Checking the accuracy of

P H A R M AC E U T I C A L
application note
Checking the accuracy of
PERKIN-ELMER POLARIMETERS
H. Stenz, Bodenseewerk Perkin-Elmer GmbH
Abstract
Quartz standards, sucrose solutions
and optically active pure liquids are
discussed as potential polarimetry
standards. It is suggested to preferably use quartz standards for checking the accuracy of Perkin-Elmer
polarimeters. Quartz standards are
absolutely stable, have a high
accuracy and are easy to use.
For laboratories working in an
environment which is governed by
GLP or GMP principles, it is compulsory to regularly check the accuracy
of the analytical instruments used.
The methods to be employed for
these checks inevitably depend on
the type of instrument concerned.
This paper reviews the methods and
standards which are commonly used
(or have been considered in the
literature) to check the accuracy of
Perkin-Elmer precision polarimeters
(Model 341 and Model 241 families
of instruments).
Before entering into this discussion, it is helpful to briefly describe
the operating principle underlying
Perkin-Elmer polarimeters. PerkinElmer polarimeters employ an
optical null principle with automatic
analyzer adjustment. An optically
active sample generates an electrical
signal which in turn drives a motor
linked to the analyzer of the instrument via a precision gear train. This
signal rotates the analyzer by exactly
the angle by which the sample has
rotated the plane of polarization of
the optical beam. During analyzer
adjustment, an optical encoder
connected to the analyzer drive
system directly monitors the size of
the angle by which the analyzer is
being rotated. Thus, similar to the
divided circle provided on classical
visual polarimeters, the angle of
rotation is directly obtained by the
primary measurement. That is, the
primary measurement does not
involve any auxiliary parameters
(such as electrical voltages or
currents) which would need to be
calibrated into angles of rotation, as
is the case with other approaches to
design polarimeters, for example
when the Faraday compensation
principle is employed. Also, high
long-term stability of the operating
wavelengths is achieved by employing spectral line sources. Thus, the
ageing of interference filters and
lamps has no critical effect on the
accuracy as would be the case with
continuum source lamps. From a
technical point of view, it is therefore highly unlikely that the accuracy of Perkin-Elmer polarimeters
degrades over time; in fact, a significant loss of accuracy is conceivable
only in the context of a major
instrument malfunction.
Nevertheless, the user of the
polarimeter is required by GLP
principles to check and document
that the instrument is functioning
properly and yields correct results. It
is therefore necessary to check the
accuracy of the instrument at regular
intervals using a reliable independent approach. Technically speaking
a monthly to yearly check is usually
sufficient for Perkin-Elmer polarimeters, depending on how intensely
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the instrument is used, but this
recommendation is not to supersede any local FDA or GLP guidelines and practices which specify
a more frequent performance
verification. Empirical experience
and good record keeping will guide
in the development of an appropriate testing schedule. The method or
standard that is used is dependent
upon the target accuracy to which
the polarimeter is to be tested, or
what target accuracy is needed in
the daily use of the instrument
(as far as the target accuracy is
determined by properties of
the polarimeter).
Doubtlessly, quartz standards
(also called quartz control plates)
are by far the best and most reliable
tools for testing the accuracy of
polarimeters. Their use is also
suggested in some of the pharmacopoeias [1]. These quartz standards consist of polished plane
parallel disks of single crystal
quartz material. The disks are cut
from the crystals in a special
crystallographic direction so that
they exhibit circular birefringence
in the direction of the optical beam
path. That is, these quartz disks
rotate the plane of polarization of
radiation in the same way as
optically active samples do [2, 3].
The amount of rotation is determined by very precisely known
optical constants of crystalline
quartz material [4]. At a given
wavelength and temperature the
amount of rotation shown only
depends on the thickness (on the
length of the beam path) of the
quartz disk. As a consequence, the
rotation value of a given quartz
standard virtually is an invariable
property which is not subject to any
changes resulting from ageing or
use of the standard, unless the
standard gets seriously damaged.
