ACKNOWLEDGMENT The authors thank Ziklag Chemicals Ltd., Haifa, Israel, for the donation of the sample of polyurethane foam.
LITERATURE CITED (1) P. Stokes, Verh. lnt. Ver. Theor. Angew. Limnol., 19, 2128 (1975). (2) R. E. Brown, J. Water Pollut. ControlFed., 47, 2863 (1975). (3) I. S. Bhat, R. S. lyer, and S. Chandramoull, Anal. Chem., 48, 224 (1976). (4) J. E. Going, G. Wesenberg, and G. Andrejat, Anal. Chim. Acta, 81, 349 (1976). ( 5 ) R. G. Smith, Jr., Anal. Chem., 46, 607 (1974). (8) K. S. Lee, D. W. Lee and W. Lee, Yonsei Non-Chong (published by Yonsei University, Korea), 13, in press (1976). (7) A. L. Clingman and J. R. Parrish, J. Appl. Chem., 13, 193 (1963). (8) K.H. Sugawara, H. H. Weetall, and G. D. Schucker, Anal. Chem., 46,489 (1974). (9) D. E. Leyden and G. H. Luttrell, Anal. Chem., 47, 1612 (1975).
(10) D. C. Gregire, and A. Chow, Talanta, 22, 453 (1975). (11) H. J. M. Bowen, J. Chem. SOC.A, 1082 (1970). (12) T. Braun and G. Ghersini, "Extraction Chromatography," Elsevier Scientific Publishing Co., London, 1975, p 344. (13) T. Braun and A. B. Farag, Talanfa, 22, 699 (1975). (14) T. Braun and A. B. Farag, Anal. Chirn. Acta 69,85 (1974). (15) T. Braun and A. B. Farag, Anal, Chim. Acta, 71, 133 (1974). (16) T. Braun and A. B. Farag, Anal. Chim. Acta, 76, 107 (1975). (17) G. N. Lypka, H. D. Gesser, and A. Chow, Anal. Chim. Acta, 78, 367 (1975). (18) E. B. Sandell, "Colorimetric Determination of Traces of Metals", Interscience, New York, 3d ed., 1959, p 668. (19) W. Rieman 111 and H. F. Walton, "ion Exchange in Analytical Chemistry", Pergamon Press, New York, 1970, p 168.
RECEIVEDfor review June 28, 1976. Accepted August 26, 1976. One of us (DWL) thanks the German Academic Exchange Service (DAAD) in West Germany for a scholarship.
Analysis of Complex Aromatic Hydrocarbon Mixtures with Solid Silver Nitrate Columns Stanley P. Wasik" and Robert L. Brown Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234
A highly efficient chromatographic column consisting of glass beads coated with solid silver nitrate and modified by water vapor In the carrier gas can be used at room temperature to analyze, In a reasonable time, mixtures containing aromatic hydrocarbons having boiling points up to 180 'C. The construction of this column is described and its properties discussed. Its eff iclency is compared with conventional gas-liquid chromatographiccolumns.
The principal advantages of gas-solid over gas-liquid chromatography arise from the high selectivities possible with solid substrates and the stability of such substrates at high temperatures. Its 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 (I).Another disadvantage arises from the large partition coefficients generally found for 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 AgNOB. This compound was chosen because of its ability to complex with unsaturated hydrocarbons (1). By modifying this surface with water vapor ( Z ) , we produced a highly efficient column that could analyze in a reasonable time at room temperature aromatic hydrocarbons with boiling points as high as 180 "C. The paper describes how the column was made and how i t compares in efficiency with two conventional gas-liquid chromatographic columns, di-n-propyl tetrachlorophthalate ( D P T P ) , and 7,8-benzoquinoline
(BQ). EXPERIMENTAL Columns. The glass beads used were "Class M" in the 60/80 mesh range (Hewlett-Packard Co., Rockville, Md. 20850). They were washed once with 1M nitric acid and ten times with distilled water. A known amount of AgN03 was dissolved in a slurry of water and beads and the water evaporated until the beads were dry. The coated beads were then sieved (60/80 mesh) and the column packed in the 2218
usual manner. Dry helium was passed through the column at 150 "C to further dry the packing. Column dimensions were 3.6 m (12 ft) X 5.4 mm i.d. (%-in.0.d.). Apparatus. The column sat in a water bath whose temperature was controlled to f0.1 "C. Upstream from the column was a water-saturator in a constant temperature bath (&O.l O C ) . To prevent the AgN03 from going into solution, the partial pressure of the water vapor in the carrier gas was kept lower than the partial pressure of a saturated AgN03 solution at the column temperature. In line with the saturator was a valve 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 a 14-componentmixture of aromatic hydrocarbons and was injected onto the column with a 1O-pl gas syringe.
