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V.Suhasini M.PHARM(PHARMACEUTICS)

THERMOGRAVIMETRIC ANALYSIS

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Introduction In the early years of analytical chemistry ,

practically all analytical precipitates were dried 100- 120 degree in an oven ignited at red heat by using undefinable temp of the burner

All changed in recent year due to the development of modern recording thermobalance according to Duval (1953)

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Principle TGA is a technique in which, upon heating a

material, its weight increases or decreases TGA is a method of thermal analysis in which

changes in physical and chemical properties of materials are measured as a function of increasing temperature.

By using this analysis, one can predict the thermal stability of given sample and it is used to determine the composition of given sample. This technique is used for those substances which can exhibit weight loss or gain due to certain reactions like decomposition, oxidation, and dehydration.

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Weight loss A B C D

E

F temperature

MCO3 2H2O

MCO3H2O

MCO3

CO3+ MO

MCO3 - Metal carbonate

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The principle of the technique can be illustrated by the weight loss curve of hypothetical compound MCO3.2H2O

The temperature at A is called minimum weight loss temperature

At B due to MCO3 .H2O Further heating results in the formation of

anhydrous MCO3 weight levels from C & D At point D the compound MCO3 starts to

evolve CO3 giving MO weight level from E & F

The thermal stability of the original compound

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Difference between DTA, DSC, TGA

DTA = Differential Thermal Analysis: You heat a sample in a crucible and a reference in another (but identical) crucible simultaneously. The DTA signal is the temperature difference between sample and reference, typically plotted versus temperature T

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DSC = Differential Scanning Calorimetry: Very similar to DTA, but due to a more sophisticated construction not only T itself, but additionally the heat flow into (or from) the sample can be measured.

TGA = Thermogravimetric Analysis (often only TG): Sometimes performed simultaneously with DTA/DSC, sometimes separately. measured the weight loss or gain as a function of temperature

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Type of thermo gravimetric analysis

Dynamic TGA - in this type of analysis the sample is subjected to conditions of continuous increase in temperature usually linear with time

Isothermal or static TGA - in this type of analysis the sample is maintained at a constant temperature for a period of time during which any change in weight are noted

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Gravimetric analysis : concerned with the process of

producing and weighing a compound or element in as pure form

Traditional gravimetric : essentially manual in character

and labor intensive Electro gravimetric : partially instrumental Thermal gravimetric : highly instrumental

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Instrumentation TGA are based on the act that the

sample be continuously weighted as it is elevated temp

The sample is continuously weighted by an instrument , called thermo balance

Both manual as well as automatic recording balances

4 general component

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A recording balance A furnace Furnace programmer or controller A recorder device

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A TGA consists of a container which is in the form of a crucible for holding the sample (sample pan)

a furnace for heating the given sample at a high temperature, and an appropriate precision balance which can continuously monitor the sample

That pan resides in a furnace and is heated or cooled during the experiment.

A sample purge gas controls the sample environment

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This gas may be inert or a reactive gas such as Helium or Argon that flows over the sample and exits through an exhaust

Finally the weight is recorded as a function of increasing temperature

TGA instrument continuously weighs a sample as it is heated to temperatures of up to 2000 °C

. As the temperature increases, various components of the sample are decomposed and the weight percentage of each resulting mass change can be measured

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Results are plotted with temperature on the X-axis and mass loss on the Y-axis. The data can be adjusted using curve smoothing. Instrument can be coupled with FTIR and Mass spectrometry ,  gas chromatography analysisTGA-FT-IR TGA-MS TGA-GC/MS

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What is the best way to prepare a sample? The sample size should be between

2 and 50 mg. If you have minimum amount of

sample, run at least 1 mg. If possible, cover the bottom of the

pan with the sample material. The sample pans, ceramic or

platinum, can accommodate liquids, powders, films, solids or crystals

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Once you have decided on the sample form, then for best results, use approximately the same sample weight during each experiment. This will ensure reproducibility.

Many small pieces of sample are better than one large chunk. It is better to have a large surface area exposed to the sample purge.

