Fluid sample cell

A fluid sample cell comprising a sample compartment of precisely predetermined depth bounded by opposed surfaces of a transparent window and a diffuse mirror is disclosed, and comprises inlet and outlet ports for the flow of a series of successive samples into and through said sample compartment and the spectroscopic analysis thereof by irradiation and detection of transmitted and reflected radiation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a new and improved fluid sample cell for 
spectroscopic analysis of a wide variety of different fluid samples. 
2. Description of the Prior Art 
A wide variety of sample cells are known for use in accordance with 
conventional transmission spectroscopy techniques for analyzing fluid 
samples to determine the concentration of radiation energy-absorbing 
sample constituents. Such analysis is based on the selective attenuation 
of radiation energy at different predetermined wavelengths passing through 
the sample in accordance with the absorption characteristics of the 
sample, the concentration of the radiation energy-absorbing constituents 
of the sample, and the radiation energy transmission path length through 
the sample. Certain fluid samples, for example, liquids, contain 
suspensions of fine particles which independently attenuate radiation 
energy transmitted therethrough by diffusely scattering the same. 
Depending upon particle size distribution and concentration of particles 
in the fluid medium, a portion of the incident beam of light, instead of 
being directly transmitted therethrough, will be diffusely scattered into 
forward- and back-scatter components of variable geometric distribution. 
In conventional transmission spectroscopy, the back-scatter component, as 
well as a portion of the forward-scatter component, will not be sensed by 
the radiation sensor. The resultant loss of signal appears as part of the 
attenuation measurement. Variations in turbidity, therefore, will 
introduce a variable parameter in the measurement due to variations in the 
loss of the diffusely scattered components, as opposed to variations in 
the purely absorptive properties of the constituent of interest in the 
fluid. 
Furthermore, the spectral energy attenuation characteristics of true 
solutions is a logarithmic function of molecular concentration, whereas 
the diffuse scattering of light by finely suspended particulate matter is 
a non-linear function of particle size and concentration as well as 
wavelength. Such suspension, therefore, will introduce an error in the 
analysis results, unless time-consuming and complex (and oftentimes 
inaccurate) corrections are made for the diffusely scattered radiation 
energy components. In addition, a wide variety of fluid samples, for 
example, heavy process food syrups, ice creams or the like, do not 
transmit sufficient radiation energy to be appropriately processed in 
accordance with conventional transmission spectroscopy techniques and, 
hence, require special sample processing, such as dilution, with attendant 
introduction of further possibility of error. 
Also, a wide variety of sample cells are known for use in the analysis of 
generally solid samples by conventional reflectance spectroscopy 
techniques. By such techniques, solid samples, generally reduced to a 
powdered or finely ground consistency, are illuminated by a source of 
spectral radiation and analyzed for their surface spectral reflectance 
properties. Light striking and penetrating the sample surface is partially 
absorbed in accordance with the concentration of sample constituents and 
their spectral absorbance characteristics; also, such light is diffusely 
scattered in a similar manner as with turbid liquid samples, into forward 
and back-scatter components. It is the back-scatter component, selectively 
attenuated by sample spectral absorbance characteristics, which is 
measured in a reflectance instrument and used to determine constituent 
concentration. However, the forward-scatter component is lost by 
absorbtion within the sample and, hence, not available to determine 
constituent concentration, although such component contains worthwhile 
information. Such sample cells would be generally inapplicable for use in 
the analysis of fluid samples, wherein the spectral reflective surface 
properties per se of the sample are not indicative of, for example, the 
concentration of a particular sample constituent. 
In addition, and because of the basic differences in optical requirements 
between transmission and reflectance spectroscopy, currently available 
spectroscopy instruments are, in general, limited to only one type of 
measurement or would require complicated and costly accessory equipment 
for conversion from one type of measurement to the other. Also, because of 
the limitations set forth hereinabove, a wide variety of existing 
semi-liquids or semi-solids simply cannot be conveniently analyzed with 
suitable precision by either existing transmission spectroscopy or 
reflectance spectroscopy instruments. 
OBJECTS OF THE INVENTION 
It is an object of this invention to provide a new and improved fluid 
sample cell which enables the precise analysis of fluid samples by 
reflectance techniques, for a constituent of interest or other such 
physical chemical or optical property of the sample. 
