Study on the retention behavior of sulfur compounds in reversed phase ion-pair chromatography

Pang Xiuyan, Sun Hanwen,Shen Shigang
(Key laboratory of analytical science and technology of Hebei province, College of Chemistry and Environmental Science, Hebei University, Baoding, 071002, China)

Abstract Based on the reversed phase ion-pair chromatography, the retention behaviour of thiourea, polysulfide, ammonium sulfide and ammonium thiocyanate were discussed under different conditions such as ion pair reagent and its concentration, ion intensity and organic reagent. It was testified that the concentration of ion pair reagent, pH and the volume ratio of tetramethylene-oxide in mobile phase played important roles in the retention behavior of ammonium sulfide and ammonium thiocyanate. The best analytical results were obtained by using a C18 column with a mobile phase comprising a 20:79:1 (% v/v) mixture of methanol-water-tetramethylene-oxide, pH 6.64, containing 3.0 mmolL-1 ammonium tetrabutyl bromide and 6 mmolL-1 KHPO-KHPO buffer. Column temperature and detection wave were controlled at 25ºCand 220nm, respectively. Linearity of the working curve was investigated with correlation coefficients of 0.9989-0.9999, and the relative standard deviation was below 4.9%. The proposed method had been successfully used in the samples determination.

Ammonium thiocyanate and thiourea were two kinds of important chemicals and medicine materials[1,2]. And the conventional production process to obtain them were[3,4] :
CS + 2NH = NHSCN + HS   [1]
NHSCN= NHCSNH               [2]
The reaction [1] was often accompanied with a secondary reaction [3], which provided a by-product of polysulfied (NHCS. (NHCS could be changed into ammonium thiocyanate described as equation [4] when heated[5]
2NH + HS + CS = (NHCS     [3]
(NHCS = NHSCN + H         [4]
In the preparation of ammonium thiocyanate, the mixture of ammonium thiocyanate, ammonium sulfide, and polysulfide was often gained, and when thiourea was prepared through isomerization of ammonium thiocyanate, the mixture of thiourea and ammonium thiocyanate was obtained. So the development of the qualitative and quantitative method for simultaneous determination of the components would provide a powerful means for their kinetics and mechanism study. In the previous work, Pertunina gave the titrimetric method for the detection of NHSCN, (NHS, NH and HS step by step[6]. While Berestetskii reported a potentiometric titration of S2-, SCN and S2- using a carbon indicator electrode[7]. NHCSNH was usually detected by its UV absorption spectrometry[8]. But none of these methods could be directly used for the simultaneous determination of the four components. Reversed ion pair HPLC had been well used in the determination of organic acid, heterpolyacid, inorganic compounds and specific analysis[9-12]. But it had not been employed in the determination of NHSCN, (NHS, (NHCS and NHCSNH at the same time. In this paper, the reversed phase ion pair chromatography method was developed and used for the simultaneous and direct determination of the four components. The optimal separation condition was obtained through the study of retention behavior of the components under different conditions. The method had been successfully used in the samples detection.

2.1 Instrument
A LC-6A HPLC was coupled to a SPD-6A ViS detector and C-R3A data processing system. The Shim-Pack CLC C18 column (10
m, 150×6.0mm) was employed (Shimadzu, Japan). PHS-3C acidometer and UV8500 Spectrophotometer were used (Shanghai).
2.2 Reagents
Ammonium sulfide was guarantee reagent (Tianjin, China). Polysulfide was prepared with carbon disulfide and ammonium sulfide according to the reference[13]. The yield of polysulfide was calculated according to the amount of carbon disulfide. All the reagents and solvents used were analytical grade.

3.1 The selection of ion pair reagent and its concentration

The UV absorption of the four components between 190 and 350nm was detected. To insure the sensitivity, the analytical signal (peak area) was monitored at 220nm. In mobile phase, ion pair reagent should have weak absorption at 220nm and have enough eletrostatic attraction with the ion compounds. Four kinds of positive ion pair reagents such as benzyl trimethyl ammonium chloride, cetyl trimethyl ammonium bromide, ammonium tetrabutyl iodide and ammonium tetrabutyl bromide (TBA) were used, and TBA gave the higher sensitivity than the others.
To study the influence of the concentration of TBA on retention behavior, the fixed volume ratio of water, methanol and tetramethylene-oxide was used; the ion intensity and pH were controlled by KHPO – HPO buffer solution. From Fig.1 we could see that with the increasing of TBA concentration, the k’ of NHCSNH and (NHCS had no obvious change, but the retention times of (NHS and NHSCN were increased. So the concentration of ion pair reagent played an important role in the separation of the four components.
3.2 Influence of pH on the retention behavior
The pH value of mobile phase had no effect on thiourea and polysulfide, but in order to keep ammonium of thiocyanic acid and hydrogen sulfide in ionic state, it should be controlled at above 6.0. With the increasing of pH from 6.10 to 7.17, the retention times of the ammonium thiocyanate and ammonium sulfide decreased, the results were shown as Fig. 2. This might be caused by the increasing of negative charge of phosphate, which promoted the complexing of phosphate with TBA, then decreased the complexing of TBA with thiocyanate and sulfide[11,14]. In the separation of the four components, pH played an important role.

