Trace Explosives Vapor Generation and Quantitation at Parts per

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Trace Explosives Vapor Generation and Quantitation at Parts per Quadrillion Concentrations Braden C. Giordano, Christopher Ryan Field, Benjamin Andrews, Adam Lubrano, Morgan Woytowitz, Duane A Rogers, and Greg Earl Collins Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b04581 • Publication Date (Web): 14 Mar 2016 Downloaded from http://pubs.acs.org on March 22, 2016

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Analytical Chemistry

1

Trace Explosives Vapor Generation and Quantitation at Parts per Quadrillion

2

Concentrations

3 4

Braden C. Giordano1*, Christopher R. Field1, Benjamin Andrews2, Adam Lubrano2, Morgan

5

Woytowitz2, Duane Rogers1, Greg E. Collins1

6 7

1

Chemistry Division, U.S. Naval Research Laboratory, Washington, D.C. 20375

8

2

Nova Research, Inc. Alexandria, VA 22308

9 10

* Corresponding author: [email protected]

11 12

1

ACS Paragon Plus Environment


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Abstract

2

The generation of trace 2,4,6-trinitrotoluene (TNT), cyclotrimethylenetrinitramine (RDX)

3

and pentaerythritol tetranitrate (PETN) vapors using a pneumatically-modulated liquid delivery

4

system (PMLDS) coupled to a polytetrafluoroethylene total-consumption micronebulizer is

5

presented. The vapor generator operates in a continuous manner with final vapor concentrations

6

proportional to the explosive concentration in aqueous solution delivered through the nebulizer

7

and the diluent air flow rate. For quantitation of concentrations in the parts per billionvolume

8

(ppbv) to parts per trillionvolume (pptrv) range, Tenax-TA thermal desorption tubes were used for

9

vapor collection with subsequent analysis on a thermal-desorption system programmable-

10

temperature vaporization gas chromatograph (TDS-PTV-GC) with a µ-ECD detector. With 30

11

minute sample times and an average sampling rate of 100 mL min-1, vapor concentrations of 38

12

pptrv for TNT, 25 pptrv for RDX, and 26 pptrv for PETN were determined. For parts per

13

quadrillionvolume (ppqv) vapor quantitation of TNT and RDX, an on-line PTV-GC system with a

14

negative-ion chemical ionization mass spectrometer (methane reagent gas) was used for direct

15

sampling and capture of the vapor on the PTV inlet. Vapor concentrations as low as 160 ppqv

16

and 710 ppqv for TNT and RDX were quantified, respectively, with an instrument duty cycle as

17

low as 4 minutes.

18 19

Key

20

explosives, GC/MS, vapor generation, TNT, RDX

Words:

trinitrotoluene,

cyclotrimethylenetrinitramine,

pentaerythritol

tetranitrate,

21 22

2


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1


Introduction

2

In order to develop and evaluate the performance of new and emerging trace-explosive

3

vapor sensors, stable and quantifiable trace vapors must be available. Numerous examples of

4

trace-explosive vapor generators have been discussed in the literature. These vapor generation

5

techniques usually fall into two broad categories: (1) permeation from solid material, either neat

6

or deposited onto an inert surface

7

surface

8

concentration at or below the saturated head-space vapor concentration, with higher

9

concentrations possible provided the sample can be heated. Additionally, the acquisition of neat,

10-12

.

1-9

or (2) generation from a liquid deposited onto a heated

Typically, generation from a solid material limits the maximum vapor

10

solid material can be prohibitive, due to safety and regulatory considerations.

These

11

considerations can limit the feasibility of this method for routine laboratory use. Alternatively,

12

liquid deposition onto a heated surface is relatively easy to implement and purified explosive

13

standards in solution are readily available. However, when implemented in a continuous fashion,

14

this method for vapor generation has been found to yield significant pulsations. These pulsations

15

have been attributed to the imbalance between the rate of evaporation and solution introduction.

16

In these arrangements a droplet forms at the end of a supply line and falls onto a heated surface

17

where the solvent evaporates and the analyte is vaporized. Consequently, there is an inherent

18

pulsation associated with how the sample is introduced to the heated surface and vaporized.

19

These pulsations can be exacerbated by liquid delivery systems such as peristaltic pumps or

20

syringe pumps that are driven by stepper motors. Additionally, direct deposition onto a heated

21

surface is not compatible with thermally-labile explosives, where the compound of interest may

22

degrade due to thermal effects and the resulting vapor stream may contain a mixture of

23

degradation products. A detailed description of many of these vapor generation techniques has 3



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1

been described in a recent review by Atkinson and coworkers

2

with a wide dynamic range that uses readily available analytical-standard solutions and does not

3

degrade the analyte during vaporization would dramatically simplify and promote the

4

development, characterization, and calibration of next-generation trace-explosive vapor sensors

5

and sensing materials.

6

. A continuous vapor generator

Research at the United States Naval Research Laboratory has recently focused on the 11,14-18

7

generation and validation of a variety of trace-explosive vapors

8

generation technique has been developed to overcome the weaknesses of the aforementioned

9

techniques, notably a lack of vaporization efficiency, narrow operating conditions for avoiding

10

vapor pulsations, limited dynamic range, and the potential for thermal degradation of analytes.

