Analytical Techniques

Foundation for Advancement in Conservation


© 2008

Introduction

Conservators will find a seemingly endless number of analytical techniques in the conservation literature or may even have access to some of them themselves.

This tutorial covers the most common analytical techniques that are available to analytical chemists.

  • Spectroscopy
  • Chromatography
  • Scanning Electron Microscopy
  • The "X factors" (XRF, XRD, EDS)

Understanding analytical techniques requires a knowledge of basic atomic structure and chemical bonding. Organic Chemistry for Dummies by Arthur Winter is a useful information source.

Contents

This tutorial is divided into the following sections:

Please complete each section in order, as the information builds on that covered in previous sections. You can return to any section later.

Press Ctrl+Shift+F to search.

Learning Objectives

After completing this tutorial, you will be able to:

  • Describe the kind of information you can learn about a material through different analytical techniques
  • Understand how absorption of energy is key to gaining information from spectroscopic methods
  • Recognize that chromatographic techniques are used primarily for separating mixtures of compounds
  • Know which techniques destroy the sample and which do not
  • Differentiate among the uses for x-ray techniques

Foreword

Image of different graphs

First of all we should be clear that the actual test procedure is only one of a number of important steps.

  • Choosing the correct technique
  • Preparing the samples
  • Running the tests
  • Interpreting the results

Of all the steps the last one, interpreting the results, usually takes the longest and is by far the most difficult.

Spectroscopy

In this section you will be introduced to:

  • Ultraviolet/visible spectroscopy
  • Infrared spectroscopy
  • Raman spectroscopy
  • Mass spectroscopy

These techniques are used to find out what kind of chemical structure a material has and any changes to it after treatment or aging.

FTIR spectrum of copper(I) chloride by Izmaelt. FTIR spectrum showing modern beeswax compared to the filling material on a Neolithic human tooth, from Bernardini et al. (2012)

Spectroscopy: Electromagnetic Spectrum

First of all we need to remind ourselves about the electromagnetic spectrum. Energy travels in waves. Each wave contains a quanta (packet) of energy. As the waves get shorter the energy gets higher.

Diagram of electromagnetic spectrum, with short wavelengths and higher energy gamma rays on the left and long wavelengths and low energy alternating currents on the right

Spectroscopy: Electromagnetic Spectrum

A simple diagram of the electromagnetic spectrum

Diagram of the electromagnetic spectrum. Wavelengths shorter than visible light have enough energy to break chemical bonds. Infrared rays will not break bonds but will make them vibrate.

Ultraviolet/Visible Spectroscopy (UV/Vis)

Image of electromagnetic spectrum with ultraviolet rays highlighted

Spectroscopy: UV/Vis — Bond Energy

Long chain, organic molecules can have single, double and occasionally triple bonds. These all have different energy levels.

Chemical diagram of single and conjugated bonds

Alternating double and single bonds are called conjugated bonds. Energy increases from single, double, to triple bonds, but single bonds are much higher in energy than conjugated bonds.

Spectroscopy: UV/Vis — Conjugated Bonds

As the number of conjugated bonds increases their overall energy drops. Eventually, instead of having energy levels in the far/mid UV range, the molecule has energy levels in the near UV/visible light range. It can absorb UV or visible light and appears colored.

Chemical diagram of retinal

Retinal, a dark red molecule

Spectroscopy: UV/Vis — Obtaining Spectrum

UV-Vis spectroscopy is usually carried out on samples in the liquid phase.

Diagram of UV-Vis spectrometer. From the UV or Visible light source, light is split equally between the sample in solvent and solvent with no sample, called a blank. The blank spectrum is subtracted from the sample spectrum to give the UV or Visible light spectrum of the sample.

Spectroscopy: UV/Vis — Measuring Color Change

In conservation UV-Vis spectroscopy can by used to measure color change during aging or chemical changes such as increased conjugation or crosslinking.

