The four absorbance bands analyzed were at 799, 780,
524, and 467 cm? The two bands closer to the far-IR,
524 and 467 cm, were as useful in quantifying quartz as
those at the 799- and 780-cm‘ positions. The far-IR bands
also overlap less than the 799-780 doublet. The 799- and
780-cm" bands, however, were used by most earlier ana-
lysts to quantify quartz for several reasons. These two
bands fit into a low-absorbance section of PVC-acrylic
filter spectra. Previous analysts experienced good linearity
in calibrations with these bands, and they generally desired
to examine the minimum number of bands possible to pro-
portionally minimize their own work. Also, technological
limitations of instruments made far-IR examinations more
challenging in the past. The bands toward lower wave-
numbers, however, have advantages prominent enough to
merit augmenting measurements of the traditional bands
with the far-IR bands. Because of recent advances in
spectroscopic hardware and software, there has been a
trend toward examining more than a minimum one or two
absorption bands in spectra. Advances in data processing
also make such practices more feasible.

Ultimately, automated analysis of spectral data will
probably play a role in reaching the goal of the research—
the fastest and most accurate analytical method that can
be achieved. Interactive manipulations by the instrument
operator give opportunities for error at each step of
analysis,

First, the filter must be mounted in the sample com-
partment. For this step, there is no possible error for the
custom accessory because the filter fits tightly in its
receptacle and aligns precisely with the IR beam. For
mounting devices at the beam focus, a visual alignment
must be done. This is particularly difficult if the quartz

deposit is light. Even if the quartz deposit is consistently
placed in the analytical beam, minor changes in the filter
orientation can affect both light-scattering and baseline
properties in the final spectrum.

Second, when interactive subtraction of a blank refer-
ence filter is done, the operator cannot totally rely on the
relative masses of the sample and reference filters for the
subtraction factor. For analyses at the beam focus, only a
portion of the filter is examined. A calculated subtraction
factor is a good starting place, but then visual judgment of
the operator is needed to evaluate the spectral display.
Calculated subtraction factors are considerably more re-
liable when using the custom accessory because whole fil-
ters are being examined. In any case, subtraction anoma-
lies can result that make subsequent baseline correction
more difficult.

Third, a baseline must be electronically drawn on the
FTIR monitor screen under each absorption band of in-
terest. This involves the operator making a best guess for
appropriate end points of the baseline (absorption min-
ima) on either side of the absorption band. If different
baselines are drawn, different peak heights will result.

These three errors occur with dispersive instruments as
well as FTIR. A means must be found to circumvent op-
erator error, if quartz analysis is to be improved with any
instrument. For example, if a 10% error is introduced at
each analytical step, 30% total error may result in worst
cases. Experimentally, much data have been gathered in
this research with 15% variability or less, while the re-
mainder of spectra collected show greater variability.
Proof of concept for on-filter analysis by FTIR has been
demonstrated, but the technique needs much development
work.

CONCLUSIONS

Because of the higher energy throughput characteristic
of FTIR instruments, they are more sensitive and versatile
than dispersive IR hardware. If the instrument beam ge-
ometry is appropriate, this advantage is exploitable ir the
design and use of a beam-matching accessory for analysis
of dust collection filters. In comparing on-filter analyses
for quartz deposited on polymer membrane filters by
various IR techniques, FTIR analyses performed at the
beam focus are the most sensitive, with a detection limit
of 2 ug, This is of particular interest to analysts who
prefer sample concentration involving filter ashing and
redeposition of residue onto a smaller deposit area, FTIR
analyses performed with a customized beam-matching

accessory compare favorably with analyses done with dis-
persive instruments, according to information available in
the literature. While the custom accessory cannot fully
achieve the sensitivity of FTIR beam focus analyses, the
detection limit is an acceptable 20 ug quartz, similar to
current dispersive methods, with future research likely to
improve this limit, The custom accessory is more vulner-
able to light-scattering and random noise phenomena than
FTIR beam focus analyses, but less susceptible to inter-
ference fringes or optical alignment problems. Many of
the problems encountered are unimportant in the wave-
number range of interest for quartz analysis, particularly
‘below 1,000 cm. That is, spectral quality is much better


efficiency with Min-U-Sil 5 quartz dust. Quartz-bearing
IPA solutions were passed through the filters using vacu-
um techniques identical to those used for sample prepara-
tion. PVC backup filters with 0.2-ym pore size were used.
FTIR analysis of the backup filters showed no evidence of
quartz breakthrough or bypass; that is, no quartz was de-
tected on the backup filters. The examination of backup
filters shows that wet deposition may be done on a variety
of filters without major risk of poor collection efficien-
cy. Care must be taken, however, to ensure a tight seal
around the filter area collecting the dust—a rule of thumb
for all filtration work.

PREPARATION AND ANALYSIS
OF CALIBRATION STANDARDS

‘Two series of calibration standards were prepared—one
for analysis at the beam focus of the FTIR and the other
for analysis with the custom filter holder. Both series
were prepared by wet deposition using quartz suspended
in IPA. In each series, the diameters of the deposits,
75 or 33 mm, were chosen to be appropriate for the beam
diameter at the filter mounting site. Different deposition
apparatuses were used for achieving the different deposit
diameters, but the same wet deposition technique was
used. Dust was always deposited on the rougher, duller
side of filter membranes to lock dust deeper into the filter,
making it more secure.

To prepare calibration standards for the custom filter
holder, 37-mm diameter, 0.8 ym-pore PVC-acrylic filters
were placed in the base of ordinary polystyrene 37-mm
cassettes, backed by a cellulose support pad. A 2-in-long
cylindrical cassette extension was tightly pressed into the
cassette base, holding the filter and support pad in place
and forming a reservoir above the filter surface. The cir-
cumference of the cassette base and extension was wrap-
ped with fluorocarbon polymer tape (commonly used with
threaded fittings) to ensure a no-leak fit. The filter
was then wetted with a small amount of HPLC-grade IPA
(pure grade of IPA appropriate for high-pressure liquid
chromatography), and brief suction to the cassette drew
the pure IPA through the filter. Next, the quartz-IPA
solution was added to the cassette, and again suction was
applied to drain the solution through the cassette. Last, a
wash bottle with IPA was used to rinse the walls of
residual quartz and a final suction ensured that all quartz
was finally on the filter. The cassette was carefully pried
apart, and the filter was placed in a polystyrene petri slide
to dry, warmed by a light box of the type commonly used
for viewing films or photographic slides. This procedure
ensures a 33-mm-diameter deposit on a 37-mm-diameter

filter. Extensive FTIR examination of filter samples
treated in this manner showed no evidence of any material
leaching out of the cassette or petri-slide components and
contaminating the filters; extra IR absorbance bands in-
dicative of contamination never occurred. Since the appa-
ratus components are cheap and expendable, the method
is well suited for sample preparation. Any polystyrene
component that cracked in use was discarded and replaced
with a fresh component.

