Multiple internal reflectance liquid sampling

A liquid cell for IRS comprising a rod-shaped crystal having a cylindrical sampling surface and integral conical end portions forming end entrance and exit faces for a radiation beam. The center of the end faces are masked. All radiation entering the entrance face internally reflects from the conical surface and propagates down the IRE by multiple internal reflections from the cylindrical sampling surface. The IRE can be sealed into a trough or flow-through housing for the liquid sample. The seals are at insensitive regions of the IRE.

This invention relates to multiple internal reflection spectroscopy (IRS), 
and in particular to a novel accessory for the application of this 
analysis technique to the analysis of liquids. 
BACKGROUND OF THIS INVENTION 
Reference is made to a book authored by N. J. Harrick entitled "Internal 
Reflection Spectroscopy", published by Interscience Publishers in 1967. 
This book provides a complete description of the principles underlying 
this technology, and also describes the construction and configuration of 
so-called internal reflection elements (IREs) used in such analysis 
equipment. Attention is especially drawn to pages 223-227 which describes 
the application of IREs for use in liquid cells. Reference is further made 
to U.S. Pat. Nos. 4,730,882 and 4,602,869, which are also directed to 
different IRE and liquid cell geometries, the contents of which are hereby 
incorporated by reference. 
In general, it is important to many companies that process liquids to be 
able to conduct in-line analysis or analyses of samples in the simplest 
and most economic manner. As is known, many conventional spectrometers 
generate a radiation beam which upon emerging from the instrument 
converges to a region in the so-called sampling space or compartment of 
the instrument, and if not intercepted or used will continue back into the 
instrument for detection and spectral analysis. It is common to locate the 
IRE element and transfer optics for the IRE element in an accessory in the 
sampling space so as to maintain the original focussing conditions. At the 
same time, it is desirable that the IRE element, which must physically 
contact the liquid, be suitably positioned for effective and efficient 
use. 
The U.S. Pat. No. '869 referenced above describes a single reflection prism 
liquid cell for IRS. This construction offers limited interactions with 
the liquid sample and thus reduced signal-to-noise (S/N) ratios. A recent 
paper in Applied Spectroscopy, Vol. 44, No. 1, 1990, pps. 50-59, describes 
a cylindrical IRE cell using an IRE rod with a circular cross-section, 
which can be located in line with the optical beam in the sampling 
compartment. Disadvantages, however, of this construction include reduced 
light throughput due to the need for the entering and exiting beam to 
reflect off metallic surfaces, and the possibility of spurious spectra due 
to beam interaction with the seals for sealing the IRE rod within the 
cell. 
SUMMARY OF INVENTION 
One object of the invention is an IRE construction that is suitable for 
in-line analysis of liquids, providing increased S/N ratios and reduced 
spurious spectra. 
Another object of the invention is an IRE and liquid sampling accessory for 
IRS using a cylindrical IRE which is simple to use while maintaining the 
original focussing conditions of the spectrometer. 
There and other objects and advantages of the invention are achieved in 
accordance with one feature of the invention with a novel cylindrical IRE 
crystal that is configured with widening conical crystal ends integral 
with the cylindrical crystal rod. Preferably, the ends terminate in narrow 
cylindrical portions. The centers of the outside of both rod ends are 
masked off to block radiation. 
In accordance with another feature of the invention, the crystal rod is 
mounted in a trough or flow-through cell such that liquid provided inside 
the cell wil contact all of the rod cylindrical surfaces except for the 
end cylindrical portions. The latter are used to support seals for sealing 
the IRE within the cell. When the cell is placed inside the sample 
compartment, the novel IRE allows entrance of the analyzing IRS radiation 
beam into the crystal via an annular region surrounding the center mask at 
one rod end. The entering beam totally internally reflects off of the 
conical surface, which redirects the beam to propagate down the rod via 
multiple internal reflections from its cylidrical major surface until it 
encounters the opposite conical end, from which it exits from the opposite 
rod end via a corresponding annular region after reflection from the 
conical surface. The exiting beam, modulated by interaction with a 
sampling in contact with the rod cylindrical surface, is re-aligned in the 
sampling compartment for ready detection and processing. 
The advantages of the invention include: higher light throughput since all 
reflections are by total internal reflection except where the beam 
interacts with the sample; no spurious spectra due to beam interaction 
with the seals, as the seals can be located at insensitive edges of the 
IRE; increased sensitivity as a standard IRE length allows 10 reflections 
from the sample; very easy to use and align, as no other optics are 
necessary to transfer the beam and the sample need only be poured into the 
trough or flowed through the cell; and gives excellent spectra of any 
contacting liquid due to the multiple interactions of the beam with the 
liquid sample.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 illustates one form of an IRE in accordance with the invention for 
direct immersion into a liquid to be analyzed. The IRE 10 is a generally 
rod-like single crystal of one of the well-known IRE materials. It 
comprises a central section 11 having a circular cross-section forming a 
cylindrical sampling surface 12, which is optically polished in the usual 
way. Integral with the central section 11 are opposite end sections 13, 
14, each of which are widening conical portions axially aligned with that 
of the central section 11. The end portions 13, 14 each terminate in a 
narrow cylindrical part 16, 17 forming transverse end faces 18, 19. 
