Validation of an analytical method for cyanide determination in blood, urine, lung, and skin tissues of rats using gas chromatography mass spectrometry (GC-MS)

Abstract

This study was conducted to establish the analytical method for the determination of cyanide in blood, urine, lung and skin tissues in rats. In order to detect or quantify the sodium cyanide in above biological matrixes, it was derivatized to Pentafluorobenzyl cyanide (PFB-CN) using pentafluorobenzyl bromide (PFB-Br) and then reaction substance was analyzed using gas chromatography mass spectrometer (GC/MS)-SIM (selected ion monitoring) mode. The analytical method for cyanide determination was validated with respect to parameters such as selectivity, system suitability, linearity, accuracy and precision. No interference peak was observed for the determination of cyanide in blank samples, zero samples and lower limit of quantification (LLOQ) samples. The lowest limit detection (LOD) for cyanide was $10{\mu}M$. The linear dynamic range was from 10 to $200{\mu}M$ for cyanide with correlation coefficients higher than 0.99. For quality control samples at four different concentrations including LLOQ that were analyzed in quintuplicate, on six separate occasions, the accuracy and precision range from -14.1 % to 14.5% and 2.7 % to 18.3 %, respectively. The GC/MS-based method of analysis established in this study could be applied to the toxicokinetic study of cyanide on biological matrix substrates such as blood, urine, lung and skin tissues.

1. Introduction

Cyanide exists as a single-constituent substance or a mixture, in the solid, liquid, or gaseous form. Because it is known to be a highly toxic substance, very few cases of poisoning occur from intentionalingestion of cyanide. However, there is a risk of cyanide poisoning from chemical processes such aselectroplating and precious metal refinement, industrialaccidents during manufacture, and everyday activities and events such as smoking, automobile exhaust, and fire.^1-3 Cyanide ions (CN^-) are generated by pyrolysis when products containing cyanide are burned, and ifinhaled, they may lead to carbon monoxide and cy anidepoisoning in humans.³ The time of manifestation of cyanide toxicity differs depending on the route of exposure; toxic effects of cyanide are exhibited whenit enters the body through inhalation, ingestion, absorption through skin, or injection.^2,3 When cy anide is introduced into a living organism, it exhibits toxiceffects by mechanisms that include inhibition of cytochrome c oxidase activity; it can also cause cellular hypoxia and cytotoxic anoxia, which may even lead todeath in severe cases.⁴ Cyanide is highly volatile and reactive and has a very short half-life, which makesit difficult to measure cyanide exposure when a certainamount of time has elapsed after the exposure.⁵Therefore, it is important to identify the route of cy anideexposure and estimate the cyanide concentration indifferent types of living tissues so that hazards associated with cyanide exposure can be assessed.

Previous studies that measured cyanide concentrationsused ion chromatography (IC),⁶ spectrophotometry,⁷gas chromatography (GC),^8-10 gas chromatography-mass spectrometry (GC-MS),^1,11-13 and high-performanceliquid chromatography-mass spectrometry (HPLC-MS). ¹⁴ Analytical methods that use mass spectrometry typically include a derivatization step to enhance the physicochemical attributes for sensitivity and specificity of the analytes.¹⁵ Analytical methods reported in previous studies also included GC-MS with aderivatization reagent called pentafluorobenzyl bromide (PFBBr) as an alkylating agent to introduce alkyl groupsinto organic compounds. Recently, this analytical method has been used to identify the cause of deathin vehicle fire victims.¹⁶ PFBBr dissolves in organicsolvents, is thermally stable, and is highly volatile, which makes it an ideal derivatization reagent for high-sensitivity GC-MS.^4,5

The present study used a PFBBr-based derivatization method ^1,11-13 for derivatization of CN^- to obtain PFB-CN (Fig. 1), which was analyzed by GC-MS. The study also validated an analytical method for determination of cyanide in blood, urine, lung, and skin tissues, which could have been absorbed owing to cyanide exposure, in order to assess hazards associated with cy anideexposure.

 

Fig. 1. Chemical structures of before and after derivatization.