Once the filters were prepared, they were placed in the
37-mm custom filter holder to be analyzed by the FTIR
spectrometer. Computerized routines subtracted the spec-
tral contributions of IPA-leached filter blanks from those
of quartz-deposited filters to arrive at the spectra of the
quartz. The mass ratio of the filter blank to the deposition
filter provided the subtraction factor for such routines.
The resulting residual spectra of quartz were analyzed
using interactive computer baseline correction and by
measuring the peak heights of quartz absorbance bands.
Operator judgement was actively used during the measure-
ment of each quartz band height. Appropriate minima on
either side of an absorbance band were chosen as points
through which to place a baseline. The wavenumber po-
sition of the minima could vary slightly from sample to
sample. Once the minima were chosen, an automatic
straight-line baseline correction function was executed for
the particular band being examined. The resultant screen
graphics would present a spectrum with one absorbance
band over a horizontal baseline, and the rest of the
spectrum unmodified for baseline drifts. The maximum of
the absorbance band over the synthetic baseline was meas-
ured using a screen cursor accurate to 0.0001 absorbance
unit. The maximum wavenumber position of an absorb-
ance band could also slightly vary from sample to sample.
The effects of finite spectral resolution and residual ran-
dom noise cause minor shifts in band position. Each band
of each spectrum was examined in turn by this procedure.

For samples analyzed at the FTIR beam focus, 47-mm-
diameter, 0.45-ym pore uncut PVC-acrylic filters were
placed on common 25-mm fritted glass filtration supports,
backed by glass fiber pads. A glass chimney with a
7.5-mm bore was placed on top of the filter and pad and
clamped into place with a common aluminum filtration
clamp. The method of depositing the quartz is the same
as that described for the cassette apparatus, but a 7.5-mm-
diameter deposit is the result.

Once the filters were dry and ready for analysis, they
were mounted on a magnetic sample holder, typical of
many common film holders for FTIR spectrometers. The
deposition area was visually aligned in the center of the
film holder aperture. The routines for determining quartz


cm

cmt

mg

UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

centimeter beg microgram
centimeter reciprocal (wavenumber) min minute
inch mm millimeter

milligram pm micrometer



at low wavenumbers than at high wavenumbers for filter
samples. Because of this, future work with far-IR optics
is a promising option. It is undetermined whether the cus-
tom accessory is the definitive means to achieve on-filter
analyses of field samples. However, the accessory per-
formance is satisfactory, conforming to expectations based
on basic principles of spectroscopy, and it merits continued
work. A significant research variation that may give
optimum results and minimize sample matrix problems

involves use of sampling cassettes that deposit dust uni-
formly. Filter samples from such cassettes might be used
in conjunction with or apart from the custom accessory, al-
though wholesale adoption of new cassettes would likely be
a challenging undertaking. With continued research, on-
filter analysis of field samples seems achievable through
use of readily obtainable equipment and accessories. Many
avenues remain open for continued progress toward this
goal.

REFERENCES

1. Bureau of National Affairs, Inc. Need for Surveillance, Training
Seen Emphasized in NIOSH Strategies. Occup. Saf. and Health Re-
ener, May 9, 1985, p. 971.

- Decade of Coal Worker Autopsy Data Shows Black Lung
Deatt ae Rate "Relatively Low." Mine Saf. and Health Reporter, Oct. 17,
1984, p. 184,

3. Tomb, T. F., R. G. Peluso, and P. S. Parobeck. Quartz in Unit-
ed States Coal Mines. Ann. Am. Conf. Gov. Ind. Hyg., v. 14, 1986,
pp. 513-519.

4. Bureau of National Affairs, Inc. Bureau Predicts Quartz Dust
Will Affect Two-Thirds of Coal Work Force Next Year. Mine Saf. and
Health Reporter, Oct. 17, 1984, p. 185.

5. U.S. Mine Safety and Health Administration, Instrumentation and
Analytical Branch. Infrared Determination of Quartz in Respirable Coal
‘Mine Dust, Chemical Method No. P-7, Aug. 3, 1989, 19 pp.

6. U.S. Mine Safety and Health Administration. MSHA’s Procedure
for Determining Quartz Content of Respirable Coal Mine Dust. MSHA.
IR 1152, 1984, 13 pp.

7. Courtney, W. G. Dust Control. Ch. in Underground Mining
Methods Handbook, ed. by W. A. Hustrulid. Soc. Min. Eng. Am. Inst.
Min., Metall., and Petroleum Eng,, Inc., 1982, pp. 1687-1710.

8. Ainsworth, S. M., P. S. Parobeck, and T. F. Tomb. Determin-
ing the Quartz Content of Respirable Coal Mine Dust by FITR. MSHA.
IR 1169, 1989, 11 pp.

9. Smith, A. L. Applied Infrared. /. Wiley, 1979, 322 pp.

10. Griffiths, P. R, and J. A. deHaseth. Fourier Transform Infrared
Spectrometry. Wiley, 1986, 656

11. Ferraro, J.R., and L. J. Basile, (eds.). Fourier Transform Infrared
Spectroscopy. Acad. Press, v. 1: Applications to Chemical Systems,
1978, 311 pp.; v. 2: Applications to Chemical Systems, 1979, 321 pp.
v. 3: Techniques Using Fourier Transform Interferometry, 1982, 215 pp.
v.4: Applications to Chemical Systems, 1985, 406 pp.

12, Toma, S. Z., and S. A. Goldberg. Direct Filter Infrared Analysis
of Alpha Quartz Deposited on Filters. Anal. Chem. v. 44, No. 2, Feb.
1972, pp. 431-432.

13. Freedman, R. W., S. Z. Toma, and H. W. Lang. On-Filter Analy-
sis of Quartz in Respirable Coal Dust by Infrared Absorption and X-
Ray Diffraction. Am. Ind. Hyg. Assoc. J., v. 35, July 1974, pp. 411-418.

14, Anderson, C. C. (SRI International). Collaborative Tests of Two
Methods for Determining Free Si Airborne Dust (NIOSH contract

210-79-0059). Final Rep., Feb. 1983. U.S. Dep. Health and Human
Serv. (Natl. Inst. for Occup. Saf. and Health), No. 83-124, Oct. 1983,
157 pp.

15, Hunsacker, H. A. R. A. Buchan, T. J. Keefe, and W. E. Marlatt,
Characterization of Asbestos Fiber Distribution on Membrane Filters
From 25- and 37-mm Sampling Cassettes. Appl. Ind. Hyg., v. 3, No. 10,
(Oct. 1988, pp. 284-290,

16. Winn, C., and H. A. Mercer. The Effect of Filter Holder Design
‘on Particle Deposition Patterns. Presented at AIHA Conference, Port-
land, OR, May 25-29, 1981, 7 pp.; available from Glasrock Medical
Services, Fairburn, GA.

17. Harris, G. W., and G. S. Revell. Environmental Monitoring: 5
(Notes on the Analysis of Free Silica). Health and Safety at Work
(Croydon, United Kingdom), v. 3, Jan. 1981, pp. 52-56.

18. Foster, R.D., and R. F. Walker. Quantitative Determination of
Crystalline Silica in Respirable-Size Dust Samples by Infrared Spec-
trophotometry. Analyst (London), v. 109, No. 9, Sept. 1984, pp. 1117-
1127.

19. Cahn, L. Dynamic Weight Changes of a Membrane Filter With
Humidity. Mater. Res. and Stand,, v. 3, No. 5, May 1963, p. 377.