UV-Vis range spectrometers can measure the 200–1000 nanometer range of the spectrum. Below 200 nanometers, oxygen in the air absorbs the light, making measurement more difficult.

Spectroscopy: UV/Vis — Sample Spectrum

Image of a UV-vis spectrum, with samples with n equals 3, n equals 4, and n equals 5 increasing in wavelength

\(n\) = number of conjugated bonds

As \(n\) increases, the spectra move to lower wavelengths (lower energy).

Spectroscopy: UV/Vis — Summary

You have learned that:

  • Molecules that have energy levels in the near UV/visible light range can absorb UV or visible light.
  • UV-Vis spectroscopy is usually carried out on samples in the liquid phase.
  • UV-Vis spectroscopy can be used to measure color change during aging or chemical changes such as increased conjugation or cross linking.

Spectroscopy: Infrared Spectroscopy (FTIR)

Diagram of electromagnetic spectrum with infrared rays highlighted

FTIR: Vibrations

This technique looks at the ways the atoms in a molecule move in relation to each other.

Although we draw diagrams of chemical bonds with rigid, straight lines,

C — C — C

It would be much better to imagine them joined with springs that allow the atoms to move (vibrate) in various ways.

Animated GIF of stretching vibrations. Symmetric vibrations move together, while asymmetric vibrations alternate

FTIR: Origins

The Fourier Transform in FTIR refers to the mathematics used to sort out the spectra into something that we can use.

Joseph Fourier was a French mathematician who died in 1830. Before his mathematics could be applied to scientific techniques, however, the microprocessor needed to be available. Fourier transform takes the original spectrum, which looks like a chaotic mess, and transforms it into its individual components. The usual analogy is of taking a musical chord and separating out the individual notes.

By the way, Joseph Fourier also showed that the atmosphere could contribute to the surface temperature of the Earth (Greenhouse Effect).

FTIR: Vibration Energy

Animated GIF of symmetric and asymmetric stretching vibrations, and the bending vibrations in-plane rocking, in-place scissoring, out-of-plane wagging, and out-of-plane twistingThe energy a molecule uses to vibrate is in the same part of the electromagnetic spectrum as infrared. Each different molecule vibration will use a slightly different level of energy.

FTIR: Energy Absorption

When a beam of infrared light is shone through a sample it causes the molecules to vibrate. These vibrations subtract some energy from the original beam.

By comparing the final beam to the original beam we can see how much energy has been absorbed by the molecule and in exactly which part of the spectrum.

Animated GIF of stretching vibrations. Symmetric vibrations move together, while asymmetric vibrations alternate

In a hydrocarbon, these vibrations would absorb energy at around the 350 × 10-6m (the near infrared range).

FTIR: Functional Groups

In addition to the different vibrations, the mass of the atoms and the strengths of the bonds will also affect the energy levels involved.

A strong bond like O — H where the oxygen atom is attached to the light hydrogen atom will vibrate at a higher frequency (energy) than a C — C bond.

All of this could lead to very complicated spectra. Luckily chemists realized that most functional groups within a molecule will vibrate at specific frequencies and these could be recognized in the IR spectrum.

FTIR: Hooke's Law

If we think of two bonded atoms as two vibrating masses joined by a spring then their frequency will be given by Hooke's Law.

A strong bond (high \(K\)) connected to a light mass will vibrate at a high frequency.

O — H and N — H absorb energy at 3600–3200 cm-1.

\(\bar{v}\) = frequency

\(c\) = speed of light

\(K\) = force constant

\(\mu\) = reduced mass


\( \bar{v} = \frac{1}{2 \pi c}\sqrt \frac{K}{\mu}\)

FTIR: Reduced Mass Calculation

Reduced Mass \(\mu\): This is calculated from:

\( \mu = \frac{m_1 \times m_2}{(m_1 + m_2)}\)

Where:

\(m_1\) = relative atomic mass of the first atom × 1.66 × 10-27 kg

\(m_2\) = relative atomic mass of the second atom × 1.66 × 10-27 kg

FTIR: Ethanol Spectrum

Here is the spectrum of ethanol showing the O — H, C — H, and C — O stretches. Note the units of measurement.

