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    Title: FLAME ATOMIC SPECTROSCOPY

    section 1: Instrumental settings

    section 2: Wavelength scan, atomic emission and absorption

    section 3: Chemical interferences

    section 4: Detection limit, external standard vs. standard addition

    Full name: NAUMAN MITHANI

    Student no.: 301016320

    Sections: LA02: group C

    Date of expt.: Feb. 7, 2008

    ABSTRACT:

    This atomic spectroscopy experiment employs a flame furnace-absorption

    spectrophotometer in showing the following: the highest signal:noise ratio at a

    different burner height than the highest signal:background; the spectroscopy runs

    show strongest signals for atomic Ca absorption and ionic Mg absorption; the

    chemical interference trials demonstrate the hindering effect of phosphoric acid and

    the beneficial, preventative effect of strontium and EDTA in the Ca solutions destined

    for spectral analysis; by the external standards method, the concentration of Ca (aq) in

    the unknown was determined to be 1.457 0.097ppm ( 6.657 %). There is,

    however, limited data in certain sections and so conclusions are not as binding.

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    INTRODUCTION:

    In order to study the absorption/emission spectra (thus the related properties

    and applications) of a substance, it must be excited; as the substance de-excites, it

    releases energy, which is characteristic of the substance. This series of atomic

    spectroscopy experiments employed the use of a flame furnace (coupled with an

    atomic absorption spectrophotometer) to excite the analyte atoms and measure their

    absorption/emission spectra.

    The first section was the optimising the instrumentation setup; the second

    section saw the main experiment where the analyte (Ca and Mg) was volatilised,

    ionised and its spectra measured as per the sub-species (atomic or ionic Ca or Mg)

    under study, this was done by adjusting the wavelengths corresponding to the sub-

    species literature wavelengths. The practical scenario of interferences was measured

    in the third section where the effect of additives such as K (aq), EDTA, phosphoric

    acid and strontium were studied. The fourth section involved application of the

    technique as a quantitative analytical tool.

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    EXPERIMENTAL:

    Apparatus: Perkin-Elmer 1100 B Atomic absorption spectrophotometer,

    Ca and Mg hollow cathode lamps.

    FAS section 1: The experiment was commenced with the preparation of

    250mL of a 10 ppm Ca (aq) solution from the stock solution of 1,000 ppm. This was

    done by diluting 2.5 mL of the stock solution with de-ionised water to the mark in a

    volumetric flask. 94.51 g of the Ca solution and 55.03 g of water were added to two

    separate plastic bottles. The sample tube was inserted in the Ca solution bottle and the

    drain tube in the water bottle. The flame was then lit. Aspiration and nebulisation

    were conducted for five minutes (blue flame), at the end of which the flame was shut

    off and the bottles were re-weighed at 70.34 g and 76.70 g for the Ca (aq) and water

    respectively. The sample uptake rate was calculated at 0.5 mL/min and the experiment

    was carried on.

    The next section involved the process for selecting the optimum burner height.

    The burner heights of 2, 2.5, 2.75, 3, 3.25, 3.5 and 4 were tried and for each height

    value, the mean detector response was noted for the 10 ppm Ca solution and the

    blank. The optimal burner height was deemed to be 3.25. During this aspiration, the

    flame was seen to change to red-orange colour.

    FAS section 2: The 1,000 ppm Ca and Mg stock solutions were used to

    prepare 100 mL solutions of 10 ppm concentration. This was done by diluting 1 mL

    extracts of the solutions with de-ionised water to the mark in separate volumetric

    flasks.

    The solutions were analysed and the emission intensities recorded; the

    scanning wavelengths were set from 1 nm less to 1 nm beyond the literature emission

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    wavelength for the particular species to be analysed: 421.5 nm! CaI! 423.5 nm,554.0 nm! CaOH! 556.0 nm and 284.2 nm! MgI! 286.2 nm; the scan rate was setat 1 nm/min. The next step was measuring the signal intensities of the CaI, CaII, MgI

    and the MgIIspecies at their respective wavelengths of 422.5 nm, 393.3 nm, 285.2

    nm and 279.5 nm respectively. The (H)ollow (C)athode (L)amp was turned on, set to

    the suitable current and the signals were recorded for CaIand MgIat their respective

    absorption line wavelengths.