20. Drolet, D., and G. Perrault, Applications of a Mathematical Mod-
el of Weight Correction for Several Membranes Used for Dust Sam-
pling. Presented at AIHA Conference, Montreal, Quebec, Canada,
May 31-June 5, 1987, 23 pp; available from Institut de Recherche en
Santé et en Sécuritite du Travail du Québec, Montreal, Quebec, Canada.

21. Beaulieu, P., G. Perrault, and C. Roy. Mesure de poussieres dans
Vair ambiant: Correction de la masse des membranes filtrantes ester
de cellulose en function de "humidité atmosphérique. (Measurement of
Dust in the Ambient Atmosphere: Weight Correction as a Function of
Humidity of Cellulose Ester Filters). Analusis, v. 14, No. 2, 1986, pp. 74-
78 (Engl. sum.)

22. Meltzer, T. H. Choosing Membrane Filters—In-Use Assessment
Is Best. Microcontamination, v. 6, No. 8, Aug. 1988, pp. 30-33.

23. Brock, T. D. Membrane Filtration: A User's Guide and Refer-
‘ence Manual. Sci. Tech Inc., Madison, WI, 1983, pp. 15-25.

24. Kaufman, R.A. Evaluation of FT-IR Spectrometry as a Method
for the Determination of Crystalline Silica in Respirable Dust Samples.
MS. Thesis, Grad. Sch. of Public Health, Univ. Pittsburgh, Pittsburgh,
PA, 1987, 73 pp.



2,200 cm’, The importance of this type of interference for
analyses at the beam focus is about the same as random
noise. Interference fringes pose a problem for accurately
measuring very small quantities of quartz; strategies for
alleviating the problem were given above. Current good
results with interference fringes, however, are not a guar-
antee for future specimens that may be examined. Discus-
sion of this type of interference is important in that earlier
researchers investigating on-filter analysis have failed to
address the topic.

Detection Limits and Sensitivity

As described above, a series of calibration curves were
prepared using standards with Min-U-Sil 5 quartz dust de-
posited on polymer filters. Figures 11 and 12 show the
results for 7.5-mm-diameter deposits analyzed at the beam
focus. Figures 13 and 14 shiow the results for full-face
deposits analyzed at the 37-mm insert position of the cus-
tom filter holder. Data points in figures 11 through 14 are
all the averages of two spectroscopic measurements. The
appendix presents all relevant data in tabular form.

The detection limit at the beam focus is approximately
2 wg quartz. The detection limit with the custom acces-
sory is approximately 20 wg quartz. These detection limits

2 T T T T T
KEY
—o— 799 cm!
Lok 7-780 cm! 4

ABSORBANCE

Ll 1
400 500 600

L L 1
° 100 200 300
MASS QUARTZ, yg

Figure 11.—Calibration curves for quartz 799- and 780-cm?
bands analyzed at beam focus.

are defined as those quartz concentrations with at least
two absorbance bands having absorbance heights approxi-
mately three times the background noise. In examining
figures 11 through 14, it can consistently be said that read-
ings taken with the custom filter holder are approximately
one order of magnitude less sensitive than beam focus
readings for all absorbance bands tested. If random noise
and interference fringes could be reduced, the detection
limits would be improved. The detection limit of the cus-
tom accessory, while significantly higher than the beam
focus measurements, is still in the range of usefulness and
very comparable to the detection limit of dispersive meth-
odologies. Further research will improve the custom ac-
cessory performance. Additionally, it can be said that the
FTIR beam focus sensitivity is much better than that seen
with dispersive instruments, as recorded in the literature.
The recent adoption of FTIR for IR quartz analysis will
greatly improve the detection limit, as long as problems
common with FTIR, such as interference fringes, can be
resolved.

Based on fundamental principles of spectroscopy, the
above observations were expected. That is, results with
the custom accessory would not be as sensitive as meas-
urements done at the beam focus. Analytical sensitivity is
inversely proportional to the cross-sectional area of the

12 T T T T T
KEY
—o— 467 cm”
lok 7 524cm"! 4d

ABSORBANCE,

1 1 1 L
° 100-200-300. 400 500 600
MASS QUARTZ, yg

Figure 12.—Calibration curves for quartz 524- and 467-cmTM
bands analyzed at beam focus.


As seen in figure 1, respirable dust samples from mines
cover the full face of the 37-mm-diameter filter, except for
a narrow 2-mm band around the edge where sampling cas-
settes form a seal. Full-face IR examination of the filter
is required to average over the nonuniform deposit. Areas
of mild coloration on a filter indicate arcas of light dust
deposit (low mass per unit area) or lightly colored dusts.
Quartz and other minerals may be pale colored and may
be present on pale portions of any filter.

Figure 2 shows a fairly typical FTIR beam geometry.
The beam converges to the focus as it enters the sample
compartment and diverges from the focus as it exits to-
ward the IR detector. By moving the sample filter toward
or away from the focus, the beam diameter can be match-
ed to the diameter of the deposit on the filter. Generally,
matching the diameter of the IR beam to that of the dust
deposition on the filter is preferred, achieving a ratio of
approximately 1.0, Placing the filter in the appropriate
position on the detector side of the beam focus will allow
for maximized recovery of light scattered by the sample.
The particular dimensions of an FTIR’s sample compart-
ment, as well as its beam geometry and energy throughput,
will vary from manufacturer to manufacturer. Although
these may be limiting factors, the beam-matching tech-
nique should be achievable with many FTIR instruments.

The beam-matching accessory was designed to be as
versatile as possible. Figure 2 shows intended positions for
mounting standard 37-mm, 25-mm, and 13-mm-diameter
filters. All research to date has been with the 37-mm
position because it is the size filter used to collect dust
samples from mines. The custom filter holder cannot ac-
commodate 47-mm-diameter filters. The position corre-
sponding to a 47-mm beam diameter would be too close

Sample compartment interior

Fixed position
accessory baseplate

i 25mm 13 mm
wee aa «
sete =

a

PLAN VIEW

Figure 2.—Conceptual detail of custom accessory fully as-
sembled and mounted in instrument sample compartment.

to the exit of the sample compartment to engincer this
option.

Figure 2 also shows the fixed position baseplate of the
custom filter holder. The bottom of the baseplate has
metal pins to mount the filter holder in a precise optical
alignment with the IR beam in the sample compartment.
Every FTIR spectrometer’s sample compartment is actual-
ly a portion of the instrument’s optical bench. The base-
plate and its pins ensure precise and reproducible align-
ment in the optical bench scheme. A pair of vertical
mounting brackets for each filter size ensures proper align-
ment of the filters. The mounting brackets do not inter-
cept the diverging IR beam. The beam is free to diverge
throughout the whole accessory. Only when a particular
filter and its mounting apparatus are fully assembled and
inserted in place will interception of the IR beam occur.

Figure 3 shows an exploded view of a typical filter
mounting assembly with full perspective and features.
Figures 4 through 6 further illustrate the FTIR filter
holder. Figure 4 shows the filter holder accessory from a
rear view with full detail, including the IR beam of the
FTIR spectrometer. The circular crosssections of the
beam match the mounting positions and diameters of dust
deposits on filters. For 37-mm filters, the dust deposit is
33 mm in diameter. For 25-mm filters, the dust deposit is
21 mm in diameter. For 13-mm filters, the dust deposit is
9 mm in diameter. At each filter mounting position, the
IR beam is a few millimeters larger than needed to cover
the dust deposit, When a filter is mounted in the custom
holder, the outer fringe of the beam is intercepted so that
there is an adequate match between the penetrating beam

Retainer

yoo
Spring

Mounting plate

Figure 3.—Exploded view of typical filter mounting assembly.


