speed of analysis, and adequate sensitivity were attained. And the results obtained were comparable to the results of an established ASTM method. Concerning cost of equipment, a GC already owned by a laboratory can be used if it is equipped with an FPD and a gas mixing device, or a very inexpensive GC may be purchased. Even if all the necessary equipment has to be purchased, costs are only a little more than those required for the Lamp Method. Amortization of the equipment can readily be seen in the following rationale: The Lamp Method can analyze 12 samples per man day and a lead correction is necessary. The FPD method can analyze 60 samples per man day and no lead content correction is necessary. A fivefold increase in throughput is anticipated using the same staffing level. At an assumed figure of $80/man day the costs for replacing the
Lamp Method by the GC/FPD are amortizable in less than three weeks.
LITERATURE CITED (1) "1973 Annual Book of ASTM Standards", Part 17, American Society for Testing and Materials, Easton, Md.. Nov. 1973. (2) Ref. i,,p 951. (3) S. S. Brody and J. E. Chaney, J. Gas Chromatogr.,4, 42 (1966). (4) R. K. Stevens, A. E. O'Keeffe, and G. C. Ortman, Enviroon. Sci. Techno/.,3, 652 (1969). (5) R. H. Devonald, R. S.Serenius, and A. D. Mclntyre, Pu$ Pap. Can., 73, 3 (March 1972). (6) T. Sugiyama, Y. Suzuki, and T. Takeuchi, J. Chromatogr., 80, 61-67 (1973).
RECEIVEDfor review May 19,1976. Accepted September 24, 1976.
Gas Chromatographic Determination of Trace Amounts of Sulfur Compounds in Industrial Effluents A. G. Vitenberg" and L. M. Kuznetsova Chemistry Department, Leningrad State University, 199004 Leningrad, USSR
1. L. Butaeva and M.
D. lnshakov
All-Union Research institute of Paper Industry, Shvernik St., 49, 19402 1, Leningrad, USSR
A method is described for the trace determination of hydrogen sulfide, mercaptans, sulfides, and disulfides In Industrial effluents which is based on a combinationof head-space analysis with microcoulometry. This method increases the analytical sensitivity 102-103 times without any preliminary concentration of the sample. The factors affectlng the precision and sensitivity of the analysis are studied. A simple device is developed permitting injection and gas chromatographic aiiaiysis of samples of the solution under study without losses of the compound of interest.
One of the most urgent problems in environmental protection consists of the determination of sulfur compounds, such as hydrogen sulfide, mercaptans, organic sulfides, and disulfides in industrial wastes and particularly in the effluents of the paper industry. Because of the high toxicity of these pollutants, the corresponding tolerance limits for their content are very low, their analysis thus requiring extremely sensitive methods which should permit selective determination of impurities at the 10-% level or 0.1 ppm. These requirements are best met by gas chromatography (1)which is widely used in the analysis of sulfur compounds ( 2 ) ,in particular in trace amounts ( 3 ) . A specific feature of the analysis of sulfur impurities in industrial effluents is the requirement of determining trace amounts of unstable and fairly volatile compounds against a strong background of concomitant impurities that may considerably exceed that of the compounds to be determined. Besides, of considerable importance for the precision of the results to be obtained is the correct choice of the sampling technique and a proper preparation of the sample for analysis. However, these aspects of trace gas chromatographic analysis of sulfur compounds do not receive adequate attention a t the present stage. This paper is devoted to the description of a gas chroma128
ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
tographic method developed for the microdetermination of sulfur compounds in kraft mill effluents providing their selective determination at the tolerance concentration level and reducing the losses involved in the sampling and preparation for analysis to a minimum. A high sensitivity and good selectivity in gas chromatographic analysis can be attained by employing special detecting devices possessing sensitivity only toward certain elements or functional groups (4).As for the sulfur compounds, the flame photometric ( 2 )and the microcoulometric (5) detectors have received particularly wide use. We have chosen for the work the microcoulometricdetector with the selectivity coefficient for hydrocarbons of lo5 (5) which is capable of determining sulfur-containing impurities down to 1 ppm by direct injection of the object to be analyzed (5).However, in most cases, such a sensitivity turns out to be insufficient for direct determination of sulfur compounds in concentrations corresponding to the official regulations for the disposal of waste waters. The existing traditional methods for the concentration of organic impurities in solutions based on distillation, extraction, and removal by gas flow with their subsequent trapping by adsorption (6)are of limited use for the analysis of labile sulfur compounds since the inherent losses of the compounds of interest may result in considerable error.
