 |
Description  |
|
|
FIELD OF THE INVENTION
The present invention relates to sample holders for use in infrared
spectroscopic analysis and their use.
BACKGROUND OF THE INVENTION
In infrared ("IR") spectroscopy a beam of light from an infrared source is
passed through a sample. The light that is transmitted through the sample
is evaluated in comparison with the incident light and its intensity
plotted as a function of wavelength or wavenumber. Wavenumber is expressed
herein as centimeters.sup.-1 or "cm.sup.-1 ". This spectral plot or
spectrum can provide information regarding the functional groups and
structural features of the sample and, accordingly, IR spectroscopy has
become a valuable tool in analytical chemistry for certain types of
samples.
The infrared region of the electromagnetic spectrum extends from the upper
end of the visible region (wavenumber of approximately 14,300 cm.sup.-1)
to the microwave region (near 20 cm.sup.-1). The region which is typically
of most interest to analytical chemists for determination of structural
features of an organic sample is from about 4000 cm.sup.-1 to about 400
cm.sup.-1. In this region of the spectrum, organic compounds absorb
incident infrared light at frequencies corresponding to the vibrational
frequencies of the compound. These absorbed frequencies are characteristic
of the structural features of the compound or compounds in the sample and
can permit rapid identification. The intensities of the peaks in the
spectral plot or spectrum are a function of the concentration of the
sample, extinction coefficient, and path length of the incident light
through the sample.
To obtain an infrared spectrum of a sample, the sample is typically applied
to a sample holder or "cell". This sample holder or cell holds the sample
in the path of the incident beam of infrared light. It is essential that
the material used for the sample holder be highly transmissive in that
region of the IR spectrum which is of interest. Also, the sample holder
should not be soluble in, or reactive with, either the sample or solvent
(if any). Illustrative examples of materials used in sample holders
include inorganic salts, glasses, and quartz.
Sodium chloride (NaCl) is perhaps the most commonly used material since it
does not absorb infrared light in the range of 4000 to 625 cm.sup.-1 and
is relatively less expensive than some alternatives. However, NaCl
crystals are very susceptible to moisture and easily broken. For a
discussion of cell materials see Pasto and Johnson, Organic Structure
Determination, Prentice-Hall, Inc., 1969, pp. 145-147.
In the majority of analyses, the holder (or cell) is a pair of plates made
from crystals of an inorganic salt that has been precisely machined and
polished for maximum optical clarity. A sample is then placed between the
pair of plates and mounted by a variety of techniques in the beam of
infrared light. Solid samples are often ground and intimately mixed with
an inorganic salt such as potassium bromide, pressed into a thin wafer or
pellet, applied to a sample holder, and mounted in the infrared beam.
Alternatively, samples may be mulled with an oil such as NUJOL.TM. mineral
oil, applied to a sample holder, and analyzed as a thin film. Liquid
samples, either neat or in solvent, may also be analyzed using a sealed
cell in which a pair of plates are sealed together with a spacer to
provide a chamber in which the sample is held. In addition to the use of
plates, other sample preparation techniques have been developed. For
instance, liquids or solutions having a relatively high surface tension
such as aqueous solutions have been analyzed by suspending a thin film
from a loop of wire. Also, a solution may be coated and dried to form a
film, e.g., a solution may be coated on a film of polytetrafluoroethylene
and dried, and the resulting thin film peeled from the
polytetrafluoroethylene and analyzed.
Due to the susceptibility of many known cell materials to degradation by
moisture and the long drying time necessary for preparation of some
samples, analysis of aqueous samples is difficult. Increasingly stringent
regulations have prompted many industries to reduce or eliminate organic
solvent use and emissions, prompting the development of water-based
processes and products. Illustrative examples of materials that have been
used for cells for use with aqueous samples include silver bromide,
calcium fluoride, and barium fluoride. Use of such materials is limited by
the typically high expense, limited useful spectral ranges, burdensome
maintenance, and difficult sample preparation associated with such
materials. Typically, aqueous samples are analyzed using a horizontal
attenuated total reflectance ("ATR") crystal to which a sample is applied.
A beam of infrared light is reflected repeatedly through the sample before
being evaluated in a detector. Use of this technique is hampered by the
high cost of sample holders and difficulties encountered in sample
preparation and maintenance. In part due to these problems, IR
spectroscopy has not reached its potential as a routine tool for analysis
of aqueous samples.
In addition to the problems described, namely cost, sensitivity to moisture
and fragility, commercially available cells have high maintenance
requirements. In view of the high costs, disposal of these cells is
prohibitive. Accordingly, sample holders must be carefully cleaned,
typically with organic solvents, after each analysis to prevent
contamination from one sample to the next. In some instances, the solvents
may present health risks to operators. In addition, the high cost of
sample holders tends to inhibit retention of samples on a long term basis.
