﻿ Thermogravimetric analysis

# Thermogravimetric analysis

Thermogravimetric analysis (TGA), also known as thermogravimetric analysis, is an analytical method or method of thermal analysis or thermal analysis, in which the change in mass of a sample as a function of temperature and time is measured. The sample is heated to a small crucible of temperature stable and inert material ( eg, platinum or alumina) in an oven at temperatures up to 2400 ° C. The sample holder is coupled to a microbalance, which registers the mass changes during the heating process. A thermocouple close to the crucible measures the temperature. Modern TGA devices allow a connected computer, a setting of the final temperature, heating rate, the gas flow or the like. During the analysis of the sample chamber is purged as required with different gases. Most used to using pure nitrogen to avoid oxidation (ensure vacuum tightness of the entire apparatus ). In some cases, however, purged with air, oxygen or other gases. On heating, the sample may decompose to give reactions or evaporation of volatile components to the environment or absorb from the environment, for example by oxidation reactants. Weight loss or increase, and the temperature at which occurs the change in weight can be specific to a sample under investigation. Resulting conclusions can be drawn about the composition of the substance.

## Principle of measurement

Thermogravimetry the change in mass of a solid sample during a heating or cooling process known is observed. The most common application is the heating of the sample at a constant heating rate. Mass change can be triggered by the following causes:

• Mass loss by physical processes (eg evaporation, sublimation )
• Mass loss of a sample by decomposition (decomposition with formation of volatile products )
• Mass loss by reaction ( eg, reduction )
• Increase in mass by reaction (eg, oxidation)

Usually a thermobalance of the following components:

• An adjustable temperature in the furnace
• The scale
• Feed lines for hydrogen, nitrogen, oxygen and helium
• Evaluation device for processing the measured values

### Oven with temperature control

The most important property that needs to have the furnace of a thermobalance, is to produce a homogeneous temperature field at the sample, since even small temperature variations in the experimental procedure may have an impact on the output curve. It should be noted that the homogeneous field decreases with increasing temperature.

### The scale

The scale most frequently used based on the principle of electromagnetic compensation. Here, the metallic timber is held by means of two scales mounted on both sides of the balance arms electromagnetic coils in the same position always. Each deflection of the balance out of its rest position is detected by a photoelectric sensor and the voltage of the solenoid is controlled so that the scale is held at the initial position. In order to keep the forces acting on the solenoid current as low as possible, a counterweight is mounted on the opposite side of the sample arm. It corresponds in mass like that of the crucible. During the measurement of the change in voltage is measured linearly related to the mass change.

### The gas supply lines

Affixed to the thermobalance gas lines allow charging of the apparatus ( oven and balance head) with different gases and gas mixtures. As the inert gas nitrogen is generally applied. Connections and design of the thermal balance must be vacuum tight so sensitive samples do not react with of introduced ambient air (oxygen). When coupling the thermobalance with a mass spectrometer may be more appropriate the use of helium as it does not appear as nitrogen in the detection range of the mass of carbon monoxide. Possible reaction gases are synthetic air for oxidation or hydrogen for reductions.

### Influences on the measurement

There are a number of apparatus and physical effects that have an influence on the test results. A blank measurement is therefore often added prior to the start, as a result of temperature effects apparatus variables, such as the conductivity of the coils in the scale head, the density or the viscosity of the gas used may change.

## Coupling methods

For analysis of the volatilized substances in the furnace or the reaction products and decomposition products further analysis devices can be connected to the exhaust pipes. The most common mass spectrometer or infrared spectrometer will be used here. There are also constructions in which a separation is carried out by gas chromatography and the spectrometer between thermobalance (TG -GC- MS). A simple method for the analysis of the exhaust gases is the use of absorption tubes. Using special Desorptionsöfen can be analyzed spatially separated at other measuring systems, the exhaust then.

## Calibration

The temperature calibration of the thermal balances may be done by metals or alloys, which show at a defined temperature, a Curie transition. Suitable materials may be nickel ( up 360 ° C ) and iron ( up 768 ° C) here. In practice, the measuring cell in the area of ​​influence of a strong external magnetic field needs to be. The conversion is detected as an apparent change in mass. The temperature deviation is heizratenabhängig. Thus, the temperature calibration must be performed for different heating rates. However, it results in a linear relationship between temperature and heating rate variation. The temperature differences may be dependent on the temperature to produce a calibration at different temperatures and thus necessary with several calibrators makes ( multi-point calibration ).

A mass calibration is carried out by suitably appropriate ( calibrated ) weights. A simple review of the detected mass differences can be effected by calcium oxalate monohydrate. The compound shows under inert conditions (no oxygen) defined three stages of decomposition:

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