NBSIR 74-573

Development of Contaminant Generation

Systems for Certification of Portable

Air Sampling Instruments

B. C. Cadoff, B. Greifer. P. A. Pella, and J. K. Taylor

National Bureau of Standards

Department of Commerce

Washington. D, C. 20234

February 1975

Annual Report

Prepared for National Institute for Occupational Safety and Health

Testing and Certification Laboratory

Morgantown, West Virginia

NBSIR 74-573

DEVELOPMENT OF CONTAMINANT GENERATION

SYSTEMS FOR CERTIFICATION OF PORTABLE

AIR SAMPLING INSTRUMENTS

B. C. Cadoff, B. Greifer. P. A. Pel la, and J. K. Taylor

National Bureau of Standards

Department of Commerce

Washington, D. C. 20234

February 1975

Annual Report

Prepared for National Institute for Occupational Safety and Health

Testing and Certification Laboratory

Morgantown, West Virginia

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

NATIONAL BUREAU OF STANDARDS. Richard W. Roberts. Director

TABLE OF CONTENTS

Page

1. INTRODUCTION 1

2. TECHNICAL APPROACH 2

3. PROJECT PLAN 2

3.1 Bulk Mixtures 2 3.2 Gas Analysis 4

3.3 Stability. 4

3.4 Performance Evaluation 4

3.5 Delivery of Working Standards 5

4. EXPERIMENTAL DETAILS ..... 5

4.1 Preparation of Compressed Gas Mixtures .... 5

4.1.1 Vapor Distillation 6

4.1.2 Liquid Injection 10

4.1.3 Estimation of Target Concentrations. . . 11

4.1.3.1 Vapor Distillation 11

4.1.3.2 Liquid Injection 12

4.2 Charcoal Tube Gravimetry . . • 15

4.2.1 Summary of Method 15

4.2.2 Experimental 15

4.2.3 Calculations 18

4.3 Gas Chromatographic Analysis of Gas Mixtures . 19

5. EXPERIMENTAL RESULTS 20

5.1 Results of Gravimetric Analysis 20

5.2 Results of Gas Chromatographic Analysis. ... 25

6. SUMMARY AND CONCLUSIONS. 25

i
LIST OF TABLES

Page

1. Organic contaminants 3

2. Vapor pressures of solvents of interest 3

3. Concentrations of volatile solvent mixtures 12

4. Concentrations of styrene gas mixtures 14

5. Charcoal tube gravimetry - chloroform . 21

6. Charcoal tube gravimetry - toluene 21

7. Charcoal tube gravimetry - methylene chloride. ... 22

8. Charcoal tube gravimetry - ethylene dichloride ... 22

9. Charcoal tube gravimetry - 1,1,1- tr ichloroethane . . 23

10. Charcoal tube gravimetry - styrene - low and high

concentration 23

11. Properties of compressed gas mixtures 24

12. Gas chromatographic analysis of vapor-air

mixtures -chloroform (1210 ppm} 28

13. Gas chromatographic analysis of vapoi*-air

mixtures -toluene (650 ppm) 29

14. Gas chromatographic analysis of vapor-air

mixtures -methylene chloride (4850 ppm) 30

15. Gas chromatographic analysis of vapor-air

mixtures -ethylene dichloride (980 ppm) 31

16. Gas chromatographic analysis of vapor-air

mixtures -1 , 1, 1 - tr ichloroethane (1940 ppm) ..... 32

17. Gas chromatographic analysis of vapor-air

mixtures -styrene (475 ppm) 33

/ LIST OF FIGURES

1. Instrumentation for filling compressed gas cylinders 7 2. Solvent vapor filling system 9

3. Vapor pressure of solvents 13

4. Charcoal tube gas sampling system 16

5. G.C. response vs. gas concentration 26

ii
Development of Contaminant Generation Systems

for Certification of Portable Air Sampling

Instruments >

ABSTRACT

The development of gas generation systems is

described. Stable mixtures of contaminant gases with

air are stored in compressed gas cylinders at concentrations about five times the Threshold Limit Values

(TLV) . The cylinders arc analyzed by charcoal tube

gravimetry in which the weight increase of tared char-

\ coal tubes is used to determine the organic component

of the gas mixture. For use, the compressed gases

are dynamically diluted with air to the needed concentrations in a gas-flow system.

Data are presented to demonstrate (1) linearity

of the gas dilution system as measured by gas chromatographic determinations of vapor concentrations;

(2) precision of charcoal tube gravimetric measurements; and (3) stability of compressed gas mixtures

of air with chloroform, toluene, methylene chloride,

ethylene dichloride, 1 , 1

, 1 - trichloroe thane , and

styrene at concentrations of 1210, 650, 4850, 980,

1940, and 475 ppm respectively.

It is concluded that the methods of preparation

and verification of these compressed vapor-air mixtures have general applicability. A wide variety of

stable single-component vapor-air mixtures may be

prepared and analyzed to one percent accuracy rapidly

and routinely.

1. INTRODUCTION

This report summarizes work concerned with the development

of systems to generate test atmospheres useful for verifying

the performance of gas analysis instrumentation. It is part

of an ongoing program at the National Bureau of Standards

under sponsorship of the National Institute for Occupational

Safety and Health (NIOSH) to assist the latter agency in its

measurement methodology certification program. A previous

report - NBSIR 73-292, Gas Generation Systems for the

Evaluation of Gas Detecting Devices, October 1973 - deals

with the general philosophy and describes gas generation

systems for a number of mixtures based on several generating

principles. The present report is an extension of this work

and is devoted to gas mixtures containing organic substances,

and especially organic solvents.

1
2. TECHNICAL APPROACH

Some organic substances commonly found as contaminants

of industrial work atmospheres are listed in table 1. The

present investigation is concerned with the first six of

these. Test atmospheres of these substances should be

readily generated by the dilution of concentrated bulk mixtures

with air in dynamic blending systems such as described in

the earlier report, if the bulk mixtures are of sufficient

stability. Even in the case of limited long-term stability,

this approach is feasible, provided the bulk mixture can be

analyzed at the time of use.

The work involved the following tasks: preparation of

bulk mixtures; investigation of stability of bulk mixtures;

development of analytical methodology; evaluation of the

dilution system. ,

3 . PROJECT PLAN

^3.1 Bulk Mixtures . Gas mixtures consisting of a single component in air

or other diluent gas are readily prepared in compressed gas

cylinders of the type used for industrial gases, to give longlasting supplies of the known mixtures. Liquids may be introduced into the gas cylinders by vapor distillation or by

direct injection in an amount calculated to give the desired

concentration of the organic ingredient after the cylinder is

pressurized with diluent gas. Vapor pressure considerations

are important, for too high an organic vapor concentration

can cause condensation within the cylinder (with resultant

change of mixture composition) at lowest operating temperatures.

