ASTM D8460 2022 Standard Test Method for Quantification of Volatile Organic Compounds Using Proton Transfer Reaction Mass Spectrometry

Designation D8460 − 22 Standard Test Method for Quantification of Volatile Organic Compounds Using Proton Transfer Reaction Mass Spectrometry1 This standard is issued under the fixed designation D8460.

Designation: D8460−22

Standard Test Method for

Quantification of Volatile Organic Compounds Using Proton

This standard is issued under the fixed designation D8460; the number immediately following the designation indicates the year of

original adoption or, in the case of revision, the year of last revision A number in parentheses indicates the year of last reapproval A

superscript epsilon (´) indicates an editorial change since the last revision or reapproval.

1 Scope

1.1 This test method describes a technique of quantifying

the results from measuring various volatile organic compound

contents using a chemical ionization mass spectrometer

result-ing in the production of positively charged target compound

ions Depending on the nature of production of so-called

primary ions, the associated instruments having the capability

to perform such analyses are either named Proton Transfer

Reaction Mass Spectrometers (PTR-MS), Selected Ion Flow

Tube Mass Spectrometers (SIFT-MS) or, in the most generic

term, Mid-pressure chemical ionization mass spectrometers

(MPCI-MS) Within this standard, the term PTR-MS is used to

represent any of these instrumentations

1.2 Either of the instrument types can be used with the two

main mass analyzers on the market, that is, with either

quadrupole (QMS) or time-of-flight (TOFMS) mass analyzer

This method relates only to the quantification portion of the

analysis Due to large differences in user interfaces and

operating procedures for the instruments of the main

instru-ment providers, the specifics on instruinstru-ment operation are not

described in this method

1.3 Details on the theoretical aspects concerning ion

pro-duction and chemical reactions are included in this standard as

far as required to understand the quantification aspects and

practical operation of the instrument in the field of vapor

intrusion analyses Specifics on the operation and/or calibration

of the instrument need to be identified by using the user’s

manual of the individual instrument vendor A comprehensive

discussion on the technique including individual mass-line

interferences and in-depth comparison with alternate methods

are given in multiple publications, such as Yuan et al (2017)

( 1 ) and Dunne et al (2018) ( 2 )2

1.4 Units—Values stated in SI units are to be regarded as

standard No other units of measurement are included in this

standard Reporting of test results in units other than SI shall not be regarded as nonconformance with this standard 1.5 All observed and calculated values shall conform to the guidelines for significant digits and rounding established in Practice D6026

1.5.1 The procedures used to specify how data are collected/ recorded or calculated in the standard are regarded as the industry standard In addition, they are representative of the significant digits that generally should be retained The proce-dures used do not consider material variation, purpose for obtaining the data, special purpose studies, or any consider-ations for the user’s objectives; and it is common practice to increase or reduce significant digits of reported data to be commensurate with these considerations It is beyond the scope

of this standard to consider significant digits used in analysis methods for engineering data

1.6 This standard may involve hazardous materials,

operations, and equipment This standard does not purport to address all of the safety concerns, if any, associated with its use It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental prac-tices and determine the applicability of regulatory limitations prior to use.

1.7 This international standard was developed in accor-dance with internationally recognized principles on standard-ization established in the Decision on Principles for the Development of International Standards, Guides and Recom-mendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

2 Referenced Documents

2.1 ASTM Standards:3

D653Terminology Relating to Soil, Rock, and Contained Fluids

D1357Practice for Planning the Sampling of the Ambient Atmosphere

D3740Practice for Minimum Requirements for Agencies

1 This test method is under the jurisdiction of ASTM Committee D18 on Soil and

Rock and is the direct responsibility of Subcommittee D18.21 on Groundwater and

Vadose Zone Investigations.

Current edition approved May 1, 2022 Published June 2022 DOI: 10.1520/

D8460-22

2 The boldface numbers in parentheses refer to a list of references at the end of

this standard.

3 For referenced ASTM standards, visit the ASTM website, www.astm.org, or

contact ASTM Customer Service at service@astm.org For Annual Book of ASTM Standards volume information, refer to the standard’s Document Summary page on

the ASTM website.

Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959 United States

Engaged in Testing and/or Inspection of Soil and Rock as

Used in Engineering Design and Construction

D5314Guide for Soil Gas Monitoring in the Vadose Zone

(Withdrawn 2015)4

D5730Guide for Site Characterization for Environmental

Purposes With Emphasis on Soil, Rock, the Vadose Zone

and Groundwater(Withdrawn 2013)4

D6026Practice for Using Significant Digits and Data

Re-cords in Geotechnical Data

D8408/D8408MGuide for Development of Long-Term

Monitoring Plans for Vapor Mitigation Systems

E2600Guide for Vapor Encroachment Screening on

Prop-erty Involved in Real Estate Transactions

3 Terminology

3.1 Definitions:

3.1.1 For ease of reading, the term PTR-MS is used to

reflect any variations of instrumentation as described in1.1and

1.2

3.1.2 For definitions of common technical terms used in this

standard, refer to the guidelines in PracticeD1357and

Termi-nologyD653

3.2 Definitions of Terms Specific to This Standard:

3.2.1 gas analysis, n—involves multiple gas measurements

including calibration and zero gas background subtraction,

therefore involves multiple gas measurements

3.2.2 gas measurement, n—an analysis performed with a

PTR-MS without calibration nor zero gas background

subtrac-tion.5

3.2.3 ion molecule reactor, n—the instrument part within the

PTR-MS where ionization reactions of the target molecules

using primary ions happen

3.2.4 ZeroAir, n—a gas determined to be free of any

interfering substances at the reporting limit of the project

3.2.4.1 Discussion—For example, the PTR-MS can be used

to perform the analysis or an equivalent methodology or the

certificate can be used in case of a certified cylinder.6

3.3 Abbreviations:

3.3.1 CI-MS—chemical ionization - mass spectrometer or

spectrometry

3.3.2 DOD—United States Department of Defense

3.3.3 DOE—United States Department of Energy

3.3.4 EPA—United States Environmental Protection Agency

3.3.5 FEP—fluorinated ethylene propylene 3.3.6 GC—gas chromatography

3.3.7 ICAL—initial multipoint calibration 3.3.8 IMR—ion molecule reactor

3.3.9 LCS—laboratory control sample 3.3.10 MDL—method detection limit 3.3.11 MS—mass spectrometer 3.3.12 NIST—National Institutes of Standards and

Technol-ogy

3.3.13 PEEK—polyetheretherketone 3.3.14 PFA—polyfluoroalkoxy alkane 3.3.15 PTFE—polytetrafluoroethylene 3.3.16 PTR-MS—proton transfer reaction - mass

spectrom-eter or spectrometry

3.3.17 QMS—quadrupole mass spectrometer 3.3.18 SDS—safety data sheet

3.3.19 SIFT-MS—selected ion flow tube - mass

spectrom-eter or spectrometry

3.3.20 TOFMS—time-of-flight mass spectrometer 3.3.21 VI—vapor intrusion

3.3.22 VOC—volatile organic compound 3.4 Symbols Used in Equations:

3.4.1 A—a target compound (analyte)

3.4.2 AH+ —a protonated target compound 3.4.3 R—reagent ion (primarily hydronium) 3.4.4 C V N (A)= [A]—number concentration (molecules/mL)

of a neutral A in the ion molecule reactor

3.4.5 C V V (A)—mixing ratio or concentration of a

constitu-ent ion sample air (mL/L)

3.4.6 I(AH + )—signal intensity, that is, ion count rates

(ions/s)

3.4.7 E(AH+)—ion transmission efficiencies through the

mass spectrometer

3.4.8 k—ion–molecule reaction rate constant (molecules

mL–1s–1)

3.4.9 t—reaction time (s) 3.4.10 τ—dwell time (s) 3.4.11 [air]R—C V (air) number concentration of air in the ion molecule reactor (molecules/mL)

3.4.12 C cal —calibration concentration (nL/L) 3.4.13 pR—pressure of the ion molecule reactor (mbar) 3.4.14 TR—temperature of the ion molecule reactor (K) 3.4.15 UR—voltage of the ion molecule reactor (V) 3.4.16 µ—ion mobility (m2V–1s–1)

3.4.17 µ0—reduced ion mobility (cm2V–1 s–1) at standard

conditions of p0and T0 3.4.18 p0—air pressure in standard conditions (mbar) 3.4.19 T0—temperature in standard conditions (K) 3.4.20 NA—Avogadro constant

4 The last approved version of this historical standard is referenced on

www.astm.org.

5 Background—Signal is caused by contaminations in the sampling system and

the ionizer This is different from the base line signal, which is caused by electronic

noise, stray ions, and/or peak tails of very abundant compounds.

6 In practice this means that a gas mixture can have 20 components present at 10

nL/L (ppb) each These components shall not produce interfering signals or

contribute significantly to the consumption of the reagent ion Commercially sold

ZeroAir cylinders and generators usually guarantee the content to have <0.1 nL/L

hydrocarbons The actual amount of hydrocarbons within a given air needs to be

identified separately In case of the use of a ZeroAir generator, the feedline might

require additional scrubbers Despite of these aspects, a ZeroAir generator is

preferred over bottled air for the system blank (see Chapter 12) since the ambient

humidity can be an important factor for the calibration in some systems Zero

Nitrogen is an option, with the same conditions as described above.

3.4.21 l—length of the ion molecule reactor (m)

3.5 Quantities, their symbols and SI units, non-SI units

accepted with the SI and equivalent non-SI units are often used

in the scientific literature of this field In this standard we try to

use SI units where possible and indicate scientific jargon units

in parenthesis An overview of the quantities and units used in

this field is listed in Table 1

4 Summary of Test Method

4.1 This method describes the practical aspects of

quantifi-cation of a proton transfer reaction - mass spectrometer

(PTR-MS) in quantifying various volatile organic compounds

in ambient air samples Ambient air samples are drawn through

inert tubing and routed to the PTR-MS for analysis Sampling

can be performed either through direct input of the sample gas

into the instrument or by using a secondary pump system for

sampling from more distant areas by using PTFE, PFA or

equivalent sampling tubing; by using the latter approach,

distances between sampling spot and instrument of several

hundred feet can be achieved Limitations in terms of distance

are described in Sears, et al (2013) ( 3 ) The inlet can be set up

to handle either continuous sampling or for discreet sample

intake of previously acquired air samples in, for example,

canisters or bags Instruments configured with a multiport

valve allow different sample flows from discreet, separate locations to be programmatically measured at a single instru-ment location

4.2 The instrument is calibrated either from manual input of calibration standards and zero air or through the use of an automated calibration and zero system Automatic systems are commercially available and can be linked to the PTR-MS through inert tubing Such systems usually produce zero air for blanks and use a calibration mixture through dynamic dilution

of that calibration standard into the zero air Whether manual or automatic, the concept of calibration remains the same, and is described in detail later in this method

