Note: Descriptions are shown in the official language in which they were submitted.
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INFRARED SPECTROPHOTOMETRIC APPARATUS
This invention relates to infrared
spectrophotometric analysis apparatus.
This application is a division of copending
Canadian Patent application Serial No. 495,013 filed
November 12, 1985. The parent application claims an
optical cell and detector assembly for a
spectrophotometric analyzing apparatus while the present
application is directed to a spectrophotometric
analyzing apparatus.
Infrared spectrophotometric analyzers are
employed for the analysis of emulsions and suspensions,
for example in the dairy industry for analyzing the fat,
protein and lactose content in milk and milk products.
An example of such apparatus is described in US patent
No. 4,310,763. In particular, Figure 1 of this patent
illustrates the optical lay-out of one form of such
apparatus known as the "Multispec" (Trademark)
instrument.
In this apparatus two beams of infrared light,
each of a different wavelength are led alternately
through an optical cell containing the sample under test
to an ellipsoidal mirror from where it is focussed on to
an infrared detector. The signal from the detector can
be analyzed in accordance with well known technology in
this field to give a reliable estimate of the component
being analyzed for in the sample.
The above instrument, using the so-called
'double-beam-in-wavelength' system analyzes milk for
~at, protein and lactose and requires filters of
suitable wavelength for each component together with
reference filters for each sample wavelength.
Furthermore, a null-balance system is usually employed
involving a relatively expensive attenuator comb and
associated servo mechanism and electronics.
Various factors affect the performance of such
apparatus, for example the presence of water vapor in
the optical path of the apparatus causes interference by
selectively absorbing part of the infrared signal and
thus, as well as attempting to reduce the water vapor
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within the apparatus, for reasons of conserving energy,
optical path lengths should be kept to a minimum.
Furthermore the presence of large globules in the
emulsion or suspension under test scatters infrared
radiation, thus causing errors in the optical
measurements of the components in the milk sample. As
the amount of radiation scatter is proportional to the
globule diameter it is necessary to force the sample
through a high pressure homogenizer to break down the
globules. The necessity for homogenization of, for
example, milk samples greatly increases the cost of a
suitable analyzing instrument. Ellipsoidal mirrors are
relatively expensive pieces of squipment and the use of
these tends to increase the cost of the instrument.
There is a great need for analyzing apparatus
which is inexpensive and yet has sufficient accuracy to
be useful in dairy industries.
The invention seeks to provide economic
apparatus for analyzing samples which overcomes or
reduces the above disadvantages.
According to the present invention there is
provided an infrared spectrophotometric analyzing
apparatus which comprises an infrared source and a
detector therefor, optical means for focussing the beam
onto a sample cell, chopper means for periodically
obscuring the beam, and filter means for selecting one
or more wavelengths from the beam, wherein the sample
under test is an aqueous emulsion and that the filters
are chosen to determine the concentration of the non-
aqueous components of the emulsion by measuring thewater displaced by the components.
Placing the detector closely adjacent, or
indeed abutting the sample cell reduces the path length
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of the radiation, allows the ellipsoidal mirror formerly
used to be dispen6ed with, and, since the temperature of
the cell is controlled (for example as described in US
4,310,736), enables the detector temperature to be
monitored and controlled also. In particular, the
effect of scattering owing to large globules within the
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sample under tast is very much reduced since even
scattered light is likely to be collected by a detector
so close to the sample and thus homogenization of the
sample under test may be reduced or eliminated entirely.
The detector may even replace part or all of one cell
wall.
The apparatus of the invention is particularly
applicable to the analysis of milk and dairy products.
The most important practical measures required here are
the amounts of fat and of solids, non-fat (SNF) in the
sample. In the apparatus of the invention, by using a
'fat' filter such as a 5.7 micron filter and a 'water'
filter, e.g. at 8.64 or 4.7 microns, these parameters
can be simply and accurately determined.
In one embodiment, the apparatus of the
invention includes a microcomputer, small display and
keypad, an optics unit, control electronics and a
pump/homogenizer. The optics unit is straightforward/
having an infrared source and detector at either end of
each light path, with a chopper, filter and cell or
cells in between. The chopper has an equal area of
'windows' and reflective areas: the speed of said
chopper is variable and is controlled by the
microprocessor. The filter is located between the
~5 chopper and the cell.
Different filters may be employed in the
apparatus by means of a rotating wheel, the filters
being mounted thereon, which rotates so as to place the
desired filter into position. The cell is plumbed into
the pump/homogenizer.
Within the optic unit infrared~radiation is
emitted from an infrared source. his radiation is
incident upon a convex lens which focusses the beam onto
a chopper which is driven by a motor. The chopper, with
its equal number of reflective areas and windows,
alternately transmits then blocks the beam at a rate
dependent upon the chopping frequency. The transmitted
beam, after having traversed a suitable filter, is then
focussed by a second convex lens onto a sample cell; the
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radiation transmitted thereby being incident upon an
infrared detector. A resulting A.C signal is obtained
which is indicative of the absorption which has taken
place within the cell.