For maximum stability the mixture concentration should

be as high as possible consistent with vapor pressure limitations because low- concentrat ion mixtures in the ppm range

are especially susceptible to loss of material through vapor

sorption on the cylinder walls. While high- concentrat ion

mixtures are subject to the same sorption losses, there is

opportunity for eventual stability to be attained through

surface saturation; also, the higher concentration mixtures

would be subject to smaller relative errors from this cause.

The following working standards were prepared with air

as the diluent gas: chloroform (1210 ppm), toluene (650 ppm),

methylene chloride (4850 ppm), ethylene dichloride (980 ppm),

1 , 1

, 1 - tr ichloroethane (1940 ppm), and styrene (70 and 475

ppm). Details of preparation are given in Section 4.

2
Table 1. Organic contaminants

Concentration Range

(PPm) *

12.5 to 125

50 to 500

250 to 2500

25 to 250

175 to 1750

50 to 500

10 to 100

100 to 1000

500 to 5000

100 to 1000

37.5 to 375

2 . 5 to 25

2 . 5 to 25

^These values from Federal Register 36i (157J, Aug. 13, 1971.

^Formerly 500 ppm, then 50 ppm, now reported to be zero.

Table 2. Vapor pressures of solvents of interest^

Condensation Temperature (°C) at Indicated Pressures (mm Hg)

Solvent 1mm 1 Omn 4 0mm 100mm 4 00mmI 76 0mm 152 0mm

1. Chloroform -58.0 -29. 7 -7. 1 10. 4 42. 7 61. 3 83.9

2. Toluene -26. 7 6. 4 31 . 51. 9 89. 5 110.6 136. 5 3. Methylene chloride -70.0 -43. 3 -22. 3 - 6. 3 24. 1 40.7

4. Ethylene dichloride -44.5 -13. 6 10. 0 29. 4 64. 0 82.4 108.1

5. 1,1,1-Trichloroethane -52.0 -21 . 9 1. 6 20. 0 54. 6 74.1

6. Styrene -7.0 30. 8 59. 8 82. 0 122. 5 145.2

7. Carbon disulfide -73.8 -44. 7 -22. 5 - 5. 1 28. 0 46,5 69.

1

8. Vinyl chloride 105.6 -83. 7 -66. 8 -53. 2 -28. 0 -13.8

9. Acetone -59.4 -31. 1 - 9. 4 7. 7 39. 5 56.5 78.6

10. Methyl ethyl ketone -48.3 -17. 7 6. 0 25. 0 60. 0 79.6

11

.

Chlorobenzene 13.0 22 . 2 49. 7 70. 7 110. 0 132.2 160. 2 12. Aniline 34 . 8 69. 4 96. 7 119. 9 161. 9 184.4 212.8

13. Phenol 40.1 73. 8 100. 1 121 . 4 160. 9 181 . 9 208 . 0

TLV ^ Contaminant (ppm)

1

.

Chloroform 50

2 . To luene 200

3

.

Methylene Chloride 500

4 . Ethylene Dichloride 50

5. 1,1,1-Trichloro ethane 350

6. Styrene 100

7. Carbon Disulfide 50

8. Vinyl Chloride 02

9. Acetone 1000

10. Methyl Ethyl Ketone 200

11

.

Chlorobenzene

12. Aniline 5 13. Phenol 5

^Handbook of Chemistry and Physics, Chemical Rubber Co., 49th ed.

(1968-1969), pp D-120 to D-136.

3
3 . 2 Gas Analysis

The method of preparation can serve as a means for determining the initial composition of the resultant gas mixtures

since known quantities of solvent vapor are diluted with

knpwn volumes of air. However, this calculated composition

is good to only a first approximation because (1) there is

no assurance that all of the vapor or liquid introduced into

the compressed gas tank is available since loss can occur

due to sorption on the interior tank surface, and (2) there

are uncertainties regarding the equat ion-of- state for the

particular system. Furthermore, it is necessary to be able

to analyze the bulk mixtures at any later time to verify

their concentrations.

An unambiguous and absolute procedure involving sorption

on activated charcoal was developed. The procedure, referred

to as charcoal tube gravimetry, should prove useful for a wide range of organic solvents, giving results that are

accurate to about one percent. A charcoal tube of the type

used for sampling industrial atmospheres is used to sample

the gas mixture for a precisely determined time and at a constant flow rate. The weight of the sorbed material and

the total volume sampled are used to calculate the gas

concentration. Details are given in Section 4.2,

3. 3 Stability >

The stability of the prepared gas mixtures was measured

by charcoal tube gravimetry over a period of 2 to 6 months

between October 1973 and July 1974. Four measurements made

in 1973 were not included in the final data averages because

refinements in the measurement techniques (i.e. precise calibration of the flowmeter, measurement of actual gas flow

through each charcoal tube, elimination of static electricity

effects during weighings, etc.) made these first analyses

less dependable than desired, even though they differed by

only one to five percent from the 1974 analyses.

3.4 Performance Evaluation

The analysis of the working standards and the output

mixtures of a gas dilution system using a gas chromatograph

gave straight line plots passing very near the origin, thus

demonstrating the reliability of the dynamic dilution procedure. Charcoal tube gravimetry was also used to measure the

concentrations of some of the mixtures produced by gas dilut ion

,

4
The relative standard deviation of all the charcoal tube

measurements and the gas chromatographic analyses did not

exceed two percent, and in most cases was one percent.

3 . 5 Delivery of Working Standards

Compressed gas cylinders delivered to NIOSH at the

completion of this program included chloroform (1210 ppm)

,

toluene (650 ppm), methylene chloride (4850 ppm), ethylene

chloride (980 ppm), 1 , 1

, 1 -t r ichloroethane (1940 ppm) and

three cylinders of styrene (475 ppm average). The 70 ppm

styrene tank whichhad been prepared especially for analytical

experimentation and stability studies was not delivered.

4. EXPIiRIMENTAL DETAILS

4 . 1 Preparation of Compressed Gas Mixtures

Volatile solvents are distilled into evacuated compressed

gas cylinders to the desired partial pressure as measured by

a mercury manometer. The partial pressure must not exceed

the saturation vapor pressure of the solvent at the lowest

temperature of use, for the resulting condensation will

change the mixture composition. Solvents of low volatility

are injected directly into the evacuated cylinders through

a rubber septum, using a weighed hypodermic needle. The

cylinders are pressurized with air to a suitable total

pressure to dilute the inserted solvent vapors to the desired

trace concentrations.

Compressed gas cylinders of chloroform, toluene,

methylene chloride, ethylene dichloride, and 1,1,1-

trichloroethane were prepared by distilling the solvent vapors

into evacuated tanks to the appropriate vapor pressures, then

filling the tanks with breathing air to yield mixtures containing the organic solvents at the desired concentrations.

Styrene was injected directly into cylinders through a rubber septum because the low vapor pressure would have

required inordinately long distillation time with resultant

risk of room air contamination through chance leaks. The

weight of styrene injected was determined by weighing the

hypodermic syringe before and after injection.