EXPERIMENTAL The work was carried out on a Model 102, Tsvet-100 gas chromatograph employing a flame ionization detector. For selective determination of sulfur-containing compounds, we used a model KDS-2 microcoulometric detector. When both detectors were used, the gas flow emerging from the chromatographic column was divided by means of a T-shaped stream splitter into two parts in a ratio close t o 1:1. The contents of the component of the mixture under analysis were calculated taking into account the accurate value of the ratio of the gas flows delivered to the flame ionization and the microcoulometric detectors. A mixture of the simplest mercaptans, sulfides, and disulfides was
bo V
I
I
II $ 14
6 12 B 10 E
0 V
a 6
z7
8
4
1
2
pc
IO
Figure 1. Chromatogram of (a) a water solution of diethyl disulfide and (b) the gas
above the solution
20 30 46 50 TXM'LiiATUlLE, "C
Figure 2. Partition coefficient of sulfur compounds vs. temperature for a pH 2 buffer solution with 7 % of sodium sulfate
2-m X 3-mm glass column, 15% PEGA on C-22 Celite, 120 'C. flame ionization detector
separated in a glass column 2 m long with an inner diameter of 3 mm. The packing consisted of 15% polyethylene glycol adipate on C-22 celite, 80-100 mesh grain size. The column temperature was 90 "C, the flow rate of the carrier gas (helium) was 50 ml/min. Liquid samples were injected into the chromatographic column with a 1O-pl Hamilton microsyringe, the volume of the injected solution being determined by taking into account the evaporation of the liquid from the syringe needle. Gas samples were injected by means of a gas sample valve. Quantitative analysis was carried out by the absolute calibration method. For the flame ionization detector, the measured parameter on the chromatograms was the peak height, while for the microcoulometric detector, it was the peak area calculated by an analog integrator supplied with the detector. For calibration of the instrument, we used standard ethanol or water solutions of the sulfur compounds of interest with concentrations ranging from 100 to 0.1 ppm which were prepared by proper dilution of concentrated (0.1%) solutions obtained by introducing an accurately weighed amount of the compound in question into the solvent of known volume. Calibration solutions of volatile compounds (e.g., methyl mercaptan) were prepared by crushing sealed thin-walled ampoules containing a precisely known amount of the compound in a solution in a closed vessel. The mixture of hydrogen sulfide with helium was prepared by the technique described in Ref. (7). The calibration graphs were obtained from the measurements by the least-squares method. A KC1-HCl buffer solution of pH 2 was used to maintain a constant magnitude of pH. The magnitude of pH was checked against a glass electrode calibrated on a series of standard solutions.
RESULTS AND DISCUSSION To increase the sensitivity of microimpurity determination in solutions, we used head-space analysis. This technique finds increasingly extensive use in the trace determinations ( 3 )of sulfur compounds (7-10). The head-space analysis is fairly simple to carry out. A volume VL of the solution to be analyzed, is placed in a closed vessel with the volume of the gas phase V Gand maintained a t constant temperature until thermodynamic equilibrium has set in. Next, one determines gas chromatographically the equilibrium concentration of the analyte in the gas phase CG which is connected with that in the liquid CL by the simplest partition law.
The concentration of the microimpurity in the original solution CL', i.e., in the object under analysis is defined by the expression (8).
