VIETNAM JOURNAL OF CHEMISTRY DOI: W.15625/0866-7144.2014-0076
VOL. 52(6) 794-798 DECEMBER 2014
DEVELOPMENT OF ANALYTICAL PROCEDURE FOR DETERMINATION OF FREE CYANIDE IN WATER USING AUTOMATIC SAMPLE INJECTION
CAPILLARY ELECTROPHORESIS SYSTEM COUPLED WITH CONTACTLESS CONDUCTIVITY DETECTOR
Nguyen Bich Ngoc, Duong Hong Anh, Pham Hung Viet Research Center for Environmental Technology and Sustainable Development,
VNU-University of Science
Received 21 September 2014; Accepted for Publication 15 October 2014
Abstract
Background electrolyte conditions and setting condition of auto capillary electrophoresis system were studied to develop an analysis procedure for free cyanide determination in environmental water samples. CAPS buffer with added CTAB at pH 11 was selected as the mobile phase. The sample injection amount is adjusted by changing needle valve position and sample injection time. Separation process is performed at +20 kV. Developed procedure allows the quantification of cyanide with high accuracy, short analysis time, less chemicals consumption and more importantly avoiding the dangerous and complex sample handling process. The detection and quantification limit of this method are respectively 0.9 and 2.9 mg/L.
Keywords. Capillary zone electrophoresis, coupled contactless conductivity detector (C'D), auto sample injection, cyanide.
1. INTRODUCTION
Cyanide has long been known as a highly toxic substance, usually appears in effluent of some industries such as plating, leather tanning, mining and processing of cassava starch.The most toxic form of cyanide is free cyanide which includes ion CN" and hydrogen cyanide HCN. Total cyanide is determined by total concentration of free cyanide and Weak Acid Dissociable (WAD) containing complexion of cyanide and transition metals (Zn, Cd, Cu, Ni, Ag ...). Cyanide has been banned and included in the regulations on water quality of many counfries including the U.S., European countries and Vietnam (QCVN 40/BTNMT).
The conventional method for cyanide detection is optical method using photometric red complex of cyanogen chloride with pyridine and barbituric acid.
This method has been standardized in TCVN 7723:2007 (ISO 14403:2002) for free cyanide and total cyanide analysis. Its detection limit is very low (0.002 mg/L); however the distillation process is very complex and dangerous. Several studies using capillary electrophoresis (CE) coupled with UV detector for separation and quantification of cyanide complexes is published [1,2]. Capacitively coupled contactless conductivity detector (C*D) was first
used in cyanide analysis in electroplating water by Professor Peter Hauser's group [3]. This study initially demonstrated the possibility of using CE- C'D system to rapidly analyze free cyanide as well as the metal complexes of cyanide.
Portable CE-C D system using membrane pump and compressed air for auto sample injection was developed under the support of Vietnam National University Hanoi (QGTD.12.03) for rapid quantitation of major ions in water. This paper presents the development of a cyanide analysis procedure which is suitable for the home built equipment. This procedure is then used to verify the system performance.
2. EXPERIMENTAL 2.1. Chemicals
Chemicals used in this research including sodium cyanide, N-cyclohexyl-3- aminopropanesulfonic acid (CAPS), sodium chromate, sodium bicarbonate, di-potassium phosphate, sodium hydroxide, diethylamine, triethylamine, tert-butyl amine and cetrimonlum bromide (CTAB) were of analytical reagent grade and purchased from Fluka (Buchs, Switzerland) or
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Merck (Darmstadt, Germany). Stock solution 1000 ppm of CN" was at pH 11. Standard sample was prepared dally by diluting stock solution in NaOH ImM. Stock solution Is also used to spike in surface water sample taken from Day River at Phu Ly city.
Ha Nam province to make spiked sample in real matrix. All solutions were filtered through 0.45 pm filters (BGBAnalytik, Anwil, Switzerland) and degassed in an ultrasonic bath for 5 min before performing measurements.
2.2. Instrumentation
A home-built capillary electrophoresis system (Figure 1) consisting of a high voltage supply unit (Spellman CZEIOOOR Start Spellman, Pulborough, UK), home-made fluidic system a new version of contactless conductivity detector and an analog digital converter (Ecorder 401) were used in this research. Silica capillary with 50 |JM I.D, 50 cm length and 43 cm effective length is treated with IM NaOH for 10 minutes and deionizer water for 10 minutes before using. Samples were auto-injected by membrane pump into sample loop, then buffer under pressure of compressed air pushes the sample through the ground interface and into the capillary.
Detector is placed near high voltage electrode.
Pham Hung Viet, et aL and retention time were used to compare and evaluate different conditions for selection of suitable buffer.