Image of FTIR spectrum of ethanol, with troughs at O-H stretch, C-H stretch, and C-O stretches

FTIR: Units of Measure

Infrared spectra are measured in cm-1. This is called wave number. Wave numbers are the reciprocal of the wavelength. (How many wavelengths would fit in one centimeter?)

\(\text{Wave number } = \frac{1}{\text{Wavelength}}\)

The scale on the \(x\) axis therefore runs:

Large wavenumbers are short wavelengths, small wavenumbers are long wavelengths.

FTIR: Frequency Table

Frequency tables for functional groups can be found in any textbook on organic spectroscopy or analytical techniques. These are taken from Interpreting Organic Spectra by D. Whittaker.


The cm-1 scale runs from 4000 to around 600.

Blank graph showing x-axis scale
Functional Group Absorption range (cm-1)
N—H 3500–3250
CON—H 3500–3000
O—H 3700—3200
COO—H 3200—2200
C—H 3100—2700
C=H 1850—1640
Aromatics 1600–1500

FTIR: Distinguishing Polymer Types

In this example of an FTIR spectrum, Lomax and Fisher (1990) used FTIR to distinguish between different methacrylate polymers. The different methacrylates show different peaks in the 950–100 cm-1 range.

Overlay of FTIR spectra of poly (iso-amyl methacrylate), poly (n-butyl methacrylate) and poly (iso-butyl methacrylate) increasing in absorption

FTIR: Summary

In this section you learned that:

  • Molecular vibration energy is in the infrared part of the electromagnetic spectrum.
  • Looking at how much energy has been absorbed by the molecule in which part of the spectrum helps to identify functional groups within a molecule.
  • Infrared spectra are measured in wave numbers.
  • Frequency tables for functional groups can be found in any textbook on organic spectroscopy.

Raman Spectroscopy

Diagram of electromagnetic spectrum with visible light range highlighted

Raman Spectroscopy: Background


Photo of Raman

Raman spectroscopy is named after Chandasekhara Venkata Raman, an Indian physicist who died in 1970.

In this technique, laser light is shone on, or through, the sample and one specific part of the scattered light is collected and analyzed.

Like infrared spectroscopy, the inter-molecular vibrations are studied to find the chemical structure.

Raman Spectroscopy: Background

Raman spectroscopy is a complimentary technique to infrared spectroscopy. It has also proved to be very useful for inorganic analysis such as paint or varnish identification.

Since Raman spectroscopy is not affected by water in the atmosphere and needs a very small sample (less than 1 µ), it can be used with a microscope (Raman microscopy).

Raman Spectroscopy: Diagram

Diagram of Raman spectroscopy. A laser is shone through the sample, scattering light onto a detector perpendicular to the beam

The much smaller amounts of Raman scattered light on either side of the main (Raleigh) light are the ones collected in Raman spectroscopy.

Raman Spectroscopy: Challenges

Historically Raman spectroscopy had some difficulties which made it the "poor sister" of infrared spectroscopy.

Because the amount of light collected is so small, an intense light source was necessary. This became much easier when lasers operating in the visible region became available.

Unfortunately many colored samples fluoresce in this range and the weak Raman scattering is lost under the much stronger fluorescence.

Raman Spectroscopy: Challenges

Infrared lasers overcame this problem but the intensity of the scattering decreases as the wavelength gets longer so detection was difficult.

This final problem was solved with the advent of the ubiquitous desktop computer which could do the Fourier transform calculations very quickly.

Raman spectrometers can now produce a single spectrum, made from several individual, co-added scans.

Raman Spectroscopy: Example

Here is an example of Raman spectra, in which Leona et al. (2004) compared the spectrum of pure indigo (a) with Maya Blue (b).

