AlllOb EbESll I Publication

480-43

Reference

NBS

Publications

Hydrocarbon Vapor

Standards for Performance

Evaluation of Combustible

Gas Detectors

Law Enforcement

Technology

U.S. DEPARTMENT OF

COMMERCE

National Bureau of

Standards

.C LOO

J57

,^0.^80-^3

L981
ABOUT THE TECHNOLOGY ASSESSMENT PROGRAM

The Technology Assessment Program is sponsored by the Office of Development, Testing, and Dissemination of the National Institute of Justice (NIJ), U.S. Department of Justice. The program responds to the mandate of the Justice System Improvement Act of 1979, which created NIJ and directed it to encourage research and development

to improve the criminal justice system and to disseminate the results to Federal, State, and local agencies.

The Technology Assessment Program is an applied research effort that determines the technological needs of

justice system agencies, sets minimum performance standards for specific devices, tests commercially available equipment against those standards, and disseminates the standards and the test results to criminal justice agencies

nationwide and internationally.

The program operates through:

The Technology Assessment Program Advisory Council (TAPAC) consisting of nationally recognized criminal justice

practitioners from Federal, State, and local agencies, which assesses technological needs and sets priorities for research programs and items to be evaluated and tested. The Law Enforcement Standards Laboratory (LESL) at the National Bureau of Standards, which develops

voluntary national performance standards for compliance testing to ensure that individual items of equipment are

suitable for use by criminal justice agencies. The standards are based upon laboratory testing and evaluation of representative samples of each item of equipment to determine the key attributes, develop test methods, and establish minimum [)erformance requirements for each essential attribute. In addition to the highly technical standards, LESL

also produces user guides that explain in nontechnical terms the capabilities of available equipment.

The Technology Assessment Program Information Center (TAPIC) operated by the International Association of Chiefs of Police (lACP), which supervises a national compliance testing program conducted by independent agencies.

The standards developed by LESL serve as performance benchmarks against which commercial equipment is measured. The facilities, personnel, and testing capabilities of the independent laboratories are evaluated by LESL

prior to testing each item of equipment, and LESL helps the Information Center staff review and analyze data. Test

results are published in Consumer Product Reports designed to help justice system procurement officials make

informed purchasing decisions.

All publications issued by the National Institute of Justice, including those of the Technology Assessment Program, are available from the National Criminal Justice Reference Service (NCJRS), which serves as a central information and reference source for the Nation's criminal justice community. For further information, or to register with NCJRS, write to the National Institute of Justice, National Criminal Justice Reference Service, Washington, DC

20531.

Paul Cascarano, Assistant Director National Institute of Justice

ACKNOWLEDGMENTS

This report wa.s prepared by the Law Enforcement Standards Laboratory of the National Bureau of Standards under

the direction of Paul H. Krupenie, and Lawrence K. Eliason, Chief of LESL. This work was sponsored by the National

Institute of Justice, Lester D. Shubin, Standards Program Manager.
NBS Special

Publication

480-43

NiTlONAL BOBSA^

OF STANDABUa

NOV 2 1981

Hydrocarbon Vapor

Standards for Performance

Evaluation of Combustible

Gas Detectors

by

James E. Brown

Center for Fire Research

National Engineering Laboratory

and the

Law Enforcement Standards Laboratory

National Engineering Laboratory

National Bureau of Standards

Washington, DC 20234

prepared for the

National Institute of Justice

U.S. Department of Justice

Washington, DC 20531

Issued October 1981 U.S. DEPARTMENT OF COMMERCE,

Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS

Ernest Ambler, Director
Library of Congress Catalog Card Number: 81-600135

National Bureau of Standards

Special Publication 480-43

Nat. Bur. Stand. (U.S.), Spec. Publ. 480-43, 15 pages

CODEN; XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1981

For sale by the Superintendent of Documents

U.S. Government Printing Office, Washington, DC 20402

Price $2.00

(Add 25 percent for other than U.S. mailing)
FOREWORD

The Law Knforcetiient Standards Laljoratory (l,KSI,) ol tlu' National I5iirraii ol Slandard-

(NBS) furnishes technical support to the National Institute of Justice (NIJ) jirofirain to strengthen law enforcement and criminal justice in the United States. LESI.'s function is to conduct research

that will assist law enforcement and criminal justice agencies in the selection and prociucincnl of quality equipment.

l.KSL is: (1) Subjecting existing equi|)ment to ial)()ratorv testing and evaluation and (2) conducting research leading to the development of several series of documents, including national voluntary equipment standards, user guides, and technical reports. This document is a law enforcement equipment report developed in LL.SI, under the sponsorship of NIJ as part of the Technology Assessment Program, which is described on the

inside front cover of this report. Additional reports as well as other documents are being issued under the LESL program in the areas of protective equipment, communications equipment,

security systems, weapons, emergency equipment, investigative aids, vehicles, and clothing.