INTRODUCTION

Silicosis is one of the best known of the occupationally
related pneumoconioses. Despite the many years of work
that have gone into its investigation and control, the
disease remains a genuine concern for America’s mining
work force. Public health organizations specifically name
silicosis as one of the major occupational diseases that
require more attention (1)? The National Coal Workers
Autopsy Study records silicosis lesions among 12% of US.
miners (2-3). Coal miners are still receiving significant
doses of silica dust in their work environment. The pres-
ent 12% prevalence level can be reduced, but improved
laboratory techniques must parallel all the administrative,
research, and enforcement efforts to do so, Improving the
method by which quartz in respirable dust is measured is
inherent for the health and enforcement community to
pursue improved epidemiology, detail dose-response rela-
tionships, clarify low-exposure situations, and verify control
technology. Increased speed of analysis and a low detec-
tion limit would be very helpful for the progress of such
work.

Recent U.S. Mine Safety and Health Administration
(MSHA) data indicate that airborne quartz levels in mines
are higher than thought in previous years (3). Increasing
numbers of mines now face more stringent dust standards
as quartz levels exceeding 5% are detected (4). Improve-
ments achieved with the current analytical methodology for
quartz have helped the mining community to accurately
recognize the presence and degree of health risk. They
have also helped to appropriately assign more stringent
dust standards to specific mines that need them.

The foundational method for determining quartz con-
tent in respirable coal mine dust is based on dispersive IR
spectroscopy (5-6). The method has a working range of
measuring 25 to 250 yg of quartz with a precision of 13%
to 22%. This multistep analysis has been adequate for
many years, comparing favorably with X-ray methods.
However, dispersive IR methodologies had limitations (5-
7), such as exacting sample preparation requirements and
few options to cope with interfering substances. These
factors could be ameliorated by use of more current in-
strumentation, that is, FTIR. This newer class of instru-
ment has improved the precision of the longstanding meth-
odology (8). Although there has been an upgrade in
instrumentation, the existing technique for analyzing dust
samples for quartz continues to have much potential for
improvement. It remains quite labor intensive and time

Italic numbers in parentheses refer to items in the list of references
preceding the appendix at the end of this report.

consuming, still involving the use of low-temperature ash-
ing, sample filtration, and redeposition before actual IR
analysis. A substantial portion of dust samples collected
at coal mines must be discarded rather than analyzed be-
cause current techniques are too inconvenient to evaluate
low-mass samples. Research into on-filter analysis of
samples will attempt to resolve these ongoing issues.

In the classical dispersive instrument, radiation from the
source passes through the sample and is dispersed by some
optical element, usually a diffraction grating. The beam
then proceeds through a slit mechanism that ensures that
the radiation reaching the detector is a narrow band of
frequencies. Slit controls are used to determine wave-
length resolution. A more recent development is the
FTIR spectrometer. The core of an FTIR is a Michelson
interferometer, which consists of a beam splitter, a fixed
mirror, and an oscillating mirror. Collimated radiation
from the source impinges on the beam splitter, which
equally divides the beam, directing half to the fixed mirror
and the other half to the oscillating mirror. After re-
flecting off the mirrors, the beams recombine at the beam
splitter where spectral interferences occur. The resultant
beam passes through the sample and onto the detector.
Wavelength resolution is determined by the maximum
range of motion of the oscillating mirror, accurately meas-
ured by laser techniques.

FTIR spectrometers are a substantial advancement over
dispersive IR instruments, especially when accompanied by
specialized data processing and control electronics. Data
manipulation routines provide for great versatility. Fur-
thermore, an FTIR does not have a monochromating de-
vice that limits energy intensity of the IR beam. The high-
energy throughput of FTIR, that is, the undiminished level
of IR radiation that actually passes through samples, often
achieves much lower detection limits. Wavelength resolu-
tion is also greatly improved. High scanning rates permit
rapid sample processing, and coadding scans reduce ran-
dom noise in collected spectra. These features are sig-
nificant technical advances in the collection and analysis of
IR spectra. As a result, FTIR spectrometers are rapidly
replacing aging dispersive hardware. A sound base of
information has been accumulated on principles and
performance of FTIR instruments (9-11).

Determining quartz without multistep preparation of
the original collection filter has been a goal for analytical
chemists for a long time (12-13). The terms “direct analy-
sis" and "on-filter analysis’ as used in this report, refer to
the ideal situation of being able to analyze material de-
posited on a filter with minimal or no sample preparation.


RESEARCH TOWARD DIRECT ANALYSIS OF QUARTZ DUST
ON FILTERS USING FTIR SPECTROSCOPY

By Donald P. Tuchman’

ABSTRACT

The U.S. Bureau of Mines is investigating Fourier transform infrared (FTIR) spectroscopy for on-
filter quartz analysis of respirable dust. A custom accessory is described for full-face examination of
filters utilizing a large-diameter infrared (IR) beam. The accessory positions samples to match
diameters with that of the diverging analytical beam. Sample absorbance is then measured. With
nonuniform deposition of dust on collection filters being a major issue for such analyses, this approach
is the most direct way to accomplish sample area averaging. The approach is unconventional since it
utilizes large-beam geometries instead of the usually desired minimized beam dimensions. The issues
and problems involved in the analysis of quartz on a filter matrix are discussed. Absorption bands
chosen, light-scattering effects, curved baselines, random noise, interference fringes, and possible
solutions to technical difficulties are the topics covered. The more significant findings include a 20-yg
detection limit for quartz when the custom accessory is used and minimal occurrence of light-scattering
effects at low wavenumbers. The custom accessory performance was satisfactory and merits further

work. With continued research, an on-filter method for quartz analysis of respirable dusts seems
achievable.

*Research industrial hygienist, Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA.


O12 T T T T T
KEY
—o— 799 em"!
lol 7-#- 780 cm"! J
8
oe
8
2
a
2 06+
S
B -
8 -
< uc
04 o* 4
ia
’
’
’
,
2b ’ 4
7
?
aye
Z
1 1 1 L L
° Too 200-300. ~<400+~=«500 «600

MASS QUARTZ, 4g

Figure 13.—Calibration curves for quartz 799- and 780-cmTM
bands analyzed with custom filter holder at 37-mm position.

beam at the location of analysis. To be specific, since
the beam focus measurements involve a beam diameter of
12 mm and the full-face measurements involve a beam di-
ameter of 33 mm, the ratio of areas predicts a ratio of 7.6
for the relative sensitivities. This is an approximation
that would apply to both the quartz levels at the detection
limits and quartz concentrations much higher than the
detection limits, The ratio between the two would never

2 T r ;
KEY
—o— 467 cm!
tok --- 524 cm"! 4
os 4
w
gS
2
a
@ 6+ 4
5
8
@
oak vacvl
2b - 4
\ 1 n Hl L
° 100-200. 300. 400 500 600

MASS QUARTZ, yg

Figure 14.—Calibration curves for quartz 524- and 467-cm*
bands analyzed with custom filter holder at 37-mm position.

