Question

Analytical Chem Tutorial: Atomizers

Answer 1

\({\bf{Flame~Atomizers:}}\)

> works by nebulizing the sample with a gaseous oxidant, mixing it with fuel, and moving them into a flame (typically by laminar flow)
> pressure and flow rate must be strictly monitored with double-diaphragm pressure regulators
> rotameter: measures flow rates
> basic steps of atomization:
1. desolvation: solvent evaporates into molecular aerosol
2. volatilization: formation of gaseous molecules
3. dissociation: formation of gaseous atoms

> basic flame structure: innermost region is the primary combustion zone (blue if hydrocarbon flame); outermost is secondary combusion zone, and interzonal region (contains the most free atoms, and where most spectroscopy is done)


> limitations: a lot of the sample is wasted through the draining system, and the residence time is brief

Answer 2

\({\bf{Electrothermal~Atomizers:}}\)

> evaporate sample at low temp
> ash in graphite tube (can cover the tube w/ pyrolytic carbon to decrease permeability)
> greatly increase current and measure the resulting absorption/fluorescence
> can work on solids; typical methods are to grind the solid into a fine powder or form a slurry
> limitations: slow, narrow analytical range

Answer 3

\({\bf{Other~Atomizers:}}\)

1. glow-discharge atomizer
> fires argon gas around the sample
> ionizes the gas, where the sample itself acts as the cathode
> sample itself must be conductor or ground into a powdered conductor such as graphite/copper


2. hydride atomizer
> gas and NaBH4 are combined w/ sample and moved into a heated, quartz tube



3. cold-vapor atomizer:
> mercury only
> mercury is converted into its cationic form and treated w/ nitric + sulfuric acids, then reduced SnCl2

Answer 4

\({\bf{Instrumentation:}}\)
> problems with atomic spectroscopy - nonlinar calibration curves, poor sensitivity, narrow atomic absorption line
- possible solutions: using line sources with very narrow bandwiths compared to absorption line width. doppler broadening.
> two main types of sources: hollow cathode lamps and electrodeless discharge lamps

1. hollow cathode lamps:
- tungsten anode and cylindrical cathode, enclosed in glass
- tube filled w/ inert gas
- gas is ionized by applying voltage
- atoms from cathode are ejected (sputtering)

2. electrodeless discharge lamp:
- sealed quarts tube w/ inert gas
- excited by radio/microwave radiation

\({\bf{Source~Modulation:}}\)
- emission from the flame can interfere w/ readings
- solution: modulate the source output (alter the signal back and forth at constant frequency)
- can insert chopper between source and flame, and then rotate the disc at regular frequency

Answer 5

Source material is section 9A-9B of Principals of Instrumental Analysis, 6th edition by Skoog, Douglas A., Holler, James F., Crouch, Stanley R.