Circular beam masks 20, 21 are placed at the center of each end face, 
forming at the left end face a narrow annular entrance window 23 for an 
axially incident radiation beam 25, and at the right end a correspondingly 
shaped annular exit window 26 for the exiting beam 27. The result of the 
masking is to form a hollow beam, but only the optical paths on one side 
are shown. 
As will be clear, the size of the masks 20, 21 relative to that of the end 
faces, and the cone angle 29 and conical section length 30 are chosen such 
that all non-masked entering radiation internally reflects from the 
conical surface 32 and thus will propagate as shown at 33 by multiple 
internal reflections from the cylindrical sampling surface 12 before 
undergoing symmetrical redirection at the exiting end to form the exiting 
beam 27. No incident radiation passes through the IRE without reflecting 
from the sampling surface 12. Thus, all of the radiation will efficiently 
interact with the sample on the sampling surface. 
As an example, which is not intended to be limiting, the IRE may have an 
overall length between end faces of 7.16 cm, a diameter of the cylindrical 
sampling surface 12 of 0.63 cm, a length 30 of each of the conical ends of 
1.1 cm, a cone angle of 45.degree. (which can vary between 
30.degree.-60.degree.), a length of the narrow cylindrical end portions of 
2 mm, and a mask diameter of 6 mm. The rod is symmetrical as shown. With 
these dimensions for a standard zinc selenide crystal material, an IR beam 
will undergo 10 reflections before exiting. With other IRE materials, the 
IRE can be given the same or different dimensions to achieve the optical 
performance described above. 
In use, the IRE 10 is mounted inside a sealed trough-like cell structure 
(FIG. 2) or flow-through cell structure (FIG. 3) to form a liquid cell. 
The FIG. 2 cell 40 comprises a base member 41 for supporting the cell 40 
in-line with the optical beam in the spectrometer sampling compartment. On 
the base 41 is mounted a generally cylindrical housing member comprising a 
central, section 42 to which are removably connected, as by screw threads, 
end sections 43, 44. The central section 42 has at its top an oval opening 
45. The end sections 43, 44 each have outer projections 47 engaging an 
annular shoulder 48 on the central section, forming between the facing 
inner surfaces an annular seal area 50. In the seal area 50 is located an 
annular gasket 51 for supporting an O-ring 52 which sealingly engages the 
narrow cylindrical portions 16, 17 of the IRE 10. The end sections 43, 44 
of the housing member each have bevelled openings 54 to accommodate a 
converging entering beam and a diverging exiting beam. 
The FIG. 3 flow-through cell 60 is similar, the same refernce numerals 
being used for similar parts. The only difference is the central section 
42' which is without an opening, and the sample is supplied via standard 
fittings 61,62 to the cell interior. Both cell housing parts may be made 
up of stainless steel, and conventional gasket and O-ring materials may be 
used suitable for the liquid being analyzed. 
In use, the IRE 10 is placed within the respective cell housing 40, 60, the 
O-rings 52 installed, and the ends 43, 44 screwed on to form a sealed cell 
asembly. The trough cell in FIG. 2 is used by simply pouring the sample 
through the opening 45 so as to surround the IRE cylindrical sampling 
surface 12. The flow-through cell is used by simply flowing the liquid 
through the cell. The seals prevent leakage. 
FIG. 4 is a sample spectra of isopropanol taken with the FIG. 2 cell. The 
excellent spectra obtained will be evident. 
As will be clear from FIGS. 1 and 2, the radiation does not reflect from 
the narrow cylindrical end regions 16, 17 where the seals are located. 
Those regions, being outside the radiation path, are insensitive thus 
eliminating O-ring spectral interference. Moreover, all reflections are 
total internal reflections (except at the sampling surface) so that no 
reflection losses are encountered. The result is higher sensitivity and 
better spectra. The ease of use is evident from the description. 
It is also very easy to attach to the outside of the cell housing hoses or 
tubes leading to the spectrometer beam exit and entering ports, through 
which a purge medium is available. Thus, the entire beam path can easily 
be maintained in a purge atmosphere, as described in our copending 
application, Ser. No. 07/831,529, whose contents are hereby incorporated 
by reference, thus permitting easy sample exchanges while maintaining the 
purge atmosphere through the spectrometer and cell accessory. 
The IREs of the invention can be constituted of any of the radiation 
transparent materials described in the Harrick book, and are useful with 
any of the radiations conventionally used in this field, including IR, UV 
and visible. 
While the invention has been described in connection with preferred 
embodiments, it will be understood that modifications thereof within the 
principles outlined above will be evident to those skilled in the art and 
thus the invention is not limited to the preferred embodiments but is 
intended to encompass such modifications.