be an exact 7.6 because of light-scattering complications
and a variety of practical considerations not accounted for
in the ideal inverse area rule. Generally, however, it
would be expected that measurements done in the full-face
mode should be about an order of magnitude less sensitive
than beam focus measurements, The experimental results
conform to this expectation,

DISCUSSION

There remain quite a few options for improving sen-
sitivity with the custom filter holder for on-filter analysis.
First, and most simply, a more open aperture from the
source might be tried, allowing more IR energy through
the samples. Next, work with far-IR optics might be done
for scanning 250 to 1,500 cm". This is the spectral region
of most interest, and using optics designed for that region
will reduce noise. Beyond this, there is also the option of

trying a different type of interferometer with continuous,
self-correcting alignment that would also reduce noise.
Finally, data processing features could enhance the spectra.
Generally, no aspect of the analyses performed with the
custom accessory was unexpected or disappointing.
However, further research is required to determine the full
potential of the accessory and the role it may play in
finally achieving on-filter analysis of quartz.


ggagegorerss:

because of nonuniform airflow through different parts of
the filter. Variation in filter texture, thickness, and pore

ly
tt
HUE

Figure 1.—Examples of filter samples collected at coal mines.

size may contribute to these flow differences. Sometimes,
heavier deposition occurs at the center of a filter. Some-
what like in an impactor, relatively fast airflow at the
center of the cassette tends to concentrate dust deposition
at the center of the filter.

Nonuniform deposition is the first and primary technical
hurdle to overcome in achieving on-filter analysis. Using
cassettes that are designed to collect dust evenly on fil-
ter surfaces could eliminate nonuniform deposition. Some
25-mm cassettes are known to collect dusts with good uni-
formity. Furthermore, since the area of collection would
be reduced in comparison to 37-mm cassettes, the more
concentrated sample would increase sensitivity. Addi-
tionally, smaller beam geometries are generally more ad-
vantageous, as discussed in the "Detection Limits and Sen-
sitivity’ section. Practical implications of converting all
sampling devices used in the mining industry, however, led
to the pursuit of on-filter analysis with samples as they
now exist: nonuniform, highly variable, and 37 mm in di-
ameter. No simple correction factor or mathematical
modeling can deal with the nonuniform deposition of dust
on collection filters because cach filter sample is unique.

PURPOSE AND DESCRIPTION OF CUSTOM FILTER HOLDER

An alternative to requiring inherently uniform deposits
is to attempt to average the analysis over the whole filter
area. Several averaging techniques are possible options.
Spinning the sample in the IR beam might accomplish
area averaging, but would introduce new problems. The
filter-spinning device would increase both vibrational and
spectral noise in the spectrometer. Furthermore,
harmonic interaction between the rhythm of the FTIR os-
cillating mirror and sample spinner might occur. An
alternate technique would involve taking several spectra
from different sections of the sample and then averaging
the results. This technique, however, would increase sam-
pling time and further complicate the analytical procedure.
Furthermore, examining a filter in a reproducible manner
would require marking or cutting it. This approach makes
double-checking the analysis of a filter difficult. One
alternative, however, has considerable potential. Matching
the instrument beam diameter to the sample diameter
would achieve area averaging, This approach requires
some means of placing a 37-mm filter sample in the di-
verging IR beam where the beam diameter is also nearly
37 mm and then achieving a match between the beam and
dust deposit diameters. A custom-designed filter holder
was fabricated for that specific purpose.

This approach is optically simple to engineer, but very
unconventional. In spectrometer design, it is almost al-
ways preferable to place a small, compact sample in the
path of a narrow, high-intensity analytical beam. This is
because the analytical sensitivity of the measurements is
inversely proportional to the cross-sectional area of the
analytical beam used (the general beam geometry). Ina
series of samples containing identical masses of an analyte,
the physically smallest sample that can be analyzed with
the narrowest beam will show the highest absorbance
reading. By analyzing large-area samples with a wide-
diameter analytical beam, the problem of nonuniform dep-
osition is solved and the speed and convenience of on-filter
analysis may be gained. An initial compromise in sensitiv-
ity, however, must be accepted during this early phase of
the research. Questions to resolve are whether the ana-
lytical sensitivity is still adequate and whether this ap-
proach is viable enough to merit further research on its
improvement. If the analytical sensitivity experienced is
comparable to dispersive instruments, despite the optical
modifications to the FTIR, the viability of the approach
will be demonstrated. Also of significance is the availabil-
ity of research options that may later improve sensitivity.


BUREAU OF MINES
INFORMATION CIRCULAR/1992

Research Toward Direct Analysis
of Quartz Dust on Filters Using
FTIR Spectroscopy

By Donald P. Tuchman

UNITED STATES DEPARTMENT OF THE INTERIOR


Figure 6.—Placing 37-mm mounting assembly in appropriate mounting bracket.


25-mm_ ossembly,

Typical
filter mounting

g

Spring

37-mm
mounting plate

37mm
mounting
bracket

To
IR detector

~ 13-mm assembly

Pa

~~ From IR source

Fixed position
accessory baseplate

Figure 4.—Detailed construction plan for custom accessory, with rear view showing IR beam.

and the dust deposit diameter. Considerable cooperation
between the instrument manufacturer and the accessory
fabricator is needed to devise a successful geometry for
such an accessory.

The widths of the three mounting plates and the cor-
responding widths of the three mounting brackets are sig-
nificantly different. Each mounting plate can fit in only
one mounting bracket, making it impossible to place a
mounting plate in a wrong position or mismatch the beam
diameter to the filter diameter. Only the 37-mm mounting
plate is in place for research recorded in this report. The

accessory is constructed of aluminum that has been an-
odized black. Figure 5 shows the manual mounting of a
37-mm filter in its mounting plate. Experience bears out
that many filters are up to 0.8 mm larger than nominally
stated size. To prevent filters from warping or puckering
during mounting, machined recesses in mounting plates
must be slightly larger than the filter diameters declared
by the manufacturers. Figure 6 shows manual insertion of
the 37-mm mounting plate into its bracket. Use of the
custom filter holder requires little hand work and no
particular skill.


Achieving true on-filter analysis would provide the analyst
with many benefits. These benefits would include speed,
convenience, and productivity. If analysis time was short-
ened, the analyst would be able to process more samples
per laboratory workday, without increased cost or work
staff. The result might be a better characterization of the
work environment and better avoidance of health risks to

the miners. With the advent of inexpensive FTIR instru-
mentation accompanied by sophisticated data processing,
the goal of on-filter analysis may be within reach.

This research with on-filter analysis reflects the U.S.
Bureau of Mines goals in service to the mining community,
protecting worker health from respiratory disease and ef-
ficiency in evaluating the mine environment.

APPARATUS AND MATERIALS

The FTIR spectrometer used in this investigation was
a Digilab? model FTS-40 (Bio-Rad, Digilab Div.) equipped
with a dry-air purge accessory. The interferometer has an
air-bearing design, allowing spectral resolution as fine as
0.5 cm’, if desired. This instrument was equipped for the
mid-IR range (400 to 4,400 cm’) with a deuterated tri-
glycine sulfate detector. The source was cooled with a
circulating constant-temperature water bath. Although
polyvinyl chloride (PVC) filters were considered, calibra-
tion samples were prepared using PVC-acrylic copolymer
membrane filters and Min-U-Sil 5 (US. Silica Co.) stand-
ard quartz dust. Samples with small-area dust deposits
(7.5-mm diameter) were mounted in a standard magnetic
film holder. Samples with full-face dust deposits (33-mm
diameter) were mounted in a customized accessory, which
is discussed in detail in this report. The sizes of the dust
deposits were appropriate for the specific experiments
conducted. Other analysts with different instruments and
hardware may choose other size deposits.

