Laser light scattering photometer

A laser light beam is projected through a fluid measuring cell having a fluid accommodating chamber and inlet and outlet pathways for the fluid. The light is separated into transmitted light and scattered light. Separate light detectors sense the transmitted light and scattered light. An electric control circuit uses the variations of the intensity of the transmitted light to compensate for fluctuations in the intensity of the scattered light to provide an output related to the intensity of the scattered light.

BACKGROUND AND DETAILED EXPLANATION OF INVENTION 
The present invention relates to a laser light scattering photometer for 
the determination of molecular weight of the solute component in the 
subjected liquid by detecting the intensity of the scattering light from 
the liquid. This invention offers equipment for the measurement of the 
intensity of the scattering light by means of a novel optical system and 
measuring "cell" structure. 
The light scattering method is playing an important role for determining 
the molecular weight and size of solute-molecule in the solution. This 
method is not widely used because of the complicated operative techniques 
for measurement. For instance, it is necessary to extrapolate the 
concentration and the angle to zero by the Method of the Zim Plot, for 
example, by measuring the intensity of the scattering light at the angle 
in the range from 30.degree. to 150.degree. usually as to several samples 
with different concentrations of the solutions. 
In laser light scattering method it is possible to determine the molecular 
weight without extrapolating technique of both factors as in the above. 
That is, in the use of laser light scattering photometer, it is possible 
to measure the intensity of scattering light at a small angle of 5.degree. 
or lesser angles. 
In practice, therefore extrapolation to zero-angle is unnecessary. 
Futhermore, because of higher intensity of the incident light, very dilute 
solution can be used as a sample for measurement, thus usually there is no 
necessity to extrapolate to zero-concentration. 
In such a case, the intensity of the scattering light to be measured is 
proportional to the product of the molecular weight of the solute and the 
concentration of the solution, therefore, by combined use of laser light 
scattering photometer as a molecular weight detector with a concentration 
detector, it is possible to measure the molecular weight of the solute in 
the subjected solution continuously. 
Conventionally known laser light scattering photometers have specific 
structures especially on the measuring "cell" part, which require special 
processing acuity, and thus, lacking in practicability. Moreover, the 
fluctuation of the intensity of the incident light becomes a cause of 
variation of the intensity of scattering light, thereby, it is difficult 
to obtain stable measuring values as the dermerit. Then further practical 
photometers are required. 
The present inventors have been making efforts for improving the above 
defects, and have completed a laser light scattering photometer in 
featuring the equipment provided with the optical system removing the 
fluctuation of the itensity of scattered light due to the change of the 
intensity of the incident light, and its corresponding electronic system 
as well as the measuring cell with much simpler structure than that of the 
conventional ones. 
In other words, in a laser light scattering photometer for measuring the 
intensity of scattered light of a subjected testing liquid, a measuring 
cell and an annular slit equipped with a light trap at the central 
position are aligned so as to match the axis of the incident beam. A pair 
of detectors in a receiving optical system monitors the intensity of the 
light transmitted through the measuring cell and is reflected by the light 
trap. The second light detector detects the intensity of scattered light 
at the scattering angle restricted by the annular slit. An electronic 
circuit eliminates the intensity fluctuation in which the the fluctuation 
in the intensity of the transmitted light is detected by the detector for 
transmitted light. The electronic circuit has an output signal 
corresponding to the fluctuation in intensity of the transmitted light. 
This output signal is put into the detector for scattered light as input 
signal. Thus, by the output signal, the fluctuation in the output of the 
scattered light detector caused by the fluctuation in the intensity of the 
incident beam is compensated. 
