Elemental Analysis - Atomic Spectroscopy A) Introduction

Based on the breakdown of a sample into atoms, followed by the measurement of the atom’s absorption or emission of light.

i. deals with absorbance fluorescence or emission (luminescence) of atoms or elemental ions rather then molecules

- atomization: process of converting sample to gaseous atoms or elementary ions

ii. Provides information on elemental composition of sample or compound

- UV/Vis, IR, Raman gives molecular functional group information, but no elemental information.

iii. Basic process the same as in UV/Vis, fluorescence etc. for molecules

Eo

E1h

Absorbance Fluorescence

iv. Differences for Molecular Spectroscopy

- no vibration levels much sharper absorbance, fluorescence, emission bands

- position of bands are well-defined and characteristic of a given element - qualitative analysis is easy in atomic spectroscopy (not easy in molecular spectroscopy)

Examples:carbon

oxygen

nitrogen

B) Energy Level Diagramsenergy level diagram for the outer electrons of an element describes atomic spectroscopy process.

i. every element has a unique set of atomic orbitalsii. p, d, f split by spin-orbit couplingiii. Spin (s) and orbital (l) motion create magnetic fields that perturb each other

(couple)- parallel higher energy; antiparallel lower energy

Na Mg+

• Similar pattern between atoms but different spacing

• Spectrum of ion different to atom

• Separations measured in electronvolts (eV) 1eV = 1.602x10-19 J

= 96.484 kJ mol-1

• As number of electrons increases, number of levels increases emission spectra more complex Li 30 lines

Cs 645 linesCr 2277 lines

Note slight differences in energy due to magnetic fields caused by spin

C) Desire narrow lines for accurate identification

Broadened by

i. uncertainty principle

Uncertainty principal:

t . E ≥h

t . ≥ 1

t – minimum time for measurement – minimal detectable frequency difference

Peak line-width is defined as width in wavelength at half the signal intensity

Doppler effect

- emitted or absorbed wavelength changes as a result of atom movement relative to detector

- wavelength decrease if motion toward receiver

- wavelength increases if motion away from receiver

Usage in measurement of velocity of galaxies, age of universe and big bang theory

C) Desire narrow lines for accurate identificationBroadened by

ii. Doppler effect

Pressure broadening:

Collisions with atoms/molecules transfers small quantities of vibrational energy (heat) - ill-defined ground state energy

Effect worse at high pressures:

• For high pressure Xe lamps (>10,000 torr) turns lines into continua!

C) Desire narrow lines for accurate identificationBroadened by

iii. Pressure broadening

- temperature changes number of atoms in ground and excited states

- need good temperature control

Boltzmann equation

N1 and No – are the number of atoms in excited and ground states k – Boltzmann constant (1.28x10-23 J/K)

T – temperature E – energy difference between ground and excited statesP1 and Po – number of states having equal energy at each quantum level

Na atoms at 2500 K, only 0.02 % atoms in first excited state!

Less important in absorption measurements - 99.98 % atoms in ground state!

D) Effect of Temperature on Atomic Spectra

i. Need to break sample into atoms to observe atomic spectra

ii. Basic steps:

a) nebulization – solution sample, get into fine droplets by spraying thru thin nozzle or

passing over vibrating crystal.

b) desolvation - heat droplets to evaporate off solvent just leaving analyte and other

matrix compounds

c) volatilization – convert solid analyte/matrix particles into gas phase

d) dissociation – break-up molecules in gas phase into atoms.

e) ionization – cause the atoms to become charged

f) excitation – with light, heat, etc. for spectra measurement.

E) Sample Atomization – expose sample to flame or high-temperature

E) Sample Atomization – expose sample to flame or high-temperature

iii. Types of Nebulizers and Atomizers

F) Atomic Absorption Spectroscopy (AAS)

– commonly used for elemental analysis

– expose sample to flame or high-temperature

– characteristics of flame impact use of atomic absorption spectroscopy

Flame AAS:

• simplest atomization of gas/solution/solid

• laminar flow burner - stable "sheet" of flame

• flame atomization best for reproducibility (precision) (<1%)

• relatively insensitive - incomplete volatilization, short time in flame

i. Different mixes and flow rates give different temperature profile in flame

- gives different degrees of excitation of compounds in path of light source

ii. Types of Flame/Flame Structure – selection of correct flame region is important for optimal performance

a) primary combustion zone – blue inner cone (blue due to emission from C2, CH & other radicals)

- not in thermal equilibrium and not used b) interconal region

- region of highest temperature (rich in free atoms)- often used in spectroscopy- can be narrower in some flames (hydrocarbon) tall in others (acetylene)

c) outer cone- cooler region- rich in O2 (due to surrounding air)

- gives metal oxide formation

Not in thermal equilibrium and not used for spectroscopy

Primary region for spectroscopy

Flame profile: depends on type of fuel and oxidant and mixture ration

Temperature varies across flame – need to focus on correct part of flame

Consequences:- Sensitivity varies with element- must maximize burner position- makes multi-element detection difficult

Most sensitive part of flame for AAS varies with analyte

iii. Basic instrument design (Flame atomizer)

Single beam

Double beam

a) atomizer

1) Laminar Flow Burner

- adjust fuel/oxidant mixture for optimum excitation of desired

compounds

- usually 1:1 fuel/oxidant mix but some metals forming oxides use

increase fuel mix

- different mixes give different temperatures.