4.3 This method is used to quantify the concentration of VOCs in the gas phase using ambient air as the carrier gas In the standard case this method will draw VOCs into the PTR-MS using air as the carrier gas, but gasses that are inert to the method can be substituted as the carrier gas (N2or noble gasses) Calibrations and blanks are either conducted automati-cally using an appropriate calibration system or manually using auxiliary standards

5 Significance and Use

5.1 Vapor intrusion testing has been performed traditionally using multiple canister samples or thermal desorption tube

TABLE 1 Comparative Listing of SI and Common Units as Applicable to PTR-MS Analyses

Volumetric

Concentration

C N

Mixing

Ratio

C Number

Concentration

mL

Pressure

Independent

Gas Flow

hPa

L ⁄ min

sccm bar mL ⁄ min mbar L ⁄ min

mL/min is confusing, because

it is pressure dependent It should be called standard mL/min, which is not an SI unit At the standard pressure

of 1.013 bar, all these units are the same.

unified atomic mass unit atomic charge unit =

predominantly +1, therefor m/z is equivalent to the atomic mass unit of the charged molecule.

Signal intensity,

ion count rate

ion ⁄ s

the literature Resolving

powerB

sometimes also referred to as resolution

difference at which two neighboring peaks can be distinguished

mass range

A Sensitivity s = signal intensity I per concentration C of a compound = I/C.

B

Mass resolving power R = M ⁄∆M50%: for an isolated peak, observed mass divided by the peak width at 50 % height (FWHM, or full-width-at- half-maximum).

samples These discontinuous measurements have been shown

to be snapshots and provide averages of exposure In many

cases a higher temporal resolution is desirable to identify peaks

of emissions due to specific occupancy or environmental

changes For these cases, a continuous real-time monitoring

solution is desirable These continuous monitoring setups can

be either short-term or be part of a long-term monitoring plan

as described in ASTM guide “Standard Guide for the

devel-opment of LongTerm Monitoring Plans for Vapor Mitigation

Systems” (E2600)

5.2 The PTR-MS provides real-time measurement of

mul-tiple VOCs at ultra-trace levels, that is, in the µL/L (ppm) to

less than pL/L (ppt) range Its strengths lie with the ability to

measure VOCs in real-time and continuously (that is, ~1 Hz or

faster, using time-of-flight analyzers), and with limited sample

pre-treatment, compared to a gas chromatograph (GC) system,

which is commonly the method of choice to measure VOCs

using a variety of detectors In case of PTR-MS with

quadru-pole analyzers, the terms would be nearreal-time and

semi-continuous The high temporal resolution of the PTR-MS

measurement in the range of second(s) is often desired when

studying the atmospheric chemistry or source emissions that

result in unpredictable, sudden, and short-term fluctuations

For a detailed description on the design and theory and

practical aspects of operation for the different types of

PTR-MS, please refer to Yuan et al (2017)( 1 ).

5.3 For ambient air measurements, such as vapor intrusion

(VI) related emission testing, the PTR-MS can be used in three

different modes of operation: (1) in scanning mode to identify

sources and VI entry points within buildings; (2) in variation

identification mode, as a continuous monitoring instrument

with seconds to minutes of temporal resolution covering a large

number of VOCs; (3) in source tracking mode, as a scanner of

indoor and outdoor sources and as a rapid tracking device for

external emissions; this requires the instrument to be mounted

on a moveable platform, such as on an (autonomous) vehicle or

trolley The same operation can be used to identify various

other constituents in air, depending on the application—be it

fugitive emissions from toxic materials or illicit materials, or

metabolic reactions to infections expressed in different breath

emissions

5.4 Spatial and temporal variability are two common

chal-lenges with ambient air measurements and source assessments

Within a given building, the sources for vapors can be few or

many and are generally irregularly spaced; they may be

obscured from view by floor coverings, furniture or walls, which in itself can be a large source of VOC The current methods of choice require the use of time-discreet monitoring

or time-averaged monitoring of a specific sampling spot Real-time monitoring provides a method to assess the spatial distribution of vapor concentrations, which may help to rapidly and efficiently identify the location of vapor entry points 5.5 Real time assessment is valuable as a component of a program of assessment with two or more supporting lines of evidence and can be used to:

5.5.1 Provide support for real-time decisions such as where and when to collect long-term samples for fixed laboratory analysis using canisters or sorbent tubes;

5.5.2 Verify data quality (for example, monitoring the effi-cacy of soil gas probe purging prior to sampling, providing leak checks; and

5.5.3 Measure changes in VOC vapor concentrations in response to changes in building pressure, temperature, solar irradiation, or other weather conditions and factors affecting vapor fate and transport, including secondary chemistry occur-ring within the building

5.5.4 Identify alternative pathways based on prior identified intrusion compounds or based on emissions within such pathways, such as stormwater drains

5.6 Screening of a property prior to a real estate transaction based on site specific potential sources of concern The option for voluntary investigative assessments of potential VI in the real estate business is described in ASTM methodE2600-15

N OTE 1—The quality of the result produced by this standard is dependent on the competence of the personnel performing it, and the suitability of the equipment and facilities used Agencies that meet the criteria of Practice D3740 are generally considered capable of competent and objective testing/sampling/inspection/etc Users of this standard are cautioned that compliance with Practice D3740 does not in itself assure reliable results Reliable results depend on many factors; Practice D3740 provides a means of evaluating some of those factors.

6 PTR-MS Instrument

6.1 This chapter only describes the steps necessary for understanding the quantification of PTR-MS generated data For general description of the instrument, please refer to Yuan

et al (2017) ( 1 ) and Dunne et al (2018) ( 2 ).

