1

DSC:

Differential Scanning Calorimetry

A bulk analytical technique

What Does a DSC Measure?

A DSC measures the difference in heat flow rate

(mW = mJ/sec) between a sample and inert

reference as a function of time and

temperature

-0 .4

-0 .3

-0 .2

-0 .1

0 .0

0 .1

Heat Flow (W/g)

0 2 5 5 0 7 5 1 0 0 1 2 5 1 5 0

Tem pe ra tu re (°C )E xo U p

Endothermic Heat Flow

• Heat Flow

�Endothermic: heat flows into the sample as

a result of either heat capacity (heating) or

some endothermic process (glass

transition, melting, evaporation, etc.)

-0 .1

0 .0

0 .1

Heat Flow (W/g)

0 20 40 60 80 100 120 140 160

Temperature (°C )Exo Up

Exothermic Heat Flow

• Heat Flow

�Exothermic: heat flows out of the sample

as a result of either heat capacity

(cooling) or some exothermic process

(crystallization, cure, oxidation, etc.)

Temperature

• What temperature is being measured and

displayed by the DSC?

�Sensor Temp: used by most DSCs. It is

measured at the sample platform with a

thermocouple, thermopile or PRT.

Constantan Body

Chromel Wire

Chromel Area Detector

Constantan Wire

Chromel Wire

Base Surface

Thin Wall Tube

Sample Platform

Reference Platform

Temperature

• What temperature is being measured and

displayed by the DSC?

�Pan Temp: calculated by TA Q1000

based on pan material and shape

� Uses weight of pan, resistance of pan, &

thermoconductivity of purge gas

�What about sample temperature?

� The actual temperature of the sample is

never measured by DSC

Temperature

• What other temperatures are not typically

being displayed.

�Program Temp: the set-point temperature

is usually not recorded. It is used to control

furnace temperature

�Furnace Temp: usually not recorded. It

creates the temperature environment of

the sample and reference

Understanding DSC Signals

Heat Flow

• Relative Heat Flow: measured by many

DSCs. The absolute value of the signal is

not relevant, only absolute changes are

used.

• Absolute Heat Flow: used by TA’s Q1000.

Dividing the signal by the measured heating

rate converts the heat flow signal into a heat

capacity signal

DSC Heat Flow

t)(T,dt

dT Cp

dt

dHf+=

signal flowheat DSC dt

dH=

Weight SampleHeat x Specific Sample

CapacityHeat Sample Cp

=

=

Rate Heating dt

dT=

(kinetic) re temperatuabsolutean at

timeoffunction is that flowHeat t)(T, =f

Tzero Heat Flow Equation

R s R r

q s q r

C s C r

T r

T 0

T s

Heat Flow

Sensor Model

( )ττ d

TdC

d

dTCC

RRT

R

Tq r

ssr

rsr

∆−−+

−∆+

∆−=

110

How do we calculate these?

Besides the three

temperatures (Ts, Tr, T0);

what other values do we

need to calculate Heat

Flow?

Measuring the C’s & R’s

• Tzero™ Calibration calculates the C’s & R’s

• Calibration is a misnomer, THIS IS NOT A

CALIBRATION, but rather a measurement of

the Capacitance (C) and Resistance (R) of

each DSC cell

• After determination of these values, they can be

used in the Four Term Heat Flow Equation

showed previously

Measuring the C’s & R’s

• Preformed using Tzero™ Calibration Wizard

1. Run Empty Cell

2. Run Sapphire on both Sample & Reference

side

Measuring the C’s & R’s

Empty DSC constant heating rate

Assume: 0≡= rs qq

Heat balance equations give sensor time constants

τ

τ

d

dT

TRC

ssss

0∆==

ττ

τ

d

Td

d

dT

TTRC

srrr ∆

−

∆−∆== 0

Measuring the C’s & R’s

Repeat first experiment with sapphire disks on sample

and reference (no pans)

Assume:τddT

cmq ssapphss = τd

dTcmq rsapphrr =

Use time constants to calculate heat capacities

10 −∆

=

ss

sapphs

s

d

dT

T

cmC

ττ

10 −

∆

−

∆+∆=

rs

sapphr

r

d

Td

d

dT

TT

cmC

τττ

Measuring the C’s & R’s

Use time constants and heat capacities to calculate

thermal resistances

s

s

sC

Rτ

=r

r

rC

Rτ

=

A few words about the Cs and Rs

• The curves should be smooth and continuous,

without evidence of noise or artifacts

• Capacitance values should increase with

temperature (with a decreasing slope)

• Resistance values should decrease with

temperature (also with a decreasing slope)

• It is not unusual for there to be a difference

between the two sides, although often they are

very close to identical

Good Tzero™ Calibration Run

Can see that it is bad during

Tzero™ cal run

Bad Tzero™ Calibration Run

Before Running Tzero™ Calibration

• System should be dry

• Dry the cell and the cooler heat exchanger

using the cell/cooler conditioning template and

the default conditions (2 hrs at 75°C) with the

cooler off

�Preferably enable the secondary purge

�Do not exceed 75°C cell temperature with

the cooler off, although the time can be

extended indefinitely

Stabilization before Calibration

• System must be stable before Tzero™

Calibration

• Stabilization is achieved by cycling the baseline

over the same temperature range and using the

same heating rate as will be used for the

subsequent calibration

• Typical systems will stabilize after 3-4 cycles, 8

cycles recommended to ensure that the system

has stabilized

Example of Typical Results

40

50

60Reference Resistance (°C

/Watt)

0.02

0.03

0.04

Reference Capacitance (Joule/°C)

0.01

0.02

0.03

0.04

0.05

Sample Capacitance (Joule/°C)

30

40

50

60

70

Sample Resistance (°C

/Watt)

-200 -100 0 100 200 300

Temperature (°C)

Characteristics of the thermal resistances and heat capacities:

Both curves should be smooth, with no steps, spikes or inflection points.

