Phosgene (carbonyl dichloride) and diphosgene (trichloromethyl chloroformate) are toxic compounds that damage the alveolar air/capillary air–blood barrier in the lungs. The change in the permeability of this barrier due to the action of the compounds gives rise to typical pulmonary edema, and the reduced resistance against secondary infection supports the development of bronchopneumonia. Inhalation toxicity of phosgene and diphosgene (LCt50
) is 3200 mg·min/m3
, the maximum vapor concentration (volatility) is 4,300,000 mg/m3
for phosgene (7.6 °C) and 45,000 mg/m3
(20 °C) for diphosgene [1
]. Phosgene is an important industrial chemical in the organic synthesis domain. Both phosgene and diphosgene were one of the main types of chemical warfare agents used (CWA) in the deployment of major chemical weapons, mainly during World War I. Currently, phosgene is listed on Schedule 3 of the Chemical Weapons Convention. Hazards may arise from phosgene and, partly, diphosgene leaks during chemical industrial accidents and from civil wars and terrorist attacks in which the compounds are used as efficient chemical weapons.
Great attention is constantly paid to analytical methods for phosgene and diphosgene. Apart from technically advanced physical and physico-chemical detectors and analysers, attention is paid to rapid, simple, and inexpensive detection methods for phosgene/diphosgene in air based on color reactions using detection papers, strips, chalks, and detection tubes [2
]. The traditional procedures and methods are based on a nucleophilic substitution reaction (acylation) of phosgene/diphosgene with analytical reagents such as p
], 4,4′-bis(dimethylamino)benzophenone [6
], or anabasine [10
]. A number of novel reagents providing colored reaction products that exhibit visible, often predominating fluorescence were developed during the past decade. Examples include chemosensors based on pyronine, rhodamine, iminocoumarin, BODIPY, and other organic compounds [11
]. Their research is only at the laboratory stage, commercial evaluation is pending.
Detection tubes are among the most widespread simple tools to detect phosgene/diphosgene. They have different designs (the indication charge can be completed with a detection solution in an ampoule) and different sensitivity and selectivity levels, and they also have different resistance to the weather conditions and to ageing. Existing detection tubes are almost solely based on the reaction with 4-(p
-nitrobenzyl)pyridine and N
-phenylbenzylamine or with p
-dimethylaminobenzaldehyde in combination with aniline derivatives [10
]. Given the existence of new colorimetric and fluorescence reagents (chemosensors), it is not surprising that efforts are made to examine the feasibility of using them in detection tubes. Such devices can be referred to as ‘second generation detection tubes’. The present work is devoted to the development of a detection tube with a chemosensor based on o
] in combination with a specifically prepared composite carrier. Such a device could be used for the orientational detection of phosgene/diphosgene in the armed forces and rescue services. We previously used this chemosensor for the preparation of a detection paper for use on liquid CWA [26
2. Materials and Methods
Materials used for the preparation of carriers included menthol (Dr. Kulich Pharma, Hradec Králové, Czech Republic), camphor (Dr. Kulich Pharma, Hradec Králové, Czech Republic), magnesium aluminum metasilicate (Neusilin® US2, Fuji Chemical Industry, Toyoma, Japan), microcrystalline cellulose (Avicel® PH-101, FMC Biopolymer, Wallingstown, Ireland), microcrystalline cellulose and sodium carboxymethylcellulose (Avicel® RC-581, FMC Biopolymer, Brussels, Belgium), o-phenylenediamine-pyronin (PY-OPD, synthesized by the University of Chemistry and Technology, Prague, Czech Republic; the spectral data corresponded to the literature), chloroform (Merck, Darmstadt, Germany), and purified water (Ph. Eur. 10).
The tests were made with calibrated 1% phosgene (Linde Gas, Prague, Czech Republic), diluted as required with air in 1 dm3 Tedlar bags (SKC, Eighty Four, PA, USA). Diphosgene (Merck, Darmstadt, Germany) was used for the gas chamber test.