6.2 A mass spectrometer is usually considered as consisting

of a sampling system, an ionizer, a mass analyzer and data analysis electronics This is illustrated inFig 1

FIG 1 Definition of Mass Spectrometer (Hardware View) and Analysis (Procedural View) and Their Correspondence

6.3 A gas analysis consists of the following procedures:

ionizing the sample gas, mass analysis of the ions, quantifying

the mass peaks, correcting for transmission differences of ions

with different mass/charge, assigning fragment ions to their

parent ions and assigning isotopes to their compound Those

processes are also illustrated in Fig 1 The last three

proce-dures are not always required For example, if a measurement

of isotope ratios is to be done, the de-isotoping procedure will

be omitted

6.4 Gas sampling is usually done with inert tubes, mostly

made of PTFE, PFA, PEEK or equivalent These tubes are

usually temperature controlled When measuring semi-volatile

organic compounds (SVOC) the temperature should be above

100 in order to minimize condensation It is preferable to

keep the sampling lines short and move the mass spectrometer

to the sample

6.5 Chemical ionization (CI) is chosen in this method

because it is soft, selective and sensitive

6.5.1 Soft ionization means that only a small number of

fragments are produced from a target compound and a higher

likelihood of production of the charged complete molecule

This results in simpler spectra and therefore is key for direct

mass spectrometry, for example, analysis without

chromato-graphic separation

6.5.2 Selective ionization means the main gases (N2, O2) of

the atmosphere are not ionized This is important to reach low

detection limits Without selective ionization, the mass

spec-trometer would be overwhelmed and saturated by the highly

abundant air compounds

6.5.3 Sensitive ionization means that the signal intensity I

per concentration C of a compound A is large This allows for

fast measurements and reduces signal-to-noise

6.6 Chemical ionization means that a compound A is

ion-ized via the chemical reaction with a reagent R In most cases

the reagent is an ion, which is indicated as Rzwhere z is the

charge state of the reagent In some cases, the reagent can be a

neutral, metastable molecule or element, which is indicated as

R●

6.7 The reaction can be of many different types Common

reaction types are proton transfer ionization, electron transfer

ionization, or adduct ionization

6.7.1 PTR-MS uses the reagent ion H3O+ and therefore

ionizes organic analytes (A) via the following proton transfer

reaction:

A1H3O 1 →AH 1 1H2O 6.7.1.1 Only compounds that have a proton affinity greater

than that of water (693 kJ/mol) can be ionized when using the

hydronium mode of ionization

6.7.1.2 This reaction may also take place with water cluster

ions (H2O)nH3O+as reagent ions or adduct ions, which can be

helpful in untargeted analytical approaches

6.7.2 Electron transfer ionization (ETI) can also result in

positive ionization:

A1R 1 →A 1 1R 6.7.3 Adduct chemical ionization is sometimes preferred to

H3O+because it is even “softer”:

A1R 1 →AR 1 6.7.4 The chemical ionization reaction takes place within the ion molecule reactor (IMR), that is, where the sample air stream interacts with the reagent ions produced by the reagent ion source (Fig 1) The ion molecule reactor (IMR) is pressure, temperature and voltage adjusted to control reaction kinetics 6.7.5 The IMR gas pressure determines the reaction dynam-ics We differentiate three different pressure regimes:

6.7.5.1 Low pressure chemical ionization (LPCI): pR < 0.01

mbar: in this pressure regime, ion molecule reactions are rare Therefore, secondary reactions are very unlikely This is the purest form of chemical ionization, but also not very sensitive 6.7.5.2 Medium pressure chemical ionization (MPCI): 0.01

mbar < pR < 10 mbar: in this pressure regime collision energies

are sufficiently high to allow disintegrate water clusters; therefore, the medium pressure regime is used for H3O+

ionization However, some secondary reactions do happen, especially with samples of high VOC loads

6.7.5.3 High pressure chemical ionization (HPCI): 10 mbar

< pR: In this pressure regime secondary ionization is very

likely and the highest sensitivities are achieved since the reaction collisions are very numerous This is best suited for very clean air measurements

6.8 Mass analyzers come in different varieties with different properties:

6.8.1 Quadrupole mass analyzers (QMA) are the “tradi-tional” analyzers in PTR-MS Their resolving power is limited

to “unit mass,” which means isobars cannot be resolved, which usually is important for CI-MS due to the lack of chromato-graphic separation PTR-MS with QMS are ideally deployed when the monitoring duration is over multiple days or weeks and a temporal resolution of multiple minutes is acceptable

6.8.1.1 Quadrupole MS—Analyte ions are measured

se-quentially in a measurement cycle In a multiple ion detection cycle the measurement cycle consists of measuring the reagent ion (H3O+), other diagnostic ions (O2 , NO+, (H2O)2H+), and

up to 50 analyte ions The instrument repeats this cycle indefinitely storing the data to file

6.8.1.2 The dwell time (τ) is the length of time the mass spectrometer spends measuring an ion and can be varied to improve signal to noise; typically, dwell time is 1 second for analyte ions A measurement cycle is the sum of the dwell times of all analytes being measured, which ultimately deter-mines the time resolution of measurements

6.8.2 Time-of-flight mass analyzers (TOF) have widely replaced the QMA in chemical ionization mass spectrometry The key differences of TOF-MS instrumentation are the mass resolution7of 1 Th (nominal mass) meaning that no elemental composition identification can be performed; and the staggered (nonconcurrent) measurement of individual analyte ions They

can reach high resolving power (R > 10,000) which allows

separation of many isobars which is very useful to compensate the lack of chromatographic separation Their high mass

7 Mass resolution: ∆M50% = peak width at 50% height, which is approximately the smallest difference between two peaks M1 and M2 so that they can be identified

as separate signals.