Quartz standards for small
rotations (smaller than approximately 5 degrees of arc) require a
special approach. Quartz disks that
would produce this small degree of
rotation would be so thin that they
would be very fragile and extremely
difficult to manufacture and
Perkin-Elmer Model 341 Polarimeter.
handle. This difficulty is overcome
by ‘dual’ quartz disks. A quartz disk
of the desired sense of rotation,
being sufficiently thick to be
mechanically stable (thickness
approximately 0.7 to 0.8 mm), is
combined with a second disk, which
has the opposite sense of rotation
and which is thinner than the first
disk by a certain small amount.
The second disk thus compensates
for most of the optical rotation
introduced by the first disk, and
the thickness difference between
the two disks determines the
resulting optical rotation of the
pair. Using this approach, it is
even possible to manufacture
quartz standards for small optical
rotations, with all the advantages
which quartz control plates offer
as standards for optical rotations.
The optical rotation of a quartz
disk depends only on the mechanical thickness of the disk and the
refractive indices of the quartz
material at a fixed wavelength and
temperature. We should therefore
be able to calculate the optical
rotation of a quartz disk at a certain
operating wavelength from these
data. However, these calculations
are not sufficiently accurate, since
the thickness of the quartz disks is
usually not known with the required
accuracy and precision. This
statement is true especially for the
‘dual’ quartz disks, where the optical rotation is determined by the
small difference in thickness of
the two disks. Therefore, the optical
rotation of quartz standards must
be determined by calibration
experiments. The PhysikalischTechnische Bundesanstalt (PTB) at
Braunschweig, Germany (a German
bureau of standards), performs these
calibrations for a wide range of
wavelengths in the visible spectral
range. Under P/N B091-2009,
Perkin-Elmer offers primary quartz
standards calibrated by the PTB.
The standards are provided in a
thermostattable housing which is
compatible with all Perkin-Elmer
polarimeters, and include the
corresponding certificate of the PTB.
The desired nominal rotation and
the operating wavelength (visible
spectral range only) is to be
specified when ordering.
As an alternative to quartz
standards calibrated by the PTB,
Perkin-Elmer offers secondary
quartz standards for nominal optical
rotations of +1° and –1° 589 nm
(P/N B009-8800 and B009-8799,
respectively). These secondary
quartz standards are calibrated by
Perkin-Elmer using a computer-aided
procedure. The procedure always
includes measurements taken with
corresponding primary standards
which have been calibrated by the
PTB. This approach permanently
monitors the accuracy of the instrument used for the calibration, and
provides full traceability of the
secondary standards. Comprehensive
error checking of the data acquired
during the calibration and proper
correction calculations guarantee
a high degree of accuracy of the
calibration obtained. The secondary
standards are calibrated at the NaD
spectral line (589 nm) and at the
Hg spectral lines at 578 nm, 546 nm,
436 nm and 365 nm. They offer a significant price advantage over primary
quartz standards, while their calibration is only scarcely less accurate
than it is for primary standards.
For all standards used to check
the calibration of analytical instruments, the ideal condition would be
that the potential calibration error of
the standard is significantly smaller
than the permissible inaccuracy of the
instrument to be tested. Unfortunately,
this requirement usually cannot be
met in polarimetry. Even when using
quartz standards, the inevitable
tolerances of the certified rotation
values are in the same order of
magnitude as the accuracy which can
be achieved with modern automatic
polarimeters. This situation must be
taken into account when evaluating
the results of a calibration check.
The tolerance interval of the quartz
standard must be added to the
specified potential inaccuracy of
the instrument in order to obtain
the permissible range of rotation
readings. The instrument being
evaluated is clearly out of specification only when the standard yields
readings outside of the sum of
tolerance intervals.
Some pharmacopoeias suggest
the use of sugar solutions to test the
linearity of polarimeters [1]. In fact,
the optical activity of sucrose is
so well known [5, 6] that precisely
prepared sugar solutions may also be
considered as standards for checking
the absolute accuracy of polarimeters
[7]. Sucrose reference material used
for this purpose is available from
several institutions [8, 9]. The
sucrose solutions should always be
prepared freshly since they tend to
degrade after several days of storage.
