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Description  |
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BACKGROUND OF THE INVENTION
Copending application Ser. No. 07/166,211 filed on Mar. 10, 1988, is
incorporated herein by reference.
FIELD OF THE INVENTION
The present invention relates to sample holders or reflectors for use in
near-infrared reflectance spectrophotometers.
PRIOR ART
Near-infrared reflectance spectroscopy ("NIRS"), often used for analyses of
agricultural products, C.A. Watson, Anal. Chem. 49(9). 835A (1977), is now
being used for analyses of pharmaceuticals as well. In recent reports,
NIRS has been used to determine the particle sizes of pharmaceutical raw
materials and to perform qualitative analyses of powdered product
mixtures. E.W. Ciurczak, R.P. Torlini, and M.P. Demkowicz, Spectroscopy
1(7), 36 (1986); E.W. Ciurczak and T.A. Maldacker, Spectroscopy 1(1), 36
(1986).
In the past, it has been difficult, if not impossible to use NIRS to
analyze typical pharmaceutical tablets and capsules. For example, an
aspirin tablet is too small for analysis in an ordinary NIRS instrument
because it weighs only about 400 milligrams and occupies a volume of about
250 microliters. A tablet the size of an aspirin would not begin to fill
conventional solid-sample holders. In addition, capsules are difficult to
analyze with NIRS because they have odd shapes and require unique
positioning and support for analysis. Furthermore, most solid-sample
holders are designed for powdered samples, requiring that the integrity of
the tablets or capsules be destroyed and the resulting powders pooled
before analysis may be accomplished with NIRS. Potential NIRS
applications, and even routine quality control, such as the detection of
product tampering, are unnecesearily complicated by this pooling and/or
grinding requirement.
Focusing reflectors for small samples have been designed for use in NIRS
instruments. To date, however, these reflectors have required grinding or
pooling of the sample. T. Hirschfield, Paper 1093. presented at the
Pittsburgh Conference, New Orleans, LA, Feb. 1985. Focusing reflectors
also are problematical because many NIRS samples are inhomogeneous. Thus,
if the incident radiation is focused on too small a spot, the probability
of obtaining an unrepresentative result increases.
In addition to problems experienced in obtaining holders or reflectors for
analyses of certain solid sample types, such as intact capsules and
tablets, problems in obtaining holders to measure small liquid samples
have been experienced as well. Most available liquid analysis holders or
reflectors are cumbersome and expensive. A typical liquid holder or
reflector requires that a relatively large volume of sample be used (on
the order of milliliters) and that complex purge/fill and wash cycles be
utilized to prevent clogging. If clogging does occur, cleaning can be
difficult.
Accordingly, it is an object of present invention to provide a sample
holder or reflector for small samples, such as pharmaceutical tablets, for
analysis using NIRS.
It is another object of the present invention to provide a sample holder or
reflector for capsules for analysis using NIRS.
It is a further object of the present invention to provide a sample holder
or reflector which permits a sample, such as a tablet or capsule, to be
analyzed using NIRS, without grinding or destroying the integrity of such
a sample.
It is an additional object of the present invention to provide a holder or
reflector for a liquid microcell for analysis using NIRS.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of the preferred embodiment of the holder
or reflector for capsules.
FIG. 2 is a top view of the preferred embodiment of the reflector or holder
for capsules.
FIG. 3 is a cross-sectional view of a preferred embodiment of the holder or
reflector for tablets.
FIG. 4 is a cross-sectional view of the preferred embodiment of the holder
or reflector for liquid microcells.
FIG. 5 is the near-infrared calibration for sodium chloride in water,
obtained with the preferred embodiment for liquids.