Laminar – non-turbulent streamline flow

i. sample, oxidant and fuel are mixedii. only finest solution droplets reach

burneriii. most of sample collects in wasteiv. provides quite flame and a long path

length

2) Electrothermal (L’vov or Graphite furnace)

- place sample drop on platform inside tube

- heat tube by applying current, resistance to current creates heat

- heat volatilizes sample, atomizers, etc. inside tube

- pass light through to measure absorbance

Place sample droplet on platform

Po

P

b) Laminar Flow Burner advantages:

- good b (5-10 cm)- good reproducibility

disadvantages:- not sample efficient (90-99% sample loss before flame)- small amount of time that sample is in light path (~10-4 s)- needs lots of sample

3) Comparison of atomizers

b) Light source - need light source with a narrow bandwidth for light output

- AA lines are remarkably narrow (0.002 to 0.005 nm)- separate light source and filter is used for each element

1) problem with using typical UV/Vis continuous light source - have right , but also lots of others (non-monochromatic light) - hard to see decrease in signal when atoms absorb in a small bandwidth - only small decrease in total signal area - with large amount of elements bad sensitivity

Hollow Cathode Lamp

2) Solution is to use light source that has line emission in range of interest - laser – but hard to match with element line of interest - hollow cathode lamp (HCL) is common choice

Coated with element to be analyzed

Process: use element to detect element

1. ionizes inert gas to high potential (300V)

Ar Ar+ + e-

2. Ar+ go to “-” cathode & hit surfaces

3. As Ar+ ions hit cathode, some of deposited element is excited and

dislodged into gas phase (sputtering)

4. excited element relaxes to ground state and emits characteristic radiation

- advantage: sharp lines specific for element of interest

- disadvantage: can be expensive, need to use different lamp for each element tested.

c) Source Modulation (spectral interference due to flame) - problem with working with flame in AA is that light from flame and light source both reach detector

- measure small signal from large background- need to subtract out flames to get only light source signal (P/Po)

i. done by chopping signal:

ii. or modulating P from lamp:

Flame only

Flame + P

P

time

Flame only

Flame + P

d) Corrections For Spectral Interferences Due to Matrix

- molecular species may be present in flame

- problem if absorbance spectra overlap since molecular spectrum is much broader with a greater net absorbance

- need way of subtracting these factors out

Methods for Correction

1) Two-line method

- monitor absorbance at two close together

one line from sample one from light source

second from impurity in HCL cathode, Ne or Ar gas in HCL, etc

- second must not be absorbed by analyte

absorbed by molecular species, since spectrum much broader

- A & are ~ constant if two close

- comparing A1, A2 allows correction for absorbance for molecular species

A1 (atom&molecule) – A2 (molecule) = A (atom)

Problem: Difficult to get useful second with desired characteristics

2) Continuous source method- alternatively place light from HCL or a continuous source D2 lamp thru flame

- HCL absorbance of atoms + molecules- D2 absorbance of molecules

advantage: -available in most instruments-easy to do

disadvantage: -difficult to perfectly match lamps (can give + or – errors)

3) Zeeman Effect- placing gaseous atoms in magnetic field causes non-random orientation of atoms- not apparent for molecules- splitting of electronic energy levels occurs (~ 0.01 nm)- sum of split absorbance lines original line

- only absorb light with same orientation

- can use Zeeman effect to remove background

place flame polarized light through

sample in magnetic field get absorbance (atom+molecule) or

absorbance (molecule) depending on how light is polarized

Background

Background+Absorbance

e) Chemical Interference - more common than spectral interference

1) Formation of Compounds of Low Volatility- Anions + Cations Salt Ca2+ +SO4

2- CaSO4 (s)- Decreases the amount of analyte atomized decreases the absorbance signal- Avoid by:

increase temperature of flame (increase atom production) add “releasing agents” – other items that bind to interfering ions

eg. For Ca2+ detection add Sr2+

Sr2+ + SO42- SrSO4 (s)

increases Ca atoms and Ca absorbance add “protecting agents” – bind to analyte but are volatile

eg. For Ca2+ detection add EDTA4-

Ca2+ + EDTA4- CaEDTA2- Ca atoms 2) Formation of Oxides/Hydroxides

M + O MO

M + 2OH M(OH)2

- M is analyte- Avoid by:

increase temperature of flame (increase atom production) use less oxidant

non-volatile & intense molecular absorbance

3) IonizationM M+ + e-

- M is analyte- Avoid by:

lower temperature add ionization suppressor – creates high concentration of e- suppresses M+ by shifting equilibrium.