Deionized or distilled water should
be used; use of HPLC grade or milliQ
water is highly recommended. Also,
the concentration of the solutions
should preferably be determined by
weight (for highest accuracy, buoyancy correction will be needed), as
the accuracy of volumetric
flasks may often be insufficient for the testing
accuracy that is required.
For solutions with higher
concentrations, great care
should be taken to make
sure that the solutions are
completely homogeneous,
and thermostatting of the
solutions is strongly
suggested.
As compared to quartz
standards, use of sucrose
solutions may produce
misleading test results, due
to potential errors which
might occur in the preparation of the solutions. Also,
effects of the polarimeter
cell used might influence
the test results when testing
Perkin-Elmer secondary quartz standard with certificate.
polarimeter accuracy using sucrose
solutions. In the literature, inert and
optically active pure liquids have
also been discussed as standards for
polarimetry; especially the use of
(–)-2-methyl-1-butanol has been
suggested [10, 11]. This material is
sufficiently stable in chemical and
physico-chemical respects, and being
a pure liquid, it does not involve the
risk of preparation errors. Therefore,
the material may be considered a good
canditate for a polarimetry standard.
However, it can be used as a relative
standard only, because the material,
in general, is not sufficiently pure. It
would be difficult to expect the optical
rotation of two samples taken from
two different manufacturing batches,
for example, to be equal within narrow
tolerance limits. Thus, (–)-2-methyl-1butanol may be useful to monitor the
reproducibility of readings of a given
instrument or to compare instruments,
as long as material from the same stock
reagent container is used. Checking the
absolute accuracy of polarimeters
down to narrow tolerances, however,
will not be possible with this material.
In summary, it can be stated that
quartz standards are highly precise
and reliable tools to check the accuracy of polarimeters. They are easy to
use, and their lifetime is almost
unlimited. Considering the high
accuracy of Perkin-Elmer polarimeters,
thermostatting of the quartz standards
is strongly suggested for the test
procedure to provide the most accurate and reproducible results. When
employing sucrose solutions, greatest
care should be taken in the preparation of the solutions. Also, attention
should be given to the polarimeter
cell used. When aiming at highly
accurate measurements the residual
cell rotation should preferably not
exceed about 10 millidegrees. Optically active pure liquids (such
as (–)-2-methyl-1-butanol) will, in
general, not be sufficiently pure
(chemically as well as enantiomerically) to be used as absolute
standards and should be employed
as relative standards only.
References
[1]
European Pharmacopoeia, 3rd Edition,
published 1996 by the European Council,
Strasbourg (published in English, French
and German).
[2]
Ewing, G.W., Editor, Analytical Instrumentation Handbook, Marcel Dekker, New York
1990.
[3]
Flügge, J., Grundlagen der Polarimetrie, De
Gruyter, Berlin 1970 (in German).
[4]
Lowry,T.M., and W.R.C. Coode-Adams,
Phil.Trans. A226, 391 (1927).
[5]
Hermann, G., and J. Haus, Encyclopedia of
Applied Physics,Vol. 14,VCH Publishers Inc.,
1996.
[6]
Organisation Internationale de Métrologie
Légale, Recommendation OIML R14,
Edition 1995(E).
[7]
United States Pharmacopeia, USP 23,
United States Pharmacopeial Convention
Inc., Rockville MD 1995.
[8]
National Institute of Standards and
Technology, Gaithersburg, MD 20899, USA,
Standard Reference Material 17e Sucrose.
[9]
Institut für Technologie der Kohlehydrate
Zuckerinstitut e.V., D-38106 Braunschweig,
Germany, Saccharose-Referenzmaterial.
[10] Vandenbelt, J.M., Rotation Standards for
Polarimetry, Reports on Analysis Techniques
No. 36 E, Bodenseewerk Perkin-Elmer
GmbH, D-88662 Überlingen 1974.
[11] Fluka AG, Buchs, Switzerland, Material
No. 65979 Überlingen, 20. 11. 98.
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