SUMMARY OF THE INVENTION
The foregoing objects, advantages and features of the present invention may
be achieved with NIRS holders or reflectors for pharmaceutical samples and
liquids. The present invention is a holder for samples, such as tablets
and capsules, and liquid samples, such as blood. The holder comprises a
main body comprising a metal disc with polished substantially 90.degree.
right-circular cone-shaped receptacle therein. This main body is of a size
and shape which fits into a sample holder drawer of a near-infrared
reflectance spectrophotometer. The preferred embodiment for tablets also
comprises a polished insert, comprising a cylindrical portion and a
substantially right-circular 90.degree. insert cone portion which is
oriented in an opposing direction to the cone-shaped receptacle of the
main body. The insert is inserted into the cone-shaped receptacle. The
preferred embodiment for liquids also comprises an insert comprising a
cylindrical portion and a substantially 135.degree. insert cone portion
which is oriented in an opposing direction to the cone-shaped receptacle.
The holder for the tablets and capsules also comprises a wire for
positioning and stabilizing a sample suspended above and across the
cone-shaped receptacle of the main body, while the holder for liquids has
a cavity slide and cover slip for holding a sample also suspended above
and across the cone-shaped receptacle.
DETAILED DESCRIPTION OF THE INVENTION
A. Preferred Embodiment For Capsules
FIG. 1 shows a holder or reflector generally denoted by numeral 1. Holder 1
comprises main body 2, which comprises a metal disc with a substantially
90.degree. right-circular polished cone-shaped receptacle 3 therein. Main
body 2 should be constructed from a material which is reflective in the
near-infrared region of the spectrum or plated with such a material.
Gold-plating is preferred. In addition, if main body 2 is not plated, it
preferably should be constructed from a single block of such material.
Main body 2 is round and of a size and shape which fits into a
solid-sample drawer of a near-infrared reflectance spectrophotometer, such
as an InfraAlyzer 400 spectrophotometer. In addition, the dimensions of
main body 2 must comport with the diameter of the incident beam of the
near-infrared reflectance spectrophotometer.
A second smaller hole or receptacle 4 of a small diameter may be located at
the vertex of cone-shaped receptacle 3, but is not required for this
embodiment. A capsule 5 is secured in cone-shaped receptacle 3 of main 2
body by a wire 6 which is suspended above and across said cone-shaped
receptacle 3. Wire 6 forms a loop 7 directly above the center of
cone-shaped receptacle 3 of main body 2 and is for holding and stabilizing
capsule 5. A cylindrical quartz sample holder with a cavity therein also,
may be placed in loop 7 for the purpose of holding and stabilizing capsule
5. As more fully shown in FIG. 2, wire 6 may be secured at opposing sides
of main body 2, preferably by screws 8 and 9. Capsule 5, also, may be
positioned and secured in cone-shaped receptacle 3 by means other than
wire 7, such as by a container made from quartz, glass or diamond which is
capable of fitting directly into cone-shaped receptacle 3. Other means
also may be used.
This holder or reflector is especially configured to reduce specular
reflectance. When empty, holder or reflector 1 reflects radiation back
toward a source 10, parallel to the incident beam of that source. When
capsule 5 is positioned along the axis of rotation of main body 2, the
specular reflectance can be minimized while the diffuse reflectance is
maximized. Radiation reflected from the surface of capsule 5 is returned
to source 10 when the incident radiation is perpendicular to the base of
main body 2--this is the configuration used in most spectrophotometers.
Radiation is then focused along the length of capsule 5. Any radiation
that might pass through capsule 5 without being scattered is also returned
to source 10. Therefore, the bulk of the radiation which reaches a
detector on a NIRS instrument is radiation scattered by the contents of
capsule 5.
If the base of main body 2 is uniformly illuminated by collimated radiation
(as is the case with most spectrophotometers), the amount of radiation
incident on any given segment of capsule 5 is directly proportional to the
curved surface area of a frustum (a conic section taken parallel to the
base of cone-shaped receptacle 3 of main body 2) in which it lies. In
turn, the frustum near the vertex of cone-shaped receptacle 3 and the
frustum near the base of the same cone-shaped receptacle do not have the
same curved surface area. (The curved surface area of a frustum is given
by .pi.s(r.sub.1 +r.sub.2), where r.sub.1 and r.sub.2 are the radii of the
base and top of a right-circular frustum, respectively, and s is the
length of the generator line, i.e., the length between the top and bottom
measured along the surface of cone-shaped receptacle 3).