Image of Raman spectra of indigo and Maya Blue, with arrows pointing to two peaks present in indigo but absent in Maya Blue

Raman Spectroscopy: Summary

You learned that:

  • Laser light is shone on, or through, the sample and one specific part of the scattered light is collected and analyzed.
  • Inter-molecular vibrations are studied to find the chemical structure.
  • Raman spectroscopy is very useful for inorganic analysis such as paint or varnish identification.
  • The spectra are not affected by water in the atmosphere and need a very small sample to acquire.

Mass Spectroscopy

This technique is unlike the three spectrographic techniques we have looked at so far.

Mass spectra are produced by bombarding the sample with electrons so that it splits into smaller fragments or ions.

UV-Vis, FTIR, and Raman are regarded as non-destructive techniques and the sample can be rescued and used again.

Mass spectroscopy destroys the sample and therefore should be classed as a destructive technique. Mass spectroscopy is often coupled with another analytical technique such as gas chromatography.

Mass Spectrometry: Fragmentation

Diagram of electrons hitting a sample molecule M, knocking off an electron, turning M into a radical cation. The cation decomposes into pairs of fragments, which can also decompose. Only the ions and radical cations can be detected.

Samples must be in the vapor phase.

The sample molecule M is bombarded by high energy electrons. It loses an electron and becomes a radical cation.

Mass Spectroscopy: Radical Cation

By losing an electron (negative charge) the original molecule gains an overall positive charge (a cation).

Losing one electron from an electron pair means that an unpaired electron is left on the molecule (a radical).



M·+

Mass Spectroscopy: Spectrum

The fragments are accelerated into a magnetic field which separates them according to their mass. The fragments with the lighter mass are deflected more by the magnetic field than those with heavier mass. This has the effect of spreading out the fragments when they hit the detector.

Image of mass spectrum. The y-axis is a scale of the ratio of all the fragments relative to the most abundant fragment, and the fragments are listed by their mass on the x-axis.

Mass Spectroscopy: Mass to Charge

Specifically they are separated by the ratio between their mass \(m\) and the charge \(z\).

\(\frac{m}{z}\)

Since the charge is usually 1 they appear on the spectrum as mass.

Mass Spectrometer: Diagram

Diagram of mass spectrometer. The sample is introduced with an injector, the heater vaporizes the sample, the sample is ionized by an electron beam, it is accelerated into the magnetic field, turning 90 degrees under the magnet. The magnetic field separates the particles in the charged sample with the lightest particles deflected the most and the heaviest particles the least when they reach the detector.

Mass Spectroscopy: Decane Spectrum

Mass spectrum of decane, with different fragments highlighted at regular intervals. The most abundant fragments are C3H7 and C4H9, with only a small number of full C10H22 molecules remaining

Here we can see some of the fragments that split off from the whole molecule.

Mass Spectroscopy: Patterns

Molecules tend to split in certain, well defined ways. Long chain aliphatics, like the decane example, tend to lose methyl groups.

Carboxylic acids, CH3(CH2)nCOOH, will lose the acidic end group COOH.

Mass spectroscopy is very good for detecting functional groups. It can also be used for some elemental analysis.

Most of the calculation needed to identify fragments and from this to work out the identity of an unknown molecule is done by the computer. Scientists can buy "libraries" of mass spectra for comparison, or make their own by running examples of known chemicals.

Mass Spectroscopy: More Information

For anyone wishing to delve further into this technique (and gas chromatography) there is, at the moment at least, a tutorial managed by the University of Akron.

This site is copyrighted to James K. Hardy and the University of Akron. A link is provided on the web page for this tutorial. [Link is broken.]

Mass Spectrometry: Summary

In this section you learned that:

  • Mass spectra are produced by bombarding the sample with electrons so that it splits into smaller fragments or ions.
  • Mass spectroscopy destroys the sample.
  • The fragments are accelerated into a magnetic field which separates them according to their mass.
  • Molecular splitting follows particular patterns for different classes of organic compounds.