Besides buffer components and concentration, working condition of fluidic system Including sample injection time, flow-splitting needle valve position which affect to the sample volume injected to capillary were also studied. Sample injection time was varied by changing injection time from 10 seconds to 20 seconds and/or turning needle valve position from 0-0 position to 4-0 position. In this design, needle valve which is divided into 4 rounds each round consists of 50 smaller graduations, will determine the opening of discharge valve and at the same time control the back pressure which pushes the sample into the capillary. Finally, high-voltage was adjusted from +10 kV to +25 kV.
Analytical method was then assessed by detection limit - LOD (S/N = 3), calibration range, accuracy (trueness and precision).
3. RESULTS AND DISCUSSION 3.1. Bacl^round electrolyte
As mentioned above, four buffer prepared at pH 11 were used for cyanide separation. CAPS buffer with bulky molecular structure has much lower conductivity (~ 2 jxS/cm before and - 750 jiS/cm after adding NaOH) than other three buffers (2000- 2500 ^iS/cm after adding NaOH).
Figure I: Schematic drawing of the home-built portable automated capillary electrophoresis system
using the pneumatic flow 2.3. Methods
To avoid the formation of high the volatile and highly toxic hydrogen cyanide, four stable buffers (CAPS, carbonate, phosphate, and chromate) at pH 9 to 11 were tested. All buffers were prepared at the concentration of 10 mM and added NaOH 2.5 M to reach pH 11. Base substance used to adjust pH Is then replaced by diethylamine (100 %), h-iethylamine (100 %), and tert-buthylamine (100
%). Each factor was in turn varied while keeping others unchanged. The parameters of the peak height, peak area, full width at maxima, resolution
Mitigation time [i)
Figure 2: Cyanide separation using different buffer at pH 11
Low conductivity buffer will raise the differences between the analyst signal and the background signal; therefore increase peak heights as well as the sensitivity of the method. Moreover, the Joule effect causing background noise could be eliminated by lowering buffer conductivity.
Therefore in case of using CAPS buffer, the cyanide
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peak is better than other three as shown in Figure 2.
The conductivity of the background electrolyte can still be reduced when replacing NaOH with organic amines which have bulkier structures such as diethylamine, triethylamine, tert-butylamine (Figure 3). In this study, CAPS buffer with added tert-butyl amine which has lowest conductivity (~ 600 jlS/cm) at pH 11 was chosen for the analysis procedure.
Development of analytical procedure for...
'Jr
Jl
Terbuthyl Amine
Triethyi Amine
Diethyl Amine
. NaOH
Mitigation time (s)
Figure 3: Cyanide separation using CAPS buffer (10 mM) at pH 11 with different pH adjustment
chemicals
Volume of added tert-buthyl amine was increased from 50 p.L to 450 [iL per 100 mL buffer to adjust pH of background electrolyte from 10.2 to 11.2. Low pH buffer will release CNfrom the sample leading to decreasing cyanide concentration and lower peak height. However, pH increasing also has its negative effects such as increasing the solution conductivity and the electro-osmotic flow (EOF) speed. Therefore, the pH range suitable for determining free cyanide should only be between
10.9 and 11.1.
10,18 10,75 10,92 11,12 11,24
Figure 4: Variations of cyanide peak height and mitigation time at different pH of the background
electrolyte
" v-
—<- 0.1 "a •Pea<afej
•"I 3 • P*»< height
0-0 1-0 2-0 3-0 -H) Mlve Position
Figure 5: Variation of cyanide peak area and peak height at different splitting-valve positions 3.2. Injection and separation parameters
The opening of needle split valve P470 (Upchurch) used in this research was increase from round 0 to round 4. The greater the valve opens, the more sample goes to waste stream and the less sample goes into capillary for separation. Peak area therefore decreases from ~ 1.6 V.s at closed position to ~ 1.16 V.s at fully open position (Figure 5).
However, the height of the signal does not notably change (from 113 to 123 mV). Therefore, needle valve customizing does not significantly affect the detection limit of the method. However, when a large amount of sample goes into the capillary, the full width at maxima of cyanide peak tends to be broadening, that makes it is difficult for accurately calculating the peak areas. The 0-20 split valve position is therefore selected for the cyanide determination process.
Sample injection time was also increased from 10 to 20 seconds (Figure 6) in order to increase the amount of sample injected into the capillary to Increase the sensitivity of the method. However, with a fixed back pressure controlled by valve position, the volume of Injected sample only raised to a fixed amount. The experimental results showed that the sample injection time longer than 17.5 seconds does not increase peak height.
