BASIC
HIGH PERFORMANCE LIQUID CHROMATOGRAPHY
EFFECTS
OF pH ON THE RETENTION OF NEUTRAL, BASIC AND ACIDIC COMPOUNDS IN A
REVERSED-PHASE CHROMATOGRAPHIC SYSTEM
The main purpose of
this experiment is to observer and understand the effects of different pH on
the retention of neutral, basic and acidic compounds in a reversed-phase
chromatographic system.
The pH is
generally defined as pH = - log [H+], where [H+] is the
activity of H+. In most approximate applications, the pH is take as
– log [H+]. A solution is acidic if [H+] [OH-].
At 25oC, an acidic solution has a pH below 7, and a basic solution
has a pH above 7. Although pH generally falls in the range of 0 to 14, these
are not limits of pH. A pH of –1.00, for example, means – log [H+]
= 1.00; or [H+] = 10 M.
The pH of solution can be controlled by a buffering solution. A
buffering solution resists changes in pH when acids or bases are added or when
dilution occurs. The buffer is a mixture of an acid and its conjugate base.
There must be comparable amounts of the conjugate acid and base to exert
significant buffering.
In reversed-phase chromatographic system the component’s pH measured
are acid, neutral and basic and the effects of pH in a chromatographic
separation is noted when one changes the pH of a mobile phase and observe the
effects of such change. At low pH, the acidic components have a higher
capacitor factor together with the neutral components. However, the basic
components of a reversed-phase chromatographic system have a lower capacitor
factor at low pH. In the pH 5.5 the acidic components and the basic components
have the same capacitor factor and it is known as co-elution point. At pH over
the co-elution point the basic components have a higher capacitor factor than
the acidic ones. Consequently the acidic components eluted first that the
basic at pH higher than 5.5.
When one is using buffers in column packed at reverse-phase
chromatography needs to be aware that it can not use buffers in mobile phase
with a high concentration of methanol because the salts from the buffer
solution will precipitated consequently causing damage to the column.
Materials
and System Set-up:
Column: C18, 5 cm x
4.6 mm, 3 m m.
Detection: UV 254 nm
Chart Speed: 0.5cm/min.
Sample:250 m g/ml phenol
250 m g/ml aminophenol
12.5 m g/ml p-hydroxybenzoic acid
50 m g/ml Uracil
Mixture of the above.
All samples prepared individually in 50% MeOH and 50% water.
Injection Volume : 20 m l
Flow rate: 1.5 ml/min (depending on pressure)
Mobile phase: 200 ml each:
40% MeOH in 50mM buffers of pH 2.5; 4.0; 6.0 and 7.5 respectively. (80
ml MeOH + 120 ml buffers).
Procedure:
The Seneca College
technician already prepared the buffer solutions p 2.5; 4.0; 6.0 and 7.5. So,
it was mixed 80 ml of MeOH with 120 ml of each respectively buffer solution.
After that the mixture was filtered through filter in paper by vacuum to avoid
formation of bubbles of air and to do the filtration faster and in addition to
purify against a particles that could contaminate the solution.
All solutions to the mobile phase were prepared in the high standard of
purity to avoid the damage the column and consequently to lose the efficiency
of the column by contaminating the column with solutions containing big
particles.
The standards 250 m g/ml phenol, 250 m g/ml aminophenol, 12.5 m g/ml
p-hydroxybenzoic acid and 50 m g/ml Uracil were prepared individually in 50%
MeOH plus 50% water.
When in our case the solution, which contained a buffer pH 6.0 the base
line equilibrated in 5 minutes after start to run in the HPLC. However, it was
observed that other solutions with different pH it had a longer time to reach
a satisfactory base line.
Back Pressure for each run at each pH mobile phase:
|
pH 2.5
|
pH 4.0
|
pH 6.0
|
pH 7.5
|
Pressure
|
246~260
|
260~270
|
273~280
|
287~300
|
The pressure
increases with pH increases.
The
column was clean with a solution, which contained ACN (15%) because the high
concentration of water dissolves the salt from the buffer and after 5 minutes
of purge system is ready to be shut down.
The pH of a mobile phase is an important tool in HPLC because
silica-based packing are stable, in general, to all common organic solvents
and to aqueous buffers in a pH range of 2.0 to 7.5. At pH values below 2.0,
the bonded phases can be stripped off the packing, in extreme cases leaving
the bare silica substrate. At pH values above 7.5, the base silica can
dissolve, even to the extent that voids or channels are created in the packed
bed. However, some newer silica-based packing have extended pH tolerance.
