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# Application of UV/Vis Spectroscopy Common Problems : a) Mixtures : - PowerPoint PPT Presentation

Application of UV/Vis Spectroscopy Common Problems : a) Mixtures : - blank samples often contain multiple absorbing species. - the absorbance is the sum of all the individual absorbencies A= A 1 + A 2 +A 3 + … = e 1 bc + e 2 bc + e 3 bc …

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Presentation Transcript

Common Problems:

a) Mixtures:

- blank samples often contain multiple absorbing species.

- the absorbance is the sum of all the individual absorbencies

A= A1 + A2 +A3 + … = e1bc + e2bc + e3bc …

- substances in both the blank and sample which absorb can be “blanked out” in both double and single beam spectrometers.

But, If the blank absorbance is high, Po will decrease too much, the response will be slow and the results inaccurate

If blank absorbance too high:

- dilute the sample

- use a different l where the analyte absorbs more relative to the interference.

- use a different method of separation

Large blank

absorbance

l scan of substance

b) Instrumental Deviations from Beer’s Law:

- polychromatic light (more then a single l)

‚since all instruments have a finite band pass, a range of l’s are sent through the sample.

‚e may be different for each l

Deviation’s from Beer’s law at high concentration

Let us say that exactly 2 wavelengths of light were entering the sample

l = 254 nm e254 =10,000

l = 255 nme255 = 5,000

let Po = 1 at both l’s

What happens to the Beer’s law plot as c increases?

A =A254 + A255

A = log Po/P (total) = log (Po254 + Po255)/(P254 + P255)

At the individual l’s:

A254 = e254bc = log Po254/P254

10ebc = Po254/P254

P254 = Po/10ebc = Po10-ebc

For both together:

A = log (Po254 + Po255)/(Po25410-e254bc + Po25510-e255bc)

Since Po254 = Po255 =1:

A = 2.0/(Po25410-e254bc + Po25510-e255bc)

A = 2.0/(10-10,000x1.0xc + 10-5000x1.0xc)

The results are the same for more l’s of light. The situation is worse for greater differences in e’s (side of absorption peak, broad bandpass)

Always need to do calibration curve! Can not assume linearity outside the range of linearity curve!

b) Chemical Deviations from Beer’s Law:

- Molar absorptivity change in solutions more concentrated than 0.01M

‚due to molecular interactions

‚Beer’s law assumes species are independent

‚electrolytes may also cause this problem

‚e is also affected by the index of refraction

- association, dissociation, precipitation or reaction of analyte

‚c in Beer’s law is the concentration of the absorbing species

‚commonly use the analytical concentration – concentration of all forms of the species.

Ka

phenolphthalein:

H+ + In-

HIn

Red, l =600nm

colorless

If solution is buffered, then pH is constant and [HIn] is related to absorbance.

But, if unbuffered solution, equilibrium will shift depending on total analyte concentration

Ka

example: if Ka = 10-4

H+ + In-

HIn

“Apparent” deviation since can be accounted for by chemical equilibrium

But, if unbuffered solution, equilibrium will shift depending on total analyte concentration

Ka

example: if Ka = 10-4

H+ + In-

HIn

Isosbestic point

At the isosbestic point in spectra:

A = eb([HIn] + [In-])

c) depending on total analyte concentrationNon-constant b:

- worse for round cuvettes

- use parallel cuvettes to help

B2

P2

P0

B1

P1

A = log10 Po/P = ebc

d) depending on total analyte concentrationInstrument noise:

noise - short term baseline fluctuations, which decrease the precision of the analysis

‚can not measure A precisely

‚the various sources of noise each cause some uncertainty

in the absorbance measurement and can be treated as individual standard deviations.

1) 0%T noise:

- noise when light beam is blocked

- seldom important

- typically ±0.01%T

- especially with a meter

- typically ±0.5%T  1-3% error in concentration

3) Shot Noise:

- occurs when e- transfers across a junction (like the space between cathode & anode in PMT).

- causes random fluctuations in current since individual e- arrive at random times

- increases with increase current (%T). Especially bad above 95%T.

4) depending on total analyte concentrationFlicker Noise:

- noise from the lamp due to intensity changes

- important at high transmittances.

5) Cell positioning uncertainty:

- not really noise, but affects precision

- minor imperfections, scratches or dirt change %T

- may be the major cause of imprecision

Taken together, these noise sources depending on total analyte concentration

indicate that the intermediate absorbance

and transmittance ranges should be used.

