Background correction in Atomic Absorption Spectroscopy

“Tell me and I’ll forget : show me and I may remember; involve me and i’ll understand”

— Chinese Proverb

Before you move to background correction it is necessary to understand what is background absorption. The main reason for background absorption is presence of undissociated molecules of matrix that have broadband absorption spectra and tiny solid particles in the flame which may scatter light over a wide wavelength region. When this type of non-specific adsorption overlaps the atomic absorption wavelength of the analyte the ground state absorption is cut. The problem is overcome by measuring and subtracting the background absorption from the total of measured absorption to determine true atomic absorption component

Background Correction

A number of background correction approaches have been proposed but we shall limit our discussion to two main approaches which have found widespread application in commercial instruments

Continuuum Deuterium Source Background Correction

Continuum background correction measures and compensates for any background component present in atomic absorption measurements

D2 Lamp Background Correction Schematic

D2 Lamp Background Correction Schematic

The broadband contimuum source emits light over a broad spectrum of wavelengths instead of specific line.Spectral interferences which are caused by atoms with absorption lines very close to the analyte absorption line or by fine structure in the molecular absorption profile can result in either positive or negative errors in the measurement of the concentration of the analyte. Fortunately spectral interferences are rare in flame atomic absorption which is a distinct advantage over emission techniques

A common inexpensive technique for background correction in flame Atomic Absorption Spectroscopy is deuterium background correction. The correction is effective over the wavelength range of 180nm -420nm. Background level becomes significant at lower wavelength range.

The cathode lamp and the deuterium lamp are sequentially pulsed with a chopper or electronically with delay of about 2ms. When hollow cathode lamp is on and deuterium lamp off total absorbance (AA + BG) is measured. When the HCL is off and the deuterium lamp on the continuum energy recorded is (BG). The atomic signal is automatically calculated by subtracting background from total absorbance.

Limitations of Continuum D2 background correction :

  • The D2 lamp has a finite lifetime and requires periodic replacement
  • The D2 lamp and HCL light may not view the same portion of atom cloud in the flame due to time lag. This could become significant at high background levels.
  • Proper alignment of both light sources is required for right background correction
  • Correction is limited to the specific wavelength range of the two lamps
  • Intensities of both lamps if not similar will result in errors

Zeeman Background Correction

Zeeman Background Correction Schematic

Zeeman Background Correction Schematic

Zeeman Background Correction is used mainly in graphite furnace atomic absorption systems. When an atom is placed in a magnetic field and its absorption of observed in polarised light, the normal single line is split into three components – б-, π and б +displaced symmetrically about the normal position

Magnetic Field Off

Magnetic Field Off

Magnetic Field on

Magnetic Field on

Free atoms show Zeeman splitting in a magnetic field but molecules, liquid droplets or solid particles show no Zeeman splitting and so advantage can be taken of polarised light. The π component is linearly polarised parallel to the magnetic field while the б components are circularly polarised perpendicular to the magnetic field. A polariser is positioned in the optical system to remove the π components of the transmitted radiation. This affords background measurement at the exact analyte wavelength when magnetic field is applied. Since the background is measured at the analyte wavelength and not averaged as in D2 system structural molecular background and spectral interferences are easily corrected.

In AC modulated Zeeman system the combined atomic and background absorption is measured while magnetic field is off. When the magnetic field is on, the detector measures only the background absorption as the π component is not detected. The difference between the two is the Zeeman corrected atomic absorption signal

The magnetic field may be applied to the graphite tube with either transversally( perpendicular) or longitudinal ( parallel) to the optical axis. A major advantage of longitudinal Zeeman background correction is that the polariser is not needed to eliminate the π component, thereby providing higher light throughput by eliminating polarizer absorption

Advantages of Zeeman background correction :

  • Corrects high levels of background
  • Corrects at exact analytical absorption line
  • Requires only a single standard light source. Alignment problems of multiple light sources are not encountered

Limitations of Zeeman background correction :

  • More expensive than continuum background correction
  • Loss in sensitivity for some elements due to splitting of б and π components which may overlap

So far you have been introduced to the basic concepts and principles of Atomic Absorption Spectorscopy. The next module is a set of questions which you may face at time of interview when you apply for a suitable opening in a laboratory equipped with an Atomic Absorption Spectrometer.

