Provides an overview of many methods of analysis. We will concentrate on the ones you will use in your laboratories.
The others are there as a survey or overview. We will discuss
them in general but will describe only several in depth.
Overview of methods:
Some Categories of Analysis:
Analytical Technique Related
Matrix Specific
Use mostly manual equipment and older methods
Examples of classical methods:
Titration example - The titration with an indicator of Ca (and/or
Mg) ions in water using EDTA are examples of a classical analysis.
Ca2+ (or Mg2+) + H2Y2- ---->
CaY2- (or MgY2- ) + 2H+
Gravimetric example - Precipitation of silver with Chloride
ion is an example of a classical gravimetric analysis.
Ag(aq) + Cl(aq) ---> AgCl(s)
Combination of several classical methods produce complex
reaction groups of dependent reactions.
DO (dissolved oxygen)
Volumetric example - Oxidation-reduction-titration-volumetric,
dissolved oxygen (DO) (Winkler method) being done in constant
volume bottles and comparing relative iodine color is an example
of volumetric and titrimetric analysis.
Mn+2+ 2OH- 1/2 O2 ---> MnO2(s) + H2O
MnO2(s) + 2I- + 4H+ ---> Mn+2
+ I2 + H2O
I + 2S2 O32- ---> S4 O62- + 2I-
Oxygen dissolved in water oxidizes manganese from +2
to +4 in basic solution. The acidification of the hydrated
MnO2 in the presence of I- to release free
I2, then titrate free I2 with thiosulfate
with starch providing the endpoint.
BOD
Biochemical Oxygen Demand
Air saturate water samples in a standard size bottle (usually). Then add a biological "seed"and incubate for 5 days. Estimate the amount of biodegradable organic matter in the water to determine the remaining dissolved
oxygen.
This is usually expressed in mg/l; where mg is of oxygen.
Absorption and emission are two fundamental principles
Beer's Law is one of the most common relationships applied
Beer's Law: A=abc
Where:
A = Absorbance
a = absorptivity, (wavelength-dependent)
b = is the path length the light travels through the absorbing
solution
C = concentration of the absorbing material in solution
This fundamental relationship is the key parameter in:
Molecular absorption
Colorimetric analysis
Atomic absorption
Review for familiarity in general.
These will produce the data used
in environmental evaluation.
Examples:
Atomic absorption Traditional Lab. method
Atomic emission
Flame
ICP-OES
X-ray fluorescence optical emission Spect.&
a new field method, Survey only!
Electrochemical methods
note: many interferences when using real samples
Mass Spectrometry (MS)
Organic
Inorganic
Chromatography
Gas Chromatography (GC)
Liquid Chromatography (LC)
also call high performance L.C. or (HPLC)
Supercritical Fluid Chromatography (SFC)
Ion Chromatography
Hyphenated Techniques
GC-MS
LC-MS
ICP-MS
LC-ICP-MS
Sample preparation must be done on real environmental samples prior to analysis.
Extraction
Organic------------ organic solvent extraction
Inorganic---------- acid extraction
Dissolution
Organic------------ organic solvent
Inorganic---------- acid decomposition
Purge and Trap
for volatile organic components, such as, in soil (gasoline)
In water for example:
Turbidity
Temperature
Color
Conductance
Specific Gravity
In soils and sediments:
Particle size
Color
Physical phases
Obvious structure
Tests Specific to the Sample:
BOD (biological oxygen demand) in water
DO (dissolved oxygen) in water
Extraction from matrix in soils, and in sediments
pH in water
Total Alkalinity in water
O3 (ozone) in air
NOx in air (ps)
SO2 in air, (ps), point source
Example:
X-ray fluorescence
Example:
Method 7473 for Hg analysis (RCRA update VI)
Example:
Field portable laboratory trailers take lab to the field.
| Microwave Sample Decomposition | Standard EPA Procedure |
| EPA RCRA Methods | 3051a, 3015a, 3052, 3050B, (and others) |
| Inactively Coupled Plasma-Mass Spectrometry | (ICP-MS) Standard EPA Procedure |
| EPA RCRA Methods | 36020 (update VI 6020a) |
National Science Foundation (NSF) Problem Based Learning, PBL discussion:
(applies to sampling and laboratory philosophy).
References for Laboratory:
"Curricular Developments in the Analytical Sciences, A Report
from the Workships"
National Science Foundation, Washington DC,
September 12, 1997, pages 2-6.
