Spectroscopy - Bryn Mawr College

download report

Transcript Spectroscopy - Bryn Mawr College

Investigating art
Why??
to help direct restoration
to help conservation
to aid interpretation
just to “know”
( see: “Is a painting ever done?” Ball, Time as Painter, Chapter 11)
Methods:
1. spectroscopy (“quantify interaction with light”)
2. chemical behavior ( “does it react?”)
3. microscopy (“visual with a microscope”)
lots
Tickling Out the Truth
Objectives for today:
Objectives for Tuesday:
Understand:
1. what is spectroscopy
Lab
1.
2. how it quantifies color
Measure visible spectra of
mauveine and
pseudomauveine
3. what is meant by light absorption,
reflectance, transmittance
4. how to generate a reflectance
(transmittance) spectrum from an
absorption spectrum and vis
versa.
5. how broad spectroscopy is, e.g.,
how infrared spectroscopy is
possible, UV.
little
2. Obtain Infrared spectrum
of carmine and of mauve
3.
Run ESI-MS on mauveine
and pseudomauveine
The Basics of Spectroscopy:
light source
sample
detector
the “black
box”
the brains for the box
A= e*b*c
lots
little
lots
little
Absorption Spectrum of Blue Filter
lots
little
Transmittance Spectrum of Blue Filter
Transmission
Absorption
light
color
“thrown away
(observed)
Reflection
Absorption
Transmission and Reflection effectively do the same thing: throw away unabsorbed colors
lots
lots
Transmission
Spectrum
little
Absorption
Spectrum
little
Our eyes are pretty good, but not as quantitative as a spectrum from spectroscopy
Example 1.
Shiraz wine
does look
different
from Chianti (!)
Shiraz
Chianti
 wavelength
550
600
650
700
750 nm
the fate of an open bottle of Shiraz:
Example 2.
Black Swan Shiraz, fresh:
530 nm
Shiraz does taste different when left open.
Black Swan Shiraz, open 12 days:
510 nm
Watercolor Pigment
Reflectance Curves:
How does
ultramarine blue
Reflectance curve for ultramarine blue watercolor
pigment differ from
phthalocyanine
blue?
Reflectance curve for phthalocyanine cyan
http://www.handprint.com/HP/WCL/color2.html#reflectance
© handprint.com, 2004, Bruce MacEvoy
Watercolor Pigment
Reflectance Curves:
Reflectance curve for ultramarine blue watercolor
How does
ultramarine blue
pigment differ from
A yellow pigment?
Reflectance curve for hansa yellow watercolor
http://www.handprint.com/HP/WCL/color2.html#reflectance
© handprint.com, 2004, Bruce MacEvoy
“Light” is a term that refers to electromagnetic radiation.
And electromagnetic radiation are waves of different energies that extend over a broad range:
frequency, Hz 10 26 10 24
cosmic
10 22
gamma
wavelength, nm 10 -8 10 -6
10 -4
10 20 10 18 10 16
X-rays
10 -2
1
10 14 10 12
UV
Vis
10 2
10 10
IR
10 4 10 6
10 8 10 6
Radio
10 8
10 4
induction
10 2 Hz
power
10 10 10 12 10 14 10 16 nm
If the electromagnetic spectrum were a piano keyboard…..
the visible spectral region would be just one key!
light source
sample
detector
interacts with light;
“removes” (absorbs)
some light components
X-rays
Interaction type:
Observed results:
UV
Vis
electronic
none
none
IR
vibration
color
heat
Chromate, CrO4 2“chromophore” in
chrome yellow
X-rays
Interaction type:
UV
Vis
electronic
Observed results:
none
X-ray
affects
inner
Electrons:
Higher energy
none
IR
vibration
color
UV/vis
affects
outer
electrons:
Lower energy
heat
X-rays
Interaction type:
UV
Vis
electronic
Observed results:
none
where in painting:
penetrates all
none
IR
vibration
color
surface
X-rays
X-rays interaction
strength depends
on number of electrons:
Pb >> Zn, Ti
Lead white scatters X-rays
more than zinc white or titanium white
or most other pigments
(except HgS or Ba-pigments)
heat
below paint
IR
wavelength selected
to interact most with
black of underdrawing
Infrared Spectroscopy
Instrumentation
An infrared spectrophotometer is composed of:
1) an IR light source, 2) a sample container, 3) a prism to separate light by wavelength, 4) a detector,
and a recorder (which produces the infrared spectrum).
SEE:
Alizarin
Carmine
Indigo
http://www.chemistry.ccsu.edu/glagovich/teaching/472/ir/instrumentation.html
Investigating the Renaissance
a Harvard site with examples of spectroscopic methods
Feast of the Gods
An analysis by WebExhibits
begin with historical context:
•when did artist/work occur
•workshop/atelier context and influences?
•location
•information on commission/motivation for work
Electrospray Ionization Mass Spectrometry ESI-MS
‘black box’
Molecules in gas phase as ions + or -
magnetic field
separates
light and heavy
mass ions
mass
detector
Electron impact ionization
Calculated mass of mauveine
C24H19N4 = 363 g/mol
24 C = 24 x 12 = 288
19 H = 19 x 1 = 19
= 363
Calculated mass of pseudomauveine
C26H23N4 = 391 g/mol
4N=
26 C = 26 x 12 = 312
4 x 14 =2356H = 23 x 1 = 23
4N=
4 x 14 = 56
= 391
Electrospray ionization method
makes tiny droplets
Vacuum removes solvent
Electric potential
creates + and - ions
All Mo molecules are identified by a characteristic series of lines:
Mo isotopes: 92Mo, 94Mo, 95Mo, 96Mo, 97Mo, 98Mo,