Instrumentation used for the preparation of carriers included a Stephan UMC 5 electronic mixer (A. Stephan & Söhne, Hameln, Germany), a PharmTex 35T single-screw extruder and spheronizer (Wyss & Probst Engineering, Ettlingen, Germany) and an AS200 Basic sieve shaker (Retsch, Haan, Germany). A MIRA3 scanning electron microscope (SEM) (Tescan Orsay Holding, Brno, Czech Republic) was used to obtain SEM images of the pellets. The samples were coated with a 20 nm gold layer by metal sputtering in argon atmosphere on a Q150R ES Rotatory-Pumped Sputter Coater/Carbon Coater (Quorum Technologies, Laughton, UK). A secondary electron detector was used, and accelerating voltage of 3 kV was applied for the SEM measurement itself. The specific surface area of the samples was measured with the BET method on a Quantachrome Autosorb iQ3 gas porismeter (Quantachrome Instruments, Boynton Beach, FL, USA). The samples were degassed at 80 °C, which was reduced to 30 °C after attaining the pressure increase rate of 20 mtorr/min, and the samples were kept under vacuum until the measurement.
The performance of the detection tubes was tested in a 0.5 m3 gas chamber equipped with a thermostat and a fan. The samples were taken with a manual aspiration system, stroke volume 100 ± 5 cm3 (Kavalier, Votice, Czech Republic).
An LMG 173 tristimulus colorimeter (Dr. Lange, Düsseldorf, Germany) was used for the objective measurement of the indication charge color intensity. A 366 nm radiation wavelength was obtained from an L92 UV lamp (Leuchtturm, Geesthacht, Germany).
2.3. Carrier Preparation
A powder mixture of 44 g of Avicel®
PH-101 microcrystalline cellulose, 11 g of Avicel®
RC-581 microcrystalline cellulose with sodium carboxymethylcellulose, 25 g of magnesium alumino metasilicate (Neusilin®
US2) and 20 g of menthol (batch M20.0) or camphor (batch C20.0) was homogenized in a mixer at 1500 rpm for 5 min and subsequently wetted with purified water (mixture/water ratio 1:1.15) at a flow rate of 100 mL/min. The pellets were prepared from the moistened mass by extrusion/spheronization on an axial single-screw extruder/spheronizer at 60 rpm through a grid of 1.25 mm mesh. The extrudate was then spheronized at 1000 rpm for 20 min. The product was dried in a hot air oven at 60 °C for 24 h. The reference pellet batch (N) contained neither menthol nor camphor. The physical properties of the pellets (bulk density, tapped density, Hausner ratio, hardness, friability, pycnometric density, interparticular and intraparticular porosity, and sphericity) were determined by conventional methods [27
2.4. Carrier (Pellet) Impregnation and Detection Tube Preparation
A 100 g batch of 0.8–1.25 mm pellets was impregnated with 50 mL of 1% PY-OPD solution in chloroform. The impregnated pellets were allowed to air dry to complete solvent evaporation. Dry and non-agglomerated carriers, light-yellow in color, were stored in a hermetically sealed container protected from sunlight.
The detection tubes were prepared as follows: The charge was poured into a 5 mm diameter glass tube to obtain a 10 mm long layer. This was stabilized by using polyethylene (PE) elements to enable air to flow through the system. The tubes were hermetically sealed (the total length of 100 mm). The construction of the detection tube and its use are shown in Figure 1
2.5. Detection Tube Testing
We selected testing in a gas chamber in which a specific volume of diphosgene solution in hexane had been evaporated, as the basic method of detection tube performance testing. The detection tube response to phosgene/diphosgene was evaluated based on (i) VIS color intensity assessed with the naked eye and measured on a tristimulus colorimeter; and (ii) intensity of fluorescence excited under a UV lamp (assessed with the naked eye). The stability of charge was tested on detection tubes exposed to elevated temperatures in a dryer for a prolonged period of time. The maximum temperature applied was 60 °C (selected with regard to the material of structural elements—polyethylene), the maximum heat burden duration was 2 months. The ageing process of the charge (evaluated with the naked eye and on the tristimulus colorimeter) and its response to phosgene/diphosgene in air were evaluated periodically. The color changes were evaluated based on the experience gained by the printing industry, where the human eye is capable of noting ΔE values measured with a tristimulus colorimeter as follows: ΔE < 0.2 imperceptible; ΔE = 0.2–0.5 very weakly; ΔE = 0.5–1.5 weakly; ΔE = 1.5–3.0 clearly, ΔE = 3.0–6.0 very clearly, ΔE = 6.0–12.0 strongly, ΔE > 12.0 very strongly.