Why is the study of polymorphism important for pharmaceutical compounds
Pharmaceutical Polymorphism Studies by DSC
good morning everyone I'm sorry that we
are starting just about eight minutes
later than planned there are a few
technical errors in the internet
connection on my side I hope this will
not prevent us from continuing my name
is John Ernie
I'll be presenting today's webinar on
polymorphs and pseudo polymorphism by
means of differential scanning
calorimetry just as an introduction I
wanted to let everyone know that you can
use your GoToWebinar tool panel to ask
questions I would very much look forward
to receiving questions from you the
beginning of this presentation the first
45 minutes will be presentation
component and then we'll follow that
with a question and answer session with
that being said and because I know we
were beginning just a few minutes late I
want to go ahead and just get right into
this so here at net we the niche family
comprises three different groups of
three different companies the analyzing
and testing portion is the group that I
work for this includes thermal analysis
instrumentation and determination of
thermo physical properties so we're
definitely an old old company we've been
around since the 1870s
we're mid-sized company and apart from
the analyzing and testing groups
especially for pharmaceuticals it's good
to know that we also are on the process
side of things with grinding and
dispersing as well as pumps and systems
for pharmaceutical grade applications
very small a bit about your host today
my name is again John Ernie I am a
regional Technical Manager and regional
technical sales associate for neck
covering here in the u.s. Texas
Louisiana Oklahoma as well as Mexico my
background is in chemistry I've worked
in the polymers oil and gas and
pharmaceuticals industry primarily from
an analytical chemistry standpoint I've
been with net since 2017 in my current
role here based in Houston Texas and
covering pharma for the US
but most accounts here within within my
local region so let's get right into it
what we're doing is we're looking at
polymorphs using differential scanning
calorimetry if you're not familiar with
the technique of DSC many of the we're
not going to go into fundamentals
because we covered that in the previous
component of this series in part one we
are now in part two but even if you're
unfamiliar with DSC we'll be talking
about the concepts throughout because
that's primarily the data that we'll be
investigating so what we have here is
what you would generally expect in a DSC
trace of a pure pharmaceutical compound
on the y-axis of course we have what is
called the heat flow this is either
endothermic or exothermic events
associated with the heating or cooling
of a compound what we have here is a
ribavirin this is an anti antiviral
compound that we've got here we've got a
sample mass of 1.0 two milligrams we're
heating or heating our sample at ten ten
degrees Kelvin or 10 degrees centigrade
per minute in aluminum crucibles
nitrogen atmosphere and what we can see
in the range of 140 degrees C up through
180 degrees C is we have a very clearly
defined onset of our of our melting so
as we're heating the material we have no
no endothermic or exothermic events
until we reach the melting of this
crystalline compound so we have the
so-called onset temperature which is an
extrapolation of the baseline and the
tangent here with the this the slope of
this melting peak we also have the maca
point of maximum heat flow so you can
see the onset and the peak temperature
and you're in your pharmacopoeia you
should have both of these very well
defined as well as one thing we can look
at is the area under the curve here so
this is again a very well defined
melting of a crystalline compound very
straightforward but let's look into
let's look into a little bit more you
know more complex examples
this particular compound can exist as
two different polymorphic forms a and B
and as I mentioned we're looking at
polymorphs using DSC so we'll be
defining polymorphs and looking more
into DSC throughout the course of this
of this webinar so how do we get these
different forms of this same material
form a we're looking at a material
that's crystallized out of aqueous
ethanol so ethanol with a with a water
component and it has a given melting
range from 166 to 168 degrees Celsius
but then there's also a form be
crystallized from pure ethanol and that
has a melting range that is a bit higher
174 to 176 degrees Celsius so for this
particular substance it has the same
stoichiometry the same chemical formula
but the physical structure of the
substance is different so for certain
substances we have not only one melting
point but we can have multiple melting
points the example that we showed before
is of course form a based on the melting
point that onset and the peak
temperature that we saw but one of the
ways that we can distinguish between
polymorphs as we can see if I have a and
B and I can I can find out what their
melting is of my real sample this allows
me to distinguish between the two
different polymorphic forms and this is
a very basic very first form of using
DSC for polymorph studies so let's dig
in just a little bit deeper into what is
polymorphism so this is the exact same
material and we're looking at solid
materials existing in different crystal
structures so again same stoichiometry
same chemical composition you might be a
little bit more familiar if this is a
difficult concept to grasp or this if
this is new to you
chemical elements also exhibit this not
just compounds not just maliki not just
you know hetero molecules but also just
individual elements so for example
example carbon can exist as a graphite
as diamond or as these so-called
fullerenes we have the same carbon
molecules they're simply arranged
differently
in a stacked and layered form either
small groups with graphite or large
sheets with graphene we can have a
tetrahedral arrangement of these same
molecules with diamond or of course then
these buckyballs or fullerenes
