chem 454

Optical Atomic Spectra. Mostly Absorption

• A usage seems to be declining; nm favored
• 1 A = 10^-10 m
• 1 A= 0.1 nm

• as same as in vacuum, c
• (precise work needs a correction)

• 5890 and 5896 A
• why a doublet ?
  • electron spin
    • couples with 3p
      • change with energy
      • no change with 3s
  • two transition energies

Intensity of radiation follows Boltzmann expression

• N(3 p)/ N(3s) =3.0 * exp [-(hc/l)/(kT)]

• 1000 7.271E-11
• 1500 2.512E-07
• 2500 0.0001702
• 3000 0.0008682
• 3500 0.0027804
• 4000 0.0066564
• 5000 0.0225952
• 7500 0.1152704
• 2510 0.0001769
  • text: 10 degree temp change at 2500 K
  • produces 4% change in intensity
  • contrast: in absorption, signal is proportional to 3s population
    • 0.999829 at 2500 K
    • 0.9991328 at 3000 K
  • absorption is much less affected by temperature fluctuations

• population of excited state reduced
• need hotter source to excite

Line Width (of the emitted light)

• text... with 10 nsec lifetime
• line width is 1 x 10^-4 A (1x10^-5 nm)

• if molecule changes state on collision
• lifetime reduced to collision period
• not a problem at reduced pressures and lower temp
• worsens as pressure rises
• pressure broadening effect
• some- temperature effect
• (often it's every 5-20 collisions)

Doppler Broadening

• Dl/l = vel /c
• if vel =3 x 10³ m/sec
• Dl/l= 1 x 10-5
• at 589 nm thst's 0.006 nm
  • much more than natural line width
• as Temperature rises, velocity rises
  • (goes as square root of Temp);
• Can give rise to unusual effects
  • assume light source-is hot (broadened emission)
  • but surrounded by its own vapor (narrower absorption band)
  • emerging spectrum has a dip near the center of the band
    • also sample of cooler vapor
    • won't absorb as much light near the "wings"

Atom Sources

• (major player)

• mix with fuel / oxidizer / plasma gas
• nebulizer
  • pneumatic
  • ultrasonic

• (Occasional, specialized Player)

• As, Also Sn, Sb, Hi (Pb) as hydride
• Hg (as metal vapor)
• fed into flame as vapor

• heat a small "boat" in a flame (rare}

• Electrothermal (common)
  • rapid heating of graphite usually
  • graphite oven in AA

Flames

oxidizers:

• air (wastes energy heating N2)
• oxygen (may be harder to control than air flame)
• nitrous oxide (more energetic oxidizer than 02)

• burner premixes fuel and most of the oxidized
• flame travels 40 2000 m/sec
  • gas had better travel out just as fast
  • faster and the flame blows out
  • slower and torch backfires
  • each flame has a range of gas flow rates at which it -can form stable flames

• max temp around 1 cm (0.2-2 cm)
• cool below the cone (unburnt gas)
• cooler above the cone

• but, some species also oxidize in the falme
  • (get better signal right above the cone)
  • may heIp to have flame (oxygen poor with other elements
    • (max signal is higher)
• most burners have height adjustments

Flame AA

• set it up, introduce a sample
• vary height for max signal
• adjust nebulizer for maximum signal
• adjust fuel / oxidizer ratio (flame temperature)
• align lamp and burner for largest signal)
• or rely on typical settings, past experience

AA Instruments

• Flame (or Graphite Oven)
• Cold Cathode Lamp
  • need a supply of suitable lamps
  • best are single element
  • but mixed cathodes (alloys) are more economical
• Chopper
  • shift to lower noise frequency
  • can cancel out emission from flame itself
  • (may be single beam or double beam design)
• Sample Chamber (flame)
• Monochromator
  • mainly to select which line
  • instrument bandwidth >> line width
• Detector, usually a PMT
  • typically signal averaging and log computation

• Best analysis is typically + 1%

• How much of the absorbance signal is not atomic?

Deuterium Lamp Correction

• On alternate chopper cycles use D-lamp
  • UV continuum radiation
  • (correct signal for any absorbance of this light)
  • is quite effective when flame has small tendency to scatter light

Zeeman Effect

• Light is of the same frequency

• Some peaks split into two components at different freq
  • splitting is proportional to the field
  • For light of the desired polarization the absorption band changes
    • it is no longer absorbed by the samples
• Basically can turn on/off atomic absorption
  • what's due to AA and what's due to Artifacts?

Quick tricks

• most elements absorb at several wavelengths
  • typically to different final states
  • generally, much lower absorbance at these secondary wavelengths
  • can be used with much more concentrated solutions

• also, with concentrated solutions analyst often rotates burner by 90^o
  • reduces path length by 20-50 fold

• for standards and samples
• dilute as required
• dangerous for absorption of sample to lie
• beyond calibration samples
• calibration is often not linear at high end

What are the interferences?

• if so, select another line

Refractory Elements, Oxide Formation

• Most atom that form very stable oxides
• cannot be detected in cooler flames
• Al is example-- needs acetylene and N20 flame)
  • These can also cause light scattering when measuring other elements

Phosphates

• can produce extremely stable compounds
  • these do not dissociate well in the flame
  • (may be able to avoid by forming other complexes first-- treat with chelating agent)

Ionization

• Any ions that form decrease the population of atoms
  • (they have different absorption spectra)
• Easy to ionize alkali and alkaline earth elements
  • can form ions even in the cooler flames
• Matrix species (like K)
  • can affect degree of ionization of other components
  • text example in the text- Sr shows low response (loss to ions)
  • Add K and K⁺, e- suppresses Sr⁺ and Sr²⁺ formation
  • so Sr response is much higher

• These are part of reason for Standard Additions Method
  • But one can often reduce possible interferences
  • add complexing agents
  • add source of K⁺ (as above)

Atomic Fluorescence Spectroscopy (skip coverage)

• Author probably too involved in the field
• No commercial instruments
• Pretty good alternative instruments already exist
  • unless replacement is much better, easier to use, more reliable, cheaper
    • by a factor of 10 or more.
    • not likely to get established

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