A chopperless spectroanalytical system of the double beam type in which radiation from a common source is split into reference and analysis beams. The two beams are directed along similar paths such that the analysis beam passes through an analysis region and the reference beam bypasses that analysis region. A monochromator has two spaced aperture regions such that one aperture region provides an entrance aperture for the analysis beam and an exit aperture for a dispersed portion of the reference beam; and the other aperture region provides an entrance aperture for the reference beam and an exit aperture for a dispersed portion of the analysis beam. The beams exiting from the two exit apertures are simultaneously monitored and compared to compensate for errors due to source fluctuations and the like.

This invention relates to spectroanalysis systems and more particularly to 
spectroanalysis systems of the double beam type. 
In spectroanalysis systems of the double beam type, two beams of radiation 
are provided, an analysis beam that is modified by the sample to be 
analyzed and a reference beam that is not modified by the sample. By 
comparing signals from the reference and analysis beams while making 
measurements, errors that arise from fluctuations in source intensity are 
largely eliminated. Frequently, a moving component such as a chopper is 
used to alternately close and open the two beam channels at a steady rate, 
a common type of chopper being a rotating shutter. Such a chopper 
alternately provides radiation along the reference and analysis paths such 
that there cannot be simultaneous measurement of the two beams which makes 
difficult to analyze transient events. Also, with such a device, there is 
an edge interval in which each radiation beam is partially blocked and 
therefore that interval cannot be used for analytical purposes. 
Complexities are introduced by the addition of this moving component as 
the chopper mirror must be flat and precisely positioned relative to the 
axis of rotation of the chopper and to the beam axis or monochromator 
system axis. The chopper may be used with a beam splitter or two choppers 
which are accurately synchronized may be used. The use of movable members 
in spectroanalytical systems frequently results in loss of accuracy over a 
period of time. 
In accordance with the invention there is provided a spectroanalytical 
system of the double beam type in which radiation from a common source is 
split into reference and analysis beams. The two beams are directed along 
similar paths such that the analysis beam passes through an analysis 
region and the reference beam bypasses that analysis region. A 
monochromator is arranged with two spaced aperture regions such that one 
portion of one aperture region functions as an entrance aperture for the 
analysis beam and another portion of that aperture region functions as an 
exit aperture for a dispersed portion of the reference beam, and one 
portion of the other aperture region functions as an entrance aperture for 
the reference beam and another portion of that other aperture region 
functions as an exit aperture for a dispersed portion of the analysis 
beam. The beams exiting from the two exit apertures may be concurrently 
monitored and compared to compensate for errors due to source fluctuations 
and the like. 
In a particular embodiment, a hollow cathode type radiation source is 
employed in an atomic absorption type of analysis system, the output beam 
from the source is divided by a beam splitter into reference and analysis 
beams that are passed along generally parallel paths that lie in a plane 
that is angularly offset from the monochromator axis less than one degree. 
While separate entrance and exit apertures may be employed, in that 
particular embodiment the apertures are defined by spaced elongated curved 
slits of a monchromator of the stigmatic type. The analysis beam passes 
through the lower portion of one slit and is dispersed into a spectrum 
with a portion of the dispersed spectrum being passed as an exit beam 
through the upper portion of the second slit. Similarly, the reference 
beam passes through the lower portion of the second slit and is dispersed 
into a spectrum with a portion of the dispersed spectrum of the reference 
beam being passed as an exit beam through the upper portion of the first 
slit. The monochromator includes a masked collimating mirror and a 
reflection grating, and a mirror and detector assembly is supported at 
each slit for sensing the dispersed radiation exiting through that slit.

DESCRIPTION OF TICULAR EMBODIMENT 
The spectroanalytical system shown in the diagram of FIG. 1 includes 
radiation source 10 in the form of a hollow cathode tube that generates a 
beam of radiation along part 12. A first (analytical) portion 14 of the 
radiation in beam 12 from tube 10 is passed through quartz beam splitter 
16 and is focused by spherical quartz lens 18 for passage through analysis 
zone (flame 20 from burner 22) so that the image of the aperture of tube 
10 is located in the center of flame 20. A second lens 24 focuses the beam 
of radiation that passes through flame 20 on the entrance slit 26 of 
stigmatic monochromator 30. Beam splitter 16 and mirror 32 reflect a 
second (reference) portion of the output beam 12 along path 34 through 
lenses 36, 38 for passage through a second entrance slit 40 of 
monochromator 30. 
In this embodiment, slits 26 and 40 are two of a series of ten slits of 
graduated width that are formed in planar disc 42 of the type shown in 
Smith et al., U.S. Pat. No. 3,508,813. In this embodiment, disc 42 
comprises a copper substrate in which apertures are formed and that 
carries a nickel film in which five pairs of matched slits are formed 
along a circle that is about 6.8 centimeters in diameter, the slits being 
of equal length (about 1.7 centimeters), and graduated in width from ten 
microns to four-hundred microns to permit slit width adjustability by 
rotation of disc 42. It will be apparent, of course, that other slit 
arrangements may be used. 
