L/C detector cell assembly

A spectrophotometer detector cell assembly having improved sensitivity, has a cell defined by a bore through a body and closed at the ends by radiation transparent windows with inlet and outlet passages through the body to the bore, so that radiation passed through the sample fluid flowing through the bore is detected by a photodetector. The body, which is made of a thermally conductive material, is a large thermal mass is relation to the volume of the cell and a tubular inlet conduit, also made of a thermally conductive material, wraps around the body and connects to the inlet passage so that fluid flowing into the bore will tend to reach a stable temperature due to the heat sink effect of the body and conduit thereby stabilizing the refractive index of the fluid in the bore and enhancing the sensitivity of the photodetection. Sensitivity is further enhanced by focussing the radiant energy entering the bore and by concentrating the exiting radiation in a planar area in which the diameter of the bundle of exiting energy rays does not change with changes in the refractive index of fluid in the cell, and by having the latter area substantially coincide with the photodetector surface.

BACKGROUND OF THE INVENTION 
The present invention relates to detectors for spectrophotometers and 
particularly to a detector for a spectrophotometer of the flow-through 
type utilizing variable wavelength radiation for liquid chromatography. 
In the subject type of spectrophotometers for liquid chromatography a 
substance whose quantitative presence in a sample is to be determined is 
dissolved in a suitable carrier solvent and flowed through a detector 
cell, which has end windows through which ultraviolet or visible light 
radiation is directed. Radiation exiting from the cell falls on a 
photodetector whose output is recorded by suitable instrumentation which 
is calibrated to indicate the amount of radiation absorbed by the fluid 
flowing through the cell. Absorbance is customarily indicated by a graph 
continuously recorded on a strip chart by a pen recorder. The quantitative 
presence of a substance of interest is determined by measuring the area 
under the graph peaks which represent the amount of radiation of a 
particular wavelength that is absorbed, particular materials being 
identified by particular wavelengths characteristically absorbed by them. 
The sensitivity of a spectrophotometer detector cell is a function 
particularly of the stability of the base line of the graph; the graph 
base line is established by the absorbance of the solvent used, and will 
change in relation to any change in the refractive index of the solvent, 
which in turn will be changed by a change in the temperature of the 
solvent in the cell. When the baseline changes the true peak area can not 
be measured accurately and the peaks themselves become less clearly 
defined and hence difficult to identify and measure with any reliable 
degree of accuracy. 
A particular problem affecting sensitivity in conventional 
spectrophotometers is the inefficient utilization of the radiation 
applied. In instruments adapted to take measurements with radiation of 
various wavelengths, the beam characteristics of the source are different 
for different wavelengths (which may be provided by substituting different 
radiation sources or by selecting a particular wavelength with a 
monochromator or with a filter). Therefore, it has been the custom to 
dimension and mount the detection cell and photodetector in relation to 
the source so that a minimum diameter source beam of radiation will fill 
the cell entrance, which means a larger diameter beam will lose some 
radiation to vignetting at the entrance and, at the other end, the 
diametric area of exiting radiation impinging on the detector may increase 
beyond the area of the detector surface, and thus be wasted due to a 
change in the index of the solvent in the cell. Consequently, much of the 
radiation applied is lost and only a portion of the exiting beam is 
recorded. Consequently, the sensitivity of the instrument which is rather 
limited in the best circumstance when the index of refraction of the 
solvent remains constant (ie. when flow noise is at a minimum) is 
disproportionally reduced by any change in temperatures of the solvent, 
which alters its index of refraction and thus increases flow noise. 
SUMMARY OF THE INVENTION 
Objects of the present invention are: to provide a spectrophotometer 
detector cell for liquid chromatography that is more accurate and 
sensitive than previously known types of such detector cells, to provide a 
detector cell assembly which incorporates an optical system for applying 
available radiation more efficiently and for more efficient utilization of 
the exiting light for increasing the sensitivity of measurement, to 
provide such a detector cell assembly which renders the spectrophotometer 
substantially insensitive to flow noise due to changes in the refractive 
index of the solvent, and to provide such a cell in which temperature 
changes in the sample flowing through the cell are minimized so that the 
refractive index of the solvent remains substantially constant thereby 
further enhancing the sensitivity of the absorbance measurement. 
