Monograph 645 of the United States Pharmacopeia (USP) is American
legislation that has been in force a few short years.
It has a
direct impact worldwide on water analysis and quality in the
pharmaceutical manufacturing industries.
Water is the
world's most abundant solvent and naturally is one of the
primary vehicles for drug administration in pharmaceutical
Water for injection and purified water need to be
rigorously tested for trace impurities.
Before the updated
monographs, WFI and PW tests for chloride, sulphate, calcium,
ammonia, and carbon dioxide were done off-line in laboratories.
For example, the test for chloride ion was carefully to take a
grab sample, neutralise it with nitric acid and then to add
Depending on the chloride content, the sample
became more or less cloudy.
To determine whether the sample of
WFI or PW exceeded the chloride limits laid down, the turbidity
of the sample was compared against a standard solution.
this is a very labour-intensive method, and when added to the
requisite tests for sulphate, calcium, ammonia, and carbon
dioxide, proved highly inefficient.
Being a pass/fail only
methodology, it also gave no indication of how far the sample was
from the acceptable limit.
An alternative method was needed to
maintain or improve the existing water quality, improving the
reliability of testing, reducing the number of manual tests,
permitting qualitative in-line control, and ensuring quality
standards continue to apply in WFI and PW production methods
destined for the USA.
The alternative - electrolytic conductivity
Enter the measurement of electrolytic conductivity.
This is a well-established means of measuring water quality or
the concentration of a chemical solution.
The technique involves
measuring the ability of a liquid to pass an electrical current.
It is employed in industries as diverse as detecting trace
impurities in power generation steam turbine cycles, in
semiconductor etch rinse duty, food and beverage industry control
of product/water interfaces and detergents strength monitoring,
and general chemical industry concentration measurements.
Conductivity measurement has evolved over the years to provide
some very specific industry techniques so that it is now a very
simple to use method backed by highly sophisticated technology.
It is interesting therefore to see why the United States
Pharmacopeia chose to request devices without these advances.
Firstly it is necessary to review how conductivity is measured.
There are two basic requirements: a means of passing, or
inducing, a small alternating current through a precise volume of
liquid to be measured - this function is carried out by the
conductivity measuring cell; and an instrument capable of
supplying power to the conductivity measuring cell and measuring
small changes of electric current passing through the solution,
thereby providing means of indication, recording or transmission
of a current proportional to the electrolytic conductivity.
opposed to electron flow in 'metallic' conductors,
current in liquids is transported by ions.
conductivity it is necessary to have a conductivity meter and a
The cell embodies the electrodes and provides a
fixed geometry between the electrodes and the sample.
In view of
the wide variety of samples suitable for measurement and control
by their conductivity, and the wide range of measurable
conductivities, many types of cell are available with different
geometries, electrode sizes and cell materials.
requirement of USP Monograph 645 is that the cell constant is
This is an onerous task not fully appreciated by the
In relatively high conductivities, say over 500mS/cm
(microsiemens/cm - this is the standard unit of measurement for
conductivity), it is comparatively easy to source a calibration
solution and verify the validity of measurement.
It is even
possible to trace it back to national standards.
level, when one approaches the sub-10mS/cm range of WFI and PW
samples, it is impossible.
No realistic calibration solution is
Even if it were, it would be grossly unstable,
absorbing carbon dioxide from the atmosphere and rapidly
increasing its apparent conductivity value.
The solution is for
instrumentation suppliers to substantiate the validity of their
cell constants by comparing to traceable transfer standards.
During manufacture consistency of the engineering tolerances is
controlled via calibrated dimensional gauges and statistical
The in-house primary standard conductivity cells
are traceably calibrated on a regular schedule using a certified
National Institute of Standards and Technology (Nist)
conductivity solution transfer standard.
standard cells for the production line are then calibrated from
these primary standard cells.
Cells from the production batches
are calibration checked against secondary standard cells to
ensure conformity of manufactured cell constant (K) values to the
certified Nist standard.
These are measurements not taken lightly
and must be made under strict regimes.
Measurements are made at
elevated levels in a temperature-controlled environment and
readings compared with a traceably validated standard cell.
result is a sensor that has been verified to be within the +/-2%
tolerance demanded by Monograph 645.
If the supplier is
effective, the actual tolerance will be better, at +/-1% of K.
Some suppliers offer a correction factor to input into the
instrument enabling correction of manufacturing inefficiencies.
Others are guaranteed to be within tolerance without any further
A similar but altogether easier requirement is
validation of the instrument's accuracy and resolution to
better than 0.1mS/cm.
Modern microprocessors and signal
processing technology mean this kind of performance is readily
By replacing the cell with precision resistances
traceable to Nist, meter calibration is simply performed.
Re-calibrations, or rather re-validations, of the cell constant
and instrument performance are generally recommended on an annual
This is usually performed back at base by the instrument
There are a significant number of users who prefer to
do this themselves and have invested in traceable, portable
These very handy tools provide a valuable
service but they are not without their problems, as will become
evident further on.