Technical comments and suggestions concerning this report are invited from all interested

parties. They may be addressed to the Law Enforcement Standards Laboratory, National Bureau of Standards, Washington, DC 20234.

Lawrence K. Eliason. Chief

Law Enforcement Standards Laboratorv

iii

CONTENTS

Page

Foreword iii

1. Introduction 1 2. Selected Hydrocarbons for Va|)or Standards 2

3. Instrumentation , 2

3.1 Standards Generator 2

3.2 Gas (^hromatograph (GC) 4

4. Generation of Selected Hydrocarbon Vapor Standards 4

4.1 Calibration of Diffusion Tubes 5

4.2 Method for the Determination of the Vapor Standard Concentrations 7

5. Conclusions 11

6. References 11

V

HYDROCARBON VAPOR STANDARDS FOR PERFORMANCE

EVALUATION OF COMBUSTIBLE GAS DETECTORS

James E. Brown*

Niiliiiniil Hiiri-iiii (if Sliiiiddrds. II (laliiiifildii. /)(. 202:1

1

(Calibration mixtures of hydrocarbon vapor were prepareil for the calibration and performance evaluation of portable combustible gas detectors which are used to detect the presence of accelerant vapors in cases of suspected arson. The standards contained vapors of aliphatic and aromatic compounds common to petroleum

distillates such as gasolines, which are frequently employed as accelerants in arson fires. These standards were

prepared by a dynamic dilution technique in which the vapor evolving from diffusion tubes containing liquid hydrocarbons was diluted with air. The concentrations of the vapors were calculated from gravimetrically determined diffusion rates of the hydrocarbons and from measured flow of air at three temperatures: 30°, .'SO", and 70 °C. (ias chromatographic

analyses of the vapor-air mixtures generated at 30° and 50 °C showed that the vapor concentrations are

essentially constant within measurement periods of 1 to 3 d and from one period to another, and that they agreed with calculated concentrations.

Key words: accelerants; arson; combustible gas; diffusion rates; diffusion tubes; detectors; gas chromatography;

hydrocarbons; vapor detectors; vapor generator; vapor standards.

1. INTRODUCTION

This is a final report on the research and evaluation of a generator of hydrocarbon vapor

standards. A summary is presented of the research resuhs obtained during the period of January 1 through October 30, 1979.

The purpose of the research was to develop standards of low concentrations of organic vapors

that could be used by fire investigators to calibrate and evaluate the performance of hydrocarbon

vapor detectors, also known as combustible gas detectors or "sniffers." Until this work, there

existed no readily available means for calibrating the sensitivity of these detectors. Numerous jurisdictions have initiated or are anticipating major programs to investigate

suspicious fires. These programs will include the use of scientific techniques in the investigations of fire scenes when arson is suspected, and may involve the use of combustible gas detectors. The

most common accelerants for initiating incendiary fires are reported to be petroleum distillates, gasoline being the most available and most frequently used [1]'. Hence, there is need for calibration standards to evaluate the response of the detectors to low concentrations (parts per

million, ppm) of hydrocarbon vapors which are common to petroleum distillates. The sensitivity of most effective vapor detectors is variable. In addition, the sensitivity of some detectors will decrease by an unpredictable factor when the sensing element of the detector becomes contaminated, through continued use, with residues from particulate matter or with vapors of organo-metallic compounds (e.g., tetraethyl lead in leaded gasoline) which ina\ be

*Onter for Fire Research, National Kngineering Laboratory.

' Niinil>ers in brackets indicate literature references in section 6.

1
encountered in the course of repetitive fire-scene investigations. Knowledge of the vapor detector performance by the investigator is essential, because the primary use of these devices is for the

location of accelerant residues in incendiary fire debris, and the location of residual accelerants is a step in providing the physical evidence necessary to establish the crime of arson [2]. Thus, the detector performance must be reliable, and reliability can be assumed through frequent

calibrations using readily available standards. In this report a method is described for generating

predictable concentrations of hydrocarbon vapors at constant rates for long periods of time. The

report presents data on gravimetrically measured rates of diffusion of hydrocarbon vapors through

precision bore tubing. In all cases, the vapors evolved from liquids maintained at constant temperature. Gas chromatographic analyses of the vapors at various intervals within a diffusion period are also reported, in order to confirm the constancy of the vapor composition.

