Chem. 133 – 4/7 Lecture

Announcements I

• Lab – Should be starting Set 2 Period 1– Set 2 Period 2 Lab Reports due Today

• Pass Out TH Cheng Award Letters + Return Lab Reports + Last Week’s Assignment

• Quiz scores somewhat low• Additional Problem

– Made error on board so 4/4 pts unless done all right (then +1 bonus)

– C = -logT/eb (not -eblogT) and sC = [(dC/dT)2sT

2]0.5

– sC = [(-1/ebTln10)2sT2]0.5 = sT/ebTln10

Announcements II• Exam 2

– Format similar to Exam 1– Will cover: Ch. 13 (starting at sect. 4), 14, 17,

19, and 20 (covering overview, theory, and atomization means; parts on interferences will be on Exam 3)

– Will Review Topics– Possible Help Session Wed. 12 – 1 PM in

Sequoia 446• HW Set 2.3 Solutions Posted

Announcements III• Today’s Lecture

– Chapter 20: Atomic Spectroscopy• Theory (Boltzmann Distribution)• Atomization (flame, graphite furnace, and ICP)

– Review for Exam 2– Chapter 20: Atomic Spectroscopy (parts not

on Exam 2 – if time)• Optical Parts of Instruments + Interferences

Atomic SpectroscopyTheory

• For emission measurements, a key is to populate higher energy levels

• In most cases, this occurs through the thermal methods also responsible for atomization

• Fraction of excited energy levels populated is given by Boltzmann Distribution

• More emission at higher temperatures and for longer wavelengths (smaller DE)

Na(g)o (3s)

4pE

kTEeg

g

N

N /

00

**

N = number atoms in ground (0) and excited (*) states

g = degeneracy (# equivalent states) = 3 in above example

k = Boltzmann constant

Atomic SpectroscopyTheory

• Example problem:Calcium absorbs light at 422 nm. Calculate

the ratio of Ca atoms in the excited state to the ground state at 3200 K (temperature in N2O fueled flame). g*/g0 = 3 (3 5p orbitals to 1 4s orbital).

Atomic SpectroscopyAtomization

• Flame Atomization– used for liquid samples– liquid pulled by action

of nebulizer– nebulizer produces

spray of sample liquid– droplets evaporate in

spray chamber leaving particles

– fuel added and ignited in flame

– atomization of remaining particles and spray droplets occurs in flame

– optical beam through region of best atomization sample in

fuel (HCCH)

oxidant (air or N2O)

burner head

spray chamber

nebulizer

light beam

nebulizerair

liquid

Atomic SpectroscopyAtomization

• Atomization in flames – Processes– nebulization of liquid: MgCl2(aq) →

MgCl2(spray droplet)

– evaporation of solvent: MgCl2(spray droplet) → MgCl2(s)

– Volatilization in flame: MgCl2(s) → MgCl2(g)

– Atomization (in hotter part of flame): MgCl2(g) → Mg(g) + Cl2(g)

Target species for absorption measurement

Atomic SpectroscopyAtomization

• Complications/Losses– Ideally, every atom entering nebulizer

ends up as gaseous atom– In practice, at best only a few % of

atoms become atoms in flame– The nebulization process is not that

efficient (much of water hits walls and goes out drain)

– Poor volatilization also occurs with less volatile salts (e.g. many phosphates)

Atomic SpectroscopyAtomization

• Complications/Losses (continued)– Poor atomization also can occur due to

secondary processes such as:• Formation of oxides + hydroxides (e.g. 2Mg (g) + O2

(g) → 2MgO (g))• Ionization (Na (g) + Cl (g) → Na+ (g) + Cl- (g))

– If the atomization is affected by other compounds in sample matrix (e.g. the presence of phosphates), this is called a matrix effect (discussed more later

Atomic SpectroscopyAtomization

• Electrothermal Atomization– Atomization occurs in a graphite furnace– Process is different in that a small sample is

placed in a graphite tube and atomization occurs rapidly but in a discontinuous manner

– Electrothermal atomization is more efficient; atoms spend more time in the beam path, and less sample is required resulting in much greater sensitivity• Concentration LODs are typically ~100 times lower

(e.g. 100 ppt for EA vs. 10 ppb for flame)• Mass LODs are even lower (100 pg/mL*0.01 mL = 1

pg for EA vs. 10 ng/mL*2 mL = 20 ng for flame)

Atomic SpectroscopyAtomization

• Electrothermal Atomization (Process)– Sample is placed

through hole onto L’vov platform

– Graphite tube is heated by resistive heating

– This occurs in steps (dry, char, atomize, clean)

Graphite Tube in Chamber (not shown)

L’vov Platform

Sample in

T

time

dry char

atomize

Clean + cool down

Ar in chamber flow stops and optical measurements made

Atomic SpectroscopyAtomization

• Inductively Coupled Plasma (ICP)– A plasma is induced by radio

frequency currents in surrounding coil

– Once a spark occurs in Ar gas, some electrons leave Ar producing Ar+ + e-

– The sample is introduced by nebulization in the Ar stream

– The accelerations of Ar+ and e- induce further production of ions and great heat production

– Much higher temperatures are created (6000 K to 10000 K vs. flames)

ICP Torch

Quartz tube

Argon + Sample

RF Coil

Atomic SpectroscopyAtomization

• Advantages of ICP Atomization– Greater atomization efficiency than in flame AA

(partly because better nebulizers are used than with flames due to higher total instrument cost and partly due to higher temperatures)

– Fewer matrix effects because atomization is more complete at higher temperatures

– High temperature atomization allows much greater emission flux + more ionization allowing coupling with emission spectrophotometers and mass spectrometers

– Emission and MS allow faster multi-element analysis

Chapter 20 Questions

1. Why would it be difficult to use a broadband light source and monochromator to produce light used in AA spectrometers?