Dove and Hallett, Chemistry and Industry, 1966, pp. 2051-53, describe an
all-plastic evacuable cell to be used for infrared or ultraviolet
spectroscopic analysis of gases. The cell has windows that can be made
from RIGIDEX.TM. Type 35 polyethylene. The relative thickness of the
windows, i.e., about 3 millimeters, would preclude the use of such sample
holders in most routine IR spectroscopic analysis due to the strong
absorbances. Andrede, J.Chem. Ed., 66(10), p. 865, 1989, describes using
polyethylene film as windows in a sample cell. For sampling of liquids the
author suggests applying the sample to a film stretched over a ring,
covering the sample with a second film, and securing both stretched films
with a second ring.
IR spectroscopy has been used as a tool in the analysis of polymer films.
Osland, Laboratory Practice, 37(2), p. 73, 1988, describes a heated press
used to prepare plastic films for analysis by IR. Love and Wool, A.C.S.
Polymeric Material Science and Engineering, analyzes semi-crystalline
polymer films by Fourier Transform Infrared Spectroscopy (FTIR). Benson,
European Plastics News, p. 26, 1989, describes using IR radiation to
measure the thickness or gauge of polymer films.
Owen and Wood, J.Chem. Ed., 64(11), 1987, pp. 976-79, describe the use of
tissue paper as a support matrix to obtain infrared spectra of solids and
non-volatile liquids. This method would appear to be impractical due to
the fragility of the paper and the strong interfering absorbances of the
cellulose. As a result, the signal-to-noise ratio or sensitivity is quite
low.
Jackson, "Novel Sampling and Support Media for the Infrared Analysis of
Water-immiscible Oil-based Environmental Pollutants", Analyst, vol. 109,
March 1984, pp. 401-02, discloses the use of stretched
polytetrafluoroethylene tapes as a support medium for recovery and
infrared spectroscopic analysis of water-immiscible organic pollutants.
U.S. Pat. No. 4,942,297 (Johnson et al.) discloses an apparatus for
collection and infrared spectroscopic analysis of aerosol-borne
particulates.
Thus, there is a need for a commercially available sample cell that is
inexpensive, easy to use, insensitive to or non-reactive with liquids such
as water or organic solvents, and has a useful spectral range for most
routine analysis.
SUMMARY OF THE INVENTION
The present invention provides a novel sample holder for use in manual and
automated transmission infrared ("IR") spectroscopic analysis and a novel
method for using such sample holders. Sample holders of the invention are
simple to use, permit simplified sample preparation, and provide precise
and accurate spectra of samples. The sample holders provided herein can be
sufficiently inexpensive to permit being discarded or stored after a
single use. They also eliminate the need for sample clean up and
post-analysis reconditioning of the sample holder. The elimination of such
clean up and reconditioning provides improved safety, particularly in
cases of hazardous samples and cleaning agents, as well as greater
convenience and time economy. In some embodiments, sample holders of the
invention may be used with aqueous samples. The sample holders provided
herein can provide exceptional spectral accuracy, with embodiments that
are essentially inert with the sample and that exhibit minimal substrate
absorbances or artifacts so as to not interfere with the spectra obtained.
In brief summary, a sample holder of the invention comprises a microporous
sheet having two major surfaces or faces and a support member which
facilitates mounting the holder in a spectrometer. In some embodiments,
the holder further comprises an aperture shield.
BRIEF DESCRIPTION OF DRAWING
The invention will be further explained with reference to the drawing,
wherein:
FIG. 1 is a plan view of one face of an illustrative embodiment of a sample
holder of the invention;
FIG. 2 is a plan view of one face of another illustrative embodiment of a
sample holder of the invention;
FIG. 3 is a plan view of one face of another illustrative embodiment of a
sample holder of the invention comprising an aperture shield; and
FIG. 4 is a cross-sectional view of the sample holder shown in FIG. 3 along
axis 4--4.
These figures, which are idealized, are not to scale and are intended to be
merely illustrative and non-limiting.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
FIG. 1 illustrates sample holder 10 comprising microporous sheet 12 and
support member 14. Sheet 12 is preferably inert, i.e., non-reactive, with
the samples to be applied thereto, including any solvents they may
contain.
Sheet 12 is preferably very thin, typically being less than about 150
microns, preferably between about 2.5 and about 25 microns, thick. Thicker
films tend to lead to greater interference due to the stronger spectral
absorbances of the films. Polymeric sheets used in the invention may
typically have a basis weight between about 0.03 and 1.0 grams/square
meter. Sheets with lower basis weights may be used in some instances, but
may tend to be too weak to support sample material. Sheets with higher
basis weights may be used in some instances, but may tend to interfere
undesirably with spectroscopic analysis.
The observed transmittance of the sheet is a function of sheet thickness,
porosity, infrared light scattering characteristics, and composition. It
may also depend in part upon the particular wavelength or wavenumber
region of interest. The standard deviation (n=20) of the sheet
transmittance variability, i.e., the variation in transmittance of the
sheet at different locations, is preferably less than about 25 percent
relative, more preferably less than about 10 percent relative. In order to
ensure highly probative evaluation of sheet transmittance variability, it
is typically measured at a wavenumber at which the sheet has an absorbance
of about 0.7 to about 1.0 absorbance units, e.g., at the 1460 cm.sup.-1
absorbance for polyethylene sheets. When using a dual beam (dispersive)
instrument a small standard deviation in sheet variability facilitates
more accurate subtraction of the absorbances of the sample holder from
those of the sample on the holder. Similarly with FTIR instruments a small
standard deviation in variability permits subtraction of one standard
reference spectrum from those of later analyses.