Chromatography

In this section you will learn about:

  • High Performance Liquid Chromatography (HPLC)
  • Gas Chromatography
  • Thin Layer Chromatography (TLC)

These techniques separate complex mixtures into their component parts.


Chromatograms of Bergamot orange using liquid chromatography from Gardana et al. (2008)

Chromatography: Definition (HPLC or GC)

Chromatography exploits the differences in attraction between the sample and the mobile phase and the sample and the stationary phase.

The sample (analyte) is injected into a gas or liquid mobile phase which is passed over a solid stationary phase that has been packed into a column. The sample molecules are adsorbed/desorbed onto the stationary phase as the mobile phase carries them down the column. The smaller the affinity the analyte has for the stationary phase, the faster it will move down the column.

Chromatography: Diagram of Liquid or Gas Column

Diagram showing first the column with the filled with the mobile phase (gas or liquid) and the stationary phase on the sides of the column. The mixed analyte begins at the top of the column, and moves downward. In this example there is red component and a smaller quantity of blue component. The blue component has a greater affinity for the stationary phase and travels more slowly down the column. The red component is eluted first followed by the blue component.

Chromatography: Resulting Chromatogram

The result of our simple experiment would look something like this.

Diagram of chromatogram with the first components to appear on the right and the last on the left. The amount of red component is indicated by a higher peak towards the right side of the graph, followed by the smaller peak for the blue component further to the left.

Chromatography: Gas Chromatography Example

In this example, Schilling et al. (1996) compared an unknown sample to the chromatogram of a known sample that they have run with the same mobile and stationary phases.

Chromatography: Thin Layer Chromatography (TLC)

This technique can be run with little or no dedicated equipment.

It is surprisingly accurate and is often used as a quick "look and see" method before more sophisticated analytical techniques are tried.

Conservators use it mostly to separate out the different components of dyes and inks.

TLC: Diagram of Set-up

Diagram of setup for thin layer chromatography, showing a glass container with a lid and a small quantity of solvent at the bottom, with a sample plate leaning against the side. The sample line is slightly above the solvent level.

TLC Supplies Description

The stationary phase is usually a thin layer of silica gel, aluminum oxide, or cellulose bound to an inert holder such as glass or plastic.

These are available at most chemical suppliers. The sample plates or plastic can be cut to size.

TLC Solvents

The choice of solvent, or solvent mixture, can be difficult. Usually a search of the conservation literature will tell you what other people have used.

A mixture of ethanol and water is a good starting point. For more advanced work the solvent classification list in Table 1 from Nyiredy, Sz; "Planar chromatographic method development using the PRISMA optimization system and flow charts" Journal of Chromatographic Science 2002, 40, 1–11 along with Figure 4 and the first part of the section "Selection of the individual solvents" should solve a lot of problems.

TLC: Photo of Set-up

A tiny amount of the sample (about the size of a period) is spotted near the bottom of the plate, which is then put in the solvent chamber. The solvent is pulled up through the plate by capillary action, taking the sample with it.

Photo of the separation of black ink on a TLC plate in ethanol and water, image by Natrij

The original sample of black ink has separated into its component dyes.

There is a piece of blotter soaked in solvent to help saturate the container with solvent vapor.

Notice the container, the lid, the solvent.

TLC: Marking the Chromatogram

Once the plate is removed from the container the solvent front is marked, before it dries and disappears, and the top edges of all the components.

Diagram of marking the chromatogram. The top edge of each component is marked, as well as the highest level the solvent reached (solvent front). Sometimes it is easier to see the components under UV light.

TLC: Comment on Marking Top Edge

Most, if not all, chemistry textbooks ask you to mark the center of the component rather than the top edge. This is fine if the component moves up the plate in a reasonably compact shape.

A lot of components will "smear" up the plate and it is difficult to decide where the center is.

TLC: Rf Values (Retardation Factor Values)

These are calculated for each component from the ratio of how far the component traveled versus the solvent front.

Diagram showing the distance from the sample line to the solvent front (a) and the distance from the sample line to the top edge of the blue component (b).


