Food Chemistry 172 (2015) 681–684

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Food Chemistry journal homepage: www.elsevier.com/locate/foodchem

Analytical Methods

Rapid measurement of free cyanide in liquor by ion chromatography with pulsed amperometric detection Wenlin Wu ⇑, Quanwei Xiao, Ping Zhang, Mei Ye, Yuping Wan, Hengxing Liang Department of Food, Chengdu Institute of Product Quality Inspection Co., Ltd., Chengdu, Sichuan 610100, China Department of Food, Chengdu Product Quality Supervision and Inspection Institute, Chengdu, Sichuan 610100, China

a r t i c l e

i n f o

Article history: Received 15 October 2013 Received in revised form 13 August 2014 Accepted 10 September 2014 Available online 17 September 2014 Keywords: Liquor Free cyanide Ion chromatography Pulsed amperometric detection

a b s t r a c t This study investigated the measurement of free cyanide in liquor by ion chromatography coupled with pulsed amperometric detection (IC–PAD). Eluent concentration, interferent evaluation and method performance were discussed. Results show that free cyanide in liquor can be rapidly determined by the optimised IC–PAD method. A sample requires only 1:100 dilution and simple filtration before being subjected to IC–PAD. The linear range is 1–5000 lg/L with an R value of 0.9998. The detection limit is 1 lg/L for a 25 lL injection loop. The overall relative standard deviation (RSD) of the method is less than 5%, and the recovery range is from 98.1% to 105.0%. This study has been proven significant and may have potential applications in liquors analysis. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Free cyanide is classified as a hazardous material and is highly harmful to humans (Dash, Gaur, & Balomajumder, 2009). From a toxicological perspective, free cyanide is a non-reversible inhibitor of cytochrome C oxidase, thus preventing cellular respiration (Nambisan, 2011). Free cyanide in liquor is formed in the fermentation process mainly from various raw materials such as cereal and cassava (Burns, Bradbury, Cavagnaro, & Gleadow, 2012). Free cyanide must also be detected at low concentrations (lg/L) because of its significant toxicity (Rezaul Haque & Howard Bradbury, 2002). The maximum contaminant level for free cyanide set by the US EPA in drinking water is 200 lg/L; whilst the European Union has an even lower limit of 50 lg/L. The measurement of free cyanide concentration in liquor is hindered by many obstacles. Thus, the development of a rapid and simple method to determine free cyanide in liquor is important. Several methods such as spectrophotometry (EPA, 1996), titration (Breuer, Sutcliffe, & Meakin, 2011), sequential injection method (Themelis, Karastogianni, & Tzanavaras, 2009) and atomic-absorption spectrophotometry (Xu, Xu, & Fang, 1984), have been investigated to measure free cyanide. Spectrophotometry is considered a ‘classic’ technique and is widely applied amongst ⇑ Corresponding author at: Department of Food, Chengdu Institute of Product Quality Inspection Co., Ltd., Chengdu, Sichuan 610100, China. Tel.: +86 15208427050. E-mail address: [email protected] (W. Wu). http://dx.doi.org/10.1016/j.foodchem.2014.09.052 0308-8146/Ó 2014 Elsevier Ltd. All rights reserved.

official methods for the measurement of free cyanide in liquor. The National Standard of the People’s Republic of China (NSPRC) approved the use of spectrophotometry to measure free cyanide; this process involves the conversion of the analyte to cyanogen chloride by chloramine-T followed by a reaction with isonicotinic acid, thus offering a maximum contamination level of 8 mg/L free cyanide (converted to 100% alcohol). This method requires distillation and has many measurement complications, including difficulty with high-pH solutions, oxidisers, low recovery and sulphur-bearing compounds. Compared with the abovementioned methods, ion chromatography (IC) is used extensively in the field (Kontozova-Deutsch, Krata, Deutsch, Bencs, & Van Grieken, 2008), food industry (Zhong et al., 2012) and pharmaceutical industry (Shen, Ouyang, Baeyens, Delanghe, & Yang, 2005), etc. IC exhibits unmatched advantages because of its simple operation, fast analysis and non-use of toxic solvents (Lucy, 1996). The development and application of pulsed amperometric detection provide IC with high selectivity and improved accuracy; these benefits have prompted the widespread application of pulsed amperometric _ detection to IC (Błazewicz et al., 2011). Studies have recently reported on the use of IC to measure free cyanide (Giuriati, Cavalli, Gorni, Badocco, & Pastore, 2004). Unfortunately, the measurement of free cyanide by IC exhibits many disadvantages such as manual eluent preparation, time-consuming operation, inaccurate concentration, high noise, unstable base line and low reproducibility. Thus, adopting a new approach that can generate accurate and stable eluents is important. To date, the eluent generator is widely used in producing eluents, thus making IC