Table I. Constancy of the Partition Coefficient of Dimethyl Sulfide at Various Concentrations in the Buffer Solution Containing 7% Sodium Sulfate (at 20 C) O
Dimethyl sulfide concentration in buffer solution
8x
Partition coefficient
lo-'
7.6 7.0 7.6 7.2
2 x 10-3 4 x 10-5 1 x 10-5
CLO = C G
3
(K +-
Equation 2 implies that the gain in sensitivity a of the analysis of an impurity in solution obtained by determining CG gas chromatographically as compared with direct injection of the liquid depends o n the magnitude of the partition coefficient and is 103 times (3) K + VG/VL (Here lo3 is the limiting volume ratio of the gas and liquid injected in a gas chromatograph.) Besides the gain in sensitivity, the analysis of an equilibrium gas phase has additional advantages over direct injection of the solution under study consisting in a strong reduction of the content in the sample introduced in the chromatograph of the major solvent (water) and of the compounds with large values of K . Eventually, it improves the quality of separation and increases the accuracy of the quantitative treatment of Chromatograms (Figure 1). The trace analysis of solutions by this method requires the knowledge of the values of K for extremely dilute water solutions of sulfur compounds. However, these data are not available in the literature. To determine K , we used the technique of introducing a pure gas (e.g., air) into a closed vessel of known volume containing a known amount of the solution with the preliminarily measured concentration of the compounds of interest (9). It is known that the area of application of the analytical methods based on phase equilibria is limited by dilute solutions whose activity coefficients are constant, or are close or equal to unity in the concentration range covered (3, 11). Thus, the upper limit of the concentrations determined in solutions is restricted by the value at which the dependence a=
ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
129
I
16
14
------- _ _ _ _ _ - -
12
10
CONCEfiRATIOI ORGANIC SUBSlWCES. P
Flgure.4. influence of the content of organic compounds on the magnitude of the partition coefficient of dimethyl sulfide in a pH 2 buffer solution containing 7 % sodium sulfate
Table 11. Limiting Concentrations of Concomitant Compounds at Which the Accuracy of Determining Sulfur-ContainingCompounds in Water Solutions is Still Not Impaired
2
i
6
8 ?O I2
Compound
i4
C O ~ C E ~ 'oazso4, I O ~
Methanol Acetic acid Ethanol Lactic acid Sodium sulfate
a
Figure 3. Partition coefficient of dimethyl sulfide vs. content of sodium sulfate in solution (20 OC)
Concentration, %
2.2 2.6 3.4 1.6 0.9
I
CG = ~ ( C Lis) observed to deviate from the linear law. Indeed, as we have shown for dimethyl sulfide ( D h S ) within the accuracy of determining K in the system considered, the isotherm of partition of a compound bet&eeh the liquid and the gas is linear to a few percent of the compound in solution (Table I). The lower limit of concentrations determined in solution depends on the threshold sensitivity of the method employed for the analysis of the gas Phase and can be calculated by Equation 2. For the microcoulometric detector with a threshold sensitivity for sulfur of 5 X g ( 5 ) ,the chosen conditions of gas chromatographic analysis, the injection of 10 ml of equilibrium gas in the phromatograph and VG5 VL, it is possible to analyze sulfur compounds in water solutions a t concentrations down to 10-6-10-7%. When using the head-space technique to analyz? sulfur compounds in water solutions, one should bear in mind that CG depends on a number of factors determined by the experimental conditions under which equilibrium distribution of a compound between the liquid and gas is established. Of particular importance for the accuracy of the analysis is the correct choice of pH of the solution. In the case of a water solution of the sulfur compounds, this factor affected essentially their content in the equilibrium gas phase, since for dissociating compounds the vapor pressure above the solution is proportional to the nondissociated fraction of the compound (3, 7). Therefore prior to equilibrating the solution under study with the gas phase, ope should bring pH to 2 using a KC1-HC1 buffer solution. The choice of this buffer is explained by the fact that a t pH 2 the dissociation equilibrium of each of the sulfur compounds considered in water is displaced practically completely to the left (12). Since the distribution of a compound between the liquid and the gas phases depends on temperature, one should maintain it constant during equilibration with the accuracy determined by the character of the temperature dependence of the partition coefficient and the allowable analytical error. Figure 2 shows such a dependence for a nbmber of sulfur compounds. These data show the low temperature region to be characterized by a steeper dependence K = f ( T )which means that for a given accuracy the requirements for the 130
*
ANALYTICAL CHEMISTRY, VOL. 49, NO. l,JANUARY 1977
precision of thermostating at elevated temperatures (above 20 " C ) may be less stringent than a t lower temperatures. Equilibrating a t elevated temperatures results also in an improvement of sensitivity due to the decrease of the partition coefficient (Equation 3); however it involves the use of special equipment. The presence of concomitant impurities with broadly varying composition and content may affect substantially the accuracy of the analysis of sulfur-containing pollutants in industrial effluents. In the analytical version considered, combining a sufficiently effective column and selective microcoulometric detection, the errors associated with the superposition on a chromatogram of peaks of the concomitant compounds and of the sulfur substances of interest are practically excluded. However, the concomitant compounds may distort the analytical results by changing the composition and properties of the solution and thus affect the magnitude of the partition coefficient. This effect is most of all pronounced for mineral salts a t low concentrations (Figure 3). Therefore the technique developed by us for the trace determination of sulfur coinpounds in industrial waste waters involves introducing sodium sulfate in the sample to be analyzed a t a concentration of 7%. In addition to reducing the errors associated with the change of properties of the solution due to the salting out action, such a mineralization increases sensitivity of the analysis (Figure 3, Equation 3). The influence of concomitant organic impurities on the distribution of sulfur compounds between the liquid and the gas phases was studied by us on methanol, ethanol, acetic and lactic acids, i.e., water-soluble compounds present in kraft mill effluents. Dimethyl sulfide was chosed as the analyte; however the results obtained are equally applicable, within the accuracy of the method, to the other sulfur compounds considered (10).
