ASTM E2520-21

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Designation: E2520 − 15 E2520 − 21
Standard Practice for
Measuring and Scoring Performance of Trace Explosive
Chemical Detectors
This standard is issued under the fixed designation E2520; 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 practice may be used for measuring, scoring, and improving the overall performance of detectors that alarm on traces of
explosives on swabs. These explosive trace detectors (ETDs) may be based on, but are not limited to, chemical detection
technologies such as ion mobility spectrometry (IMS) and mass spectrometry (MS). Technologies that use thermodynamic or
optical detection are not specifically addressed, but may be adapted into future versions of this practice.
1.2 This practice considers instrumental (post-sampling) trace detection performance, involving specific chemical analytes across
eight types of explosive formulations in the presence of a standard background challenge material. This practice adapts Test
Method E2677 for the evaluation of limit of detection, a combined metric of measurement sensitivity and repeatability, which
requires ETDs to have numerical responses.
1.3 This practice considers the effective detection throughput of an ETD by factoring in the sampling rate, interrogated swab area,
and estimated maintenance requirements during a typical eight hour shift.
1.4 This practice does not require, but places extra value on, the specific identification of targeted compounds and explosive
formulations.
1.5 The functionality of multi-mode instruments (those that may be switched between detection of trace explosives, drugs of
interest, chemical warfare agents, and other target compounds) may also be tested. A multi-mode instrument under test shall be
set to the mode that optimizes operational conditions for the detection of trace explosives. This practice requires the use of a single
set of ETD operational settings for calculating a system test score based on the factors described in 1.2, 1.3, and 1.4. A minimum
acceptable score is derived from criteria established in Practice E2520 – 07.E2520 – 07, and an example of such a test is presented
in Appendix X1 (Example 2).
1.6 Intended Users—ETD developers and manufacturers, testing laboratories, and international agencies responsible for enabling
effective deterrents to terrorism.
1.7 Actual explosives as test samples would be preferable, but standard explosive formulations are not widely available, nor are
methods for depositing these quantitatively and realistically on swabs. This practice considers sixteen compounds that are available
from commercial suppliers. This does not imply that only these sixteen are important to trace detection. Most ETDs are able to
This practice is under the jurisdiction of ASTM Committee E54 on Homeland Security Applications and is the direct responsibility of Subcommittee E54.01 on CBRNE
Sensors and Detectors.
Current edition approved Feb. 1, 2015Feb. 1, 2021. Published February 2015March 2021. Originally approved in 2007. Last previous edition approved in 20072015 as
E2520 – 07.E2520 – 15. DOI: 10.1520/E2520-15.10.1520/E2520-21.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2520 − 21
detect many other compounds, but these are either chemically similar (hence redundant) to the ones considered, or are unavailable
from commercial suppliers for reasons of stability and safety. Under typical laboratory practices, the sixteen compounds considered
are safe to handle in the quantities used.
1.8 This practice is not intended to replace any current standard procedure employed by agencies to test performance of ETDs for
specific applications. Those procedures may be more rigorous, use different compounds or actual explosive formulations, employ
different or more realistic background challenges, and consider environmental sampling procedures and other operational
variables.
1.9 This practice recommends one method for preparation of test swabs, pipetting, because this method is simple, reproducible,
quantitative, documented, and applicable to most current detection technologies. Other methods, such as inkjet printing and dry
transfer, may generate more realistic analyte distributions and particle sizes, but these methods are not widely available and less
familiar. They may be used if the procedures are validated and documented properly.
1.10 With any deposition method, some compounds are difficult to present to the ETD inlet quantitatively due to volatility and
loss during the swab preparation process. Problematic issues pertinent to this practice are identified along with recommended
instructions. The user should be aware of the possibility that untested scenarios may lead to failure in the determination of reliable
test scores.
1.11 Units—The values stated in SI units are to be regarded as standard. No other units of measurement are included in this
standard.
1.12 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 safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.13 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
E1154 Specification for Piston or Plunger Operated Volumetric Apparatus
E2677 Test Method for Estimating Limits of Detection in Trace Detectors for Explosives and Drugs of Interest
E2771 Terminology for Homeland Security Applications
3. Terminology
3.1 Definitions of Terms Specific to This Standard:
3.1.1 alarm, n—visual or audible response, or both, from an ETD that signifies the detection of an explosive.
3.1.2 ambient background, n—particular mixture of environmental substances (dust, dirt, etc.) that is collected during swab
sampling.
3.1.2.1 Discussion—
The chemical background collected on swabs is expected to be highly variable, compositionally and temporally, and comprised
of a nearly unlimited number of possible chemical species and formulations. Background challenge materials (BCMs) should
mimic important types of chemical background found in ETD deployment areas.