Standard EPA Methods:
EPA RCRA Method 3051a (update VI, 1998)
"Microwave Assisted Acid Dissolution of Sediments, Sludges,
Soils, Oils"
EPA RCRA Method 6020 (update III, 1996) "Inductively Coupled
Plasma - Mass Spectrometry"
EPA RCRA quality assureance chapter 3 (update VI, 1998)
"Clean Chemistry and the Analytical Blank"
Hyphenated techniques are frequently formed using mass spectrometry
coupled with GC and HPLC. MS came later and increased the utility
of the methods.
Method includes:
GC-MS
LC-MS (or HPLC-MS ), Organic or Inorganic
Direct introduction of liquid or gas samples for inorganic ICP-MS
The Method - Mass Spectrometry Fundamentals
The method includes and depends on:
1. Production of a gaseous ion and/or ion fragments [Uniform fractionation
patterns from specific instrument configurations producing identical
patterns]
{Standardized and Comparable Patterns}
2. The measurement of a change to mass ratio in a vacuum [m/z] (collision free path requires at least 10-6 torr)
3. A stable uniform and consistent ionization source to produce reproducible ionization
4. Measurement of relative abundance of ions or fragments of each
mass without bias
Sample Introduction System (inlet system)
Ionization System
Vacuum System
Mass Analyzer (Mass Separator)
Ion Collection and (Mass Detection)
Basis of measurement is mass-to-charge ratios
Data Manipulation and Molecular Identification System
(sometimes associated with sophisticated data base, searching and spectral
analysis capability)
Configurations and Hyphenation
Frequently the mass spectrometer is interfaced with some sample treatment and separation device.
1.Direct Liquid Introduction lab will use this method
2. Liquid Chromatography
3. Gas Chromatography
4. Gas, Aerosol, Particle Introduction
5. Laser Ablation
5. Solid Samples
6. Inductively Coupled Plasma
Sector Mass Analyzers
Magnetic Sector Analyzers
Single focusing
Double focusing
Triple focusing
Electrostatic Sector Analyzers
Dynamic Mass Analyzers
Quadrupole Both Lab ICP-MS's use this one
Cyclotron Resonance Mass Analyzer
Ion Trap
Time of Flight
Three main types:
1. Electron Multiplier Both Lab ICP-MS's use these
2. Faraday Cup Collector Both Lab ICP-MS's use these
3. Channel Electron Multiplier Array
An electrically connected cage into which the ions are directed
Electrons are supplied through a circuit and the potential change in the cage is amplified to obtain a signal.
The Faraday Cup detector is very stable but it has no internal
amplification ( a one for one - one ion produces one electron).
(see figure below)
This type is the most important and most common.
Two discrete types:
Continuous dynode electron multiplier (most popular)
Discrete dynode electron multiplier
In discrete dynode multiplier each dynode is held at successively higher voltages amplifying the signal.
Up to 20 dynodes are available with a current gain of 107 - 108 electrons per ion
Continuous dynode is glass doped with Pb with a potential of 1.8-2kV across the length of a trumpet shaped detector. Ions striking the surface near the entrance eject electrons that skip along the surface ejecting electrons. Current gains of 105 to 108 are possible.
Figures S&L pg. 426-427 Figures 18-3 - 18-4.
2. Discrete dynode electron multiplier and
3. Continuous dynode electron multiplier (most popular)
These detectors require vacuum to prevent arcing and to preserve
their electrical coatings.
Scanning is one way to produce peaks of the different mass ions
(or more correctly the charge to mass ratios m/z).
The mass analyzer can be set to look for the mass of interest based on an elemental ion or a molecular form expected in the sample.
Analysis of the intensity of the mass signal compared against
some standard is the usual way of quantifying the unknown concentration.
A very important ion source that you should be able to explain.
Inductively Coupled Plasma is an important Ion Source and is mainly useful in inorganic mass spectrometry as the temperature is too high to be of use in organic mass spectrometry.
It is based on a Torch and Plasma generator
The Inductively Coupled Plasma (ICP) torch is an electrodeless discharge in a gas at essentially atmospheric pressure maintained by energy coupled to it from a radio frequency generator.
The energy is coupled by a coil functioning as a primary radio frequency transformer and the secondary is created by the discharge itself.
The plasma is generated at the open end of the torch which is
a quartz tube assemble of 2 inner tubes, one to transport the
sample and carrier gas, and an outer tube to transport the gas
volume necessary to supply the plasma with gas to ionize.
