NBSIR 73-259

A Gas Dilution System for Methyl Bromide

Eugene P. Scheide, Ernest E. Hughes, and John K. Taylor

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 73-259

A GAS DILUTION SYSTEM FOR METHYL BROMIDE

Eugene P. Scheide, Ernest E. Hughes, and John K. Taylor

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

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

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

A Gas Dilution System for Methyl Bromide

ABSTRACT

A system capable of producing well-defined

test atmospheres of methyl bromide in air (or

any other desired diluent) and an analytical

system for the analysis of these gas mixtures

is described. Using a bulk mixture of 1000 ppm

methyl bromide in nitrogen, and accurately

blending this with a stream of clean, dry air,

concentrations between 5 and 100 ppm can be

produced. This system provides a means of calibration of the various analytical systems for

methyl bromide now in use.

1 . INTRODUCTION

This report describes a system capable of producing

well-defined test atmospheres containing concentrations

of methyl bromide for the calibration of analytical instruments and evaluation of analytical methodology. The system

developed and described was designed to produce concentrations in the industrial hygiene important range of 5 to

100 ppm but other concentrations could be achieved by

suitable adjustment of the operational parameters.

2. EXPERIMENTAL

Methyl bromide is a relatively stable substance so that

little difficulty is encountered in preparing low concentration

mixtures of it in air by dilution of high concentration mixtures. Furthermore, high concentration mixtures of methyl

bromide in air prepared at high pressure in gas cylinders appear

to have sufficient stability so that frequent re-analysis is

not required. Consequently, the simplest system for providing

a variety of concentrations of methyl bromide in air consists

of a cylinder at high pressure containing a known concentration

of methyl bromide and a system for diluting this high concentration with accurately measured volumes of air. Systems for

the accurate blending of gas mixtures have been built and

evaluated and it is intended that such systems will be used

for the dilution of methyl bromide.

The dilution system is described in detail elsewhere [1]

and is shown schematically in Figure 1. It consists of two
controlled flow systems, one for the diluent air and one for

the bulk mixture of methyl bromide, two flow meters for

measuring the flow of each gas stream, a mixing chamber where

the two streams are combined, and a manifold from which the

mixture may be withdrawn for analysis or for calibration

purposes

.

2 . 1 Gases, Gas Mixtures

A pressurized cylinder containing a bulk mixture of methyl

bromide in nitrogen was prepared manometr ically to have a methyl bromide concentration of 1000 ppm. The cylinder was

pressurized to a total pressure of 1200 psi corresponding to

3600 liters of the 1000 ppm mixture at standard conditions. The

mixture also contained 350 ppm of propane as an internal

calibrant

.

The source of air for dynamic dilution of the bulk mixture

can be either a pressurized cylinder of "breathing" air or

"house" air from the bench. In either case, the air is pretreated by passing through a silica gel-charcoal dryer to

remove major contaminants.

2 . 2 Analys is

A gas chromatograph with a flame ionization detector can

be used to analyze the concentration of either the bulk mixture

of methyl bromide or any of the dilutions produced with the

system. The dilution, sampling and analytical systems are

shown diagramatically in Figure 2. Sampling is accomplished by

inserting a Teflon tube, connected at one end to a gas sampling

valve, into one port of the sampling manifold. Samples are

drawn through the gas sampling valve by suction, and are then

injected into the gas chromatograph by the carrier gas.

The recommended gas chromatographic operating parameters

are as follows:

column 6 ft x 1/4 in Porapak Q,

80-100 mesh, #5750

column temp. 145 °C

detector FID at 225 °C

lie flow 50 ml/min

sample 10 ml

sampling rate 40 ml/min
Precautions should be taken to optimize the detector

sensitivity before each set of analyses. A significant decrease

in the detection signal for samples of the same concentration

is an indication that the detector must be optimized again.

The decline in detector response is not the same for methyl

bromide and propane hence the need to optimize the detector

each time it is used.

2 . 3 Calculation of Bulk Mixture Concentration

The bulk mixture concentration is calculated using the

following equation:

' MeBr g.c. peak area n fr n C, = jz ^ t

t x R x propane concentration. (Eq. lj

b Propane g.c. peak area r r

where

,

C, = bulk concentration of methyl bromide (ppm)

.

R = g.c. response factor (ratio of signals for equal

concentrations) = 3.309 (experimental value).

P'or example, using a bulk mixture of methyl bromide containing 350 ppm propane, if the propane g.c. peak area = 4220

and the methyl bromide peak area = 4045, then the bulk mixture

concentration of methyl bromide would be 1110 ppm, e.g.,

4 04 S C, = x 3.309 x 550 = 1110 ppm.

The methyl bromide concentration of the various dilutions

is calculated using Equation (2).

F, • C,

C x

= f" + F . « 2 > b air

where, C = methyl bromide concentration produced in the

manifold (ppm)

.

F, = measured flow rate of bulk mixture (cc/min)

,

F . = measured flow rate of diluent air (cc/min)

.

air . *tV i

• j i

= concentration of bulk mixture (ppm).