buckminsterfullerene x' as they're
sometimes known in a completely
different orientation but still the same
still the same element
so that's polymorphism so the same
material existing in different crystal
structures we also have pseudo
polymorphism and this is going to be a
special case of polymorphism in which
the different crystal types are simply
the result of water addition so
hydration or salvation so crystal
structures that have incorporated within
them some of their some of the local
solvent molecules be that water or be
that another solvent molecule so no no
overview is complete without a very
brief historical view going back all the
way into the 80 end of the 18th century
so 1798 Martin Clapp Roth looked and saw
that calcite and aragonite are just
simply two different forms of calcium
carbonate so the exact same chemical
structure this calcium carbonate exists
in an orthorhombic or in a rhombohedral
geometry and based on thermodynamics and
kinetics
so basic basically looking at what
temperatures is the material existing at
and at what time how long has it been at
that given temperature one form can then
be converted into the other form within
a few days we can also see that there's
then even a third polymorph with a
different crystal structure which is
even of course less stable then and then
the actual term polymorphism come
meaning multiple shapes coming from that
that Greek base was defined already
early in in the 1820s so this is nothing
new
but we do have new means of studying
these tech you know these different
structures and why is it important and
this is really a the question ultimately
when we're going to be look using
analytical instrumentation or spending
our time investigating substances what's
what's the real critter
reason for us to do this so having the
same material the same substance within
with difference different organization
you know different polymorphic forms
this can adjust the melting point the
sublimation temperature as you can see
here heat capacity volume viscosity so
those are all physical properties but
from a pharmaceuticals perspective the
real impact is that the process ability
of drug substances and the shelf-life so
we're looking at stability how
hygroscopic the material is that is how
much water does this absorb and then of
course that result that relates again
back to shelf life and that can relate
to bioavailability and we're also we
also need to look at dissolution so the
same crystal strike you know say the
material with different crystal
structures can dissolve differently they
can be available for for activity within
the body differently and they can have
different shelf lives even though they
have they came same chemical chemical
structure so I mentioned we would go
into DSC and we will be looking a little
bit more into the DSC I apologize here I
actually have some German slides content
I know this is the English language
version but we'll work through it as is
the convention we can display the DSC
traces either with endotherms so heat
flow into the sample up or downwards it
is the standard European convention to
display endotherms in an upward fashion
so melting Peaks will be displayed in an
upward fashion I know that here in the
US where I'm broadcasting from many many
providers of thermal thermal analytical
instrumentation have the opposite
convention all software packages can
allow you to reverse this so really we
get the exact same information it's just
a question of is endo up or endo down so
we also have this nicely
color-coordinated red and blue red
intuitively for heating blue more
intuitively for cooling and so we have
here is indium this is one of the most
commonly known calibration standards
used for looking at DSC now what makes
it a calibration standard this is just a
pure metal and it has a very very
clearly defined onset temperature
literature one 56.7 we can see or see
here the onset temperature of melting
again as the tangent of the intersection
of the tangents of the baseline and this
rise in the melting this onset
temperature of 156 point six so very
very close to literature here the other
important point is the area
sorry that's again German here but the
area under the curve in joules per gram
so how much heat energy is required how
much energy is required to go from solid
to a completely liquid state so that's
the area under the curve here and you
can see as we heat the material up we
get a melting that is then complete we
can then take the material and go from
these high temperatures down to low
temperatures and we go through then the
crystallization we can then see very
very closely matched the area under the
curve but what you might also notice
then is that we have an offset there's
going to be a different process that
describes or that or that dictates
crystallization from melting melting is
a thermodynamic event whereas the
crystallization is a kinetic event if
these terms are a little unfamiliar to
you the thermodynamic event is going to
be is going to take place at the given
temperatures relatively independent of
of time scaling however when we go from
the liquid to a solid we're going
through a crystallization what's going
to happen is that crystallization is
going to be time dependent it actually
it takes time for those there's liquid
molecules to orient themselves in
relation to each other and form that
crystal lattice so that's why we're
seeing this offset known as hysteresis
between the onset of melting and then
the onset of cooling but really what's
important for for a calibration standard
of any type is that you have
repeatability repeatability
reproducibility precision and so what we
have is simply seven runs of the same
material heated cooled heated cooled
overlaid and very nicely consistently
showing us the exact same properties
this is this is one of the ways that
we're going to not only calibrate our
system but we're also going to use this
as a consent
chill tool to understand what's going on
in our system with a simple metal as
we're heating and cooling we're getting