G) Atomic Emission Spectroscopy (AES) – similar to AA with flame now being used for atomization and excitation of the

sample for light production

1) Atomic Processes

heat

Degree of Excitation Depends on Boltzmann Distribution:

N1 and No – are the number of atoms in excited and ground states

k – Boltzmann constant (1.28x10-23 J/K) T – temperature E – energy difference between ground and excited statesP1 and Po – number of states having equal energy at each quantum level

Increase Temperature increase in N1/No (more excited atoms)

I (emission) N1, so signal increases with increase in temperature

Need good temperature control to get reproducible signaleg. For Na, temperature difference of 10o 2500 2510

results in a 4% change in N1/No

Temperature Dependence Comparison between AA and AES:

- AA is relatively temperature independent. Need heat only to get atoms, not atoms in excited state.- AA looks at ~ 99.98% of atoms- AES uses only small fraction (0.02%) of excited atoms

2) Comparison of AA and AES Applications

Flame Emission More Sensitive

Sensitivity About the Same

Flame Absorption More Sensitive

Al, Ba, Ca, Eu, Ga, Ho, In, K, La, Li, Lu, Na, Nd, Pr,Rb, Re, Ru, Sm, Sr, Tb, Tl, Tm, W, Yb

Cr, Cu, Dy, Er, Gd, Ge, Mn, Mo, Nb, Pd, Rh, Sc, Ta, Ti, V, Y, Zr

Ag, As, Au, B, Be, Bi, Cd, Co, Fe, Hg, Ir, Mg, Ni, Pb, Pt, Sb, Se, Si, Sn, Te, Zn

Comparison of Detection Limit

Some better by AA others better by AES

AES - emission from multiple species simultaneously

3) Instrumentation- Similar to AA, but no need for external light source (HCL) or chopper

look at light from flame

flame acts as sample cell & light source

Source Temperature (oC)

Flame 1700-3150

Plasma 4,000-6,000

Arc/Spark 4,000-5,000/40,00

Atomization Sources:

Electrothermal usually not used – too slow and not as precise

a) Flame Source:- used mostly for alkali metals

easily excited even at low temperatures- Na, K- need internal standard (Cs usually) to correct for variations flame

Advantages- cheap

Disadvantage- not high enough temperature to extend to many other elements

b) Plasma (inductively coupled plasma - ICP)- plasma – electrically conducting gaseous mixture (cations & electrons)- temperature much higher than flame- possibility of doing multiple element analysis

40-50 elements in 5 minutes

Advantages- uniform response- multi-element analysis, rapid- precision & accuracy (0.3 – 3%)- few inter-element interferences- can use with gas, liquid or solids sample

Inductively Coupled Plasma (ICP) Emission Spectroscopy- involves use of high temperature plasma for sample atomization/excitation- higher fraction of atoms exist in the excited state, giving rise to an increase in emission signal and allowing more types of atoms to be detected

Magnetic fieldIons forced to flow in closed path, Resistance to flow causes heating

Ar charges by Tesla coil

(high voltages at high frequency)

Temperature Regions in Plasma Torch

Overall Design for ICP Emission Spectrometer

Rowland circle:

- curvature corresponds to focal curve of the concave grating.

- frequencies are separated by grating and focused onto slits/photomultiplier tubes positioned around the Rowland circle

- slits are configures to transmit lines for a specific element

Arc & Spark Emission Spectroscopy- involves use of electrical discharge to give high temperature environment- higher fraction of atoms exist in the excited state, giving rise to an increase in emission signal and allowing more types of atoms to be detected- can be used for solids, liquids or gas phase samples- types of discharge used:

DC arc: high sensitivity, poor precisionDC spark: intermediate sensitivity and precisionAC spark: low sensitivity, high precision

Because of difficulty in reproducing the arc/spark conditions, all elements of interest are measured simultaneously by use of appropriate detection scheme.

Arc created by electrodes separated by a few mm, with an applied current of 1-30 A

Concave grating disperse frequencies, photographic film records spectra

Comparison of ICP and Arc/Spark Emission Spectroscopy- Arc/Spark first instrument used widely for analysis- all capable of multielement detection with appropriate instrument design (e.g. 40-50 elements in 5 min for ICP- ICP tends to have better precision and stability than spark or arc methods - ICP have lower limits of detection than spark or arc methods- ICP instruments are more expensive than spark or arc instruments

Example 11: For Na atoms and Mg+ ions, compare the ratios of the number of particles in the 3p excited state to the number in the ground state in a natural gas-air flame (2100K)