The gas cylinders were pressurized with air to give

the desired vapor concentrations according to the following

expression derived from the gas laws:

5
P (14.7 X lOM

76C

where = concentration of vapor in tank (ppm)

P^ = partial pressure of vapor in tank (mm Hg)^

P^ = total tank pressure (psia)^

14.7/760 = psia/mm conversion

10^ = factor for parts per million conversion

This is an approximate calculation because it assumes

that the ideal gas laws hold at both low and high pressures.

However, this equation is sufficient for the purpose.

4.1.1 Vapor Distillation

Select a new compressed gas cylinder (preferably) or one

that has been used only for air or water-pumped nitrogen.

The tank may be filled with solvent vapor using pressurevacuum instrumentation similar to that diagrammed in figure 1.

1. With a full (2000 psig) cylinder of breathing air

on line 1, and the empty or new cylinder on line 4 but with

all valves closed, start the vacuum pump, then open valves

5, 7, 8, 9, 10 and 4 in that , order, to evacuate the

system down to minimum attainable pressure as read on the

absolute pressure gauges.

2. ' Check for vacuum leaks by closing valve V-5 and

noting any pressure rise in the system. If the pressure

does not increase after 30 min the system is considered to

be vacuum tight. Close all valves,-

3. Check for pressure leaks by opening valve V-1,

close it immediately, then open V-7 slowly to admit less

than 1000 psi then close it, open V-8 slowly to admit less

than 500 psi then close it, then V-9 to less than 100 psi,

n mm Hg = pascals = 133.32 Pa

^1 psia = "^^^^^^ pascals = 6892.9 Pa

6
0£

<

• mm u.

7
and V-10 to less than 800 mm, close all valves rapidly to

avoid overpres sur ing the gauges. Now all the valves are

closed and all the pressure gauges are reading near their

maximum scale readings. Observe the system for 30 min

and note that there is no pressure drop. The system is

ready to be used. Open valve V-6, then V-7, 8, 9, and 10

in order to bleed the system down to atmospheric pressure.

Close all valves.

4. Flush the sample tank three times by applying

vacuum and pressurized air alternately using only valves

V-1, 4, 5, and 6 for the purpose. This removes traces of

moisture found occasionally in new tanks after factory

pressure testing. Leave the tank fully evacuated, disconnect it from line 4 and connect it to the solvent vapor

filling system shown in figure 2.

5. Connect the solvent filling system (figure 2) to

the pressure-vacuum instrumentation through line 4 (figure

1) and open all valves, except V-13, 1, 2, 3, 6 to evacuate

the system. Check for leaks under vacuum as outlined in

step 2, keeping valves 4 and 12 open and valves 11 and 13

closed. The mercury manometer as read with a cathetometer

is a suitable leak detector; it should show no pressure

increase from the minimum attainable pressure even after

30-60 min. It is safe now to open the sample cylinder to

the system, noting that there should be no pressure increase;

if there is, check for leaks between the system and sample

tank. Re-evacuate the system and assure that there are no

vacuum leaks. Record the mercury manometer reading.

6. Close valve V-4 and open V-il to admit solvent vapor

to the system. The manometer pressure rises as solvent

boils and distills into the sample cylinder. Admit the

vapor slowly, chilling or warming the glass bulb of the

solvent reservoir as required,

7. When the desired partial pressure of vapor has

been attained close V-11 and allow the system to equilibrate

for 30 min. Then, close the sample cylinder and open vent

V-13 cautiously to avoid forcing the mercury out of the

manometer, and open V-11 to relieve the vacuum over the

solvent. Now, close valves V-11, 12, 13, and the sample

cylinder

.

8. The cylinder is ready to be pressurized. Tape a thermometer to the cylinder and cover the bulb with

several layers of insulating material so that the tank temperature not the room temperature is read. With the air

tank still on line 1 and the sample filling system still

on line 4, all valves closed, open valve V-1 slightly and

8
9
allow the pressure to build up slowly on the 2000 psia gauge.

Open V-4 and V-13 to flush out the lines; then, close V-13

and open the sample cylinder.

9. Keep a close control over V-4 to prevent the compressed air from surging too rapidly into the evacuated

sample tank. When the pressure is nearly at the desired

value, say 1000 psia, close the sample tank, then valves

V-1 and V-4 and allow the tank which has been heated by

compression of its contents to cool for several hours or

overnight if possible.

i- 10. After temperature equilibration, the pressure is

adjusted to the final value. The valve adjustments must be

made in such a way that the gauge pressure a3>ways exceeds

the sample tank pressure so that the flow is always from the

air supply to the sample tank and never in the reverse

direction (loss of organic vapor might otherwise occur).

This is accomplished by assuring that the sample tank valve

is opened last and closed first in any pressure adjustment,

11. Set the air supply to the desired tank pressure

with valve V-1, using as many gauges as necessary to read

the final pressure most accurately. With V-1 closed and

V-4 open, crack the sample tank valve until the gauge

readings stop falling, and close it immediately. Repressurize

with V-1 and repeat the process in small pressure steps

until the sample tank pressure is at the desired value.

Record the pressure and temperature of the tank,

12. Vent the air supply system to atmospheric pressure

with either V-6 or V-13,

13. Mount the sample tank in an inclined position (use

a cylinder cart) and heat the bottom gently with an infrared

light for at least three hours to mix the contents. Overnight is a convenient mixing period. The tank is ready

for use after suitable labeling of the contents.

4.1.2 Liquid Injection

Styrene and other solvents of low volatility are

handled somewhat differently because their low volatility

would require an inordinately long distillation time for the

partial pressures to build up to the desired values. The

effects of minute system leaks, undetectable during 30-60 min

of leak testing would be magnified during a 3-4 hr vapor

distillation, and any pressure increase due to air would be

read as vapor partial pressure. Warming the distillation

flask to accelerate distillation is not an acceptable

procedure for styrene because of the danger of polymerization. It is best to inject the liquid directly into the

sample tank in these cases.

10
A rubber septum of the type used for gas chromatographic

injection is mounted in one arm of a tee compression tube

fitting which is threaded onto a CGA fitting that fits

directly into the sample tank inlet. The third arm of the

tee is attached to valve V-4 of the pressure-vacuum instrumentation Cfigure 1) and the following procedure is

employed. *

1. The previously evacuated and air-flushed tank is

leak tested for 15-30 min under pressure (40 psia maximum)

and for 30-60 min under vacuum to assure that the rubber

septum and associated connections are tight.

2. Fill a hypodermic syringe with a suitable volume

of styrene and seal the syringe by pushing the tip into a spare rubber septum to cut down evaporation losses. Weigh

the filled syringe.

3. Plunge the syringe deeply into the rubber septum

fitting past the tee side-arm into the throat of the opened

evacuated sample tank, and inject the contents rapidly

into the fitting. The vacuum inside the fitting will assist

the operation. Remove the syringe, cap with the same rubber

septum, and weigh to determine the amount of styrene injected.

