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Transcript of FAS report
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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 %).