Materials Characterization & Analysis (MCA)

molecular weight below 1000, advances in the techniques are continually

BMEn 5001

pushing this limit upwards...

Mass Spectroscopy ("Mass spec") probably more properly called: Mass Spectrometry

Use Qualitative If the compound is pure and reference spectra are available, identification can be unequivocal. Even without reference spectra are not available, identification can often Application

be made with a high degree of certainty.

Technique provides qualitative and quantitative infromation about organic compounds including ionics. Initially limited to compounds with BMEn5001, Class Notes 10b

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Unfortunately some compounds are fragile enough that the molecular ion is not observed. Higher molecular weight compounds may not be volatile enough to be analyzed by EI/MS.

Many other ionization techniques have been developed, such as Chemical Ionization, Field desorption, Laser Desorption, Fast Atom Bombardment... FAB one technique to ionize polar, non-volatile or thermally labile compounds Nature of sample (pure or ...) is important consideration.

of both low and high molecular weight.

For mixtures, hypenated techniques (GC-MS or LC-MS) are valuable.

Samples are dissolved in a matrix (one example, glycerol) and bombarded with "fast neutrals" - e. g. 5-10 KV Ar atoms. Molecules from

Principle:

the matrix are "sputtered" from the surface, the matrix molecules ionizing

Many variations are possible but three primary components are usually

first to generate both positive and negative ions. These matrix ions react

present:

with the analyte to form analyte ions, which can then be mass-analyzed.

a) a source where neutral molecules are converted to ions b) an analyzer component where ions are separated according to their

FAB/MS has been used industrially for characterization of surfactants,

mass to charge (m/z) ratio.

organic acids and salts, synthetic polymers and additives, aromatics and

c) a detector where an output signal is generated which is related to the

organometallics.

number of ions reaching the detector. MS-MS Most widely used ionization method: excitation in the vapor phase using

(The ultimate hyphenated technique?)

an electron beam: Electron Impact (EI) ionization. The excited molecule may undergo bond cleavage and fragmentation to yield ions which are characteristic of the molecule. Computerized reference libraries (50,000+ compounds) can aid in analysis.

Probably the most important peak is the so-called "molecular ion" (M+) from which the molecular weight of the material can be deduced. BMEn5001, Class Notes 10b

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Chromatography Gas chromatography

Application - Separation of complex volatile mixtures into individual components.

Principle materials are separated based on volatility (~boiling point) and interaction with a stationary phase in a chromatography column. The amount of interaction with the stationary phase is characteristic of each compound. The greater the interaction, the longer the retention time. A carrier gas is used to elute the compounds. As the compounds leave the column they are detected by suitable means. (Output peaks could go to a mass spec or ir detector or...) Readily adaptable to quantitative analysis. With proper calibration the area under the analyte curve can be related to its concentration in the original analyte sample. Problem: stuff must be volatile

High Pressure Liquid Chromatography (HPLC)

(can make "derivatives") Crudely speaking - instead of the carrier gas in GC, here a liquid is used...

Useful for non-volatile or thermally labile materials.

Quantitation as in GC.

Detectors are typically visible/uv or refractive index based -> attractive for general polymeric applications.

BMEn5001, Class Notes 10b

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Mult-dimensional detection for HPLC

BMEn5001, Class Notes 10b

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X-ray Diffraction Powder diffraction

technique used to obtain structural information on crystalline or polycrystalline material. Can be used to determine the presence of amorphous phases. Analysis of multicomponent systems are possible but difficult..

Use Obtaining the powder diffraction pattern of a crystalline component and comparing it (computer) with diffraction patterns of the JCPDS (Joint Committee on Powder Diffraction Standards) permits identification (in favorable circumstances) of the material.

Principle: A beam of monochromatic X-rays (copper, usually) is directed at a powder sample. The crystalline material diffracts X-rays according to Bragg's law (nλ = 2dsinθ). d is the spacing between crystallographic planes in the powder sample which are characteristic of the unit cell size and its contents. The intensities of reflection depend on the electron density of the scattering planes and thus lead to a unique set of diffraction peaks for each different crystalline compound.

BMEn5001, Class Notes 10b

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BMEn5001, Class Notes 10b

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Methods of thermal analysis

DSC allows these events to be monitored and can be done on samples

Differential Scanning Calorimetry (DSC)

in the mg range. Applications for the technique have been found in almost every material class examples including: glass transition, melting, solidification, decomposition, polymerization, gelation, curing, evaluation of processing, thermal, and mechanical history...

In a typical experiment a small sample of polymer (milligrams) is sealed in an aluminum pan and heated electrically. A temperature sensor is used for a feedback control of the heater in order to achieve a strictly constant rate of temperature increase. The electrical heat is measured precisely and it provides a record of the heat capacity of the sample as a function of temperature.

measures of heat capacity - depends on the nature of molecular movement large when many modes of movement are activated.

Usually the sample is observed twice.

First scan, a) around 0 C, water that is absorbed and adsorbed on the sample melts and an endotherm results. b) The next observable transition is usually the glass transition. Applications: used for studying the thermal behavior of materials as f(T) as they undergo physical or chemical transformations that absorb or evolve heat or with changes in heat capacity.

c) At 100 C water leaves yielding another endotherm. d) The next endotherm is the melting temperature. e) Samples that were incompletely polymerized will kick off (an exotherm). f) Further heating will lead to polymer decomposition - avoided on the

Many physical or chemical transformations are associated with heat

first run.

absorption (endothermic process) or heat being given off (exothermic). BMEn5001, Class Notes 10b

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A second scan may be easier to interpret because water and cure related phenomena may not be observed.

Thermogravimetric Analysis used to determine changes in sample weight as f(T).

Study thermal decomposition of inorganic, organic, and polymeric substnces Study oxidative degradation of polymers Determine moisture and volatile(s) contents of a sample Measure rates of evaporation and/or sublimation Study decomposition kinetics

Principle: A very sensitive balance is used to determine the change in sample weight as it is heated.

BMEn5001, Class Notes 10b

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