2.3. X-ray absorption & fluorescence

X-ray absorption is classical spectroscopy technique originating from

absorption line analysis of solar chromo-sphere spectrum

Technique has been utilized for a wide range from visible, UV and

X-ray light by absorption of light or X-ray spectrum in vapor.

Disadvantage: probe

must be chemically

prepared & vaporized

X-Ray Fluorescence X-ray fluorescence is based

on the excitation of material

& subsequent X-ray emission

- fluorescence -

bremsstrahlung

Excitation or

Ionization

by X-ray

bombardment

Ka

Ka

Kg

Kb

Only characteristic lines for material are emitted

Analysis of X-ray energies Advantage of X-ray fluorescence is the unique determination of

element characteristic X-ray radiation, Ka , Kb , or La , Lb, Lg lines

Critical component for X-ray fluorescence studies is the necessity

for high energy resolution in the X-ray spectrum to separate

energetically the characteristic X-ray lines of different elements.

Characteristic x-Ray Energies The characteristic X-ray energies

correspond to the energies of the

K or L transitions in the atom:

Ex=h·=h·c/=ΔE=En(i)-EK/L(f)

or it corresponds to the difference

of the absorption edge energies:

Ex =EK,L -En

Data for Pb; Z = 82

atomic weight = 207.210;

density = 11.3400

Absorption edges K-edge: 88.0060 keV

L-edges: 15.8600, 15.1980, 13.0350 keV

M-edge: 3.85000 keV

Characteristic energies Ka1,Kb1: 74.9570 84.9220 keV

La1,Lb1: 10.54900 12.6110 keV

Analyzing the strength of characteristic x-ray transitions gives the

abundance of element in sample: good energy resolution is required

The Lithium drifted Silicon

Si(Li) detector When an X-ray strikes the detector,

it will generate a photoelectron

within the body of the Si. As this

electron travels through the Si, it

generates electron-hole pairs. The

electrons and holes are attracted to

opposite ends of the detector with

the aid of a strong electric field.

The size of the current pulse thus

generated depends on the number

of electron-hole pairs created,

which in turn depends on the

energy of the incoming X-ray. Thus,

an X-ray spectrum can be acquired

giving information on the elemental

composition of the material under

examination.

Applications for XRF techniques

Non-destructive applications

Analysis of small amounts <100 mg of material

Determination of chemical content and structure

• ancient coin material - metal composition

• ancient oil and varnish material

• pigment in ancient paint

• medieval scripts and miniatures

Lucretius Trio

136 BC

Publius Clodius

42 BC

Marcus Antonius

31 BC

Roman Economy

0

20

40

60

80

100

120

-150 -100 -50 0 50 100 150 200 250

Per

cen

t C

om

po

siti

om

Year [AD]

Copper

Silver

The average of the measured Silver and Copper

content in the Roman Denarii as a function of time.

Clearly visible are the debasement ordered by

Octavianus Augustus and Nero, as well as the

continuing debasement of the Denarii during the

Severan period of the empire in the first half of the

third century.

The Andrea Guarneri Violin

The secret of the varnish of the violins made

around 1660 by Andrea Guarneri in Cremona

has been solved by XRF analysis of the varnish

contents - oil, resin, wax. Additional inorganic

substances influence color, hardness etc. The

varnishing technique of the Cremona violin

builders has been lost! The detailed XRF analysis

of all the contents:

• Provides the recipe for the varnish.

• Detects forgery

What kind of varnish does he use?

Violin varnish is applied with a brush in two or more coats. It is composed of sun-thickened oils such as

linseed or walnut, oxidized resins and added coloring matter. Good historic Italian varnish is highly

translucent and soft, yet friable. The surface has texture and a delicate patina, not a gloss. The colors should

be warm and rich.

20 different elements detected:

Fe, As, Pb, pigments in varnish

Zn,Cu,Pb drying substrate in oil

Total x-Ray Fluorescence

Reflector

Si(Li) detector

Sample Holder

X-ray beam

X-ray tube

slits

Alex von Bohlen, Historical Violin Varnishes Spectroscopic Studies and Characterization – Actes de la journee d`etude les

vernis de violin – Cite de la Musique -2006, pg. 60

Alex von Bohlen Friedrich Müller, Microanalysis of old violin varnishes by total-reflection X-ray fluorescence;

Spectrochimica Acta Part B: Atomic Spectroscopy 52, 1053–1056 (1997)

Characteristic X-ray energies

for varnish element K-transitionss

E Z eV

E eV eV

E eV eV

E eV eV

E

x

x

x

x

x

( ) . [ ]

( ) . [ ] . [ ]

( ) . [ ] . [ ]

( ) . [ ] . [ ]

1 136 11

2

15 136 11

22 29

19 136 11

2368

21 136 11

24 50

2

2

2

2

2

2

2

2

for S: Z = 16;

for Ca: Z = 20;

for Ti: Z = 22;

for Fe: Z = 26;

( ) . [ ] . [ ]

( ) . [ ] . [ ]

25 136 11

26 37

29 136 11

2858

2

2

2

2

eV eV

E eV eVxfor Zn: Z = 30; keV

keV

keV

keV

keV

Hair & Hemoglobin:

C3032H4816O872N780S8Fe4

For additional color, lakes are ground fine and stirred into the liquid varnish or applied as a

glaze between coats of varnish. Lakes are organic pigments made by precipitating natural

dyestuffs usually onto alum. Three common lakes are "robbia" made from madder root,

"verzino" made from brazilwood shavings and "carmina" made from cochineal. Grinding is

done with a glass muller over a sheet of roughened glass. The microscopic colored crystal lake

particles remain isolated and suspended within the varnish layer allowing light to freely

penetrate between the crystals, maintaining the translucency of the varnish. Yet, as light passes

through the varnish layer on its course to and from the wood, it is refracted in multiple

directions by the colored crystals, lighting up the varnish as if it were one

Varnish

Stradivari violin

varnish, ground layer and wood

A. Von Bohlen, F. Meyer; Spectrochimica Acta B 52 (1997) 1053

Heavy element components vary from violin

to violin, indicating that different secret

recipes have been used by violin makers!

Modern versus old varnish

Bogdan Constantinescu, Catalina Pauna, Daniela Stan, Authentication of ancient stringed musical instruments – the

contribution of X-Ray Fluorescence analysis 2011

K, Ca, Fe, Cu, Zn

Pb

Comparison between Violins Recipes

Fe2O3 in umber or ochre; As2S3 golden auripigmentum,

As4S4 red realgar; PbO yellow massicot, Pb3O4 red lead