As the limiting concentration of a concomitant impurity that still practically does not affect the accuracy of determination of the sulfur compounds, we have chosen its content a t which the magnitude of the partition coefficient of the analyte in the buffer solution deviates from the mean by the magnitude of the error in the coefficient's determination ( K f ( A K I K ) ) This . maximum allowable concentration was de-
3
-$-------Tho,
2 1
mia.
Figure 5. Chromatogram of vapors of the simplest sulfur compounds in air contents in the range 10-100 ppm (1) mercaptan; (2) dimethyl sulfide
+
ethyl mercaptan; (3) dimethyl disulfide; (4) diethyl disulfide. Detector: flame ionization. Conditions of the gas chromatographic analysis are specified in the Experimentalsection
1 5 1 2 9 6 3
duced graphically from the intercept of the dependence of K on the concomitant impurity concentration in solution with the ordinate corresponding to the maximum allowable deviation of K from its mean value which in our case is 10% (Figure 4). The dependence of K on the content of concomitant compounds in the buffer solution was determined by measuring gas chromatographically the analyte concentration in the liquid phase of an equilibrium system with known volumes of the liquid and the gas by varying properly the content of the concomitant substances in the buffer solution. If we introduce in the liquid phase of an equilibrium system a compound changing the properties of the solution so as to maintain the quantity of the analyte distributed between the two phases constant, and if we neglect the change in the volumes, the partition coefficient for the solution obtained can be found from the formula (5) (the primes referring to the parameters of the system obtained after introducing the concomitant compound in question). Table I1 lists the results of determining the limiting concentration of the concomitant organic admixtures and of the mineral salt in a buffer solution containing 7% sodium sulfate. The data obtained show that for a 10%relative error in the determination of the sulfur compounds, the effect of concomitant organic substances on the magnitude of K manifests itself at sufficiently high concentrations, so that in the case of effluents which need a preliminary treatment it should lie within the analytical errors. When determining impurities in effluents with a high content of concomitant compounds, one should introduce the corresponding corrections for the change of the partition coefficient (Figures 3 and 4). Of considerable importance for the achievement of the maximum sensitivity and a reasonably high accuracy in the analysis of trace impurities is the correct choice of conditions for the gas chromatographic separation of the mixture (column efficiency, temperature, carrier gas flow rate), characterized by the spread of the peak in the eluate which increases with increasing retention time of the compound of interest under isothermal conditions. To reach the highest possible sensitivity, we chose the conditions of separation under which the duration of the analysis did not exceed 20 min. Figure 5 presents an example of separating an air-vapor mixture of the simplest mercaptans, sulfides, and disulfides in concentrations 10-100 ppm. The compounds most difficult to separate here are DMS and ethyl mercaptan (EM) which are not separated under the conditions specified in the Experimental section. Complete separation of these compounds can be effected after a fairly long time using a 6-m long column with a nonpolar stationary phase (ApiezonL) (13).A recently published paper (2) reports on a separation of DMS and EM taking only a few
0
i 5 U Y 6
A
1
0
a
Figure 6. Chromatogram illustrating the change of ethyl mercaptan
content in an alkaline solution in time (A) pH 2; (B) pH 10; (1) after 20 min; (2) after 45 min; (3) after 70 min; (4) after 90 rnin. Conditions of the chromatographicanalysis are specified in the text
