Transcript Atomic Absorption

Buck Scientific, Inc.
Seminar in ANALYTICAL
INSTRUMENTATION for
Spectroscopy & Chromatography
ATOMIC ABSORPTION (AAS)
Theory & Principle of OPERATION
• Using a LIGHT SOURCE made from a
Pure METAL ELEMENT, the
CONCENTRATION of that METAL in a
Solution can be determined by the
ABSORBANCE of this Light by Excited
ATOMS of the same METAL
Basic AA Theory
• There are THREE
Primary Modes of
measuring an
ATOMIC SIGNAL:
– Emission
– Absorption
– Fluorescence
Basic AA Theory
While “PLASMA EMISSION” was also
being investigated at the same time,
several companies developed WORKING
AAS instruments rapidly, within 2 years
Basic AA Theory
The COMPLEX
nature of Atomic
FLUORESCENCE
made it difficult to
FULLY develop as a
working Analytical
technique for
Quantitative
Analytical Chemistry
Basic SPECTROMETER Design
ALL Analytical Spectrometers have several
BASIC design components in common:
Light Source
Sample “Chamber”
Optical System
Detector
Output
Basic AA Design
• The BASIC Instrument was developed back in
1947 as part of a U.S. Government Grant
Basic AA Design
• First instruments were
DIRECT Current, SINGLE
Beam designs
• Next generation were
ALTERNATING Current to
try to compensate for high
Drift & Instability
• Final designs had to “split”
the signal into a “DOUBLE
Beam” path to fully correct
for Noise, Drift & Errors
Basic AA Design
• Light Source =
– Buck lamps have
the highest output
(energy) at the
lowest current for
best stability and
lifetime
– Amp-hours will
determine HCL
operating life
Lamp Power effects on Line Signal quality
Basic AA Design
• BUCK Scientific
manufacture’s its
OWN HollowCathode Lamps to
provide the BEST
possible Energy to
achieve the
highest Sensitivity
and Reproducible
Sample Data
Basic AA Design
“SAMPLE” chamber
to deliver the Solution
is made of a
DYNAMIC
NEBULIZER, a
SPRAY CHAMBER
and a BURNER
HEAD Assembly
Basic AA Design
BURNER HEAD
uses Dynamic
Impact-Bead
NEBULIZER for
maximum
Sample Aerosol
delivery to Flame
for ALL types of
Solutions (high
salt, organic)
Basic AA Design
OPTICAL SYSTEM
for the BUCK AAS
consists of a compact,
med-high Resolution,
SINGLE-BEAM, 1/4M
Ebert Monochromator
with variable Slits to
maximize separation of
Resonance Lines that
come from the HollowCathode Lamps
Basic AA Design
Adjustable SLITS
are used to increase
the Resolution /
Separation of HCL
signals from Metals
with “rich” Spectra
(many lines); such
as Fe, Ni, Mn & Co
Basic AA Design
The DETECTOR
used to measure
the Absorbance
SIGNAL is a highGain / low-Noise
Photo-Multiplier
Tube (PMT) that is
OPTIMIZED for
the UV region
Basic AA Design
DATA OUTPUT
can be read
directly from the
Graphics LCD,
sent to a Printer or
be transferred to a
Computer as an
ASCII file to
process in Excel,
Lotus, etc.
BUCK AA Hardware
The features of high-energy, compactness &
economy developed into the 210-series AAS:
BUCK AA Hardware
3-Lamp
Turret with
Self-Aligning
Lamps, high
Safety AutoGas Box &
Ignition...
…low-cost
manual
controls...
…precision
Wavelength and
Slit adjustments
for Rapid,
Simple &
Reliable analyses
AA Techniques
There are several MODES of Analysis for
Atomic Absorption Spectroscopy:
Flame AAS
Graphite Furnace AAS
Hydride Generation AAS
Cold-Vapor Mercury AAS
AA Techniques
FLAME AAS:
o
o
Air-Acetylene (~1800 C) or Nitrous Oxide (~2300 C)
Flames allow the determination of most Metals with
high-PPB / low-PPM Detection Limits
AA Techniques
GRAPHITE FURNACE
uses extremely SMALL
sample volumes (20µL),
then runs a Temperature
PROGRAM to DRY any
liquid, then CHAR or ASH
away any organic or
matrix, then ATOMIZE
the Analyte for PPB levels
AA Techniques
GFAAS
requires a
source of
Cooling
Water and
Argon Gas
to operate
The BUCK 220-GF used a proprietary high-frequency,
back-mounted Power Supply for maximum Efficiency
AA Techniques
For Environmental work,
the low-level HYDRIDE
GENERATION method
is excellent for some
Metals; such as As, Se,
Sb; and occassionally Bi,
Ge & Sn. A carefully
controlled Reaction with
a STRONG Reducing
Agent is used.
Hydride Generation Reaction :
To FLAME
AA Techniques
The Metal, MERCURY,
has the unique ability to
ABSORB light in the
Vapor state at ROOM
Temperature. Since it
does not require any
HEAT to generate an
ABS signal, this method
is called COLD-VAPOR
or Flameless AAS
BACKGROUND Correction
In AAS, there are
several types of
INTERFERENCES
that can affect the
Accuracy and
Precision of the
Data due to various
effects on the Metal
or the Instrument
BACKGROUND Correction
SPECTRAL Interferences
are addressed by using a
good quality HCL.
PHYSICAL effects are
corrected by using proper
Sample Preparation.
CHEMICAL errors are
minimized by using Matrix
Modifiers and Reagents…
BACKGROUND Correction
IONIZATION can be
reduced by adding a Buffer
to the Solutions.