11

This alternative method for trace-explosive vapor generation uses a pneumatically modulated

12

liquid delivery system (PMLDS) coupled to a nebulizer

13

eliminate pulsations in the liquid delivery, a problem when testing sensors with rapid time

14

responses, and to provide a continuous flow of solution to a total-consumption micronebulizer.

15

The commercially available total-consumption micronebulizer is commonly used when a low-

16

flow separation technique is coupled to an inductively coupled plasma (ICP)

17

liquid flow range droplets are less than 10 µm in diameter. Because a large number of small

18

droplets are generated rather than one large droplet cooler operational temperatures can be used.

19

The high surface-area to volume ratio of these micron sized droplets also allows more efficient

20

vaporization. Consequently, this approach is feasible for the vapor generation of thermally-labile

21

energetics, such as PETN. This mode of vapor generation shares some similarities with efforts

22

described by Martinez-Lozano et al. where secondary electrospray ionization (SESI) was used to

23

generate trace PETN and TNT vapors for analysis via atmospheric pressure ionization mass

19

. An alternative vapor

. The PMLDS was implemented to

20

. In the target

4


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21

1

spectrometers

. Briefly, the author’s generated explosives vapor by electrospraying TNT and

2

PETN methanol solutions at known concentrations into a heated CO2 stream. That vapor was

3

then electrosprayed into the MS for analysis.

4

Equally important to vapor generation is the development of appropriate analytical

5

techniques to validate these vapor streams. Typical vapor validation methods rely on the use of

6

thermal desorption tubes for the collection of sample with subsequent analysis via hyphenated

7

gas chromatography techniques. In such an approach, vapor is first adsorbed by sampling onto a

8

large volume of sorbent material (e.g. Tenax-TA) in a sorbent tube; the sorbent tube is then

9

manually inserted into a thermal desorption system (TDS); the vapor is desorbed from the

10

sorbent and trapped on a low-volume programmable temperature vaporization (PTV) inlet

11

(located where a GC inlet liner would typically be found), followed by a second desorption step

12

for delivery into the GC column 14,16-18. For very low vapor concentrations, this approach can be

13

a time consuming process limited by sample flow rate and the presence of chemical noise

14

associated with the sampling media. An alternative to the TDS-PTV-GC method is online PTV-

15

GC instrumentation, where analyte is sampled directly to a PTV inlet held at a low temperature

16

(with or without a stationary phase for analyte capture) and subsequently desorbed for GC

17

analysis 15.

18

This paper presents the successful generation and quantitation of TNT, RDX, and PETN

19

vapors using a PMLDS-nebulizer system. For parts per billionvolume (ppbv defined as nanomoles

20

explosives/moles of air) to parts per trillionvolume (pptrv defined as picomoles explosives/moles of

21

air) vapors, TDS-PTV-GC-µECD methodologies were used for vapor quantitation. In order to

22

extend limits of quantitation to parts per quadrillionvolume (ppqv defined as femtomoles

23

explosives/moles of air) levels for TNT and RDX, an online PTV-GC-MS method was 5



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implemented. Calibration curves for the quantitation of the explosives vapor were derived from

2

direct liquid injection onto thermal desorption tubes or liquid injection into the online PTV inlet.

3

Accurate methods for the generation and quantitation of trace explosives are essential for the

4

proper evaluation of sensor technologies.

5 6

Experimental

7

Vapor Generation using PMLDS-Nebulizer

8

A polytetrafluoroethylene (PTFE) nebulizer designed to self-aspirate 30 µL min-1 with a

9

sheath air flow of 1 L min-1 (ME2020, Meinhard Glass, Golden, Colorado, USA) was selected.

10

As received, the solution introduction tubing of a PTFE nebulizer is designed to be submerged

11

into a sample vial where it can draw solution out of the vial through self-aspiration. However,

12

the self-aspiration of a PTFE nebulizer is susceptible to disruption from bubbles and

13

environmental changes when used for continuous, extended vapor generation. This prompted the

14

coupling of the nebulizer to the PMLDS for continuous, pulse-free operation. The construction

15

and characterization of the PMLDS has been previously described 19. The solution flow rate for

16

the PMLDS was set to 40 µL min-1 to avoid self-aspiration by the nebulizer.

17

The PMLDS was coupled to the solution introduction tubing of a PTFE nebulizer using

18

0.0625 in. (1 in. = 25.4 mm) outer diameter (OD), 0.008 in. inner diameter (ID) PTFE tubing and

19

a polyether ether ketone, PEEK, union (P-742, IDEX Health & Science, Oak Harbor,

20

Washington, USA). A mass flow controller, MFC, (M100 Smart Trak 2, Sierra Instruments,

21

Inc., Monterey, California, USA) was connected to the sheath gas inlet of the nebulizer by

22

connecting the 0.25 in. OD tubing from the MFC to the 0.125 in. OD tubing provided with the

23

nebulizer through a stainless steel reducer union (Swagelok, Cleveland, Ohio, USA). The 6


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0.125 in. OD tubing provided by the nebulizer manufacturer was press-fit onto the sheath gas

2

inlet of the nebulizer.