    FAS section 3: Three 100 mL solutions of Ca and Mg of 10 ppm

    concentration each were prepared (as before). To one pair of Ca and Mg solutions,

    0.1 mL of 10,000 ppm K (aq) were added to bring its K (aq) concentration to 10 ppm;

    10 mL of the stock K (aq) solution were added to another pair to bring the K (aq)

    concentration to 1,000 ppm and no K (aq) was added to the third pair. The emission

    intensities of the Ca (CaI) solutions were measured and the absorption intensities of

    the Mg (MgI) solutions were measured. Four more 100 mL Ca (aq) of 10 ppm were

    prepared in the following manner. 10 mL of 0.01 mol/L H3PO4 was added to one 100

    mL volumetric flask, bringing its concentration to 0.01 mol/L in that Ca (aq), 10 mL

    of 0.01 mol/L H3PO4 + 10 mL of 10,000 ppm Sr2+

    to have 0.01 mol/L concentration

    of the acid and 1000 ppm of the Sr2+ (aq) was added to another, the same volume of

    the acid (and thus concentration) + 50 mL of 0.05 mol/L EDTA were added to the

    third flask, and nothing was added to the fourth flask; 1 mL of the stock Ca solution

    was added to each flask and were all diluted to the mark with de-ionised water. Their

    intensities were measured at the CaIline (422.5 nm). The presence of Sr (aq) caused

    the colour of the flame to become red, orange with EDTA.

    FAS section 4: The vessels used as the containers for the following are

    volumetric flasks. Ca (aq) solutions of 10, 5, 1, 0.1 and 0.01 ppm were prepared. The

    10 ppm solution was prepared as before; the 5 ppm one was prepared by diluting 0.5

    mL of the 1,000 ppm Ca solution to the 100 mL mark; the 1 ppm solution was

    prepared by diluting 10 mL of the 10 ppm solution to the mark; 0.1 ppm solution by

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    diluting 1 mL of the 1 ppm to the mark and so on and so forth for further lower

    concentrations. The unknown solution was prepared by diluting 25 mL of

    phosphoric acid + 2.5 mL of the 1,000 ppm Ca sol to the 250 mL mark with de-

    ionised water. The HCL lamp was turned off and the emission intensities of CaIwere

    measured using the Ca solutions prepared. Multiple readings were taken for each

    concentration and the averages recorded. The unknown was measured five times as

    well.

    25 mL aliquots of the unknown was made, to which 0, 0.2, 0.4, 0.6 and 0.8 mL

    of the 1,000 ppm Ca stock solution was added then diluted to the 50 mL mark. Their

    emission signals were also measured.

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    DATA AND RESULTS:

    !FAS section 1: -

    before nebulisation ! 10 ppm Ca (aq): 94.51 g ; water: 55.03 g

    after nebulisation ! 10 ppm Ca (aq): 70.34 g ; water: 76.70 g

    difference ! 10 ppm Ca (aq): (-)24.17 g ; water: (+) 21.67 g

    Ca (aq) utilised ! 24.17 g 21.67 g = 2.5 g

    density of water" 1 g/mL

    10 ppm Ca (aq) blank

    burner height mean signal # {noise} mean signal # {noise}

    2 0.963 0.001 1.109 0.001

    2.5 1.308 0.001 1.237 0.001

    2.75 1.018 0.002 0.856 0.001

    3 0.504 0.004 0.307 0.001

    3.25 0.384 0.002 0.144 0.0013.5 0.318 0.002 0.133 0.001

    4 0.254 0.001 0.087 0.001

    " Q1: rate of delivery of Ca atoms to the flame:

    24.17 mL of 10 ppm Ca (aq) " 24.17 mL #10 mg

    L 40.08 g

    mol#NA

    = 3.63#1018

    atoms delivered in 5 minutes

    =1.21#1016

    atoms / s

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    ! '!" Q2: Nebulisation efficiency:

    calcium utilised

    duration (min)=

    2.5g

    5"2.5mL

    5

    = 0.5gmin#1" 0.5mLmin#1

    "Q3, Q4: Signal:noise andsignal:backgroundratios and graphsCaIemission line

    burner height signal:noise signal:background

    2 963 0.868349865

    2.5 1308 1.057396928

    2.75 509 1.189252336

    3 126 1.641693811

    3.25 192 2.666666667

    3.5 159 2.390977444

    4 254 2.91954023

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    !FAS section 2: -CaIemission intensity MgIemission intensity CaOHemission intensity