complete reproducibility not necessarily
so with our polymorphic materials you
know this is a simple metal when we're
looking at more complex structures and
we get into polymorphism it's not always
so straightforward and it doesn't have
to be a complex structure for us to get
something that's that's not 100%
straightforward if we take ice very very
pure deionized filtered water we make
sure that we use a very clean crucible
this is an aluminum crucible that's been
it's been a solvent cleaned baked we
make sure that the water is extremely
pure and as we go from solid to liquid
of course everyone understands the
melting of ice very nice and easy
conceptual process we have that onset
right near zero degrees Celsius and then
it's completed but then what we see
under cooling is we get the material all
the way down here to negative 15 degrees
Celsius and there's still no
crystallization that's taking place
again that that melting is a
thermodynamic process the cooling is a
kinetic process and this is something
that you might have seen before this is
known as super cooling we reach here
some critical point where there's the
onset of crystallization after which
point we have a small seed crystal and
then that provides a lattice after which
the rest of the water molecules which
are already well below their actual
freezing point then arrange themselves
around that we get a very rapid rapid
cooling so looking at polymorph so we've
we've looked at some basic calibration
materials and very common materials
indium and water when we're talking
about polymorphs it would be very nice
if there were a standardized way of
describing these different forms of the
same substance and in the beginning what
we what was what was attempted was to
use you know Greek letters alpha beta
Roman numerals one two latin uppercase a
B to characterize these forms in a
systematic way and the idea was
the modification or the form of the
material with the highest melting point
isn't generally named form one alpha or
a unfortunately this is not always the
case sometimes a more stable form is
found later and then things are changed
so while in general you can see form one
or a or alpha as as the highest melting
point temperature or highest melting
point form this is of course this is not
always the case so take this with a
grain of salt you'll also say things
metastable modifications so things that
are not pure that are not permanently
stable sometimes marked with with this
Prime and so we need to form let's
formulate some some concepts around
polymorphism before we actually look at
the DSC data what I'm going to do is go
through a few more conceptual ideas
before we go through and look at about
four different examples of polymorphism
studies and what can what we can pull
out of it from the DSC data so let's
take this empirical observation so we
take a material and Kulim the material
from the melt what we end up finding is
that the thermodynamically least stable
modification crystallizes first and then
you get this rearrangement this
recrystallization and you get a step by
step formation of more stable forms so
these are these are discrete steps this
isn't the material partially
crystallizing and then partially
crystallizing depending on the kinetics
depending on the rate of cooling what we
can arrive at is a discrete series of
these metastable States and then we end
up in this more stable phase which again
will have that higher melting point
along with these crystalline forms again
depending on the cooling because it's a
kinetic process a time-based process we
can also end up with an amorphous phase
so phases a morphic crystalline let's
look at a little brief overview of
different states of material different
phase transitions so that we're all on
the same page this is coming from a text
it's a text from the 1980s very common
very commonly used in Germany also
translated into other languages but what
have here on the left-hand side we've
got entropy and enthalpy of our
materials increasing as we go up we have
density of our material decreasing then
as we go down so if we look at a
crystalline form of a material let's say
the Alpha form the most stable form this
is something we're very familiar with we
know this terminology we have melting
and crystallization as we go from that
crystal form to a liquid form we could
also have a less stable form and we
could have that solid transition so we
can have that crystal transition we can
have a solid solid transition from
crystal form a to crystal form B of
course we also know the terminology here
basic physical chemistry going from
liquid to a gaseous state we've got
evaporation and condensation but these
don't describe the full the full range
of possibilities we have sublimation and
and then D sublimation going from
gaseous directly just solid forms you'll
see this with with things like liquid
nitrogen and solid and dry ice but then
we have the so called liquid crystals
which are a little bit more liquid but
have retained some crystalline structure
to them we also have glassy state
materials which are more solid but which
retain some degree of that liquid that
liquid form so there is quite a large
range of different material phases and
the important thing is that based on
their thermal thermodynamic stability
and the kinetics we can use the
differential scanning calorimeter to
pull out what phase of material do I
have or what phases of material do I
have so let's get into some of the
examples this is what I know a lot of
you are waiting for and this is to me
the really exciting part is we've looked
at some of some of the background let's
look and see the the data what can we
actually pull out from some some
concrete examples you might recognize
sorbitol this is a a c6 sugar this is
something that you'll find in a lot of
different food products also a sweetener
for various different pharmaceuticals
and what all what I will show at the at
the bottom are just experimental
conditions often with these materials
we'll have somewhere between one and
five milligrams
um most of the USP the u.s.