3

The nebulizer was inserted into a 0.375 in. OD, 0.25 in. ID passivated (SilcoNert, Silco-

4

Tek, Bellefonte, Pennsylvania, USA) stainless steel tube wrapped in a polyimide heater

5

(35475K363, McMaster-Carr, Princeton, New Jersey, USA) using a bored-through 0.25 in. to

6

0.375 in. passivated stainless steel coupler. A PTFE nut (751-FN4, Savlillex, Eden Prairie,

7

Minnesota, USA) was hand-tightened to securely seal the nebulizer within the coupler without

8

causing damage to the plastic nebulizer. Figure 1A shows a schematic diagram of the fluidic

9

connections for the PMDLS and the nebulizer. During operations the relative standard deviation

10

in liquid flow rate was 0.5% 19.

11

Shown in Figure 1B is the placement of the K-type thermocouple (SA1XL-K-SRTC,

12

Omega Engineering, Bridgeport, New Jersey, USA) used to monitor and control the temperature

13

in the heated tube in which the nebulizer is placed. A proportional-integral-derivative (PID)

14

temperature controller (CSi32, Omega Engineering, Inc., Bridgeport, New Jersey, USA) was

15

used to control the polyimide heater with the thermocouple firmly attached to the outer surface

16

of the 0.375 in OD passivated stainless steel tube, a.k.a. the heat tube. Careful attention was

17

given to ensure the total tube temperature never exceeded 150°C, which was the maximum

18

operating temperature of the polyimide heater. The operating temperature of the heat tube was

19

130°C unless otherwise specified. A black-anodized, aluminum housing was placed over the

20

heat tube and packed with insulation.

21

Vapor Delivery Structure

22 23

A simple test structure was used to evaluate and study the vapor generation of the PMLDS-nebulizer and has been previously described

15

. Briefly, it included a dual manifold 7



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(one line is designated for analyte, while the other serves as a dedicated clean air line)

2

constructed from passivated stainless steel (SilcoNert®, SilcoTek, Bellefonte, PA). Zero-grade

3

humidified air (30-80% relative humidity, RH) was provided using two zero air generators

4

(Environics, Inc., Tolland, CT) and two humidity control units (Miller-Nelson, Livermore, CA).

5

Solutions of explosives were prepared by depositing trace analytical standards prepared in

6

acetonitrile into a 50 mL centrifuge tube and allowing the solvent to evaporate. The explosive

7

was reconstituted in water and nebulized. Water is an ideal solvent when one considers the long

8

term goal of this work, the evaluation of emerging sensing technologies. Nonaqueous carrier

9

solvents could serve as interferents in the presence of trace explosives vapors, especial when

10

they are present at several orders of magnitude higher concentration. As this system incorporates

11

the use of a dual manifold system and two independent humidity control units, the additional

12

water present due to vapor generation in the analyte manifold can be offset by adjusting the

13

humidity in the “clean air” manifold. The vapor stream from the nebulizer as described in the

14

previous section (1 L min-1) was introduced into the analyte line of the dual manifold. Further

15

dilution of the vapor was possible by adjusting the flow rate of the humidity control unit (2-20 L

16

min-1). The entire manifold was housed in a custom oven set to 130°C in order to limit analyte

17

adsorption to the inner surface of the manifold. However, given the low volatility of the analytes

18

of interest, some retention of analyte in the system is expected. A 0.25 in. OD, 0.125 in. ID

19

passivated stainless steel tube with a copper sheath was connected to a 3-port valve to allow for

20

sampling of the vapor stream. The copper sheaths were added to the tubing to enhance thermal

21

conductivity to ensure maximal temperature of the stainless steel tubing as it exits the oven.

22

Vapor Concentration Validation – TDS-PTV-GC/µECD

8


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1

Vapor samples from the PMLDS-nebulizer were collected by first attaching clean vapor

2

sampling tubes filled with Tenax-TA (part No. 009947-000-00; Gerstel GmbH & Co., KG,

3

Mulheim an der Ruhr, Germany) to the sample port while the valve was rotated to the Clean Line

4

of the manifold. Sampling began by rotating the 3-port valve to the Analyte Line; sampling

5

ended by rotating the valves back to the Clean Line. A vacuum pump (DIVAC 1.4 HV3,

6

Oerlikon Leybold Vacuum, Switzerland) and a MFC (M100 Smart Trak 2, Sierra Instruments,

7

Monterey, California, USA) were used to regulate the flow rate from the manifold through the

8

sample port to the vapor sampling tubes at 100 mL min-1. Sample time changed as a function of

9

vapor concentration using previously established calibration methods for thermal desorption

10

tubes, as demonstrated elsewhere 11,16-18. Briefly, each sample tube was spiked with 3,4-DNT to

11

serve as an internal standard, and the sample was desorbed from the tube and trapped on a cryo-

12

cooled PTV inlet and subsequently desorbed for GC/µElectron Capture Detector (ECD) analysis

13

(7898A GC, Agilent). Analyte response was normalized to the 3,4-DNT signal and quantified

14

using a calibration curve prepared per previous references 14,16-18.

