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Spectroscopic Methods

Spectroscopy
is the study of matter by investigating light, sound, or particles that are emitted, absorbed or scattered by the matter under investigation


Spectroscopy may also be defined as the study of the interaction between light and matter

Physical quantity measured

The type of spectroscopy depends on the physical quantity measured.

Normally, the quantity that is measured is an amount or intensity of something

Three main types of spectroscopy

Absorption spectroscopy uses the range
of electromagnetic spectra in which a
substance absorbs.

commonly used in atomic absorption spectroscopy, the sample is atomized and then light of a particular frequency is passed through the vapour

the amount of absorption can be related to the concentrations of various metal ions through the Beer-Lambert law

widely used to measure concentrations of ions such as sodium and calcium

Other types of spectroscopy may not require sample atomization

For example, ultraviolet/visible (UV/ Vis) absorption spectroscopy is most often performed on liquid samples to detect molecular content

Emission spectroscopy uses the range of
electromagnetic spectra in which a
substance radiates

The substance first absorbs energy and then radiates this energy as light

This energy can be from a variety of sources, including chemical reactions

Scattering spectroscopy

measures certain physical properties by measuring the amount of light that a substance scatters at certain wavelengths

Common types of spectroscopy

Flame Spectroscopy

Liquid solution samples are aspirated into a burner or nebulizer/burner combination, desolated, atomized, and sometimes excited to a higher energy electronic state

The use of a flame during analysis requires fuel and oxidant, typically in the form of gases

Common fuel gases used are acetylene or hydrogen

Common oxidant gases used are oxygen, air, or nitrous oxide

methods are often capable of analyzing metallic element analytes in the part per million, billion, or possibly lower concentration ranges

Atomic Emission Spectroscopy

uses flame excitation; atoms are excited from the heat of the flame to emit light

commonly uses a total consumption burner with a round burning outlet

A higher temperature flame than atomic absorption spectroscopy (AA) is typically used to produce excitation of analyze atoms

Since analyte atoms are excited by the heat of the flame, no special elemental lamps to shine into the flame are needed

Atomic Fluorescence Spectroscopy

This method commonly uses a burner with a round burning outlet

The flame is used to solvate and atomize the sample, but a lamp shines light at a specific wavelength into the flame to excite the analyte atoms in the flame

The atoms of certain elements can then fluoresce emitting light in a different direction. The intensity of this fluorescing light is used for quantifying the amount of analyte element in the sample.

A graphite furnace can also be used for atomic fluorescence spectroscopy. This method is not as commonly used as atomic absorption or plasma emission spectroscopy.

Alternatively, a monochromator can be set at one wavelength to concentrate on analysis of a single element at a certain emission line

Plasma emission spectroscopy is a more modern version of this method.

Atomic Absorption Spectroscopy (often called AAS)

commonly uses a pre-burner nebulizer (or nebulizing chamber) to create a sample mist and a slot-shaped burner which gives a longer path-length flame

The temperature of the flame is low enough that the flame itself does not excite sample atoms from their ground state

The nebulizer and flame are used to desolvate and atomize the sample, but the excitation of the analyte atoms is done by the use of lamps shining through the flame at various wavelengths for each type of analyte

In AA, the amount of light absorbed after going through the flame determines the amount of analyte in the sample


A graphite furnace for heating the sample to desolvate and atomize is commonly used for greater sensitivity

The graphite furnace method can also analyze some solid or slurry samples.

Because of its good sensitivity and selectivity, it is still a commonly used method of analysis for certain trace elements in aqueous (and other liquid) samples.

Spark or arc (emission) spectroscopy

can be used for the analysis of metallic elements in solid samples

An electric arc or spark is passed through the sample, heating the sample to a high temperature to excite the atoms in it

The excited analyte atoms glow emitting light at various wavelengths which could be detected by common spectroscopic methods

Since the conditions producing the arc emission typically are not controlled quantitatively, the analysis for the elements is qualitative.

Visible spectroscopy

Many atoms emit or absorb visible light. In order to obtain a fine line spectrum, the atoms must be in a gas phase. This means that the substance has to be vaporized.

Spectrum is studied in absorption or emission.


UV spectroscopy

All atoms absorb in the UV region because photons are energetic enough to excite outer electrons. If the frequency is high enough, photoionisation takes place.

Infra-red spectroscopy

In organic chemistry different types of interatomic bond vibrate at different frequencies in the infra-red part of the spectrum.

The analysis of IR absorption spectra shows what type of bonds are present in the sample.