2. SELECTED HYDROCARBONS FOR VAPOR STANDARDS

For the preparation of a vapor standard, four relatively pure compounds were selected from

which vapors could be generated simultaneously and at constant rates. The selection of the four compounds was based on the following considerations:

1) the compounds are constituents of most gasolines,

2) the vapor pressures are such that, at convenient temperatures, variable concentrations can be generated which approximate the concentrations that would be found at fire scenes

at which accelerants had been employed, and

3) the compounds have vapor pressure-temperature relationships such that the vapors are produced at concentrations within the detectable limits of commercial vapor detectors.

It is recognized that gasoline formulations [3,4] contain both aliphatic and aromatic

compounds, and that both of these classes of compounds should be included in the vapor

standards. Consequently, four commercially available compounds were selected. Spectrograde isooctane

(2,2,4 trimethylpentane) and «-hexane (99 mole percent) were chosen to represent, respectively,

branched and linear chain-aliphatic hydrocarbons. Scintillation-count-grade toluene and o-xylene were chosen to represent the aromatic hydrocarbons. The purity of these compounds was presumed to be that claimed by the manufacturer. The vapor pressures of these compounds are such that they may enable detection by sniffers, as the vapors evolve from the liquids absorbed in a variety of substrates [5]. Similarly, the vapor pressure-temperature relationships of the selected compounds are such [6] that vapor standards of various concentrations can be generated easily

from the pure liquids.

3. INSTRUMENTATION

3.1 Standards Generator

A survey of the literature on commercial vapor generators showed that instruments are

available which, with slight modifications, could meet the requirements for the generation of dynamic vapor standards for the evaluation of combustible gas detectors. Based on this search, a vapor generator. Analytical Instrument Development Model 350 Standards Generator", was chosen

and modified. This involved replacing the diluent (air) flow meter normally provided on the instrument with one that can measure a diluent flow of up to 2 L/min. With this instrument,

" Ortain commercial e(]uipment. Instruments, or materials are identified in this paper in order to adequately specify the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau

of Standards, nor does it imply that the equipment or material identified is necessarily the best available for the purpose.

2
vapors are generated from liquids contained in diffusion or permeation tubes, wfii. l. ar<- als<, available from the manufacturer of the generator. For this work, the diffusion tube tc( li,ii<|uc was

chosen as the most appropriate. A cross-sectional view of a precision bore diffusion tube and its reservoir containing a liquid sample is shown in figure 1. The liquid reservoir has a capacity of about 4 milliliters (mL); tube bores are available with diameters of 0.5, 2, or .5 mm and lengths to approximately 8 cm.

<— Diffusion tube

-*— Bulb

Liquid Sample

hl(.l UK !. Crii.ss-.seilioiiii/ ilidgnini of ii dijfusion tiihc.

Vapors evolve through the diffusion tube at a constant rate when the liquid is maintained at constant temperature (isothermal) and constant pressure (isobaric) conditions. These conditions are achieved in the sample chamber of the standards generator. As many as four diffusion tubes can be accommodated simultaneously. A diluent gas, for example, air, flowing at a controlled rate is used to dilute the evolving vapor to pre-selected concentrations. A schematic diagram of the flow system is illustrated in figure 2. Air under pressure enters the instrument at the inlet and then

passes through a pressure regulator and is adjusted to a lower working pressure. The air passes through a charcoal filter to remove common contaminants, to yield essentially hydrocarbon-free

air. Next, the air is split into two streams, one providing diluent and the other flowing through the chamber. Both are regulated by valves and are monitored by rotameters. The chamber air is brought to thermal equilibrium with the chamber by passing it through a heat exchanger before it enters the chamber containing the diffusion tubes.

3
The air from the chamber passes through a three-way solenoid valve which directs the flow

either to the sample and vent ports or to the vapor outlet trap. The vapor outlet trap is used when

the generator is operating in a standbv mode.