pharmacopoeia as well as other other
standards recommend for doing basic
studies we're looking at about 10 Kelvin
per minute heating and cooling but if we
want to do particular studies we can of
course change this we can go slower we
can go faster depending on the
particular goal of our experiments and
whether or not we are using a particular
standard or we're doing our own
individual Rd we might see aluminum
crucibles either pierced or oh you know
pierced and open to the environment or
closed and sealed since we're showing
nitrogen atmosphere of course this would
then be a not sealed component not
sealed experiment
so as we're heating our material what we
see is a single crystalline phase we
have the onset onset here and then we
have one simple melting but then after
we cool let's say we cool this material
we take it from solid to liquid we then
cool the material and we cool it at 10
degrees Kelvin per minute then we heat
it up one more time now this looks
completely different what's what's going
on
what I mentioned is that cooling you
know the crystallization is is a kinetic
of that event or a series of kinetic
events it's not simply that we reach a
certain temperature and the material
crystallizes it's a time-based process
so if we're cooling this material at 10
degrees Kelvin and then we immediately
start heating the material we go through
this step change we go through a glass
transition and then we don't see a
crystallization what this means with a
little bit of knowledge of the materials
and and DSC is that we were actually in
an amorphous state glass transitions are
associated with materials with with an
amorphous phase however if we then go
through and store this material one day
at room temperature we allow the
kinetics of crystallization to occur we
then see a different we do see a
difference we see a different story
we heat up the material again and then
we have not this melting you know at
this onset of 95 degrees but then we
have an onset melting of you know down
in the upper 70s we have an onset of
melting down here around 50 or 45
so we have different phrases of this
material simply after being stored for a
number of days so depending on them on
the shelf life that we're interested in
we need to look at storage conditions we
need to look and see well if we want to
maintain an amorphous phase do we store
it at room temperature or do we need to
store at a lower temperature and what
are the actual physical properties of
these of these you know what is my goal
what is what form do I actually want to
have so this is going to dictate our
storage conditions but this is how we
can use DSC as a very simple tool to
evaluate what form have I received my
material in vs. how do I want to then
process that material and send it out so
this leads us into a few more
definitions we don't only have one
simple type of polymorph there are
multiple different types of polymorphs
there are those that are reversible they
go through reversible phase transitions
there are also those which go through
irreversible phase transmissions so we
have the so-called and anti tropic
transitions these are reversible so as
we go from a low temperature state to a
high temperature modification we have
one form that is under all conditions
more stable at higher temperatures and
one that is more stable at lower
temperatures so if we look at a plot of
the Gibbs free energy of the two
different phases are the two different
polymorphs below a transition
temperature this TT we can see the pink
material has a lower Gibbs free energy
ie will be more stable below this
transition temperature whereas this
alpha the Alpha form above the
transition temperature has the lower
Gibbs free energy and this will be more
stable and at this higher temperature
range so depending on whether we want to
let's say that we want to have primarily
polymorph a what I can do is allow the
kinetics of recrystallization to happen
above the transition temperature and
this higher temperature this at this
higher temperature I'll end up with more
of this polymorpha a if I'm interested
in in polymorph B then what I can do is
maintain my material down at this lower
temp in this lower temperature
to ensure that I have the more stable
material down here and we can formulate
we can formulate you know reverse
reversible and irreversible polymorphic
transitions and look at these different
in ENT tropic forms there's a few
different rules that came out of the
1970s these were published in
microquimica ACTA by a professor Berger
and these are so-called the Berger rules
and the RAM Berger rules and these are
these relate to solid solid
transformations again going between
different solid states of of these
materials going between different
polymorphic forms and so what are these
rules say how do they guide us the only
reason we want to have rules is to help
us think through and understand what's
going on so the transition enthalpy of
these two in anti tropic forms that is
reversible forms so above the transition
point we're going to have an endothermal
effect going and below the transition
temperature
we actually have an exothermic event
endothermic and exothermic that's when
we're looking back to our DSC trace and
seen do I have heat flow into my
material endotherm or heat flow out of
my material exothermic so this is going
to be a nice guide for us looking at us
at a DSC trace and knowing whether I'm
getting heat flow into or out of am i
above or below my that transition
temperature so that's the heat of
transition rule if we're looking at the
heat of fusion rule so this is instead
of looking at transition heats what
we're doing is here is looking at
differences in the melting heat so the
melting enthalpy so if I have form a
versus form B am I going to get am I
going to have a higher heat of fusion or
higher heat of melting of form a or form
B how am I going to know well the higher
melting form so this is the material
that melts at a higher temperature it's
going to have a lower melting enthalpy
so that's the area under the curve for
this for this particular and antium a nd
sorry enantiomorphic pair so it's a
really nice simple rule the higher
melting form has a lower melting
enthalpy if the higher melting form
though also has a higher melting
enthalpy
the relationship is mana topic what does
that mean it means instead of
enantiotopic having this reversible
transition if the higher melting form
also has the higher melting enthalpy we
all we have a situation which is mono
tropic and we'll talk about monitor
traffic forms here in just a little bit
so you can imagine we have a lot of
different ways that DSC traces could
look going and we're looking here at
some data that's very very nice this was
again published in the Journal of
