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Description  |
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FIELD OF THE INVENTION
This invention pertains generally to spectral analysis, and more
particularly to apparatus and a method for making absorbance measurements
over a wide range of absorptivities and concentrations.
BACKGROUND OF THE INVENTION
Liquid chemical absorbance measurements using conventional spectrometers
employ cuvettes of fixed path length for holding the sample of the
analyzed. If the concentration or absorbance of the sample is too high,
the instrument may not measure the correct absorbance value due to limited
polychromatic light scattering from the diffraction grating in the
instrument. If the absorbance is too low, the resolving capability of the
instrument may be exceeded due to noise limitations established by the
detector, amplifiers and/or source or too low a light level measured by
the detector. For each instrument there is an optimum range of absorbance
values which have the best resolution, lowest noise and best linearity.
However, with the concentrations of liquids available and the fixed path
length of light passing through the liquid, the absorbance does not always
fall within the optimum range for the instrument. In addition, if the path
length of the cell is too short, it may be difficult to get the liquid
sample into the cell. It is also possible to have within a given spectrum
two or more absorbance peaks located in different portions of the spectrum
which cannot both be accurately measured because the cell path length has
been optimized for the absorbance typical of only one of them. A single
path length can only quantitate optimally one absorbance peak if there is
a large difference is absorbance values between peaks.
Another problem which may arise in the course of measurement of certain
types of fluids is the formation of a coating on the window of the
spectrophotometry cell. Such a coating may substantially reduce the light
transmission through the cell, resulting in a change in signal level which
is confounded with changes in sample absorption in conventional
spectrophotometers.
It is in general an objective of the invention to provide a new and
improved apparatus and method for making absorbance measurements.
Another objective of the invention is to provide an apparatus and method of
the above character which overcome the limitations, problems and
disadvantages of the absorbance measuring instruments as discussed
hereinabove.
SUMMARY OF THE INVENTION
This and other objectives are achieved in accordance with the invention by
varying the length of the path of light passing through the sample, to
optimize the amount of light absorbed by the sample, and to calculate and
differentiate between the fixed absorbance of a film on the windows and
the variable absorbance with distance of the fluid being measured. The
amount of light passing through the sample can be monitored, and the
length of the path can be adjusted to maintain this light at a level which
is optimum for the instrument which analyzes the sample. The path length
can be modulated about this optimum length (or some other suitable
length). The relationship between the amount of light absorbed and the
length of the path is known, and the true absorbance of the sample can be
calculated from the measured absorbances and the lengths of the paths. In
addition, the path length may be decreased after the liquid is introduced
into the cell, thereby insuring that the liquid will fill the light path
regardless of how short the path may be.
In accordance with a more specific aspect of the present invention, a
spectrophotometry system may include an elongated cell insertion tube
having an integral enclosing housing, a variable length spectrophotometry
cell having a cylindrical external surface making a close fit with the
insertion tube, arrangements such as pipe threads for securing the
insertion tube to a container or conduit containing the fluid to be
measured, a valve mounted on said insertion tube adjacent the securing
arrangements for selectively closing off the insertion tube; and a seal
between the insertion tube and the outer surface of the cylindrical cell
for permitting withdrawal of the cell beyond the valve, and closure of the
valve to permit full removal of the cell without loss or escape of the
fluid being tested.
Further, the variable length cell may include external spacing control and
path length measurement arrangements operable to change the optical test
spacing within the container or conduit when the cell unit is fully
inserted. The cell may include a reflective prism spaced from the input
and output optical waveguides, and with the prism being mounted on a pair
of guide rods to maintain optical alignment as the prism is advanced and
retracted.
From a method standpoint, it is desirable to initially calibrate the
associated spectrophotometer relative to at least one known spectral
frequency using monochromatic light routed through the entire optical
system. In addition, an amplitude vs. frequency sample should be taken,
preferably with at least two different spacings of the prism to provide
different path lengths, with the sample cell being free of the fluid to be
measured. Subsequently, using these background measurements, and amplitude
vs. spectral frequency characteristics taken at two different spacings,
the effect of possible filming of the sensor surfaces (constant with
different spacing) may be separated from the response characteristic of
the fluid sample (linear increase of absorbance with distance).
