Determination of inorganic ions in explosive residues using capillary zone electrophoresis

CHINESE JOURNAL OF CHROMATOGRAPHY Volume 26, Issue 6, November 2008 Online English edition of the Chinese language journal Cite this article as: Chin J Chromatogr, 2008, 26(6): 667–671.

RESEARCH PAPER

Determination of inorganic ions in explosive residues using capillary zone electrophoresis FENG Junhe1,2, GUO Baoyuan3,*, LIN Jin-Ming2,*, XU Jianzhong4, ZHOU Hong4, SUN Yuyou4, LIU Yao4, QUAN Yangke4, LU Xiaoming1 1 2

Department of Chemistry, Capital Normal University, Beijing 100037, China Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology, Department of Chemistry, Tsinghua University, Beijing 100084, China

3

State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences,

4

Institute of Forensic Science, Ministry of Public Security, Beijing 100038, China

Chinese Academy of Sciences, Beijing 100085, China

Abstract: Five anions (chlorate, perchlorate, nitrate, nitrite, and sulfate) and two cations (ammonium and potassium) in explosive residues were separated and determined using capillary zone electrophoresis (CZE) with indirect ultraviolet (UV) detection. The electrolyte buffer solution for the cation separation was 10 mmol/L pyridine (pH 4.5)-3 mmol/L 18-crown-6-ether. Ammonium and potassium ions were baseline separated in less than 2.6 min with the detection limits of 0.10 mg/L and 0.25 mg/L (S/N = 3), respectively. The electrolyte buffer solution for the anion separation consisted of 40 mmol/L boric acid-1.8 mmol/L potassium dichromate-2 mmol/L sodium tetraborate (pH 8.6). Tetramethyl ammonium hydroxide (TMAOH) was used as an electro-osmotic flow modifier. All five anions were well separated in less than 4.6 min with the detection limits of 0.10 to 1.85 mg/L (S/N = 3). This method was successfully used in the investigations of real samples for the confirmation of explosive types. Key Words: capillary zone electrophoresis (CZE); indirect ultraviolet detection; inorganic ions; explosive residues

In recent years, the existence and spread of terrorism activity and crime worldwide have occurred frequently. As one of the most common and harmful mode of terrorism activities, explosion attack has caused worldwide attention. Therefore, understanding and grasping the detection methods of explosive residues is an important facet of counterterrorism procedures. Many analytical techniques have been used to determine the explosive residues, such as ion chromatography (IC) [1], gas chromatography (GC) [2], mass spectrometry (MS) [3], scanning electron microscopy-energy dispersion X-ray (SEM/EDX) [4], infrared spectrometry (IR) [5], capillary electrophoresis (CE) [6,7], and so on. IC and CE are the main analytical techniques for inorganic explosives. Compared with IC, CE has major advantages of experimental simplicity, higher separation efficiency, faster separation, and lower cost. CE may also be more tolerable than IC towards variations in the sample matrix without the column fouling problem. CE has shown important

applications in trace analysis of inorganic explosive residues and gunshot residues. Several reviews have been reported in recent times describing the role of CE in forensic investigations [8–12]. Several explosives are composed of inorganic compounds (such as ammonium nitrate explosive, black powder, chlorate/perchlorate explosive, detonator, and so on). The determination of the corresponding residue ions, such as ammonium, potassium, sodium, sulfate, chlorate, perchlorate, nitrite, or nitrate, is crucial for rapidly identifying the types of explosives at a terrorist scene [13]. For example, when potassium, chlorate, and perchlorate exist in the explosive residues, it can be defined as chlorate/perchlorate explosives. However, the types of ammonium nitrate explosives are identified when both qualitative and quantitative analysis results have to be offered. Ammonium nitrate is not only a composition of ammonium nitrate explosives, but also a composition of chemical fertilizers, which are

Received August 18, 2008; revised September 30, 2008 * Corresponding authors. LIN Jin-Ming. Tel/Fax: +86-10-62792343, E-mail: [email protected]. Guo Baoyuan. Tel/Fax: +86-10-62841953, E-mail: [email protected]. This work was supported by the National Basic Research Program of China (“973” Program, No. 2007CB714507) and the Research Fund for the Doctoral Program of Higher Education (No. 20070003026). Copyright © 2008, Chinese Chemical Society and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.

