ABSORPTION SPECTROPHOTOMETER

Atomic Emission

EXCITATION

DECAY

Energy +

Ground state atom

Excited state atom

+Ground state

atomLight EnergyExcited state

atom

l

• Atomic emission can be used when no lamp is available

• Flame emission has better detection limits than atomic absorption for several elements including Al, Ba, Li, K, Na, and V

Atomic Absorption

+Ground state

atomExcited state

atomLight Energy

l

Atomic Emission vs Atomic Absorption

• There are some basic differences between atomic emission and atomic absorption.

• In atomic emission, the processes which involved are excitation and decay.

• In atomic absorption, the “ground state” atom absorbs light energy of a specific wavelength as it enters the “excited state”.

Atomic Absorption Instrumentation

There are five basic components of an atomic absorption instrument :1. The light source which emits the spectrum of

the element 2. An “absorption cell” in which atoms of the

sample are produced (flame, graphite furnace)

3. A monochromator for light dispersion4. A detector which measures the light intensity

and amplifies the signal5. A display that shows the reading after it has

been processed by the instrument electronics

THE USE OF FLAME IN AAS

In atomic emission : It converts the sample aerosol into an atomic vapor and then thermally elevates the atoms to an excited state. When these atoms return to ground state, they emit light which is detected by the instrument. The intensity of light emitted is related to the concentration of the element of interest in solution.

In atomic absorption :The only function of the flame is to convert the sample aerosol into atomic vapor which can then absorb light from the primary light source (hollow cathode lamp or discharge lamp).

FLAMES

• There are oxidant and fuel gas combination used in atomic absorption is air – acetylene.

• Other flames that can be used are air – hydrogen, nitrous oxide – acetylene and argon – hydrogen – entrained air.

AIR - ACETYLENE

• Can be used for determination of approximately 35 elements

• The temperature is about 2300 oC• The acetylene flow is about 4 liters/minute• The air flow is about 24 liters/minute• For many elements, the fuel/oxidant gas

ratio must be adjusted for maximum sensitivity.