Other objects, features and advantages of the invention will become
apparent from a consideration of the following detailed description and
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of one embodiment of apparatus for measuring
absorbance according to the invention;
FIG. 2 is a cross-sectional view, somewhat schematic, of one embodiment of
a sensing head for use in the apparatus of FIG. 1;
FIG. 3 is a cross-sectional view, somewhat schematic, of another embodiment
of a sensing head for use in the apparatus of FIG. 1;
FIG. 4 is a cross-sectional view, somewhat schematic, of another embodiment
of a sensing head for use in the apparatus of FIG. 1;
FIG. 5 is a partial cross-sectional top view of a spectrophotometry cell
illustrating the principles of the present invention;
FIG. 6 is a cross-sectional side view of the cell of FIG. 5; and
FIGS. 7 and 8 are cross-sectional views taken along lines VII--VII and
VIII--VIII of FIG. 5.
DETAILED DESCRIPTION OF THE INVENTION
As illustrated in FIG. 1, the apparatus comprises a spectrum analyzer 11
with a remote sensing head 12 connected to the analyzer by optical fiber
waveguides 13, 14. The spectrum analyzer can be of the type disclosed in
Ser. No. 644,325, filed Aug. 24, 1984, now U.S. Pat. No. 4,664,522, and is
has a computer 16 associated with it. Waveguide 13 carries light from a
source within the analyzer to the sensing head where it is passed through
a sample (not shown), and waveguide 14 carries the light from the sample
back to the analyzer. Each of the optical waveguides is preferably a
single strand optical fiber for purity of optical transmission, and low
cost, as compared with optical channels including bundles of optical
fibers.
As discussed more fully hereinafter, sensing head 12 includes means for
varying the length of the light path through the sample. A servo mechanism
17 controlled by computer 16 is connected to the sensing head to control
the length of the light path.
FIG. 2 illustrates one embodiment of a sensing head which can be utilized
in the apparatus of FIG. 1. This sensing head comprises a pair of probes
21, 22 having optical fiber waveguides 23, 24 connected respectively to
waveguides 13, 14. The confronting ends 26, 27 of waveguides 23, 24 are
aligned axially on opposite sides of a gap 28 for transmitting and
receiving light through the sample to be analyzed. The length of the path
of light passing through the sample is determined by the width of gap 28,
and the position of one or both of the probes is adjustable to vary the
width of the gap. This sensing head is particularly suitable for use in
applications where it is submerged in the liquid sample to be analyzed
such as a vat or tank or in a stream of flowing liquid.
The sensing head of FIG. 3 includes a pipe 31 which defines a chamber or
cell 32 which contains the sample to be analyzed. Probes 33, 34 are
connected to waveguides 13, 14 and are aligned axially within the pipe on
opposite sides of chamber 32. Each of the probes includes a cylindrical
lens 36 which collimates the light, and a light transmissive window 37 at
its inner end. The length of the light path 38 in the sample is determined
by the distance between the probes, and one or both of the probes is
movable to vary the length of the light path. The liquid sample flows into
the chamber through an inlet 41 and flows through an outlet 42. Seal rings
or rubber O-rings 43 close the ends of the chamber and provide fluid-tight
seals between the outer walls of the probes and the inner wall of the
pipe.
The sensing head of FIG. 4 includes a single probe 46 to which optical
waveguides 13, 14 are connected. Collimating lenses 47 and windows 48 are
aligned with the waveguides. A light reflector or prism 49 is spaced
axially from the ends of windows 48. The reflector 49 receives light
passing through the sample from waveguide 13 and directs this light back
through the sample to waveguide 14. The separation 51 between the end of
the probe and the reflector determines the length of the light path in the
sample, and this distance can be adjusted by moving either the probe or
the reflector, or both. The reflector can be of any suitable type, and in
one presently preferred embodiment, it comprises a corner cube or prism.
In all of the sensing heads described herein, the movable probe or probes
can be driven by any suitable mechanism such as mating screw threads, a
rack and pinion gear, a cam and piston, a linear motor, a piezoelectric
crystal or a hydraulic ram. The drive mechanism can be driven by any
suitable means such as a stepper motor or a servo motor, and the positions
of the movable elements can be monitored by suitable means such as an
optical encoder, a conductive plastic linear or rotary potentiometer; or,
for extremely critical path length determinations, an interferometer may
be used. This sensing head configuration of FIG. 4 is particularly
suitable for use in a fluid path where the entire sensing head is immersed
in the fluid being measured.
Absorbance is given by the equation A=ebc, where "e" is the molar
absorptivity of the sample in liter/cmmole, "b" is the path length of the
cell in centimeters, and "c" is the concentration of the absorbant
material in moles/liter. From this relationship, it will be noted that
absorbance is linearly proportional to the path length. A short path
length would reduce the light loss through the sample for a highly
absorbing fluid, while an extended path length would reduce the loss in
absorbance resolution for dilute samples.