RESULTS AND DISCUSSION Column Properties. At a fixed column temperature, the retention volume of an aromatic solute was found to be dependent upon the partial pressure of the water vapor in the helium carrier gas ( I ) . Increasing the partial pressure resulted in smaller retention volumes while dry helium caused the hydrocarbons t o 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 60/80 mesh AgN03 crystals gave identical results but was much less efficient. For columns (3-meter) containing the same amounts of glass beads but with different AgN03 coverage (0.3, 0.5, 0.7, and 1.Wo w/w) gave approximately the same retention volumes for the aromatic solutes at a given column temperature and water vapor concentration. The four columns had approximately the same number of theoretical plates (9000 calculated for n-butylbenzene). The eluted peaks were symmetrical except a t low column temperatures (-8 "C) and low water vapor concentrations (saturator at 0 "C).For these cases, the peaks had sharp fronts and diffuse tails, typical of those resulting from a Langmuir-type absorption isotherm. The columns were efficient in the temperature range 10 to 70 "C,but above this range the selectivity was low and decreased at still higher temperatures. Comparison with Other Columns. H&sz (3) has derived the following equation for calculating the number of theoretical plates, Nreq, required to separate two peaks:
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
Table I. Comparative Data for the Retention of Aromatic Hydrocarbons by Several Columns BP ("C) 4AgN03) 4 B Q ) Benzene 80.1 4.12 1.66 Toluene 110.8 2.87 1.89 Ethylbenzene 136.1 1.41 1.058 8750 10000 24 000 138.5 p -Xylene 1.077 1.064 139.1 m-Xylene 1.36 1.240 144.4 o-Xylene 1.30 1.28 159.5 n-Propylbenzene 25300 68000 1.50 1.106 1-Methyl-4-ethylben162.0 zene 1.042 1.027 (1.048)5 1-Methyl-3-ethylben162.5 zene 1.067 1.19 (1.093)6 164.6 1,3,5-Trimethylbenzene 1.18 1.03 1.120 1-Methyl-2-ethylben162 zene 1.23 1.29 1.13 1,2,4-Trimethylbenz169.2 ene 1.04 1.14 1.30 2500 26 500 n-Buthylbenzene 180 1.25 1.24 1.34 1,2,3-Trimethylbenz164.6 ene These numbers label the peaks shown in Figure 1. Retention data on the BQ and DPTP columns were taken from Ref. 4. For the DPTP column some of the a values refer to different solute pairs than those listed for the AgNO3 and BQ columns. These values are given in parentheses. The superscripts 1-6 identify the particular solute pair: (1)ethylbemenelm -xylene, (2) m-xylenelp-xylene, (3) p-xylene/o-xylene, (4) n-propylbenzenell-methyl-3-ethylbenzene,(5) l-methyl-3-ethylbenzene/l-methyl-4-ethylbenzene, (6) 1-methyl-4-ethylbenzene/l,3,5-trimethylbenzene. [I
2
1
where a is the separation factor, VI is the retention volume of the compound eluted later and vd is the dead volume of the columns. When a is near unity, Nreqcan be greatly reduced if a can be increased even slightly by changing the nature of the substrate. However, a is not the sole quantity to consider in judging column performance, the factor involving v d and VI can also be important, particularly for early peaks where the ratio vd/vr can be large. A chromatogram of the 14-component mixture of aromatic hydrocarbons using a column (0.8%w/w AgN03) at 15 "C appears in Figure 1. Table I identifies the components and their correspondence to various peaks in Figure 1.I t also gives the boiling points, values of a for successive pairs of peaks in order of elution time for three solvents (AgN03, BQ, and DPTP), and values of Nreqfor the difficult separations. T h e retention volumes for the aromatic solutes were measured at 110 O C for the DPTP and BQ columns and a t 15 O C for the AgNO3 column. The times required for the three different analyses were approximately the same. For most solute pairs, the a values given in Table I are greater for the AgN03 column than for the gas-liquid columns. The most notable exception is for the l-methyl-3ethylbenzene/l,3,5-trimethylbenzene separation where the D P T P and BQ columns gave higher a values. T h e most difficult separation for the AgN03 column was the l-methyl4-ethylbenzene/l-methyl-3-ethylbenzenepair, requiring
I
1
6
9
12
15
18
21
24
27
30
33
36
39
T I M E , MIN.
Figure 1.
Separation of aromatic hydrocarbons on solid
AgN03
col-
umn Column temperature, 15 OC. Water-saturator temperature, 10 OC. Carrier gas, helium. Inlet pressure (gauge), 14 Ib/in.* (9.7 X IO4 N/m*). Rate of flow, 80 ml/min
25 300 theoretical plates. The DPTP column gave a slightly higher value of a for this separation. The most difficult separation for the D P T P column was the 1,4-trimethylbenzeneln-butylbenzene pair. It required 26 500 theoretical plates while the AgNOB column required only 2 500. Although the BQ column compared favorably with the AgN03 column for the m-xylenelp-xylene separation, it had difficulty separating ethylbenzene from m-xylene ( a = 1.06). The AgN03 column easily made this separation ( a = 1.41). The advantage of working a t lower column temperatures is well known. Purnell(4) has shown that the BQ and D P T P columns would be more efficient a t lower temperatures, but t o obtain a reasonable analysis time it would be necessary to use very low solvent coverage. Often however, packed columns
ANALYTICAL CHEMISTRY, VOL. 48,
NO.
14, DECEMBER 1976
2219