4. Flush the styrene into the tank by piercing the

septum with a hypodermic needle, leaving it to bleed

atmospheric air into the sample tank for 30-60 min. The

fitting may be warmed with an infrared heat lamp during this

operation. When the sample tank is almost at atmospheric

pressure but still under slight vacuum, close the tank and

remove the special fitting.

5. Connect the tank to valve V-4 of the pressurevacuum instrumentation board and pressurize to the desired

value with breathing quality air as described in steps

8-13 for vapor distillation. The tank contents are mixed

for several hours or overnight using a heat lamp directed at

the base to generate convection currents.

4.1.3 Estimation of Target Concentrations

4.1.3.1 Vapor Distillation

The compressed gas mixtures were prepared at the highest

concentrations possible short of causing condensation inside

the sample tanks. The limiting concentration is that at

which the partial pressure of the solvent vapor reaches the

saturation pressure at the temperature of use, which may be

in the range 20-25 °C. (However, if the cylinder is outdoors

on a cold day, condensation may occur with resultant change

of gas composition; and it is not known how long it may take

11
to re-vaporize and mix the contents when the cylinder is

returned to room temperature.) Consequently, the vapor

pressures of the inserted solvents were kept below the 20 °C

levels in most cases to permit the tanks to be unharmed by

temperature excursions somewhat below this value.

Table 2 shows the condensation temperatures of 13

solvents for seven pressures between 1 mm Hg and 2 atm.

Figure 3 plots these data and shows the maximum partial

pressures that can be tolerated in the gas cylinders at any

given temperature. To provide a margin of safety, the

cylinders were filled to somewhat lower partial pressures

so that they might be used at temperatures of 15-18 °C if

necessary

.

The required partial pressure of the solvent vapor

may be estimated from the pressure-volume relationships

obtained by rearranging Eq. (1); or, knowing the partial

pressure to which solvent vapor was permitted to distil, the

equation permits calculation of the pressure to which the

tank should be filled with breathing air for chosen values

of mixture concentration. Table 3 summarizes the calculated

and measured concentrations for the five volatile solvents.

Table 3. Concentrations of volatile solvent mixtures

Average

Partial Cylinder Calculated measured

pressure pressure concentration concentration

Ingredient (mm) (psia) (ppm) (ppm)

Chloroform 65.4 1000 1265 1210

Toluene 27.0 875 597 650

Methylene Chloride 259.0 1000 5000 4850

Ethylene Dichloride 44.3 837 1024 980

Trichloroethane 79.75 786 1963 1940

4.1.3.2 Liquid injection

Since the saturation pressure of styrene is about 5.5 mm

at 20 °C it is desirable that the partial pressure P not

exceed about 4 mm to avoid the possibility of condensation,

especially since it is not known how difficult it may be to

revolatilize the styrene inside the gas tank without heating

and attendant risk of polymerization.

The volume of liquid styrene needed to yield a partial

pressure of 4 mm in a compressed gas tank having an average

volume of 42 liters is given approximately by the expression:

^ ^ ^ 24. 5 X 760^ ^^-^

12

volume of liquid styrene (ml)

partial pressure of styrene vapor = 4 mm

total volume = 42 liters

molecular weight of styrene = 104.14 g/mole

density of styrene = 0,909 g/ml

volume of one mole of ideal gas at 25

and one atm,

atmospheric pressure (mm)

The volume needed is 1.0 ml, or 0.9 g styrene and this

information is useful for choosing the proper size of

syringe ...

Equation (1) may now be used to calculate the total

pressure to which the styrene tanks should be filled to

reach a concentration of 500 ppm. This is approximately

150 psi for the assumed 4 mm vapor pressure, which is not

enough styrene mixture for the required exploratory gas

chromatographic experiments and charcoal tube gravimetric

analyses. Consequently, an additional mixture pressurized

to over 800 psi was prepared for the exploratory experiments. Three gas tanks pressurized to the required

150 psia were prepared also for final analysis and delivery

to NIOSH. .

, Table 4 presents the calculated and measured concentrations for the styrene working standards.

Table 4. Concentrations of styrene gas mixtures

where: V^^

P

V

M

2 4.5

760

Weight Cylinder Calculated Measured

injected pressure concentration concentration

Ingredient (g) (psia) (ppm) (ppm)

Styrene 0.980 843 97 70

Styrene-1 1.025 150 . 562

Styrene-2 1.040 150 571 475

Styrene-3 1.034 150 567)

14
4,2 Charcoal Tube Gravimetry

\.2.1 Summary of Method

Gas mixtures containing organic vapors that are capable

of being quantitatively adsorbed by activated charcoal may

be analyzed for their organic vapor content by determining

the weight gain of the charcoal corresponding to known volumes

of gas. The gas mixtures are sampled at precisely measured

flow rates through tared charcoal tubes of the type used for

air pollution analysis and their weight gain, translated into

volumes of gas sampled, gives the organic vapor concentrations in ppm. Figure 4 is a diagram of the charcoal tube

gas sampling system..

4.2.2 Experimental

1. Select three (3) charcoal tubes that have been

stored in the presence of silica gel. Since the tube ends

are not fire polished, avoid tubes that exhibit rough edges

from which bits of glass can break off and change the tares.

2. Wipe the tubes with a lintless tissue to remove surface film and dust. Thereafter, the tubes may be handled

lightly with bare hands except when gripping them tightly

(as for insertion* into the plastic end caps) when they should

be protected with a tissue.

3. Insert the tubes into holders connected to critical

flow orifices (figure 4) and insert the open ends into a gas

manifold through which breathing quality air is already

flowing from a compressed gas cylinder at a rate about twice

the total sampling rate through all the orifices (e.g. 6-8

L/min for three 1-L/min orifices). Open the vacuum sampling

cock

.

4. After 10-15 min flow turn off the sampling vacuum,

remove the tubes, cap them (use tissue) and set them near

the balance to equilibrate for temperature and humidity.

This may be as long as 1-hr. Disconnect the compressed air

from the manifold.

5. Weigh the tubes. It is recommended that the

balance be equipped with an ionizing device to minimize the

effects of static electrical charges on the glass tubes;

this is especially important on days of low humidity, i.e.

less than 30 percent R.H.

6. Repeat the conditioning treatments until two

successive weighings show that the tubes have attained

constant weight within ± 50 yg. Repetitive conditioning

15
16
treatments are recommended for each new lot of charcoal

tubes to establish the total air pretreatment required for

the tubes to attain constant weight. Thereafter, the tubes

may be conditioned once for this total time. The initial

repetitive weighings for each lot of tubes may be avoided

by conditioning for a long time, say 1-hr, but this is

risky because some lots of tubes may require even longer

conditioning, and some may require only a short conditioning time.

7. Record the constant weight (average of three

readings for each weighing) as the tare. Place the tubes

in separate envelopes with identifying numbers corresponding to the three orifices to be u-sed for sampling.

8. Flush the gas manifold with the contaminant gas

mixture for 3-5 min at 6-8 L/min. Remove the plastic

end caps from the tubes, saving them in numbered envelopes.

Pay special attention to the end caps to avoid mixing them.