Monochromator 30 includes collimating mirror 50 with an aligned mask 52 
that has aperture 54 aligned with the analysis beam 14 of radiation that 
passes through the lower portion of slit 26 and aperture 56 aligned with 
the reference beam 34 of radiation that passes through the lower portion 
of slit 40; and a dispersing element 60 in the form of a reflection 
grating mounted for rotation about an axis perpendicular to axis 44 of the 
monochromator. Dispersed radiation from reference beam 34 exits along path 
64 through the upper portion of slit 26 and is reflected by mirror 62 
mounted adjacent the upper portion of slit 26 along path 65 to photosensor 
66. Similarly, dispersed radiation from analysis beam 14 exits along path 
70 through the upper portion of slit 40 and is reflected by mirror 68 
mounted adjacent the upper portion of slit 40 along path 71 to sensor 72. 
Further understanding of the spectroanalytical system may be had with 
reference to FIGS. 2 and 3. Monochromator 30 is of the Ebert type and of 
1/3 meter focal length. The entrance beams 14, 34 are each located at an 
angle of twelve minutes below the monochromator axis 44 and the exit beams 
64, 70 are similarly each located at a corresponding similar angle above 
the monochromator axis 44 as indicated in FIG. 3. The beams 14 and 34 are 
spaced about nine centimeters apart at beam splitter 16 and mirror 32. 
Each mirror 62, 68 is mounted on a support 74, 76 respectively that is 
fastened to housing 78 for slit disc 42. Thus, analysis beam 14 enters 
monochromator 30 through the lower part of slit 26 and, after dispersion, 
exits in beam 70 through the upper part of slit 40 for sensing by 
photomultiplier tube 72, as indicated in FIG. 3. Similarly, reference beam 
34 enters monochromator 30 through the lower part of slit 40 and a 
correspondingly dispersed component (beam 64) exits through the upper 
portion of slit 26 for concurrent sensing by photomultiplier tube 66. 
Further details of the support member 76 for mirror 68 may be seen with 
reference to the perspective view of FIG. 4. That member provides 
coordinated support for mirror 68 and photomultiplier tube 72 in accurate 
alignment with slit 40. A horizontal baffle plate (not shown in FIG. 4) is 
disposed between entrance beam 34 and exit beam 70. Mirror 62 and 
photomultiplier tube 66 are accurately positioned relative to slit 26 by a 
similar support member 74. Each support member 74, 76 is of aluminum and 
has a planar face 80 that is fastened against a face of housing 78 by 
fasteners which are received in threaded holes 82, face 80 having a height 
of about three centimeters and a width of about three centimeters. Formed 
in face 80 is an elongated opening that has an upper portion 84, a lower 
portion 86 and a slot 88 that is about 0.8 millimeter in width. Each 
opening 84, 86 has a width of about 0.6 centimeter and a height of about 
one centimeter. Channel 90 extends rearwardly from opening 86 through 
support 76 (a length of about 21/2 centimeters) to the rear surface and 
provides a passage for entrance beam 34. A baffle plate (not shown in FIG. 
4) has one edge received in slot 88 and is seated on surface 92 to define 
the upper boundary of channel 90. 
A similar channel 94 extends rearwardly from upper opening 84 to surface 96 
that is disposed at an angle of sixty degrees to face 80 and against which 
mirror 68 is fastened. Cylindrical surface 98 of about two centimeters 
radius is formed in the rear of block 76 and photosensor 72 is seated 
against that surface. A third channel 100 extends from channel 94 to the 
curved surface 98 along a path that is disposed in an angle of thirty 
degrees to face 80 and provides a path for the exit beam 71 (reflected at 
102 by mirror 68) to sensor 72. 
In use, the beam of radiation from tube 10 is divided by beam splitter 16 
into analysis beam 14 and reference beam 34. The beams 14, 34 are directed 
along similar paths (analysis beam 14 passing through burner flame 20) and 
the beams pass through the lower portions of entrance slit 26, 40 
respectively. Each beam is collimated by mirror 50 and dispersed into a 
spectrum by grating 60 with a portion of each resulting spectrum being 
directed along an exit path 70, 64 respectively through the upper portions 
of slits 40, 26 respectively, and then reflected by its respective mirror 
68, 62 for concurrent sensing by the respective photomultipliers 72, 66. 
Thus, there is provided a double beam spectroanalytical system that does 
not require a chopper mechanism. 
While a particular embodiment of the invention has been shown and 
described, various modifications will be apparent to those skilled in the 
art and therefore it is not intended that the invention be limited to the 
disclosed embodiment or to details thereof and departures may be made 
therefrom within the spirit and scope of the invention.