Separation of Some Benzene Derivatives by Gas – liquid Chromatography

Hadi Hessen Jusim
College of Science, Al-Mustansirya University 

(NJC)

(Received on     12 / 6/2005)	         (Accepted for publication on  29  / 4 /2006)

Abstract
        Using chromatographic column of glass beads coated with solid lithium nitrate and modified by water vapor in the Helium carrier gas can be used at room temperature give a highly efficient separation, in a reasonable time of range (10-40) min., mixture containing different types of xylene and some benzene derivatives. The column glass beads used were (40 / 50 ) mesh . Its high separation efficiency was obtained compared with conventional gas – liquid chromatography.
الخلاصة  
            استخدم في هذا البحث عمود من الكروموتوغرافي محشو بحبيبات او كريات صلبة مع نترات الليثيوم. كما استخدم الهيليوم كطور غازي، وبدرجة حرارة الغرفة، تم الحصول على أعلى كفاءة فصل ممكنة وبزمن تراوح ما بين (10-40) دقيقة لمزيج يحتوي على العديد من انواع الزايلين وبعض مشتقات البنزين الاخرى، حيث كانت حجوم الكريات او الحبيبات المستخدمة كحشوة بحدود (40 / 50 ) مش. بحيث حصلنا على كفاءة فصل عالية مقارنة مع الطرق التقليدية للفصل بأستخدام كروموتوغرافيا الغاز – سائل. 

Introduction 
Advantages of gas liquid chromatography arise from the high selectivities possible with solid substrates and the stability of such substrates at high temperatures(1- 4). It’s principal disadvantage is the nonlinearity of the absorption isotherms usually found for gas – solid interactions. This problem can often be  solved by modifying the surface with a suitable chemical substance (5-8) another disadvantage coefficient found for arises from the large distribution adsorption. To achieve reasonable retention times, high column temperatures must be used and the analyses restricted to low boiling mixtures.
We have constructed a column for which this latter disadvantage has been eliminated . It consists of glass beads coated with solid LiNO3 . This salt was chosen because of its ability to complex with benzene or Xylene by modifying this surface with water vapor (4-8), we produced a highly efficient column that could analyze in areasonable time at room temperature aromatic hydrocarbons with boiling points as high as 165 oC. The study describes how the column was made and how it compares in efficiency with two conventional gas – liquid chromatographic columns, di-n-propyl tetra chlorophthalate (DPTP ) and 7,8 – benzoquinoline (BQ). 
Experimental  
Columns 
The glass beads used in the (40 -50 ) mesh range. They were washed two time with (0.5 N) nitric acid and seven times with distilled water. 
A known amount of (0.4 M) Lithium nitrate was dissolved in distilled water and a mixture of (beads and water ) evaporated until the beads were dry at (105 oC). The column packed in the usual manner. The carrier gas of dry helium was passed through the column at 120 oC to further dry the packing . Column dimensions were 160 cm ×  0.9 mm.   
Apparatus and Materials  
	The column sat in a water bath whose temperature was controlled to (800.05) oC. Upstream from the column was a water saturated in a constant temperature bath (800.05) oC. To prevent Lithium nitrate from going into solution, the partial pressure of the water vapor in the carrier gas was kept lower than the partial pressure of saturated LiNO3 solution at the column temperature. In line with the saturator was avalve which directed the helium either through the saturator or directly into the column. the effluent was monitored by a hydrogen flame detector. The sample was composed of 7 component mixture of aromatic hydrocarbons and was injected on to the column with 10 l gas syringe. 
Results and discussion
Column Properties 
The retention volume of benzene derivative solute was found to be dependent upon the partial pressure of the water vapor in the helium carrier gas, at a fixed column temperature. In areasing the partial pressure resulted in smaller retention volumes while dry helium caused the solute to be retained completely. The relative retention volumes were independent of the partial pressure of the water vapor but dependent on the column temperature. 
A column packed with 40 / 50 mesh LiNO3 gave identical results but was muchless efficient. For columns containing the same amounts of glass bead but with different LiNO3 coverage (0.2, 0.3, 0.4, 0.5, 0.7 % w / w ) gave approximately the same retention volumes for the aromatic solutes at a given column temperature and water vapor concentration. The columns had the same number of theoretical plates ( 4 × 103 calculated for O-xylene ) 
Comparison with other columns 
         Halasz (6)  has derived the following equation for calculating the number of theoretical plates (Nreq ) required to separate two peaks. 