3.1.3 background challenge material, BCM, n—a standard natural matrix material applied on a test swab to challenge the detection
performance of an ETD.
3.1.3.1 Discussion—
A BCM should be a well-documented material that closely mimics the ambient background typically collected during swab
sampling. Many certified reference materials, derived from a variety of natural matrices and processed to offer stable and
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.
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reproducible characteristics, are internationally available from standards suppliers. The BCMs recommended here are Standard
Reference Materials (SRMs). While these represent a limited number of natural matrices, they are compositionally complex and
offer fair detection challenges to ETDs.
3.1.3.2 Discussion—
For the purpose of verifying that an ETD meets minimum performance requirements, the presence of a BCM on the test swab is
optional.
3.1.4 clear-down, n—the process of allowing an ETD to recover from an alarm through a repeated sequence of automated
cleansing to clear out the residual sample from the instrument until the signal is reduced below a set threshold.
3.1.4.1 Discussion—
May also be used as a verb, for example: “Enough time was allowed to clear-down the ETD.”
3.1.5 compound identity calibration (CIC), n—act of providing the detector with a known substance so that the internal software
parameters may be adjusted to identify explosive compounds correctly.
3.1.5.1 Discussion—
Manufacturers of explosives detectors often provide so-called calibration media. In an IMS instrument, CIC allows the instrument
to adjust the present values of the mobility (or drift) time of the calibrant to the most current conditions. For explosives detectors
based on MS, CIC is often called tuning. Some IMS and MS explosives detectors may have built-in materials and software to
perform CIC automatically.
3.1.6 explosive trace detector (ETD), n—a system designed to detect trace amounts (micrograms or less) of explosive compounds.
3.1.6.1 Discussion—
In the context of this practice, an ETD under test will require the use of sample swabs. Some ETDs may sample vapors or particles
directly from air or surfaces without swabs. This type of sample introduction involves environmental sampling procedures that this
practice does not consider.
3.1.7 limit of detection (LOD), n—the lowest quantity of a substance that can be distinguished from the absence of that substance
within a stated confidence limit.
3.1.7.1 Discussion—
The LOD90A is the limit of detection for alarm, the mass of a particular analyte that elicits a detection alarm 90 % of the time
(90 % CL) in a particular ETD, while process blanks elicit alarms less than 10 % of the time.
3.1.7.2 Discussion—
LOD90A values will be dependent on the alarm rules and response thresholds set in an ETD for each analyte. By default, these
rules and thresholds are normally established by the manufacturer, butand may be changed by the users.
3.1.7.3 Discussion—
LOD90A values are distinguished from LOD90 values (the subject of Test Method E2677) in that the latter are 90 % limits of
detection for channel signals, intrinsic to the ETD, and independent of alarm rules and alarm thresholds.
3.1.7.4 Discussion—
LOD90A values are usually higher greater in value than LOD90 values, sincevalues because the alarm rules and thresholds in
ETDs are normally set to avoid false alarms from a wide range of ambient background substances.
3.1.7.5 Discussion—
LOD90A or LOD90 values may be calculated from appropriate measurement data through the website http://pubapps.nist.gov/
loda.https://www-s.nist.gov/loda.
3.1.8 process blank swab, n—sample swab that has been dosed with the chosen BCM.
3.1.8.1 Discussion—
For the sole purpose of verifying that an ETD meets minimum performance requirements, a process blank swab may be dosed with
pure solvent.
3.1.9 swabs, n—sampling media that are made from various types of materials, including fabric and paper, that are supplied by
the equipment manufacturer or secondother parties.
3.1.9.1 Discussion—
Also referred to as sample traps, sample tickets, swipes, wipes, coupons, filters, tokens, and substrates by some manufacturers of
ETDs.
3.1.9.2 Discussion—
Swabs are used either manually (held with gloved fingers) or placed in wands to collect sample residues for analysis in ETDs.
3.1.9.3 Discussion—
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With manual or wand use, swabs have an active area where sample is collected. Additionally, swabs have an interrogated area that
is analyzed by the ETD, either through thermal desorption, scanning, or other means. These two areas are not always spatially
congruent. The intersection of the active sampling area and the ETD interrogation detection area is called the effective area (EA).
3.1.9.4 Discussion—
After swab sampling, the only collected sample that is effectively analyzed is in the EA, so a larger EA is beneficial to trace
detection and is therefore factored into the scoring criteria. The location and size of the EA may vary considerably in different
ETDs, and may be identified by the manufacturer of the ETD.
3.1.10 swab support, n—holder for a swab that prevents contact of the back side of the EA with any surface that might contaminate
the swab or wick away solution.
3.1.11 test score, n—a metric of general detection performance for an ETD, which combines factors of scope, measurement
sensitivity, selectivity, repeatability, and EA throughput.