With a conventional single beam spectrometer, absorbance is commonly
measured by first scanning a reference at every wavelength of interest,
then scanning the specimen through the same spectrum. The absorbance for
each wavelength, .lambda., is given by the relationship:
##EQU1##
where P.sub.o (.lambda.,L.sub.o) is the reference light amplitude for path
length L.sub.o, and P(.lambda.,L.sub.o) is the sample light amplitude for
path length L.sub.o.
When the length of the light path is varied, as in the present invention,
the measured absorbance is given by the relationship:
##EQU2##
where L.sub.o is the length of the path for the reference at wavelength
.lambda., and L is the length of the path for the sample at wavelength
.lambda..
As an example, consider a sample solution which displays an absorbance as
follows with a light path length of 1 cm:
______________________________________
Absorbance
Wavelength A (1 cm)
______________________________________
200 nm 20,000
225 nm 12,000
250 nm 5,000
275 nm 1,000
300 nm 500
325 nm 250
350 nm 50
375 nm 1
400 nm 0.3
______________________________________
In the foregoing table the wavelengths are given in nanometers (nm).
For a spectrum analyzer with a maximum linear dynamic range of 1.5 Au
(Absorbance Units), it is desirable to use a path length which limits the
absorbance of the sample to no more than 1.5 Au. If the reference is
scanned at a path length L.sub.o and the sample is scanned at a path
length L, then the measured absorbance A.sub.m can be determined from the
reference absorbance A by the relationship:
##EQU3##
In the previous example, if the reference is taken at a path length of 1 cm
in either air or a solvent that is low in absorbance (i.e. less than 0.5
Au) and the sample is taken at varying path lengths to limit the maximum
difference in absorbance between the reference and the sample, to 1 Au or
less at each wavelength, the relationship between the path length,
measured absorbance and reference absorbance is as follows:
______________________________________
Reference
Measured Absorbance
Wavelength
Path Length Absorbance
A @ 1 cm
______________________________________
200 nm 0.5 .mu.m 1 20,000
225 nm 0.833 .mu.m 1 12,000
250 nm 2 .mu.m 1 5,000
275 nm 10 .mu.m 1 1,000
300 nm 20 .mu.m 1 500
325 nm 40 .mu.m 1 250
350 nm 200 .mu.m 1 50
375 nm 1 cm 1 1
400 nm 1 cm 0.3 0.3
______________________________________
In the foregoing table, the designation ".mu.m" means micro-meters or
10.sup.-6 meters as compared with centimeters (cm) which are 10.sup.-2
meters.
In addition to keeping the absorbance measurements within the optimum range
for a given instrument, the variable path length has a further advantage
in that it makes it relatively easy to get the sample fluid into gaps or
light paths of any length. With cells having fixed path lengths, it can be
difficult to get a highly viscous fluid sample into a path of relatively
short length. With the variable path length, the fluid can be introduced
when the gap is relatively wide, and the gap can be closed to provide the
desired path length with the fluid in position.
In the event that the light throughput of the cell is affected by the path
length due to light losses not related to absorption by the sample, any
resulting errors can be eliminated by either rerunning the reference after
the sample is measured, using the same path lengths that were established
for the sample, or storing a correction table with corrections for various
path lengths or calculating the corrections with a predetermined empirical
equation.
If desired, the servo system can be set to produce a null for a given value
of absorbance for each wavelength to be scanned, and this will simplify
the calculation required to determine the actual absorbance. For a null
value of 1 Au or 1/10 of the reference and a path length of 1 cm, for
example, the relationship for determining the absorbance becomes:
A(L)=(1Au)(D/1 cm) (4)
Referring now to FIGS. 5 through 8 of the drawings, they illustrate a
preferred specific configuration for implementing the probe arrangement
shown schematically in FIG. 4 of the drawings.
FIG. 5 is a top view, and FIG. 6 is a side view of the probe assembly.
Referring first to FIG. 6 of the drawings, the reflector 49 appears at the
left-hand end of the drawings and it is mounted in an assembly 52 which is
supported by the two rods 54 which in turn are secured to the threaded
plate 56, actuated by the threaded rod 58 which is in turn rotated by the
handle 60 within the protective housing 62 at the opposite end of the
entire assembly.
Incidentally, the fluid which is being measured spectrally is within a
container or conduit generally indicated by the dashed lines 64. The probe
assembly is provided with pipe threads 66 by which it is secured to the
conduit or container 64. The single strand waveguides 68 and 70 are
coupled at the left-hand end of the probe to the lenses 72 and 74, which
couple the optical waveguides 68 and 70 to the window structures 76 and
78, respectively. The assembly 52 may be moved from the indicated position
to that shown in dashed lines at 52', where the path length through the
fluid being sampled has been reduced substantially to zero.