For example if the length of capsule 5 is divided into 1 millimeter
segments and these segments are numbered from 1 to 20, starting at the end
of capsule 5, i.e., toward the vertex of cone-shaped receptacle 3, the top
segment of the capsule (i.e.. segment no. 20) will receive 39 times more
light than the bottom segment (i.e., segment 1). In fact, the amount of
light (P) received by a particular segment numbered R (the height of the
section above the inverted vertex of cone shape receptacle 3) is given by:
P=k(2.pi..sqroot.2)R-.pi..sqroot.2 (1)
Because each 1 millimeter section of capsule 5 does not have a separate
detector in the NIRS instrument, the detector inside the integrating
sphere of the NIRS instrument integrates the signal from the entire
capsule to produce the detector response:
detector response=k'(.pi..sqroot.2)R'.sup.2 -(.pi..sqroot.2R'), (2)
where k and k' are proportionality constants that depend principally on the
amount of incident radiation and the nature of the material in the
capsule, and R' is the total number of vertical capsule segments filled
(i.e., from R=1 to R').
The diameter of the incident beam of an InfraAlyzer 400 spectrophotometer,
for example, is 26 millimeters. Such a proportionment causes direct
illumination of the upper segments of capsule 5 (i.e., R=13 to 20) by the
incident beam to be the predominant factor in producing a signal from this
region. The amount of light on each segment decreases exponentially as the
segment number is decreased in this zone. Of course, the entire
cone-shaped receptacle 3 is filled with scattered light, and the thickness
and composition of the wall of capsule 5 are not uniform over the capsule
length. These two factors, combined with the probable sample
inhomogeneity, prevent a simple analysis from completely explaining the
signal observed from an individual capsule. The overall response, however,
follows the trends outlined above.
B. Preferred Embodiment For Tablets
FIG. 3 shows a sample holder or reflector for small samples such as
tablets, which is generally denoted by numeral 1A. The holder has a main
body 2A comprising a metal disc with a substantially 90.degree.
right-circular polished cone-shaped receptacle 3A therein. Main body 2A
should be constructed from a material which is reflective in the
near-infrared region of the spectrum or be plated with such a material.
Gold-plating is preferred. In addition, if main body 2A is not plated, it
preferably should be constructed from a single block of such material.
Main body 2A is round and fits into a solid-sample drawer of a
spectrophotometer, such as an InfraAlyzer 400 spectrophotometer, in place
of the conventional closed sample cup. In addition, the dimensions of main
body 2A must comport with the diameter of the incident beam of a
near-infrared reflectance spectrophotometer.
A second smaller diameter receptacle 4A, preferably 2 millimeters in
diameter, is located at the vertex of cone-shaped receptacle 3A in main
body 2A and serves to receive and stabilize an insert 5A. Insert 5A
comprises a cylindrical portion 6A, which may be inserted into second
receptacle 4A, and a substantially 90.degree. right-circular polished
insert cone portion 7A. Insert 5A preferably should be constructed from a
material which is reflective in the near-infrared region of the spectrum
or plated with such a material. Gold-plating is preferred. Oriented in an
opposing direction to cone-shaped receptacle 3A of main body 2A, insert
cone portion 7A directs light passing around a sample tablet 8A up
underneath the tablet. Tablet 8A is suspended above and across cone-shaped
receptacle 3A on a wire 9A, containing a loop lOA for holding and securing
tablet 8A directly above insert 5A. A cylindrical quartz holder with a
cavity therein also may be placed in loop lOA to hold tablet 8A. Wire 9A
preferably should be made from a sturdy metal wire and be gold-plated and
is preferably 8 millimeters in diameter. Wire 9A is attached to main body
2A at opposing sides of main body 2A, preferably by screws IIA and 12A.