thermal analytical calorimetry this was
from a Swiss researcher working with
pharmaceuticals and this shows heating
curves from a DSC and a lot of different
potential ways that the curves could
look and we'll break them down one at a
time so what do we have so in this case
we have two different forms we have a
pharmaceutical compound which has two
different forms simply a and B and so
what's going on as we're heating our
material we have in this first case an
endotherm small endotherm and then
another larger endotherm so what we have
here is stable form a transforms and
form B so this is an endothermic effect
and then that form B melts what about in
conditioned to the next one down stable
form a simply melts and then the
transformation in to form B is hindered
kinetically what does that mean that
it's it's kinetically hindered it means
that we're going through this process
too quickly where we might transition
into into a into B this heating is
happening too quickly and this process
would take time so kinetically hindered
means that the process is happening too
quickly and the thermodynamic process
takes over so what about example 3 so we
have stable form a that's melting form B
so stable form a melts here form B is
growing from that melt so we're getting
a recrystallization into form be that
recrystallization is providing this with
a Morse from a dynamically stable
compound so that's giving off heat
that's why we have an exotherm and then
a form B is melting if we look down to
to this fourth example what we could
have is a meta state
Formby it's transforming into form a so
metastable form b has a higher Gibbs you
know has a higher Gibbs free energy then
that stable form a so we're giving off
heat to trans to transition from B to a
but then a melts a trans for re a
transforms into B and then B melts so we
can see a lot of different transitions
metastable be transforming to a a
transforming to B be melting and then of
course at the very end we have the most
simple version which is metastable form
B melts we have no transition to a no
transition back back to form b so let's
look at let's go ahead and look at this
let's let's go back to some DSC data if
i if i want to look at this enantiotopic
transition i have two different phases
of cesium chloride III so I have one
phase that is a face centered cubic and
one that is a body centered cubic so
this would be a structure that is more
like sodium chloride you know whether we
have a cubic like a straight square
versus something that looks more a
little bit more orthorhombic so cesium
chloride goes through these reversible
phase transitions just like the indium
or in a similar fashion to the indium
whereas the indium melts and
recrystallizes into a single form the
cesium chloride goes between two solid
forms so on the first heating we've got
the first teethin in red and i have a
nice onset temperature that occurs here
and then cooling I then go back into the
body centered polymorphic phase
polymorphic structure and this is then
something that occurs repeatedly I then
have a second heating and once again I
have this nice stable onset we can see a
slight difference we can see a slight
difference here and you might say well
this looks very unrepeatable what is
going on why do I have this difference
between this first and second heating as
I'm going through this first insight the
first and second heating my material is
physically changing and physically
adapting to the DSC pan as I'm heating
it so
if I have a second a third fourth fifth
heating I'm going to see a nice overlap
between those those subsequent heating's
and Cooling's whereas on that first
heating maybe just the way that I've
prepared the sample means that it's not
conformed to the DSC pan in an ideal
fashion or it simply rearranged itself
you know via gravity and this transform
it transition so I'm going this is not a
solid liquid transition this is simply a
solid solid transition and then a solid
solid transition back to the previous
phase so again enantiotopic this is
completely reversible and it's happening
we're having a thermodynamic process
this in this particular one and then I'm
sorry that was a miss that was me miss
speaking so we have both of these are
actually kinetic kinetically driven
processes we talked about an anti tropic
and those were those were situations
where we had two compounds that crossed
at some transition point if we look at
mono tropic polymorphic transitions
these ones are irreversible and so what
this means is that only one of these
solid state transformation
transformations is going to be observed
so if we begin with the less stable form
once we trend once we convert it into a
more stable form that's the only
transition that we're going to see so if
you have these polymorphs alpha and beta
of a particular material so a below a
given transition temperature the pink
form is more stable but above a given
transition temperature the form B is
formed and then we never actually go
back we never actually convert back into
the to the to the modification B to this
to this beta form so once we've
processed the material into the alpha
form it stays in that alpha form as
opposed to the N anti-entropic situation
where we have multiple transitions where
we can go back and forth between a and B
in these and a numeric situations with
these monitor Opik transitions it's a
little simpler so instead of five or six
examples we now just have three and we
will start with the most same the most
simple example which is this number two
we've got stable form a
simply melting this is the example that
we saw at the very beginning when we
were looking at something like sorbitol
or the ribavirin we have one stable form
that is melting and we have that
crystalline structure melting and and we
go from solid to liquid what we could
however have is that metastable form b
that is less stable at all temperature
at all temperatures than a transitioning
and wheres if we see this exotherm so we
go from a less stable to a more stable
form we will see that exothermic event
so we're transitioning to form a and
then form a melts in a similar fashion
to a form a melting here number two the
last example that we can see here is the
metastable form b doesn't convert to a
but but for a particular compound what
might happen is the meta stable form
melts and through the melts we then have
the freedom within the melt within that
molten structure that the stable form a
can form and then form a melts and so
you might wonder so I have a neck I have
an endotherm I have an exotherm was
there this overlap of these endothermal
and exothermic events so b melting is
going to absorb energy any any solid to
liquid transition that we're going to be
looking at here is going to involve heat
put into the system but then the