Insert the tubes in their correspondingly numbered

orifices while the contaminant gas is flowing.

9. With a stopwatch already running, open the vacuum

cock to the sampling orifices when the pointer passes an

index, and time for exact Iv 2, 4, 10, 20 min etc. to

collect about 6-15 mg on tne charcoal. Hnd the sampling

when the pointer passes the index after the desired time

by turning the vacuum cock to simultaneously shut off the

vacuum and open the orifices to the atmosphere. (It is

not sufficient to merely shut off the vacuum because the

critical flow orifices would continue to sample for

several seconds afterward, until the pressure across them

equalizes.) Shut off the contaminant gas flow.

10. Cap the charcoal tubes with their own end caps,

permit them to equilibrate near the balance (1-hr used

in this laboratory), and weigh them to determine the

sample weight. The tubes may be used for a second sample

collection if their breakthrough capacity has not'been

reached.

11. After the gas sampling experiments are concluded,

return the charcoal tubes to their critical flow orifices

and confirm the sampling flow rates of the orifices plus

charcoal tubes with a wet-test meter. Connect the orifices

one at a time to the downstream side of the meter, leave

the upstream side open to the atmosphere. When the vacuum

sampling valve is turned and vacuum is applied, air flows

through the meter into a charcoal tube and orifice. Time

the flow rates for at least one complete revolution of the

meter pointer.

17
4.2.3 Calculations

The concentration of the organic constituent in a gas

mixture is calculated from the weight of adsorbed material,

sampling time, and flow rate by a simple relationship derived

from the gas laws:

• = RFT (24,450) (3)

where C = concentration (ppm)

W = weight increase of charcoal tube (mg)

M = molecular weight of organic vapor (g/mole)

r = flow rate (liters/min) corrected to STP^

; t = sampling time (min)

24,450 = volume of one mole of gas (ml) at STP^

^Since the flow rates are measured with a wet-test meter

their correction to STP (standard temperature and pressure,

25 °C and 760 mm) are carried out conveniently by application of the wet-test meter equation reproduced here for

convenience

:

r = r . V . 298. 16 . ^bar " ^g ... ^STP "^^m ^Mtr "~T ^60

m

•

where: ^STP ~ '^°^^^ctcd flow rate (liters/min)

r^^ = measured flow rate (liters/min)

^Mtr ~ ^^''-"^^ wet-test meter determined by

calibration = 1.024 liter/rev for meter used

= temperature of water in wet'test meter (°K)

^bar

~ barometric pressure (mm Hg) corrected for

scale expansion referred to 0 °C

P = back pressure on wet-test meter (mm Hg)

Since the critical flow orifices are downstream of the wettest meter during the flow calibrations, the vapor pressure

of water at the measured temperature does not enter into

the equation,
4 . 3 Gas Chromatographic Analysis of Gas Mixtures

The concentrated gas mixtures which have been analyzed

by charcoal tube gravimetry, i.e. the working standards, are

diluted to the desired concentrations with the aid of gas

mixing systems such as have been described in previous

reports. The diluted gas mixtures may be analyzed rapidly

and effectively by gas chromatography to confirm the proper

operation of the gas dilution system. This section describes

the analytical procedures used, and demonstrates the feasibility of using a gas chromatograph to analyze gas mixtures

at all dilutions from the original working standard concentration to the lowest concentrations of interest, about 1/2

TLV.

The response of a gas chromatograph equipped with a flame ionization detector is directly proportional to the

amount of sample introduced. Thus, a single calibration

point is all that is required to permit the gas chromatograph to be used for the required analyses. This calibration

point is provided by the independent analysis of the undiluted working gas mixture. In theory, this linear plot

should pass through the origin since the absence of organic

vapor should correspond with zero peak area. In practice,

the best straight line fit through the data points

approaches the origin closely but it may not intersect the

origin in all cases.

All measurements were made with a gas chromatograph

equipped with a flame ionization detector (FID). The

analytical column was a 1/8-in diameter 6-ft long stainless

steel tube packed with 10 percent OV-17 on Chromosorb W

(AW-DMCS) 80-100 mesh. This column gave sharp peaks and

short retention times for the six solvent vapors examined.

The He flow was 80 ml/min, air flow 800 ml/min. Ha flow

adjusted to maximize the detector response for each vapor.

The FID temperature was 210 °C and the column temperature

was adjusted for each analysis.

The tank mixtures were diluted with air using a gas

dilution system. A tube inserted loosely into the

^See, for example, Hughes, E.E. et al., "Gas Generation

Systems for the Evaluation of Gas Detecting Devices,"

National Bureau of Standards Report No. NBSIR 73-292,

October 1973.

19
output gas manifold was connected to a double-loop GC

sampling valve with 5-ml loops, and thence to the house

vacuum.

The double-loop valve permitted the sample in one loop

to be flushed into the GC while the sample gas flowed

through and conditioned the second loop, thereby permitting

a more rapid sample introduction than could be attained

with a single-loop GC valve.

The sample gas was drawn from the gas manifold at

atmospheric pressure through a loop of the GC valve, at a rate of 600 ml/min as measured by a rotameter. Just before

introducing a sample into the GC, the flow through the

valve was stopped by shutting off the vacuum, A 10-sec

wait allowed the sample to equilibrate at ambient atmospheric

pressure. After the GC valve was turned to flush 5 ml of

sample into the instrument, the vacuum was turned on to draw

additional sample through the second loop. This stop-flow

method was found to give better reproducibility of measurements than the procedure of sampling the gas while it

flowed through the GC valve since it assured that all the

samples were at the same (atmospheric) pressure.

Several determinations were made at each concentration,

starting from the undiluted working standard at 5 x TLV or

higher, down to zero concentration (air only). The results

given in the next section show that the gas chromatograph

response was linear with the gas concentrations as measured

with the flowmeters of the gas dilution system, thereby

demonstrating that the gas dilution flow controllers and

meters were functioning in the expected manner.

5. EXPERIMENTAL RESULTS

5 . 1 Results of Gravimetric Analysis

Tables 5-10 present the results of the charcoal tube

determinations of the vapor concentrations in the working

standard gas tanks. The average gas concentrations rounded

to the nearest 5-10 ppm are summarized in table 11.

The earliest 1973 determinations of chloroform,

toluene and methylene chloride were not included in the

final data averages because tlie early measuring techniques

were relatively crude compared with the later measurements

started in January 1974. Beginning at this time the individual flow rates were measured through each charcoal tube

(instead of using an average flow calibration for the limiting flow orifice). The wet-test meter used to measure

these flows was calibrated by two independent operators

: 20
Table 5. Charcoal tube gravimetry

chloroform

Date

Sample

weight Orifice

Flow

rate

Sampl ing

time

Gas concn

.

Average

concn.

(mg) (1/min) (min) (ppm3 CppmJ

10/16/73 9 .622 PP--C 1 .626 1. 0 1213

9 . 592 PP--c 1 ,626 1. 0 1209

9 . 858 PP-D 1 . 680 1

.