- at low %T, 0%T, noise and readout precision

are important

- at high %T, shot and flicker noise are large.

Keep A in range of 0.1 – 1.5 absorbance units (80 -3%T)

Applications depending on total analyte concentration:

A) Molar Absorptivities (e) in UV-Vis Range:

e=8.7 x 1019 PA

P – transition probability (ranges from 0.1 to 1, for likely transitions)

A – cross-section area of target molecule (cm2)

- ~10-15 cm2 for typical organics

- emax = 104 to 105 L/mol-cm

- e < 103 – low intensity (P £ 0.01)

- For Compounds with Multiple Chromophores: depending on total analyte concentration

‚If greater then one single bond apart

- l constant

CH3CH2CH2CH=CH2lmax= 184 emax = ~10,000

CH2=CHCH2CH2CH=CH2lmax=185 emax = ~20,000

‚If conjugated

- shifts to higher l’s (red shift)

H2C=CHCH=CH2lmax=217 emax = ~21,000

Example 6: depending on total analyte concentrationThe equilibrium constant for the conjugate acid-base pair

HIn + H2O H3O+ + In-

Calculate the absorbance at 430 nm for an indicator concentration of 3.00x10-4 M

K = 8.00x10-5

e = 0.755x103

e = 8.04x103

Applications depending on total analyte concentration:

B) Absorbing Species in UV/Vis:

1)Electronic transitions involving organic compounds, inorganic compounds, complexes, etc.

Basic process:

M + hn M*

10-8 – 10-9s

M*  M + heat

(or fluorescence, light, or phosphorescence)

or

10-8 – 10-9s

M*  N

(new species, photochemical reaction)

Note: excited state (M*) is generally short and heat produced not generally measurable.

Thus, get minimal disturbance of systems (assuming no photochemical reaction)

2) depending on total analyte concentrationAbsorption occurs with bonding electrons.

- E(l) required differs with type of bonding electron.

- UV-Vis absorption gives some information on bonding electrons (functional groups in a compound.

- Most organic spectra are complex

‚electronic and vibration transitions superimposed

‚detailed theoretical analysis not possible, but semi-quantitative or qualitative analysis of types of bonds is possible.

‚ effects of solvent & molecular details complicate comparison

- Single bonds usually too depending on total analyte concentrationhigh excitation energy for most instruments (£185 nm)

‚vacuum UV

‚most compounds of atmosphere absorb in this range, so difficult to work with.

‚usually concerned with functional groups with relatively low excitation energies (190 £l £850 nm).

- Types of electron transitions:

i) s, p, n electrons

Sigma (s) – single bond electron

Low energy bonding orbital

High energy anti-bonding orbital

Pi ( depending on total analyte concentrationp) – double bond electron

Low energy bonding orbital

High energy anti-bonding orbital

Non-bonding electrons (n): don’t take part in any bonds,

neutral energy level.

Example:Formaldehyde

depending on total analyte concentrations s* transition in vacuum UV

‚n s* saturated compounds with non-bonding electrons

l ~ 150-250 nm

e ~ 100-3000 ( not strong)

‚n p*, p  p* requires unsaturated functional groups (eq. double bonds)

most commonly used, energy good range for UV/Vis

l ~ 200 - 700 nm

n p* : e ~ 10-100

p  p*: e ~ 1000 – 10,000

Absorption Characteristics of Some Common Chromophores depending on total analyte concentration

Other Examples of Some Common Chromophores depending on total analyte concentration

ii) d/f electrons (transition metal ions) depending on total analyte concentration

‚ Lanthanide and actinide series

- electronic transition of 4f & 5f electrons

- generally sharp, well-defined bands not affected by associated ligands

‚ 1st and 2nd transition metal series

- electronic transition of 3d & 4d electrons

Crystal-Field Theory

• In absence of external field d-orbitals are identical

• Energies of d-orbitals in solution are not identical

• Absorption involves e- transition between d-

orbitals

• In complex, all orbitals increase in energy where

orbitals along bonding axis are destabilized

Magnitude of depending on total analyte concentrationD depends on:

- charge on metal ion

- position in periodic table

- ligand field strength :

