AAS Handout

download AAS Handout

of 30

Transcript of AAS Handout

  • 7/28/2019 AAS Handout


    Atomic Absorption / Emission / FluorescenceSpectroscopy

  • 7/28/2019 AAS Handout


    Atomic Absorption Spectroscopy

    The analyte concentration is determined from theamount of absorption.

    Io IT

  • 7/28/2019 AAS Handout


    Atomic Absorption Spectroscopy

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

    Lambert Law:

    Abs logI


    Abs cb

    = extinction coefficient

  • 7/28/2019 AAS Handout


    Atomic Absorption Spectroscopy

    But what ifis unknown?Concentration measurements can be made from

    a working curve after calibrating the instrument

    with standards of known concentration.

  • 7/28/2019 AAS Handout


    Atomic Absorption Spectroscopy


    Light Sources


    Detection Methods

  • 7/28/2019 AAS Handout


    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.

  • 7/28/2019 AAS Handout


    Hollow-Cathode Lamps

  • 7/28/2019 AAS Handout


    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)

  • 7/28/2019 AAS Handout



    Atomic Absorption Spectroscopy (AAS) requires that

    the analyte atoms be in the gas phase.

    Vaporization is usually performed by:



  • 7/28/2019 AAS Handout


    Flame Atomization

    Flame AAScan onlyanalysesolutions.

  • 7/28/2019 AAS Handout


    Flame Atomization

    Degree of atomization is temperaturedependent.

    Vary flame temperature by fuel/oxidant


    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

  • 7/28/2019 AAS Handout



    process in a


  • 7/28/2019 AAS Handout



  • 7/28/2019 AAS Handout


    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.

  • 7/28/2019 AAS Handout



  • 7/28/2019 AAS Handout


    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


    Atoms are generated in excited states and

    spontaneously emit light.

  • 7/28/2019 AAS Handout



    Photomultiplier Tube (PMT).

  • 7/28/2019 AAS Handout


    Cold Vapor Atomic Absorption Spectrometry.

  • 7/28/2019 AAS Handout


    Hydride Generation

    Atomic Absorption Spectroscopy

  • 7/28/2019 AAS Handout


    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.

  • 7/28/2019 AAS Handout


    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.

  • 7/28/2019 AAS Handout


    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%.

  • 7/28/2019 AAS Handout


    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.

  • 7/28/2019 AAS Handout


    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.

  • 7/28/2019 AAS Handout


    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

  • 7/28/2019 AAS Handout


    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 =


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

    MDL = 3.14 S

  • 7/28/2019 AAS Handout


    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

  • 7/28/2019 AAS Handout



    some times represented by the quantity called

    percentage recovery (%R) defined as below

  • 7/28/2019 AAS Handout



  • 7/28/2019 AAS Handout