The spectrometer operates by changing filters
and taking readings with, say, a sample of milk in the
cell and comparing these with readings taken, say, with
a sample of water in the cell. The readings are a
simple ratio of the transmission (absorbance) at one,
two or more selected wavelengths of the sample and
reference. At one time ratio methods were too
inaccurate owing to drift in the sensitivity of the
detectors, and null-balance methods have been favored.
However, recent improvements in detector technology
enable the ratio method to be used herein with
sufficient accuracy and great cost saving.
For each sample, the cell is filled by the
pump/homogenizer, each filter is moved into position in
turn, and measurements are taken. From these
measurements the concentrations of components af
interest in the sample, for example fat and SNF, or fat,
protein and lactose, in milk, are then calculated and
displayed. In order to perform this calculation it is
necessary to have some reference measurement. For this
purpose a distilled water sample is put through from
time to time. Since there is a microcomputer with on
board clock/calendar in this embodiment, the instrument
may itself request a reference sample if one has not
been forthcoming for some time. The frequency with
which the reference sample is required is dependent upon
the stability of temperature and humidity within the
instrument.
However, before any measurements are made, it
is necessary to convert the optical signal arriving at
the detector into computable form. This is done in four
stages:
a) The detector produces a small electronic signal;
b) A pre-amplifier amplifies this to a convenient level;
c) A programmable gain amplifier under the control of
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the computer compensates for different signal levels
from different filters; and
d) A 12 bit analogue to digital converter converts the
signal to digital form at a rate in excess of lOoo
samples per second.
Conventionally a tuned filter and synchronous
rectifier (lock-in amplifier) is used, coupled with a
low pass filter, and the resultant DC signal is sampled
by the computer. By sampling the AC signal and
processing it in the computer there are fewer electronic
components which clearly means increased reliability.
The method also leads to a lower noise level (there are
fewer noise generators), lower cost, and an ability to
vary the chopping frequency with no hardware changes.
The signal is sampled for some time, and
between one and two thousand data points are stored.
These data are then filtered, using a finite impulse
response digital filter with a narrow pass band at the
chopping frequency, and the amplitude of the resulting
signal is then proportional to the optical signal
magnitude.
Alternatively a fast Fourier transform may be
done and the amplitude at the chopping frequency
computed directly. Although fast fourier transforms are
time consuming, in this case we require that only one
point in the frequency domain is computed, and in
certain circumstances this method may be faster than
filtering and estimating the amplitude.
In either case, the method of ascertaining the
optical signal magnitude is by digital signal processing
carried out by the computer. In a typical method of
operation, for example in the analysis of milk samples,
the cell is filled with milk and readings taken at 5.7
microns and at 8.64 microns. Similar measurements will
have already been made with distilled water in the cell.
The difference between the readings for milk
and water at 5.7 microns gives a measure of the fat
content of the milk sample. The difference between the
milk and water readings at 8.64 microns gives a measure
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of the total solids in the milk. Consequently the
difference between these two readings, i.e. total solids
and fat, gives a value of the 'solids non-fat' of the
milk. Other wavelengths of "water filter" can be
employed, e.g. 4.7 microns, but we have found that 8.64
microns gives good results, with a standard deviation of
less than 0.04%.
The fat and the SNF levels are by far the most
important measures needed in the analysis of milk. The
microprocessor within the embodiment is programmed to
give a direct read-out of these two levels without the
operator having to perform any calculations.
One embodiment of the invention has only two
lenses and no ellipsoidal or concave mirrors in its
optical system. Furthermore, owing to the cell and
detector arrangement, a homogenizer may be dispensed
with. Thus the cost of the apparatus of this embodiment
is only a fraction of that of conventional analyzers.
The spectrometer operates by changing filters
and taking readings with, say, a sample of milk in the
sample cell and comparing these with similar readings
with, say, a reference of water in the sample cell. A
microprocessor may be incorporated as before to compare
and integrate the readings automatically without greatly
increasing the cost. The readings are a simple ratio of
the transmission (absorbance) at one, two or more
selected wavelengths of the sample and reference.
An advantageous form of this apparatus
comprises only one cell. This may firstly be filled
with distilled water and measurements taken at, say,
8.64 and 5.7 microns. The cell is then filled with a
milk sample and the measurements repeated. Since the
measurements on water may be stored by the instrument
there is no need to repeat the water measurements with
each succeeding milk sample. Thus a single beam at a
single wavelength (at any one time) is passed through a
single cell. This leads to savings in cost of the
optical system needed as compared with double-beam
instruments - without incurring penalties in speed of
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operation owing to the stored water reference
information.
Instruments embodying either or both aspects
of the invention may be made to any desired level of
sophistication from very basic ~and hence inexp~nsive),
e.g. for use in developing countries, to
multi-functional employing, for example, filters for
protein and lactose as well as fat and having powerful
computing facilities built in.