Tablet 8A also may be positioned and secured in cone-shaped receptacle 3A
by means other than wire 9A, such as by a container made from quartz,
glass or diamond which is capable of fitting directly int cone-shaped
receptacle 3A. Other means also may be used.
The bottom of tablet 8A is illuminated by a double reflection. First,
collimated light from a light source 13A is directed perpendicularly onto
holder 1A and reflected off main body 2A. Second, reflections from insert
cone portion 7A of insert 5A recollimate the light back in the direction
of light source 13A. At this point the light is intercepted by sample
tablet 8A and scattered into the integrating sphere and the detector of a
NIRS instrument. The preferred embodiment of the present invention for
tablets operates in the same manner as that of the preferred embodiment
for capsules.
C. The Preferred Embodiment For Liquid Microcells
FIG. 4 shows a holder or reflector for a liquid microcell, generally
denoted by numeral 1B, for use in a near-infrared reflectance
spectrophotometer, such as a Technicon InfraAlyzer 400. Liquid microcell
holder 1B comprises a main body 2B comprising a metal disc with a
substantially 90.degree. right-circular cone-shaped receptacle 3B therein.
Cone-shaped receptacle 3B preferably has a height and a base radius of 13
millimeters, however, other dimensions may be satisfactory. Main body 2B
should be constructed from a material which is reflective in the
near-infrared region of the spectrum or plated with such material.
Gold-plating is preferable. In addition, if main body 2B is not plated, it
preferably should be constructed from a single block of such material.
Main body 2B is of a size and shape which fits into the solid-sample
drawer of a near-infrared reflectance spectrophotometer in place of the
standard closed sample cup. In addition, the dimensions of main body 2B
must comport with the diameter of the incident beam of the near-infrared
reflectance spectrophotometer.
A smaller second receptacle 4B is located at the vertex of cone-shaped
receptacle 3B in main body 2B and serves to stabilize an insert 5B. Insert
5B comprises a cylindrical portion 6B, which may be inserted into second
receptacle 4B, and a substantially 135.degree. insert cone portion 7B.
Insert cone portion 7B is oriented in the opposing direction to that of
cone-shaped receptacle 3B of main body 2B and preferably has a vertex of
135.degree.. Insert 5B preferably should be constructed from a material
which is reflective in the near-infrared region of the spectrum or plated
with such a material. Gold-plating is preferred.
A standard single-cavity microscope slide 8B with a cavity therein
(preferably 25.times.76 millimeters) is centered with cover slip 9B
(preferably 22.times.22 millimeters) over cone-shaped receptacle 3B. The
position of slide 8B may be made stable and reproduoible by resting it
against screws lOB and llB fastened onto main body 2B, screws lOB and llB
preferably being placed at either end of slide 8B.
The use of cavity slide 8B in the present invention has some distinct
advantages over a conventional flat microscope slide: (1) it provides a
longer and more reproducible optical pathlength, (2) cover slip 9B acts as
a lid on the cavity in cavity slide 8B and lowers the liquid-sample
evaporation rate, and (3) the cavity shape acts as a lens to scatter
transmitted light into the integrating sphere of a near-infrared
reflectance spectrophotometer. When completely filled with a liquid cell
sample single-cavity slide 8B (which can be obtained from Dickinson and
Company, Parsippany, NJ, #3720) and an ordinary cover slip 9B (which can
be obtained from American Scientific Products, McGaW Fark, lL, #M6045-2)
can contain from about 70 to 110 microliters of sample. However, different
slides and cover slips with different masses can be used to vary the
optical pathlength and the sample cell volume. Cover slip 9B actually
floats on the sample, while heavier cover slips tend to squeeze the sample
and reduce the cell volume.