About Dr. Deepak Bhanot

Dr Deepak Bhanot is a seasoned professional having nearly 30 years expertise beginning from sales and product support of analytical instruments. After completing his graduation and post graduation from Delhi University and IIT Delhi he went on to Loughborough University of Technology, UK for doctorate research in analytical chemistry. His mission is to develop training programs on analytical techniques and share his experiences with broad spectrum of users ranging from professionals engaged in analytical development and research as well as young enthusiasts fresh from academics who wish to embark upon a career in analytical industry.

Comments

  1. calbiration the standard of atomic

  2. maintainance the atomic

  3. Dear Mr. Deepak,

    When choosing a graphite furnace, how to determine if a Zeeman correction is necessary or if deuterium correction is sufficient.

    Thank you.

    Regrads

    • Hi Amgad,
      Zeeman background correction is a more expensive option but there are some situations when its use will provide higher sensitivities. It depends on the sample matrix.A matrix containing higher level of interfering element or a biological sample specimen which have more complex composition are such examples. For routine determinations deuterium background correction is sufficient.
      Thanks

  4. Dear Dr Deepak,
    Pl inform that either we can use air-natural gas flame instead of air-acetylene for sodium metal analysis.
    Regards,
    Muhammad Ayub

    • Dear Ayub,
      Sodium and other alkali metals get excited at lower temperatures than those reached when using acetylene-air . Conventionally in flame photometry natural gas -air mixtures have been used for emission studies but established validated methods on AAS for sodium estimation are based on acetylene-air mixtures so it is advisable to use recommended methods for authentic results.
      Regards

  5. abdalla says:

    I have problem in Deuterium lamp of AAS , it ‘snot work
    The electricity not reached the socket of Deuterium lamp(wires of Deuterium lamp) ?
    what I do it ? why the Deuterium lamp was stopped from work ?
    what the tips for this assure ?

    Best wishes

    abdalla mohammed

    eng. maintenance of equipment’s

    • Hi abdalla,
      It is possible that your lamp is not lighting up for the following possible reasons:
      1. Check on age of lamp.It has a shelf life.
      2.There is a problem with the socket or connector due to electrical contact loss.
      3.short circuit in the adaptor cable
      You could begin trouble shooting by first trying another lamp and see if it lights up and then investigate for the real cause.

  6. Sanaz Narimani says:

    Hi, my problem is this, after background correction by D2 lamp mean absorbance become very small or even minus. element that I try to determinate is antimony and I have lots of iron oxide and hydrochloric acid in matrix of sample. please guide me if it is possible. thanks a lot

    • In response to Sanaz
      Your sample matrix seems to be creating the problems. Try filtering the solution if you are able to see any suspensions of iron oxide. This may possibly provide the solution to your problem.

  7. Very superb visual appeal on this site, I’d rate it
    10.

  8. Sibasis Dhar says:

    Dear Dr. Bhanot,

    I am glad to come across such a huge open source resource for learning Atomic spectroscopy.
    Currently we are using graphite furnace atomic absorption with D2 background correction and encountered following problems :

    1)while running element Pb(EDL lamp) it prompts D2 lamp intensity too low but instrument is 6 months old.
    2)Signals shows considerable high as recommended by the system for a certain concentration.

    We are using matrix modifier ammonium dihydrogen phospahte in standard also.Is it suggested ?

    Please advice.

    Thanks.

    • Dear Sibasis,
      It is advised that the issue is first taken up with your supplier.Your instrument is still under warranty and if deuterium lamp needs replacement the supplier will replace it free of cost.

  9. Dear Dr.Deepak,
    I face a major problem when try to quantify the levels of zinc from serum samples, it is always low. i use a flame atomic spectrophotometer. could you suggest any method for

  10. Will this apply even to fuel oil analysis of metal?

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