Another object of this invention is the provision of a fluid sample cell 
which enables the precise, spectroscopic quantitative analysis of a very 
wide variety of fluid samples of markedly different turbidities. 
Another object of this invention is the provision of a fluid sample cell 
which enables the precise, spectroscopic quantitative analysis of a wide 
variety of fluid samples which, because of their opaqueness, cannot be 
accurately measured by conventional transmission spectroscopy and which, 
because of their non-solid state, cannot be accurately measured by 
conventional reflectance spectroscopy. 
Another object of this invention is to provide a new and improved fluid 
sample cell which enables the precise quantitative analysis of both clear 
and turbid fluid samples by measurement of both the transmitted attenuated 
radiation due to sample absorbance and both the forward and backward 
diffusely scattered radiation due to sample turbidity, such that the 
measurement is independent of variations in turbidity, except for any 
spectrally absorptive effects of the particulate matter in the sample. 
Another object of this invention is the provision of a sample cell which 
insures accurate fluid sample analysis despite the presence of some 
quantity of air bubbles in the sample. 
A further object of this invention is the provision of a sample cell which 
is of relatively simple and inexpensive construction and which may be 
readily and conveniently disassembled for periodic cleaning. 
A still further object of this invention is the provision of a sample cell, 
as above, which is particularly adaptable for use in automated, 
continuous-flow sample analysis systems. 
SUMMARY OF THE DISCLOSURE 
As disclosed herein, the new and improved fluid sample cell of our 
invention comprises a sample compartment of precisely determined depth 
bounded by opposed surfaces of a transparent window and a diffuse 
reflector, for example, a diffuse mirror, without appreciable 
spectral-absorbant characteristics. Access ports are provided to enable 
the flow of fluid samples into and out of the sample compartment. 
Preferably, the mirror surface is configured to maintain any air bubbles 
present in the fluid sample without the viewing area. In operation, the 
sample portion within the viewing area of the sample compartment is 
irradiated with radiation energy at a selected wavelength(s). A 
measurement is taken of the total diffuse reflected radiation energy from 
the sample, which is composed of: radiation transmitted through the sample 
and diffusely reflected back from the diffuse mirror; radiation which is 
scattered by particles in the sample in a forward direction and diffusely 
reflected back from the diffuse mirror; and radiation which is diffusely 
scattered by the sample in a backward direction, without reflection off 
the diffuse mirror. Each of these radiations will have undergone 
attenuation by the fluid sample in accordance with the spectral absorption 
characteristics of the sample constituents, concentration of sample 
constituents, and actual path length traversed through the sample, thus 
permitting a precise quantitative analysis of the sample.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawings, the fluid sample cell is indicated generally 
at 10 and comprises a generally cup-shaped body member 12 and an annular 
cover member 14. The body member 12 and cover member 14 are complementally 
threaded, as indicated at 16 in FIG. 3. A circular viewing aperture, as 
indicated at 18, is defined in cover member 14 and a circular access 
aperture, as indicated at 20, is formed as shown centrally of body member 
12 at the bottom wall thereof. 
A generally circular diffuse reflector or mirror 22 is fabricated from a 
ceramic or other suitable material of appropriate light dispersing 
characteristics to insure little, if any, spectral absorption by the 
mirror. Diffusing mirror 22 may, for example, exhibit the optical and 
physical characteristics of the ceramic spectral reflectance standard, 
disclosed in U.S. Pat. No. 4,047,032, assigned to a common assignee. As 
utilized herein, diffuse mirror 22 comprises a raised annular mounting 
ridge 24 which extends, as seen in FIG. 3, from the upper, reflecting 
surface 25 of the mirror. Further, diffuse mirror 22 comprises an annular 
groove 26 for the mounting of an O-ring seal or gasket 28; stepped, 
diametrically opposed access ports 30 and 32, as best seen in FIG. 3; and 
an annular access groove 34 which connects said access ports, as best seen 
in FIG. 2. A generally circular, transparent window is indicated at 36 and 
overlies the diffuse mirror 22, as best seen in FIG. 3. 
Diffuse mirror 22, O-ring 28 and window 36 are disposed, as shown in FIG. 