Typical Plasma Torch Configuration:
The torch usually consists of 3 concentric tubes.
1. inner,
2. middle, and
3. outer.
Typical dimensions are in 1.5 mm inner diameter
tube (supplying the sample); contained inside a 13 mm middle
tube supplying the gas to make up the bulk of the plasma usually
10 times the flow of the sample stream but is usually supplied
in a spiral direction to help form the plasma. Both the inner
and middle tubes terminate within a centimeter of the end of the
torch. The outer quartz tube is usually 10 mm and contains
the other two tubes. The radio frequency (RF) generator coils
are located around the outer tube.
Carrier and plasma gases are usually Argon, the torch is usually quartz and the RF (radio frequency, actually it is a microwave frequency) coil is water cooled copper to prevent it from melting.
The RF energy is 27 or 40 Mhz coupled to it by the load coil.
The generator is usually operated at a 1-1.5 kW supplyng the RF to the induction coils.
Power requirements to maintain the induced plasma are between 0.75 and 2.0 kW.
The Argon plasma is maintained at 5,000 to 9,000 °C.
The torch is mounted with its axis horizontally for extraction
of the ions into the mass spectrometer.
The ICP-MS usually requires samples to be introduced
into a flowing stream of argon (sometimes He and other inert gases
are used) such as:
Gas
Vapor
Aerosol
Solid Particles (very fine size)
Figure 2.8 Demonstrates the entire assembly for Torch, two Cones
(Sampling and Skimmer), and the accelerations of the positive
ions by a negative voltage field behind the second cone.
The gas is drawn into the region between the
two cones by a vacum and reaches supersonic velocities as it expands
in the vacuum chamber and reaches the skimmer orifice in a few
micro seconds (µs).
Figure 2.12 JGH illustrates the sampler and skimmer with the ions
in between where they are propelled at supersonic speeds and how
light and heavy ions are accelerated at the same velocity (v)
as neutral Ar in the jet. Figure 2.13 not reproduced there also
helps illustrate this effect.
The positive ions are accelerated by charged plates in a vacuum.
Only negative acceleration plates are customarily used.
where:
The kinetic energy (KE) that is added to the ion is due to the
acceleration potential V and is given by:
The KE of mass m and charge z is expressed as:
where: v is the velocity after the ion is accelerated and e is the charge of the ion (e = 1.60 x 10-19 C {coulombs})
Thus trajectories are based on the laws of physics used in the design of the Mass Analyzers or mass separators used to discriminate m/z ions Figure 18-19 S&L (Skoog and Lerie)
Lense collection and acceleration
A field is formed by four electrically conducting, parallel rods.
The rods work as opposite pairs.
The mass analyzer is in a section of the mass spectrometer with progressively lower vacuum of between 10-7 - 10-8 Pa or 10-5 - 10-6 torr.
The quadrupole mass analyzer can operate in relatively low vacuum such as 10-4 torr but there must be enough vacuum to prevent arching from the quadrupoles.
The ions enter on a linear flight path in the z direction and are directed by means of the electric fields in the x and y direction. Unwanted ions of non selected m/z ratio oscillate with increasing amplitude and collide with the quadrupoles acting as electrodes and become neutral particles.
Selected ions proceed through the quadrupole mass analyzer and are detected.
By controlling the potential on the quadrupole, only one m/z ion will be permitted at a time to emerge and proceed to the detector.
m/z ratio - is mass/charge ratio
The only moving parts are in the vacuum system.
Figure 20.6 (SH) demonstrates how the quadrupole blocks all ion
mass to charge ratios except the one being analyzed, which is sent
on to the detector.
The quadrupole functions as two sets of mass filters similar to optical cut-off filters. Sets of filters called Set (a) and Set (q).
Each set has different cut-offs for high m/z or low m/z.
This section discusses options for sample introduction
including:
Nebulizers
Spray Chambers
Plasma Torches, and
Sampling Cones
Note:
The great majority of analysis done on ICP-MS instruments
are done on liquid samples.
A gas stream is required for introduction into the ICP-MS, and thus a liquid stream must be converted.
An aerosol is the most convenient form of liquid sample at present.
Related rational for the laboratories structured.
Other experts who suggest using the approach that we are taking
in these laboratories (modeling, sampling, and analysis) have
studied your needs as future professionals and how to help you
gain these skills.
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