Food Analysis: Types of Spectroscopy

Types of Atomic Spectroscopy…

2
Types of Atomic Spectroscopy

3
4
Principle of Flame Emission Spectroscopy
• Flame emission spectroscopy employs a nebulizer-burner system to atomize and
excite the sample
• The instrument may be either
• A spectrophotometer (which uses a monochromator to isolate the desired emission
line)
• A photometer (which uses a filter to isolate emission lines)
• Flame emission is most useful for elements with relatively low excitation energies
including;
• Sodium
• Potassium
• Calcium

5
Basic Components of AES

6
 Excitation sources used in AES

1. Plasma
• Term plasma, as used in conjunction with light emission instruments, refers to hot gas in which
a significant percentage of atoms have been ionized
• In plasma source usually use inert Ar gas
• Temperature of Ar plasma source range from 5000-8000 K
• High temperature of plasma allows complete sample vaporization causing formation of free
atoms and ions in the plasma
Types of plasma for (AES)
• Inductively coupled plasma (ICP)
• Direct current plasma (DCP)
• Microwave induced plasma (MIP)

7
 Excitation sources used in AES…

2. Arc

• The arc source relies on the passage of electricity across a gap between two
electrodes to cause excitation of the sample

• Graphite electrodes are used

• Although AC or DC electric current can be used but the AC supply gives more
precision

8
 Excitation sources used in AES…
3. Spark
• Alternating current spark source have been used for analysis of sample
• Rotating disks pick up the sample solution and place it into the high voltage
discharge
• The spark discharge is cooled and the sample solution is not consumed to any
significant extent
• The spark source instruments exhibit better precision but less sensitivity than
arc source

9
 Excitation sources used in AES…
4. Flame
• Flame emission source has been used for determination of many elements in
plants tissue extracts and soil extracts
• Instruments are called flame photometers
• Flame photometry has the advantage of simplicity and reduced cost when
compared to other excitation source

Flame Oxidant Temperature

Gas Air ~1800 °C
Acetylene O2 ~3000 °C
H2 O2 ~2600 °C
10
 Flame Photometer

• Atoms in gaseous state in the flame absorb thermal energy from the flame itself

• Some of the atoms get excited & as they return back to the ground state they
emit radiation having energy equal to that absorbed.

• The emission is proportional to the number of excited atoms, which is

proportional to the total number of atoms in the flame i.e. the sample
concentration
11
 Flame Spectra

• The spectra of gaseous, atomic particles

• Consist of well defined narrow discrete lines arising from electronic transition of outermost
electrons
• Since there is no bonds, atoms undergo electronic transition only, no vibrational or rotational
transitions
• The energy to which the atoms are subjected must be less than the ionization potential

• The resonance wavelength

• (At which the most intense absorption and emission occur) is 671 nm for lithium, 589 nm for
sodium and 767 nm for potassium
12
The number of atoms of an element excited by the flame
depends on
1. Flame temperature

2. The energy difference between the excited and ground states

• The number of excited atoms in the flame is considerably small, even in the case of alkaline
metals which are easily excited
• Sodium at 2500 K, 0.017% of the atoms are excited
• Other metals the number of excited atoms is extremely small e.g. in case of zinc only 10-9 are
excited
• Any increase of the flame temperature is accompanied by great increase in the number of
excited atoms 13
 Instrument for Flame Emission
a. Flame atomizer
b. Monochromator
c. Detector
d. Readout meter (Photomultiplier tube)

14
15
 Flame Atomizer
The atomizer is composed of
1. Nebulizer 2. Burner
1. Nebulizer
• Device to form fine droplets of sample solution by aspiration into fine spray or
aerosol
• As the oxidant flows it withdraws the sample from the capillary in very fine
droplets
• Then mixed in the premixing chamber with the fuel gas
• The fuel-oxidant-sample aerosol mixture passes to the burner producing the
necessary heat for atomization and excitation

16
 2. Burner

• The combustion of fuel occurs producing the necessary heat for atomization
and excitation but not ionization
• The temperature of the flame produced depends on fuel-oxidant ratio and
kind 17
The flame temperature must be
• An increase by 100°C is accompanied by increase of 4% in the excited atoms
• It must be sufficient to cause atomization only and not ionization

Monochromator
• Either grating or interference filters which allow the resonance wavelength to pass to
the detector

18
Applications of AES in Food Industry

• Trace elements determination in food samples

• Al, B, Ca, Cu, Fe, K, Mg, Mn, Na, P, S, Sr, and Zn etc.

• Determination of calcium and iron in beer

• For determination of alkali and alkaline earth metals

20