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FLUORIMETRY

A fluorometerorfluorimeteris a device used to measure parameters offluorescence: its intensity

andwavelengthdistribution ofemission spectrumafterexcitationby a certain spectrum of light.[1]

These

parameters are used to identify the presence and the amount of specific molecules in a medium. Modern

fluorometers are capable of detecting fluorescent molecule concentrations as low as 1 part per trillion.

TYPES OF FLUORIMETRY

TWO TYPES PF FLUORIMETRY

Filter fluorometer

Spectrofluorometer

FILTER FLUORIMETRY

A filter fluorometer will use filters while a spectrofluorometer will use grating monochromators. Filter

fluorometers are often purchased/built at a lower cost but are less sensitive and less resolute than

spectrofluorometers.

Images ofspectrofluorometer

SPECTROFLUORIMETRY

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The spectrofluorometeris an instrument which takes advantage of fluorescent properties of somecompounds in order to provide information regarding their concentration and chemical environment ina sample. A certain excitation wavelength is selected, and the emission is observed either at a singlewavelength or a scan is performed to record the intensity versus wavelength also called an emissionspectra.

http://en.wikipedia.org/wiki/Spectrofluorometer

http://en.wikipedia.org/wiki/Fluorometer#Fluorometer_types

Differential scanning calorimetry

Differential scanning calorimetry orDSC is athermoanalyticaltechnique in which the difference in the

amount ofheatrequired to increase thetemperatureof a sample and reference is measured as a function

of temperature. Both the sample and reference are maintained at nearly the same temperature

throughout the experiment. Generally, the temperature program for a DSC analysis is designed such that

the sample holder temperature increases linearly as a function of time. The reference sample should

have a well-definedheat capacityover the range of temperatures to be scanned.

The technique was developed by E.S. Watson and M.J. O'Neill in 1962,[1]

and introduced commercially at

the 1963 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy. The

firstadiabaticdifferential scanning calorimeter that could be used in biochemistry was developed by P.L.

Privalov and D.R. Monaselidze in 1964.[2]

The term DSC was coined to describe this instrument which

measures energy directly and allows precise measurements of heat capacity.

Differential Scanning Calorimeter

Principle of Operation

When a sample undergoes a physical transformation such as aphase transition, more

or less heat will need to flow to it than to the reference (typically an empty sample

pan) to maintain both at the same temperature. Whether more or less heat must flow to

the sample depends on whether the process isexothermicorendothermic. For

example, as a solid sample melts to a liquid it will require more heat flowing to the

sample to increase its temperature at the same rate as the reference. This is due to the

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absorption of heat by the sample as it undergoes the endothermic phase transition

from solid to liquid. Likewise, as the sample undergoes exothermic processes (such

ascrystallization) less heat is required to raise the sample temperature. By observing

the difference in heat flow between the sample and reference, differential scanning

calorimeters are able to measure the amount of heat absorbed or released during such

transitions. DSC may also be used to observe more subtle phase changes, suchasglass transitions.

Applications

DSC is commonly used to measure a variety of properties in both organic and

inorganic materials, from metals and simple compounds to polymers and

pharmaceuticals. The properties measured include:

Glass transitions

Phase changes

Melting

Crystallization

Product stability

Cure / cure kinetics

Oxidative stability

Heat capacity and heat of fusion measurements

Detection of phase transitions [edit]

The basic principle underlying this technique is that when the sample undergoes a physical

transformation such asphase transitions, more or less heat will need to flow to it than the reference to

maintain both at the same temperature. Whether less or more heat must flow to the sample depends on

whether the process isexothermicorendothermic. For example, as a solid samplemeltsto a liquid it will

require more heat flowing to the sample to increase its temperature at the same rate as the reference.

This is due to the absorption of heat by the sample as it undergoes the endothermic phase transitionfrom

solid to liquid. Likewise, as the sample undergoes exothermic processes (such as crystallization) less

heat is required to raise the sample temperature. By observing the difference in heat flow between the

sample and reference, differential scanningcalorimetersare able to measure the amount of heat

absorbed or released during such transitions. DSC may also be used to observe more subtle physical

changes, such asglass transitions. It is widely used in industrial settings as a quality control instrument

due to its applicability in evaluating sample purity and for studying polymer curing.[4][5][6]

DTA [edit]

An alternative technique, which shares much in common with DSC, isdifferential thermal analysis(DTA).

In this technique it is the heat flow to the sample and reference that remains the same rather than the

temperature. When the sample and reference are heated identically, phase changes and other thermal

processes cause a difference in temperature between the sample and reference. Both DSC and DTA

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provide similar information. DSC measures the energy required to keep both the reference and the

sample at the same temperature whereas DTA measures the difference in temperature between the

sample and the reference when they are both put under the same heat.

DSC curves [edit]

The result of a DSC experiment is a curve of heat flux versus temperature or versus time. There are two

different conventions: exothermic reactions in the sample shown with a positive or negative peak,

depending on the kind of technology used in the experiment. This curve can be used to

calculateenthalpies of transitions. This is done by integrating the peak corresponding to a given

transition. It can be shown that the enthalpy of transition can be expressed using the following equation:

where is the enthalpy of transition, is the calorimetric constant, and is the area under the

curve. The calorimetric constant will vary from instrument to instrument, and can be determined by

analyzing a well-characterized sample with known enthalpies of transition.[5]

Applications [edit]

Differential scanning calorimetry can be used to measure a number of characteristic properties of a

sample. Using this technique it is possible to observefusionandcrystallizationevents as well asglass

transitiontemperatures Tg. DSC can also be used to studyoxidation, as well as other chemical

reactions.[4][5][6][7]