Thermal resistances should always have negative slope that gradually decreases.

Heat capacities should always have positive slope that gradually decreases.

This cell is very well balanced. It is acceptable and usual

to have larger differences between sample and reference.

-0.4

-0.2

0.0

0.2

0.4

0.6

Heat Flow (mW)

-100 0 100 200 300 400

Temperature (°C)

Conventional BaselineT zero Baseline

Tzero™ vs Conventional Baseline

Indium with Q Series Heat Flow Signals

Q1000

Q100

Q10

Keeping the DSC Cell Clean

• One of the first steps to ensuring good data is

to keep the DSC cell clean

• How do DSC cells get dirty?

�Decomposing samples during DSC runs

�Samples spilling out of the pan

�Transfer from bottom of pan to sensor

How do we keep DSC cells clean?

• DO NOT DECOMPOSE SAMPLES IN THE DSC CELL!!!

• Run TGA to determine the decomposition

temperature

�Stay below that temperature!

• Make sure bottom of pans stay clean

• Use lids

• Use hermetic pans if necessary

TGA Gives Decomposition Temperature

Cleaning Cell

• If the cell gets dirty

�Clean w/ brush

� Brush gently both sensors and cell if

necessary

� Be careful with the Tzero™ thermocouple

� Blow out any remaining particles

Brushing the Sample Sensor

It Does Matter What Pan you use

Monohydrate

Pharmaceutical

sample

Sample Shape

• Keep sample thin

• Cover as much as the bottom of pan as

possible

Sample Shape

• Cut sample to make thin, don’t crush

• If pellet, cut cross section

Sample Shape

• Cut sample to make thin, don’t crush

• If pellet, cut cross section

• If powder, spread evenly over the bottom of the

pan

Using Sample Press

• When using crimped pans, don’t over crimp

• Bottom of pan should remain flat after crimping

• When using Hermetic pans, a little more

pressure is needed

• Hermetic pans are sealed by forming a cold

wield on the Aluminum pans

Crimped Pans Hermetic Pans

Good BadSealed

Not

Sealed

Sample Size

• Larger samples will increase sensitivity

but…………….

• Larger samples will decrease resolution

• Goal is to have heat flow of 0.1-10mW going

through a transition

Sample Size

• Sample size depends on what you are

measuring

�If running an extremely reactive sample (like

an explosive) run very small samples (<1mg)

�Pure organic materials, pharmaceuticals

(1-5mg)

�Polymers - ~10mg

�Composites – 15-20mg

Effect of Sample Size on Indium Melt

Size: 0.4900 mg

Size: 1.2100 mg

Size: 5.7010 mg

-25

-20

-15

-10

-5

0

Heat Flow (mW)

150 152 154 156 158 160 162 164

Temperature (°C)

Weight Onset Peak Width

(mg) (°C) (°C) (°C)

0.49 156.41 156.56 0.17

1.21 156.45 156.76 0.29

5.70 156.61 157.17 0.55

Purge Gas

• Purge gas should always be used during DSC

experiments

�Provides dry,inert atmosphere

�Ensures even heating

�Helps sweep away any off gases that might

be released

• Nitrogen

�Most common

�Increases Sensitivity

�Typical flow rate of 50ml/min

Purge Gas

• Helium

�Must be used with LNCS

�High Thermo-conductivity

�Increases Resolution

�Upper temp limited to 350°C

�Typical flow rate of 25ml/min

• Air or Oxygen

�Used to view oxidative effects

�Typical flow rate of 50ml/min

Sample Temperature Range

• Rule of Thumb

�Have 2-3 minutes of baseline before and

after transitions of interest - if possible

� DO NOT DECOMPOSE SAMPLES

IN DSC CELL

�Temperature range can affect choice of pans

�Just because the instrument has a

temperature range of –90°C to 550°C (with

RCS) doesn’t mean you need to heat every

sample to 550°!

Start-up Hook

Do not attempt to interpret transitionsbefore Heating rate has stabilized

9.56mg PET @ 10°C /m in

0

2

4

6

8

10

12

Deriv. Temperature (°C

/min)

-0 .25

-0 .15

-0 .05

Heat Flow (W/g)

-5 5 15 25 35

Temperature (°C )Exo Up

Heating Rate

• Faster heating rates increase sensitivity

but…………….

• Faster heating rates decrease resolution

• Good starting point is 10°C/min

Effect of Heating Rate

PMMA

10.04mg

Thermal History

• The thermal history of a sample can and will

affect the results

• The cooling rate that the sample undergoes can

affect :

�Crystallinity of semi-crystalline materials

�Enthalpic recovery at the glass transition

• Run Heat-Cool Heat experiments to see effect

of & eliminate thermal history

�Heat at 10°C/min

�Cool at 10°C/min

�Heat at 10°C/min

Heat-Cool-Heat of PET

Second Heat

First Heat

Cool

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Heat Flow (W/g)

20 60 100 140 180 220 260

Temperature (°C)