\(R_f = \frac{\text{Distance traveled by component}}{\text{Distance traveled by solvent}}\)

\(R_f \text{ of blue component } = \frac{b}{a}\)

TLC: Uses of Rf Values

Rf values are sensitive to:

  • The solvent system
  • The absorbent (grain size, water content, thickness)
  • The amount of material spotted
  • The temperature

There are no published tables of Rf values. Thin layer chromatography can be used as a:

  • Comparative test (comparing one ink to another)
  • Test of solubulity (seeing which component is most soluble in which solvent)
  • Quick method of determining how many components are present in an unknown substance

TLC: Example

Image from Schweppe (1979) of a polyamide sample plate with six dye materials separated into their components.

In this example Schweppe (1979) used thin layer chromatography to produce a reference chromatography of different natural yellow dyes. Notice that he has carefully listed the stationary phase (Polyamide) and the mobile phase (chloroform : methanol : butanone : formic acid 6:2:1:1).

Chromatography: Summary

In this section you learned that:

  • Chromatography exploits the differences in attraction between the sample and the mobile phase and the sample and the stationary phase.
  • While HPLC and GC require specialized equipment, TLC can be run with little or no dedicated equipment.

Scanning Electron Microscopy

In this section you will learn that:

  • Unlike optical microscopy, SEM uses a stream of electrons which interact with the surface of the sample.
  • The samples must be conductive in order to produce an image.
  • Samples are usually covered in a very thin layer of gold in a sputter coater.
  • Environmental SEM (ESEM) operates under a much lower vacuum and does not require gold coating the specimens.

Scanning Electron Microscopy (SEM): Definition

SEM image of spray dried polymer, image by Dimitra GScanning electron microscopy has become a very common examination technique in conservation research.

Unlike optical microscopy, SEM uses a stream of electrons which interact with the surface of the sample.

SEM: Cost

Nowadays a scanning electron microscope is comparatively inexpensive (around $6000 at the moment). The technical upkeep however is expensive and unremitting.

Scanning electron microscopes are usually available for loan or hire at most universities.

SEM: Description

A person loading a sample into a scanning electron microscope, image by Daniel Schwen.

Electrons are produced by heating tungsten at the top of the column.

They are accelerated down the column and focused on the sample.

The column and the sample chamber have to be kept under high vacuum so that the electrons are not stopped by atmospheric gas.

The motorized sample stage carries the sample into the chamber and positions it under the electron beam.

SEM: Sample Preparation

Image of sputter coater from NASA, 1999The samples must be conductive in order to produce an image. They are usually covered in a very thin layer of gold in a sputter coater.

Gold coating the specimens means that the sample can't be used for any other tests.

Some scanning electron microscopes, called environmental SEM (ESEM) operate under a much lower vacuum. In these systems it is not necessary to gold coat the specimens.

SEM: Diagram of Process

When the electrons strike the sample, several things can happen.

Diagram showing electron beam striking sample. Primary backscattered electrons are scattered more parallel to the beam, with secondary electrons and x-rays scattered at wider angles.

SEM: Sample Electron Micrograph

The primary backscattered electrons are used to determine different chemical areas or give information on crystalline areas. The x-rays are used for x-ray micro analysis (EDS). The secondary electrons are used to form the pictures we normally see in the literature.

SEM image of smooth uniform nylon fibers at 500x magnification. Image from Nicola Angeli at MUSE of Trento, Italy. SEM image of irregular 'collapsed tube' filter paper fibers at 500x magnification. Image from Nicola Angeli at MUSE of Trento, Italy. SEM images of synthetic nylon and cotton filter paper at 500x magnification.

SEM: Literature Example

SEM image of the surface of a brick from Kavenaugh and Wheeler (2003)

This is one of the many examples to be found in the conservation literature, Kavenaugh and Wheeler (2003) used SEM to examine the soiling of brickwork in a historic building.