minutes under conditions of programmed temperature control and the use of the flame ionization detector. However, complete separation was likewise not reached here which practically excludes the possibility of using such a technique with the inertial microcoulometric detector. The conditions of the gas chromatographic analysis chosen by us for separate determination of DMS and EM involve analysis of the gas phase above solution at various pH. At pH 2, one determines the total content of these compounds, after which pH is increased up to 10 where mercaptan is mostly converted into mercaptide, the remainder of EM undergoing oxidation to diethylsulfide at room temperature (Figure 6). The key condition for a successful use of the head space analysis of the sulfur impurities in solutions is the exclusion of adsorption losses of the compounds of interest, as well as of losses involved in sampling, injection in the chromatograph, and in the chromatographic process itself. The existing devices for establishing equilibrium distribution of a compound between a liquid and gas and the techniques of injecting the equilibrium gas in the chromatograph do not preclude such losses since they have rubber membranes and the injection is accompanied by a change in the analyte concentration in the gas phase ( 3 ) . Losses of small amounts of sulfur compounds in the course of the gas chromatographic analysis can be prevented only by using chromatographs in which the vapors of the compounds of interest do not come in contact with metal. Some authors, see e.g., Ref. (IO), believe that the use of metal in columns and the evaporator may result in losses of hydrogen sulfide and of mercaptans. Therefore we employed a gas chromatograph whose parts coming in contact with the vapors are made of glass or Teflon. To avoid losses of the analyte in the course of sampling, preparation for analysis, and injection in the chromatograph we used glass vessels of variable volume. For this purpose, we chose large volume hypodermic syringes (50-100 ml) in which a sealing device is attached to the tip (14). This glass device with a variable inner volume permits sampling practically without any loss of volatiles, achieves equilibrium distribution of compounds between the liquid and the gas, allows addition of any liquid or gaseous reagents, storage for long times, and even to transport samples to remote places if desired. When used in combination with a gas sample valve, this device permits injection of the equilibrium gas into the chromatographic column (14). When utilizing a thermoANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
131
~~
~
Table 111. Analytical Data on Test Solutions of Sulfur Compounds Approaching the Composition of Concomitant Impurities to Kraft Mill Effluents Flame ionization detector
Microcoulometric detector
Analytical error Compound
1.Methyl
mercaptan 2.Ethyl mercaptan 3. Dimethyl
sulfide 4.Dimethyl disulfide 5. Diethyl disulfide
Introduced, %
3.8 x 1.6 x 7.4 x 1.5 x 4.0 x 4.0 x 5.5 x 9.9 x 7.1 x 5.9 x 7.1 x 6.4 x 6.5 x 4.9 x 9.9 x
10-5 10-4 10-5 10-4 10-6 10-4
10-4 10-5 10-6
6.Hydrogen sulfide
Found, o/oa
3.5 x 1.7 x 8.2x 1.6x 3.8 x 4.4 x 5.4 x 1.0 x 6.3x 6.2 x 6.3x 5.8 x 6.5 x 4.8 x 1.1 x
lo+ 10-5
10-4 10-5 10-4 10-5 10-4 10-4 10-5 10-5
Analytical error
Absolute
Rel., %
-0.3 x 10-5 +o.i x 10-5 +0.8 x +o.i x 10-4 -0.2 x 10-5 +0.4 x -0.1 x +0.1 x -0.8 X +0.3 x 10-4 -0.8 x -0.6 x
7.9 6.3 10.8 6.7 5.0 10.0 1.8 1.0 11.3 5.1 11.3 9.4
-
-0.1 x 10-5 +0.2 x 10-6
2.o
2.0
Introduced, %
3.8 x 1.6x 7.4 x 1.5 x 8.6 X 4.0 x 5.5 x 7.4 x 6.9 x 5.9 x 3.9 x 7.1 x 6.5 x 4.9 x 9.9 x 4.2 x 3.5 x 5.7 x
10-5 10-5 10-6 10-5 10-4 10-5
10-4 10-4 10-4 10-5 10-6 10-4 10-5 10-6
Found, %a
Absolute
Rel., %
3.3 x 10-5 1.4 x 10-5 8.8 x 1.4 x 10-4 1.0x 10-4 3.2 x 10-5 4.7 x 10-4 8.6 x 7.4 x 10-6 5.6 x 4.3 x 10-4 5.7 x 10-6 7.3 x 10-4 5.7 x 10-5 8.3 x 4.5 x 104 3.9 x 10-5 4.9 x 10-6
-0.5 x 10-5 -0.2 x 10-5 +1.4 x -0.1 x 10-4 +1.4 X -0.8 X -0.8 X +1.2x 10-5 +0.5 x -0.3 x 10-4 +0.4 X -1.4 X
13.2 12.5 18.9 6.7 16.3 20.0 14.5 16.2 7.2 5.1 10.3 19.7 12.3 16.3 17.2 7.1 11.4 14.0
+0.8 X
+0.8X -1.7 X +0.3 x 10-5 +0.4 x
-0.8 x
a Mean of 10 determinations.
