1 ENVE 450 – Environmental Sampling & Analysis, Lecture Notes, İsmail ANIL, 2014

ATOMIC ABSORPTION SPECTROMETRY

1. INTRODUCTION Atomic absorption spectrometry (AAS) is an analytical technique that measures the

concentrations of elements. Atomic absorption is so sensitive that it can measure ppm (mg/L)

and down to ppb (µg/L). The technique makes use of the wavelengths of light specifically

absorbed by an element. They correspond to the energies needed to promote electrons from

one energy level to another, higher, energy level. Atomic absorption spectrometry has many

uses in different areas of environment.

2. HOW IT WORKS? Atoms of different elements absorb characteristic wavelengths of light. Analyzing a sample

to see if it contains a particular element means using light from that element. For example

with lead, a lamp containing lead emits light from excited lead atoms that produce the right

mix of wavelengths to be absorbed by any lead atoms from the sample. In AAS, the sample is

atomized – ie converted into ground state free atoms in the vapor state – and a beam of

electromagnetic radiation emitted from excited lead atoms is passed through the vaporized

sample. Some of the radiation is absorbed by the lead atoms in the sample. The greater the

number of atoms there is in the vapor, the more radiation is absorbed. The amount of light

absorbed is proportional to the number of lead atoms. A calibration curve is constructed by

running several samples of known lead concentration under the same conditions as the

unknown. The amount the standard absorbs is compared with the calibration curve and this

enables the calculation of the lead concentration in the unknown sample.

Samples containing particulate matters must be removed by using 0.45 µm filter in order

to prevent any clogging in the system. If elements in particulate matter is measured,

microwave acid digestion technique may be used prior to analysis.

Consequently an atomic absorption spectrometer needs the following three components:

a light source; a sample cell to produce gaseous atoms; and a means of measuring the specific

light absorbed.

Schematic diagram of an atomic absorption spectrometer

2 ENVE 450 – Environmental Sampling & Analysis, Lecture Notes, İsmail ANIL, 2014

2.1. Light Source (Hollow Cathode Lamp) The light source is usually a hollow cathode lamp (HCL) of the element being measured.

Its purpose is to provide the spectral line for the element of interest. The HCL uses a cathode

made of the element of interest with a low internal pressure of an inert gas. A low electrical

current (10 mA) is imposed in such a way that the metal is excited and emits a few spectral

lines characteristic of that element (For instance, Cu has 324.7 nm and a couple of other lines;

Se has 196.0 nm and other lines.) The light is emitted directionally through the lamp’s window,

which is made of a glass transparent in the UV and visible wavelengths.

A typical HC Lamp

2.2. Atomization of the sample Two systems are commonly used to produce atoms from the sample. Aspiration involves

sucking a solution of the sample into a flame (in FAAS); and electrothermal atomization is

where a drop of sample is placed into a graphite tube that is then heated electrically (in

GFAAS). Some instruments have both atomization systems but share one set of lamps. Once

the appropriate lamp has been selected, it is pointed towards one or other atomization

system.

2.3. Monochromator The role of a monochromator is to isolate photons of various wavelengths that pass

through the flame or furnace and remove scattered light of other wavelengths from the light

source. In doing this, only a narrow spectral line impinges on the photomultiplier tubes (PMT)

detector. The monochromator of atomic absorption uses UV–VIS radiation.

2.4. Detector The photomultiplier tubes (PMT) detector determines the intensity of photons in the

analytical line exiting the monochromator. Since the basis for both the flame and flameless

system is atomic absorption, the monochromator seeks to allow only the light not absorbed

by the analyte atoms in the flame or furnace to reach the PMT. That is, before an analyte is

atomized, a measured signal is generated by the PMT as light from the HCL passes through

the flame or furnace. When analyte atoms are present in the flame or furnace—while the

sample is atomized—some part of that light is absorbed by those atoms. This causes a

decrease in PMT signal that is proportional to the amount of analyte. In a double-beam

3 ENVE 450 – Environmental Sampling & Analysis, Lecture Notes, İsmail ANIL, 2014

system, the readout represents the ratio of the sample to reference beams. Therefore,

fluctuations in source intensity do not become fluctuations in instrument readout, and signal

stability is enhanced.

3. TYPES of AAS AAS consists of 3 main parts: Flame atomic absorption spectrometry (FAAS), Graphite

furnace atomic absorption spectrometry (GFAAS) and Hydride system atomic absorption

spectrometry (HSAAS).

3.1. Flame Atomic Absorption Spectrometry (FAAS) The typical concentration range of the FAAS is mg/L but for some elements it can detect

also µg/L concentrations. The main parts of the AAS is same in the FAAS but the atomization

of elements is performed by a nebulizer and a flame burner. The liquid sample is aspirated

into a nebulizer (4-7 ml/min) and the formed aerosol/spray is introduced into flame, where

aerosol/spray desolvation, evaporation and analyte atomization occur.

The total analysis time for one element & one sample is about 15 seconds. Totally 1-2 ml

sample is required for single element analysis in FAAS.

