Instant Notes: Analytical Chemistry

1/l=a(Z -b)^2

For example, the copper-Ka 1 line has a wavelength of 0.154 nm, and Z =29,

whereas for nickel the values are 0.166 nm and 28.

The absorption of X-rays must be considered, since certain elements may act

as filters for removing components of the X-ray spectrum, and also the sample

itself will absorb. The absorption of X-rays depends upon the thickness of pene-

tration into the target and a mass absorption coefficient determined by the

atomic number of the element and the wavelength of the X-rays. However, the

absorption does not follow a smooth curve, but shows a series of absorption

edgeswhich appear when the ionization energy for a K, L or M electron is

reached. This happens because more energy is absorbed in exciting the electrons

in the target. For nickel, the edge occurs at 0.148 nm, which means that a nickel

filter will absorb the copper Kblines, around 0.139 nm, strongly, but absorb the

copper Kalines at 0.154 nm very little. This is shown in Figure 1(c). In a complex

matrix, all the elements will contribute to the absorption.

Instrumentation Two major types of instrumentation are used for X-ray emission spectrometry.
These are illustrated in Figures 2(a)and (b).
In a dispersive instrument, the specimen is the target for bombardment by
high energy X-rays from the source, generally an X-ray tube containing a target
such as tungsten, onto which electrons are accelerated by a 50 kV potential
difference. These primary X-rays excite the specimen to produce X-rays

E6 – X-ray emission spectrometry 215

hn or e–

• 
  
  
  
  • 
    
    
    
    • 
      
    
    
    
    
  
  
  
  


• – –


• 
  


• ––


• –


• –


• –

+ M

'hole'

KL

M

KL

• 
  
  
  
  • 
    
    
    
    • 
      
    
    
    
    
  
  
  
  


• – –


• 
  


• ––


• –


• –


• –

e– +

(a) (b) hn

Fig. 1. (a) Excitation of an atom. (b) Relaxation and fluorescent emission of X-rays. (c) X-ray

emission spectrum of copper (full line) plus the X-ray absorption for nickel (dashed line).

Intensity

0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18

Wavelength (nm)

(c)