Min-U-Sil 5 quartz dust (pure quartz particulates,
5 wm or smaller) is well accepted as a standard for IR
analysis of quartz dusts (5, 14). There are, however, some
special considerations that should be kept in mind in the
choice of any quartz standard that have significant indus
trial hygiene implications, It is well known that mineral

samples must be finely ground to produce accurate IR
spectra, The absorption strength of IR bands of mineral
dusts, including quartz dusts, is dependent on the particle
size distribution of the sample. Generally, the finer the
dusts, the stronger the absorbance bands. The relative
strengths of the absorbance bands also change with parti-
cle size distribution. Quartz produces maximum absorb-
ances when its particles are 1 to 3 ym in diameter. Par-
ticles larger than this do not produce optimum absorbance.
Below this size range, an amorphous silica layer commonly
found on crystalline quartz dust may also diminish absorb-
ance. The amorphous surface seems to be a constant
0.03-ym-thick outer layer that is a part of all sizes of par-
ticles. Therefore, for very small particles, the amorphous
layer constitutes a significant portion of the particle mass.
The surface layer mentioned is particularly interesting in
that the most recent research into quartz toxicity im-
plicates surface chemistry of the particles. Min-U-Sil 5
quartz dust is one of the better practical quartz standards
available, generally matching particle size distributions col-
lected with personal sampling apparatus. However, the
particle size distribution of any individual field sample, its
surface or spectroscopic properties, and therefore its toxi-
cological evaluation, may not match Min-U-Sil 5 quartz
dust or any commercially available material.

NATURE OF MINE DUST SAMPLES

The dust samples of interest are collected on polymer
membrane filters. Very few sampling devices, however,
deposit dust uniformly on filters. While some sampling
cassettes achieve fairly uniform dust deposition (15-16),
cassettes commonly used at mine sites do not. The 37-mm
sampling cassettes used for compliance purposes under the
Federal Coal Mine Health and Safety Act are the strong
convention for all minesite sampling. Density of dust
deposited on typical collection filters can vary strongly with
position on the filter. The masses of dust at two small.

3Reference to specific products does not imply endorsement by the
‘U.S. Bureau of Mines.

localized positions on the same filter can vary greatly,
sometimes by a factor of 2 or 3, and in some cases by a
greater factor (16-18), Figure 1 shows some filter samples
from mine sites. They represent challenging, but not un-
common, mine dust samples. Usual personal sampling ap-
paratus (10-mm nylon cyclone assembly, 37-mm preweigh-
ed filter cassette, and battery-operated portable air pump)
were used to collect these samples. Highly nonuniform
dust deposition is evident in the samples. Figure 1 in-
cludes some filters with patterned dark and light spotted
grids. These patterns are likely the result of the perfor-
ated support membrane backing the 5-ym PVC filter.
Blotches, streaks, and irregular deposition might occur


Abstract ....... se eeeee
Introduction . . .
Apparatus and materials.
Nature of mine dust samples
Purpose and description of custom filter holder
Sample preparation and analysis
Filters ...
Deposition techniques. .
Filter efficiency during wet deposition .
Preparation and analysis of calibration standards
Spectrometer resolution .
Experimental observations and results. .....
Nature of FTIR spectra for quartz on filters
Custom versus standard filter holder ..
Problems caused by light scattering
Problems caused by interference fringes .
Detection limits and sensitivity .
Discussion ......
Conclusions
References ..
‘Appendix—Experimental data .

. Beam focus measurements ...

. Differences from calculated averages of custom accessory ‘measurements

CONTENTS

ILLUSTRATIONS

Examples of filter samples collected at coal mines .
Conceptual detail of custom accessory fully assembled and mounted in instrument sampl compartment
Exploded view of typical filter mounting assembly .
Detailed construction plan for custom accessory, with rear view showing IR beam . .
Hand-mounting 37-mm filter in its mounting plate . .
Placing 37-mm mounting assembly in appropriate mounting bracket .
Characteristic FTIR spectrum of quartz on polymer membrane filter
Spectrum of PVC-acrylic filter taken at beam focus
Spectrum of PVC-acrylic filter taken with custom accessory .
Interference fringes complicating spectrum of quartz beeeeeee
Calibration curves for quartz 799- and 780-cmTM bands analyzed at beam focus
Calibration curves for quartz 524- and 467-cm" bands analyzed at beam focus
Calibration curves for quartz 799- and 780-cm" bands analyzed with custom filter holder at
position... 2... eee cece eee ee eee
Calibration curves for quartz 524- and 467-cmTM bands analyzed with custom filter holder at 37-mm
position

TABLES

Custom accessory measurements .
Differences from calculated averages of beam focus measurements .

BERBER SiasauUHe


using the FTIR spectrometer were exactly as described for
the full-face deposit filters. The 7.5-mm deposit was just
small enough to be reliably mounted within the 12-mm
beam focus. A 7.5-mm deposit in a 12-mm beam leaves
only 2.25-mm spare room anywhere around the deposit to
compensate for misalignment in standard film accessories
that depend completely on the operator’s eye for proper
placement. The deposit could not have been made larger
without risking visual misalignment for these particular
experiments. Other analysts may choose other size de-
posits for their own purposes.

SPECTROMETER RESOLUTION

The narrowest quartz absorption bands in a mid-IR
spectrum are 20 cm" wide. Spectrometer resolution was
set at 4 cm’, allowing five data points for these bands.
Finer resolution gave unnecessary detail with undesirable

noise. Coarser resolution reduced the noise, but failed to
give adequate detail of the narrow bands. To determine
the general spectral properties of a filter sample, 16 or
64 scans were adequate. For actual analytical procedures,
256 scans were taken at the 4-cm resolution. This num-
ber of scans improved noise levels in the IR region where
quartz has its absorption bands (below 2,000 cnr), Signal-
to-noise ratio improves proportionally with the square root
of the number of scans. Total analysis time per sample
was 8 to 9 min, including 4 min for scanning, 3 min for
reestablishing dry-air purge, and the remainder for filter
handling time and keypunch manipulations. This ultimate-
ly means that processing of many samples in a laboratory
workday will be achievable. The operating parameters
described may be adjusted for any particular analyst's
needs, or to optimize the procedure for the particular
FTIR used.