Nebulizer & Burner system HC Lamp light & Flame atomization

Air - acetylene flame (T: 2000-2300°C): for readily/easily atomizable metals (alkali

metals, Zn, Cu, Cd, Pb, Mn, Fe, casually Cr). Air is the oxidant, Acetylene is the fuel, and

Air/Acetylene Burner is used.

Nitrous oxide - acetylene flame (T: 2800-3000 °C): for hardly atomizable elements (Sr,

Ba, V, Cr, Mo, Al, Si, Ca, B etc.); it is also applied for determination of other elements

(Mg, Ni, Fe) in difficult matrices (those containing phosphates, silicates…). Nitrous

oxide is the oxidant, Acetylene is the fuel, and Nitrous oxide/Acetylene Burner is used.

Atomization on

Flame

4 ENVE 450 – Environmental Sampling & Analysis, Lecture Notes, İsmail ANIL, 2014

3.2. Graphite Furnace/Electrothermal Atomic Absorption Spectrometry (GFAAS) The typical concentration range of the GFAAS is µg/L but for some elements it can detect

also ng/L concentrations. The main parts of the AAS is same in the FAAS but the atomization

of elements is performed by a graphite furnace (electrothermal) atomizer.

In a flameless graphite furnace system, instead of using an aspiration device, liquid

samples (a few microliter sample) can be deposited directly to a graphite boat using a syringe

inserted through a cavity. The graphite furnace can hold an atomized sample in the optical

path for several seconds, compared with only a fraction of a second in the flame system. This

results in a significantly higher sensitivity of the GFAAS as compared with FAAS.

Graphite furnace is usually a graphite tube (diameter 3-4 mm, length 20-25 mm) with a

sampling hole (injection port). The tube is electrically heated, cooled by water and rinsed by a

flow of inert gas (Ar or N2); a graphite platform can be inserted into the tube.

HC Lamp light & Graphite Tube atomization

The total analysis time for one element & one sample is about 2-3 minutes. Totally 10-50

µl sample is required for single element analysis in GFAAS.

The details will vary with the elements but a typical graphite furnace program;

I. Drying/Evaporation: 30–40 seconds at 150 °C to evaporate the solvent,

II. Ashing: 30 seconds at 600 °C to drive off any volatile organic material and char the

sample to ash,

III. Atomizing: with a very fast heating rate to 2500–3000 °C for 5–10 seconds to

vaporize and atomize elements (including the element being analyzed).

IV. Cleaning: heating the tube to a still higher temperature – 3000 °C – cleans it ready

for the next sample.

V. Cooling: Finally graphite tube is cooled to lab temperature with water cooler at 20

°C.

During this heating cycle the graphite tube is flushed with argon gas to prevent the tube

burning away.

Atomization in a

graphite tube by

electric current

5 ENVE 450 – Environmental Sampling & Analysis, Lecture Notes, İsmail ANIL, 2014

3.3. Hydride Generation Atomic Absorption Spectrometry (HGAAS) The two atomic absorption spectrometric methods using flame and graphite furnace can

measure most elements with satisfactory detection limits. There are notable exceptions to

these techniques, including several elements (mercury, selenium, and arsenic) that are of

environmental importance.

Mercury (Hg) is a volatile metal; it does not need to be heated in a flame or furnace during

spectroscopic analysis. It can be analyzed by a ‘‘cold vapor’’ atomic absorption (CVAA)

technique.

Arsenic (As) and selenium (Se) are not enough volatile at room temperature to be analyzed

by the ‘‘cold vapor’’ techniques, but they are too volatile to be analyzed by FAA or GFAA. The

hydride generation atomic absorption (HGAA) technique, as a sample preparation method, is

applicable in atomic absorption spectroscopy for the analysis of As, Se, and several other

elements (Bi, Ge, Pb, Sb, Sn, Te).

The gaseous hydride of the analyte (e.g. AsH3 or H2Se) is formed from the corresponding

compound of the analyte (H3AsO3 or H2SeO3) by reducing reaction with NaBH4 (“sodium

borohydride”); the reaction takes place in a hydride generator, gaseous products are delivered

to the atomizer (usually a heated quartz tube)

The technique is by 2-3 orders of magnitude more sensitive than FAAS

As a consequence of analyte transfer into gaseous phase most of interferences are

eliminated (namely those manifested on analytical lines of As and Se that are located in the

region of wavelength 200 nm).

Atomizer for HGAAS is most often an externally heated quartz T-shaped tube positioned

above the air-acetylene burner (heated by flame) or in a heating block (electrically heated to

approx. 900 °C); trace amount of oxygen is necessary for splitting of the hydride molecules

into atoms.

Cold vapor mercury analyzer A hydride generation and atomization system

6 ENVE 450 – Environmental Sampling & Analysis, Lecture Notes, İsmail ANIL, 2014

Typical detection limits (DLs) for FAAS, GFAAS, and ICP-OES (all in µg/L)