15

Vapor Concentration Validation – Online PTV-GC/MS

16

Direct quantitation of vapor samples was possible using a separate, but identical make

17

and model GC (7898A GC, Agilent) coupled to a mass spectrometer (5975C, Inert EI/CI MSD,

18

Agilent) instead of a µECD. This GC was equipped with an online PTV inlet (Part No. GSS,

19

Gerstel GmbH & Co.) for direct vapor collection. The vapor manifold was coupled to the PTV

20

inlet using a heated transfer line (Clayborn Lab, Truckee, CA) set to 225°C. It should be noted

21

that the PTV inlet incorporates a complicated gas handling system coupled to a 6-port valve

22

allowing for sampling and injection onto the GC column. As provided by the manufacturer, gas

23

is transported through 0.0625 in. stainless steel tubing; all tubing that analyte vapor comes into 9



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contact with was replaced with Silconert-coated stainless steel to limit analyte adsorption to the

2

handling structure. The entire PTV vapor handling system is mounted above the GC oven at a

3

set temperature of 250°C. The PTV was Peltier-cooled for vapor sampling onto a deactivated

4

baffled liner at a temperature of 10 °C with collection on the liner occurring at a flow rate of 175

5

mL min-1 for 1-16 minutes; for desorption, the initial temperature was held for 0.1 minutes,

6

ramped to 100°C at a rate of 5°C min-1, and then ramped immediately to 250°C at a rate of 12°C

7

min-1 and held for two minutes. Desorption from the PTV inlet/injection onto the GC column

8

was facilitated by using an inlet split of 1:1, resulting in 10 mL min-1 of helium passing through

9

the liner for desorption, and a column flow of 5 mL min-1. Separation occurs concurrent to the

10

desorption step using the following oven ramp profile: an initial GC oven temperature of 100°C

11

was held for 0.5 minutes and then ramped to 250°C at 50°C min-1 with a 0.25 minute hold time;

12

total run time was 3.75 minutes. The MS was operated using negative-ion mode chemical

13

ionization with methane as the reagent gas. The source and quadrupole temperatures were

14

150°C and the transfer line was set to 280°C. The MS was operated in Selective Ion Mode

15

(SIM); after a 1 minute solvent delay ions with m/z of 227 [C7H5N3O6]-, 210 [C7H4N3O5]-, and

16

197 [C7H5N2O5]- were collected and attributed to TNT 22; after 2.75 minutes ions with m/z 129

17

[C3H5N4O2]-, 120 [CH2N3O4]-, and 46 [NOO]- were collected and attributed to RDX 23. All three

18

ions associated with each analyte were used in the establishment of calibration curves.

19

Calibration of the system was made possible by directly injecting a sample onto the

20

Peltier-cooled inlet liner, thus accounting for any loss of analyte in the on-line PTV system. A

21

specialized stage was designed and fabricated to allow for injection using a standard Agilent

22

auto-injector (G4513A). Calibration curves for TNT (from 0.025 picograms to 50 picograms at 10


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1

the detector; R2 = 0.998) and RDX (from 0.25 to 50 picograms at the detector; R2 = 0.994) were

2

achieved. It is clear from the dynamic range associated with each calibration curve that the

3

detector was an order-of-magnitude more sensitive to TNT than RDX.

4

Sample collection and separation parameters are performed independent of the two

5

detection system used. The online PTV inlet was coupled to a GC with both µECD and MS

6

detectors. We noted that the MS detector in negative-ion mode CI offered at least an order of

7

magnitude improvement in sensitivity when injecting standards; thus negative-ion mode CI was

8

chosen for all work presented herein for analysis with the online PTV.

9 10

Results and Discussion

11

Vapor Generation of TNT – Parts per Trillion Vapor Concentration

12

TNT has a saturated vapor concentration of approximately 9 parts per billion, or 80 ng

13

per liter of sampled air at 25°C 24. Initial experiments with nebulized sample solution utilized an

14

aqueous TNT solution concentration of 1 µg mL-1, which would produce a 445 pptrv vapor

15

assuming a vaporization efficiency of 100%. This vapor concentration is more than an order-of-

16

magnitude below the saturated vapor pressure of TNT. When considering sample volumes and

17

flow rates used in this work (e.g., 100 mL min-1 for one minute) approximately 400 pg of TNT

18

are sampled, a mass well above reported limits of quantitation for existing µECD methodologies

19

(i.e. 100 pg 18).

20

Table 1 summarizes the TNT vapor concentrations as a function of the sample solution

21

concentration as determined using the TDS-PTV method after sampling with a Tenax-TA tube.

22

The vapor stream was sampled for 5 to 30 minutes (at a ~100 mL min-1), depending upon the

23

vapor concentration. Lower solution concentration resulted in the expected reduction in final 11



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measured vapor concentration, exhibiting linear behavior over the tested range and an average

2

vaporization efficiency of 94 ± 3%. The measured vapor concentration is representative of what

3

a sensor would be exposed to during an evaluation.

4

Table 2 shows a comparison between TDS results and online PTV results, when vapor

5

the sampling time was 1 minute regardless of vapor concentration.