Glass transitionsmay occur as the temperature of anamorphoussolid is increased. These transitions

appear as a step in the baseline of the recorded DSC signal. This is due to the sample undergoing

achange in heat capacity; no formal phase change occurs.[4][6]

As the temperature increases, an amorphous solid will become lessviscous. At some point the moleculesmay obtain enough freedom of motion to spontaneously arrange themselves into a crystalline form. This

is known as thecrystallization temperature(Tc). This transition from amorphous solid to crystalline solid is

an exothermic process, and results in a peak in the DSC signal. As the temperature increases the sample

eventually reaches its melting temperature (Tm). The melting process results in an endothermic peak in

the DSC curve. The ability to determinetransition temperaturesandenthalpiesmakes DSC a valuable

tool in producingphase diagramsfor various chemical systems.[4]

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Top: A schematic DSC curve of amount of energy input (y) required to maintain each temperature (x), scanned across a

range of temperatures. Bottom: Normalized curves setting the initial heat capacity as the reference. Buffer-buffer baseline

(dashed) and protein-buffer variance (solid).

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Normalized DSC curves using the baseline as the reference (left), and fractions of each conformational state (y) existing at

each temperature (right), for two-state (top), and three-state (bottom) proteins. Note the minuscule broadening in the peak of

the three-state protein's DSC curve, which may or may not appear statistically significant to the naked eye.

Examples [edit]

The technique is widely used across a range of applications, both as a routine quality test and as a

research tool. The equipment is easy to calibrate, using low meltingindiumat 156.5985 C for example,

and is a rapid and reliable method of thermal analysis.

Polymers [edit]

DSC is used widely for examining polymeric materials to determine their thermal transitions. The

observed thermal transitions can be utilized to compare materials, however, the transitions do not

uniquely identify composition. Composition of unknown materials may be completed using a technique

such as IR.Melting pointsandglass transition temperaturesfor mostpolymersare available from

standard compilations, and the method can showpolymer degradationby the lowering of the expected

melting point, Tm, for example. Tm depends on themolecular weightof the polymer and thermal history,

so lower grades may have lower melting points than expected. The percent crystalline content of a

polymer can be estimated from the crystallization/melting peaks of the DSC graph as reference heats of

fusion can be found in the literature.[8]

DSC can also be used to study thermal degradation of polymers

using an approach such as Oxidative Onset Temperature/Time (OOT), however, the user risks

contamination of the DSC cell, which can be problematic.Thermogravimetric Analysis(TGA) may be

more useful for decomposition behavior determination. Impurities in polymers can be determined by

examining thermograms for anomalous peaks, andplasticiserscan be detected at their characteristic

boiling points. In addition, examination of minor events in first heat thermal analysis data can be useful as

these apparently "anomalous peaks" can in fact also be representative of process or storage thermal

history of the material or polymer physical aging. Comparison of first and second heat data collected at

consistent heating rates can allow the analyst to learn about both polymer processing history and material

properties.

Liquid crystals [edit]

DSC is used in the study ofliquid crystals. As some forms of matter go from solid to liquid they go through

a third state, which displays properties of both phases. Thisanisotropicliquid is known as a liquid

crystalline or mesomorphous state. Using DSC, it is possible to observe the small energy changes that

occur as matter transitions from a solid to a liquid crystal and from a liquid crystal to anisotropicliquid.[5]

Oxidative stability [edit]

Using differential scanning calorimetry to study the stability tooxidationof samples generally requires an

airtight sample chamber. Usually, such tests are done isothermally (at constant temperature) by changing

the atmosphere of the sample. First, the sample is brought to the desired test temperature under an inert

atmosphere, usuallynitrogen. Then, oxygen is added to the system. Any oxidation that occurs is

observed as a deviation in the baseline. Such analysis can be used to determine the stability and

optimum storage conditions for a material or compound.[4]

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7/30/2019 Fluorimetry Asign

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Safety Screening [edit]

DSC makes a reasonable initial safety screening tool. In this mode the sample will be housed in a non-

reactive crucible (oftengold, or gold plated steel), and which will be able to withstandpressure(typically

up to 100bar). The presence of anexothermicevent can then be used to assess thestabilityof a

substance to heat. However, due to a combination of relatively poor sensitivity, slower than normal scan

rates (typically 2-3 /min - due to much heavier crucible) and unknownactivation energy, it is necessary to

deduct about 75-100 C from the initial start of the observed exotherm to suggesta maximum

temperature for the material. A much more accurate data set can be obtained from anadiabatic

calorimeter, but such a test may take 23 days fromambientat a rate of a 3 C increment per half hour.

Drug analysis [edit]

DSC is widely used in thepharmaceuticalandpolymerindustries. For the polymer chemist, DSC is a

handy tool for studyingcuringprocesses, which allows the fine tuning of polymer properties. Thecross-

linkingof polymer molecules that occurs in the curing process is exothermic, resulting in a positive peak in

the DSC curve that usually appears soon after the glass transition.[4][5][6]

In the pharmaceutical industry it is necessary to have well-characterizeddrugcompounds in order to

define processing parameters. For instance, if it is necessary to deliver a drug in the amorphous form, it is

desirable to process the drug at temperatures below those at which crystallization can occur.[5]

General chemical analysis [edit]

Freezing-point depressioncan be used as a purity analysis tool when analysed by differential scanning

calorimetry. This is possible because the temperature range over which a mixture of compounds melts is

dependent on their relative amounts. Consequently, less pure compounds will exhibit a broadened

melting peak that begins at lower temperature than a pure compound.

http://www.photometrics.net/DSC.html

http://www.photometrics.net/DSC.html

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