EXPERIMENTAL OBSERVATIONS AND RESULTS

NATURE OF FTIR SPECTRA
FOR QUARTZ ON FILTERS

In the mid-IR region, quartz has absorption bands at
1,087, 799, 780, 695, 524, and 467 cm. Two of the quartz
absorption bands, however, have limited usefulness. The
absorption band at 695 cm? is very weak and is often
indistinguishable from background noise. A nearby carbon
dioxide absorption band at approximately 670 cm" is close
enough to cause some interference, particularly if the air
purge is not fully effective and the carbon dioxide band is
strong and broad. Interference fringes are also a signifi-
cant concern when trying to examine weak absorption
bands. This class of interference will be discussed in detail
in the "Problems Caused by Interference Fringes" section,
These problems make useful quantification of the 695-cm
absorption band extremely difficult. The absorption band
at 1,087 cm‘ is strong, but very broad. Its broadness,
complicated by frequent baseline anomalies and noise in-
terference, make baseline correction for this band very
arbitrary, Therefore, measuring the height of this band is
very unreliable. The remaining four bands at 799, 780,
524, and 467 cm" are quite suitable for quantifying quartz.
Although random noise is reduced by an FTIR’s capacity
to scan rapidly and coadd many scans, it is not climinated.
Furthermore, no spectrometer, including FTIR, is immune
to light-scattering effects and the anomalies they may
produce in spectra. The samples being examined, consist-
ing of filters deposited with dust, are prone to scatter light.

Figure 7 shows a characteristic FTIR spectrum of
quartz, It was obtained from analyzing approximately

10 ug of Min-U-Sil 5 quartz dust on a polymer filter ana-
lyzed at the beam focus. The most dominant influences of
the filter have been removed by spectral subtraction of a
blank reference filter. The spectrum of quartz as recorded
in figure 7 summarizes some characteristics that are prod-
ucts of both the sample matrix and errors common with
FTIR. Random noise increases above 2,000 cm. Beyond
3,000 cm", the noise level renders that portion of the
spectrum useless, Fortunately, quartz absorption bands
occur closer to the far IR region and are not obscured
by this noise. Noise at high wavenumbers while using

ABSORBANCE

29001600 1,200 800 400
WAVENUMBER, cm’!

2400

F805

-94)
3/200

Figure 7.—Characteristic FTIR spectrum of quartz on polymer
membrane filter.



quartz are below 2,000 cm where the baseline tends to be
better behaved. At lower wavenumbers, common baseline
correction procedures are usually adequate. Toward the
far-IR region, light scattering is less severe and the per-
formance of the custom accessory is very much like beam
focus analyses.

Some problems with data variability were expectable
since light scattering is not likely to be exactly reproducible
on repeat measurements. This has been reported by other
researchers (17, 24). Also, even minor difficulties in posi-
tioning a filter in the IR beam can affect the resultant
measurement, Averaging data points was chosen to pro-
duce the best calibration plots. The final issues are
whether the customized accessory performs adequately in
comparison to analysis at the FTIR beam focus and how
FTIR performs in comparison to dispersive instruments.

Problems Caused by Interference Fringes

In addition to light-scattering and noise-related phe-
nomena, baselines may also show a sinusoidal variation,
which in figure 7 is most apparent below 2,000 cm. Since
the opposite faces of membrane filters are smooth and
parallel and the refractive index changes abruptly between
the filter surface and the sample compartment air, the IR
light undergoes internal reflections within the filter. Con-
structive and destructive interferences occur between the
primary IR beam and that portion of IR light that has
gone through the extra reflections. The net effect on the
spectrum is rhythmic baseline oscillations, called interfer-
ence fringes. Interference fringes are more common with
FTIR and tend to be less of a problem in spectra collected
with dispersive instruments. The FTIR spectrometer is ac-
tually more accurately recording the more subtle prop-
erties of the sample, which the less sophisticated dispersive
IR instrument would less often display. The spectral pre-
cision of FTIR often allows the fine detail of a spectrum
to be seen more clearly.

Figure 10 shows an expansion of figure 7, the spectrum
for about 10 ug of Min-U-Sil 5 quartz dust on a polymer
filter analyzed at the beam focus. (The interference fringe
at 670 cm” also shows the presence of trace carbon diox-
ide.) Not only is it difficult to tell the peaks apart, but the
interference fringes are adding to the quartz peaks,
affecting their shape and height. When the amount of
quartz deposited on a filter is low (below 25 jug), the
heights of interference fringes begin to be a significant
percentage of quartz peak heights.

Several options that have been proposed for removing
interference fringes include

1, Using sophisticated software programs to alter the
data the FTIR collects and the mathematical processing
that follows.

2. Tilting the sample membrane in the IR beam path
or curving it into a nonflat surface.

3. Spraying a light layer of oil on one surface of the
filter to change its refractive index.

4, Using reference filters that are very carefully
matched by manufacturer, composition, batch, and mass.
Filter thickness will correlate well with mass.

5. Using a larger analytical beam over a larger cross
section of the sample area to incorporate more minor ir-
regularities of the sample and reduce the internal reflec-
tion phenomenon.

Further research is needed to determine which of these
options will work best and with greatest reliability. Work
to date has demonstrated that the use of asymmetric filters
in which the size of the pores are different on opposite
sides of the filter does not eliminate interference fringes.
The change in refractive index in asymmetric filters, vary-
ing with depth in the filter, is insufficient to prevent in-
ternal reflections of light from occurring. Filters that do
not appear mirrorlike on visual examination or by general
conjecture are still reflective in nature to the IR beam.
Well-matched reference filters seem to be helpful. Use of
the custom filter holder incorporates option 5 above, also
with some encouraging results.

Interference fringes were not found to be a severe
problem for analyses at either the beam focus or with the
custom accessory. They were hardly ever seen with cus-
tom accessory analyses. The examination of a larger filter
area includes more filter irregularities within the analytical
beam, and the filter appears less mirrorlike to the beam.
At the beam focus, interference fringes of 0.003 absorb-
ance unit in height were seen regularly between 850 to

ABSORBANCE

-.002! L
{000880 900

B50 800750 700 650600550
WAVENUMBER, cm"!

Figure 10.—Interference fringes complicating spectrum of
quartz.


Library of Congress Cataloging in Publication Data:

Tuchman, D. P. (Donald P.)
Research toward direct analysis of quartz dust on filters using FTIR spectrosco-
py / by D.P. Tuchman.
p. cm. — (Information circular / Bureau of Mines; 9309)
Includes bibliographical references (p. 16).
Supt. of Does. no.: I 28.27:9309,
1. Quartz dust—Analysis. 2. Fourier transform infrared spectroscopy. 3. Air

filters. I. Title. Il. Series: Information circular (United States. Bureau of Mines);
9309.

TN295.U4 [TN312] 622 s—de20[622’.83] 90-19576 CIP



Information Circular 9309

Research Toward Direct Analysis
of Quartz Dust on Filters Using
FTIR Spectroscopy

By Donald P. Tuchman

UNITED STATES DEPARTMENT OF THE INTERIOR
Manuel Lujan, Jr., Secretary

BUREAU OF MINES
TS Ary, Director


SAMPLE PREPARATION AND ANALYSIS

FILTERS

PVC-acrylic filters were used for all test samples used
during this research. Both PVC and PVC-acrylic filters
are good candidates for on-filter analysis. Pure PVC fil-
ters, being hydrophobic, can achieve maximum weight sta-
bility and are little affected by ambient relative humidity.
PVC-acrylic filters, on the other hand, offer better IR
transparency. Although a fair amount of commentary has
been written (18-22), no definitive advantage of one filter
composition over the other surfaced during the review of
literature. PVC-acrylic filters were chosen in this research
for several reasons. First, it was desired to duplicate the
final IR analytical step of MSHA’s P-7 method for quartz
analysis, which has demonstrated reliability over many
years. Second, other researchers have had good success in
performing IR analysis with such filters. Third, the variety
of products and pore sizes available in the PVC-acrylic
composition is adequate for the various phases of this
work—wet deposition during research and air deposition
for final implementation. Filters of 0.45-ym pore were
used in analyses performed at the instrument beam focus
to parallel the MSHA P-7 method. Filters of 0.8-ym pore
were used for full-face analyses with the custom accessory.
They were chosen because this pore size is the smallest
that might be expected for final adoption in field use,
pressure drop requirements being the limiting factor. The
chemical composition of the filters were identical.