The flow through the chamber is continuously variable from about 50 ml>/min to 1.50 mL/min. The diluent flow is variable from 0 to about 2 L/min. The chamber is operated at a fixed temperature of either 30°, 50°, or 70 °C. GAGE

INLET

CHARCOAL

TRAP

VENT SAMPLE

3.2 Gas Chromatograph (GC)

The chromatograph used in this study was a commercial instrument equipped with dual

flame-ionization detectors, which were operated at 250 °C. The glass GC column, about 2 m in length and 2 mm i.d., was packed with 3 percent SE30 on a solid support (supelcoport). The

column was maintained at 50 °C while the carrier gas, helium, flowed at 30 mL/min. The injection

ports were maintained at 150 °C. The GC was interfaced with a recording integrator

(printer/ plotter) from which the retention times and peak areas of the hydrocarbons were

obtained.

4. GENERATION OF SELECTED HYDROCARBON VAPOR STANDARDS

The diffusion rate r, in g/s, of hydrocarbon vapors through a tube has been described [7,8] as shown in eq (1):

r = (DMP/RT) {A/L) In P/{P-p) (1)

where D, M, P, R, and T respectively are the diffusion coefficient (cm"/s), molecular weight

(g/mol), of the compound, total pressure (atmospheres), gas constant (cm*-atm/mol-K) and the absolute temperature (K). The partial vapor pressure in atmospheres of the compound at the temperature T is represented by p, and A/L (cm) is the ratio of the area to length of the diffusion

tube. Hence, eq (1) may be used to predict two important factors in the preparation of dynamic

vapor standards by the diffusion method: (1) the rates and length of time that the hydrocarbon

vapors may be generated through diffusion tubes, and (2) the constancy of the diffusion rates. In addition, a theoretical issue remains: Does the observed data fit eq (1)? 4
4.1 Calibration of Diffusion Tubes

About 2 mL of the hydrocarhon were weighed into a diffusion tube and jtlaced in the chamber of the generator for 1 to 3 d at a selected temperature. During this time, the air flowing

through the chamber to remove the evolving vapor was maintained at a rate of 100 mlVmin. The

diffusion rate of each component was calculated from the gravimetrically measured lirjuid-wcight

loss during this period.

Table 1 summarizes the calibration of the diffusion tubes at 30 °C. For each componnd. the same diffusion tube was used repetitively for its rate determinations (runs). The dimensions lA the diffusion tubes are listed for each compound. Averaged rate values are shown for all measurements, although it appeared that temperature equilibrium was not obtained in runs I aiid 2 until a significant amount of the measured diffusion time had elapsed.

TmiM: I. Dil'fiision nilfs of sclcclcd Ini/riiniihin.s ill .'10 "C

Diffusion rale. )Llg/ min

1 )iriieii>i(iii> (li 1 iilie:

ln>iil<e (liaiiietfi (riirii) 2 2 3 5

l.enfjth (mm) 76 62 38 34

Run 1 ime (min) «-Hexane /.sooclane Toluene o-Xylene

1 1263 14.2 5.4 33.8 8.8 2 1575 15.2 6.2 33.9 9.2 3 1350 16.1 7.1 34.4 9.3 4 4345 16.9 7.2 37.5 9.4 5 4340 16.8 7.0 37.4 10.0

Average ± standard

deviation (Runs 1-5) 15.8±1.1 6.6±0.8 35.4+1.9 9.5±0.5

Average ± stamlard

deviation (Runs 3-5) 16.6±0.4 7.1±0.1 36.4±1.8 9.8±0.3

TAlil.K 2. Diffusion rales o)' selected lixdrorii rl>ons III 50 °C

Diffusion rate, jag/min

Dimensions of tube: Inside diameter (mm) 2 0.5 5 5 Length (mm) 76 12 38 34

Run I ime (min) /(-Hexane Aooftane Toluene o-Xylene

1 1360 43.0 6.4 91.4 25.8

2 1585 43.5 6.7 92.2 27.3

3 4325 43.7 6.6 91.7 28.0

Average ± standard

deviation 43.4±0.4 6.6 ±0.1 9 1.8 ±0.4 26.9 ±1.5

Results for the diffusion rates obtained at .50 °C are shown in table 2. Note that in this case, the tsooctane diffusion tube had an i.d. of 0.5 mm and a length of 12 mm. Table 3 gives an

estimate of the diffusion rates at 70 °C. Only one measurement was made at 70 °C because thermal

equilibrium between the liquid in the reservoir and the chamber could not be achieved, since the

boiling point of hexane is 69 °C.