3.1.11.1 Discussion—
There is no maximum limit to a test score; improvements in scope, SSRs, and ESRs will result in higher scores.
3.1.12 test solution, n—dilute solution of a single explosive compound dissolved in a semivolatile solvent.
3.1.13 test swab, n—a sample swab that has been dosed with the BCM and target compound within the EA.
3.1.14 wand, n—a hand-held narrow rod that holds a removable swab, used for probing and sampling residues on surfaces.
3.1.14.1 Discussion—
Some wands are designed by ETD manufacturers to fit into the sampling port of the ETD.
3.2 Acronyms:
3.2.1 AN, n—ammonium nitrate
3.2.2 BCM, n—background challenge material (see 3.1.3).
3.2.3 CAN, n—calcium ammonium nitrate [5Ca(NO ) +NH NO +10H O]
3 2 4 3 2
3.2.4 CIC, n—compound identity calibration
3.2.5 COTS, n—commercial off-the-shelf
3.2.6 EA, n—effective area of the swab (see 3.1.9)
3.2.7 ESR, n—combined metric for effective sampling rate performance (see 6.56.6 and 6.76.8)
3.2.8 EtC, n—ethyl centralite (IUPAC: 1,3-diethyl-1,3-diphenylurea)
3.2.9 ETD, n—explosive trace detector (see 3.1.6)
3.2.10 ETN, n—erythritol tetranitrate (IUPAC: [(2R, 3R)-1,3,4-Trinitrooxybutan-2-yl] nitrate)
3.2.11 HMTD, n—hexamethylene triperoxide diamine (IUPAC: 3,4,8,9,12,13-Hexaoxa-1,6-diazabicyclo[4.4.4] tetradecane)
3.2.12 HMX, n—high melting explosive (IUPAC: Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine)
3.2.13 IMS, n—ion mobility spectrometry
3.2.14 KClO , n—potassium perchlorate
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3.2.15 KNO , n—potassium nitrate
3.2.16 LOD90A, n—limit of detection for 90 % alarm rate (see 3.1.7)
3.2.17 MS, n—mass spectrometry
3.2.18 NaClO , n—sodium chlorate
3.2.19 NG, n—nitroglycerin (IUPAC: 1,2,3-Trinitroxypropane)
3.2.20 OEM, n—original equipment manufacturer
3.2.21 PETN, n—pentaerythritol tetranitrate (IUPAC: [3-Nitrooxy-2,2-bis(nitrooxymethyl)propyl] nitrate)
3.2.22 RDX, n—research department explosive (IUPAC: 1,3,5-Trinitroperhydro-1,3,5-triazine)
3.2.23 R-salt, n—cyclotrimethylenetrinitrosamine (IUPAC: hexahydro-1,3,5-trinitroso-1,3,5-triazine)
3.2.24 SRM, n—Standard Reference Material, certified and distributed by the National Institute of Standards and Technology,
Gaithersburg, MD, USA.
3.2.25 SSR, n—combined metric for sensitivity/selectivity/repeatability performance (see 6.76.8)
3.2.26 TATP, n—triacetone triperoxide (IUPAC: 3,3-Dimethyl-1,2-dioxacyclopropane)
3.2.27 Tetryl, n—2,4,6-trinitrophenylmethylnitramine (IUPAC: N-methyl-N,2,4,6-tetranitroaniline)
3.2.28 TNT, n—trinitrotoluene (IUPAC: 2-Methyl-1,3,5-trinitrobenzene)
3.3 General Terms:
3.3.1 Please refer to Terminology E2771.
4. Summary of Practice
4.1 Based on the capabilities of the ETD detection technology, select particular target compounds to be measured and the identity
of BCM.
4.2 Reference solutions are prepared, Prepare reference solutions, each containing a known concentration of a particular target
compound.
4.3 Assure all target compounds are programmed into the ETD under test, and thatset standard operating conditions are
set.conditions.
4.4 Each Pretreat each test swab is pretreated with 100 μg of BCM.
NOTE 1—For the sole purpose of verifying that an ETD meets minimum performance requirements, the presence of a BCM is optional.
4.5 Using the manufacturer’s instructions, perform steps to assure that the ETD is in operational readiness. This may involve
compound identity calibration (CIC), verification, and minor tuning. Note the time needed to perform these tasks.
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4.6 Twenty-five Analyze twenty-five process blank swabs are analyzed to determine the background response and the basic
sampling rate.
4.7 Determine the LOD90A for each target compound selected. Via pipette or syringe, place BCM and target compound are placed
anywhere within the EA of the swab as defined by the ETD manufacturer. Between analyses, note the time is noted to recalibrate,
retune, and troubleshoot the ETD system in order to maintain operational readiness.
