Nebulization is conversion of a sample to a fine mist of finely divided droplets using a jet of compressed gas.  The flow carries the sample into the atomization region.

PNEUMATIC NEBULIZERS (most common)

Four types of pneumatic nebulizers:

·         Concentric tube - the liquid sample is sucked through a capillary tube by a high pressure jet of gas flowing around the tip of the capillary (Bennoulli effect). This is also referred to aspiration. The high velocity  breaks the sample into a mist and carries it to the atomization region.

·         Cross-flow - the jet stream flows at right angles to the capillary tip. The sample is sometimes pumped through the capillary.

·         Fritted disk - the sample is pumped onto a fritled disk through which the gas jet is flowing. Gives a finer aerosol than the others.

·         Babington - jet is pumped through a small orifice in a sphere on which a thin film of sample flows. This type is less prone to clogging and used for high salt content samples.

ULTRASONIC NEBULIZER

The sample is pumped onto the surface of a vibrating piezoelectric crystal. The resulting mist is denser and more homogeneous than pneumatic nebulizers.

ELECTRO-THERMAL VAPORIZERS (ETV)

An electro thermal vaporizer contains an evaporator in a closed chamber through which an inert gas carries the vaporized sample into the atomizer.

FLAME ATOMIZATION

The sample is aspirated through a nebulizer in which the jet of gas is a mixture of oxidant and fuel. The flame acts as the atomization region.

Within the flame is a complex set of processes.

The nebulizated mist is desolvated (the solvent is evaporated) leaving a finely divided solid molecular aerosol. The solid is then volatized which produces an atomic gas. These gaseous molecules can be excited themselves or dissociated into atoms or atomic ions which in turn can be excited.

With the multitude of processing occurring in the flame, the atomization step is very critical to the success of the analysis. The type of flame is thus critical to process.

Types of Flames

Types of Flames (Fuel/Oxidant)

Temperature oC

Velocity (cm/sec)

methane/air

1700 - 1900

39 - 43

methane/oxygen

2700 - 2800

370 - 390

hydrogen/air

2000 - 2100

300 - 440

acetylene/air

2100 - 2400

158 - 266

acetylene/oxygen

3050 - 3150

1100 - 2480

acetylene/nitrous oxide

2600 - 2800

285

At temperatures of 1700o - 2400oC, only easily decomposed samples can be atomized. More refractory samples require higher temperatures.

Burning velocities are also important since flames are stable within only certain gas flame rates.   The gas flow rate must exceed the burning velocity otherwise a flashback occurs.   As the gas flow rate increases, the flame rises above the burner to a point where the gas velocity is equal to the burn velocity.   This is the region where the flame is the most stable.   At higher flow rates, the flame moves to high and is blown off the burner.   The flow rate of the fuel/oxidant is very important.

Flame Atomizers

A typical atomizer is a concentric tube nebulizer with a laminar flow burner.

The aerosol, which uses the oxidizer flow, is mixed with the fuel and passes a series of baffles which remove the larger mist and only passes the finer solution droplets.  The removal of the coarse mist means that most of the sample is actually drained from the mixing chamber.

The aerosol / oxidant / fuel mixture are then combusted in a slotted burner that provides a flame from 5 to 10 cm in length.

These laminar flow burners provide a relatively quiet reproducible flame with a long sample path length.

 

Burner/nebulizer used in Perkin-Elmer Spectrophotometer Model 460

 

A – Burner

B – Nebulizer

C – Rotational Adjust Knob

D – Horizontal Adjust Knob

E – Vertical Adjust Knob