The measured vapor

6

concentrations show good agreement between the two analytical techniques, albeit with a slight

7

negative bias for the online-PTV method, which we attribute to trapping analyte at 10°C in order

8

to minimize water introduction into the GC-MS.

9

precision (i.e. relative standard deviation) is observed for the online PTV method.

In addition, a significant improvement in The

10

improvement in precision is attributable to a number of factors, including reduced chemical

11

background and a more reproducible desorption process. Samples collected on Tenax-TA tubes

12

require the use of an internal standard to account for desorption flow rate variability16,18 (a

13

function of tube packing nonuniformity). In contrast, the online PTV uses an unpacked tube. In

14

addition, the wide dynamic range afforded by MS, versus µECD, allows for consistent use of a 1

15

minute sample time for each vapor concentration. Extended sample times are necessary with the

16

TDS method to ensure that the analyte concentration falls within the limited dynamic range of

17

the µECD detector (0.1 – 4 ng at the detector).

18

Vapor Generation of TNT – Parts per Quadrillion Vapor Concentration

19

Due to the long sample times necessary to quantify sub-parts per trillion vapors with

20

traditional thermal desorption tubes (e.g., 4 hours would be necessary to sample 500 parts per

21

quadrillion of TNT), vapor quantitation of ppqv level vapors was limited to the online PTV.

22

Initial efforts focused on maintaining a sample time of one minute for all vapor concentrations,

23

which were as low as 160 ppqv for the nebulized solution concentration of 0.0005 µg mL-1 TNT. 12


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1

Figure 2 illustrates the relationship between vaporization efficiency and nebulizer solution

2

concentration.

3

efficiency remained consistent with an average of 84%. As the concentration is decreased to

4

0.001 ng µL-1, the efficiency steadily declines to approximately 61%.

5

however, the efficiency increases to 73%.

For solution concentrations between 0.05 and 1 ng µL-1 the vaporization

At 0.0005 ng µL-1,

6

Obviously, one concern when quantifying trace vapors from low volatility analytes is the

7

potential for carryover within the analytical instrumentation used for analysis, as well as the

8

vapor generation and delivery system, as indicated in the experimental section. When sampling

9

from the clean air side of the manifold, no TNT was detected – ruling out PTV and GC-based

10

contributions to TNT signal. A systematic breakdown of the system was performed to determine

11

the contributor to the increased vapor concentration. First, the liquid handling tubing was

12

replaced with new tubing that had never been in contact with TNT, and no significant change in

13

vapor concentration was observed when spraying the 0.0005 ng µL-1 TNT solution. Water (18

14

megaohm) was then sprayed through the nebulizer, and a background level of approximately 20

15

ppqv TNT was detected. If the baseline level of TNT is subtracted from the concentration

16

observed when spraying 0.0005 ng µL-1 solution, the resulting efficiency is approximately 63%,

17

a value more consistent with the efficiency determined for the 0.001 ng µL-1 solution. We

18

hypothesize that during the nebulization process some liquid is directly deposited onto the heated

19

tube as shown in Figure 1B. The heated tube becomes a secondary source of TNT as it

20

evaporates from the heated tube surface in a fashion similar to our previous vaporization design

21

11

22

was generated.

23

Vapor Generation of RDX – Parts per Trillion Vapor Concentration

. For clarity, Table 3 summarizes the conditions under which each TNT vapor concentration

13



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1

In a similar fashion, the vapor generation and quantitation of RDX was compared using

2

TDS-PTV-GC and online PTV GC analyses for nebulizer spraying solutions between 1 and

3

0.1 ng µL-1 RDX.

4

concentration of approximately 5 parts per trillion (45 picograms RDX per L of air at 25°C) 24.

5

As one would expect, it is a much more difficult to reliably generate a stable vapor of RDX. One

6

important feature of the PMLDS-nebulizer system is that the entire vapor handling system is

7

uniformly maintained at 130°C; consequently, vapors can be generated in excess of their

8

saturated vapor concentrations at room temperature, should that be desired.

9

measured air temperature though the vapor handling system exceeds 66°C, based on the

10

Clausius-Clapeyron equation we would expect it to be possible to generate a vapor concentration

11

for RDX up to 2.2 parts per billion.

Compared to TNT, RDX has a significantly lower saturated vapor

Because the

12

Table 4 shows the quantitation of RDX afforded by TDS-PTV and online PTV. There

13

are clear differences between concentrations determined by the online PTV system versus the

14

TDS-PTV system. In all instances, the online PTV results are higher than the TDS-PTV results.

15

Similar to the results for TNT, the online PTV showed improved precision over TDS-PTV. We

16

believe that the lower RDX quantitation results for Tenax-TA tube samples is the result of

17

incomplete desorption from the tube 16.