0 1203

9 .854 PP-D 1 .680 i! 0 1203 1207

10/29/73 1 7 tin , J i u PP

.

r r

- r 1 .007 3. 0 1176

17 . 390 PP-F 1 .007 3. 0 1181

17 . 580 PP--F 1 , 007 3. 0 11 X1 Q4*t X X O *f

1/11/74 20 .514 BC-C 1 .7191 2. 0 1222

21 . 239 BC--D 1 . 7881 2. 0 1216 1219

4/29/74 21 .255 BG-D 0 .9051 4. 0 1202

22 . 794 BG-•E 0 .9664 4. 0 1208 1205

Average: 1202 1204

(S.D.-14)

Chloroform Analysis : Average of all 1974 measurements: 1212 ppm (S.D.*9)

Rounded value: 1210 ppm

Table 6. Charcoal tube gravimetry

toluene

Date

Sample

weight

Flow Sampling

Orifice rate time

Gas concn

.

Average

concn.

(mgj (l/min) (min) Cppm} CppmJ

11/1/73 6. 965

7 .110

PP-F

PP-F

1.007

1.007

3.0

3.0

613

626 620

1/21/74 8.400

8.856

BC-C

BC-D

1. 7346

1. 7985

2.0

2.0

643

653 648

1/22/74 8.471

8. 585

8.802

8.913

BC-C

BC-C

BC-D

BC-D

1.7463

1.7463

1 . 8049

1.8049

2.0

2.0

2.0

2.0

644

652

647

655 650

5/1/74 14.031

15.436

BG-E

BG-F

0.9715

1.0486

6.0

6.0

639

651 645

Average 642

(S.D.-13)

641

Toluene Analysis

Average of 1974 measurements

Rounded value:

- 648 ppm

650 ppm

(S.D.-6)

21
Table 7. Charcoal tube gravimetry

ethylene chloride

I/O L C7 Sample

ur 1 I 1 ce Flow Sampl ing Gas Average Wat f7 K t" rate t ime concn. concn.

I. i/ mi n J (min J (ppm) (.ppm)

10/29/73 O • U 17 V/ r r ~ r

1 n n 7 U . 0 4917

o • u ^ o PPr r -- rP 1 n n 7 U . b 4940

8.545 PP-F 1. 007 0.5 4895 4917

1 / 7\ / 1

A

J. / CO/ / H 1ID. J

7 nu 7^ BU - u u . y i i 0 1 . 0 4832

15.461 BG-D 0.9116 1.0 4883

16.365 BG-E 0.9693 1.0 4861

16 410 u . J u y J i . U 4o /4

17.844 BG-F 1.0554 1.0 4867

17.519 BG-F 1.0554 1 . 0 4779 4849

4/30/74 32. 223 BG-E 0.9585 ^ . U 4839

34.973 BG-F 1.0422 2.0 4830 4835

Average

:

4865 4867

(S.D.=45)

Methylene Chloride Analysis

:

Average of 1974 measurements : 4846 ppm (S.D.=33)

Rounded value : 4850 ppm

Table 8. Charcoal tube gravimetry

ethylene dichloride

Sample Flow Sampl ing Gas Averagi

Date weight Or i f ice rate time concn

.

concn

CmgJ tl/min) (min) (ppm) Cppm)

3/12/74 3. 860 BG-E 0. 9696 1 . 0 984

7.725 BG-E 0. 9696 2.0 984

4.100 BG-F 1 . 0519 1.0 963

8.360 BG-F 1.0519 2.0 982 978

3/13/74 15.520 BG-E 0. 9667 4.03 983

16.770 BG-F 1.0434 4.03 985 984

4/30/74 10.608 BG-D 0. 8974 3.0 974

11.415 BG-E 0. 9643 3.0 975 975

Average

:

979 979

Ethylene Dichloride Analysis : Average of all 1974 measurements « 979 ppm (S.D.=8)

Rounded value: 980 ppm

22
Table 9. Charcoal tube gravimetry

1,1,1- trichloroethane

Date

Sample

weight Orifice

Flow

rate

Sampling

t ime

Gas concn

.

Average

concn.

(.i/min J (.min J (.ppm) IppmJ

3/11/74 10.49 BG-E 0.9800 1.0 1962

in A Ck iU . 4y or* c U . y 0 UU 1 .

0

1962

DP C Bb - r

1 n A A 7 1 . UOO

J

1 . 0 1954

11 AC 11.4b or* c Db- r1 A A ^ T 1

.

Odd 3 1 .

0

1968 1961

5/1/74 20.326 BG-E 0. 9719 1.99 1926

21.925 _BG-F 1.0523 1.99 1919 1923

S/2/1A 19.260 BG-D 0.9127 2.0 1934

10.270 BG-E 0.9726 1.0 1935

20.525 BG-E 0.9726 2.0 1934

10.998 BG-F 1.0528 1.0 1915

22.292 BG-F 1.0528 2.0 1940 1931

Avg. Concn. 1941 1938

1,1,1- Trichloroethane Analysis : Average of all 1974 measurements: 1941 ppm (S.D.=18)

Rounded value: 1940 ppm

Table 10. Charcoal tube gravimetry

styrene - low concentration

Sample Flow Sampling Gas Average

Date weight Orifice rate time concn

.

concn

.

(mg) (1/min) (min) (ppm3 (ppm)

3/14/74 11. 20 BC-C 1.7511 22.0 68.3

11.59 BC-D 1.7492 22.0 70.7 69.5

5/2/74 6.235 BG-D 0.9134 23.0 69. 7 6.748 BG-E 0. 9743 23.0 70.7

7.430 BG-F 1.0605 23.0 71.5 70.6

Average Concn. 70 70

Low-Styrene Analysis: 70 ppm (S.D.=1.2)

styrene - high concentration

7/22/74 9.339

9.806

10.541

BG-D

BG-E

BG-F

0.9116

0.9718

1.0605

5.0

5.0

5.0

High-Styrene Analysis

Rounded value

474 ppm (S.D.-7)

475 ppm

481

474

467 474

23
COu

a.

CO

CO

<u. CO

CO<u tooo

CO

0) •H

4J

>-l (Uo

PL4

<u T3C3O

Pi • -o c <u o u c 3 o CO a nJ

0)e

T)

(U U • «0 ca 3 C o oa o

TJ 3

0) CO iH CO iH <U •H M

fa a.