 

2. Experimental Methods

 

2.1. Standards and reagents

The standard material used in the experiment, sodium cyanide (NaCN; 95.5 %), was purchased from Sigma-Aldrich (Saint Louis, MO, USA). The internal standard (IS), 2,5-dibromotoluene (2,5-DBT; 99.7 %), was purchased from Acros Organics (New jersey, USA). Other products used were purchased from the following sources: sodium hydroxideconcentrate (NaOH, 0.1 mol/L in water) and tetrabutylammonium sulfate solution (50 wt.% in water) from Sigma-Aldrich (Saint Louis, MO, USA); PFBBr (99.1 %) from Thermo Fisher Scientific(Haverhill, MA, USA); borate and 0.5 M buffersolution (pH 8.0) from Alfa Aesar (Haverhill, MA, USA); and ethyl acetate (99.9 %) and water from Burdick & Jackson (Brooklyn, NY, USA).

Blood, urine, lung, and skin tissue samples obtained from Sprague-Dawley rats and stored at temperature below -20 °C were used in the validation of the analytical method. An anticoagulant, EDTA-2K, was used for the blood samples.

 

2.2. Preparation of standards and reagents

The stock standard solution of NaCN was prepared by weighing sodium cyanide and using 10 mM NaOH to bring it to 100 mM, after which it was stored in a refrigerator until use. The working standardsolutions were prepared to concentrations of 200-4000 µM using 100 mM stock standard solution and 10 mM NaOH. NaOH (10 mM) used to prepare the standards was prepared by 10-fold dilution of 0.1 M sodium hydroxide concentrate using distilled water. The IS, 2,5-DBT, was prepared to a concentration of 0.2 µM using ethyl acetate, after which it was stored in a refrigerator until use.

Borate (0.5 M buffer solution), and distilled water were used to prepare a 0.1 M borate buffer solution. Tetrabutylammonium sulfate and 0.1 M borate buffer solutions were used to prepare a 10 mM tetrabutylammonium sulfate solution, whereas PFBBrand ethyl acetate were used to prepare 20 mM PFBBr.

 

2.3. Preparation of calibration standard and quality control (QC) samples

Calibration standards for the blood, urine, lung, and skin tissue samples were prepared by mixing blank blood, blank urine, blank lung, or blank skintissue, respectively, with the working standard solution at a ratio of 95:5 (v/v) to bring the calibration concentration to 10-200 µM. The blank and zeroblank were prepared using 10 mM NaOH instead of working standard solution. The blank lung and blankskin tissue samples were prepared by adding 1 mL of 10 mM NaOH per 0.1 g of sample and homogenizing in a homogenizer. The blood, urine, lung, and skintissue samples for QC were prepared by mixing blank blood, blank urine, blank lung, or blank skintissue, respectively, with the working standard solutionto bring the concentration to 10 µM (lower limit of quantitation; LLOQ), 20 µM (low QC; LQC), 50 µ M(mid QC; MQC), and 150 µM (high QC; HQC).

 

2.4. Pretreatment of samples

For pretreatment of blood, urine, lung, and skintissue samples, 100 µL of IS was added to 100 µ Leach of calibration and QC samples (blood, urine, lung, and skin tissue), including the blank and zeroblank, and mixed for 1 minute. After mixing 800 µ Lof 10 mM tetrabutylammonium sulfate solution and 500 µL of 20 mM PFBBr for 10 minutes, derivatization reaction was carried out at 70 °C for 1 hour. The derivatized sample was centrifuged (14,000 g, 4 ℃ C, 5 minutes), and the supernatant was used in subsequentanalysis.

 

2.5. Gas chromatography-mass spectrometry

For analysis of PFB-CN derivatized afterpretreatment, SCION SQ (Bruker, Billerica, MA, USA) was used. The mass values m/z for quantification of PFB-CN and 2,5-DBT were set to 207.0 and 249.8, respectively. GC column used was BR-5ms (Bruker, Billerica, MA, USA), and the parameters for GC-MS were as shown in Table 1.

 

Table 1. GC/MS operating conditions and parameters

 

2.6. Validation of analytical method

For validation of the analytical method fordetermination of cyanide in blood, urine, lung, and skin tissues of rats, the selectivity, system suitability, linearity, accuracy, and precision of the method weretested. Validation of the analytical method was carried out in accordance with the Guideline on Bioanalytical Method Validation provided by the FDA and EMA.^17,18

Selectivity indicates the ability of an analytical method to isolate and quantify the target substance when other substances are coexistent within a biological sample. Accordingly, selectivity was assessed by analyzing whether the peaks of blank, zero blank, and LLOQ samples of blood, urine, lung, and skintissues obtained from six different individuals areaffected by interference peaks. In addition, systemsuitability was examined by assessing the precision for peak area on the basis of six repeated injections of LLOQ samples.