Nuclear Magnetic Resonance (NMR)
spectroscopy

NMR spectroscopy analyzes certain atomic nuclei to determine different local environments of hydrogen, carbon, or other atoms in the molecule of an organic compound or other compound. This is used to help determine the structure of the compound.

Use of Atomic Absorption (AA) Spectroscopic methods
Metals

Significance

Effects of metals in water and wastewater range from beneficial to dangerously toxic

Some metals are essential
Fe – blood
Cu, Zn – physical performance

Others may adversely affect water consumers, wastewater treatment systems, and receiving waters
Arsenic- cancer, toxic to aquatic spp
Asbestos – asbestosis ( ‘black' lung cancer)
Cadmium, chromium, copper – liver and kidney damage

Methods

Atomic absorption (AA)
Colorimetric methods
Sampling and sample preservation


What fraction to be analyzed
Dissolved
Suspended
Total
Acid-extractable

Preservation

Acidify with 1.5 ml (5ml for alkaline samples) HNO3 /L sample to pH < 2
Filter samples for dissolved metals before preserving
Store in fridge at 4oC
Stable for 6 months


Pre-treatment of samples

Total metals – all metals inorganically and organically bound, both dissolved and particulate

Colorless, odorless water - < I NTU – acidify with HNO3 to pH <2 and analyze directly

Dissolved metals

filter sample (0.45 micrometer membrane filter)
acidify filtrate with HNO3 to pH < 2
And analyze directly

Suspended metals

Filter sample (0.45 micrometer membrane filter)
Digest filter
And analyze

Acid-extractable metals

Extract metals
And analyze extract

Pre-treatment for Acid-extractable metals

extractable metals – lightly absorbed on particulate matter
At collection acidify sample with 5 ml HNO3/L sample
To prepare sample
Mix well
Transfer 100 ml to flask
Add 5 ml HCl
Heat 15 min on a steam bath
Filter thorough 0.45 micrometer membrane filter)
Adjust filtrate volume to 100 ml with water and analyze

Pre-digestion of metals

Reduce interference by organic matter and to convert metal associated with particulates to a form that can be determined by AA

HNO3 – digestion is adequate for clean samples or easily oxidized materials

HNO3-H2SO4 or HNO3-HCl – digestion is adequate for readily oxidizable organic matter

HNO3-HClO4 or HNO3-HClO4-HF – is for the difficult to oxidize organic matter or minerals

Nitric Acid Digestion

Mix sample
Transfer 50 to 100ml to 125 ml conical flask
Add 5 ml conc. HNO3 and few boiling chips
Boil and evaporate on hot plate to lowest volume (10 to 20 ml) before precipitation occurs
Continue heating and adding HNO3 until digestion is complete (light-coloured, clear solution)

Wash down flask walls with water and then filter
Transfer filtrate to 100 ml flask , cool, dilute to mark, mix and analyze.

Direct Air-Acetylene Flame Method

Applicable for Ca, Cd, Cr, Co, Cu, Fe, Mg, Mn, Ni, Zn, etc

Principle

The technique of flame atomic absorption spectroscopy (AA) requires a liquid sample to be aspirated, aerosolized, and mixed with combustible gases, such as acetylene and air or acetylene and nitrous oxide

The mixture is ignited in a flame whose temperature ranges from 2100 to 2800 oC

During combustion, atoms of the element of interest in the sample are reduced to free, unexcited ground state atoms, which absorb light at characteristic wavelengths


RISK ASSESSMENT
Environmental Engineer

objective

Reduce risk from hazards to the environment and public health

Estimate future risks, then use science to prevent or mitigate them

To determine the comparative risks from various environmental pollutants and which risks it is most important to decrease or eliminate

risk analysis

Risks associated with various hazards must evaluated and quantified


Risk

1970s – environmental laws were enacted to protect public heath
E.g. US Clean Air Act

No mention of risk in US Clean Air and Clean Water Acts

Acts required that pollution standards be set that would allow adequate margins of safety to protect public health

Assumption was that pollutants have thresholds and exposure to concentrations below these thresholds would produce no harm

Problems of toxic waste were finally recognized

Toxic substances are suspected carcinogens

Even smallest exposures creates a risk

If any exposure to a substance causes some risk, how can air quality and water quality standards be set

Emergence of the field of environmental risk assessment

1980s - environmental policy - was acceptance of the role of risk assessment and risk management in environmental decision making

Risk assessment

Is a system of analysis that includes four tasks

Identification of substance (toxicant) that may have adverse health effects
Scenarios for exposure to toxicant
Characterization of health effects
As estimate of the probability (risk) of occurrence of these health effects


Is the gathering of data that are used to relate response to dose

Such dose-response data can then be combined with estimates of the likely human exposure to produce overall assessment of risk

Risk management

is the process of deciding what to do

It is the decision making, about how to allocate national resources to protect public health and the environment

Is a one-in-a-million lifetime risk of getting cancer acceptable and, if it is, how do we go about trying to achieve it?