5
The results listed in tables 1-3 show that, for the same tubes, raising the temperature 20 °C

results in an expected increase in the rates at which the vapors are generated. The predictable

temperature effect on the rates of the vapor generation can be understood quantitatively by

calculating the ratio of the vapor diffusion rates at 70°, .50°, and 30 °C. The results of these

calculations are given in table 4.

I'viii 1, 3. Dtjllisintl rales (if .v(7iT/(v/ Inilriicdrliiiiis iil 7ll°('

Difliisidri ralfs. /Xf;/ riiiii

'

1 iiiie (ruin) /i-He\ane /.vjiiiclaiip Tdliiene (i-\\lene

1 445 1.{()'' 17.9 2:54 71

riie cliliusldii tiihe-i listed In tal)le 2 were also used In these measnrements.

'' A|i[ir()\lmate \ alue.

TmiI I-: 4. R/iliii.s iif diffusion rules ill lempernturc inlervals of 20 °(.

rem|ieratures (°(^) «-He\ane /.sYjoctane Toluene f-Xvlene

5l)/3() 2.6 2.5 2.7 70/50 2,7 2.6 2.6

' Not calculated.

The magnitudes of the observed changes in the diffusion rates of n-hexane and toluene were compared with the expected rate changes based on eq (1), using known values of diffusion constants [7| and vapor pressures [6] at 30° and 50 °C. These comparisons were made as a means

of evaluating the observed temperature dependence of the diffusion rates. For these calculations, diffusion coefficients of 0.0804 cm"/s and 0.0903 cm"/s were obtained for hexane diffusion through a 0.11-cm i.d. tube, respectively, at 30° and 50 °C. Similarly, diffusion coefficients of 0.0932 cm"/s and 0.0993 cm"/s were obtained for toluene diffusion through a 0.78-cm i.d. tube

at the respective temperatures. The vapor pressures of «-hexane are 0.241 atm (24.4 kPa) at 30 °C and 0.525 atm (53.2 kPa) at 50 °C; and for toluene, the pressures are 0.048 and 0.121 atm (4.86

and 12.3 kPa) at the indicated temperatures. The total pressure was assumed to be one atmosphere (~10' Pa) in all cases. The calculations show that the ratio of the diffusion rates of hexane at 50 °C to that at 30 °C is 2.85, and the ratio of the rates for toluene vapor is 2.63 at the same temperatures. These calculated values agree within 10 percent with the experimental values

of 2.6 and 2.5, respectively. The agreement indicates that the experimentally determined diffusion

rates agree with the theoretical ratios predicted by eq (1). The useful life of a specific diffusion tube as a vapor generator may be estimated from the

diffusion rate and sample volume. This period, the life expectancy in days, was calculated for 2 mL (about one-half of capacity) of each compound contained in the referenced diffusion tubes,

using the diffusion rates reported in tables 1-3. The results of these calculations are listed in table

5. Although the effect of the tube dimension on the diffusion rate was not investigated in this study, eq (1) indicates that the rate should be proportional to the square of the tube diameter and

inversely proportional to the tube length. Thus, diameters of 0.5, 2, and 5 mm should change the

life expectancy for a specific length in the ratio of 100:16:1 respectively.

6
'I'MM.K 5. Hdlf-lifc of •.'(ijiiir f;cniT(iltirs (in iliixs)

(il viiridi/.s Irrii/M'mliirc.s

Temperature (°(;) /7-Hexane /.Wdclane

"

Toluene i-Xylene

30

50

70

55

21

135 14653

13

125 45

17

' For 30 °C the diameter of the fliffusion lube was 2 mm and its lenjith was 62 mm; for 50° and 70 °C the tube diameter was 0.5 mm and its length was 12 mm.

4.2 Method for the Determination of the Vapor Standard Concentrations

The concentrations of the selected hydrocarbons in the vapor standards were calculated from

the gravimetrically determined diffusion rates and the air flow. The air flows measured with a bubble meter were frequently monitored by the rotameters for constancy.