The probe structure which has been discussed up to this point is mounted in
or to the tube 82, which has a cylindrical outer surface. In addition, the
assembly 52 on which the reflector 49 is mounted, is of a slightly smaller
diameter than the tube 82. The outer assembly 84 includes the mounting
plate 86, the valve assembly 88, and an enclosing tubular housing 90
provided with a pair of seals 92 and 94 which engage the outer surface of
the cylindrical tube 82 at two locations to prevent the escape of the
fluid to be measured from the container or conduit 64. With the valve 88
in the open position as shown in FIG. 6, the probe including the tube 82
may be readily slid into position as indicated in this figure. In the
event that there is liquid under some pressure within the chamber 64, the
inner tube 82 may be inserted partially into the outer assembly 84, so
that it is sealed at least at the seal 94, with the valve 84 being closed.
Then, with the sample cell being forced from right to left as shown in
FIGS. 5 and 6, the valve is opened, and the cell is pushed the remaining
distance from right to left to the position shown in FIGS. 5 and 6. The
seal 94 is then tightened up to hold the unit in position. Of course, when
the valve 88 is closed, the inner tube 98 is rotated to a position 90
degrees displaced from that shown in FIG. 6, so that the channel is
closed, and the vessel 64 is sealed.
When it is desired to shift the position of the reflector 49, the housing
62 is removed, and the handle 60 is rotated, thus turning the threaded
shaft 58, and moving the threaded plate 56 in one direction or the other.
Of course, this moves the rods 54 axially, carrying the housing 52 and the
reflector 49 to the desired position.
The change in the gap length is, of course, twice the distance by which the
unit 52 is moved. Half length calibration is accomplished by the plate 102
which has a threaded central opening which is engaged by the threads 104
on the shaft to which the handle 60 is secured. Mounted on the plate 102
are two calibrated arms 106 bearing vernier indicia so that the rotation
of the handle 60, and the corresponding position of the reflector 49 may
be accurately measured and determined.
Instead of the handle 60, and the rods 106 bearing indicia, the threaded
shaft 104 may be rotated by a worm and gear arrangement in place of the
handle, and with digital code wheel arrangements or other similar
automatic read-out mechanisms being provided to indicate the position of
the reflector. Various types of automatic shifting and sensing
arrangements were mentioned hereinabove in connection with blocks 12 and
17 in FIG. 1 of the drawings.
The problem of light losses from causes other than absorption by the
sample, such as formation of films or coatings on the windows to a cell,
will now be considered. The formation of a coating on the cell window will
be used as an example of losses of this type. If over a period of time a
coating is formed on windows of the cell, a false reading may result
because the coating itself is also absorbing. Some coatings absorb at
substantially higher values than their thickness indicates because the
coating condense on the windows, react with the windows, or are light
sensitive and react with the light beam. To differentiate these losses
which occur after the reference has been taken, a two path length probe
approach has been developed. That is, by varying the probe path length
over known distances any fixed serial addition of absorbance can be
removed. Additional loss variations may be introduced with two separate
path lengths, and a known reference without a sample is therefore taken to
compensate for length dependent variations.
The following mathematical analysis shows how a filmed window need not
prevent accurate determination of fluid absorbance.
A.sub.1 =ecD.sub.1 +A (5)
A.sub.2 =ecD.sub.2 +A (6)
A.sub.T =A.sub.1 -A.sub.2 =ec(D.sub.2 -D.sub.1)K (7)
Where:
A.sub.1 : Total absorbance at Length D.sub.1
A.sub.2 : Total absorbance at Length D.sub.2
e: molar absorbtivity
c: concentration
A: absorbance due to window coating or other cause not related to
absorption of light by the sample
A.sub.T : total absorbance less window absorbance
K: correction factor between the two path lengths.
Accordingly, the fact that the bulk absorbance varies linearly with path
length, while film absorbance is constant, is employed to calculate the
true absorbance of the fluid corresponding to the effective path length,
D.sub.2 -D.sub.1.
It is apparent from the foregoing that a new and improved apparatus and
method for measuring absorbance have been provided. While only certain
presently preferred embodiments have been described in detail, as will be
apparent to those familiar with the art, certain changes and modifications
can be made. Thus, by way of example and not of limitation, other
mechanical arrangements may be employed to shift the position of the
reflector 52 in FIGS. 6 and 7; and the variable spacing sensing head 12
may have its spacing varied by a motor without using servo control to
implement the film checking procedure, for example. Accordingly, the
present invention is not limited to the arrangements as specifically shown
and described hereinabove.
* * * * *
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