4.8 An Calculate an ETD score is calculated through a formula using the LOD90A values achieved for each target compound, the
selectivity of each alarm, and the effective rate of sample throughput.
5. Significance and Use
5.1 TheThis practice may be used to accomplish several ends: to establish a worldwide frame of reference for terminology,
metrics, and procedures for reliably determining trace detection performance of ETDs; to give developers tangible benchmarks
designed to improve detection performance of next-generation ETDs; as a demonstration by the vendor that the equipment is
operating properly to a specified performance score; for a periodic verification by the user of detector performance after purchase;
and as a generally-acceptable template adaptable by international agencies to specify performance requirements, analytes and
dosing levels, background challenges, and operations.
5.2 It is expected that current ETD systems will exhibit wide ranges of performance across the diverse explosive types and
compounds considered. As in previous versions, this practice establishes the minimum performance that is required for a detector
to be considered effective in the detection of trace explosives. An explosives detector is considered to have “minimum acceptable
performance” when it has attained a test score of at least 80.
5.3 It is not recommended to use scores exclusively to compare different ETD systems in order to make procurement or
deployment decisions. The scores themselves signify ratings based on general detection performance, but do not necessarily reflect
capabilities with specific analytes or BCMs, nor do scores consider many factors that users may also consider important:
procurement and operating costs, robustness and dependability, training requirements, ease of use, security features, size and
weight constraints, network capabilities and interoperability, and radioactive material management.
6. Procedure
6.1 Selections:
6.1.1 Given a particular ETD system running under a single set of operational conditions (or automated control of those
conditions), choices must be made regarding the analytes and BCM to be used for the tests. This flexibility in the practice allows
a significant increase in scope of the explosives considered without requiring an excessive test workload, and also allows avoidance
of any particular BCM that causes difficulties with any particular detection technology. Eight types of explosives are identified in
Table 1, along with sixteen chemical compounds that are associated with these types. No more than one compound from each type
may be chosen for a maximum of eight compounds for testing. One BCM must also be selected from the list in Table 2. unless
the sole purpose of the test is to verify minimum acceptable performance.
6.2 Reagents and Materials:
TABLE 1 Compounds Associated with Explosive Types
Chemical Class or Explosive Type Target Compounds
Nitramines RDX, HMX
Nitro-esters PETN, ETN
Nitro-aromatics TNT, Tetryl
Nitrosamines R-salt
Peroxides HMTD, TATP
Inorganic nitrates AN, CAN, KNO
Nitrates AN, CAN, KNO
(Per)chlorates NaClO , KClO
3 4
Smokeless powders NG, EtC
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TABLE 2 Standard Materials Associated with Background Types
BCM Type Standard Materials
Background Type of BCM Standard Materials
Watershed sediment (integrated SRM 2703 (Sediment for solid
large-area chemical background) sampling)
SRM 1646a (Chesapeake Bay
sediment)
SRM 1944 (NY-NJ waterway
sediment)
Agricultural soil SRM 2709a (San Joaquin soil)
SRM 2586 (Garden soil)
Domestic dust SRM 2585 (House and hotel dust)
Atmospheric particulate matter SRM 1648 (St. Louis air particulate)
(contain nitrates from combustion SRM 1649 (Washington DC urban
processes) dust)
SRM 2975 (Diesel particulate matter,
industrial forklift)
6.2.1 Swabs—A Procure a sufficient quantity of clean swabs (that are designed for the ETD model under test) shall be procured
from the OEM or second-party other party provider. At a minimum, expect to use 30 swabs per target compound plus 25 swabs
to measure sampling rate and process blank response.
6.2.2 Swab Supports—Trays Obtain trays or other items designed to hold (and organize) the swabs so that BCM and target
compound may be dispensed onto the EA and solvent evaporated quickly without risk of contamination.
6.2.3 BCM Suspension—Prepare the BCM suspension by weighing out 400 mg of the solid BCM and placing it into an
appropriately sized squeeze bottle with conical lid, then adding 100 mL of analytical-grade isopropanol. Seal and shake well. Pure
isopropanol may be used without BCM if the sole purpose of the test is to verify minimum acceptable performance (see Appendix
X1As needed, the suspension may be transferred , Example 2). In this case, subsequent references to “BCM” and “suspensions”
shall refer to a process blank prepared with pure isopropanol. As needed, transfer the suspension quickly into a small plastic
squeeze dropper bottle for dispensing. Properly shaken, a typical drop of 25 μL will contain about 100 μg of suspended (and
partially dissolved) BCM. This amount is ten times higher than the highest trace analyte testing level, and represents a reasonable
amount of ambient background collected during swab sampling. As isopropanol can form peroxides over time, only freshly opened
bottles and freshly prepared suspensions should be used.
6.2.4 Test Solutions—Prepare test solu
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