Fig. 2 Influence of pH of mobile phase on retention
■┈ thiourea, ●┈polysulfide, ▲┈ammonium sulfide,▼┈ammonium thiocyanate

image Fig. 1 Influence of TBA concentration on retention
■┈ thiourea, ●┈polysulfide, ▲┈ammonium sulfide,▼┈ammonium thiocyanate

3.3 The influence of organic reagents on the retention behavior
When the volume ratio of methanol in water was increased, the retention times of the components could be decreased. But the influence was weak in adjusting the retention behavior of (NHS and NHSCN. Only when the value was above 20%, could ammonium thiocyanate flow out from the column within 20min. With the addition of 1% tetramethylene-oxide in mobile phase, the retention time of NHSCN could be decreased greatly (about 30%). This might be caused by the coordinated action of strong dispersion force of tetramethylene-oxide and the increasing solubility of complexes in mobile phase. So the content of tetramethylene-oxide played an important role in the separation.
3.4 The influence of ion intensity on the retention behavior
When the ion intensity in mobile phase was improved, it has weak influence on the retention times of NHCSNH and (NHCS. But the influence on (NHS and NHSCN were just the same as that of TBA concentration.

Fig. 3 Chromatogram of four compositions
R,=3.113 min, tR,II=5.263 min, tR,III=9.795 min, tR,IV=14.263 min.

3.5 Optimization of the mobile phase
With the modification of the TBA concentration, pH, ion intensity, the volume ratio of water, methanol and tetramethylene-oxide, the optimal separation conditions were gained through uniform design experiment U (7) provided by Fang[15]. The mobile phase was 20:79:1 of methanolwatertetramethylene-oxide, containing 3.0 mmolL-1 TBA and 6 mmolL-1 KHPOHPO buffer solution with pH 6.64.
Under the proposed condition, the flow rate and column temperature were controlled at 1ml/min and 25º, respectively, the chromatogram of NHCSNH (I), (NHCS (II), (NHS (III) and NHSCN (IV) was obtained as Fig. 3.
3.6 The detection of linearity and recoveries
The relation between the peak area and the mass of the four components was examined. The linearity regression equations along with linearity ranges and correlation coefficients for determination of the four components are given in Table 1. The calibration curve was linear for the four components with correlation coefficients of 0.9989-0.9999.
A series of mixtures of the four components with different mole ratio, were determined, and the recoveries for the four components were examined. The results with relative standard deviation (RSD) were listed in Table 2. Recoveries and relative standard deviations were in the range of 94.0-106.0% and 0.76-4.9%, respectively, for the four components.

Table 1. The regression equation and linear scopes

Composition Regression equation linearity (g) Correlation coefficient
NHCSNH Y=18665+228560x 0.01905-18.27 0.9999
(NHCS Y=34871+128660x 0.03809-36.54 0.9995
(NH Y=-3249.7+105633.7x 0.01703-35.81 0.9989
NHSCN Y=77398+106675x 0.07213-36.54 0.9992

Y: area of peak, X: the mass of composition (g)

Table 2. Recoveries of NHCSNH (I), (NHCS (II), (NHS (III), NHSCN (IV)

Composition II III IV
Recoveries 98.6–100.7% 101.3 –102.9% 94.0 –97.3% 98.0 –106.0%
RSD % (n=4) 0.76 1.95 4.65 4.9

Fig. 4 The chromatogram of (NHCS, (NHS and NHSCN detected in synthesis mixture

Table 3
. NHSCN isomerization temperature and the yield of NHCSNH

Temperature(ºC) 100 120 127 140 150 170
Yield of NHCSNH % 6.32 14.50 17.95 34.21 27.93 25.67

3.7 Sample analysis
The method described above was used to the samples analysis. The product which was gained through the reaction between carbon disulfide and ammonia was analyzed, the chromatogram of (NHCS, (NHS and NHSCN was detected as Fig.4. The isomerization products of NHSCN under different temperature were determined with the highest yield of thiourea obtained at 140ºCaslisted in Table 3. The result was consistent with the reference[5,16]
3.8 Conclusion      
The proposed method was simple and easy with good accuracy and precision. It could successfully be used for the simultaneous determination of ammonium thiocyanate synthesis products and the isomerization mixture. The research provided a powerful means for the kinetics and mechanism study.