    $(nm) signal $(nm) signal $(nm) signal

    421.5 -0.05 284.2 0.17 552 0.154

    421.6 -0.048 284.3 0.163 552.1 0.147421.7 -0.046 284.4 0.166 552.2 0.156

    421.8 -0.045 284.5 0.163 552.3 0.156

    421.9 -0.039 284.6 0.165 552.4 0.162

    422 -0.036 284.7 0.165 552.5 0.165

    422.1 -0.015 284.8 0.165 552.6 0.183

    422.2 0.094 284.9 0.17 552.7 0.193

    422.3 0.212 285 0.173 552.8 0.213

    422.4 0.317 285.1 0.182 552.9 0.246

    422.5 0.471 285.2 0.182 553 0.272

    422.6 0.578 285.3 0.183 553.1 0.303

    422.7 0.698 285.4 0.177 553.2 0.347

    422.8 0.728 285.5 0.174 553.3 0.379

    422.9 0.649 285.6 0.171 553.4 0.39

    423 0.517 285.7 0.163 553.5 0.408

    423.1 0.409 285.8 0.155 553.6 0.414

    423.2 0.293 285.9 0.151 553.7 0.438

    423.3 0.197 286 0.146 553.8 0.43

    286.1 0.147 553.9 0.422

    286.2 0.151 554 0.402

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    "Q1:

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    max emission intensity: atomic Ca > ionic Ca > atomic Mgaverage emission intensity: ionic Ca > atomic Ca > atomic Mg

    table ofnet emission signal strength

    net signal error error %CaI(422.5 nm) 0.001 0.000000 0.000

    CaII(393.3 nm) -0.060 0.002236 3.727

    MgI(285.2 nm) -0.310 0.002828 0.912

    MgII(279.5 nm) -0.290 0.002236 0.771

    table ofnet absorption signal strength fromHCL

    net signal error error %

    CaI(422.5 nm) 0.226 0.003162 1.399

    MgII(285.2 nm) -0.022 0.001414 6.427

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    !FAS section 3: -

    " Ionisation equilibria: -net emission signal strength for10 ppm Ca (aq)

    net signal error error %

    no K (aq) 0.001 0 0

    10 ppm K (aq) 0.529 0.004 0.756with

    100 ppm K (aq) 0.312 0.002 0.641

    net absorption signal strength for10 ppm Mg (aq)

    no K (aq) 0.31 0.002 0.645

    10 ppm K (aq) 0.415 0.001 0.241with

    100 ppm K (aq) -0.021 0.001 4.762

    " low volatility compounds: -net emission signal strength for10 ppm Ca (aq)

    net signal error error %

    no additives 0.001 0 0

    0.01 mol/L H3PO4 (aq) 0.312 0.002 0.641

    0.01 mol/L H3PO4 + 1000 ppm Sr

    2+

    0.206 0.003 1.456

    with

    0.01 mol/L H3PO4 + 0.05 mol/L EDTA 0.79 0.001 0.1265

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    !FAS section 4: -

    " Q1: -

    CaI emission intensity

    concentration (ppm) mean net emission intensity error error %

    10 0.693 0.003 0.433

    5 0.538 0.003 0.558

    1 0.145 0.002 1.379

    0.1 -0.001 0 0.000

    0.01 -0.001 0.001 -100.000

    lowest concentration values arent considered due to apparent erroneous readings

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    " Q2: -limit of detection =

    mean signal - lowest signal

    gradient of calib. curve

    =

    0.3437 " 0.145

    0.059=

    3.367ppm

    " Q3: -

    emission signal= 0.069concentration+ 0.065

    0.226 is the unknown signal

    0.226 = 0.059concentration+ 0.140

    concentration=1.457 ppm

    of the unknown

    error :"

    sc =sy

    m

    1

    M+

    1

    N+

    yc "y( )2

    m2Sxx

    =0.005

    0.059 1+

    1

    4+

    0.226" 12

    0.693" 0.145( )( )2

    0.0592# 40.67

    = 0.084 4

    3+("0.048)

    2

    0.145

    = 0.084 1.35 = 0.084 #1.162 = 0.097ppm

    concentration ofunknown: 1.457 0.097ppm

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    " Q4, Q5: -standard addition "unknown" emission intensity

    concentration (ppm) mean net emission intensity error error %

    0.8 0.403 0.003 0.744

    0.6 0.416 0.002 0.481

    0.4 0.535 0.002 0.3740.2 0.481 0.004 0.832

    0 0.226 0.001 0.442

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    DISCUSSION:

    " FAS 1:in the calculation of the nebulisation efficiency, plastic bottles were used and

    not glass. Plastic is lighter, easier to handle and wield and that it would still allow for

    mass measurements on analytical balances without overloading them.

