NBSIR 73-400

Preparation of Simulated

Environmental Filters Containing

Quartz and Clay

J. K. Taylor, E. R. Deardorff, C. D. Olson, and R. W. Burke

National Bureau of Standards

Department of Commerce

Washington, D. C. 20234

(

October 1973

Final Report

Prepared for National Institute for Occupational Safety and Health

Division of Laboratories and Criteria Development

Cincinnati. Ohio 45202

NBSIR 73-400

PREPARATION OF SIMULATED

ENVIRONMENTAL FILTERS CONTAINING

QUARTZ AND CLAY

J. K. Taylor, E. R. Deardorff, C. D. Olson, and R. W. Burke

National Bureau of Standards

Department of Commerce

Washington, D. C. 20234

October 1973

Final Report

Prepared for National Institute for Occupational Safety and Health

Division of Laboratories and Criteria Development

Cincinnati, Ohio 45202

or

U. S. DEPARTMENT OF COMMERCE. Frederick B. Dent. Secretary

NATIONAL BUREAU OF STANDARDS. Richard W. Roberts. Director

Preparation of Simulated Environmental Filters

Containing Quartz and Clay

ABSTRACT

This report describes techniques for the

preparation of simulated environmentally collected

samples consisting of small amounts of mixtures of

quartz and clay deposited on membrane filters.

The amounts correspond to that which would be

collected during an 8-hour period in an atmosphere

containing quartz below and above TLV concentrations. An improved speetrophotometr ic method for the

determination of quartz in environmental samples,

developed during the course of the work, is also

described.

1. INTRODUCTION

Tlie purpose of this investigation was to develop techniques

to produce filters on which a mixture of quartz and clay are

deposited to simulate environmentally collected samples. Such

filters would be useful for evaluating the reliability of

various analytical methods for determination of industrial

exposure to quartz dust in the atmosphere. Except for the

collection step, they should simulate all other problems of a typical analytical field sample.

2. PRELIMINARY EXPERIMENTS

2 . 1 Preparation of Filters

In conference with the NIOSH Project Officer, it was

decided that the samples should consist of a two -component

mixture of quartz and clay. The total amount of material

on each filter should be approximately 400 yg with three

different levels of quartz. The target concentrations

selected were:
r'
Preparation of Simulated Environmental Filters

Containing Quartz and Clay

ABSTRACT

This report describes techniques for the

preparation of simulated environmentally collected

samples consisting of small amounts of mixtures of

quartz and clay deposited on membrane filters.

The amounts correspond to that which would be

collected during an 8-hour period in an atmosphere

containing quartz below and above TLV concentrations. An improved spectrophotometr ic method for the

determination of quartz in environmental samples,

developed during the course of the work, is also

described.

1. INTRODUCTION

Tlie purpose of this investigation was to develop techniques

to produce filters on which a mixture of quartz and clay are

,| deposited to simulate environmentally collected samples. Such

! filters would be useful for evaluating the reliability of

various analytical methods for determination of industrial

exposure to quartz dust in the atmosphere. Except for the

collection step, they should simulate all other problems of a typical analytical field sample.

2. PRELIMINARY EXPERIMENTS

2. 1 Preparation of Filters

In conference with the NIOSH Project Officer, it was

decided that the samples should consist of a two -component

mixture of quartz and clay. The total amount of material

on each filter should be approximately 400 yg with three

different levels of quartz. The target concentrations

selected were:
Series Quartz, yg

100 0

200 30

300 80

400 200

The first approach which came to mind was to use natural

mixtures of clay and quartz, but no suitable substances were

discovered. Accordingly, the decision was made to use

synthetic blends.

The clay used was NBS Standard Reference Material 97a,

Flint Clay, which has the following composition:

Const ituent Percent by Weight

Si02 (as silicate) 43.7

AI2O3 38.8

Fe^Oj 0.45

TiO^ 1.9

CaO 0.11

SrO .18

MgO .15

^^2^3 . 03

Na^O .037

K^O .50

Li^O .11

ZrO^ .063

BaO .078

^2^5 .36

Loss on Ignition 13.32

Chemical analysis at NBS indicates that substantially all

of the silicon in this sample is silicate. The quartz content

Clay, yg

400

370

320

200

Total, yg

400

400

400

400

2
is less tlian 0.5 percent. Prior to issuing as an SRM the

clay was ground to pass a 100 mesh sieve with 80 percent passing

a 200 mesh sieve. Particle size measurements made at NIOSH, by

Mr. Charles Gorski (Toxicology Branch), indicate the geometric

mean diameter (50 percentile) is 0.39 ]M.

