Question

Q: Discuss why matrix effects can be important in AAS.

Q: How can I determine if matrix effects were significant in a sample (let say a tap water).

Answer 1

Background Correction Based on Zeeman Effect:

Zeeman has observed that when gaseous atoms (but not molecules) are placed in a strong magnetic field (~ 1 tesla), splitting of electronic energy levels takes place. The simplest splitting of one energy level results in three energy levels, one at a higher energy, another at a lower energy (two s satellite lines) and the third remains at the same energy as the level in absence of the magnetic field (central p line). Furthermore, the p line has twice the absorbance of a s line and absorbs polarized light parallel to direction of the magnetic field while the two s lines absorb light perpendicular to magnetic field.

Light from a HCL lamp will pass through a rotating polarizer that passes polarized light parallel to external magnetic field at one cycle and passes light perpendicular to field in the other cycle. The idea of background correction using this method is to allow light to traverse the sample in the graphite furnace atomizer and record the signal for both polarizer cycles using the wavelength at the p line.

a. First cycle: light parallel to field; the p line of the analyte absorbs in addition to absorbance by matrix (molecular matrix absorb both polarized light parallel or perpendicular to field)

Signal a = A_p + A_Matrix

b. Second cycle: light perpendicular to field; the p line of analyte will not absorb light perpendicular to field and s lines will also not affect absorbance at the p line wavelength. Only matrix will absorb.

Signal b = A_Matrix

The overall signal is the difference of the two signals = A_p

Therefore, excellent background correction is achieved using the Zeeman Effect.

Matrix effects are not considered for most method validations if they do not influence reproducibility or assay linearity. Since matrix effects can strongly suppress ionizaton efficiency and therefore reduce sensitivity, they must be evaluated (and discussed in the context of method development)—prior to method qualification. The severity of matrix effects is directly dependent upon chromatographic performance.