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 KH₂PO₄-K₂HPO₄ buffer. Column temperature and detection wave were controlled at 25ºC and 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.
Keywords Reversed phase ion-pair chromatography, thiourea, polysulfide, ammonium sulfide, ammonium thiocyanate,retention behavior

1 INTRODUCTION     
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] :
CS₂ + 2NH₃ ＝ NH₄SCN + H₂S   [1]
NH₄SCN ＝ NH₂CSNH₂               [2]
The reaction [1] was often accompanied with a secondary reaction [3], which provided a by-product of polysulfied (NH₄)₂CS₃. (NH₄)₂CS₃ could be changed into ammonium thiocyanate described as equation [4] when heated^[5].
2NH₃ + H₂S + CS₂ ＝ (NH₄)₂CS₃     [3]
(NH₄)₂CS₃ ＝ NH₄SCN + H₂S         [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 NH₄SCN, (NH₄)₂S, NH₃ and H₂S step by step^[6]. While Berestetskii reported a potentiometric titration of S^2-, SCN^- and S₂O₃^2- using a carbon indicator electrode^[7]. NH₂CSNH₂ 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 NH₄SCN, (NH₄)₂S, (NH₄)₂CS₃ and NH₂CSNH₂ 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 EXPERIMENTAL
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 (10mm, 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 RESULTS AND DISSCUSSION
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 KH₂PO₄ – K₂HPO₄ buffer solution. From Fig.1 we could see that with the increasing of TBA concentration, the k' of NH₂CSNH₂ and (NH₄)₂CS₃ had no obvious change, but the retention times of (NH₄)₂S and NH₄SCN 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. 1 Influence of TBA concentration on retention ■┈ thiourea, ●┈polysulfide, ▲┈ammonium sulfide,▼┈ammonium thiocyanate
	Fig. 2 Influence of pH of mobile phase 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 (NH₄)₂S and NH₄SCN. 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 NH₄SCN 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 NH₂CSNH₂ and (NH₄)₂CS₃. But the influence on (NH₄)₂S and NH₄SCN were just the same as that of TBA concentration.

Fig. 3 Chromatogram of four compositions
t_R,I =3.113 min, t_R,II =5.263 min, t_R,III =9.795 min, t_R,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 U₇ (7⁶) provided by Fang^[15]. The mobile phase was 20:79:1 of methanol-water-tetramethylene-oxide, containing 3.0 mmolL^-1 TBA and 6 mmolL^-1 KH₂PO₄-K₂HPO₄ buffer solution with pH 6.64.
    Under the proposed condition, the flow rate and column temperature were controlled at 1ml/min and 25ºC, respectively, the chromatogram of NH₂CSNH₂ (I), (NH₄)₂CS₃ (II), (NH₄)₂S (III) and NH₄SCN (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

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

Table 2. Recoveries of NH₂CSNH₂ (I), (NH₄)₂CS₃ (II), (NH₄)₂S (III), NH₄SCN (IV)

Fig. 4 The chromatogram of (NH₄)₂CS₃, (NH₄)₂S and NH₄SCN detected in synthesis mixture

Table 3. NH₄SCN isomerization temperature and the yield of NH₂CSNH₂