accuracy enables identification of compounds without

frag-ment libraries They measure all masses simultaneously and

therefore are quite sensitive In addition, they can be quite

compact and robust

6.8.2.1 TOF analyzers require a pulsed and cyclic ion

extraction into the field free region of the MS All ions are

measured in each extraction cycle which repeat at a rate of

typically 10 to 50 kHz dependent upon instrument specifics

Data from multiple extractions are accumulated into spectra for

predefined time periods (typically 0.01 to 10 seconds) to

improve signal to noise in the spectra

6.9 The PTR-MS with TOF analyzers are ideal when rapid

changes (bolus events, fugitive emissions) in vapor

concentra-tions are anticipated which require high temporal resolution

Other mass analyzers such as Fourier transform ion cyclotron

resonance mass analyzers are used primarily in academic

research settings and are not used in field deployments Data

acquisition system (DAQ) usually includes electronics for

recording the signals from the mass analyzer and a computing

unit

6.9.1 The recording electronics can be either a

time-to-digital converter (TDC) or an analog-to-time-to-digital converter

(ADC) Whereas TDCs count the ions individually, ADCs

measure the current produced by the ions TDCs are faster, less

expensive but have a limited dynamic range With ADCs

becoming faster and mimicking TDC properties, the ADCs

gradually replace the TDCs Modern ADCs have on-board

processing, which means some data analysis can be done

on-board

6.9.2 The main processing steps done in the computing unit

are listed in Fig 1:

6.9.2.1 Peak selection can be done in two different ways:

• Peaks are selected from a pre-defined peak list This is

referred to as “targeted analysis.”

• Peaks are selected using a peak finder algorithm in

addition to the pre-defined peak list or from scratch This is

referred to as “non-targeted analysis.”

Many standards include a list of compounds to be measured,

which amounts to a “targeted analysis.”

In many cases, the “peak finding” of peaks that are not in the

peak list is done in post-processing and even manually The

new peaks can then be added to the predefined list and the

complete data analysis can be repeated This blurs the line

between targeted and non-targeted analysis

6.9.2.2 Peak integration collects all signal of an ion species

into a single intensity for that species This process includes

mass calibration, integration of a signal peak sometimes using

peak fitting, and massspectral baseline correction These

pro-cesses can be done either real-time during recording or in

postprocessing

6.9.3 Transmission correction means accounting for the fact

that the total ion transmission depends on the mass/charge of

an ion This step does not need to be done when a compound

is quantified using a calibration gas with a known

concentra-tion of the compound

6.9.3.1 De-fragmenting means assigning the signal of

mul-tiple fragment ions to their precursor ion

6.9.3.2 De-isotoping means accounting for the mass spectral signal of the various isotopes of a given compound during peak integration, and potentially assigning the integrated signal of less-abundant isotope ions to the monoisotopic ion pertinent to that compound

6.10 Based on reaction kinetics, the number concentration (in molecules/mL) of neutral VOC [A], in the IMR can be determined by the following equation:

@A#5 1 kt

I~AH 1!

I~H3O 1!

E~H 3 O 1!

where k is the ion—molecule reaction rate constant

(molecules/mL s–1), t is the reaction time (s), I(AH+) and I(H3O+) are the respective ion counts rates (ions/s), and E(AH+) and E(H3O+) are the ion transmission efficiencies through the ion optics and the mass spectrometer The mixing ratio or concentration of the organic A in the sample air is then determined by the following equation:

X~A!5 @A#

@A I R#IMR5

@A#

@A I R#IMR109nL⁄L 5

@A#

@A I R#IMR109ppb

(2) where [AIR]IMR is the number concentration of air (molecules/mL) in the IMR; this equation may also be adjusted

to take water cluster ion reactions into account

6.11 In practice the sensitivity of the PTR-MS to various VOCs is determined by using multicomponent compressed gas standards to establish the sensitivity s = I/S (signal intensity per concentration); this sensitivity s is measured in (ions/s)/(nL/L)

= cps/ppb

6.12 In practice, due to differences in ion-molecule reaction rate constant and transmission efficiency, and different degree

of fragmentation, different species have different sensitivities For example, sensitivities are typically larger for polar oxy-genated compounds

7 Special Skills

7.1 This method aims at post-analytical quantification aspects Personnel must be competent in the operation of the PTR-MS instrument, calibration and blank procedure 7.2 The user must be educated in the steps to calculate the normalized sensitivity of VOCs using data collected from the PTR-MS and calibration and zero system Ultimately, this requires the knowledge to determine ambient concentrations of VOCs from the calculated sensitivities Personnel should also

be able to estimate the concentrations of tentatively identified compounds (TIC’s) using calculated sensitivities and proton transfer reaction rate constant data

8 Safety

8.1 Components of the PTR-MS are at a high voltage and protected from accidental human contact However, care should be taken to avoid contact with energized parts and only qualified PTR-MS technicians should attempt repair or main-tenance within potentially energized areas of the instrument 8.2 The multi-component VOC blend is stored inside a pressurized aluminum bottle with an attached regulator Before