The quartz used for the samples was a commercial

material known as "Hin-u-sil 15" v/ith an average particle size of

4.5 ym. This material is more than 99 percent Si02.

Experience with the preparation of filters containing

toxic metals (NBS Report NBSIR 73-256), suggested the use of

a similar technique in the present case. Preliminary experiments

were encouraging and the following procedure was adopted. The

finely divided solid was dispersed in 1 percent aqueous tragacanth

solution and 100 yl portions were delivered to the filter by

means of a push-button type calibrated pipette.

A simple technique for quality control of the filter

preparation would have been weighing before and after deposition,

using a direct-reading elec trobalance . Experiments were

performed to determine the reliability of this procedure by

weighing filters that had been alternately wetted and dried. ,

i Variabilities of 10 to 100 micrograms were found and therefore

a more precise method was sought. While it is believed that | this procedure could be improved by careful control of drying

and weighing conditions and possibly by the use of less

hygroscopic filters, it was abandoned in favor of direct analysis.
The clay and quartz were dried at 110°C for four hours

prior to mixing. The mixtures of the two solids were made

using a ball mill, a V-blender, and finally a mortar and pestel

to assure uniformity and to break apart balls of material formed

due to electro-static charges. A total charge of 200 g was

used, with appropriate amounts of each component to produce

the target concentrations. This procedure was used because

it was desired to prepare both bulk samples and deposits on

filters. The material proved to be difficult to blend due

to sticking on the walls of the mixers. Frequent tapping

apparently corrected this, so the process was repeated through

many cycles to achieve apparent homogeneity.

Suspensions of the mixtures were made by weighing 400 mg

and adding this to 100 ml of water. It was observed that the

suspension settled rapidly when not stirred and that stirring

j by immersion in aii ultrasonic bath was not completely effective,

j A magnetic stirrer, used continuously, left small amounts of

material unsuspended.

Experiments to determine the uniformity of suspension

were made in which 100 yl of slurry were pipetted into microbeakers, dried and weighed. This showed a relative standard

deviation of 15 percent which was considered to be unacceptable. I Inspection of the suspension showed that, although constantly

j stirred, visible amounts of solid material were settling and/or

distributed in patterns in the liquid. Accordingly, a means

to prevent this was sought.

4
It was found that the addition of gum tragacanth to the

suspension prevented settling or at least retarded it for

periods of days. The gum also was considered to be beneficial

in serving as a binder for the deposit, after drying, to

prevent loss by abrasion. A 1-percent solution appeared to

be most satisfactory. The suspension was prepared by heating

the 1-percent tragacanth solution to about 90°C, then adding

the Si02/clay sample mix, and letting the solution cool. This

operation was done using a hot plate - stirrer and a magnetic

stirring bar. Although no visible settling occurred, the

suspensions were stirred continuously, during filter preparation.

3. PREPARATION OF FILTERS

The deposits were placed on membrane -type filters, type

flAWP , 47 mm in diameter. A single filter was placed in the

bottom portion of a plastic petri dish-type container and

0.1 ml of the suspension was transferred by a push-button

pipette. The cover was loosely placed on the dish, and the

filter was air dried before closure. The filter containers

were given a serial number, indicating the order of preparation,,

to assist in identifying any systematic error of preparation.

The filters were arranged in rows of 10, which were

prepared in sequence. Then a second set of 10 was prepared.

It was possible to make the final transfers to 50 filters in

about one hour.

5
The numbering system used was as follows:

Blank - 100 Series - 000101 to 000199

Level 1 - 200 Series - 000201 to 000299

Level 2 - 300 Series - 000301 to 000399

Level 3 - 400 Series - 000401 to 000499

4. ANALYTICAL METHOD

The filters were analyzed by a spectrophotometric method

to determine the amount of quartz present. The NIOSH method

was used initially but difficulties were experienced. For

example, apparent assay values of 20 to 50 yg were found on

series 100 filters which were not believable. Also, unacceptable values were obtained at all assay levels with very large

standard deviations. Accordingly, research was conducted to

determine the cause of this and to develop an analytical

procedure to eliminate the problems. Some of the details of

this and the method finally developed and used are given below.