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How is a TGA Thermal Curve displayed? (X-axis) can be displayed as

temperature and (Y-axis) can be displayed as weight (%)

95.11°C96.62%

185.05°C73.60% 295.89°C

60.00%

398.06°C50.51%

Residue:41.10%(12.49mg)

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What does a TGA Thermal Curve look like?

  TGA thermal curve is displayed from left to right. The descending TGA thermal curve indicates a weight loss occurred.

TGA instruments can be divided into two types:

Vertical balance horizontal balance VERTICAL BALANCE : single pan not

suitable for DTA and DST only suitable TGA

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Horizontal balance instruments (TA, Perkin Elmer, etc.) normally have two pans (sample and reference).

TGA 4000. Pyris 1 TGA.

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Suggested Protocol Heating rate 5 o C/min. in air.

Maximum temperature sufficient to stabilize sample weight (typically 800 o C).

Sample size at least 2 - 4 mg, more if possible.

Three separate TGA runs on each sample.

Ash content measured independently on microbalance.

provides much greater accuracy, precision and speed and sensitivity. 

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Type of recording thermo balance

Null point type Deflection type

Helical

Beam

Cantilever

Torsion

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Null point More commonly used. In these

balance , a sensor is employed to detect the deviation of the beam from its null point position .

A restoring force of electrical or mechanical weight load is then applied to the balance beam in order to restore its null point position from the horizontal or vertical norm

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Deflection type Essentially based on either a

conventional analytical balance consisting of helical, beam, cantilever , torsion analytical balance involving the conversion of deviation directly into a record of weight change

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Factors affecting the TGA(1) Instrumental factors

(a) Furnace heating rate(b) Furnace atmosphere

(2) Sample characteristics(a) Weight of the sample(b) Sample particle size

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Instrumental factors Furnace Heating rate: The temperature at

which the compound (or sample) decompose depends upon the heating rate. When the heating rate is high, the decomposition temperature is also high. A heating rate of 3.5°C per minute is usually recommended for reliable and reproducible TGA.

Furnace atmosphere: The atmosphere inside the furnace surrounding the sample has a profound effect on the decomposition temperature of the sample. A pure N2 gas from a cylinder passed through the furnace which provides an inert atmosphere.

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Sample characteristics(a)Weight of the sample: A

small weight of the sample is recommended using a small weight eliminates the existence of temperature gradient throught the sample.

(b) Particle size of the sample: The particle size of the sample should be small and uniform. The use of large particle or crystal may result in apparent, very rapid weight loss during heating

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Applications of TGA TGA is used as a technique to

characterize materials used in various environmental, food, pharmaceutical, and petrochemical applications

TGA can be used to evaluate the thermal stability of a material In a desired temperature range

Can determine the purity of a mineral, inorganic compound, or organic material.

TGA is used to study the kinetics of the reaction rate constant.

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Thermogravimetric analysis(TGA) is a powerful technique for the characterization of the decomposition or weight loss properties of materials. The technique provides the following useful information:

Decomposition temperatures Thermal degradation properties Oxidative degradation characteristics Quantitative weight losses Compositional analysis Long term stabilities Flammability properties Rates of degradation

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Thermal Oxidative degradation characteristics

1. cigarette tobacco(approximate mass of 14 mg) was heated at a rate of 40 C/min under an air purge (40 mL/min) which helps to simulate the

2. real-life thermo-oxidatve degradation properties of the cigarette tobacco

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TGA Using Kinetics Studies The assessment of product lifetimes

is easily performed using the PerkinElmer TGA Decomposition Kinetics Software

The TGA kinetics approach uses the well-known variable heating method developed

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With the TGA decomposition kinetics approach, the sample is heated at several different heating rates ranging between 40 and 1 C/min.

Typically, 3 to 6 different heating rate experiments are performed to assess the TGA decomposition kinetics.

the corresponding temperature is determined for each different heating rate.

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Reference Instrumental method of chemical

analysis B.K Sharma page no : 308 Pharmaceutical drug analysis

Ashutosh Kar page no : 194

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