Neutral DesorptionUsing a Sealed Enclosure to Sample Explosives on Human Skin for Rapid Detection by EESI-MS
Huanwen Chen,*a,b Bin Hu,a Yan Hu,a Yanfu Huanb,Zhiquan Zhouc,Xiaolin Qiao*c
a College of Chemistry, Biology and Material Science, East China Institute of Technology, Fuzhou 34400 P. R. China; b College of Chemistry, Jilin University, Changchun, 130023 P. R. China; c College of Information Science and Engineering, Harbin Institute of Technology at Weihai, Weihai 264000 P. R. China
Supporting Information
Effect of skin temperature and gas temperature on signal levels
Firstly, experiments were done on skin surfaces of different temperature. Under normal ambient conditions (22ºC, atmospheric pressure),it was measure that the temperature of the right hand skin of a healthy volunteer was 31.2ºC , which was lower than the body temperature (37.2 ºC), which was measured as usual under his tongue using a thermometer. To obtain different hand skin temperature, the hand was marinated in sufficient water at 35ºC, 40ºC, and 45ºC for 20 min, respectively, allowing the thermal equilibrium between the hand skin and the water was reached, so that the hand skin temperature wasat the same level of the water temperature. Wet hand was then wiped quickly using a dry towel heated to 60 ºC. Aninfrared lamp (200 W) was used to irradiate the hand skin at a proper distance, preventing notable temperature drop on the skin surface. A certain amount of RDX (1ng, 10 μL, and methanol solution) was deposited on skin to form a sample spot area less than 1 cm2. The RDX sample spot was dried readily by exposing to the experimental conditions. The sample spot was placed under the ND sampling device by following the procedure described in the main text when the neutral desorption gas (room temperature, 22ºC) was turned on or off. Table S1 summarizes the experimental data. Alternatively, the sample spot was placed underneath the ESI plume, however, no signal of explosives was detected even a much higher amount of RDX (e.g., 10 μg) was deposited on the skin surface.
On the other hand, the neutral desorption gas was cooled down by incubation of the gas transfer line (copper tube, ID. 1.6 mm, OD. 2 mm) using ice and water mixture. The total length of the cooled gas line was 3 m, which was Z-stacked inside the ice box, providing a relatively long residence time for the gas inside the transfer line for heat exchange. The cool gas beam was used for neutral desorption as usual, and the results are summarized in Table S1. For safety purpose, no experiment was executed under a temperature lower than 0 ºC.
Table S1 Summary of the signal responses to different temperature of the skin and the neutral desorption gas
ExperimentCode / Neutral Desorption Gas / a Skin temperature(ºC) / Amount of RDX (ng) / b Signal Level
ON/OFF / Temperature(ºC)
1 / OFF / 22 / 31 / 1.0 / c N/A
2 / OFF / 22 / 35 / 1.0 / N/A
3 / OFF / 22 / 40 / 1.0 / N/A
4 / OFF / 22 / 45 / 1.0 / N/A
5 / OFF / 22 / 45 / 10 / N/A
6 / OFF / 22 / 45 / 100 / N/A
7 / ON / 22 / 31 / 1.0 / 3.7×103
8 / ON / 22 / 35 / 1.0 / 3.5×103
9 / ON / 22 / 40 / 1.0 / 3.8×103
10 / ON / 22 / 45 / 1.0 / 3.3×103
11 / ON / Ice cooled / 31 / 1.0 / 3.2×103
12 / ON / Ice cooled / 35 / 1.0 / 3.5×103
13 / ON / Ice cooled / 40 / 1.0 / 3.7×103
14 / ON / Ice cooled / 45 / 1.0 / 3.6×103
15 / OFF / Ice cooled / 45 / 100 / N/A
a: The alteration of temperaturewas less than ±1 ºC.
b: The signal was measured using the characteristic fragment (m/z 237) generated in MS2 experiment.
c: N/A means the signal was not detectable (under the noise level) during the experiments.
Effect of the length of the sample transfer line on the signal levels
Figure S1.Effect of the distance for neutral analyte transportation on the normalized signal intensities of RDX (20 pg) and TNT (20 pg). To visualize the details of the signal distribution using a short sample transfer line (e.g., 1~30 cm), data obtained using a sample transfer line longer than 2 m is not included in the figure. However, the data showed the signals were maintained at similar levels.
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