73

60 3 0) •H CT" t-J :2 -H

0) T) tH >^ 0)

tO 3 4J •H CO 1-1 4-J CO (U

>-i (U CO

03 >-i

C 0

Pu

•H

COa

00 • U (U

(U oc o O 0)

4-1

u c C

•H >

rH 0) o

C/3 •H o o o m in 00 ^ CO <y\ o o in

in

cNi in o rH o u o o c^ in in in <u in rH x:

4-1 (U00oH o m o CO o o O o o en 00 in m in T) o 00 o 00 00 rH r-i rH (U iH CO I I I 00

in o <r

CV( vj- CO o o o

»^ 00 1 1 in On 1 1 in CU

>H in O * rH o o o o o o in o rH iH rH .d

in 00 CN in nO CN o »H

V

VO « rH CO 1^ CN iH A rH

>N

m

d - •H o fH O in O in in o o o sO OO iH rH >

in CNI o o* 00

VV

rH CN rH CM rH A

CUoX

CU in o O o O o o O o o O in m in o O in in o o O m o O CN CM rH rH in in in in rH CN o o CO

CM rH in rH cyi rH o o o o O o o O o O m m in CO m o o in in o o CN o O rH

CN in CO rH rH o CM

rH

(30 3

00 cn CO vO rH <r •cr o 00 rH vO CO iH fn rH On rH rH rH in o rH in rH rH

CTi CN -J- OO OO 00 CN CM CO

m rH 0^ 00 o o v£> in rH C?N rH rH rH iH rH

(Uc /~\ c CU

0) d* o d (U T3 •H iJ o d o 4-1 •H u OJ t3 o 0) 4-1 CO

»-l o o o CJ T3 (U •H o rH >-l o •H 0) ^ bO rH x: o x: <<-l t3 0) (U jC o iH > 00 rH •H rH d ea o •H x: o •H 3 M >% 0)

g o rH x: CO

O J2

N rH

•H <^ •H rH 4J

d CCJ o c CU >-l T» JC 0) 0) M

iw (U 0) c H a» o 0) XJ 0) o C rH (U 1 c d d d iH o d rH

0) rH rH <U (U o rH o u •H

O o 3 x: u >> o rH d fa .H rH rH u d CU 4-1 rH •H J3 O (U CO •H CJ <U X! 45 O H w iH cn u > <: O ^1 0

a.ao

0)N

CO •H

(U •HMO

rH

x:od>H

X)

(U

COoo

(X

(U x:H

24
using entirely different methods. Special precautions

were taken to eliminate static electricity effects during

weighings, and temperature and humidity effects were

minimized by adopting a standard operating procedure of

permitting the charcoal tubes to equilibrate for 30-60 min

before every weighing.

It is apparent that the vapor content of the compressed

gas cylinders was relatively stable over the 2-6 months

that measurements were made. The greatest apparent drop in

concentration was a 1.5 percent decrease in t r ichloroethane

concentration from 1961 to 1931 ppm. The standard deviation

of the measurements (S.D. on tables 5-10) was about one

percent of the concentrations.

5 . 2 Results of Gas Chromatographic Analysis

Tables 12 through 17 list the peak areas and retention

times for chloroform, toluene, methylene chloride, ethylene

dichloride, tr ichloroethane , and styrene together with the

gas concentrations corresponding to the flow settings on the

gas dilution system controls. Tigure 5 is a plot of all

the GC responses against the gas concentrations normalized

as fractions of original concentrations of undiluted working standards. The linearity of the curves is a confirmation of the calibration of the flow controllers and meters

on the gas dilution system instrumentation.

The standard deviation (SD) of each group of peak area

measurements shown in tables 12-17 is about one percent

with occasional excursions to 1.4 percent. The 12 ppm

styrene mixture shows the widest variation in peak areas

with a standard deviation of 1.8 percent.

6. SUMMARY AND CONCLUSIONS

A system has been developed for generation of test

mixtures of organic solvent in air. This system consists

of (1) working standards of compressed gases in cylinders,

(2) a means of verifying the composition of these working

standards at any time, (3) a demonstration that these

standards may be diluted to any desired working concentration, which can be rapidly confirmed by gas chromatography.

An absolute method of gas analysis, charcoal tube

gravimetry, showed the working standards to be stable over

periods of 2 to 6 months; that is, gas mixtures measured

over these periods showed no appreciable losses or decomposition. Should losses of vapor ever occur due to sorption

on interior cylinder surfaces, the capability of the

25
6

10

1—Chloroform

2—Sfyrene

3—Toluene

4 —Methylene chloride

5—Trichloroefhane

6—Ethylene dichloride

Fraction of Tank Concentration

FIG. 5 G.C. RESPONSE VS. GAS CONCENTRATION

26
charcoal tube method to measure absolute vapor concentrations

would permit these losses to be measured accurately, so that

the integrity of the working standards would be maintained.

The standard deviation of all the charcoal tube measurements and the gas chromatographic analyses was one percent

except for slight excursions of occasional data points to

nearly 2 percent.

27
Table 12. Gas chromatographic analysis of vapor-air mixtures

chloroform (1210 ppm)

Percent of Concn of

Flow of Flow of working solvent G.C. Average

working diluent standard vapor peak of peak

Date standard in mixture in mixture area

(1/min)

air areas S.D,

[l/min) Tp e rcent) CppnJ

11/13/73 0.055

0. 055

0.22

0.22

0.22

11/16/73 0.22

0.22

11/27/73 0.57

2.12

4.55

3. 59

3.63

1.49

0.653

1,49

0.653

0.22

1.19

1.51

' 5.71

12.9

24.1

12.9

25.2

72.2

100

14.4

18.3

69.1

156

292

156

305

874

1210

4739

4674

4698

5882

5825

5800

5731

4704

5810

22030

22230

22140

22030 22110

51000

51100

51110

51080 51070

101600

102700

103100

103000 102600

50150

50220 50190

103100

103800 103500

327200

327600

327600 327500

460800

462500

461000 461400

33

63

97

50

688

50

500

235

930

Operating Conditions

Column Temperature 70 °C

Gases: He 60 psig. Air 42 psig, H2 11.2 psig

Peak retention times: Air 36 sec, chloroform 89 sec.

28
Table 13. Gas chromatographic analysis of vapor

toluene (650 ppm)

-air mixtures

Percent of Concn of

Flow of

working

s tandard

tl/minj

0.219

0.219

Flow of

d iluent

a ir

Cl/minJ

3.48

2.029

working

s tandard

in mixture

(percent

)

5.92

9. 74

solvent

vapor

in mixture

(ppm)

38. 5

63.3

G.C.

peak

area

13570

13560

14000

13640

13720

23740

23920

23700

23980

Average

of peak

areas S.D,

13700 181

23840 136

0.219 0.89 19.75 128. 48090

48580

48340

49380 48600 450

0.219 0.489 30.9 201 77650

78900

78390

78960 78480 607

0. 575 0.219 72.4 471 196100

198900

199000

199600 198400 1564

0. 93 0. 219 80.9 526 228900

229800 229400 640

2.135 0 100 650 282700

289700

284500

287800 286200 3160

Operating Conditions : Column Temperature 100 °C

Gases: He 60 psig, Air 42 psig, H2 11.2 psig

Peak Retention Times: Air 19 sec^ toluene 82 sec.

29
Table 14. Gas chromatographic analysis of vapor-air mixtures

methylene chloride (4850 ppm)

Percent of Concn of

Flow of

working

s tandard

(1/min)

0.041

Flow of

d iluent

air

(l/min)

2.029

working

standard

i n mixture

Xpercent

)

.1.98

solvent

vapor

in mixture

(ppm)

96

G.C.

peak

area

4505

4574

4463

4576

4471

4546

Average

of peak

areas S.D.