    " FAS 1 Q5, Q6:The best S/N ratio is found at a lower burner height than the for the best S/B

    ratio. As the burner height is raised, the inter-zonal region is reached, which is the

    most efficient region for atomisation and contains the highest concentration.

    " FAS 2 Q6:Mg: from the negative emission intensities, it can be seen that absorption is

    more suited to Mg. The best signal is from the ionic emission since it has a higher net

    signal and a lower error margin.

    Ca: from the limited data, the absorption mode for atomic Ca provides the

    best signal! lowest error margin and a reasonable net signal strength.

    table ofnet emission signal strength

    net signal error error %CaI(422.5 nm) 0.001 0.000000 0.000

    CaII(393.3 nm) -0.060 0.002236 3.727

    MgI(285.2 nm) -0.310 0.002828 0.912

    MgII(279.5 nm) -0.290 0.002236 0.771

    table ofnet absorption signal strength fromHCL

    net signal error error %

    CaI(422.5 nm) 0.226 0.003162 1.399

    MgI(285.2 nm) -0.022 0.001414 6.427

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    " FAS 3 Q1 to Q5:

    Ionisation equilibria: -

    net emission signal strength for10 ppm Ca (aq)net signal error error %

    no K (aq) 0.001 0 0

    10 ppm K (aq) 0.529 0.004 0.756with

    100 ppm K (aq) 0.312 0.002 0.641

    net absorption signal strength for10 ppm Mg (aq)

    no K (aq) 0.31 0.002 0.645

    10 ppm K (aq) 0.415 0.001 0.241with

    100 ppm K (aq) -0.021 0.001 4.762

    low volatility compounds: -

    net emission signal strength for10 ppm Ca (aq)

    net signal error error %

    no additives 0.001 0 0

    0.01 mol/L H3PO4 (aq) 0.312 0.002 0.641

    0.01 mol/L H3PO4 + 1000 ppm Sr2+

    0.206 0.003 1.456with

    0.01 mol/L H3PO4 + 0.05 mol/L EDTA 0.79 0.001 0.1265

    K ionises more easily than Ca or Mg and so acts as an ionisation suppressor. It

    consumes more energy of the system, so Ca or Mg has less and that by its own

    ionisation, it releases electrons and inhibits the ionisation of Ca or Mg by Le

    Chatiliers principle. As the concentration rises even further, other factors such as

    energy loss due to inter-atomic collisions come into play, since there exist more

    particles to physically interact with each other.

    A hotter flame would raise the rate of decomposition and volatilisation of the

    analyte, and so artificially higher signal intensities may be observed.

    The greater the concentration of H3PO4, the higher the concentration of the

    compound CaHPO4 and consequently lesser the concentration of unbound Ca (the

    analyte). This particular substance does not break apart as easily in the flame, nor

    does it volatilise; hence the signal intensity is, in a way, doubly reduced.

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    Sr would react and bind with the HPO42-

    ion, thereby allowing for the Ca

    analyte to remain free. In other words, it would block the interfering acid and prevent

    a lesser signal intensity.

    EDTA forms volatile complexes with Ca thereby saving it from forming the

    non-volatile CaHPO4. Ca is eventually released.

    " FAS 4 Q5 to Q7:

    due to the non-linearity of the graph, further evaluation is not feasible.

    Due to likely experimental errors and the more cumbersome calculations in the

    evaluation of error margins with the standard addition method, the external standard

    method is deemed more favourable.

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    CONCLUSION:

    It is proven that due to the inherent characteristics of a flame, the best S/N is

    obtained at a different burner height than the best S/B. The subsequent data shows the

    absorption mode for atomic Ca and ionic Mg as the most reliable signals, though the

    data is limited and this cannot be satisfactorily proven. The interference section shows

    the hindering effect of phosphoric acid and the beneficial, preventative effect of

    strontium and EDTA in the Ca solutions destined for spectral analysis. Lastly, by the

    external standards method, the concentration of Ca (aq) in the unknown was 1.457

    0.097ppm ( 6.657 %).