Insert cone portion 7B returns collimated light that passes through slide
8B, back through slide 8B and parallel to the walls of cone-shaped
receptacle 3B. This design allows the bulk of the light that passes
through the liquid in the cavity to be reflected directly into the
instrument's integrating sphere at a 45.degree. angle from the source
light. In this design the 135.degree. insert cone 7B portion of insert 5B
is placed atop a small cylinder because the sample is actually below the
integrating sphere; if insert cone portion 7B were to be lowered to the
bottom (vertex end) of cone-shaped receptacle 3B much of the reflected
light would miss the window of the integrating sphere.
Applications of the Licuid Microcell Embodiment
In the initial tests of this liquid cell holder or reflector, a set of
aqueous sodium chloride solutions was run. The determination of sodium
chloride in water can be difficult for several reasons. These reasons
include: (1) that sodium chloride has no absorption bands in the
near-infrared; (2) that water has very strong absorption bands in the
near-infrared; and (3) that these water absorption bands are very
temperature-dependent. Nevertheless, successful determinations of aqueous
sodium chloride in concentrations from 30-38 grams per liter have been
reported by using four wavelengths selected in a standard multiple linear
regression procedure.
Twenty aqueous solutions of reagent-grade sodium chloride (ten for the
training set and ten for the validation set) were prepared for analysis in
the new liquid cell. Solutions ranged in concentration from 5 to 38 grams
per liter. Each solution was loaded into a single-cavity slide two times,
and four spectra were taken from each sample loading. Spectra were
recorded at 16 wavelengths and the data were transformed to their
principal axes to avoid the need for a time-consuming
all-possible-combinations of wavelengths regression. In order to
demonstrate that one need not be very particular about the initial
selection of analytical wavelengths, the wavelength data near water
absorption peaks were deliberately deleted from the recorded spectra
(which contained data from 19 wavelengths). This also shows that
relatively complex instruments, utilizing scanning monochromators to
collect data at hundreds of wavelengths, are often unnecessary in NIRS.
Multiple linear regression was then carried out on the 80 training spectra
using only the data along the first five principal axes (these axes
accounted for over 99.9% of the total spectral variation). Data from five
axes were required because evaporative loss from the cell produced
pathlength variations that called for an additional degree of freedom in
the system. The results of the training process are summarized in the
calibration line in FIG. 5. The r.sup.2 for the training set that produced
the line is 0.97, and the r.sup.2 value for the 80 validation spectra
(shown superimposed on the calibration line, with error bars) is also
0.97. The detection limit for sodium chloride, calculated from both the
error in the validation spectra and from four solvent blanks, is I gram
per liter (1000 parts per million). This value corresponds to an absolute
detection limit of approximately 100 micrograms in the 110 microliter
sample cell.
The liquid microcell holder or reflector that has been described herein has
a number of practical advantages It is faster and easier to use than an
ordinary liquid holder. No heating or thermostatting is required because
the volume of liquid used with this liquid micro cell holder rapidly
reaches thermal equilibrium. No purging/filling or wash cycles are
required. Any number of cells can be rapidly filled with a precision
pipette if desired, and the cells can be easily cleaned or simply
discarded afterward (an advantage for potentially dangerous and toxic
samples). The configuration of the cell permits sensitive detection by
enhancing transmission through the sample in a near-infrared reflectance
instrument. The apparent lack of pathlength reproducibility for volatile
samples is compensated simply by using a random selection of pathlengths
when the training-set spectra are recorded and by letting the calibration
process take care of the rest. This microcell design adds a versatility to
liquid analysis in near-infrared reflectance instruments that complements
the flexibility of the near-infrared calibration procedure.
While the foregoing has been described with respect to preferred
embodiments and alternatives thereto, they are not intended nor should
they be construed as limitations on the invention. As one skilled in the
art would understand many variations and modifications of these
embodiments may be made which fall within the spirit and scope of this
invention.
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