I- < Br- < Cl- < F- < OH- < C2O42- ~ H2O < SCN- < NH3

< ethylenediamine < o-phenanthroline < NO2- < CN-

D increases with increasing field strength, so wavelength decreases

iii) Charge Transfer Complexes depending on total analyte concentration

‚ Important analytically because of large e (> 10,000)

‚Absorption of radiation involves transfer of e- from the donor to orbital associated with acceptor

- excited state is product of pseudo oxidation/reduction process

‚ Many inorganic complexes of electron donor (usually organic)

& electron acceptor (usually metal)

- examples: Iron III thiocyanate

Iron II phenanthroline

(colorless)

(deep red color)

C) depending on total analyte concentrationQualitative Analysis:

1)Limited since few resolved peaks

- unambiguous identification not usually possible.

2) Solvent can affect position and shape of curve.

- polar solvents broaden out peaks, eliminates fine structure.

Loss of fine structure for acetaldehyde when transfer to solvent from gas phase

Also need to consider absorbance of solvent.

2) Solvent can affect position and shape of curve. depending on total analyte concentration

- polar solvents broaden out peaks, eliminates fine structure.

(a) Vapor

Loss of fine structure for 1,2,4,5-tetrazine as solvent polarity increases

(b) Hexane solution

(c) Aqueous

3) Solvent can also absorb in UV-vis spectrum. depending on total analyte concentration

3) Can obtain some functional group information for certain types of compounds..

- weak band at 280-290 nm that is shifted to shorter l’s with an increase in polarity (solvent) implies a carbonyl group.

‚acetone:

in hexane, max = 279 nm ( = 15)

in water, max = 264.5 nm

- solvent effects due to stabilization or destabilization of ground or excited states, changing the energy gap.

‚since most transitions result in an excited state that is more polar than the ground state

- 260 nm with some fine structure implies an aromatic ring.

Benzene in heptane

More complex ring systems shift to higher l’s (red shift) similar to conjugated alkenes

UV Spectral Nomenclature types of compounds..

C) types of compounds..Quantitative Analysis (Beer’s Law):

1)Widely used for Quantitative Analysis Characterization

- wide range of applications (organic & inorganic)

- limit of detection  10-4 to 10-5 M (10-6 to 10-7M; current)

- moderate to high selectivity

- typical accuracy of 1-3% ( can be ~0.1%)

- easy to perform, cheap

2) Strategies

a) absorbing species

- detect both organic and inorganic compounds containing any of these species (all the previous examples)

b) non- absorbing species types of compounds..

- react with reagent that forms colored product

- can also use for absorbing species to lower limit of detection

- items to consider:

l, pH, temperature, ionic strength

- prepare standard curve (match standards and samples as much as possible)

Non-absorbing

Species (colorless)

reagent

(colorless)

Complex

(red)

When all the protein is bound to Fe3+,

no further increase in absorbance.

As Fe3+ continues to bind protein

red color and absorbance increases.

Standard Addition Method (spiking the sample) types of compounds..

- used for analytes in a complex matrix where interferences in the UV/Vis for the analyte will occur: i.e. blood, sediment, human serum, etc..

- Method:

(1) Prepare several identical aliquots, Vx, of the unknown sample.

(2) Add a variable volume, Vs, of a standard solution of known concentration, cs, to each unknown aliquot.

Note: This method assumes a linear relationship between instrument response and sample concentration.

(3) Dilute each solution to an equal volume, V types of compounds..t.

(4) Make instrumental measurements of each sample to get an instrument response, IR.

(5) Calculate unknown concentration, types of compounds..cx, from the following equation.

Note: assumes a linear relationship between instrument response and sample concentration.

Where:

S = signal or instrument response

k =proportionality constant

cs =concentration of the standard

Vx =volume of the sample aliquot

cx =concentration of the sample

Vt =total volume of diluted solutions

c) Analysis of Mixtures types of compounds..

- use two different l’s with different e’s

A1 = e1MbcM + e1NbcN(l1)

A2 = e2MbcM + e2NbcN(l2)

Note: need to solve simultaneous equations

d) Photometric titration types of compounds..

-can measure titration with UV-vis spectroscopy.

- requires the analyte (A), titrant (T) or titration product (P) absorbs radiation

Example 7: types of compounds..Given:

Calculate the concentrations of A and B in solutions that yielded an absorbance of 0.439 at 475 nm and 1.025 at 700 nm in a 2.50-cm cell.