Filters presented no outstanding problems during the
sample handling process. Weight stability was always
adequate. Filters were easily deionized of static charges.
Spectral stability was excellent for filters mounted in the
FTIR sample compartment. Exposure to the dry-air purge
of the FTIR did not cause spectral changes with time.
Either equilibration with the purge atmosphere was very
quick, or any spectral modifications resulting from a
thermal and atmospheric equilibration are insignificant.
The purge for the FTIR is reestablished within 3 min of
closing an opened sample compartment. Although ‘the
purge is always imperfect, with residual water and carbon
dioxide peaks being found in most spectra, the spectra of
the filters themselves do not change with longer exposure
to FTIR purge.

DEPOSITION TECHNIQUES
When the limits of performance of an analytical tech-

nique are investigated, predictable, reproducible, and ac-
curate production of samples is required. Wet deposition,

again paralleling the MSHA P-7 method, was the only
workable method of sample preparation. To determine
detection limits, very low quantities of quartz had to be
deposited on filters. Air deposition depends on mass bal-
ances to measure the amount of dust deposited. Experi-
ence has shown that errors of 20 to 30 ug in weighing of
filters are common with conventional analytical balances.
Variation in preparation procedures, static charges, mois-
ture pickup, and operator error are contributing factors as
well as the performance of the balance itself. Therefore,
preweighing and postweighing of filters cannot accurately
determine mass deposits of 100 ug or less. Consequently,
wet deposition is the only means to reliably and accurately
produce the full range of desired quartz deposits.

Wet deposition is accomplished by weighing a substan-
tial amount of quartz dust (1 mg or more) and diluting it
in an appropriate volumetric flask with isopropyl alcohol
(IPA). The range of concentrations may be 1 to 100 ug
of quartz per milliliter of IPA. The dust is dispersed in
the IPA by placing the flask in an ultrasonic bath for 10 to
20 min. A precision pipette (volumetric or graduated, ac-
curate to plus or minus 1% of full volume) is then used to
deliver one or more aliquots of the dilution to the re-
ceiving filter. All weighings, dilutions, and transfers must
be done with care to ensure that the amount of quartz
placed on the filter is very accurately known.

Attempts to accomplish wet deposition with water solu-
tions were unsuccessful. Water has poor surface tension
properties, so dusts do not disperse easily even with
prolonged sonication or use of surfactants. IPA allows
dusts to disperse very easily. PVC and PVC-acrylic filters
tolerate IPA very well. The spectra of IPA-leached and
nonleached filters were identical. At most, a minor
component may be leached from the filters, such as an
antistatic agent or a trace of solvent remaining from the
original polymer manufacture. As a matter of good prac-
tice, IPA-leached filters were used for spectral reference
specimens.

FILTER EFFICIENCY DURING WET DEPOSITION

Approximately 35% of the mass of Min-U-Sil 5 quartz
dust is attributable to submicrometer-size spherical equiva-
lent diameter particles (14), The question arose as to
whether the filters used during wet deposition adequately
retain the smaller particles since liquid filtration is usually
regarded as being less efficient than air filtration (23).
Both PVC and PVC-acrylic filters with nominal pore rat-
ings from 0.2 to 0.8 um were tested for liquid filtration


polymer membrane samples results from light scattering.
The high-frequency light is more likely to refract while
passing through the many pores of the filter. Such a light
path involves many transitions through areas of differing
density and refractive index—solid to air and air to solid.
Also, a sinusoidal variation in the baseline can add or sub-
tract from the major peaks, These periodic, nonrandom
oscillations are called interference fringes.

CUSTOM VERSUS STANDARD FILTER HOLDER
Problems Caused by Light Scattering

Figures 8 and 9 show typical FTIR spectra of polymer
filters, referenced to air. Figure 8 shows both low and
high wavenumber regions of a spectrum in which a filter
was analyzed at the beam focus position. Figures 9 shows
both low and high wavenumber regions of another spec-
trum in which a filter was analyzed with the custom ac-
cessory. These spectra display more clearly some of the
effects of light scattering. Each filter, even before dust
deposition, is a separate specimen having its own mass,
texture, and pore distribution. Therefore, each filter will
scatter light to different degrees. There are, however,
general trends in all the filter spectra.

Light scatter does not affect random noise at low wave-
numbers. At 2,000 cn, all the FTIR spectra were quite
noise free, showing random variation in the background
as low as 0.001 absorbance unit. Toward the far-IR, at
about 500 cm", random background variations for all
FTIR spectra were still only about 0.002 absorbance unit.
The random fluctuations recorded are affected by the ratio
of energy throughput to detector noise (or more generally
speaking, the entire electronic system noise). At the ends
of the mid-IR range, either high or low wavenumber, the
source will have a lower energy output, which will affect
the energy level reaching the detector. At low wavenum-
bers, however, there is no light-scattering phenomenon to
further diminish light reaching the detector. Therefore,
the detector still records a stable signal at low wavenum-
bers. At high wavenumbers, diminished source output and
high light scatter combine to greatly reduce light reaching
the detector. The result at high wavenumbers is that de-
tector noise has a substantial impact and the signal record-
ed is unstable.

In figures 8 and 9, it can be seen that the baselines
beneath the absorbance peaks are close to flat below
1,600 cm, and in general, the baseline curvatures are not
very pronounced below 2,000 cm", Light scatter does not
affect the shape of baselines of spectra much at low
wavenumbers.

In figures 8 and 9, steep slopes in the baselines can be
seen from 2,000 to 3,400 cm. This slope in the baselines

Figure 8.Spectrum of PVC-acrylic filter taken at beam focus,

\
Soo 2800

L n n
2,200 11600 1,000) “400
WAVENUMBER, em”!

400 to 3,400 cm.

ABSORBANCE,

L i hi
2,200 1,600 1000 ‘400
WAVENUMBER, em”!

ol L
3400 200

Figure 9.—Spectrum of PVC-acrylic filter taken with custom
accessory, 400 to 3,400 cm.

also results from light scattering. As the wavenumber
increases, more light is scattered, causing a false absorb-
ance manifested in a rising baseline. At 3,400 cm’, the
beam focus absorbance is about 1.8 while the custom ac-
cessory absorbance is about double that at 3.6 absorbance
units. The slope is steeper with the custom accessory and
much more light is being scattered at higher wavenumbers
when the custom accessory is being used.

At high wavenumbers, random noise and the resulting
baseline problems caused by light scattering are consistent-
ly worse while using the custom filter holder rather than
using standard sample placement at the beam focus. This
is because the IR beam passes through a much higher
mass of filter material when using the custom holder. As
discussed previously, however, the absorbance bands for