Table (1): Comparative for the Retention of benzene
derivative by several columns
No.	Benzene derivatives	Separation factor	Boiling Point oC	
	Structure	Name	LiNO3	DPTP	BQ		
1-		

	Benzene	_____	_____	_____	80oC	
2-		

	Toluene	3.09	2.10	1.92	111 oC	
3-			O – xylene	1.42	1.33	1.25	
144 oC
	
4-		

	P-xylene	1.11	1.08	1.10	
138 oC

5-		

	Ethyl
benzene	1.61	1.26	1.37	136 oC	
6-		

1- methyl 3- ethyl –benzene 	1.18	1.04	1.06	163 oC	
7-		

	1 , 3 , 5 – Trimethyl benzene	

1.40	

1.31	

1.27	

165 oC	

Where S is the separation factor, Vr is the retention volume of the compound eluted later, Vd is the dead volume of the columns. When S is near unity, Nreq can be greatly reduced, if S can be increased even slightly by changing the nature of the substrate. However, S is not the sole quantity to consider in judging column performance, the factor involving Vd and Vr can also be important, particularly for early peaks where the ratio Vd / Vr can be large(5).
Figure 1. appear a chromatogram of the seven components of the benzene derivative using column (30% LiNO3) at 27 0C. But table 1 identifies the components and their correspondence to various peaks in Figure 1.

ppm

  

Fig  (1) : Separation of benzene derivative on LiNO3 column

In this study two mixtures used, the first one content (benzene, toluene, o-xylene and p-xylene). The second mixture content (o-xylene, 1,3,5-trimethyl  benzene, ethyl benzene and m-ethyl toluene).

Table (2) : Comparative data for the Retention time of Benzene
derivative on LiON3 column 
No.	Components	Retention time (min)	Conc.
Mg / L	
	Name	Strcture			
1.		Benzene	

	11.5	5	
2.		Toluene	

	17	2	
3.		P – xylene	

	20.1	6	
4.		O-xylene	

	22.3	3	
5.		1,3,5 – trimethyl
benzene	

	31.2	6	
6.		Ethyl benzene	

	35.5	2	
7.		3- ethyl – 1- methyl benzene	

	41.3	3	

Values of S for successive Pairs of peaks in order of elution time for three solvents (LiNO3, BQ and DPTP ) and values of Nreq for the difficult separations. The retention volumes for the bezene derivative solutes were measured at 100 oC for the DPTP and BQ columns and at 27 oC for LiNO3 column. The times required for the three different analysis were aproximately the same (show in table 2 ). The separation factor values are greater for LiNO3 column than for the gas – liquid columns. The DPTP column gave a slightly higher value of (S) for this separation. The BQ column compared favorable with the LiNO3 column for the O – xylene / P- xylene separation, It had difficulty separating  1- methyl -3- ethyl benzene from ethyl benzene ( S value was 1.06 ) , but LiNO3 column easily made this separation ( S value was 1.18 ) . The DPTP column again had difficulty separating 1- methyl -3- methyl benzene from ethyl benzene ( S value was 1.04 ). The advantage of working at lower column temperatures is well known. 
Purnell et al.(9), has shown that the BQ and DPTP Columns would be more efficient at lower temperatures, but to obtain areasonable analysis time it would be necessary to use very low solvent coverage. Often however, packed columns operating at such low solvent / support ratios require non adsorptive supports and very small sample injections. Another advantage of the LiNO3 columns comes from the ability one has to change Vr without     affecting (S) or Vd by changing the partial pressure of water in the carrier gas. In cases where the ratio Vd / Vr is not negligible, Equation 1 shows that the value of Nreq decrease as Vr is increased. This provides an additional parameter for the analyst to use to achieve desirable separation . The mechanism of aromatic hydrocarbon separation is obscure. Solid LiNO3 must complex with unsaturated hydrocarbons in the same manner as (Li+ ) ion complexes with unsturated hydrocarbons in solution(10,11). The effect can be large because benzene derivative are not eluted by dry carrier gas. Water vapor in the carrier must compete with the hydrocarbons for the active sites on the solid LiNO3. Higher partial pressures of water result in fewer available sites for the hydrocarbons and lead to smaller elution times.   

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