18

Vapor Generation of RDX – Parts per Quadrillion Vapor Concentration

19

With an eye towards parts per quadrillion levels of RDX vapor generation, the RDX

20

solution concentration was systematically reduced to 0.0005 ng µL-1 in a similar fashion to that

21

described for TNT. The quantified vapor concentration as a function of nebulizer solution

22

concentration is presented in Figure 3. The black dotted line represents the theoretical vapor

23

concentration assuming 100% vaporization efficiency. The inset in Figure 3 allows for a closer 14


Page 15 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

look at vapor quantitation results for solution concentrations at and below 0.01 ng µL-1. The

2

vapor quantified from the 0.001 and 0.0005 ng µL-1 nebulizer solutions exceed the nominal

3

concentrations afforded by 100% vaporization efficiency. Like TNT, carryover in the nebulizer

4

heat tube explains these high vapor concentrations. RDX deposited onto the heat tube is

5

subsequently vaporized, providing a secondary source of material in the vapor stream. Clearly,

6

the amount of carryover on the nebulizer heat tube is related to the concentration of the nebulizer

7

solution being sprayed, and the proportion of carryover to subsequent vapors would be more

8

pronounced as one transitions from a high concentration to a low concentration. Carryover

9

mitigation strategies, including solvent rinsing and/or bake-outs of the nebulizer system are

10

viable solutions to this issue. Table 5 summarizes the conditions under which each RDX vapor

11

concentration was generated. It should be noted that as TNT and RDX have the same nebulizing

12

conditions, it is possible to generate mixtures of TNT and RDX vapor simultaneously by

13

nebulizing a solution containing both TNT and RDX. No difference in final vapor concentration

14

was observed as a function of spraying both analytes simultaneously [data note shown].

15

Vapor Generation of PETN – Parts per Trillion Vapor Concentration

16

The vapor generation and quantitation of PETN was determined using TDS-PTV-GC.

17

Similar to RDX, PETN has a low saturated vapor concentration of approximately 10.7 parts per

18

trillion (140 picograms PETN per L of air at 25°C)

19

thermally labile when compared to TNT and RDX, making generation and quantitation difficult.

20

Previous efforts have focused on developing analytical methodologies that preserve intact

21

molecular PETN, with little or no thermal degradation, by deposition of PETN solution on TDS

22

tubes and subsequent desorption and analysis by GC 14. In the current work it was necessary to

23

operate the nebulizer heat tube at 100°C when generating PETN vapor, as the normal operating

24

. In addition, PETN is exceedingly

15



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Page 16 of 25

1

temperature of 130°C caused excessive thermal degradation of PETN.

As was the case with

2

RDX, due to the elevated temperature of the air in the vapor handling system, a vapor

3

concentration well in excess of the saturated vapor concentration is possible based upon the

4

Clausius-Clapeyron equation; a saturated vapor concentration of 13.7 ppb is possible for PETN

5

at 66°C.

6

Table 6 presents the quantitation of PETN afforded by TDS-PTV-GC analysis as a

7

function of solution concentration, with the observed vaporization efficiency noted. As was the

8

case with RDX, carryover is observed, as the solution concentration was transitioned from high

9

to low, and appears to be exacerbated by operating the nebulizer heat tube at a temperature of

10

100°C to limit PETN degradation.

A corresponding online PTV method for PETN vapor

11

quantitation has not been developed at this time. The system, as configured for TNT and RDX

12

analysis, causes thermal degradation of PETN.

13 14

Conclusions

15

The generation and quantitation of trace-explosive vapors of TNT, RDX, and PETN from

16

a PMLDS coupled to a total-consumption micronebulizer are presented. By using direct vapor

17

sampling with an online PTV system coupled to GC, it was possible to quantify trace-explosive

18

vapor concentrations at parts per quadrillion concentrations. With only 1 minute and 2 minutes

19

of sampling time, respectively, a 160 ppqv TNT vapor and 710 ppqv RDX vapor were quantified.

20

The instrument duty cycle for online-PTV sampling and separation was < 4 minutes for TNT and

21

RDX quantitation. Using a TDS-PTV-GC method, a 25.9 pptv PETN vapor was quantified.

22

Carryover in the vapor generation system results in efficiencies that increase as solution

23

concentration decreases, due to the generation of a second point source for vapor – vaporization 16


Page 17 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

from the walls of the passivated stainless steel tube in which the nebulizer is placed. Given the

2

nature of the analytes and the realization that practical considerations, associated with time and

3

cost, may preclude transitioning from a low concentration to a high concentration of vapor, a

4

direct measurement of vapor concentration should be reported and always presented in the

5

context of the nominal vapor concentration.

6

Acknowledgements

7

The authors would like to acknowledge the U.S. Department of Homeland Security,

8

Science and Technology Directorate, Homeland Security Advanced Research Projects Agency,

9

Explosives Division for funding this research.

10

17



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Page 18 of 25

Tables Table 1: Measured TNT vapor concentration as a function of nebulized solution TNT concentration. Vaporization Efficiency is the percentage of vapor measured relative to the theoretical vapor concentration and can be thought of as the efficiency of transfer of explosives from the nebulizer solution to the final vapor stream. Solution Concentration (µg mL-1) Theoretical Vapor Concentration (pptrv) Sample Time (min) Measured Vapor Concentration (pptrv) Vaporization Efficency (%) 0.1 45 30 42.8 95 0.5 220 10 200.5 90 1 445 5 424.3 95