4 . 1 Sample Treatment

Place the membrane filter containing the sample in a

125 ml pyrex Erlenmeyer flask and gently heat over a Meeker

burner until it chars flames. Then heat the Erlenmeyer strongly

until all visible carbon is burned off. Add 25 ml of 85 percent

phosphoric acid to the beaker. With a thermometer inserted in

the beaker, heat the solution on a hotplate to 200°C and continue heating for 12 minutes while the temperature rises to

225° to 245°C. Swirl the solution occasionally to rinse the

sides of the beaker. Allow the solution to cool to 100°C, and

6
add 100 ml of hot water. Swirl to mix and filter with suction

through a 25 mm HAWP membrane filter, pore size 0.45 y. Wash

alternately with hot HCl (1+10) and hot water at least five . times and finish washing thoroughly with hot water. Transfer

the 25 mm filter to a wet 47 mm HAWP membrane filter, pore

size 0.45 y , on the suction flask. Wash the edges of the 25 mm

filter with water to rinse out the H^PO^ from the portion of the

small filter which was not adequately washed during the first

filtration.

Place the two filters flat in the bottom of a 250 ml Teflon

beaker. Add 1 ml of HF (1+1). Place a thin polyethylene disc,

cut to fit the bottom of the beaker, on top of the filter papers

and cover the beaker with a Teflon cover. Allow it to stand

for 30 minutes. Add 40 ml of 5 percent boric acid solution.

Remove the polyethylene disk and the two membrane filters,

washing them with small amounts of water. At this point

samples containing more than 80 yg of Si02 can be diluted to

a convenient volume with boric acid and an aliquot taken for

analysis. The aliquot is pipetted into a 250 ml Teflon beaker

and diluted to 40 ml with boric acid: Following this, 10 ml

of a 10 percent H2SO^-10 percent HNO^ solution (ultra pure

acids) is added to the solution. The molybdenum blue color is

developed and read on the spectrophotometer as described in

Section 4.4, Color Development. A blank should be run through

all steps of the sample treatment. It was observed that the

reagents and the membrane filters contributed some background

7
color to the samples. This background absorbance must be subtracted from the observed sample absorbance.

4 . 2 Determination of Free Silica

This recommended spectrophotomet ric method for the determination of silicon was developed in 1963 at NBS for use on

SRM zirconium and zircaloy. At that time, experiments were

performed to determine the concentration of reagents required,

the optimum pH and temperature for maximum complex formation

and color development, the conditions for extraction of molybdosilicic acid into butanol , and the use of a backwash step to

remove interferring elements. The molybdenum (V) complex was

reduced with stannous chloride to form the heteropoly molybdenum

blue complex whose absorbance was then measured on a spectrophotometer at 635 nm.

The first problem encountered in adapting this spectrophotometric method to the determination of quartz on the

membrane filters was separating it from the soluble silicates.

This difficulty was subsequently circumvented by dissolving the

soluble silicates with hot phosphoric acid and collecting the

insoluble Si02 on another membrane filter. However, this

procedure presented a second problem of how to remove all of

the phosphoric acid from the filter. This is necessary since

phosphate interferes with the molybdenum blue determination of

8102- A clean separation was not possible because the 25 mm

membrane filter could not be washed well enough on the edges

of the filter that were covered by the glass funnel fittings.

8
The last traces of phosphoric acid could be washed away by

transferring the 25 mm filter to the center of a 47 mm filter

placed on the 47 mm glass filter frit, and carefully washing

the clean edges of the 25 mm filter with water.

4 . 3 Calibration Curve

A standard silicon solution of 200 yg Si/ml was prepared

by fusing 213.9 mg of pure Si02 with 2 grams of sodium carbonate,

dissolving the melt, making up to 500 ml with water and storing

in a plastic bottle. A dilute standard working solution was

prepared by diluting 25 ml of the standard solution (200 yg

wSi/ml) to one liter and storing in a plastic bottle. This

dilute solution contains 5 yg Si/ml, equivalent to 10.7 yg

SiO^/ml. Aliquots of this dilute solution were used to prepare a calibration curve.

4 . 4 Color Development

The following procedure is used for color development.

Transfer 0 , 1 ml , 2 ml , 4 ml , 6 ml and 8 ml of the dilute

standard silicon solution to six 250 ml Teflon beakers. Add

10 ml of 10 percent H2SO4 - 10 percent HNO^ (use highest

purity acids) and 0.5 ml HF, 48 percent. Add 40 ml of boric

acid solution (50 g per liter) . Add potassium permanganate

solution (0.6 g per liter) dropwise to produce a permanent

pink tint. Add 4 ml of molybdic acid solution (25 g of

ammonium molybdate tetrahydrate in 200 ml of H2S0^ (1+9)

diluted to 250 ml with water and filtered through close - textured

9
paper). Adjust the pH of the solution to 1.7 - 1.8 with

ammonium hydroxide solution (prepared by bubbling ammonia gas,

using plastic tubing, into ice cooled distilled water in a polyetliylene bottle until solution is saturated ). Cover the

beaker with a Teflon cover and heat for 10 minutes on a steam

bath. Cool in water at approximately 20°C for 10 minutes.