rearrangement of the molecules into a
stable form a into a more
thermodynamically stable arrangement
gives off heat so I go from adding heat
to the system to the system and that
sample then giving off heat and then
keeping again put into the system
through that melt so along with DSC
there are a number of different ways
that we can look at polymorphs x-ray
diffraction is a really popular
technique you can look at powders or
single crystal forms if you're able to
crystallize your material down to a
single crystal we can look at at
infrared spectroscopy or Raman
spectroscopy and in all these situations
we have you know non-destructive
techniques in which we are using a form
of you know a portion of the
electromagnetic spectrum
and then specialized detectors to
directly look at either the crystal
lattice or vibrate or vibrational
effects that are going on we can also
use thermal microscopy so using
high-powered microscopes and just like
we can do the same thing with with
thermal XRD to take our material through
different phase transitions or different
solid solid transitions and then look at
the look at the material structure
directly so if we have these other
techniques why use DSC well DSC is
something which we already have in the
lab for a lot of other purposes for
looking at looking at purity studies
looking at reaction kinetics and it's
also something that's very very easy to
handle sample prep is very easy and the
experiments can be carried out very very
quickly so these different complementary
techniques can be used and you might
find them all within your lab but DSC
can provide us as we've seen a lot of
this a lot of different information so
those were those were us building up all
this information all this knowledge
about our about our our systems let's go
through and look at some some examples
like I said we've got three examples
here to close out the webinar portion
and then we can then go through and look
at questions at a Q&A session at the
very end so what we have here as one
example is that paracetamol we've got
three different literature values so
we've got our three different forms form
one two and three as we can see we've
got form one this is following the
nomenclature that we would ideally like
to see for for most of our compounds
where the form one has the highest
melting temperature and then going down
to form two lower melting temperature
and even lower in some of these
situations we don't have a set heat of
fusion that can be found if we look up
literature values often what we'll see
is kilojoules per mole what we do then
if we want to look at this in terms of
the DSC and the way that we've prepped
our sample we can see the amount of
joules per gram so the area under the
curve of melting for these two different
for this these two different forms so
let's take some paracetamol let's heat
it up so again using a very small amount
of sample 2 milligrams heating it a
relatively fast heating
1010 Kelvin per minute is it I say fast
because one of the things that I'm used
to doing is looking at purity studies
and in purity studies were often using
0.7 to 1 degree Kelvin per minute so
doing
polymorph studies we can do much much
faster so we're using aluminium
crucibles again with a pierced lid in
this case so they're open to the
environment and what do we see we see an
onset temperature of 160 9.2 for the
onset well if we go back to our table
this shows us a very nice very clear
indication that we have form one I don't
see any effects happening at 156 degrees
Celsius so I know I see no effects here
in this region so what this tells me
very clearly from the DSC is that I have
this stable form one so let's heat the
material I've got that first heating I
then cool the material down and then I
heat the material a second time this is
a situation where I now have an entirely
different event so if we if we think
back to the the N and nto tropic
examples we had five different examples
we could have stable form a metastable
form b form b so what we have here is is
we have one form we have an n we have an
endo sorry an exothermic event right
here we have an exothermic event which
means a recrystallization or a
structural transformation we're going
from a less stable to a more stable
State
we're always going to know that's
happening when we have this exothermic
event and then we have another melting
this time at 150 6.9 degrees Celsius
this then very very nicely overlays with
that form - so based on the thermal
processing on the thermal history of the
material we can very clearly see as
received we have the form one after
processing it in the finished fashion
that we have heating it up at 10k per
minute cooling it down and reheating it
we can then arrive at form at the at
this B form at this form
- rather let's take another example we
have this second compound that we're
going to be looking at which is going to
be a little bit harder to pronounce if
you want to look at the actual structure
here this carbon metazine this one has
it's not only a little bit harder to
pronounce but also has a lot more
different forms and you can see this
mixture
we've got form 1 or alpha we've got form
a 2 B 2 3 or B 4 so a lot of different
structures we've got five different
structures that are reported in the
literature with a range of different
melting temperatures and we can find
literature values for also for the heat
of fusion or the the melting enthalpy
for these for three of these different
forms so even on the very first heating
of this material as received we can see
that there's one endothermic event with
a peak temperature of 160 degrees and
then we can see a second endothermic
effect event with this onset of about
190 degrees Celsius again small sample
10k per minute heating rate pierced lid
this is gonna be critical one thing that
we can look at so we've got the first
heating we have a nice stable baseline
all the way through until we get to
these events on a second heating after
cooling with a very fast cooling rate of
40k per minute what we can see here in
the very beginning is we have a step
change we have a step change in the the
specific heat of the material as you
might remember this is a glass
transition what types of materials have
glass transitions amorphous materials so
this what this indicates is that by
cooling the material very quickly we
have kinetically hindered formation of
crystal lattice at least in a portion of
the material and so we have an amorphous
structure that goes through this glass
transition we then have this endothermal
event or sorry EXO thermal event a
little bit off this morning we have this
exit thermal event which if you recall
and we'll say it again
any of these excess thermal events are
us finding it more thermodynamically