3, within body member 12, and the cover member 14 is screwed tightly onto 
body member 12 to force the lower annular surface portion of the window 36 
tightly against the upper surface of the raised annular mounting ridge 24. 
The attendant compression of the O-ring 28 forms a fluid-tight flow cell 
sample compartment. The raised annular ridge 24 precisely predetermines 
the length of the light path through sample compartment 38, as indicated 
at l in FIG. 3. Preferably, this path length l is made as short as 
possible to insure that the operating characteristics of cell 10 fall 
within the signal-to-noise ratio bounds of radiation-energy detecting and 
processing equipment, and to insure that the radiation energy reflected 
back from the diffuse mirror 22 of cell 10, as described hereinbelow, will 
always traverse a constant path length, subject to internal scattering 
effects. 
Fluid inlet and outlet conduits 31 and 33 are connected to access ports 30 
and 32, respectively, at the bottom of diffuse mirror 22, as seen in FIG. 
3, to flow successive fluid samples into and from the sample compartment 
38. Of particular significance is the fact that any air bubbles, despite 
determined efforts to avoid the same, present in the fluid sample, due to 
surface tension, will follow the path of least resistance and flow into 
and along access groove 34 upon sample entry into sample compartment 38. 
Thus, as access groove 34 is located without the viewing area defined by 
aperture 18 in cover 14, as shown in FIGS. 1 and 3, any air bubbles in the 
fluid sample will not interfere with the accuracy of the analysis. 
For purposes of completeness of description of operation, a source of 
radiation of appropriate wavelength(s), for example in the near infrared 
region, is indicated schematically at 40. Also, an optical integrating 
sphere 42 and radiation detectors, indicated schematically at 44 and 46, 
respectively, are disposed to receive diffused radiation redirected from 
the fluid sample cell 10, when irradiated by radiation source 40. A signal 
processor 57 converts the signal output of the radiation detectors 44 and 
46 to a reflectance value, which is used to compute the concentration or 
magnitude of the constituent or property of the sample, and a display 
device 58 communicates this output information. 
A typical utilization of the sample cell 10 would, for example, involve the 
operative incorporation thereof in an infrared automated sample analysis 
system of the type disclosed in co-pending application for the United 
States Patent Application Ser. No. 15,017 filed Feb. 26, 1979 by J. F. X. 
Judge, et al., and assigned to a common assignee. 
In operation, fluid sample compartment 38 of the sample cell 10 is filled 
along access port 30 with a fluid sample 50, to be spectroscopically, 
quantitatively analyzed with regard to a particular constituent thereof, 
for example, ice cream to be analyzed for fat content taking the form of 
fine globules in a generally aqueous solution. Radiation of appropriate 
wavelength, for example a narrow band within the range of 1.4 to 2.5 
microns, is directed from radiation source 40 through an opening 43 in 
integrating sphere 42, through viewing aperture 18 and normal to the 
surface of mirror 22. Sample cell 10 exhibits the characteristics of a 
diffuse source of radiation, which is proportional to the intensity of the 
incident radiation from source 40, the diffuse reflectance characteristics 
of the fluid sample 50 and the spectral absorption characteristics of the 
fluid sample 50. The spectral absorption characteristics would be present 
only if sample 50 is sufficiently transmissive of radiation, so as to 
allow the same to be reflected back from mirror 22 into the integrating 
sphere 42. More specifically, and depending upon the optical transmission 
and/or diffuse reflectance characteristics of the fluid sample 50, 
radiation from source 40 incident, as illustrated by beams 51, upon the 
sample cell through viewing aperture 18 in cover member 14 will be: 
(a) partly reflected in substantially specular manner from transparent 
window 36, as illustrated by beams 52 in FIG. 3. Such reflected radiation 
is rejected, as it is not reflected within the integrating sphere 42, so 
as to be incident on radiation detectors 44 and 46. This reflected 
radiation contains no information relevant to the analysis of the sample 
constituent of interest; 
(b) partly diffusely reflected from sample 50, as illustrated by beams 54 
in FIG. 3, back into the optical integrating sphere 42 for detection by 
radiation detector 44; and 
(c) partly transmitted through the sample 50 and reflected from the 
reflective surface 25 of diffuse mirror 22, as illustrated by beams 56 in 
FIG. 3, back into the optical integrating sphere 42 for detection by 
radiation detectors 44 and 46. 