FENG Junhe et al. / Chinese Journal of Chromatography, 2008, 26(6): 667–671

commonly used. Furthermore, NH4 + and NO3 may also exist in decayed plants and other explosive residues. In this study, five anions (chlorate, perchlorate, nitrate, nitrite, and sulfate) and two cations (ammonium and potassium) were chosen as the target analytes, because these ions are known to be present in the post-blast residues of inorganic explosives. The indirect ultraviolet (UV) detection method has been employed, since all target ions exhibit low absorption in the UV region [14].

1

Experimental

1.1 Reagents All the reagents used were of analytical reagent grade without further purification. The 18-crown-6 (t 99.5%) was purchased from Kanto Chemicals (Tokyo, Japan). All the other reagents used in the experiments were purchased from the Chemical Reagent Company (Beijing, China). The water for the preparation of electrolyte buffer solution and standard solutions was deionized using a Milli-Q purification system with a 0.2 ȝm fiber (Barnstead, CA, USA). Actual explosive soil samples were obtained from the Ministry of Public Security of the People’s Republic of China. 1.2 Apparatus The separations were performed on a Beckman Coulter P/ACE MDQ Capillary Electrophoresis System (Beckman Fullerton, CA, USA) equipped with a UV detector. Beckman 32 Karat software (Fullerton, CA, USA) was used for data collection and processing. Uncoated fused silica capillaries, 75 ȝm i.d., and 60 cm total length (50 cm to the detector) used throughout the experiments, were purchased from Yongnian Optical Fiber Factory (Yongnian, China). 1.3 Preparation of standard solutions The standard solutions of the seven target ions (1000 mg/L) were prepared individually by dissolving their sodium salts (or ammonium nitrate) in Milli-Q water. Mixed working standard solutions were prepared by diluting the standard solutions in water before use, due to the sensitivity of the instrument and the range of linearity. All stock and standard solutions were filtered through a 0.45 ȝm pore filter and stored at 4ºC in a refrigerator. 1.4 Pretreatment of real samples Six portions of a real sample were sifted out through a 30 mesh to remove gravel and straw. Five grams of soil were weighed by an electronic scale and placed in a sample vial. After the addition of 20 mL of Milli-Q water, it was oscillated for 2 h for extraction. The upper layer was collected after centrifugation (3500 r/min, 20 min). The same extraction procedure was also performed with a blank soil. Finally, the extraction solution was filtered through a 0.45 ȝm syringe filter before use. 1.5 Electrophoretic procedures Prior to each running, the capillary was rinsed with 0.1 mol/L

NaOH for 2 min, deionized water for 2 min, and running buffer solution for 2 min. The injection time was 10 s and injection pressure was 3.45 kPa (0.5 psi). The voltage was set at 25 kV for the cation separation and 25 kV for the anion separation. The detection wavelength was 254 nm. The electrolyte buffer solution for the cation separation was 10 mmol/L pyridine (pH 4.5)-3 mmol/L 18-crown-6-ether. The electrolyte buffer solution for the anion separation consisted of 40 mmol/L boric acid-1.8 mmol/L potassium dichromate-2 mmol/L sodium tetraborate (pH 8.6). Tetramethyl ammonium hydroxide (TMAOH) was used as an electro-osmotic flow modifier. The electrolyte buffer solutions were sonicated and filtered through a 0.45 ȝm pore filter and then stored in a refrigerator at 4°C before use. To achieve reproducibility, all experiments were performed at 25°C and were run in triplicate.

2

Results and discussion

2.1 Optimization of the cation separation conditions The separation of NH4+ and K+, which can both be present in the samples, is difficult due to their similar electrophoretic mobilities at neutral pH and acidic pH. However, this problem can be solved by modifying the electrolyte with 18-crown-6 ether because potassium ions form the complex with 18-crown-6 ether and the effective mobility for potassium ion decreases, whereas, the migration time for ammonium ion has no significant changes [15]. Fig. 1 shows the electropherogram of K+ and NH4+. It is observed that the two cations are baseline separated in less than 2.6 min. The detection limits of K+ and NH4+, which are listed in Table 1, are 0.25 mg/L and 0.10 mg/L (calculated at a signal-to-noise ratio of 3), respectively. Table 1 also shows good linear relationship, sensitivity, and reproducibility of this method.