movement of the bottle from the security straps, the regulator

should be removed and the bottle head should be covered with

the supplied cap Safety Data Sheets (SDS) for chemicals, such

as analytes and solvents, should be consulted before use The

user of this test method should also be aware of the hazards

associated with the operation of the multicomponent VOC

blend that contains many toxic compounds Therefore, the

exhaust of the calibration and zero system and PTR-MS should

be vented outside the analytical workspace to avoid

contami-nation of the air with the compounds of the multi component

VOC mixture In case of primary ion sources other than

hydronium, such as O2, standard safety procedures are to be

consulted for handling gas cylinders with such content

8.3 Turbomolecular vacuum pumps can fail catastrophically

if suddenly exposed to high pressure while they are operating,

which could present a hazard to humans or property

Turbomo-lecular pumps should be turned off and allowed to come to a

complete stop before the instrument is vented

9 Setup, Sample Collection, and Handling

9.1 Fig 2 illustrates the schematic layout of a basic

PTR-MS system Due to the connection with ZeroAir, a

dilution of the actual sample can be performed in case of large

amounts of VOC emissions that can overwhelm the instrument

An example could be the investigation of alternative pathways

For calibration the 2-way valve is switched to the calibration

gas, while for measurements the valve is switched to the

sample inlet side The sample inlet side can be either a single

line of tubing or could be a multi-valve that switches between

multiple sampling lines Due to the relatively low flow rate of

the PTR-MS, which is in the range of 100 sccm, it is usually

beneficial to use a secondary pump and subsample from that

main flow

9.2 More sophisticated setups have been shown to be

adequate for specific problem settings, such as GC-PTR-MS.8

9.3 The PTR-MS does not require any pre-conditioning of

the sample While filters can be used to remove larger dust

particles, these can also interfere with the vapor content of a

sample A virtual-impactor setup is recommended, in which the

PTR-MS samples a small flow orthogonally from a much

larger flow supplied by an external pump (seeFig 2)

Depend-ing on the ambient air conditions, some advantages can also be

gained through different sampling techniques such as the use of

cold traps, nafion dryers, thermal desorption or sample dilution

using either a mass flow controller or flow orifices, however,

this is not a requirement for general indoor sampling and

analyses

9.4 The sampling line can be extended to the length required

by location Standard tubing diameters in the U.S are 1⁄4 in

(6.4 mm) or3⁄8in (9.5 mm) OD; PFA or FEP are materials with

a very good (that is, low) retention and price Sampling lines of

up to 100 feet (30.5 m) can be set up

9.5 To provide the optimal sample to the instrument guide-lines are provided by several ASTM standards, such as,D5314

and D5730 Minimal calibration requirements are shown in Table 2

9.6 The sampling line is to be kept at a stable temperature into the instrument, ideally with increasing temperature from the point of sampling to the IMR This avoids the so-called cold spots, which are areas within the sampling line colder than the ambient temperature and which potentially produce false results due to condensation on the walls However, due to the pressure difference between the ambient pressure and in the IMR, the temperature within the chamber can be reduced up to 20°C in comparison to the inlet tube temperature while still preventing condensation of sampling constituents This is beneficial to further reduce the amount of fragmentation for labile compounds during ionization

10 Operating Procedure

10.1 Startup and Operating Steps—The individual steps on

how to setup a PTR-MS run are highly dependent on the

8 Such systems have a reduced ability for real-time monitoring but an additional

layer of separation which can be beneficial in tracking very low concentrations of

target analytes; a side benefit is that this setup would fulfill the criteria to apply U.S.

EPA method 18 For comparison of such methods see Warneke et al (2015) ( 4 ).

FIG 2 Basic Configuration of a Calibration and Sampling System

for PTR-MS Analysis

individual instrument’s operating software The general steps

described below serve to assure quality control For details on

how to start the instrument and how to setup the parameters for

analysis, such as IMR temperature, pressure, and voltage,

sample inlet temperature, characteristics of the detectors and

ion optics modules (if present) and of the output files are to be

identified using the manufacturer’s guidelines

10.2 Leak Detection—Upon start-up it is necessary to tune

the ion source and identify the presence of a leak in the

instrument Leaks should not occur during normal use of the

instrument In case the vacuum chamber pressure doesn’t reach

the appropriate range within regular time frames of initial

startup (typically 15-45 minutes for QMS, 1-3 hours for

TOFMS), a vacuum leak is the cause for such a delay Should

the system fail to pump to the required vacuum, the leak must

be found and corrected

10.3 Tune Ion Source—The ion source is tuned to optimize

the H3O+count rate and keep the O2+count rate less than 2 %

of the H3O+count rate by using dry VOC-free air To tune the

ion source the following ions are measured: H3O+, O2+, NO+,

H2O+, (H2O)2H+ The ion source is tuned by adjusting the H2O

flow through the ion source, by adjusting the ion source

current, and by adjusting the voltages of the secondary IMR

lenses At this point the detector voltage can be increased to get

H3O+count rates into the desired range (actual rates of ions/s

depend on the individual instrument model and are usually

provided by the manufacturer) Equivalently, the ion ratios of

O2+ to H3O+, (H2O)2 H+ / H3O+, and NO+ / O2+ are

performance indicators, but the actual numbers of these ratios

are instrument-dependent and vary between manufacturers

10.4 Tuning of Alternative Ion Sources—If an ion source

different to hydronium is chosen, the source ion needs to be

optimized Due to the large number of potential source ions,

only hydronium is specifically described within this guideline

An individual optimization protocol shall be developed within

the sampling plan In addition, many of these ion sources

ionize the analyte by reactions other than proton-transfer

These include the use of NO+ and O2+as reagent ion

10.5 Mass Calibration (Internal Standard)—Before

mea-surements are to be made, the mass-scale calibration must be

verified The mass calibration verifies that the ion peaks are

centered over the correct value of the ion mass

10.6 Mass drift can occur for various reasons, the most

important being temperature changes and vibrations during

transportation A good practice is to perform a quick mass

calibration verification check after every transport Several

instruments provide internal “continuous” mass calibration By

injection of an inert substance such as 1,3-Di-iodobenzene into

the IMR a permanent signal is generated that the instrument

can target With such an omnipresent signal, software

algo-rithms can validate the accuracy of the peak center every

minute or less; these autocorrection features have limitations

10.7 In case the calibration is off by more than one mass in

the target region, the algorithms usually cannot identify the

appropriate peak In this case, a manual calibration with a

known standard gas mix is advised by mixing a small flow of

calibrant gas with a larger flow of zero air, such that the signals for the ions pertinent to the compounds in the calibrant mixture dominate any neighboring interferences As delineated in the chapter on mass calibration, two points or more are to be used,