In addition to injection parameters, separation voltage also affects the mitigation time, resolution and peak height. When changing the voltage from 10 kV to 25 kV, the mitigation time of cyanide decreases due to increasing of Ion velocity under stronger electric field. Resolution between cyanide peak and system peak reduced to 2 at +22.5 kV and +25 kV. At low voltage separation, long mitigation time leads to lower peak height and more broaden peak width. In the survey conditions, +20 kV and +22.5 kV gave best resuh on the peak heights (~ 130 mV) and had a reasonable separation between 796
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cyanide peak and system peak. Therefore, voltage +20 kV is used to develop analysis processes for energy saving and safer operation.
Pham Hung Viet, et al.
Figure 6: Variation of peak height and full width at maxima (FMW) at different injection times
™/J 1 / 1 : 1^
IJ 1^
^:i'i
300 400 Time (second)
Figure 7: Cyanide separation using CAPS buffer (10 mM) pH 11 at different voltage
3 3 . Analysis procedure assessment
Calibration curve was built using 5 standard samples with CN" concentration ranging from of 2.9 ppm to 260 ppm. Regression line between recorded signal and analyte concentration buiU on Graph Pad Prism software 5.0.4 gives a very high correlation coefficient (R = 0.9997). Full obtained calibration curve equation is y = (0.2634+0.0027) x + (0.3526+0.3352) with 95% reliability.
The limit of detection (LOD) in this method is 0.9 mg/L CN'and quantitative limit (LOQ) is ~ 2.9 mg/L CN". LOD and LOQ obtained from this study are lower than the Zhang research's result (Tabke 1) but still relatively high in comparison with standard method as well as the Vietnamese regulations (Waste water QCVN 40:2011/BTNMT and surface water QCVN 08:2008/BTNMT). However, the results also showed the potential to continuingly develop method that can help lower the detection limit, quick and safely determine cyanide concentration right at the contamination source.
In this study, the relative standard deviation (RSD) was used to assess the precision of the method. RSDs calculated when measuring samples containing 156 mg/L and 208 mg/L (n = 7) of cyanide are respectively 2.1% and 0.6% which are much lower than the RSD (3-7%) of manual CE systems due to sample is entirely automatic introduced and injected amount is well controlled.
Also, the trueness is tested by analyzing samples with known concentrations in deionized background and surface water background collected from Day River, Phu Ly, Ha Nam (sample HNl and HN2).
Recovery rates of deionized background samples are very high, above 99 % for both tested concentrations (156 mg/L and 208 mg/L). For two spiked surface water samples HNl and HN2, recovery rates are approximately 94-95 % which proves the high reliability of given method.
Table I: Comparison of different cyanide analytical procedures by capillary electrophoresis method
This research Zhang's research [3]
TCVN 7723: 2007 [4]
LOD, ppm 0.9 0.003
Calibration range, ppm 2.9-260 4.58-45.8
0.01-0.1
Correlation coefficient 0.9997 0.9902
Recovery, % Standard
sample 99
Spiked sample on real matrix
94-95 95
4. CONCLUSION this study demonstrated a rapid, safe and accurate on-site method to identify cyanide in water, avoiding The cyanide analysis procedure using the the complicated and error-prone sample storage CAPS/tertbuthylamin buffer (pH = 11) described in process. Nevertheless, as the LOD of this method
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was still high (0.9 mg/L), parameter optimizations have to be continued in further studies. These preliminary results suggest the potential applications of our procedure in rapid screening of cautionary cyanide emission sources, which may aid government agencies in providing quick and accurate response.
Acknowledgements. The authors gratefully acknowledge the financial support from the project grant QGTD.12.03 Viemam National University.
Hanoi REFERENCES
1. Jian Ma, Pumendu K. Dasgupta. Recent developments
Development of analytical procedure for...
in cyanide detection A review. Analytica Chimica Acta, 673, 117-125(2010).
Jermak S., B. Pranaityte and A. Padarauskas. Ligand displacement. headspace single-drop microextraction, and capillary electrophoresis for the determination of weak acid dissociable cyanide, J.
Chromatogr. A, 1!48(1), 123-7 (2007).
Zhang L. et al. Analysis of electroplating baths by capillary electrophoresis with high voltage contactless conductivity detection. Measurement Science and Technology, 17(12), 3317-3322 (2006) TCVN 7723:2007. fVater quality - Determination of total cyanide and free cyanide by continuous flow analysis. Directorate for Standards, Metrology and Quality Vietnam (2007).
Corresponding author: Pham Hung Viet
Research Center for Environmental Technology and Sustainable Deveiopmenc VNU-University of Science
E-mail: [email protected] Cell phone: 0913572589.