By way of further explanation, in those instances wherein sample 50 is 
relatively clear, the major portion of the radiation energy emitted from 
the irradiated sample cell 10 would be constituted primarily by radiation 
energy diffused and reflected by mirror 22, as illustrated by beam 56. 
Conversely, in those instances wherein sample 50 is relatively turbid, the 
major portion of the radiation energy emitted from the irradiated sample 
cell 10 would be constituted primarily by radiation energy diffused and 
scattered from within sample 50, as illustrated by beam 54. In those 
instances wherein the sample is only quasi-turbid, the radiant energy 
emitted from the irradiated cell would be comprised of diffused radiant 
energy diffused and reflected by mirror 22 and scattered within sample 50. 
Sequential irradiation as above of the fluid sample 50 with radiation at a 
predetermined number of different predetermined wavelengths is conducted 
in accordance with the spectral absorbance characteristics of the sample 
and of the particular sample property or constituent(s) of interest, and 
in accordance with the overall diffusivity of the sample. Each radiation 
wavelength is selected to provide an optimum measurement of the absorption 
and/or diffusivity of the sample in accordance with spectral absorption 
characteristics of the sample with respect to the particular constituent 
to be analyzed. Thereafter, the levels of the reflected radiation, as 
detected by radiation detectors 44 and 46, are utilized to compute the 
concentration of the particular sample constituent of interest in manner, 
for example, as fully disclosed in said co-pending application for U.S. 
patent Ser. No. 15,017. 
Following spectroscopic, quantitative sample analysis as above, it will be 
understood that the next of a series of fluid samples to be analyzed would 
be flowed into sample compartment 38, by means of an appropriate pumping 
system, not shown but indicated by arrow 59, through inlet conduit 31 and 
access port 30 to displace the analyzed sample from the said sample 
compartment 38 through access port 32 and outlet conduit 33. The sample 
analysis procedure, as described, would be repeated on the newly 
introduced sample. Preferably, a suitable quantity of wash liquid is 
passed through sample compartment 38 before loading of the next sample so 
as to prevent inter-sample contamination, in accord with conventional 
continuous-flow analytical systems, for example, as described in U.S. Pat. 
No. 3,134,263, assigned to a common assignee. 
Of particular significance is the fact that the incident radiation returned 
and detected, as described, by the optical integrating sphere 42 and 
radiation dectors 44 and 46, is composed of a purely transmitted 
component, which is attenuated in precise proportion to the spectral 
absorbance characteristics of the sample, as would occur in a conventional 
transmission instrument and both forward- and backward-scattered 
components, which are also attenuated in precise proportion to the 
spectral absorbance characteristics of the sample, but which are not 
normally measured in conventional transmission instruments with any degree 
of precision. Also, the forward-scatter component is not normally measured 
in conventional reflectance instruments. 
The resulting measurement of the spectral absorbance characteristics of the 
sample is made substantially without regard to whether the fluid sample of 
interest is optically transmitting, optically diffuse, or exhibits any 
possible combination of those optical characteristics. By providing for 
precise measurements of both the optical reflectance and optical 
transmission characteristics of the sample in the same sample cell, 
precise quantitative analysis of fluid samples is achieved substantially 
without regard to variations in sample turbidity, whereby a particularly 
wide range of fluid samples of markedly different turbidities may be 
precisely quantitatively analyzed in the same sample cell. It should be 
understood that use of the term "fluid sample" in this disclosure is by no 
means intended as limitative to freely flowing liquid or gaseous samples, 
but rather, encompasses a wide range of samples including semi-solids in 
the nature of extremely viscous syrups and limited to the capability of 
such samples to be flowed into and out of the sample compartment 38 of the 
fluid sample cell 10. 
The construction and assembly, as described, of the sample cell 10 greatly 
facilitates periodic cleaning thereof, as may be required, to remove 
residues from the sample compartment 38. More specifically, cover member 
14 would be removed by unscrewing from body member 12, to allow full 
access to sample compartment 38 and diffuse mirror 22 for complete cell 
cleaning. 
Various changes may, of course, be made in the disclosed embodiment of our 
invention without departing from the spirit and scope thereof as defined 
in the appended claims.