3
For example, using a 1000 ppm bulk mixture of methyl

bromide and a methyl bromide flow of 30 cc/min, an air flow

of 5970 cc/min would be needed to produce a 5 ppm mixture in

the manifold;

F

_ F

b

' C

b _ (50) (1000)

'air C

F

b [51 ( h

x

v J

air = 59 7 0 cm^/min.

It is recommended that the sample flow rate be maintained at a fixed value and only the air flow be varied in producing various

methyl bromide concentrations.

3. PERFORMANCE EVALUATION

3 . 1 Conditioning of Gas Cylinders

Pressurized steel cylinders containing gas mixtures of

methyl bromide in nitrogen do not need to be preconditioned before

use. The stability of the 1000 ppm bulk mixture is good showing

less than 1 percent decrease per month.

3 . 2 Tank Stability

Figure 3 shows a plot of concentration of methyl bromide

versus time. The bulk mixture contains propane in addition to

the methyl bromide. The propane (350 ppm), serves as an internal

standard by which changes in concentration of the methyl bromide

may be recognized. Many measurements in this laboratory have

shown propane mixtures to be stable.

3 . 3 Analysis of Bulk Mixtures and Dilutions

The bulk mixtures and dilutions were analyzed by gas

chromatography as described earlier. Table 1 shows the results

of replicate analyses of one of the dilutions taken from the

sampling manifold. The precision of these analyses is very

good indicating that the analytical system and sampling technique

are reliable.

Various mixtures of methyl bromide in air between 5 and

100 ppm were prepared and analyzed by gas chromatography. Figure

4 shows a plot of g.c. signal versus methyl bromide concentration.

The plot is linear and the g.c. sensitivity is sufficient to

analyze all of the dilutions directly.

4
3.4 Calibration

Methyl bromide calibration mixtures were prepared by

conventional manometric measurements.

4 . CONCLUSIONS

The system described in this report is capable of producing well-defined test atmospheres of varying concentration

of methyl bromide in air by blending a bulk mixture of methyl

bromide in nitrogen from a pressurized cylinder with a stream

of diluent air. Analysis of the bulk mixture and the dilutions

can be easily performed by gas chromatography. This gas generation system should provide a means of calibrating the various

analytical systems for methyl bromide now in use.

5 . REFERENCE

1. E. E. Hughes, W. D. Dorko, E. Scheide, J. K. Taylor,

Gas Generating Systems for the Evaluation of Gas Detecting

Devices, NBS Report NBSIR 73-292, October 1973.

TABLE 1 Precision of Gas Chromatographic Analysis

of Methyl Bromide a

Run Propane signal Methyl bromide signal MeBr /propane

1 252,8 00 248,500 0.983

2 252,500 248,300 0.983

3 252,600 248,500 0.984

4 252,700 248,300 0.983

Ave. 252,650 248,400 0.983

Std. Dev. 12 9 115 0.00058

Rcl. Std. Dev. 0.051°* 0 . 046% 0. 0 59%

a Success i ve measurements made on a 64 ppm mixture generated

by dynamic dilution of the bulk mixture.

5
6
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<D tO ^ -H o x •H r-l e « o

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CO CTj

•H 7
•H

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s

CD

CO

o

6

Q

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03

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0 20 40 GO 80 100 120

METHYL BROMIDE CONCENTRATION (ppm)

Figure 4. Response of gas chromo tagraph to various concentrations

of methyl bromide.

9
BIBI- :-raphic 0 VTA

S HE; E T NBSIR 73-259

:"l NO. I. li v.-'t Ac; (".Men No. TLL A Nil -l Li i 1 i i.

A Gas Dilution System for Methyl Bromide

3. Recipient's Access inn No. j

5. Publication Date October 1973

6. Performing Or t ;.iniz.>t ion V edc

7..-Ain-uoRis) H. P. Scheide, E. E. Hughes, and

J. K. T.ivlor

8. Performing Organization

NBSIR 7.3 - 2 5

9

9. PERFORMING OiU.ANT/.A 1 ION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/ Task, 'Aork Uni

11. Contract/Grant No.

12. Sponsoring Organization Name and Address

National Institute for Occupational Safety

and Health

Cincinnati, Ohio

13. Type of Report &i Period Covered

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 2u'J-uord or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) A system capable of producing well-defined test atmospheres of,

methyl bromide in air (or any other desired diluent) and an

analytical system for the analysis of these gas mixtures is

described. Using a bulk mixture of 1000 ppm methyl bromide

in nitrogen, and accurately blending this with a stream of clean,

dry air, concentrations between 5 and 100 ppm can be produced.

This system provides a means of calibration of the various analytical

systems for methyl bromide now in use.

17. key words (Alphabetical order, separated by semicolons) Gas Chromatography Analysis; Gas Dilution System, Methyl Bromide; Occupat ional°Safety

.

18. AVAILABILITY STATEMENT

I I UNLIMITED.

NTIS.

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

' UNCLASSIFIED

21. NO. OF PAGE;

11

22. Price

USCOMM-OC tt.<4.PM