Add 4 ml tartaric acid solution (200 g per liter) . Mix '

thoroughly, and transfer the solution to a 250 ml glass

separatory funnel with Teflon stopper and stopcock. Wash the

beaker twice with water and add the water to the funnel.

Dilute the solution to a volume of approximately 90 ml, and i

add 14 ml of H2S0^ (1+1) and mix. Cool the separatory funnel

in cold running water. Add 50 ml of normal butyl alcohol, and

shake vigorously for one minute. Allow the layers to separate;

drain, and discard the lower acid layer. Wash the butyl

alcohol layer by adding 20 ml of H2S0^ (1+99) to the separatory i funnel and shake for 30 seconds. After the layers separate,

drain, and discard the lower acid layer. Repeat this washing two

more times. Transfer the butyl alcohol layer to a dry 50 ml

volumetric flask. Wash the funnel twice with 1 to 2 ml of butyl

alcohol, and add washings to the flask. Add 0.5 ml of stannous

chloride reducing solution (Dissolve 2 g of SnCl2 * ^^2^

1 mg CUCI2 • ^^2^ "'"^^ ^^'^ store in glass stoppered

bottle. Prepare fresh solution every week.) to the flask, dilute

to the mark with butyl alcohol, and mix.

10
rill a 1 cm reference cell with normal butyl alcohol and

adjust the photometer to 635 nm. Take the photometric readings

of the calibration solutions in a 1 cm cell vs. the reference

solution. Plot the photometric readings of the calibration

solutions with respect to the micrograms of silicon per 50 ml

of solution ( yg Si multiplied by 2.139 equals yg Si02)

.

5. SUM^IARY OF RESULTS

Table 1 summarizes the analytical results obtained with

the method as finally developed. All results prior to this

were discarded due to uncertainties in some aspects of the

procedure.

The precision is reasonably good which indicates that the

filters are homogeneous. Only one value, namely that for filter

No. 410 appears to be out of line and it was not included in the

average

.

The accuracy of the values is still uncertain. It will be

noted that the average value is about 3/4 of that expected. It

is difficult to understand why the samples deviated that much

from the target values. It would be easy to conclude that the

problem arises from partial solution of the very fine fraction

of Si02 during the phosphoric acid leach. However, four

gravimetric determinations of the amount of SiO^ remaining after

leaching 1 mg samples of "Min-u-sil 15" with phosphoric acid

indicated losses of 2, 15, 6, and 2 percent, respectively, hence

this explanation does not seem reasonable.

11
Table 1 Summary of Results

Sample Number yg SiO ^ Average SiO ^ Expected Value, yig

124 2 ) 0

155 trace) 0

127 trace) 0 0

138 0.2 ) 0

239 25) 30

250 241 30

227 24) 23 30

247 20) 30

360 63) 80

315 62) 62 80

333 62) 80

410 156 200

460 137) 200

428 136) 135 200

440 133) 200

.S 97a Clay + 0.1 pi gum + 4 7 mm filter

314 Mg 2 0

420 Mg 2 0

320 trace 0

371 yg 2 0
g,s. Df PT, OF COMM. 1. PUULICATION OR REPORT NO. 2. Gev't Acceeeiea

SIBLI06RAPHIC DATA NBSIR 73-400

5n C C T

3. Reeipient's Aecession No.

, 4. TITLE AND SUBTITLE

Preparation of Simulated Environmental Filters

Containing Quartz and Clay

1

5. FublicAtion Date Oct. 1973

6.Perfotmin^ Organization Code

7. AUTIIOR(S)

; J. K. Taylor, E. R.Deardorff.C. D.Olson, R.W. Burke

8. Performing. Organ ization

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

1 2. Sponsoring Organization Name and Address National Institute for Occupational Safety § Health

Division of Laboratories and Criteria Development

Cincinnati, Ohio 45202

13. Type of Report & Period

Covered

14. Sponsoring Agency Code

h-5. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

I This report describes techniques for the preparation of simulated

I environmentally collected samples consisting of small amounts of

mixtures of quartz and clay deposited on membrane filters. The

amounts correspond to that which would be collected during an 8-

hour period in an atmosphere containing quartz below and above TLV

concentrations. An improved spectrophotometr ic method for the

determination of quartz in environmental samples, developed during

the course of the work, is also described.

17. KEY Vi'ORDS (Alphabetical order, separated by semicolons) atmospheric analysis; industrial hygiene; quartz dust analysis; reference materials.

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