The invention will be described further, by
way of example, with reference to the accompanying
drawings, in which:
Figure 1 is a partial sectional view of a cell and a
detector assembly in accordance with the invention;
Figure 2 is a simplified view corresponding to Figure 1
and illustrating the optical path; and
Figure 3 is a spectrometer incorporating the cell and
detector.
Referring to the drawings, an assembly
generally designated 10 comprises a pair of cell walls
12, 14 of optically transparent material, which, for
infrared analysis, could be silver iodide, calcium
fluoride, germanium, zinc-sulphide, or potassium
bromide. The cell walls 12, 14 are held within a body
16 by means of bolts 18 in a conventional manner, and
defined between them a sample cavity 20. The cavity 20
connects by passageways 22 and 24 to inlet and exit
ports 26, 28 for fluid samples.
On one wall 12 of the cell there is mounted a
radiation detector 30 having a radiation transparent
window 32 and a detecting element 34.
Mounted adjacent to the opposite wall 14 of
the cell is a lens 36 made of a similarly optical
transparent material to that of the cell walls 14, 12.
Radiation 38 of the appropriate wave length is, as
described more fully hereinafter, directed on to the
lens 36 from the remainder of the instrument.
The radiation 38 is focussed by the lens 36
through the sample cell and detector window 32 on to the
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detector element 34. The detector 30 is preferably of a
type having a relatively large window 32, for example
lOmm diameter or even larger. This ensures that not
only can the radiation 38 be focussed within the
receiving area of the detecting element 34 but that the
bulk of any light scattered within the cell will also be
collected thus improving the sensitivity of the
apparatus constructed in accordance with the invention.
The ellipsoidal mirror is eliminated, and the associated
optical path length also. Furthermore, since the body
16 is temperature controlled so as to control the
temperature of the cell (as described in US 4,310,763)
the detector 30, being incorporated with the cell, is
also temperature controlled so that drift owing to
temperature variations of the detector 30 is reduced or
eliminated.
In operation a reference sample is put in the
cell and absorbs varying amounts of radiation at the
different wavelengths selected by the filters, and these
levels are recorded. Subsequent milk samples absorb
more strongly than the water, and so the optical signal
at the detector 30, which is an AC signal caused by the
operation of the chopper, is of a different magnitude
dependent upon concentration of components and the
filters selected. By computation, estimates may be
obtained, from the data so recorded, of the
concentration of components of interest within the milk.
If desired other filters may be used to give
information about protein and lactose. Cross-corrections
for interfering components can be carried out using
equations known per se and, where a microprocessor is
used, these corrections can be programmed in.
Furthermore, the simplification embodied in
the instrument of the invention enables it to be
manufactured with very small dimensions. This in turn
allows it to be incorporated in plant or equipment for
process control.
In another embodiment the detector may
actually be incorporated into the cell, replacing some
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or all of the cell exit wall 12.
Referring now to Figure 3, the remainder of
the optical system for a low-cost spectrometer is
illustrated. An infrared source 40 is focussed by means
of a lens (or lens group) 42 on to a chopper 44 which
alternately allows the beam to pass and blocks it off at
a frequency determined by the speed of its drive motor
46. If the detector 30 is a photon detector a high
chopper frequency (e.g 200 c.p.s.) is preferred; but for
slower operating detectors very much lower frequencies
may be employed (e.g. 10 - 25 c.p.s.). The detector
electronics are tuned to and governed by the chopper
speed as known in the art. A replaceable filter 48 is
placed between the chopper 44 and the lens 36.
In a typical method of operation, in the
analysis of milk samples, the cell is firstly filled
with water as a reference and the filter 48 is chosen at
the Fat 'A' waveband, 5.7 microns. Measurements are
taken with the beam 38 both on and blocked off (for a
'background' level). The filter 48 is then changed to
a 'water filter' i.e. one which corresponds to high
water absorption, e.g. 8.64 microns, and the readings
repeated. This sequence is then repeated with a sample
of milk in the cell.
The difference between the readings for milk
and water at 5.7 microns (corrected for background) give
a measure of the fat content of the milk sample. The
difference between the milk and water readings at 8.64
microns (corrected for background) give a measure of the
total solids in the milk. Consequently the difference
between total solids and fat gives the 'solids, non-fat'
('SNF') of the milk. The fat and SNF levels are by far
the most important measures needed in the analysis of
milk. A microprocessor within the apparatus can be
programmed to give a direct read-out of these two levels
without the operator having to perform any calculations.
Thus sufficiently accurate measurements of the
most important parameters for milk analysis can be
- obtained with an extremely inexpensive instrument.
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While the invention has been described in
relation to instruments useful for analyzing milk and
dairy products, and the filters chosen have been of
wavelengths appropriate to this end use, it is not
so-limited. Other aqueous emulsions can be analyzed and
filters will be chosen accordingly. Even in the
analysis of milk the particular filter values described
are not the only ones which can be used; for example,
other filter wavelengths useful for the analysis of milk
10 fat include 3.46 microns and 6.84 microns.
The invention described hereabove would be a
valuable asset for the dairy industry as it provides a
quick and reliable method of ascertaining the quality
of milk samples at a relatively inexpensive price.
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