A REVIEW ON INSTRUMENTATION OF THERMAL ANALYSIS METHOD: DTA, DSC

About Authors:
Bhupender Kumar*, Assit. Prof. Prasant Beniwal, Monish Sharma, Ramchandra
Seth G.L. Bihani S.D. College of Technical Education
(Institute Of Pharmaceutical Sciences And Drug Research),
Sri Ganganagar, Rajasthan
*bhupendra.nimiwal@gmail.com

Abstract:
Thermoanalytical methods essentially techniques that are based entirely on the concept of heating a sample followed by well-defined modified procedures, such as : gravimetric analysis, differential thermal analysis (DTA) and differential scanning calorimetry (DSC). Thermogravimetric analysis measured weight change, differential scanning calorimetry measured heats and temperature of transitions and reactions, differential thermal analysis (DTA) measured temperatures of transitions and reactions.

Reference Id: PHARMATUTOR-ART-1600

1.   INTRODUCTION:
1.1 Thermal Analysis Techniques
Thermal analysis includes a group of techniques in which specific physical properties of a materials are measured as a function of temperature. Current areas of application include environmental measurements, composition analysis, product reliability, stability, chemical reactions, & dynamic properties. Thermal analysis has been used to determine the physical & chemical properties of polymers, electronic circuit boards, geological materials, & coals.^[10]

1.2 Differential scanning calorimetry (DSC):
In this technique, the sample & references materials are subjected to a precisely programmed temperature change. When a thermal transition (a physical & chemical change that results in the emission or absorption of heat) occurs in the sample, thermal energy is added to either the sample or the reference container in order to maintain both the sample & reference at the temperature. Because the energy transferred is exactly equivalent in magnitude to the energy absorbed or evolved in the transition, the energy yields a direct calorimetric measurement of the transition energy.

1.3 Differential thermal analysis (DTA):
In this technique, the difference in temperature between the sample & a thermally inert reference material is measured as a function of temperature (usually the sample temperature). Any transiton that the sample undergoes results in liberation or absorption of energy by the sample with a corresponding deviation of its temperature from that of reference.^[10]

Table:1 Thermal analysis technique

Figure:1 Differential thermogram showing types of changes encountered with polymeric materials ^[8]

2.      INSTRUMENTATIONS:
2.1  Differential scanning calorimetry (DSC):
The technique was developed by E.S. Watson and M.J. O'Neill in 1962, and introduced commercially at the 1963 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy.^[12]

Two basic types of Differential Scanning Calorimetry (DSC) must be distinguished:
·         the heat flux DSC and
·         the power compensation DSC

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Figure:2 DSC cell cross section

The heat flux DSC: Atypical DSC cell uses a constantan (Cu-Ni) disk as the primary means of transferring heat to the sample & reference positions & also as one element of  the temperature sensing thermoelectric junction. The sample & a reference are placed in separate pans that sit on raised platform on the disk. Heat is transferred to the sample & reference through the disk. The differential heat flow to the sample & reference is monitored by the chromel/constantant thermocouples formed by the junction of the constantan disk & the chromel wafer covering the underside of each platform. Chromel & alumel wires connected to the underside of the wafers form a Chromel/alumel thermocouple, which is used to directly monitor the sample temperature. Constant calorimetric sensitivity is maintained by computer software, which linearizws the cell calibration coefficient. DSC provides maximum calorimetric accuracy from -170 to 750°C. sample size range from 0.1 to 100 mg.^[10]

Figure:4 DSC^[11]

Power Compensation DSC
Sample and reference are each held in a separate, self contained calorimeter with its own heater. The advantage of the PC DSC over the HF-DSC is a very light individual furnace. The power compensated furnaces weigh 1 g. The furnaces for HF DSC weigh up to 200 g. The effect of a low mass furnace is an extremely short responding time. The heating and cooling rates can be up to 500 °C/min. When a reaction appears (exothermal or endothermal) the energy is accumulated or released to compensate the energy change in both furnaces. The power required to maintain the system in equilibrium is proportional to the energy changes occurring in the sample.