4523 50

0.219 2. 029 9. 74 472 23100

23500

23300

23700

23340

23370 23390 202

0.219 0.489 30.9 1499 75710

78050

75690

77400

76160

77360 76730 1004

0.575 0.219 72.4 3511 174500

176800

174200

177400

175000 175600 1433

2.135 100 4850 241600

238400

244600

236700

242600

236200 240000 3417

Operating Conditions : ^

^

Column Temperature 80 °C

Gases: He 60 psig, Air 42 psig, 11.2 psig

Peak retention times: Air 20 sec, methylene chloride 41 sec

30
Table 15. Gas chromatographic analysis of vapor-air mixtures

ethylene dichloride (980 ppm)

Flow of

working

standard

(l/min)

0.041

Flow of

d iluent

air

(l/min)

2.029

Percent of

working

standard

in mixture

(percent)

1.98

Concn of

solvent

Vapor

in mixture

(ppm)

19.4

G.C. Average

peak of peak

area areas S.D.

1977

1967

1989

1967

2011

1989 1983 17

0. 219 2.029 9.74 95.5 10960

10780

11020

10820

10960

10820 10890 99

0. 219 0.489 30. 9 303 36890

36450

36970

36550 36720 254

0.575 0.219 72.4 710 86020

87120

85960

87950

85610

87290 86660 952

2.135 100 980 120500

118900

122000

118400

120900

119200 120000 1376

Operating Conditions : Column Temperature 75 °C

Others: Same as table 14

Peak retention times: Air 18 sec, ethylene dichloride 83 sec.

31
Table 16. Gas chromatographic analysis of vapor-air mixtures

1 , 1

, 1-trichloroethane (1940 ppm)

Percent of Concn of

Flow of

working

standard

(,1/minJ

0. 041

0.219

Flow of

d iluent

air

(l/min)

2.029

2. 029

working

standard

in mixture

(percent J

.. 1.98

9.74

solvent

vapor

in mixture

(PpmJ

38

^ 189

G.C.

peak

area

3612

3615

3563

3574

3523

3577

18690

18410

18670

18540

18860

Average

of peak

areas

3577

S.D.

34

18630 169

0.219

0. 575

2.135

0.489

0. 219

30.9

72.4

100

599

1405

1940

64600

63270

65320

63900

64 7-2 0

63020

158700

154200

156700

153900

157000

214300

217000

214600

218400

64140

156100

216100

895

1810

1966

Operating Conditions : Column Temperature 90 °C

Others: Same as table 14

Peak retention times: Air 20 sec, tr ichloroethane 50 sec.

32
Table 17. Gas chromatographic analysis of vapor

styrene (475 ppm) ^ -air mixtures

Flow of

working

standard

(1/min)

0.055

0. 219

0. 219

2. 029

2.029

0.489

Percent of

Working

s tandard

Flow of

d iluent

air

(1/min) (percent)

Concn of

Solvent

vapor

in mixture m mixture

(ppm)

2.6

9. 74

30.9

12.4

46.3

147

G.C.

peak

area

3285

3301

3364

3331

3436

31440

31290

31790

31680

105300

105900

106900

107300

Average

of peak

areas S.D.

3343 60

31550 227

106350 915

2.135 100 475 366900

367300

370400

369800 368600 1757

LOW STYRENE CYLINDER LK 152540 70 55220

55070

55310

55370

55490 55290 158

Operating Conditions : Column Temperature 125 ''C Gases: He 60 psig, Air 40 psig, 13.5 psig

Peak retention times: Air 20 sec, styrene 75- 76 sec.

^Three styrene cylinders connected together by a gas manifold.

^Low value, should be about 8,000 by comparison with other data

points.

33

USCOMM NBS-DC
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUni.K A l iON OR RI:P0R 1 NO. NBSIR 74-573

2. Ciov 't Acifssion

N...

3. Recipient's Accession No.

4. Ti ri.I'; AND sun I I I !. I-;

Development of Contaminant Generation Systems for

Certification of Portable Air Sampling Instruments

5. I'ublication Date February 1975

6. Performing Organi/aiion (ode

7. A[;iHOK(S)g^ C, Cadoff, B. Greifer, P. A. Pella and 8. Performing Organ. Report No. NBSIR 74-573

9. Pi;RI ()RMlI^!/,*()l<^iANlT'^yi?)R'RAMl-: AND ADDRl-SS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project /Ta.sk/Work Unit No.

11. f ontract/Grant No.

12. Spcinsoring ( )r>/. .iii i/ at urn Name and < onipleto Address (Strt^ct, Cily. State, ZIP)

National Institute for Occupational Safety ^ Health

Testing and Certification Laboratory

Morgantown, West Virginia

13. Type of Report & Period Covered

Annual Report

14. sponsoring Agency (--ode

15. SUPPl.I'MI'Nl ARY NOri:s

16. AHS'IRA( r (A 200-word or less (actual summary of most si/fpifn ant in formation. If document includes a significant

bi bl iographv or literature survey, mention it here.)

The development of gas generation systems is described. Stable mixtures

of contaminant gases with air are stored in compressed gas cylinders at

concentrations about five times the TLV, The cylinders are analyzed by

charcoal tube gravimetry in which the weight increase of tared charcoal

tubes is used to determine the organic component of the gas mixture. For

use, the compressed gases are dynamically diluted with air to the needed

concentrations in a gas-flow system.

Data are presented to demonstrate (1) linearity of the gas dilution

system as measured by gas chromatographic determinations of vapor concentrations; (2) precision of charcoal tube gravimetric measurements; and

(3) stability of compressed gas mixtures of air with chloroform, toluene,

methylene chloride, ethylene dichloride, 1 , 1 , 1 - trichloroethane , and

styrene at concentrations of 1210, 650, 4850, 980, 1940, and 475 ppm

respectively.

It is concluded that the methods of preparation and verification of these

compressed vapor-air mixtures have general applicability. A wide

variety of stable single-component vapor-air mixtures may be prepared and

analyzed to one percent accuracy rapidly and routinely.

17. KhY WORDS (six to twelve entries, alphabetical order, ( apitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Air sampling; chromatography; industrial hygiene; gas analysis; gas

standards

18. AVAII.AHIl.ri Y Unlirniie-I 19. SECURITY CLASS

(THIS RLPORT)

21. NO. OF PAGES

K !•Or Official Distribution. I^o Not Release to N i IS UNCLASSIFIED

35

[.: Order I' roni Sup. of Doc, U.S. Cjovernmerit

Washintton. D.( . 20402. SD Cat. No. ( M

'r 1 lit I nj,'. Office 20. SECURII Y CLASS

(THIS PAGE)

22. Price

r Order ! rom National 1 echnical Inform. ir mii Springfield, Virginia 221'il Service IS) UNCLASSIFIED

USCOMM-I3C 29042-P74