In accordance with the invention a spectrophotometer detector cell assembly 
is formed by a bore through a body of thermally conductive material, such 
as stainless steel, brass or aluminum whose mass is at least several times 
larger in volume than of the bore. Inlet and outlet passages for fluid 
flow are provided through the body wall to the bore, and windows, normally 
of quartz, are sealed over the ends of the bore. The assembly is mounted 
for a radiation from a source, such as a deuterium arc to pass through the 
cell to a photodetector at the other side, the particular wavelength 
desired being selected by means of a monochromator or filter between the 
radiation source and the cell. 
A tubular inlet conduit from a source of sample material (in a solvent) to 
be analyzed is made of thermally conductive material, such as stainless 
steel, brass or aluminum, and connects to the inlet passage of the body 
for conducting the sample material to the cell bore. A portion of the 
inlet conduit immediately upstream from the connection to the inlet 
passage is in thermal contact with the body; in a preferred form the inlet 
conduit wraps at least once, and preferably three times around the body in 
thermal contact therewith. Thus, the body and conduit have a heat sink 
effect on the fluid sample flowing to the cell so that fluid sample 
material flowing through the cell bore is at a substantially stabilized 
uniform temperature so that the refractive index of the sample fluid 
flowing through the cell remains substantially constant, which enhances 
the sensitivity of the instrument. 
A focussing optic, normally a lens, is mounted on the assembly to focus 
radiation from the source into the cell bore, and is dimensioned and 
positioned to maximize the amount of focussed radiation within the cell. 
At the exit end of the cell, a second optical focussing element such as a 
lens is dimensioned and positioned to concentrate substantially all the 
radiant energy exiting from the cell onto the photosensitive surface of 
the photodetector.

DETAILED DESCRIPTION 
Referring to FIGS. 1 and 2 of the drawing, a detector cell assembly 10 in 
accordance with the invention may be made as a separate unit adapted to be 
inserted as the detector cell in a multi purpose spectrophotometer, or it 
may be built in. In the embodiment shown the detector cell assembly 10 
(indicated with the dash-line box) is shown and described as a composite 
unit adapted to be inserted in a multi-purpose spectrophotometer 
instrument. The unit is inserted in position for radiation from a 
spectrophotometer source 11 of radiation to pass through a windowed end 
cell 12 of the detector assembly and impinge on the photosensitive surface 
13 of a photodetector 14. The photodetector 14 produces output signals 
proportional to the radiation received, the signals thus being 
proportional to the amount of light absorbed by a sample fluid flowing 
through the cell 12. Signals from the photodetector 14 are processed by 
means well known in the art to provide an analysis in interpretable form, 
such as a continuous graph produced by a pen recorder. 
The source 11 is suitably a deutrium arc lamp and the particular wavelength 
of the radiation to be applied to the cell 12 is suitably selected by a 
monochromator or filter (not shown) in the path of the radiation from the 
source 11 to the cell 12. The windows 16, 17 are made of a suitable 
radiation transparent material such as quartz and are mounted and sealed 
over the ends of the cell bore 12 by any suitable means. In the preferred 
form shown the windows are sealed in place by means of a pressure tight 
seal of the particular type described in detail in a copending U.S. 
application Ser. No. 547,380 of Charles F. DeMey. This seal includes thin 
polytetrafluorethylene gaskets 18, captured respectively between each 
window 16, 17 and the adjacent surface of the body 15. The windows are 
each held in place by a bushing 19 having an inward flange 19a bearing 
against the outer circumferential surface of the window and an outward 
flange 19b; three or more Bellville washers 20 are placed around each of 
the bushings 19 and are held under compression against the outward flange 
19b by retaining rings 21 which are held in place on the body by screws. 
The screws 22 (only one of which is shown) are passed through one of the 
retaining rings 21 and pass through the body 15 to be threaded into the 
other retaining ring 21, as shown. 
The body 15 is made of a thermally conductive material, suitably stainless 
steel, and has a mass at least several times larger in volume than the 
cell bore 12. An inlet passage 25 and an outlet passage 26 are drilled 
through the body 15 to open into enlarged end portions of the cell bore 
12, respectively. As indicated in FIGS. 2 and 3, the enlarged end portions 
of the cell bore 12 are suitably provided by countersinking each end of 
the bore off-axis to provide a larger bevelled surface at one side through 
which the inlet and outlet passages 25, 26 open. 