Linearity was determined on the basis of calibrationcurve results obtained using standard samples with at least six different concentrations, including LLOQ and upper limit of quantitation, along with the blankand zero blank. Accuracy and precision were assessed on the basis of six repeated measurements of at least four different concentrations, including LLOQ, withrespect to the entire operation of the test method. Accuracy was compared using relative error (%RE), whereas precision was compared using coefficient of variation (%CV). %RE and %CV were calculated using the equations shown below.

%RE = [(Measured concentration − Theoretical concentration)/Theoretical concentration] × 100

%CV = [SD of measured concentration/Mean of measured concentration] × 100

 

3. Results and Discussion

 

3.1. Selectivity

To test the selectivity of the analytical methodusing GC-MS for determination of cyanide in blood, urine, lung, and skin tissues, blank and zero blanksamples using blood, urine, lung, and skin tissues obtained from six individuals and LLOQ sampletreated with NaCN and IS were assessed (Fig. 2, 3). No interference peaks were observed in the intervalin which cyanide and IS were eluted. The results demonstrated that the analytical method exhibits good selectivity for cyanide analysis and is notaffected by interfering substances

Fig. 2. Representative blank chromatogram of sodium cyanide in rat (A) blood, (B) urine, (C) lung and (D) skin tissue.

 

Fig. 3. Representative LLOQ chromatogram of sodium cyanide in rat (A) blood, (B) urine, (C) lung and (D) skin tissue.

 

3.2. System suitability

The test the system suitability of the analytical method, biological samples at the LLOQ concentration were injected six times into GC-MS. The results showed that the precision values for the analyte PFB-CN and IS was 3.8-15.4 % and 1.3-6.0 %, respectively, which satisfied the acceptance criterion of < 20 % (Table 2). Accordingly, it was determined that GC-MS used in the testing of the analytical method was properly maintained and managed.

 

Table 2. System suitability of sodium cyanide in blood, urine, lung and skin tissues in rats

 

3.3. Linearity

To test the linearity, the ratio of PFB-CN to IS concentrations (x axis) and ratio of PFB-CN to ISpeak area (y axis) were used to construct a calibrationcurve with a weight of 1/x². The correlation coefficient (r²) of each biological sample was within the range of 0.9943-0.9984 (Fig. 4), and the accuracy of the calibration curve was found to be -10.4-10.7 % (Table 3). These values were consistent with the criteria for assessing the calibration curve, that is, correlation coefficient ≥0.99 and concentration within 15 % (20 % for LLOQ) of the theoretical concentration, which were determined to be the levels suitable for cyanide determination.

 

Fig. 4. Calibration of sodium cyanide in rat (A) blood, (B) urine, (C) lung and (D) skin tissue

 

Table 3. Linearity of calibration curves of sodium cyanide in blood, urine, lung and skin tissues in rats

 

3.4. Accuracy and precision

Accuracy represents the proximity between the theoretical value and mean value obtained by analysis, whereas precision represents the proximity of values measured repeatedly when analyzing a test article at the same concentration. Table 4 shows the results of analyzing %RE and %CV from six repeated measurements of LLOQ, low QC, mid QC, and high QC samples. The accuracy and precision of each biological sample was within -14.1-14.5 % and 2.7-18.3 %, respectively, which satisfied the accuracycriterion of mean concentration being within 15 %(20 % for LLOQ) of the theoretical concentration and precision criterion of %CV <15 % (20 % for LLOQ). The results confirmed that the analytical method is suitable for the determination of derivatized cyanide.

 

Table 4. Accuracy and precision of sodium cyanide in blood, urine, lung and skin tissues in rats

 

4. Conclusions

In the present study, the analytical method using GC-MS for cyanide determination in blood, urine, lung, and skin tissues was validated by assessing its selectivity, system suitability, linearity, accuracy, and precision. In the analytical method, the quantitation range of derivatized cyanide PFB-CN was 10-200 µ M, and within this range, the correlation coefficient (r < sup>2)of the calibration curve of each biological samplecontaining PFB-CN was ≥0.99, showing good results. The results of accuracy (%RE) and precision (%CV) from LLOQ QC, Low QC, Mid QC, High QC samples were within ±15 % (20 % for LLOQ) of the valuesspecified in the FDA and EMA guidelines. Validation of the analytical method for targeted biological samplesconfirmed that the method is capable of cy anidedetermination in blood, urine, lung, and skin tissues. These results indcated that could be able to use as auseful basic data for application in the research related to routes of exposure by cyanide inhalation.