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Table 1 Free cyanide waveform. Time (s) 0.00 0.20 0.90 0.91 0.93 1.00

Potential vs Ag/AgCl (V) 0.10 0.10 0.10 1.00 0.30 0.30

Gain region

Integration

Ramp

Off On On On Off Off

Off On (start) Off (end) Off Off Off

On On On On On On

4

2. Materials and methods

D 3

Charge (nC)

C 2

B

1

A

0

-1

more convenient than ever (Sekiguchi, Matsunaga, Yamamoto, & Inoue, 2000). The simple addition of water to the measurement process generates an extremely stable baseline and high-purity eluents. Furthermore, to our knowledge, the measurement of free cyanide in liquor by IC–PAD has been never reported. Our study focused on the application of IC–PAD to achieve the rapid measurement of free cyanide in liquor by optimising factors such as eluent concentration, interferent evaluation and method performance. The main aims of this research are to investigate application value of the rapid measurement of free cyanide concentration in liquor.

0

2

4

6

8

10

14

12

Time (Min) Fig. 1. Approximately 10 lg/L of free cyanide chromatograms obtained under the following eluent composition: A-5 mM KOH, B-10 mM KOH, C-20 mM KOH and D30 mM KOH. Flow rate: 1.0 mL/min, 25 lL injection volume, 30 °C column temperature and detection waveform (Table 1).

2.1. Reagents and materials Free cyanide standard solution (50 lg/mL) was purchased from the National Institute of Metrology (China). OnGuardÒ-II-RP Sample Pre-treatment Cartridges were purchased from Dionex Co. (Sunnyvale, CA, USA). Ultra-pure water (18.2 MX cm resistivity) was obtained from Milli-Q (France), and a water deionisation system was used throughout the experiments. 2.2. Sample preparation 2.2.1. OnGuard-RP preparation Approximately 5 mL of methanol was filtered through the cartridge followed by 10 mL of ultra-pure water at 5 mL/min. 2.2.2. Sample preparation A 1 mL of sample was pipetted into a 100 mL polypropylene volumetric flask, and the mark with ultra-pure water was diluted. The sample was then filtered through a 0.22 lm MCM membrane. To remove impurity, 5 mL of sample was filtered through the

Table 2 Interference of free cyanide (0.1 mg/L). Interferent Cl

Added concentration (mg/L) 0.5 5 50

Recovery (%)

Interferent

99.9 99.8 100.1

Mn2+

Added concentration (mg/L) 0.01 0.1 1

Recovery (%) 100.5 100.2 99.8

Br

0.01 0.1 1

100.4 99.7 100.1

Pb2+

0.01 0.1 1

99.5 100.4 100.0

I

0.01 0.1 1

99.9 100.2 99.8

HCHO

0.1 0.5 1

99.6 99.4 99.7

S2

0.01 0.1 1

100.3 100.2 100.6

CH3CHO

0.1 0.5 1

100.2 100.1 99.7

SCN

0.1 0.5 1

100.2 99.8 99.9

CH3OH

50 100 200

100.8 100.2 99.9

SO23

0.5 1 2

100.0 100.2 100.8

CH3CH2OH

50 100 200

100.7 100.6 101.3

SO24

0.5 5 20

100.9 100.4 100.6

HCOOH

1 10 50

100.7 101.1 101.0

NO23

0.5 10 50

100.9 100.2 99.9

CH3COOH

1 10 50

99.6 99.9 100.1

CO23

1 10 100

99.5 101.0 100.9

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35

3

30

Peak

1

2

The free cyanide waveform is presented in Table 1. Instrument control and data processing were controlled by a Chromeleon 6.8 data management system (Dionex, USA).

3

Charge (nC)

25

3. Results and discussion

C

20

3.1. Eluent concentration

2

15

The retention time of free cyanide peak depends on KOH in the AS16 column. Approximately 10 lg/L of free cyanide chromatographs were obtained with different eluent concentration (Fig. 1). The results show that the retention time of free cyanide peak decreases from 8.7 min to 3.5 min when KOH concentration increases from 5 mM to 30 mM. The free cyanide peak shape also becomes gradually symmetrical. The peak area slows when KOH > 20 mM. A compromise between the efficiency and separation effect of free cyanide is achieved. The eluent concentration used in this study is 10 mM KOH.

B

10

1

5

A

0 -5 0

5

10

15

20

Time (Min) Fig. 2. Comparison of (A) 100-fold dilution of liquor sample, (B) 10 lg/L standard solution, (C) 100-fold dilution of liquor sample spiked with 10 lg/L free cyanide.