stable form we're going from a less
stable form to
more stable form or from an amorphous to
a crystalline of a more highly ordered
structure I then have a melting with an
onset of 186 point four degrees I can go
back and look at my table and compare
and see alright which form do I have
here in this state if I look at the same
material and instead of cooling with a
very fast cooling rate of 40 degrees
Kelvin per minute I heat my heat that
same material and I cool at 10 degrees
Kelvin per minute I have a very
different situation
so first heating this is what we saw in
the last one
now I cool down on my second heating I
no longer have that glass transition I
no longer have an exothermic
recrystallization I simply have that
same melting that I'm that I saw in the
lasts in the last curve so what's going
on its kinetics I mentioned that kinetic
hindrance can play a large part in the
way that we we have different polymorphs
in which forms exist when we cooled
really quickly we froze the material in
its amorphous state when we cooled more
slowly we allowed the material time to
find a more thermodynamically stable
form during the cooling process how is
this relevant for you as as
pharmaceuticals you know scientists or
engineers this is going to dictate the
way that we process our material is is
going to then lead to which forms are
present in our material and when we're
finished with it also looking that's at
shelf-life looking at how do we store
these materials if I need form a or form
B for bioavailability or for processing
how do I need to store these materials
so that I have the form that is desired
let's go ahead and look at another very
very common drug this is a sulfathiazole
this one has four forms reported in
literature for different melting melting
temperatures different heats of fusion
one thing that I can say is that all of
these webinars in this series will then
be posted on our website you can go back
through and download these we look
through this material at your leisure
and so I'm not going to be coming back
to these tables
I'm just gonna keep on plowing forward
through these so the sulfathiazole as
received as we're looking at this
material undergoes to two of these
endothermic events
remember if we look back to the Berger
rules if we're looking at these
endothermic events we're going from it
we're going we can look back and see
that we have a more stable going through
into a metastable form and then we're
actually getting a complete melting
right here we can also see a slight
transition we can see some effect that's
happening right here right before this
onset and this this is something that we
need to look at this could either be my
material conforming to the pan but if
this is a repeatable event this is
something that I look and say this
wasn't just my sample preparation what
we have here is is we might actually
have multiple different forms in the
material as received so I could have a
mixture of forms 1 2 3 & 4 in this case
I only have 3 forms I might have forms 1
2 3 1 3 & 4 however the case might be
but as we're heating and depending on
the heating rate we could also see
transitions from from more stable forms
or sorts from less stable forms to more
stable forms as we reach this final
melting one last example we'll go
through and look at and this is we
talked about polymorphism the last thing
I want to look at is what we talked
about in pseudo polymorphism so this is
this is something that we're going to
look at this is estradiol Hemi hydrate
and you see this second portion of the
of the of the name this Hemi hydrate
Hemi hydrate meaning that we have in the
crystal structure for every molecule of
of this pharmaceutical compound we have
half a molecule of of water of course we
don't have half molecules of water what
this means that is in bulk we have a
ratio of 2 to 1 pharmaceutical compound
to water in the crystal structure so two
simple forms form one form two let's
take this material and let's heat it up
as we've done with all of our other
material
and this is something that we haven't
seen before in the other examples now
instead of nicely nice very clearly
defined peaks with with nice peak shapes
now along with this melting peak we also
see this large broad endotherm ranging
from about 80 degrees Celsius all the
way up to about 150 degrees Celsius this
is where it's going to be critical for
us to look at the way that we process
our samples so we've got a couple
different samples we ran we ran the
samples in a couple different conditions
so we've got you know anywhere from 1 to
5 milligrams same heating rate we ran
these in aluminum and alumina so
aluminum oxide so metal and ceramic
crucibles under nitrogen atmospheres
wanted to see all right what exactly is
going on here
sometimes we need to look at these
samples from a slightly different
perspective when you look to the
material it's a Hemi hydrate hydrate
means there's water water in a lot of
cases is going to be available for
evaporation it's not going to stay with
the material throughout its whole
throughout the whole process so as we
heat up our material let's look at a
thermo gravimetric curve so if I look at
the mass of the sample as a function of
temperature as I have this endotherm
we've been looking at these solid solid
transitions so long you might be in the
mode to think this is a solid solid
transition when in fact what we have is
a solid is a solid to gas transition so
this might be solid going to liquid
liquid liquid converting to gas so that
hydrogen that Hemi hydrate that water
that's bound in the crystal lattice is
being released from the crystal lattice
and evaporating you can see very clearly
we have a very nice overlap between the
endotherm on my DSC trace and the mass
loss that we're seeing on the same
material run in this alumina crucible on
the TGA so for the DSC I ran a slightly
smaller amount on the TGA just to make
sure I got a little bit nicer signal
I ran five milligrams of this same
heating rate that way I could overlay
very nicely so what we're having is the
loss of water followed by the melting of
this crystalline structure
this is where the way that we run our
material is very very critical so if I
run my material and an aluminum crucible
with a pierced lid that is open to the
environment the water can evaporate I
have what we just saw we have the
evaporation followed by crystallization
if I run this in a closed door aluminum
crucible in which case the water has
nowhere to go that it's retained it's
retained there within the crystal
lattice then I have a more broad melting
peak influenced by the water that's
that's present so I no longer have this
nicely pure estradiol I have the
estradiol Hema hydrate which as we saw
in the last in the last webinar within