All PC DSC are in basic principles the same. But special PC DSC has also been presented in the past. One of them is Photo DSC where direct measurements of radiation flow occur under a light source. This way the degradation of material can also be observed. The maximum heating rate for not modified PC DSC is up to 500 K/min and the maximum cooling rate is up to 400 K/min. Temperature range of measurement is up to 400 °C with time constant of only 1.5 s or lower. Sample masses are around 20 mg. Crucibles of different volumes (lower than several ten cubic millimetres) are made mostly of aluminium.^[11]

Calibration of the apparatus:
Calibrate the apparatus for temperature & enthalpy change, using indium of high purity or any other suitable certified material, according to the manufacturer’s instructions. A combination of 2 metals, e.g. indium & zinc may be used to control linearity.

Table:2 Materials for temperature and heat of reaction calibration of DTA and DSC instruments. ^[1]

Hyper DSC
The high resolution of PC-DSC or new type of power compensating DSC provides the best results for an analysis of melting and crystallization of metals or detection of glass transition temperature (Tg) in medications. Fast scan DSC has the ability to perform valid heat flow measurements with fast linear controlled rates (up to 500 K/min) especially by cooling, where the rates are higher than with the classical PC DSC Standard DSC operates under 10 K/ min. The benefits of such devices are increased sensitivity at higher rates (which enables a better study of the kinetics in the process), suppression of undesired transformation like solid – solid transformation etc. It has a great sensitivity also at a heating rate of 500 K/min with 1 mg of sample material. This technique is specially proper for the pharmaceutics industry for testing medicaments at different temperatures where fast heating rates are necessary to avoid other unwanted reactions etc.

Fig.4 Power compensating DSC (Perkin – Elmer Instruments)^[11]

Modulated DSC
This new technique introduced in 1993 has been thoroughly examined. Polymer blends difficult to evaluate by conventional DSC have been successfully analysed by modulated DSC ^[5]. Main advantages are the separation of overlapping events in the DSC scans. In conventional DSC, a constant linear heating or cooling rate is applied. In modulated DSC (MDSC), the normally linear heating ramp is overlaid with a sinusoidal function (MDSC) defined by a frequency and an amplitude to produce a sinusoidal shaped temperature vs. time function. Using Fourier mathematics, the DSC signal is split into two components: one reflecting non-reversible events (kinetic) and the other reversible events.

T = To + bt + B sin(ωt)

dq/dt = C(b + Bωcos(ωt) + ƒ^’(t,T) + K sin(ωt)

where T is temperature, C the specific heat, t the time, ωthe frequency, ƒ^’(t, T) is the average underlying kinetic function once the effect of the sine-wave modulation has been substracted. K is the amplitude of the kinetic response to the sine-wave modulation and (b + Bωcos(ωt)) is the measured quantity dT/dt or ‘‘reversing’’ curve. The total DSC curve, the reversing curve giving reversible transitions and the non-reversing curve giving irreversible transitions (e.g., the glass transitions), is obtained. MDSC is a valuable extension of conventional DSC. Its applicability is recognized for precise determination of the temperature of glass transitions and for the study of the energy of relaxation, and it depends on a number of important parameters to be studied. It has been recently applied for the determination of glass transitions of hydroxypropylmethylcellulose films and for the study of amorphous lactose, as well as for the study of some glassy drugs ^[11].

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Micro-DSC
The instruments of conventional DSC allows to measure very small amounts of material. The author was able to characterize the melting peak of indium with 0.032 mg by using a DSC-7 of Perkin-Elmer. New instrument generation will permit to increase sensitivity and amount of material to be studied decrease to nanorange.^[9]

Application of DSC- the DSC is used for determination of purity of material. The presence of very small amount of impurities may reduce effective function of a drug. It is known that higher the concentration of impurity present in a substance, the lower will be its M.P. and broader will be its melting range. ^[3]

2.2. Differential thermal analysis (DTA):
DTA- le Chatelier (1887) described a new technique for the study of Clays and minerals by an examination of their temperature time curves as a substances were heated to elevated temperature. Robert- Austen (1889) improved the technique by introducing two thermocouples, one was used in the sample and the other in reference lock in furnace. The difference in temperature which was more sensitive to small temperature changes of the sample ^[7].