in the low (21.0232 Th for H3O+isotope or NO+ at 29.9987 Th) and upper range (for QMS, alpha-pinene at 137 Th, for TOFMS 203.9940 Th from the fragmentation of 1,3-di-iodobenzene if present or an equivalent standard in the range of analysis); a simple validation is to briefly breathe into the inlet and check for the mass of protonated acetone, which is 59.0865 Th

11 Interferences

11.1 The PTR-MS identifies compounds as the molecular mass of the chemical species plus the mass of one proton when using hydronium ions for ionization The technique is therefore limited by isobaric interferences for PTR-QMS and isomeric interferences for PTR-TOFMS with higher than mass unit resolution One approach to identify interferences is to use different reagent ions, such as O2 or NO+ and use the potentially different reaction mechanisms in the IMR as a separator Also, some species fragment upon ionization An-other way to separate isomers is to use GC, see9.2)

11.2 An important contributor to analyte fragmentation is the reaction with O2+; this ion is produced along with the hydronium ion (H3O+), but the IMR is tuned to increase the concentrations of the hydronium ion and reduce the concen-trations of the O2+ion As the ion source ages, the abundance

of interference ions such as O2+slowly increases (see10.3on tuning of the source) O2 ionizes the VOCs of interest mainly through charge transfer reactions The reaction is a form of hard ionization and typically fragments the VOCs of interest which can lead to either overestimation of some compound concentrations through the interference by fragment ions or the underestimation of some VOC concentrations due to the loss of the primary ion The O2+ concentration should be monitored and recorded at a minimum daily and if found out of control based on the manufacturer’s specifications, the IMR retuned according to the manufacturer’s guidelines

11.3 NO+ is also produced in the source, but to a lesser extent than O2 This ion undergoes soft ionization reaction with several common analytes resulting in detectable interfer-ences The ion can also fragment some VOC species resulting

in further interferences

11.4 Water dimers and larger clusters formed through the hydration of the reagent ion can also positively interfere with the quantification of polar species such as ketones, aldehydes and organic acids If a species has a proton affinity greater than the water dimer, then the organic compound will be ionized through proton transfer reaction from the water dimer Polar species can also be ionized through ligand switching reactions with the water dimers Because the basic calculation of the sample compounds is a function of the reagent ion only and not from ionization from any other means, the quantification of the sample compound will be positively biased due to the presence

of water dimers The formation of water dimers is controlled through tuning the IMR voltage across the IMR The drift

voltage controls the velocity the ions travel down the IMR The

water dimers break apart through random collisions with other

molecules in the flight path Increasing the voltage results in a

lower abundance of water dimers through forced fragmentation

but may also decrease the abundance of ionized sample VOCs

through loss of the proton by random collisions The IMR

voltage is tuned to minimize the water dimer interference while maintaining the sensitivity to VOCs

12 Quality Control Measures

12.1 Table 2 provides the recommended quality control

TABLE 2 Quality Control Protocol for Continuous Monitoring with PTR-MS

sample analysis

See 10.3 Initial Multipoint Calibration (ICAL) After movement of the instrument to the test site

At the beginning of a sampling campaign

Minimum of 5 concentrations, one of them being at the CCV level and the lowest being at or below the LOD Acceptable if linear least square regression for each analyte is $0.99.

If ICAL fails, rerun, if still fails, check dilution apparatus, check if zero air source is functional, verify there is no leak in the system (that is, no diluting with ambient air).

Analytes should cover as many targets as possible, however reaction kinetics approach does not require all analytes being present in calibration (see

14.2.1.3 ).

Initial Calibration Verification (ICV) Once after each ICAL to verify source standard All reported analytes of the laboratory control

sample (LCS) within ±30 % of certified value (either certified gas cylinder or pre-made canister).

If ICV fails, rerun ICV, if still fails, repeat ICAL Continuing Calibration Verification (CCV) Daily before sample analysis, if continuous, after

every 24 hours of analyses, and at the end of the batch run.

Concentration of the mid-point level of ICAL All analytes within ±30 % of the true value.

If CCV fails, analyze two consecutive samples of at least 5 seconds each If both pass in comparison with last CCV but fail with ICV, check for drastic changes in humidity Some analytes have strong ties

to humidity levels, such as formaldehyde.

If humidity had drastic changes, explain in Case Narrative In any case, since measurements are continuous and cannot be repeated, apply Q-flag to all results for the specific analytes for the duration of failure Data can be reported but must be explained

in the Case Narrative.

System (Method) Blank Once after the first CCV, and prior to starting field

analysis.

In addition, after sampling gasses of high concentration or high humidity.

The method blank is zero air – either provided through a certified canister/cylinder or through generator system.

No analytes shall be detected higher than 1 ⁄ 2 LOQ or

1 ⁄ 10 of the amount measured in any sample or 1 ⁄ 10

the regulatory limit, whatever is greater Common interferences must not be detected larger than LOQ.

If it fails, perform investigation on source and take appropriate corrective actions In some cases, running the instrument overnight, capped, under high vacuum is sufficient to remove contamination from the IMR If contamination is found in sampling system, exchange tubing if feasible; pull zero air at elevated temperatures through sampling system to clear out.