this series we saw purity determinations
we saw that solvation effects can
decrease that onset temperature and this
is this is exactly what we see going on
here very clearly so with with net
instruments we have two primary DSC
instruments that are designed for the
analysis of pharmaceutical instruments
we have the DSC two one for nevio and we
have the DSC 204 f 1 Nephi oh we can see
both of these are equipped with an
automatic sample changer one has a nice
magazine for 20 samples for
high-throughput labs or labs that are
operating both R&D and QC or that have
different groups we have this much
larger auto sampler for up to 192
samples that have these 96-well plates
accessories with a sample loader that is
barcode barcode controlled so that you
can go through and load your your
materials in if another researcher needs
to come in and load other samples they
can load theirs come back and reinsert
your samples and your your analysis will
continue on these differ slightly in
their temperature ranges but primarily
they differ in the number of samples
that can be loaded and the automation
that is that we're capable of so in
summary we look the polymorphs we looked
at polymorphs via differential scaling
scanning calorimetry and we used the
properties that the thermo physical
properties of these polymorphs there's
thermal
stability the kinetics of cooling to
look at different forms we saw that
polymorph site can have different forms
that are more or less thermodynamically
stable some forms which are metastable
so that they we can we can have
materials that are in general less
stable that are converting into more
stable compounds some that go through
transitions from stable to metastable
and back through two separate forms
before melting one of the reasons we
want to use DSC as a as a
characterization tool in combination
with some of these other techniques is
because the results are very fast and
these are these are quite fast results
instruments are very easy to operate
they're able to be automated and as we
saw in the very end for some some
materials that might not be pure they
might have solvents attached to them
looking at DSC as well as TGA or thermo
gravimetric analysis we can get a better
picture of what's going on within our
materials and one of the fantastic
things here at Natchez we also offer the
combined DSC and TGA so that both of
these at all times can be carried out
simultaneously as you know this is part
two of a four-part webinar series we
look forward to having you back with us
next month you know in October so the
9th of October of this year we're gonna
be looking at thermal stability and
shelf life of pharmaceuticals and we'll
be doing that with infrared spectroscopy
coupled to thermo gravimetric analysis
so we'll be doing we'll be looking at
kinetics of degradation and we'll be
doing some other studies using TGA
coupled to FTIR and then the final
portion we'll be looking at what many of
you might want for your laboratories
which is is fully automated routines so
basically starting with loading a
material into my sample crucible how do
I go from a simple sample prep all the
way to a finished report that just needs
approval via 21 CFR part 11 compliant
software so that's going to be the rest
of the remainder of our series and that
concludes the webinar portion of this
series now I'm going to go through and
answer a few questions that we have so I
first off again thank you for being part
of this series thank you for the
questions
that you have submitted and I will go
through and pull up those on the side
here and we'll get through to your
questions all right so first question
has to do with this first question is
from Steve in Illinois if question has
to do with the conventions the the DSC
traces why am I seeing D SH you know
endotherms up versus down Steve maybe
you missed the beginning of the webinar
this is just a convention this is in
many cases in the u.s. endotherms are
displayed and in doubt down words
whereas European Convention is
endotherms are displayed upwards this is
simply convention I have another
question an anonymous question this one
was on the TGA curve how come the TGA
was TGA curve was run in an Illumina
crucible versus the aluminum crucible
that was used for this for this Hemi
hydrate compound we could have run the
material and we could have run this in
an aluminum crucible that wouldn't have
been a problem this was just simply a
choice of what we used in the laboratory
in most cases for for selection of
crucibles what we want to look at is to
ensure that we have no interaction
between the crucible or pan material
with the pharmaceuticals or the
excipient Saur whatever we're were
analyzing the first consideration is non
interaction for a lot of pharmaceutical
ingredients this means that running in
aluminum is is ideal because aluminum
pans are relatively inexpensive and
disposable they can be pierced they can
be sealed and most of these transitions
actually I can say all of these
transitions that we're looking at are
occurring below the melting point of
aluminum at about 600 a little over 600
degrees Celsius so aluminum pans are
ideal for almost all of your
pharmaceutical applications needs lastly
if we're going to be doing purity
studies or we want to ensure that we
have highly clean crucibles if your
protocols or your methods dictate this
crucibles can be solvent cleaned using
acetone ethanol something that's 100%
purity or ACS grade pharma grade
solvents and then they can be baked so
we can ensure that we have we can we can
heat these up and we can handle them
appropriately to make sure that we don't
have any oils we don't have any residual
solvents this might be a little bit more
tricky looking at ceramic crucibles
ceramic crucibles also simply presents a
different heat transfer property
aluminum crucibles being metal have
extremely high heat transfer and so
we're going to have very very high
fidelity and a high response rate in our
DSC using aluminum pans versus I'm using
say the the ceramic crucibles that we
saw in the TGA on the TGA since we're
simply looking at mass loss and not heat
transfer per se we're not looking at
heat flow the simple mass loss of the
water is perfectly fine so really this
was a user consideration and that's it
we actually only had two questions
during today's webinar again thank you
if you have other questions that you
would like to submit that you maybe were
a little too shy to submit directly
through here please feel free to add
those in now any questions that you
submit after the fact will be addressed
by myself or by my colleagues over in
Germany again very last time thank you
so much for joining us we look forward
to having you next month for next
month's webinar and have a great rest of
your week
bye now