Scanning Electron Microscopy: Summary

In this section you learned that:

  • Unlike optical microscopy, SEM uses a stream of electrons which interact with the surface of the sample.
  • The samples must be conductive in order to produce an image.
  • Samples are usually covered in a very thin layer of gold in a sputter coater.
  • Environmental SEM (ESEM) operates under a much lower vacuum and does not require gold coating the specimens.

The "X Factors" (XRF, XRD, EDS)

In this section you will learn about what I call "the X factor" techniques, which include:

  • X-Ray Fluorescence (XRF)
    (or Energy Dispersive X-Ray Fluorescence ED-XRF)
  • X-Ray Diffraction (XRD)
    (or X-Ray Powder Diffraction)
  • Scanning Electron Microscopy Energy Dispersive Spectroscopy (SEM-EDS)

All of these techniques use x-rays to determine the elements present in an unknown sample or to examine the crystalline structure.

The "X Factors": SEM-EDS

Let's look at the last technique first since energy dispersive spectroscopy (EDS) is usually coupled with scanning electron microscopy (SEM-EDS). The electron cloud around a nucleus is arranged in "shells". The closer the shells are to the nucleus, the lower the energy of the electrons.

Diagram of an incoming electron ejecting an electron, causing a higher-energy electron to fall and release x-ray energy.

An incoming high energy electron interacts with an electron in an inner shell and displaces it leaving an empty energy level in the shell.

An electron in an outer, higher energy shell moves to fill the energy gap. In order to stay there it must lose energy in the form of an x-ray.

The "X Factors": SEM-EDS

Image of an SEMS-EDS graph of anodized titanium foil with an SEM image superimposed. Image from thesis by Megan K. Gaye at the Naval Postgraduate School in Monterey, California.Since each element has its own, distinct set of energy levels, the identity of the element can be calculated from the energy of the emitted x-ray.

EDS has been used to examine the glazes on ceramics, elemental analysis of photographs, and to determine the mordants on textiles.

The "X Factors": X-Ray Fluorescence

In this technique the sample is bombarded with energy in the x-ray part of the electromagnetic spectrum. If it removes an electron from an inner, lower energy shell around a nucleus, another electron from a higher energy shell will move to fill the gap. When it does this, it emits a photon of excess energy. This event is called fluorescence.

Image of XRF graphs of early photographic prints by Hercule Florence, showing the highest peaks for argon, gold, iron, and cobalt, but no significant silver peak. Image from Thomas-Danguin et al. (2019)The energy level of the photon is characteristic of the element that emitted it.

XRF has been used for pigment identification.

X-Ray Diffraction and X-Ray Powder Diffraction

These techniques are used to find the crystalline structure of a material and from that the chemical composition.

They have been used to determine the different mineral contents of limestone, pigment, and ground analysis of paintings and the identification of unknown crystalline deposits on objects.

X-Ray Diffraction and X-Ray Powder Diffraction

X-ray diffraction and x-ray powder diffraction are essentially the same technique, and you will sometimes find the words used interchangeably in the conservation literature.

  • X-ray diffraction studies the scattering of x-rays from a single crystal.
  • X-ray powder diffraction studies the scattering of x-rays from a collection of small crystallites that are randomly ordered.

X-Ray Diffraction: Crystal Structure

The individual atoms in many of the elements form crystal structures, or cells. Although very few of them are "ideal" it is possible to work out even complex molecules from the cells.

Diagram of crystal structure of copper, a cube with an atom in the center of each face, and iron with an atom in the center of the cube.

X-Ray Diffraction References

There are 14 different unit cells, called Bravis lattices, and it is beyond the scope of this unit to go into this in any depth.

If you are interested in studying this subject there are several books available that describe basic crystallography, including X-Ray Diffraction by B.E. Warren; Dover: Mineola, NY, 1990 and X-Ray Diffraction: In Crystals, Imperfect Crystals, and Amorphous Bodies by A. Guinier; Dover: Mineola, NY, 1994.