At their outer ends, the inlet and outlet passages 25 and 26, are connected 
respectively to an inlet tube 27 and to an outlet tube 28 which are made 
of thermally conductive material, such as stainless steel; the connection 
being suitably made by welding the ends of the tubes 27, 28 into the ends 
of the inlet and outlet passages 25, 26 respectively. Other materials, 
such as aluminum or brass might in some cases be used for the body 15 
and/or the tubes 27 and 28, but stainless steel is used to enable the 
instrument to be used for highly corrosive materials. 
A portion of the length of the inlet tube 27 immediately upstream from the 
connection into the inlet passage 25, is in thermal contact with the body 
15; in the form shown the inlet tube 27 is wrapped three times around the 
body 15 before leading off to the connection to a source of the sample 
material to be analyzed. The latter source is not shown, the connection 
thereto being indicated by a threaded bushing 29. The outlet tube 28 
suitably leads directly to a connection, indicated by threaded bushing 30, 
to a waste pipe or waste container, not shown. 
As shown, a ring 31 is suitably fixed around the outside of the 
convolutions of the inlet tube 27 around the body 15 to assure good 
thermal contact of tube convulutions and body. 
The relatively large thermal mass of the body 15 in relation to the volume 
of the cell 12 and to the inlet and outlet passages 25, 26 therethrough, 
and the thermal contact of a portion of the inlet tube 27 around the body 
15, provides a heat sink effect so that by the time the sample fluid being 
fed in reaches the cell bore 12 its temperature has become substantially 
equalized with the temperature of the body 15, which due to its thermal 
mass remains substantially constant during an operative run of the 
apparatus. In consequence the sample fluid flowing continuously through 
the cell bore 12 will be at a substantially uniform temperature; the 
refractive index of the solvent thus remains substantially constant, 
thereby eliminating flow noise so as to enhance sensitivity and accuracy 
of the measurements being taken. 
The sensitivity of the absorption measurement of a spectrophotometer 
incorporating a detector assembly 10 of this invention is further enhanced 
by concentrating more of the available radiation into the cell and by 
rendering the system substantially insensitive to any flow noise due to 
any changes in the refractive index of the solvent that may occur. These 
advantages are provided by an optical system which maximizes the efficient 
application of available radiation to the fluid in the cell bore, and 
maximizes the efficient measurement of the amount of radiation exiting 
from the cell bore. As shown in FIGS. 2 and 3 this optical system includes 
a lens 35, or other optical focussing system such as a reflective optical 
system, mounted on the detector assembly 10 (by means not shown) in 
position to focus radiation from the source 11 into the cell bore 12. In 
order to get as much of the focussed radiation as possible into the cell 
bore 12, the lens 35 is dimensioned and positioned so that its focus is as 
far along in the cell bore 12 as possible without having to lose too much 
radiation by vignetting at the entrance of the bore cell. 
At the exit end of the cell a lens 38, or other optical focussing system 
such as a reflective optical arrangement, is dimensioned and positioned to 
collect all the radiation exiting from the cell bore 12, including the 
widely diverging Schlieren rays and concentrate the radiation on the 
photodetector 14 by focussing the radiation. The Schlieren rays, indicated 
by ray traces 39, and other rays, indicated by ray traces 40, will be 
focussed at different points by the lens 38, but there will be a plane 41 
in which the focussing Schlieren rays 39 and other focussing rays 40 
intersect and in which the diameter of the area of intersection indicated 
at 42 will remain constant despite variations or changes in the angular 
direction of radiation through the cell bore 12. In the preferred 
embodiment shown, the lens 38 and/or the photodetector 14 are selected and 
positioned so that the position and diameter of the area 42 coincides with 
the position and diameter of the area of the photosensitive surface 13 on 
photodetector 14 in the plane in which the concentrated radiation impinges 
on the photosensitive surface. 
With the foregoing detector cell assembly, the temperature, and hence the 
refractive index, of the solvent passing through the cell bore 12 is 
substantially stable. Additionally by maximizing both the effectiveness of 
the application of the input radiation to fluid in the cell and of the 
measurement of exiting radiation to the photodetector, the 
spectrophotometer readings are substantially free of distorting effects of 
flow noise and are substantially insensitive to changes in the refractive 
index of the solvent. The detector cell assembly 10 of this invention thus 
provides a spectrophotometer having greatly improved sensitivity and 
accuracy.