Hazardous substances

Hazardous and toxic substances are
defined as those chemicals present in the
environment which are capable of causing harm

Risk

is the possibility of loss or
injury to people and property


Hazard

a chemical, physical, or biological agent or a set of conditions that has the potential to cause harm



Risk Assessment

Hazard identification

Is the process of determine whether or not a particular chemical is causally linked to a particular health effects, such as cancer on birth effects

Dose-response assessment

Is the process of characterizing the relationship between the dose of an agent administered or received and the incidence of an adverse effect

Dose-Response Assessment

Characteristic features of the Dose- Response
relationship:

Threshold

Is the lowest dose at which there is an observable effect

Curve A – illustrate threshold response i.e. there is no observed effect until a particular concentration is reached

Curve B - linear response with no threshold i.e. the intensity of the effect is directly proportionally to the pollutant dose, and the effect is observed for any detectable concentration of the pollutant in question

Curve C –sublinear dose-response curve and is characteristic of many pollutant dose-response relationships.

It has no clearly defined threshold, the lowest dose at which a response can be detected is called threshold limit value (TLV)

Curve D – supralinear relationship, which is found when low doses of a pollutant appear to provoke a disproportionately large response


Total body burden

An organism or a person can be exposed to simultaneously to several sources of a given pollutant

Example

inhale about 50g/day lead from ambient air and
ingest 300g/day in food and water
The concentration of lead in the body is sum of what is inhaled and ingested and what remains in the body fro prior exposure, less what has been eliminated from the body


Physiological half-life

Is the time needed for the organism to eliminate half of the internal concentration of the pollutant, through metabolism or other normal physiological functions


Bioaccumulation and bioconcentation

Bioaccumulation occurs when a substance is concentrated in one organ or the of tissue of an organism

E.g. Iodine - bioaccumulates in the thyroid gland

The organ dose of a pollutant can thus be considerably greater than what the total body burden would predict


Bioconcentation occurs with a movement up the food chain

E.g. A study of Lake Michigan ecosystem (Hickey et al. 1966) found bioconcentation of DDT as follows:

0.014 ppm (wet weight) in bottom sediments
0.41 ppm in bottom-feeding crustacea
3-6 ppm in fish
2 400 ppm in fish –eating birds

Exposure time and time vs. dosage

Most pollutant need time to react; the exposure time is thus as important as the level of exposure

Synergism

Occurs when two or more substances enhance each other’s effects, and when the resulting effect of the combination on the organism is greater than the additive effects of the substances separately

E.g. black lung disease in miners – occurs more often in miners who smoke than in those who do not


LC50 and LD50

LD50 is the dose that is lethal for 50% of the experimental animals

LC50 refers to lethal concentration rather than lethal dose

LD50 values are most useful in comparing toxicities for pesticides and agricultural chemicals


Population responses

Differ from one individual to another
Depends on age, sex, and general state of physical and emotional health


Exposure and Latency

Characterization of some health risks can take a very time
E.g.. Cancers are noticed many years or decades after exposure to potentially responsible carcinogen
Cancer in adults have a latency period between 10 – 40 years
Latency period – length of time between exposure to a risk factor and expression of adverse effect

Relative risk

Is the ratio of the probabilities that an adverse effect will occur in two different populations

E.g. relative risk of lung cancer in smokers

Ps/Pn = (Xs/Ns)/(Xn/Nn)

where

Ps = probability of fatal lung cancer in smoker
Pn = probability of fatal lung cancer in non-smokers
Xs = number of fatal lung cancer among smokers
Xn = number of fatal lung cancer among non- smokers
Ns = total number of smokers
Nn= total number on non-smoker

Relative risk of death is also called standard mortality ratio (SMR)

SMR = Ds/Dn= Ps/Pn

where

Ds = observed lung cancer deaths in a population of habitual smokers
Dn = expected lung cancer deaths in a non-smoking population of the same size

Exposure assessment

Involves determining the size and nature of the population that has been exposed to the toxicant under consideration, and the length of time and toxicant concentration to which they have been exposed

Risk characterization

Is the integration of the foregoing three steps, which results in an estimate of the magnitude of the public-health problem