Conductivity measurement essentially concerns
the mobility of ions through aqueous media.
These mobilities are
greatly affected by temperature and result in a range of specific
temperature coefficients ranging from 1-3% per C.
heat is applied dissolved ions become more mobile.
water molecule springs another surprise.
Water is readily
dissociated into highly conductive H+ and OH- ions.
and highly mobile ions have high coefficients up to 5% per C,
since the effect of temperature results in increased
Instrumentation suppliers have strived to provide
ever more accurate temperature compensation mechanisms for
These have proved a boon to users in
semiconductor manufacture or power stations where contaminants
are well defined.
The instrument measures the raw conductivity
that is, not temperature-compensated, and the process
Since the behaviour of pure water is known, its
contribution is extracted from the determined value.
resulting measurement is compensated with a dedicated curve
specific for a particular contaminant.
conductivities for concomitant species, ie, pure water and
contaminant, are re-combined to provide an accurate measurement.
Against this backdrop the pharmaceutical industry adopted
conductivity as a method of detecting levels of contaminant in
purified water and water for injection.
Monograph 645 was
established to supersede testing of trace contaminants.
coefficient should one use for chloride or sodium? The answer,
perhaps surprisingly, is none.
Rather than adopt a specific
temperature coefficient, the solution was to measure a raw
The pragmatic solution was to select the least
conductive impurity at a range of temperatures and to determine
the maximum allowable conductivity for the maximum concentration
permissible by the USP.
The supposition being that as long as one
operates below a certain band of values for low conductivity
species, then one will also be below the level of higher
Since the sodium chloride curve enshrined
in IEC746 doesn't always give the lowest conductivity
permissible by USP, this curve cannot be used as a pass/fail
Instead, USP adopted a composite table of its own.
so-called Stage 1 test requires that a manual sample is tested
for temperature and the conductivity limit is checked against the
If a measured temperature is not shown, the next
lowest is selected.
Automation enables an in-line Stage 1 test
whereby the instrument not only displays but also outputs both
parameters for permanent record.
An in-line sensor is often
mounted between the exit of a reverse osmosis system and the
inlet to a pure water storage tank.
Water is delivered to and
from the process from this tank.
To ensure rapid detection of
non-compliance, a fast re-circulating loop is also employed, in
which another validated measurement is installed.
sophisticated modern transmitters also incorporate the USP test
limits and permit an error band to be set so that the user can
operate up to a pre-set margin of safety.
An example would be
that the device is configured to alarm at 10% below the maximum
Such devices can also provide dual validated outputs
of conductivity and temperature to facilitate a permanent QA
record to be taken.
We can now return to the issue of portable
It is by now evident that when a continuous
sample is taken from a process for passing through a validation
unit, there is a risk of temperature errors.
If there is a
significant delta between process and sampled temperatures, there
could equally be a corresponding measured conductivity error.
This may not be due to actual differences in levels of
contaminants, but simply the effect of temperature.
is to keep sample lines short.
It should also be noted that
carbon dioxide has a propensity to infiltrate manual samples,
such that it will quickly raise the background conductivity.
tendency is for manual samples to read higher than expected,
particularly at low contaminant levels.
It is always better to
keep the conductivity cell enclosed in the measuring solution by
performing in-line measurements.
Quite often a manual sample will
be erroneous by the time it has reached the laboratory, purely
because of ingress from the atmosphere.
The great affinity of
carbon dioxide should also make the validation unit user wary.
Not only should sample lines be kept short, but the material of
transport line should be considered.
Either stainless steel or
nylon should be used.
Other plastic lines such as PTFE have a
tendency to permit the passage of carbon dioxide through the
walls and raise the local conductivity.
conductivity cells, the recommended orientation is with sensing
electrodes pointing downwards and attached at the top of an
Alternatively, side on connection is possible with the
Either way, effective in-line cleaning is assured by
pointing the sensor element in the direction of cleaning agents.
Conductivity cells are unaffected by their orientation per se
however they are affected by the collection of air which serves
to depress the apparent measured value.
Installation in the
branch off a line could exacerbate this situation and makes it
more difficult to guarantee effective sensor cleaning.
care has been taken, a perfectly successful Stage 1 test is
If the sample fails, then further testing is
Stages 2 and 3 are off-line and involve actively
encouraging ingress of carbon dioxide.
A sample is taken and
stirred vigorously until a stable conductivity value is
This should be below a maximum of 2.1mS/cm at 25C to
pass Stage 2.
Failing that, the subsequent Stage 3 involves
further adjustment of the ionic strength to target pHs and look
up against a check-up table.
The use of raw conductivity
measurements has provided the industry with a successful method
of determining contaminant control in WFI and PW applications.
Measurements can now be performed in-line, improving water
quality, improving reliability of tests, simplifying the
procedure and giving an insight into the degree of purity rather
than merely pass or fail.