Although the GC analysis measured the vapor concentrations, the primary use of this analysis was to establish constancy of the vapor concentrations within a run, and from run to run for a given set of experimental conditions. The response of the chromatograph was calibrated with 0.2 percent (v/v) solutions of the selected hydrocarbons in carbon disulfide. Figure 3 demonstrates the response of the GC and the column resolution for a 2-/xL sample of the carbon disulfide solution

injected from a 10-ju,L liquid syringe. The GC was calibrated for each hydrocarbon. The

calibrations were based on the assumption that the hydrocarbons in the carbon disulfide mixture

would occupy a volume predictable from ideal gas behavior when vaporized at 25 °C. The results of this calibration are given in table 6. Figure 4 shows a chromatogram of a 0.5-mL volume of vapor-air mixture (1 mL gas-tight syringe) from the vapor generator, which was operating at 30 °C

with an air flow of 100 mL/min. Table 7 shows replicate determinations of vapor concentrations

in ppm and the respective standard deviation for various runs made of standards generated at 30 °C. Table 8 lists chromatographic results which demonstrate the effect of increasing the diluent flow to about 1 and 2 L/min (at 30 °C). Chromatographic results are shown in table 9 for vapors

generated at 50 °C. Summaries of the average vapor concentrations determined by GC analyses at 30° and 50 °C

are compared in table 10 with concentrations computed from the averaged diffusion rates at the respective temperatures. The computed values were derived from the diffusion rates, expressed in microliters of vapor per minute, and the air flow rates, expressed in liters per minute. Generally,

good agreement is found between the techniques for the determination of the vapor concentrations.

In this study on the generation and calibration of vapor standards, only vapors of compounds

containing carbon and hydrogen were used. However, this method of vapor-standard preparation

and concentration determination is also suitable for standards consisting of other potential accelerants such as alcohols, ethers, and ketones, as well as selected compounds contained in other multi-component accelerants such as paint thinners (solvents) and varnishes. This method can be duplicated and conveniently used by fire investigators to calibrate and

evaluate the performance of combustible gas detectors. Only commercially available equipment

and materials are needed. Moreover, the manufacturer of a candidate vapor generator can provide

diffusion tubes of the length and bore selected by the experimenter, for generating specific vapor

standards at the desired diffusion rates and desired concentrations.

7
LO CD

CO

X

CO

CO

o

LT)

in

Fk.I UK S. Chronuuogram oj selected hydwcarbtm from a carbon disulfide solutu

TaHI.K 6. Gas chromatograph detector calibration factors obtained from carbon disulfide solutions of selected hydrocarbons

Conversion factors [(nL vapor/peak area)X 10^

| n-Hexane /sooctane Toluene o-Xylene

9.74 7.14 8.72 7.82 9.39 6.88 8.41 7.55 9.56 7.01 8.57 7.64

10.12 7.33 8.96 8.03 10.06 7.38 9.04 8.10

9.73 7.10 8.69 7.80 9.77 7.15 8.76 7.83 Average ±

standard

deviation 9.77±0.26 7.14 +0.17 8.74+0.22 7.82 +0.20

8
Figure 4. Chwmatogram of selected hydrocarbon vapor—air standard generated at 30 °C.

Table 7. Chromatographic determinations of concentrations of

vapor standards generated at 30 °C ivith carrier/diluent gas flow at 100 mL/min

Vapor concentration [ppm (v/v)] Run n-Hexane /sooctane Toluene o-Xylene

1 33.5 11.7 73.6 17.7

1 32.4 10.5 77.5 20.1

1 33.4 10.7 78.6 21.3

2 31.5 72.2 17.7 2 33.0 10.0 73.2 18.2 2 33.8 12.0 75.2 20.5

3 35.3 11.9 77.3 17.6 3 35.2 12.1 80.0 19.0

4 33.7 11.3 75.6 17.0 4 33.5 11.6 76.9 16.8

Average ±

standard

deviation 33.5±1.1 11.3±0.7 76.0+2.5 18.6±1.6

9
TmiII 8. (hr(itiiiili>frr(iphi< (IflcrniiiKilion (if sclcilcil li\ (IriKiirlion

\ fi/inr slaiii/dn/.s firncnilfil iil iO °(. ^\ilh liiltil dirrier

(ii/iirni flii^v It! ahoiit I tuiil 2 l./niin

ManilanI

fliiw rate (1,/min) K..n

Va|

//-Hexane

lor concentration ||i|ini (\/\)