2. List three methods for atomizing elements.3. List two processes that can decrease atomization

efficiency in flame atomization.4. What is an advantage in using electrothermal

atomization in AAS?5. Which atomization method tends to result in the

most complete breakdown of elements to atoms in the gas phase?

6. Why is ICP better for emission measurements than flame?

Exam 2Topics to Know

A. Electrochemistry (Ch. 13 and 14)1. Nernst Equation – know how to use to determine cell

potentials, concentrations of unknowns, and equilibrium constants.*

2. Conversion between K, DG and E (as in Quiz 3)*3. Know equipment needed for potentiometry

measurements.4. Know purpose of reference electrodes5. Know types and uses of indicator electrodes6. Understand how ion-selective electrodes work7. Some failings of ion-selective electrodes under specific

conditions

* means requires quantitative knowledge

Exam 2Topics to Know – cont.

A. Chapter 17 (Spectroscopy – Theory)1. Light defining parameters (be able to

convert between l, E, n, and for light).*2. Know processes of absorption and

emission.3. Know alternative methods of excitation

and de-excitation.4. Know regions of electromagnetic spectrum

and related transitions.5. Know basics of spectral interpretation.6. Understand and be able to use Beer’s Law

equations.*7. Know sources of deviations to Beer’s Law

+ region of best precision

~

Exam 2Topics to Know

C. Chapter 19 (Spectrometers – cont.)1. Spectrometer Design (know main components

+ designs for UV and fluoresence spectrometers)

2. Main discrete and broad band light sources3. Main methods of wavelength discrimination

(interference filters, monochromators, polychromators, Fourier methods, and through energy dispersive detectors)

4. How interference filters work*5. Components and calculations in grating

monochromators/polychromators*6. Light Detectors (basic types and how they

work)

Exam 2 Topics (cont.)

C. Ch. 19 – cont.7. Polychromators/Array detectors – how they

work8. How energy dispersive detectors work and

what types of light measurements they are used for

9. How FTIR works + advantages and disadvantages of FTIR

D. Chapter 20 (Atomic Spectroscopy)10.Methods for Elemental Analysis (solid + liquid

samples)11.Basic theory of atomic transitions

Exam 2 Topics (cont.)

D. Chapter 20 – cont.3. Atomization processes in flame, graphite

furnace, and ICP and sources of inefficiency in atomization

4. Effect of temp. on Boltzmann distribution and on emission intensity*

5. Advantages and disadvantages of various atomization methods

Exam 2: Equations Provided

• Nernst EquationE = Eº – (0.05916/n)logQ

• Monochromator angular and linear dispersion equations:

Angular dispersion = Df/Dl = n/dcosfLinear dispersion = D = Dy/Dl = FDf/Dl

• Boltzmann Distribution Equation

kTEeg

g

N

N /

00

**

Atomic SpectroscopyAbsorption Spectrometers

• The lamp is a hollow cathode lamp containing the element(s) of interest in cathode

• The lamp is operated under relatively cool conditions at lower pressures to reduce Doppler and pressure broadening of atomic emission lines

• A very narrow band of light emitted from hollow cathode lamps is needed so that absorption by atoms in flame mostly follows Beer’s law

• The monochromator serves as a coarse filter to remove other wavelength bands from light and light emitted from flames

Lamp source

Flame or graphite tube

monochromator Light detector

Atomic SpectroscopyAbsorption Spectrometers

• A narrower emission spectrum from hollow cathode lamp (vs. flame absorption) results in better Beer’s law behavior

wavelengthIn

tens

ity o

r ab

sorb

ance

hollow cathode lamp emission

Atomic absorption spectrum in flame

Sources of broadening:1. Inherent width (Heissenberg

Uncertainty Principle): dE ~ 10-25 J (see text) or dl ~ 10-4 nm

2. Doppler broadening (due to atom motion; depends on temperature)

3. Pressure broadening (shorter lifetimes at higher pressures gives broader peaks)

Atomic SpectroscopyInterference in Absorption Measurements

• Spectral Interference– Very few atom – atom interferences– Interference from flame (or graphite tube)

emissions are reduced by modulating lamp• no lamp: signal from flame vs. with lamp• then with lamp: signal from lamp + flame –

absorption by atoms– Interference from molecular species absorbing

lamp photons (mostly at shorter wavelengths and light scattering in EA-AA)

– This interference can be removed by periodically using a deuterium lamp (broad band light source)• D2 lamp signal = lamp intensity – molecular

absorption – atomic absorption (very minor)• Amolec abs = -log[I(D2)/Io(D2)] (which can be subtracted

from AMetal)

Atomic SpectroscopyInterference in Absorption Measurements

• Chemical Interference– Arises from compounds in sample matrix or atomization

conditions that affects element atomization– Some examples of specific problems (mentioned

previously) and solutions:• Poor volatility due to PO4

3- – add Ca because it binds strongly to PO4

3- allowing analyte metal to volatilize better or use hotter flames

• Formation of metal oxides and hydroxides – use fuel rich flame

• Ionization of analyte atoms – add more readily ionizable metal (e.g Cs)

– Another approach is to use a standard addition calibration procedure (this won’t improve atomization but it accounts for it so that results are reliable)

Atomic Spectroscopy Interference in Absorption Measurements

• Standard Addition– Used when sample matrix

affects response to analytes– Commonly needed for AAS

with complicated samples– Standard is added to sample

(usually in multiple increments)

– Needed if slope is affected by matrix

– Concentration is determined by extrapolation (= |X-intercept|)

Area

Concentration Added

Analyte Concentration

0 bmXA

mbX /

standards in water