Chapter 21 Atomic spectroscopy

Quantitative analytical chemistry chapter 21 atomic spectroscopy

atomic spectroscopy

the study of the electromagnetic radiation absorbed and emitted by atoms.

what happens in atomic spectroscopy

• Samples are vaporized and decompose into atoms and ions at high temperature.

• Some of the metal atoms may be raised to higher energy level and emit characteristic radiation. But large amount of metal atoms will remain in non emitting ground state and can absorb light.

• Concentrations are measured based on their absorbance or emission of light.

absorption spectrum of hydrogen in the visible region

emission spectrum of hydrogen in the visible region

electron transitions for hydrogen atom

flame atomic absorption spectroscopy

• A liquid sample is aspirated into the flame (2000-3 000 K).

• Liquid evaporates and the remaining solid is atomized in the flame. Atoms absorbs light of specific λs

• Determination of one element at a time. (one lamp one element)

hollow-cathode lamp

passes light through the sample in the flame and the intensity of the transmitted radiation is measured by a photon transducer

0\.001

Spectra of molecules have bandwidths of \~10-100 nm, whereas spectra of atoms consist of sharp lines with widths of \~___ nm.

atomic emission spectroscopy

radiation from hot atoms whose electrons have been promoted to an excited state in the flame or plasma is emitted.

• No lamp is needed, but atomization at higher temperatures (5000 – 8000 K).

atomic fluorescence spectroscopy

atoms in the flame are irradiated by a laser to promote them to an excited state from which they can fluoresce to return to the ground state.

basic diagram for atomic absorbance spectroscopy

nebulization

formation of small droplets

aerosol

A fine suspension of liquid (or solid) particles in a gas

how flame atomic absorption works

• Flames use a premix burner, in which fuel, oxidant, and sample are mixed prior to introduction into the flame.

• Sample solution is drawn into the pneumatic nebulizer by rapid flow of oxidant (usually air) past the tip of the sample capillary.

• Liquid breaks into a fine mist as it leaves the capillary. • The spray is directed against a glass bead, upon which the droplets break into smaller particles.

• The nebulizer creates an aerosol from the liquid sample.

• The mist, oxidant, and fuel flow past baffles that promote further mixing and block large droplets of liquid.

• Excess liquid collects at the bottom of the spray chamber and flows out to a drain.

5%

Aerosol reaching the flame contains only about ____ of the initial sample.

rich flame

excess fuel. increases sensitivity because excess carbon can reduce the metal oxides and hydroxides.

lean flame

excess oxidant. Gives hotter flame

liquid

flame absorption spectroscopy can only use ____ samples.

sensitive

An electrically heated graphite furnace is more ______ than a flame and requires less sample.

how graphite furnace works

• 1-100 µL of sample is injected into the furnace through a hole at the center. The sample should go to the floor.

• Light from a hollow cathode lamp travels through the window at each end of the graphite tube.

• A temperature program is applied

Steps of Graphite Furnace experiment

drying, charring, atomization, cleaning

advantages of graphite furnace vs flame absorption.

1. 1-2mL sample volume is the minimum required
for a flame compared to 1µL for a furnace.
2. Flames hold the sample in the optical path for
\~1s as it rises thru the flame; however, a
furnace confines the sample in the light path for
several s.
3. Multiple aliquots of sample can be injected on to the furnace platform and evaporated, pre concentrating the sample in the furnace prior to analysis.
4. Flames require nebulization, which can dilute sample; there is no nebulization step using a furnace.

disadvantages of graphite furnace

• have a limited lifetime.

• Memory effect – interference from previous runs.

• Flame is less expensive

direct solid sampling

when a solid is analyzed without sample preparation

matrix

Everything in the sample other than the analyte

matrix modifier

a substance added to the sample to reduce the loss of the analyte during the charring by making the matrix more volatile or the analyte less volatile.

Inductively coupled plasma

• Twice as hot as the flame

• Stable

• Inert Ar environment

• Simultaneous multi-element analysis

• Costs more to purchase and operate than the flame

how temperature affects atomic spectroscopy

1. It determines the degree to which a sample breaks
down into atoms.
2. It determines the extent to which an atom is found
in its ground, excited, or ionized state (Boltzmann
distribution)

This affects the strength of the observed signal.