3.2. Interference evaluation Interference evaluation was done in order to validate IC to the practical use in the complex system of liquor and achieve the goal of direct determination of free cyanide. A total of 17 types of components (Cl , Br , I , S2 , SCN , SO23 , SO24 , NO23 , CO23 , Mn2+, Pb2+, HCHO, CH3CHO, CH3OH, CH3CH2OH, HCOOH, and CH3COOH) were chosen (Table 2). A series of synthetic samples that contain both free cyanide (0.1 mg/L) standards and the 3 gradient interferent concentration were measured. The added amount of the interferents covered or extended their average content in liquor. The recovery of free cyanide from these interferents was used to evaluate the effect of interference. Table 2 shows good free cyanide recovery in the presence of all investigated interferents. The recovered free cyanide ranges from 99.5% to 101.3%. In general, these interferents do not interfere with free cyanide. 3 different chromatograms (liquor sample, standard solution, and liquor sample spiked with standard solution) were acquired with 10 mM KOH eluent (Fig. 2). Due to the high selectivity of pulsed amperometric detection, liquor sample chromatogram (A) contains only a free cyanide peak. And it showed good recovery.

OnGuard-RP cartridge at 5 mL/min, discarding the first 3 mL of sample. The remaining filtrate was collected for inclusion to the IC–PAD process.

2.3. Instrumentation and chromatographic conditions Chromatographic analyses were performed on a metal-free high-pressure ion chromatograph, model ICS-5000 (Dionex, USA) that included a single gradient pump, an eluent generator, a detector and chromatography module, an electrochemical detector and an AS-AP autosampler. The chromatographic conditions used in the experiment were as follows: Columns IonPacÒ AS16 Analytical (4  250 mm) and IonPacÒ AS16 Guard (4  50 mm); 1.0 mL/min flow rate; eluent source EGC III KOH with CR-ATC; 30 °C column temperature; 25 lL injection volume; detection PAD; pH–Ag/AgCl electrode in AgCl mode; certified non-disposable Ag working electrode; Ti counter electrode.

Table 3 Comparison of the analytical performance of IC–PAD and the spectrophotometric method. Methods

LOD (ng/mL)

Recovery/%

RSD/%

Linear range (ng/mL)

Spectrophotometric method IC–PAD

500 1

48.3–79.7 101.2–103.4

8.7–12.3 1.1–2.1

500–2000 1–5000

Table 4 Measurement of free cyanide in liquor. Sample no.

1 2 3 4 5 6 7 8 9 10 N.D. = not detected.

IC–PAD

Spectrophotometric method

Content (mg/L)

Added concentration (mg/L)

Recovery (%)

Content (mg/L)

Added concentration (mg/L)

Recovery (%)

0.254 0.199 0.286 0.733 0.922 1.344 0.108 N.D. N.D. 0.104

0.5 0.2 0.5 1.0 1.0 1.5 0.1 0.5 0.5 0.1

99.7 99.2 101.3 99.1 101.5 102.0 99.8 100.6 101.3 99.6

N.D. N.D. N.D. 0.502 0.618 0.954 N.D. N.D. N.D. N.D.

0.2 0.2 0.2 0.5 0.5 1.0 1.0 1.0 1.0 1.0

N.D. N.D. N.D. 67.9 67.0 70.9 53.3 68.2 64.7 70.2

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3.3. Method performance

References

A five-point calibration curve with a concentration range of 1.0– 25.0 ng/mL was established. The mean slope, intercept and linear regression coefficient square were 0.0583, 0.037 and 0.9997, respectively. The detection limit for free cyanide was 1.0 ng/mL at a signal-to-noise ratio of three. The recoveries were evaluated by adding free cyanide (5, 10 and 15 ng/mL) to each sample. The mean recovery and RSD (n = 6) for liquor were 102.3% and 1.6%, respectively. Table 3 compares the analytical performance of the spectrophotometric method reported for the measurement of free cyanide in liquor. The proposed method has advantages over the spectrophotometric method, which has been recommended as the standard method for measuring free cyanide because of its expanded linear range with low limit of detection (LOD). We analyse the same liquors by the IC–PAD and spectrophotometric method. The data show that the analysis results are in good agreement (Table 4).

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4. Conclusions This study demonstrates that IC–PAD with an optimised waveform for free cyanide is capable of measuring free cyanide in liquor rapidly and accurately. Eluent optimisation, interferent evaluation and method performance were discussed. The results show that the proposed method is sensitive enough to measure free cyanide at low levels (lg/L) with sufficient resolution even in the presence of high concentrations (mg/L) of interferents. The analysis time of the proposed method is 15 min or less. The optimised IC–PAD is more advantageous than the spectrophotometric method and may have potential applications in liquor analysis. Acknowledgements The authors would like to thank the editor for considering the manuscript for review and the esteemed reviewers for reviewing and providing valuable insights for the improvement of the manuscript.