In the pH range of 2.0 to 7.5 a pH control through buffer can be an
useful tool for improving resolution in developing a chromatographic
separation, since substances if acid or base change the capacity factor when
the pH of the mobile phase changes. A change of as little as 0.1 pH units in a
buffer can result in a retention time shift around 10% from one substance. So,
it is very important to measure and keep track of the pH of a mobile phase in
reversed-phase chromatography.
Results:
Part A:
Table #1. Raw Data:
Retention Time
|
pH 2.5
|
pH 4.0
|
pH 6.0
|
pH 7.5
|
Uracil
|
0.554
|
0.549
|
0.550
|
0.549
|
Phenol (cpd P)
|
1.392
|
1.362
|
1.395
|
1.370
|
Aminophenol (cpd A)
|
0.575
|
0.582
|
0.570
|
0.594
|
p-hydroxybenzoic (cpd H)
|
0.906
|
0.806
|
0.515
|
0.492
|
Peak 1 in mixture
|
0.559--A
|
0.567--A
|
0.513--H
|
0.525--H
|
Peak 2 in mixture
|
0.893--H
|
0.820--H
|
0.581--A
|
0.590--A
|
Peak 3 in mixture
|
1.400--P
|
1.358--P
|
1.381--P
|
1.372--P
|
A =
AMINOPHENOL; P = PHENOL H = p-HYDROXYBENZOIC
Calculated
Results:
CAPACITY
FACTOR OF COMPOUNDS
The mathematical
expression for K’ is:
K’ = T/To
-1 or (Vs – Vo) / Vo
Part
B:
Table #2. Capacity
Factor (k’) for Each Compound at Each pH.
|
pH 2.5
|
pH 4.0
|
pH 6.0
|
pH 7.5
|
Formula
|
(tR/0.554)-1
|
(tR/0549)-1
|
(tR/0.550)-1
|
(tR/0.549)-1
|
cpd A in mixture
|
0.009
|
0.033
|
0.056
|
0.075
|
cpd H in mixture
|
0.612
|
0.381
|
0
|
0
|
cpd P in mixture
|
1.527
|
1.474
|
1.510
|
1.499
|
A =
AMINOPHENOL; P = PHENOL; H = p-HYDROXYBENZOIC
A = AMINOPHENOL; P = PHENOL; H = p-HYDROXYBENZOIC
Discussion:
The graph where was plot
K’ versus pH the neutral compound (phenol) is identified by the curve with
showed that the capacitor factor (K’) did not show a strong variation when
the pH of the mobile phase was changed. However, the acidic substance (p-Hydroxybenzoic
acid) was identified because in an acidic mobile phase the substance showed a
high capacity factor (K’) but when the mobile phase change to more neutral
pH the capacity factor (K’) from the substance became drastically low.
Opposite behavior was observed to aminophenol that is an basic substance, at
low pH the aminophenol had a low capacity factor (K’) but when the pH of the
mobile phase became more neutral the capacity factor (K’) from the
aminophenol became higher.
The equilibrium constant for the ionization of a weak acid in water is
called an acid ionization constant (Ka). An example, acetic acid ionizes in
water.
CH3COOH
(aq)
« H+ (aq)
+ CH3COO- (aq)
And its ionization at 25oC
is
Ka = [H+]
[CH3COO-] / [CH3COOH] = 1.76 x 10-5
The pKa of acetic acid at 25oC is
pKa = - log 1.76 x 10-5 = 4.754
Acid is the substance
that ionizes in water to form H+ (aq) ions. Base is the substance that ionizes
in water to form OH- (aq) ions. Neutral substance in which hydrogen ion and
hydroxide ion are in equilibrium or absence of both ions.
The pKa from phenol is 9.89, from p-Hydroxybenzoic acid is 4.48 and from
aminophenol is 10.46.
A buffer contains the ion and the acid. If they are present in equal
amounts, the resulting pH is the pK of the buffer. A buffer has always has its
highest buffering capacity at its pK.
At low pH 4.48 the p-Hydroxybenzoic the substance is protonated.
Consequently the substance retains longer in the column. However when the pH
of the mobile phase increases the p-Hydroxybenzoic has a low retention because
the substance is less protonated. Opposite behavior is observed with
aminophenol that has a pK of 10.46 at low pH the aminophenol is non-polar the
consequently retains shorter. However, at a higher pH the retention changes to
a higher retention because the substance is more protonated. However the
phenol does not show a great changes in retention because it is a neutral
substance. So, in reversed phase, the retention of acid decreases and the
retention of bases increase with increasing pH. In another words when the
stationary phase does not have polarity as in reversed phase chromatography
the capacity factor (k’) of acids increases when the mobile phase has a low
pH and the basis shows a low capacity factor (k’) when the mobile phase has
a low pH.
Index
|