HL Chemistry - Option A: Modern Analytical Chemistry

ATOMIC ABSORPTION SPECTROSCOPY

A.6.1 State the use of Atomic Absorption (AA) spectroscopy.

• Atomic absorption spectroscopy is a quantitative method of analysis that is applicable to many metals and a few nonmetals.

• A few examples include: Al in blood serum Ca in blood serum, plants, soil, water Cu in alloys Cr in sea water Fe in plants

• Only a drop of sample needed• The metals need not be removed from other

components (AA is a highly selective technique)• Sensitive in the ppm range (even ppb with the right

equipment)

A.6.2 Describe the principles of AA.

• When metals are exposed to heat, they absorb light.

• Each metal absorbs light at a characteristic frequency. For example:

Metal Zn Fe Cu Ca Naλ

(nm)214 248 325 423 589

A.6.2 Describe the principles of AA.

• The metal vapor absorbs energy from an external light source, and electrons jump from the ground to the excited states

• The ratio of the transmitted to incident light energy is directly proportional to the concentration of metal atoms present

• A calibration curve can thus be constructed [Concentration (ppm) vs. Absorbance]

A.6.3 Describe the use of each of the following components of the AA spectrometer: fuel, atomizer, monochromatic light source, monochromatic

detector, read out.

• A block diagram of the AA spectrometer appears below.

• The IB does not require the inclusion of the photomultiplier tube (PMT), but it none the less is an important part of the instrumentation.

Overview of AA spectrometer.

Light Source Detector

SampleCompartment

A.6.3 Describe the use of each of the following components of the AA spectrometer: fuel, atomizer, monochromatic light source,

monochromatic detector, read out.

• The source of light is a lamp whose cathode is composed of the element being measured.

• Each analyzed element requires a different lamp.• For example, a hollow cathode lamp for • Aluminum (Al) is shown below

A.6.3 Describe the use of each of the following components of the AA spectrometer: fuel, atomizer, monochromatic light source,

monochromatic detector, read out.

• The cathode lamps are stored in a compartment inside the AA spectrometer. The specific lamp needed for a given metal analysis is rotated into position for a specific experiment.

A.6.3 Describe the use of each of the following components of the AA spectrometer: fuel, atomizer, monochromatic light source,

monochromatic detector, read out.

• The sample is made up, typically in water• A flame is created, usually using ethyne &

oxygen (fuel)• The flame gases flowing into the burner create a

suction that pulls the liquid into the small tube from the sample container. This liquid is transferred to the flame where the sample is atomized [mixing the sample with air to create fine droplets]. The metal atoms then absorb light from the source (cathode lamp).

Sample is vaporized

in the flame.

Aspirator tube sucks thesample into the

flame in thesample

compartment.

Light beam

A.6.3 Describe the use of each of the following components of the AA spectrometer: fuel, atomizer, monochromatic light

source, monochromatic detector, read out.

• The light passes through a monochromater (a device used to select a particular wavelength of light for observation)

• The intensity of the light is fairly low, so a photomultiplier tube (PMT) is used to boost the signal intensity

• A detector (a special type of transducer) is used to generate voltage from the impingement of electrons generated by the photomultiplier tube

A.6.3 Describe the use of each of the following components of the AA spectrometer: fuel, atomizer, monochromatic light

source, monochromatic detector, read out.

A typical photomultiplier tube

A.6.3 Describe the use of each of the following components of the AA spectrometer: fuel, atomizer, monochromatic light source,

monochromatic detector, read out.

• The read out specified by the user is displayed on the computer screen for each sample measured.

A.6.3 Describe the use of each of the following components of the AA spectrometer: fuel, atomizer, monochromatic light

source, monochromatic detector, read out.

The resulting data can be presented in a variety of ways, but typically a print out is made.

A.6.4 Determine the concentration of a solution from a calibration curve.

• AA can be used to identify the presence of an element (qualitative analysis), or the concentration of a metal (quantitative analysis)

• Quantitative analysis can be achieved by measuring the absorbance of a series of solutions of known concentration.

• A calibration curve and the equation for the line can be used to determine an unknown concentration based on its absorbance.

A.6.4 Determine the concentration of a solution from a calibration curve.

Sample Problem: pg. 312, #3Lead is extracted from a sample of blood and analyzed at 283 nm and gave an absorbance of 0.340 in an AA spectrometer. Using the data provided, graph a calibration curve and find the concentration of lead ions in the blood sample.

[Pb+2] (ppm) Absorbance Calculated Pb (II) concentraions (ppm) Absorbance

0.000 0.000 0.357 0.3400.100 0.1160.200 0.2160.300 0.3100.400 0.4250.500 0.520

Lead (II) Calibration Curve

y = 1.0505x

R2 = 0.9988

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.000 0.100 0.200 0.300 0.400 0.500 0.600

[Pb+2] (ppm)

Ab

so

rba

nc

e

• The data provided in the problem appears in the upper left hand corner of this MS EXCEL worksheet.

• The graph was used to calculate the best fit line.

• The equation was then used to calculate the concentration of Pb (II) ions with an absorbance of 0.340.

• The result, 0.357 ppm, is displayed above the graph.