/.sooctane Tolneiie

1

i>-\ \ lene

I.IO 4

4

4.4

3.6

1 .3 9.3

1.2 8.3

2.6

1.9

Average ±

standard

deviation 4.()±().6 l.2±0.07 8.8 +0.7 2.2 ±0.5

2.06 4

4

2.3

1.8

0.6 4.2

0.5 4.1

1.0 0.9

Average ±

standard

deviation 2.0±0.4 O.6±().07 4.2±0.07 1.0±0.07

1 Mil I- '). (.(iiin'iilriilion <il /}\i/n)ciirli(>n cii/Hir sliiiidards

pcii('ni/c{/ III SO °( IIS i/clrniiiiici/ In pas rhronialiigniphy

Klin //-Hevane

V;i|ior concentration ||i|ini (v/v)|

/.sfM)ctane Toluene «-Xvlene

3

3

3Average ±

standard

deviation

93.9

92.9

94.5

93.5+1.2

10.6

10.5

11.1

10.7 ±0.3

81.8

82.3 96.0

86.7±8.1

58.8 62.0

84.0

68.3±]3.7

Tmim-: 10. Comparison o/ iiiliiiliili'il roncenlnilions of •.Hijtor slondiirds genernled

at mid 50 "( willi loiin nlriilions determined by gii.s c/inniialogra/i/tir iinnlysis

Va|ior concentration ||i|ini (v/v)| 30 °C 50 °C

(Carrier gas. i'ercent Percent H\drocarl)on i,/niin GC Anal (Calculated difference GA'. Anal Calculated difference

/(-Hexane 2.06 2.0 2.3 13

1.10 4.0 4.3 7 0.10 34 47 29 94 7

/.WMictane 2.06 0.6 0.7 14

1.10 1.2 1.4 14

0.10 11 15 26 11 12 11

Tcduene 2.06 4.2 4.7 11 1.10 8.8 8.8 0

0.10 76 97 21 187 200 6

o-Xylene 2.06 1.0 1.1 9

1.10 2.2 2.0 10

0.10 19 23 18 68 64 7

10
5. CONCLUSIONS

It has been denioiistratecl that a simple va[)()r generator can proihicc (•on(ctilrali(iii> of vapor

on a continuous basis in the parts-per-million range. The standards nia\ contain lr((in one to (our

selected compounds. Periodic gas chromatographic analysis of the dynamic standards showed that the vapor concentrations generally are constant within 10 percent. The concentrations (p|)m) of the vapor standards can be ascertained from the diffusion rate (jU-g/min), the vapor density (g/l.)- and

carrier gas (air) flow (L/min).

6. REFERENCES

|1| DeHaan. J. I). Lalioratory asperts of arson: Accelerants, ilevices. and targets. Arson Analysis Newsletter. 2(4): 1-9; 1978.

|2| Tauber, S. J. Arson Control —A review of the state-of-the-art with emphasis on research topics. Report No. N(J.560.S6. Washington, DC: Law Enforcement Assistance Administration, U.S. Department of Justice; 1979 Tehnuary.

|3] Sanders, W. N.; Maynard, J. B. Capillary gas chromatogra|iliic method for determining the C^-C,, , hy<lrocarhon> in full- range motor gasolines. Anal. Chem. 40: 527-535; 1968.

[4] Guthrie, V. B., ed. Petroleum products handbook. New York, NY: McGraw-Hill Book Co.; I960: 4-8—4-10.

[.5] Clodfelter, R. W.; Hueske, B. S.; Hueske, E. E. A comparison of decomposition |)roducts from selected burned

materials with common arson accelerants. J. Forensic Sci. 22(1): 116-118; 1977.

|6] Jordan. T. E. Vapor pressure of organic compounds. New York. NY: Interscience Pub.; 1954.

[7] Altschuller. A. P.; Cohen, 1. R. Application of diffusion cells to the production of known concentrations of gaseous

hydrocarbons. Anal. Chem. 32: 802-810; 1960.

[8] McKelvey, J. M.; Hoelscher, H. E. Apparatus for preparation of very dilute gas mixtures. Anal. Chem. 29: 123; 1957.

S!r us. GOVERNMENT PRINTING OmCE : 1981 0—340-997 (t828)

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JOURNAL OF RESEARCH—The Journal of Research of the National Bureau of Standards reports NBS research and develop- ment in those disciplines of the physical and engineering sciences in which the Bureau is active. These include physics, chemistry,

engineering, mathematics, and computer sciences. Papers cover a broad range of subjects, with major emphasis on measurement

methodology and the basic technology underlying standardization. Also included from time to time are survey articles on topics

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citations to all recent Bureau publications in both NBS and non- NBS media. Issued six times a year. Annual subscription: domestic

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