Non-Specific absorbances,
or NSA, are due to the
BACKGROUND signal
from the Sample matrix,
and are corrected using a
DEUTERIUM [D2] Lamp
BACKGROUND Correction: D2
Pb-283nm.
HCL generates a “narrow” band radiation, that “sees”
BOTH Element-Specific AND Background Non-Specific
Absorbances
•
180-400nm.
D2 Lamp creates a “continuum” of “BROAD band
radiation, that ONLY “sees” other BROAD band
signals, such as BACKGROUND
ABSORBANCE
signals for Elements
are Narrow & Sharp;
while signals from
BACKGROUND
radiation is Broad. A
DEUTERIUM Lamp
is used to ISOLATE
Background signals.
BKG Correction: VGP
Pb-283nm.
@ 5mA
Low CURRENTS for Analytical Measurements
create a “narrow” band source of radiation
Pb-283nm.
@500mA
High CURRENTS (>500mA) are used to form the
GIANT PULSE as a “broad”, wide band; to make
it SIMILAR to the D2 Continuum for reading
ONLY signals from the BACKGROUND
For any Analytical
wavelengths outside
the 180-400nm.
Range for the D2
Lamp, Variable
GIANT PULSE
Correction will
work for many
applications
BACKGROUND Correction
Passing the HCL Beam
through a D2 Lamp
“plasma” coil allows the
Analysis of Trace Metals
in Complex Samples;
such as Cadmium in
Fertilizers; without the
high levels of Light
LOSS from Beamsplitters
ATOMIC ABSORPTION (AAS)
Sample PREPARATION & Applications
Careful techniques must be used to Collect,
Preserve and then PREPARE the Sample
so it can be run on the AAS Instrument.
This is an OVERVIEW of the Common
method for Sample Preparation
SAMPLE PREPARATION
• Samples MUST be in the form of a clear,
particle-free Solution
• These Solutions can be based on an Aqueous
(water) or Organic (solvent) matrix
• Calibration materials (Blank and Standards)
must be prepared with the SAME procedures as
the Samples
Sample PREPARATION
Some of the common procedures for AAS:
Open Beaker Acid Digestion
Closed Vessel Pressure Digestion
Soxhlet Solvent Extraction
Muffle-Furnace Ashing
Sonication
Centrifugation
Simple Filtration
PREPARATION Equipment
For the highest
Accuracy in the
Analysis, it is
important to use Labware that is Calibrated
and Certified!
Balances and Pipets
are the major sources
of Analytical error in
AAS work
PREPARATION Equipment
A reliable source of
Pure WATER is very
important for doing
low-level Trace
Analysis, such as
GFAAS work.
A “bad” Blank can
cause as much as a
50% error!!
Calibration Equipment
The proper
STORAGE
containers should
be used to
minimize any
LOSS of the
Sample METALS
or Contamination
from the
Environment
Calibration Equipment
STANDARD
Solutions for
Calibration of all
Atomic Absorption
instrumentation
should be
NIST/NBS or ISO /
IUPAC Certified
for true Accuracy
APPLICATIONS
50+ Metallic Elements of the
Periodic Table can be
determined by Flame or
Furnace AAS from the 0.0001%
to 100% concentrations
APPNOTE - Environmental
SAMPLE: Factory Effluent / Discharge
PREPARATION: Digest w/ 100ml. W/
10ml. HNO3; boil down to 100ml.; filter
and run directly (no dilution factor)
ANALYSIS: Air- & Nitrous Flames w/
2-point Calibration
RESULTS: mg/L
or PPM in Sample
APPNOTE - Environmental
ELEMENT
As [Hyd]
Cr+6 [Air]
Ag [Air]
Pb [GFAA]
Ba [N2O]
Original
85 PPB
3.7 PPM
212 PPB
1.9 PPM
450 PPM
Treated
17 PPB
0.55 PPM
< 10 PPB
0.05 PPM
< 50 PPM
AAS can monitor the Pollution in any Water, and
any Treatment. Meets ALL USEPA requirements.
APPNOTE - Metallurgical
SAMPLE: Automotive Parts Plating
PREPARATION: Simple 1:20 and 1:500
dilution of Plating Bath w/ DI Water
ANALYSIS: Air- & Nitrous Flames w/
1-point Calibration
RESULTS: Percent
(%v/v) or mg/L
(PPM) in the Bath
APPNOTE - Metallurgical
Element
Cu (acid)
Zn (alkali)
Ag (cyanide)
Cr (acid +6)
Ni (sulfamate)
New Bath
1.72%
0.35%
1250 PPM
0.60%
4800 PPM
Old Bath
1.29% [-]
0.42% [+]
490 PPM [--]
0.57% [OK]
3870 PPM [-]
AAS can be used for both the MAJOR & MINOR
Bath components, contamination build-up and for
Discharge Monitoring to avoid penalties
APPNOTE - Agricultural
SAMPLE: Rice / Corn Fertilizer & Plants
PREPARATION: 1:100 dilution of
Fertilizer. Digest 2 gram wet Plant w/
15ml HNO3+5ml H2SO4+2ml 30% H2O2
ANALYSIS: Air-Flames w/ 1-pt CAL
RESULTS: PPM in
both Samples, look at
metal RATIO in Plant
APPNOTE - Agricultural
Element
Fertilizer
Magnesium
0.15%
Potassium
1.15%
RATIO
=
7.7
Copper
65 PPM
Molybdenum 15 PPM
RATIO
=
0.23
Rice
47 PPM
195 PPM
4.2 [-]
12 PPM
3.1 PPM
0.25 [OK]
Corn
280 PPM
2250 PPM
8.0 [OK]
39 PPM
14.6 PPM
0.37 [+]
AAS + D2 correction gives the BEST Accuracy for
Trace Minerals in these Complex Samples