Boltzmann Distribution

describes the relative populations of different states at thermal equilibrium. If equilibrium exists, then N\*/N0 of any two states:

2600

At ____ K, 99.98% of the atoms are in their ground state.

drying step

graphite furnace heats up to remove any solvent from the sample (\~150)

charred step

Graphite furnace removes any interferences by heating up dried solvent

atomization

graphite furnace glows as it is \~3000 degrees. Atoms turn to gas phase so that they can be analyzed.

Cleaning

graphite furnace heats up cell even more to get rid of any sample in the furnace.

10

Varying the temperature by __ K hardly affects the ground-state population and would not noticeably affect the signal in atomic absorption

4

Atomic emission occurs from the excited state, therefore ___ % increase in the excited state would increase the emission intensity by …%

plasmas

what atomic emission typically uses.

10-3 to 10-2 nm

Monochromators generally cannot isolate lines narrower than _____ .

vapor

To produce narrow lines of the correct frequency, we use hollow-cathode lamps containing ____ of the same element being analyzed

substantially narrower

Beer’s law requires that the linewidth of the radiation source should be _______ than the linewidth of the absorbing sample.

Atomic absorption linewidths

have an intrinsic width of \~10-4 nm due to Heisenberg uncertainty, Doppler broadening and Pressure broadening

excited state

The higher the temperature, the higher the percentage of atoms in the ______ .

Heisenberg uncertainty principle

states that the shorter the lifetime of the excited state, the more uncertain its energy

Doppler broadening

• atomic motion occurs in every direction.

• An atom moving toward the radiation source “sees” a higher frequency light than one moving away.

• There is no shift for atoms moving perpendicular to the source.

• The result is a symmetric distribution of λs.

Pressure broadening

• is caused by collisions of the emitting or absorbing species with other atoms or ions in a hot environment.

• Collisions between atoms shorten the lifetime of the excited state.

• The collision frequency is proportional to the pressure.

• This is similar in magnitude to the effect of Doppler broadening and yields linewidths of 10-3 to 10-2 nm in atomic spectroscopy.

sputtering

When gaseous cations acquire enough kinetic energy to dislodge some of the metal atoms from the cathode surface producing an electron cloud.

• A portion of the _____ metal atoms are in their excited state and emit characteristic radiation as they return to their ground state.

• The metal atoms eventually diffuse back to the cathode, or deposit on the glass walls of the tube.

Background correction

• is necessary in atomic spectroscopy to distinguish the analyte signal from absorption, emission, and optical scattering of the sample matrix, the flame, plasma, or graphite furnace.

• It is critical for graphite furnaces due to residual smoke from charring.

beam chopping

• or electrical modulation of the hollow-cathode lamp (pulsing it on and off), can distinguish the signal of the flame from the atomic line at the same wavelength.

• The figure shows light from the lamp being periodically blocked by a rotating chopper.

• Signal reaching the detector while the beam is blocked must be from the flame emission.

the lamp and the flame

Signal reaching the detector when the beam is not blocked is from _____. It does not correct for scattering

analytical signal

The difference between the lamp and the flame is the ______.

Deuterium lamp background correction

broad emission from a D2 lamp is passed through the flame in alternation with that from the hollow cathode.

Light from the hollow cathode lamp is absorbed by the analyte as well as absorbed and scattered by the background.

Zeeman effect

• The splitting of a spectrum line into three components by the application of a magnetic field.

• Two are shifted and one is not.

• The unshifted one does not absorb the light because it does not have the correct polarization.

Deuterium lamp correcting

• The strong magnetic field is pulsed on and off.

• Sample and background are observed when the field is off.

• Only background is observed when the field is on.

• The difference between them is the corrected signal.

detection limit for furnaces

is typically two orders of magnitude lower than that observed with a flame because the sample is confined in the small volume of the furnace for a relatively long time.

X-Ray Fluorescence

is the emission of X-rays following the absorption of X-rays by a material. Elements are identified by their peak energies and quantified by the number of photons in each peak.