X-Ray Diffraction Diagram

If an energy in the x-ray region of the electro-magnetic spectrum is shone on a group of crystals it will be diffracted from the crystal planes.

Diagram of an x-ray beam hitting a crystal structure and electrons being scattered.

X-Ray Diffraction Definition and Databases

Diffraction is the change in the direction and intensity of a group of waves (in this case x-rays) after passing by an obstacle or through an aperture (in this case the atoms on the sides of the crystal). The size of the obstacle has to be approximately the same as the energy's wavelength.

One of the largest databases (over half a million XRD patterns) is maintained by the International Centre for Diffraction Data (ICDD, www.icdd.com).

X-Ray Diffraction Example

In this example Rodriguez-Navarro et al. (1997) used powder XRD to determine the chemical composition of Egyptian limestone.

XRF graphs of a sample of clay, showing mainly calcite and dolomite, as well as sepiolite and quartz.

The "X Factors" (XRF, XRD, EDS)

In this section you learned about what I call "the X factor" techniques, which include:

  • X-Ray Fluorescence (XRF)
    (or Energy Dispersive X-Ray Fluorescence ED-XRF)
  • X-Ray Diffraction (XRD)
    (or X-Ray Powder Diffraction)
  • Scanning Electron Microscopy Energy Dispersive Spectroscopy (SEM-EDS)

All of these techniques use x-rays to determine the elements present in an unknown sample or to examine the crystalline structure.

References

These articles have been used as examples in the text and can be found at JAIC Online.

Kavenaugh, Claudia; Wheeler, George; "Evaluation of cleaning methods for the exterior brick at the Brooklyn Historical Society"; JAIC 2003, 42 (1). 97–112.

Leona, Marco; Casadio, Francesca; Bacci, Mauro; Piccolo, Marcello; "Identification of the Pre-Columbian pigment Maya blue on works of artby non-invasive UV-Vis and Raman spectroscopic techniques"; JAIC 2004, 43 (1). 39–54.

Lomax, Suzanne Quillen; Fisher, Sarah L.; "An investigation of the removability of naturally aged synthetic picture varnishes"; JAIC 1990, 29 (2). 181–191.

References cont.

Rodriguez-Navarro, Carlos; Hansen, Eric; Sebastian, Eduardo; Ginell, William S.; "The role of clays in the decay of ancient Egyptian limestone sculptures JAIC 1997, 36 (2). 151–163.

Schilling, Michael R.; Khanjian, Herant P.; "Gas Chromatographic analysis of amino acids as ethyl chloroformate derivatives"; JAIC 1996, 35 (2). 123–144.

Schweppe, Helmut; "Identification of dyes on old textiles"; JAIC 1979, 19 (1). 14–23.

Final thoughts

This tutorial has briefly covered some of the most common analytical techniques you will find in the literature. We have tried to link them to the kind of research carried out in conservation.

It is very difficult, if not impossible, however, to give you a definite "list" headed "Strong Points" and/or "Limitations" for each technique. They overlap each other in so many ways.

Final thoughts cont.

For example, if you wanted to know the elements in an unknown sample:

  • FTIR would give you some information
  • Raman might tell you more
  • Mass Spectroscopy (especially coupled with Gas Chromatography) would be useful
  • EDX would (possibly) be best

In the end it very much depends on the analytical techniques you have available. To be honest, this applies to a lot of scientific research and researchers will often tailor their experiments around the equipment to hand.

Credits

Researched and written by Sheila Fairbrass Siegler

Instructional Design by Cyrelle Gerson of Webucate Us

Project Management by Eric Pourchot

Special thanks to members of the Association of North American Graduate Programs in Conservation (ANAGPIC) and the AIC Board of Directors for reviewing these materials.

This project was conceived at a Directors Retreat organized by the Getty Conservation Institute and was developed with grant funding from the Getty Foundation.

Converted to HTML5 by Avery Bazemore, 2021

© 2008 Foundation for Advancement in Conservation