Table 2: Comparison of TDS-PTV-GC and online PTV-GC quantitation of TNT TDS-PTV-GC-µECD Quantitation Online PTV-GC-MS Quantitation Solution Concentration (µg mL-1) Measured Vapor Concentration (pptrv) Standard Deviation (pptrv) Measured Vapor Concentration (pptrv) Standard Deviation (pptrv) 0.1 42.8 1.8 37.7 0.5 0.5 200.5 15.3 182.7 4.5 1 424.3 39.4 380.9 5.0

Table 3: Experimental conditions for the generation of trace TNT vapor. Vapor concentration is linear with respect to nebulizer solution concentration (R2 = 0.999) with an average error of quantitation of 3%. Solution Concentration (µg mL -1) Solution Flow Rate (µL min-1) Total Air Flow (L min-1) Theoretical Vapor Concentration Measured Vapor Concentration Vaporization Efficency (%) 1 40 11 445 pptrv 381 pptrv 86 0.5

40

11

220 pptrv

183 pptrv

83

0.1

40

11

45 pptrv

38 pptrv

84

0.05

40

11

22 pptrv

18 pptrv

82

0.01

40

11

4.5 pptrv

3.4 pptrv

76

0.005

40

11

2.2 pptrv

1.5 pptrv

68

0.001

40

11

445 ppqv

270 ppqv

61

0.0005

40

11

220 ppqv

160 ppqv

73

Table 4: Comparison of TDS-PTV-GC and online PTV-GC quantitation of RDX TDS-PTV-GC-µECD Quantitation Online PTV-GC-MS Quantitation -1 Solution Concentration (µg mL ) Theoretical Vapor Concentration (pptrv) Measured Vapor Concentration (pptrv) Standard Deviation (pptrv) Measured Vapor Concentration (pptrv) Standard Deviation (pptrv) 0.1 46 15.2 2.6 24.5 0.4 0.5 230 59.3 9.6 70.0 4.0 1 455 108.9 20.0 149.9 1.1

18


Page 19 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


Table 5: Experimental conditions for the generation of trace RDX vapor. Vapor concentration is linear with respect to nebulizer solution concentration (R2 = 0.995) with an average error of quantitation of 6%. Italicized concentrations indicate a significant contribution of measured vapor to emanation for the nebulizer heat tube. Solution Concentration (µg mL -1) Solution Flow Rate (µL min-1) Total Air Flow (L min-1) Theoretical Vapor Concentration Measured Vapor Concentration Vaporization Efficency (%) 1 40 11 455 pptrv 149 pptrv 33 0.5

40

11

230 pptrv

71 pptrv

31

0.1

40

11

46 pptrv

25 pptrv

54

0.05

40

11

23 pptrv

9.2 pptrv

40

0.01

40

11

4.6 pptrv

2.6 pptrv

57

0.005

40

11

2.3 pptrv

1.7 pptrv

74

0.001

40

11

460 ppq v

760 ppq v

NA

0.0005

40

11

230 ppq v

710 ppq v

NA

Table 6: Measured PETN vapor concentration as a function of nebulized solution PETN concentration. Solution Concentration (µg mL-1 ) Theoretical Vapor Concentration (pptrv) Sample Time (min) Measured Vapor Concentration (pptrv) Vaporization Efficency (%) 0.1 32 30 25.9 81 0.5 160 18 91.7 65 1 320 30 163 51

19



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Page 20 of 25

Figures Figure 1. A. Schematic diagram of fluidic connections for the PMLDS nebulizer system, including the liquid flow meter and control box. The control box sets the liquid flow rate at 40 µL min-1. The air flow rate through the nebulizer is set to 1000 mL min-1. B. Position of K-type thermocouples on a heated transfer tube prior to coupling to the analyte manifold. Temperature set-point was 130°C for TNT and RDX and 100°C for PETN.

20


Page 21 of 25

100 90 80

Vaporization Efficency TNT (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


70 60 50 40 30 20 10 0 1

0.5

0.1

0.05

0.01

0.005

0.001

0.0005

Solution Concentration (µg mL-1)

Figure 2. TNT vaporization efficiency as a function of nebulizer solution concentration. Vapor quantitation was performed using the online PTV-GC-MS. In all cases the liquid flow rate was 40 µL min-1 and the air flow rate was 11 L min-1 and sample was collected for 1 minute. Error bars represent one standard deviation (N=3).

21



500 RDX Vapor Concentration (pptr)

5

450

RDX Vapor Concentration (pptr)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 25

400 350 300

4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 0

0.002

0.004

0.006

0.008

0.01

0.012

0.014


250 200 150 100 50 0 0

0.2

0.4

0.6

0.8

1

1.2


Figure 3. RDX vaporization concentration as a function of nebulizer solution concentration. Vapor quantitation was performed using the online PTV-GC-MS. In all cases the liquid flow rate was 40 µL min-1 and the air flow rate was 11 L min-1. Sample was collected for 1 minute for vapors generated from solutions with concentrations of 1 to 0.01 µg mL-1 and 2 minutes for all vapors generated from solutions with concentrations of 0.005 to 0.0005 µg mL-1. The inset illustrates a close up view was vapor concentration generated from nebulizer solutions between 0.0005 and 0.01 µg mL-1. The dotted line indicates the nominal vapor concentration. Error bars represent one standard deviation (N=3).