If MB fails and reanalysis cannot be performed, report data with a “B”-flag to all results.

Mass Calibration (represents the internal standard) Continuous Continuous mass calibration verification is performed

by monitoring the masses that always exist within the mass spectra, such as the primary ions 19 Th,

21 Th, or 55 Th or by using the instrument specific sources as continuous internal standard, such as di-iodobenzene or chlorinated fluorocarbons For pass, the area response must be within 40 % of the mean area response.

Ionization Softness (based on fragmentation of

alphapinene/isoprene)

fragmentation ratio of selected substances such as isoprene or alphapinene The recommended fragmentation ratio of alphapinene should not be greater than 55 % for the ratio of ions/s at 81 Th/(81

Th + 137 Th) In case the fragmentation ratio is too high, the results shall be flagged with “IS.”

measures for continuous monitoring.9

13 Calibration and Standardization

13.1 Blanks are used for background subtraction

Perform-ing routine analytical blanks is important for quantifyPerform-ing the

ion counts in the absence of analyte VOCs A blank can be

performed by overflowing the inlet with zero air, which is

typically provided from a generator or from a zero air gas

cylinder Sometimes zero gas requires additional cleaning from

remaining VOCs using a scrubber/filter that contains activated

carbon

13.2 Analytical blanks are conducted before and after a

sample is taken, after sampling gasses of high concentration

(causing saturation), high humidity variations and after the first

CCV When conducting continuous sampling, analytical blanks

are conducted every eight hours at a minimum More frequent

analytical blanks should be conducted when sampling gasses

with high concentrations of volatile compounds such as

ac-etone are anticipated or experienced An analytical blank must

contain 10 sampling points (10 cycles for QMS or 10 seconds

for TOFMS) or, after a saturation event, one continues until

background count rates have returned to the original levels

Saturation is determined by the lack of primary ions in the

spectrum, so for the hydronium one uses peak intensity of m/z

= 21.0224 (O18isotope of H3O+)

13.3 The timescale of the zero measurement should be

timed with the timescale of variability required of the

measurement, as zeroing the instrument disrupts the

equilib-rium between the instrument surfaces and the sample air flow

It follows that after long timescales of measuring clean air, the

instrument is “cleaner” (has lower background) than shortly

after measuring a polluted air sample For example,

measure-ments of a vapor intrusion hot spot, which requires 1 second

measurements should not base the background on 1 hour zero

measurements as the background will be measured

systemati-cally low

13.4 Calibrations are used to determine the sensitivities of

compounds They can be performed by producing a laboratory

control sample (LCS) from a metered flow of a NIST traceable

multi component gas standard and a metered flow of the zero

air mixed through dynamic dilution using an apparatus as

sketched inFig 1 A calibration mixture at various

concentra-tions is produced by altering the flow rates of the calibration

mixture and/or zero air; seeTable 2 for the composition of a

recommended calibration mixture for ambient measurements

For the composition of a typical calibration gas mixture to be

used in the dilution series, seeTable 3 The two gas flows are

metered using mass flow controllers This mixture is

intro-duced to the PTR-MS at various concentrations to conduct a

multipoint calibration; a minimum of 5 calibration points shall

be performed The concentration range should bound the

expected concentration of the analytes under evaluation

Alternatively, liquid calibration systems (LCS) are available

that use liquid standards and nebulizers of various kinds to

provide the vapor that is then mixed with zero air for dynamic

or static dilutions The LCS is used for the initial and continuing calibration verification (ICV and CCV)

13.5 An instrument calibration curve is typically derived from a zero-calibration-zero sequence; this sequence begins by sampling five sampling points or more of zero air followed by sampling 5 sampling points or more of the calibration gas with decreasing concentrations The starting point of the calibration gas mix shall be around the anticipated maximum concentra-tion of analytes and gradually diluted to 100-fold or more; to achieve the needed concentrations an appropriate standard gas mixture has to be used and gradually diluted with zero air The example contents of one such standard are shown inTable 2 The sequence ends by sampling an adequate volume of zero air

to flush the instrument and calibration system

13.6 The sensitivity s is calculated using the following equation:

s~A!5I cal~AH 1!2 I zero~AH 1!

C cal~A! 5

I cal~AH 1!2 I zero~AH 1!

C cal~A!

13.7 The sensitivity of analyte A is calculated by taking the difference between the instrument’s response Ical(AH+) and the instrument’s background Izero(AH+) and dividing it by the

calibration concentration (C cal) The sensitivity for each com-pound A should be calculated using the average of multiple, at least 5, known concentrations from diluting a gas standard (for example, the LCS) The range of the selected concentrations during the calibration should be selected in order to span the range of expected analyte concentrations

13.8 The transmission function describes the mass depen-dent efficiency between the transfer system, the actual mass separation and the detector The transmission is a function of

the mass/charge, therefore E = E(m/Q) For the transmission

calculation a gas standard mixture is used The compounds of the gas mixture must be selected in order to cover a wide range

of masses and not to interfere with each other The QMS system has a higher transmission in the low masses while the TOFMS system has higher transmission in the higher masses

In some instances, multiple transmission curves need to be prepared if there are optional ion funnel settings Such settings can immensely increase the sensitivity, however, in some cases

9 This table is based on common laboratory practices, such as laid-out in (and

adapted from) Table B-21 of the U.S DOD/DOE QSM 5.3, Appendix B, 2019.

TABLE 3 Calibration Gas Mixture Recommended for Ambient Air Measurements (each analyte ~200 nL/L = 200 ppb)