Atomska Spektroskopija-1

8/12/2019 Atomska Spektroskopija-1

1/35

Atomic SpectroscopyEnergy Level Diagrams

Sample IntroductionSources for Atomic Absorption

Hollow Cathode LampsSources for Atomic Emission

FlamesPlasmasFurnaces

Wavelength SeparatorsComparison of Techniques

FAAS vs. ETAAS vs. ICP-AES

I would encourage you to read the following on reserve in Milne.Inductively Coupled Plasma and Its Applications 1-28, 71-92An Introduction to Analytical Atomic Spectrometry 1-47, 115-125


2/35


3/35


4/35


5/35

Sample

Introduction(common)


6/35


7/35

Sample Introduction

Venturi Effect and AtomizationPneumatic Nebulizers (for ICP

techniques)Break the sample solution into smalldroplets.Solvent evaporates from many of thedroplets.Most (>99%) are collected as wasteThe small fraction that reach theplasma have been de-solvated to agreat extent.


8/35


9/35


10/35

Flame AAS/AES Spray Chamber/Burner Configurations

Samples are nebulized (broken into small droplets)as they enter the spray chamber via a wire capillary

Only about 5% reach the flame

Larger droplets are collected

Some of the solvent evaporates

Flow spoilers

Cheaper, somewhat more rugged

Impact beads

Generally greater sensitivity


11/35


12/35

In FAAS, a key consideration is the height above the burnerthat the analyte absorption is measured at (burner positionsare adjustable)!Temperature value (adjusted using different fuel/oxidant

ratios) and consistency are important .


13/35


14/35


15/35

ElectroThermal AAS (ETAAS, GFAAS)The sample is contained in a heated, graphitefurnace.

The furnace is heated by passing an electricalcurrent through it (thus, it is electro thermal). Toprevent oxidation of the furnace, it is sheathed ingas (Ar usually)There is no nebulziation, etc. The sample isintroduced as a drop (usually 10-50 uL)The furnace goes through several steps

Drying (usually just above 110 deg. C.)Ashing (up to 1000 deg. C)Atomization (Up to 2000-3000 deg. C)

Cleanout (quick ramp up to 3500 deg. C orso). Waste is blown out with a blast of Ar.The light from the source (HCL) passes through thefurnace and absorption during the atomizationstep is recorded over several seconds. This makesETAAS more sensitive than FAAS for mostelements.


16/35


17/35

Sources (for FAAS and ETAAS)

Hollow Cathode Lamps (HCL) are the main source.These are element specific, constructed of thesame element you are analyzing.A current is passed through the lamp, exciting theelement of interest. As it returns to the ground state,it emits light which is focused through the sample.Since emission/absorption is quantized, this is thesame wavelength of light that the analytes willabsorb!Multielement lamps are available.Limited lifespan, treat carefully, do not exceedspecified maximum current!


18/35


19/35

FAAS and ETAAS Considerations Temperature level and consistency are key.Alignment of the source light is important.Since temperatures are relatively low, refractoryspecies and excessive amounts of complexescan form in the flame

Hinder ATOMIC absorption

Analytes may also be lost by volatilization prior tothe absorption of light.Matrix modifiers may overcome these twobarriers.

Reduce oxide and oxyhydroxide formationReduce sample loss from volatilizationComplex with interfering species (molecular)Ammonium chloride, palladium nitrate, magnesiumnitrate, for example


20/35

ICP-AES (ICP-OES)Inductively coupled plasmas are at least

2X as hot as flames or furnaces.The Ar plasma is the result of the flow ofAr ions in a very strong, localized radiofield.6000-10000 K are common plasmatemperatures.Hot enough to excite most elements sothey emit light.Hot enough to prevent the formation of

most interferences, break down oxides(REEs) and eliminate most molecularspectral interferences.The way to do atomic emissionspectroscopy today.


21/35


22/35


23/35


24/35


25/35


26/35

Wavelength Separators for Atomic SpectroscopyMust be able to separate light that mightbe only a fraction of nm from the nextnearest wavelength of light

Atomic spectra are complex!Usually Czerny-Turner configuration or amodification of it.

Need to incorporate backgroundcorrectionAtomic spectra are complex withmany possible spectral interferences

Lamp intensities may fluctuateFlame composition may fluctuateOver time on the same sampleFrom sample to sample


27/35

Double Beam AAS Instruments account for instability in the source.Other techniques are added to account for scattering if light in thesample and the absorption of light by non-atomic species in thesample. In the example shown here, P/Po is alternatively recordedby the instrument to cancel out short-term lamp fluctuations.


28/35

Deuterium Background Correction: A D 2 lamp (continuum lamp)alternately passes light through the sample with the HCL. Analytesdont absorb much D 2 radiation since it is a continuum source. Thelight absorbed with the HCL light passes through the sample minus

the light absorbed when the D 2 radiation passes through is the signalthat is measured.


29/35

Wavelength Separators for ICP-AES Must have greater resolving power than those for

AAS methods.Plasmas are hotter, therefore the spectra are morecomplex.

ICP techniques usually cover a wider range ofwavelengths

190 900 nmDifferent detectors for different wavelengthsPMTs still #1 choice, but CCD arrays also common.

The light emitted by an ICP is also more intense!Slits must have greater adjustability.. Sequential (scanning) ICP-AES instruments arenow less common than simultaneous (like a diodearray UV-VIS) instruments.


30/35


31/35


32/35

A Modern Sequential ICP with aMonochromator


33/35

A Modern Simultaneous ICP Design (most instruments sold now are simultaneous)


34/35

An Earlier Polychromator Design used in ICP Instruments


35/35

Documents

Atomska Spektroskopija-1