DTA data are accurate of all thermal techniques, because the thermocouple is inserted into the sample and only the temperature of transition and not the amount of heat can be measured from a DTA curve ^[6].

The various component of a DTA apparatus are as following:
a.       Furnace- this is device for heating the sample.
b.      Sample holder- this is used to contain the sample as well as the reference material.
c.       DC amplifier- generally a low level DC amplifier is employed.
d.      Differential Temperature Detector- the function of this detector is to measure differential temperature.
e.       Furnace Temperature Programme- the main function of this is to increase the temperature of the furnace at a steady rate.
f.       Recorder- this is to record the DTA curve.
g.      Control Equipment- its function is to maintain a suitable atmosphere in the furnace & sample holder.^[2]

Fig.5 Schematic diagram of the DuPont differential thermal analyser

Commercial differential thermal-analyzer system. Weighed quantities of the sample and reference material are held in the small pans labeled Sand R. The thermocouple on the right (labeled control TC) controls the rate at which the furnace must be heated in order to provide a linear temperature increase. The sample and reference thermocouples are connected in series. Any current due to a temperature difference between the two is amplified and used to determine the position of a recorder pen. With the switch in position 1$, the sample thermocouple is connected not only to the reference thermocouple but also to a reference junction, which may be at room or ice-bath temperature. The output of this circuit provides a measure of the sample temperature at any instant.

Generally, the sample and reference chamber in a differential thermal apparatus is designed to permit the circulation of inert or reactive gases. Some systems also have the capability for operation at high or low pressures ^[8].

APPLICATIONS
The various important applications of DTA are :
(i) Rapid identification of the compositions of mixed clays,
(ii) Studying the thermal stabilities of inorganic compounds,
(iii) Critically examining in a specific reaction whether a new compound is actually formed or the product is nothing but an unreacted original substance, and
(iv) DTA offers a wide spectrum of useful investigations related to reaction kinetics, polymerization, solvent retention, phase-transformations, solid-phase reactions and curing or drying properties of a product ^[4].

REFERENCES
1.    Beckett A.H., Stenlake J.B., “Practical Pharmaceutical Chemistry” 4th edition 2004, in two parts, published by CBS publisher & distribution, New Delhi, p. no. 69-74
2.    Chatwal Gurdeeep R., Anand Sham K., “Instrumental methods of Chemical Analysis”, published by Himalya Publishing house, Mumbai, p. no. 2.719 - 2.733
3.    Kamboj P.C., “Pharmaceutical Analysis volume-Π : Instrumental Methods”, 1st edition 2010, published by Vallabh Prakashans, Delhi, p. no. 445-457
4.    Kar Ashutosh, “Pharmaceutical Drug Analysis”, revised 2nd edition 2005, published by New Age International (P) Limited, New Delhi, page no. 200
5.    Mendham J., et al., “Vogel’s Textbook of Quantitative Chemical Analysis”, 6th edition 2008, published by Dorling Kindersley (India) Pvt. Ltd., p. no. 514-517
6.    “Remington the Science & Practice of Pharmacy”, vol.1, published by B.I. publication, India, p. no. 662-663
7.    Sharma B.K., “Instrumental Methods of Chemical Analysis”, published by Goel publishing, Merrut, p.no. 240-248
8.    Skoog Douglas A. and West Donald M., “Principles of instrumental Analysis” 2nd edition 1980, published by Saunders college West Washington square Philadelphia, page no. 661
9.    Swarbrick James, Boylan James C., “Encyclopedia of Pharmaceutical Technology”, vol.3, second edition, published by Informa Healthcare ,New York, p. no. 2766-2769
10.    Willard Hobart H., et al., “Instrumental Methods Of Analysis”, published by CBS publisher & distribution, New Delhi, p. no. 761-770
11.    Differential thermal analysis (DTA) and differential scanning calorimetry (DSC) as a method of material investigation. rmz-mg.com/letniki/rmz57/RMZ57_0127-0142.pdf (Accessed on 25/7/2011)
12.    Differential scanning calorimetry. en.wikipedia.org/wiki/Differential_scanning_calorimetry   (Accessed on 25/7/2011).

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