The measuring cell is a flow-type cell composed of a cell-composing 
material having cylindrical beam-passing part perforated to make the 
central axis matching the axis of a laser incident beam and cylindrically 
perforated pathway of a testing liquid, connected to the beam-passing 
part. Two glass blocks are mounted on front and rear faces of the 
cell-composing material against laser incident beam.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The major part of this equipment has a measuring cell 2 and annular slit 4 
equipped with light trap 3 at the central position thereof. The light trap 
3 is aligned so as to match the axis of the incident beam which is the 
optical axis of the system. The system includes a pair of detectors 
receiving scattered light and transmitted light. By measuring the 
intensity of the elements of transmitted and scattered light, the 
fluctuation in the intensity of scattered light due to the change of 
intensity of incident light can be compensated. 
As shown in FIG. 1, laser incident light passing through the slit 1 is 
spectroscopically divided into two elements, namely, the element of 
scattered light from the testing liquid and the element of transmitted 
light transmitted through measuring cell 2 having the central axis in 
conformity with the incident ray axis. 
The transmitted light is reflected at the right angles by the light trap 3, 
attached to the central position of the annular slit 4 whose axis is in 
accord with the axis of the incident beam light trap 3 has a ray-receiving 
plane at an angle of 45.degree. against the ray axis of the incident beam. 
A photomultiplier tube 10 (hereinafter referred to as R-PM) detects the 
transmitted light reflected by trap 3 through neutral filter 8. The 
scattered light is converged by the lens 5 after passing through the 
annular slit 4 whose axis is in accord with the axis of incident beam, 
corresponding to the specific scattering angle .theta.. 
The light beam after passing through the light-receiving slit 6 becomes a 
parallel ray by the lens 7 and is received by the photomultiplier tube 9 
(hereinafter referred to as S-PM) for detecting the scattered light. The 
size of annular slit can be selected to make the scattering angle less 
than 10.degree.. Preferably, the angle is less than 5.degree.. 
The elements of transmitted light and scattered lights are each detected at 
two detecting parts of R-PM 10 and S-PM 9 to compensate for the 
fluctuation of the intensity of the scattered light due to the variations 
of the intensity of the incident light at the electronic control circuit 
composed and illustrated in FIG. 2. 
The electronic control circuit ensures less fluctuation in the measured 
values of the intensity of the scattered light and the accuracy of the 
measurements. 
FIG. 2 shows a rough sketch of an example of the electronic control circuit 
18 for eliminating the effect of variation of the intensity of incident or 
scattered light. The control circuit 18 has an amplifier IC 2 connected in 
series with R-PM 10 and amplifier IC 3. A reference voltage setting part 
16 of the photomultiplier tube is connected to amplifier IC 3 in parallel 
with amplifier IC 2. The output of amplifier IC 3 is fed back to power 
supply source 17 of the photomultiplier tube. Power supply source 17 is 
connected to R-PM 10 to complete the fluctuation elimination circuit. The 
output voltage of power supply source 17 obtained from the fluctuation 
elimination circuit is the input to S-PM 9. The signal of S-PM 9 is fed to 
amplifier IC 1. That is, the intensity of the incident light is converted 
to an electronic signal by R-PM 10. The output signal of R-PM 10 is fed to 
amplifier IC 3 after amplification by amplifier IC 2. At amplifier IC 3, 
the input from amplifier IC 2 is compared with the voltage of the 
reference voltage supplied by the voltage setting part 16 of the 
photomultiplier tube and for making the difference of them zero. The 
supplied voltage from power supply source 17 of the photomultiplier tube 
starts the operation. For instance, when an increasing variation occurs in 
the intensity of the incident light, and the input from amplifier IC 2 
becomes greater than the voltage from voltage setting part 16, the input 
signal of amplifier 2 is reduced to make the difference of the signals 
zero. This dissolves the increasing variation of the input from S-PM 9. 
FIGS. 3 and 4 show examples of the fluid cell 2 in planes parallel to the 
laser incident light. 