Fig. 1

Electropherogram of two cation standard solution

Running buffer: 10 mmol/L pyridine (adjusting pH to 4.5 with CH3COOH)-4 mmol/L 18-crown-6. Electrophoresis conditions: injection, 3.45 kPa (0.5 psi) × 10 s; running voltage, 25 kV; capillary, 60 cm of total length, 50 cm of effective length, 75 ȝm i.d.; indirect UV detection at 254 nm. 1. NH4+; 2. K+.

FENG Junhe et al. / Chinese Journal of Chromatography, 2008, 26(6): 667–671

2.2 Optimization of the anion separation conditions Methods using electrolyte buffer solution containing chromate as the probe were used extensively for the detection of inorganic anion ions in many articles [16–18]. In this study, sodium chromate and boric acid buffer solution system with cetyltrimethylammonium bromide (CTAB) was used as an electro-osmotic flow (EOF) modifier. However, it was found that the baseline drifted severely and the buffer solution easily formed a suspension. Thus, dichromate was selected as the anion probe to solve this problem. It was better to use a weak cationic base to adjust the buffer solution pH because an appropriate amount of boric acid presented in an anionic form at pH 8.6 and reduced the detection ability of the anions. Therefore, the optimized anion electrolyte consisted of 40 mmol/L boric acid-1.8 mmol/L potassium dichromate-2 mmol/L sodium tetraborate. Diethylene triamine was used to adjust the buffer solution pH to 8.6, and tetramethyl ammonium hydroxide was used as the EOF modifier. The baseline separation of the five target analytes was accomplished in less than 4.6 min, which is shown in Fig. 2. The results of the quantitative analysis are listed in Table 1. Table 1

Analyte

The pH influences of the running electrolyte on the migration times of the five anions were investigated in this study. The separations were carried out at pH values of 7.0, 7.4, 7.8, 8.2, 8.6, and 9.0. Fig. 3 shows the electropherograms of the five anions at different pH values. It was observed that the retention times of all target anions decreased with the increase of pH value. However, the migration time of ClO4 increased when the pH increased from 8.2 to 9.0. ClO4 and NO3 were not separated if pH was lower than 7.5 (Fig. 3-a and b). The baseline separation of the five anions was obtained at pH 7.8, and the ions were eluted in the sequence of NO2, NO3, ClO4, SO42, and ClO3 (Fig. 3-c). At pH 8.2, the separation of ClO4 and SO42 was not achieved (Fig. 3-d). Five ions were also baseline separated at pH 8.6 (Fig. 3-e) with the migration times shorter than those at pH 7.8, and the ions were eluted in the sequence of NO2, NO3, SO42, ClO4, and ClO3. At pH 9.0, ClO4 and ClO3 were not separated (Fig. 3-f). Therefore, considering all kinds of factors, the pH of the running electrolyte buffer solution was optimized to 8.6 for the separation.

Retention times, detection precisions, limits of detection (LODs) and correlation coefficients of seven inorganic ions tR/

RSD of

RSD of peak

LOD/

min

tR/%

area/%

(mg/L)

r2

NH4+

2.34

0.16

2.33

0.10

0.9992

K+

2.59

0.20

2.95

0.25

0.9994

NO2

3.81

0.21

1.94

1.20

0.9994

NO3

3.93

0.20

2.47

1.52

0.9996

SO42

4.23

0.27

1.44

0.90

0.9993

ClO4

4.40

0.23

1.26

0.50

0.9994

ClO3

4.51

0.21

2.57

1.85

0.9995

Fig. 3

Electropherograms of five anions in running buffers with different pH values a. pH 7.0; b. pH 7.4; c. pH 7.8; d. pH 8.2; e. pH 8.6; f. pH 9.0. 1. NO2; 2. NO3; 3. SO42; 4. ClO4; 5. ClO3.

Fig. 2

Electropherogram of five anion standard solution

Running buffer: 40 mmol/L boric acid-1.8 mmol/L potassium dichromate-2 mmol/L sodium tetraborate, adjusting pH to 8.6 with diethylene triamine. Conditions: running voltage, 25 kV; other conditions are the same as in Fig. 1. 1. NO2; 2. NO3; 3. SO42; 4. ClO4; 5. ClO3.

The injection time is also an important factor for the separation performance. To improve the detection limits, sample-stacking techniques, including large-volume sample stacking and field-amplified sample injection have been used [19]. In this study, the influence of injection time on the migration time of target ions was studied and the results are illustrated in Fig. 4. It was evident that the migration time decreased with the prolonging of injection time; however, the injection time had little effect on the migration time when it was above 15 s.