The sheet may be of any size (area) sufficient to accommodate a sample
applied thereto and permit mounting in the desired instrument with a
suitable support member. For reasons of instrument size limitations, the
size (area) of the sheet to which a sample may be applied is typically
preferably small, ranging from less than 1.0 centimeter.sup.2 to about 6
centimeter.sup.2 per each face in many instances. It will be understood
that larger or smaller sheets may be used in accordance with the
invention. The increase in sensitivity of modern instruments enables the
taking of spectra of very small samples, therefore small sizes of
microporous sheets may be used.
The void volume of sheet 12 is typically greater than about 20 percent and
preferably greater than about 50 percent. Many useful microporous polymer
films are open structures wherein only a fraction of the total volume is
occupied by the polymer material. With sample holders made with such
films, a greater portion of the matter in the beam path is the sample
itself. Conventional non-microporous films are typically less useful and
in many instances inoperable for use herein. In many instances, samples
applied to conventional films fail to effectively wet the surface of the
film. As a result, the sample beads up on the film and tends to run off
the film when mounted in the spectrophotometer. Surprisingly, we have
found that when applied to microporous sheets made from the same polymers,
the same samples will tend to wet out the sheet, enabling the sample to be
spectroscopically analyzed. If the sample is analyzed quickly after
application to the sample holder, such as by FTIR, the solvent portion of
the sample may be spectroscopically analyzed. Alternatively, the sample
may be retained for a time to permit the solvent portion to evaporate,
leaving the non-volatile portion deposited on the sample holder for
subsequent analysis.
It has also been found that, by using microporous sheets as provided
herein, acceptable spectra may be taken of samples that readily
crystallize when put on a flat surface for a time. Prior to this invention
it had been considered difficult to obtain spectra of crystalline samples
due to the dispersive and reflective effects of the crystal lattice. It is
believed that use of a microporous sheet in accordance with the invention
either retards crystallization, or limits crystal growth due to
constrainment of the pore size, reducing the previously encountered
dispersive and reflective effects of the crystal lattice so as to permit
effective spectroscopic analysis.
Although it is believed that any microporous polymeric film may be used as
a sheet in the sample holder to provide some of the advantages of the
invention, the sheet should be selected to reduce spectral interference of
the inherent absorbances of the polymer with the bands being analyzed in
the sample. Although each film has characteristic absorbances, the
absorbances may be in regions of the infrared spectrum that do not
interfere with the absorbances of the sample. In other words, the
microporous sheet preferably exhibits relatively low absorbance, i.e., is
highly transmissive, in the spectral region(s) of interest. For instance,
as discussed below, except for the region of about 3000 to about 2800
cm.sup.-1 where its aliphatic carbon-hydrogen stretching is evident as
strong absorbances, sheets of polyethylene may be used in sample holders
of the invention to perform infrared spectroscopic analysis across the
range of about 4000 to about 20 cm.sup.-1. Polyethylene exhibits a limited
number of other signals in other portions of the range, but these are
typically narrow, well-defined absorbances that are easily taken into
account. TEFLON.TM. films and KEL-F.TM. films (chlorotrifluoroethylene
polymers and copolymers) are typically useful in the range of about 4000
to about 1500 cm.sup.-1.
The importance of this criterium may be ameliorated by use of modern
spectroscopic instruments that have the capacity to "subtract" background
absorbances due to solvents, the cell, the atmosphere, etc. In a
dispersive type instrument, the infrared beam is split into two parallel
beams, one through the sample, and the second, or reference beam, through
a "blank" cell. When taking a spectrum of a sample dissolved in solvent, a
cell containing only pure solvent is placed in the reference beam so that
the instrument can subtract the spectrum of the solvent from that of the
dissolved sample. More recent advances in electronics have allowed the
spectrum of the background of a blank or reference cell to be scanned and
electronically stored so that it may be subtracted from sample spectra
collected later.
The process of subtraction of background absorbances which may be imperfect
with conventional sample holders may also be imperfect with sample holders
of the invention because absorbances may not be cleanly subtracted and may
interfere with the absorbances of the sample, particularly when the sample
exhibits subtle absorbances which can be inadvertently masked or lost by
the subtraction process. Accordingly, the microporous sheet used in the
present invention is preferably selected to minimize, and more preferably
eliminate, interference of the absorbances of the microporous sheet with
the sample, if possible. As the IR spectra of many polymer films are well
known, it is simple to choose an appropriate sheet for use in accordance
with the present invention.
Selection of a sheet for making a sample holder for a particular
application will be dependent | | |