The measuring cell 2 in this invention is a flow-type cell composed of two 
glass blocks 11a and 11b mounted to the front and rear faces of a body 15 
of cell-composing material. Body 15 has laser beam passing pathway or 
chamber 12, an inlet-liquid pathway 13, and outlet-liquid pathway 14 
connected to pathway 12. Pathway 12 is a cylindrical liquid testing 
chamber having a central axis coincident with the ray axis of laser 
incident beam. A disc 16 in pathway 12 aligned with the axis of pathways 
13 and 14 causes the fluid, such as a liquid, to flow outwardly toward the 
outer portion of the fluid testing chamber. In other words, the fluid in 
the fluid testing chamber flows around the outer circumferential edge of 
disc 16 to the outlet pathway 14. The subjected liquid is introduced from 
the inlet-liquid pathway 13, and passes through the beam-passing pathway 
12, then it is sent to flow out of the outlet-liquid pathway 14. 
In this case, the incident light passed through the front glass block 11a, 
central axis of beam-passing pathway 12, and rear glass blocks 11b, 
thereby spectroscopically dividing the light into two elements, namely, 
transmitted light and scattered light. The body 15 is made of material 
having non-light reflecting characteristics. Preferably, the material of 
body 15 is processed synthetic resins with favorable solvent proof nature, 
such as black poly-(tetafluoroethylene) or poly-(chlorotrifluoroethylene). 
The beam-passing pathway 12, inlet-liquid pathway 13 and outlet-liquid 
pathway 14 can be easily manufactured by processing the said 
cell-composing material with cylindrical perforation. 
Referring to FIG. 4, there is shown a modification of the measuring cell 
indicated generally at 20. Measuring cell 20 has a body 21 of light opaque 
material similar to body 15. The center portion of body 21 has a 
cylindrical liquid testing chamber or pathway 22 for accommodating the 
testing of liquid. A right angle inlet pathway 23 extends upwardly into 
body 21, as shown in FIG. 4, and opens to the right side of chamber 22. An 
outlet pathway 24 is located in the upper portion of body 15 and is open 
to the right side of chamber 22. Glass blocks 25 and 26 are mounted to the 
front and rear surfaces of body 21. 
The size of the beam passing pathways 12 and 22 in the fluid flow type 
measuring cells 2 and 20 is not particularly restricted. Preferably, it is 
desirable that the diameter of each pathway 2 and 22 of the cylinder be 
less than 4 mm. In some cases, a diameter of less than 2 mm is preferable. 
The length of the cylinder of the pathways 12 and 22 is less than 20 mm. 
Preferably, this length is less than 10 mm. The flow type fluid measuring 
cells 2 and 22 of the invention have capacities of less than 100 .mu.l, 
and usually smaller than 50 .mu.l. The fluid measuring cells have the 
merit of being able to make measurement of a minor amount of a sample with 
high accuracy. Since the material of the cell body is black, it can absorb 
the reflected light from the surface between the fluid and the glass 
blocks. This makes possible the measurement of the intensity of scattered 
light with a low level of stray light. Since the beam passing pathways 12 
and 22 have the shape of simple cylinders and small dead volumes, the 
subjected testing fluid does not have a detracting flow disorder. 
Therefore, this equipment is particularly superior as a detecting device 
of successively eluted components discharged from the molecular weight 
fractionating equipments such as GPC or FFF, etc. as mentioned later. 
This flow type laser light scattering photometer of the invention measures 
the intensity of the scattered light of a subjected test fluid with high 
accuracy. The photometer is useable as a detector of fractionating 
equipments such as Gel-Permeation Chromatograph (GPC) of Field-Flow 
Fractionation (FFF). This equipment using the photometer of the invention 
is operable to make direct evaluation of the molecular weight of each 
effluent of the subjected liquid. This equipment is also operable to 
provide analytic function of molecular weight to the fractionating 
function by GPC. 
Stable measurement values were obtained in a test for the stability of the 
response corresponding to the intensity of scattered light from 
tetra-hydrofuran (THF). The extent of fluctuation of the base line had a 
variation of less than 0.1% after continuous measurement for 24 hours. 
This measurement was compared with a photometer without operating the 
fluctuation elimination circuit, as shown in FIG. 2. The result of this 
test was that the variation of the base line was 4%.