FENG Junhe et al. / Chinese Journal of Chromatography, 2008, 26(6): 667–671

Fig. 4

Effect of injection time on migration times of target ions

1. NO2; 2. NO3; 3. SO42; 4. ClO4; 5. ClO3.

Considering that the peak area broadened with the increase of injection time, the injection time was adopted as 10 s in the following experiments. The effect of separation voltage (15 kV to 30 kV) on the separation was also studied in this article, and it was found that the lower the voltage was, the longer was the separation time. Thus, the separation voltage was set at 25 kV to obtain rapid detection. 2.3 Determination of real samples and spiked recoveries The optimized conditions were successfully applied to the analysis of the real samples for the determination of explosive types. Six residue samples were collected from several explosive sites by the Ministry of Public Security of the People’s Republic of China. In considering that in the fields of explosive residues some trace elements (such as NH4+, NO3, Cl, K+, Na+, etc.) may also exist in nature, blank control experiments were carried out in this study (Fig. 5-a, Fig. 6-a). It was found that the blank control samples also contained trace amounts of NH4+, NO3, and ClO4. The electropherograms of two real samples are shown in Fig. 5-b, c (cation samples) and Fig. 6-c, d (anion samples). SO42 was not detected in the real samples; NO2 was also difficult to detect because it was easy to be oxidized under high temperature and pressure environment at the explosive moment. ClO3 was not detected in sample 1 and sample 4. Sample 2, sample 3, sample 5, and sample 6 were identified as chlorate/perchlorate explosives by comparing the retention times of detected ions with the standard ions, and comparing the real explosive samples with blank samples and typical explosive samples. Sample 1 and sample 4 were identified as ammonium nitrate explosives. Quantitative analysis experiments were needed to determine the explosive types further. The results of quantitative analysis are listed in Table 2. It was observed that ClO3 was not detected in sample 1 and sample 4, and the concentrations of K+ and ClO4 in sample 1 and sample 4 were significantly lower than other samples, and the concentrations of NH4+ and NO3 were higher than other samples.

Fig. 5

Electropherograms of cations in samples

a. blank soil; b. ammonium nitrate explosive residue; c. chlorate/perchlorate explosive residue. 1. NH4+; 2. K+.

Fig. 6

Electropherograms of anions in samples

a. blank soil; b. blank soil spiked with standards; c. ammonium nitrate explosive residue; d. chlorate/perchlorate explosive residue. 1. NO2; 2. NO3; 3. SO42; 4. ClO4; 5. ClO3.

FENG Junhe et al. / Chinese Journal of Chromatography, 2008, 26(6): 667–671

Thus, it was concluded that the sample 1 and sample 4 were ammonium nitrate explosives. Finally, in the spiking experiments of sample 1 and sample 6, the recoveries of the seven ions were from 84.4%–135.2% (Table 3). Table 2 Sample No.

NH4

Quantitative results of real samples +

+

K

NO2



NO3



SO4

2

ClO4

mg/kg 

ClO3



method is simple, rapid, sensitive, and inexpensive, and could be applied in field detection.

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1

4.08

2.26

n

6.01

n

4.52

n

2

3.25

6.43

n

5.95

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5.84

7.23

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4.21

3.57

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5.46

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5.54

7.62

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5.06

2.64

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Sample 6

found/

R/

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found/

R/

(mg/kg)

(mg/kg)

%

(mg/kg)

(mg/kg)

%

NH4+

4.08

8.84

95.2

3.94

8.53

91.8

K+

2.26

6.93

93.4

6.46

11.58

102.4

ion

back./

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NO2

n

6.22

124.4

n

5.15

103.0

NO3

6.01

10.23

84.4

6.06

10.69

92.6

n

5.85

117.4

n

6.07

121.4

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4.52

9.35

96.6

5.88

10.82

98.8

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n

6.76

135.2

7.63

12.18

91.0

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SO4

2

ClO4 ClO3



* spiking level: 5.00 mg/kg for each inorganic ion; back.: background; R: recovery; n: not detected.

3

Conclusions

Seven important inorganic ions in explosive residues were baseline separated and detected using capillary zone electrophoresis with indirect UV detection. The optimized conditions were employed for determining real samples and the types of inorganic explosives were confirmed successfully. This

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