AAS Handout

7/28/2019 AAS Handout

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Atomic Absorption / Emission / FluorescenceSpectroscopy


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Atomic Absorption Spectroscopy

The analyte concentration is determined from theamount of absorption.

Io IT


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It is possible to measure the concentration of anabsorbing species in a sample by applying the Beer-

Lambert Law:

Abs logI

Io

Abs cb

= extinction coefficient


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But what ifis unknown?Concentration measurements can be made from

a working curve after calibrating the instrument

with standards of known concentration.


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Instrumentation

Light Sources

Atomization

Detection Methods


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Light Sources

Hollow-Cathode Lamps (most common).

Lasers (more specialized).

Hollow-cathode lamps can be used to detect one

or several atomic species simultaneously. Lasers,

while more sensitive, have the disadvantage that

they can detect only one element at a time.


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Hollow-Cathode Lamps


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It is important to note that atoms producing an emission line in a

hollow cathode lamp are relatively at a lower temperature than

those in a flame. As a result emission lines in a HCL lamps are

broadened less than that the emission lines in flames.

A broadened emission line of a hollow cathode lamp (1) and a broadened

emission line of a flame (2)


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Atomization

Atomic Absorption Spectroscopy (AAS) requires that

the analyte atoms be in the gas phase.

Vaporization is usually performed by:

Flames

FurnacesPlasmas


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Flame Atomization

Flame AAScan onlyanalysesolutions.


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Flame Atomization

Degree of atomization is temperaturedependent.

Vary flame temperature by fuel/oxidant

mixture.

Fuel Oxidant Temperature (K)

Acetylene Air 2,400 - 2,700

Acetylene Nitrous Oxide 2,900 - 3,100

Acetylene Oxygen 3,300 - 3,400

Hydrogen Air 2,300 - 2,400

Hydrogen Oxygen 2,800 - 3,000

Cyanogen Oxygen 4,800


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TheAtomization

process in a

Flame


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Furnaces


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Electro-thermal Atomizers

It gives an enhancement of sensitivity by 20 to 1000 times

compared to flame technique, as the entire sample isatomized in a short period and the average residence time ofthe atoms in the optical path is a second or more.

The commercial tubes are composed of graphite withrelatively impervious pyrographite coating.

The tube is resistively heated by an external power supplyand is protected from oxidation by a flow of sheath gas

typically argon around the furnace.

The heating of the tube containing the sample is carried outthrough the three steps: drying, ashing and atomizing of the

analyte sample.


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Furnaces


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Inductively Coupled Plasmas

Enables much higher temperatures to be achieved.

Uses Argon gas to generate the plasma.

Temps ~ 6,000-10,000 K.

Used for emission experiments rather than absorption

experiments due to the higher sensitivity and elevated

temperatures.

Atoms are generated in excited states and

spontaneously emit light.


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Detection

Photomultiplier Tube (PMT).


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Cold Vapor Atomic Absorption Spectrometry.


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Hydride Generation



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Calibration of Laboratory Instruments

Initial Calibration

Either analyst prepare the standard or use commercial ones.

The concentrations of the standards must be within the optimumrange of that given by the method.

Every standards must have an expiry date & it should not be usedbeyond that.

The number of standards needed is recommended by the METHODor the MANUFACTURER of the method.

When not given use a minimum of three concentration and a blank.However six standards are more than enough.


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The 3 Standards are selected as follows

The concentration of the high standard is the upper level of

the optimum range.

The concentration of the middle standards is the of thehighest standard.

The value of the lower standard is 1/5 of the highest standard

Plot the calibration curve & this must be approved using the

corresponding correlation coefficient

Using the regression analysis

Corr. Coeff. > 0.9998

If not satisfied recalibrate.


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Calibration Verification Standards (CVS)

It is a standard (with a known value) prepared

from a source other than that used to prepare

the calibration standard.

e.g. Certified Reference Materials (CRM). The

value obtained from calibration curve cannot be

deviated by no more than 10%.


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Optimum Concentration Range

The concentration range where the calibration curve is linear.

It is defined by the method and varies with the instrument

Reagent BlankAnalytical free water analyzed with samples.

Method Blank

Analyte free water that is subjected to the same pretreatmentas samples.


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Spike sample or Matrix spikeHow are they used?

A small quantity of a known concentration of analyte stock solution is added to

the sample.

If the analyte sample is a liquid thoroughly mix after adding the stock standard.

Solid sample are mixed thoroughly in a glass/ceramic bowl before taking

aliqnot out of it.

The amount of spike should produce a signal at least 10 times that equivalentto IDL and it should not produce a signal larger than 100 times equivalent toIDL.

The concentration of the standard added generally should be 3 to 5 times theanalyte expected in the sample.

Spike solution must be added before the sample preparation.


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Detection limits

Instrument Detection Limit (IDL)

Measure instrument responds for several aliquots

of analyte free water ( e.g. Number of aliquots = 7).

calculate the standard division, S for all theresponse

IDL = 3 S

IDL varies with the analyte for a given instrument


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Method Detection Limit (MDL)

prepare a solution using analyte and analyte free water having aconcentration near the quoted minimum detectable concentration,Divide the solution in to several portions (e.g. 7).

Measure instrument response for each sample, the response for the

analyte. Use a student ttableto determine tvalue for the 7 replicatemeasurements, with the required level of confidence.

MDL = t S

e.g. for the above measurements of degrees of freedom (n1) = 7-1 =

6

If a confidence limit of 98% is required, t=3.14

MDL = 3.14 S


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Practical QuantitationLimits (PQL) or

Limit of Quantitation(LOQ) or Limit of

Reporting ( LOR)

All these 3 determinations are the same parameter.

The limit of detection ( LOD) are good enough oneto be sure that an analyte is present or absent. Ifresponse is larger the MDL or IDL then the analyteis present. However, its value can not be accurately

reported as the concentration is low.

PQL = LOQ = LOR = 10 s


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Accuracy:

some times represented by the quantity called

percentage recovery (%R) defined as below


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Precision


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