Transcript 4/12 Lecture Notes

Chem. 133 – 4/12 Lecture
Announcements I
• Strike – No Strike now planned
• Exam 2:
– Average was 77 (range 61 to 93)
• Last HW Set (Set 3 – see handout)
• Lab
– Last day for Set 2:4 today with make-up day on
Thursday
– Lab report deadlines (2:3 – 4/21 and 2:4 – 4/28)
– Term project work starts April 19th
Announcements II
• Today’s Lecture
– Atomic Spectroscopy
• Atomization (ICP)
• Atomic Spectrometers and Interferences
• Method Comparison
– Nuclear Magnetic Resonance Spectroscopy
Atomic Spectroscopy
Atomization
• 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 einduce further production of ions
and great heat production
– Much higher temperatures are
created (6000 K to 10000 K vs.
flames)
ICP Torch
RF Coil
Quartz tube
Argon + Sample
Atomic Spectroscopy
Atomization
• 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.
2.
3.
4.
5.
6.
Why would it be difficult to use a broadband light source
and monochromator to produce light used in AA
spectrometers?
List three methods for atomizing elements.
List two processes that can decrease atomization efficiency
in flame atomization.
What is an advantage in using electrothermal atomization in
AAS?
Which atomization method tends to result in the most
complete breakdown of elements to atoms in the gas
phase?
Why is ICP better for emission measurements than flame?
Atomic Spectroscopy
Absorption Spectrometers
Flame or
graphite tube
Lamp source
monochromator
•
•
•
•
Light detector
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
Atomic Spectroscopy
Absorption Spectrometers
Additional broadening in
flame from temperature
(Doppler) or pressure
Atomic absorption
spectrum in flame
Intensity or absorbance
• A narrower emission
spectrum from hollow
cathode lamp (vs. flame
absorption) results in
better Beer’s law behavior
hollow cathode lamp
emission
wavelength
Atomic Spectroscopy
Interference 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) or
using the Zeeman effect (magnetic splitting of
absorption bands)
Atomic Spectroscopy
Interference 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 PO43- – add Ca because it binds strongly to
PO43- 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
standards in water
• 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
Analyte
Concentration
Concentration
Added
A  mX  b  0
X  b/ m
Atomic Spectroscopy
Emission Spectrometers
•
•
•
•
•
In emission measurements, the plasma (or flame) is the light source
Flame sources are generally limited to a few elements (only hot enough
for low E – visible light emissions)
A monochromator or polychromator is the means of wavelength
discrimination
Sensitive detectors are needed
ICP-AES is faster than AAS because switching monochromator settings
can be done faster than switching lamp plus flame conditions
Plasma (light
source + sample)
Monochromator or
Polychromator
Light detector or
detector array
Liquid sample, nebulizer, Ar source
Atomic Spectroscopy
Emission Spectrometers
• Sequential vs. Simultaneous Instruments
• Sequential Instruments use:
– A standard monochromator
– Select for elements by rotating the monochromator grating to
specific wavelengths
• Simultaneous Instruments use:
– A 1D or 2D polychromator (Harris Color Plate 24/25)
– 1D instruments typically use photomultiplier detectors behind
multiple exit slits
– 2D instrument shown in 4/1 lecture slide 13
– Selected elements (1D instruments) or all elements can be
analyzed simultaneously resulting in faster analysis and less
sample consumption.
Atomic Spectroscopy
Interference in Emission Measurements
• Interferences
– Atom – atom interferences more
common than in atomic absorption
because monochromators offer less
selectivity than hollow cathode lamps
– Interference from molecular
emissions are reduced by scanning to
the sides of the atomic peaks
– Chemical interferences are less
prevalent due to greater atomization
efficiency
Emission
Spectrum
Atomic
peak
background
Atomic Mass Spectrometry
•
•
•
•
•
•
Most common arrangement consists of ICP torch placed to MS interface
The Ar+ ions (and electrons) collide with metals leading to ionization
The MS interface consists of skimmer cones to allow ions in, and to
drop the pressure in stages, and ion optics
ICP-MS typically is the most sensitive elemental analysis method
Interference can arise from metals (e.g. 138Ba2+ vs. 69Ga+) or from ICP
species (e.g. 40Ar+ and 40Ca+)
Use of secondary isotopic masses and collision cell reactions can
reduce these interferences
collision cell
Plasma (atomizer
+ ion source)
Mass spectrometer (e.g.
quadrupole)
Liquid sample, nebulizer, Ar source
Atomic Spectroscopy
Comparison of Instruments
Instrument
Cost
Speed
Sensitivity
Flame-AA
Low (~$10-15K)
Slow
GF-AA
Moderate
(~$40K)
Slowest
Moderate
(~0.01 ppm)
Very Good
Sequential ICP-
Moderate
Medium
Moderate
Simultaneous
High
Fast
Good
Highest
(~$200K)
Fast
Excellent
AES
ICP-AES
ICP-MS
Atomic Spectroscopy
Some Questions
1.
2.
3.
4.
5.
Why is AES with a plasma normally more sensitive
than AES with a flame?
List two ways in which a process in a flame can lead to
reduced sensitivity and a way to deal with each
process so its effect on the analysis is minimized.
Why can a simultaneous ICP-AES be more sensitive
than an sequential ICP-AES if used for analysis of 12
metals?
If a sample matrix produces molecular emissions that
interfere with atomic emissions, how would this be
observed and how can this be accounted for?
What can cause interferences in ICP-MS?
Nuclear Magnetic Resonance (NMR)
Spectrometry
Major Uses
• Identification of Pure Compounds (Qualitative
Analysis)
• Structural Determination (e.g. protein shape)
• Quantitative Analysis
• Characterization of Compounds in Mixtures
(% of C as aromatic C)
• Imaging (MRI) – not covered
NMR Spectrometry
Theory
• Spin
– a magnetic property that sub atomic particles have
(electrons, some nuclei)
– some combinations do not result in observable spin
(paired electrons have no observable spin; many
nuclei have no observable spin)
– Electron spin transitions occur at higher energies and
are the basis of electron paramagnetic spectroscopy
(EPR)
– Nuclear spin given by Nuclear Spin Quantum
Number (I)
NMR Spectrometry
Theory
• Nuclear Spin (continued)
– I = 0 nuclei → no spin (not useful in NMR) – e.g. 12C
– I = ½ nuclei → most commonly used nuclei (1H, 13C,
19F, many others)
– I > 1 nuclei → used occasionally, important for spinspin coupling
– number of different spin states (m) = 2I + 1
– examples:
• 1H (I = ½), 2 states
• 2H (I = 1), 3 states
up state (m = +1/2)
up statedown
(m = state
1) (m = -1/2)
middle state (m = 0)
down state (m = -1)
NMR Spectrometry
Theory
• Effect of External Magnetic
Field on Nuclei States
Applied Magnetic
– aligned nuclei (m = +1/2)
Field B0*
have slightly lower energy
(are more stable) than antialigned states (m = -1/2)
“up” state – m = +1/2
– the greater the magnetic field
(B0), the greater the energy
“down” state – m = -1/2
difference between the states
path made by vector tips
Note: arrows drawn at angles because
spin vectors precess about B0
*Note: technically B0 is the magnetic field at the nucleus
which is not quite the same as the applied magnetic field
NMR Spectrometry
Theory
• Energy depends on
nucleus, spin state (m),
and magnetic field
E
Energy
gmh
B0
2
g (gamma) = magnetogyric
ratio (constant for given
nuclei) and h = Planck’s
constant
• Energy difference
E  E (m  1 / 2)  E (m  1 / 2) 
ΔE
gh
B0
2
B0