22


Page 23 of 25

Table of Contents Graphic 100

MFC

1L

min-1

381 pptrv

183 pptrv

38 pptrv

18 pptrv

3.4 pptrv

90

Nebulizer

1.5 pptrv

270 ppqv

160 ppqv

0.005

0.001

0.0005

80

Zero Air 40 µL min-1 Spraying Solution in Pressurized Chamber

Flow Sensor

Vaporization Efficency TNT (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


70 60 50 40 30 20

EPC Controller

10 0 1

0.5

0.1

0.05

0.01


23



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Page 24 of 25

References (1) Bai, H.; Li, C.; Shi, G. Q. Sensor Actuat B-Chem 2008, 130, 777-782. (2) Bonnot, K.; Bernhardt, P.; Hassler, D.; Baras, C.; Comet, M.; Keller, V.; Spitzer, D. Anal Chem 2010, 82, 3389-3393. (3) Cizek, K.; Prior, C.; Thammakhet, C.; Galik, M.; Linker, K.; Tsui, R.; Cagan, A.; Wake, J.; La Belle, J.; Wang, J. Anal Chim Acta 2010, 661, 117-121. (4) Davies, J. P.; Blackwood, L. G.; Davis, S. G.; Goodrich, L. D.; Larson, R. A. Anal Chem 1993, 65, 3004-3009. (5) Eiceman, G. A.; Preston, D.; Tiano, G.; Rodriguez, J.; Parmeter, J. E. Talanta 1997, 45, 5774. (6) Jaworski, J. W.; Raorane, D.; Huh, J. H.; Majumdar, A.; Lee, S. W. Langmuir 2008, 24, 4938-4943. (7) Pinnaduwage, L. A.; Wig, A.; Hedden, D. L.; Gehl, A.; Yi, D.; Thundat, T.; Lareau, R. T. J Appl Phys 2004, 95, 5871-5875. (8) Todd, M. W.; Provencal, R. A.; Owano, T. G.; Paldus, B. A.; Kachanov, A.; Vodopyanov, K. L.; Hunter, M.; Coy, S. L.; Steinfeld, J. I.; Arnold, J. T. Appl Phys B-Lasers O 2002, 75, 367376. (9) Walker, N. R.; Linman, M. J.; Timmers, M. M.; Dean, S. L.; Burkett, C. M.; Lloyd, J. A.; Keelor, J. D.; Baughman, B. M.; Edmiston, P. L. Anal Chim Acta 2007, 593, 82-91. (10) Yang, X. G.; Du, X. X.; Shi, J. X.; Swanson, B. Talanta 2001, 54, 439-445. (11) Collins, G. E.; Giordano, B. C.; Sivaprakasam, V.; Ananth, R.; Hammond, M.; Merritt, C. D.; Tucker, J. E.; Malito, M.; Eversole, J. D.; Rose-Pehrsson, S. Rev Sci Instrum 2014, 85. (12) Chen, Y.; Xu, P. C.; Li, X. X. Nanotechnology 2010, 21. (13) Grate, J. W.; Ewing, R. G.; Atkinson, D. A. Trac-Trend Anal Chem 2012, 41, 1-14. (14) Lubrano, A. L.; Field, C. R.; Newsome, G. A.; Rogers, D. A.; Giordano, B. C.; Johnson, K. J. J Chromatogr A 2015, 1394, 154-158. (15) Giordano, B. C.; Lubrano, A. L.; Field, C. R.; Collins, G. E. J Chromatogr A 2014, 1331, 38-43. (16) Field, C. R.; Lubrano, A. L.; Rogers, D. A.; Giordano, B. C.; Collins, G. E. J Chromatogr A 2013, 1282, 178-182. (17) Field, C. R.; Lubrano, A.; Woytowitz, M.; Giordano, B. C.; Rose-Pehrsson, S. L. Jove-J Vis Exp 2014. (18) Field, C. R.; Giordano, B. C.; Rogers, D. A.; Lubrano, A. L.; Rose-Pehrsson, S. L. J Chromatogr A 2012, 1227, 10-18. (19) Field, C. R.; Terray, A. V.; Lubrano, A. L.; Rogers, D. A.; Hart, S. J.; Rose-Pehrsson, S. L. Rev Sci Instrum 2012, 83. (20) Shigeta, K.; Sato, K.; Furuta, N. J Anal Atom Spectrom 2007, 22, 911-916. (21) Martinez-Lozano, P.; Rus, J.; de la Mora, G. F.; Hernandez, M.; de la Mora, J. F. J Am Soc Mass Spectr 2009, 20, 287-294. (22) Meurer, E. C.; Chen, H.; Riter, L.; Cotte-Rodriguez, I.; Eberlin, M. N.; Cooks, R. G. Chem Commun 2004, 40-41. (23) Yinon, J.; Harvan, D. J.; Hass, J. R. Org Mass Spectrom 1982, 17, 321-326. 24


Page 25 of 25

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(24) Ewing, R. G.; Waltman, M. J.; Atkinson, D. A.; Grate, J. W.; Hotchkiss, P. J. Trac-Trend Anal Chem 2013, 42, 35-48.

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Trace Explosives Vapor Generation and Quantitation at Parts per

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