Aas

ATOMIC ABSORPTION SPECTROPHOTOMETER

(AAS)

APPLICATIONS OF AAS

• AAS is used in a variety of fields ranging from environmental to clinical.

• It allows over 60 metallic elements to be determined, in concentrations ranging from ppm to ppb level.

Note: The first modern AA was built by Dr. Alan Walsh, an Australian chemist in 1955.

AAS Determination of Heavy Metals in the Environment

• Monitoring: air, fresh water, seawater, groundwater, soils, sediments, plant material.

• Controling: process and effluent streams, waste water discharge.

SCHEMATIC OF AN AAS SYSTEM

AAS TECHNIQUES

There are 3 types of atomizers, therefore:

Flame mg/L (ppm) Graphite Furnace μg/L (ppb) Hydride Generation μg/L (ppb)

FLAME ATOMIZATION

Flame atomization is used to measure metals at concentration at ppm levels.

The two main types of flame are air/C2H2 and N2O/C2H2.

Air/C2H2 is use extensively as it enables 30 of the most common elements to be determined.

N2O/C2H2 is used to measure refractory elements as a higher flame temperature is needed.

FLAME SENSITIVITY LIMITS

The residence time of atoms:

Typically 10-5 – 10-4 seconds Less than the lifetime of many atoms

Spray Chamber

FLAME

SLOTTED TUBE ATOM TRAP

A way to improve the sensitivity and detection limit for flame system.

Slotted tube atom trap increases the resonance time of the population of atoms in the flame and therefore improves sensitivity and detection limits.

The sensitivity of lead is improved from 100 ppb to 30 ppb and the cadmium has improved from 32 ppb to 4 ppb.

GRAPHITE FURNACE Graphite furnace atomization is used if trace

elements are needed to be measured at concentration in ppb level.

This technique offers advantages in terms of sensitivity due to the increased residence time of the atoms in the light source (10-3 – 10-2

seconds). Graphite furnace are typically 1000 times more

sensitive than flames. A further advantages is that small sample

volumes are required (a few microlitres).

DETECTION LIMIT

Metal

Atomization

Lead Copper Arsenic

Flame 0.10mg/L 0.04 mg/L 0.4 mg/L

Graphite Furnace

0.07 μg/L 0.09 μg/L 0.26 μg/L

HYDRIDE GENERATION

Hydride generation (cold vapor) can be used to measure hydride forming elements such as mercury and arsenic group elements.

It has few interferences though the oxidation state of the element in question is important.

It can analyse the hydride forming elements at concentrations of low μg/L and ng/L (for mercury).

SAMPLE PREPARATION

SAMPLE PREPARATION TECHNIQUES

Wet ashing Fusion Dry ashing Pressure dissolution Microwave dissolution

WET ASHING

Acid digestion: HCl, HNO3, HClO4, HF, H2SO4, Aqua regia (mixture of 3 : 1 HCl and HNO3).

Acid grade: analar, ‘aristar’ or ‘suprapur’

Vessel used: glass/pyrex, teflon, platinum