stated gas sample valve, the reproducibility of injecting a gas with a content of sulfur-containing compounds of 1-100 ppm is not worse than 1%(rel.). It is well known that adsorption losses of analytes in trace analysis may result in systematic error amounting to tens and even hundreds of percent (15). In our device, adsorption during equilibration may occur on the surface of the elastic rubber plug and (from the solution) on rough walls of the syringes. To reduce the adsorption of the compounds of interest on the surface of the elastic plug made of silicone rubber, it is separated from the cylinder by means of a capillary Teflon tube which practically excludes the loss of the analyte from the gas phase onto the rubber plug. T o evaluate the loss of the analyte from solution through adsorption on the walls, we have developed a technique which consists essentially in measuring the change in the concentration of a compound in the solution of interest observed when it is introduced into a dry vessel. This may be easily accomplished by using two syringes connected with one double-ended needle (16). When measuring adsorption losses from a solution at 20 "C, within the range of DMS concentrations in a salt solution of 5 X 10-2-9 X 10-5%, the isotherm of adsorption is linear. This means that in the concentration range specified, the adsorption losses do not depend on the content of the analyte in solution (i.e., they have the same percentage magnitude) but rather are determined by the ratio of the liquid volume in the syringe to its surface. Indeed, in a 100-ml syringe containing 10 ml of a DMS solution, the adsorption losses constitute 0.017%. However if we place 1ml of solution in the same syringe, the losses will amount to 0.17%. Thus one can readily evaluate the adsorption losses of a compound knowing the dependence of the adsorptivity of the vessel's walls on the concentration of the compound in a known volume of the solution. Actually, our technique of determining traces of sulfur compounds in industrial effluents is as follows. A sample of the solution to be analyzed of volume VL without the gas phase is drawn into the variable-volume device (14). A 1:l KCl-HCl buffer solution of pH 2 containing 14% of sodium sulfate is added through the elastic rubber plug with the help of a hypodermic syringeto the sample. After this, air of volume VG(an inert gas is preferable for prolonged equilibration) is 132
ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
drawn into the vessel with the solution to be analyzed and the system is maintained for not less than 20 min at a constant temperature under periodic shaking. After equilibration, the concentration of the sulfur compounds in the gas phase above the solution is determined. T o do this, the equilibrium gas is displaced by the plunger from the vessel, filling the sample loop of the gas sample valve which is used to inject the analyte into the chromatographic column. On measuring the areas or heights of the corresponding peaks on the chromatogram I,, one determines the concentration of each compound of interest in the gas phase from the cali) for a given sample loop. The bration data I, = ~ ( C Gavailable conditions for the gas chromatographic analysis are specified in the Experimental section of the paper. The content of each sulfur compound in the solution is calculated by Equation 2 taking into account dilution by the buffer solution. The values of K for different temperatures for the buffer solution (pH 2) containhg 7% of sodium sulfate which are needed in the calculations are presented in Figure 2. According to Equation 2, and assuming the error in the liquid and gas determination to be much smaller than that of the concentration measurement, the error in the analysis of a microimpurity in the solution under study will be
whence it follows that the error in determining the content of an impurity in the liquid can be practically reduced to that of measuring the concentration of this impurity in the equilibrium gas phase if the analysis is performed under the condition VG> VL. However, this involves a substantial decrease in the sensitivity of the analysis (Equation 3). A t the same time, if one takes comparable volumes of the liquid and of the gas, the contribution of the error in the determination of K to the total error of analysis becomes essential increasing with increasing numerical value of K . Therefore, in cases where the detector of the chromatograph has a margin of sensitivity in determining an impurity, one should take as small liquid samples as possible. As seen from Table I11 which lists the results of an analysis of test solutions close in composition to kraft mill effluents and containing sulfur compounds a t the concentration level
(2) P. Ronkainen, J. Denslow, and 0. Leppanen, J. Chromatogr.Sci., 11,364 (1973). (3) A. G. Vitenberg, B. V. loffe, and V. N. Borisov, Zh. Anal. Khim., 29, 1795 (1974); Chromatographia, 7, 610 (1974). (4) M. Krejci and M. Dressler, Chromatogf~Rev., 13, 1 (1970). (5) H. E. Aavik, A. V. Kabun, R. A. Kollasorg, and I. A. Revel’skii, Proc. I1AllUnion Conf. on Prevention of Contamination by Pesticides of Foodstuffs. Fodder and Environmental Medium, Tallinn, 1971, p 16. (6) B. G. Berezkin and V. S. Tatarinskii. “Gas Chromatographic Techniques of Impurity Analysis” (In Russlan), Nauka, Moscow, 1970. (7) I. L. Butaeva, V. V. Tsybul’skii, A. G. Vitenberg, and M. D. Inshakov, Zh. Anal. Khim. 28, 337 (1973). (8) T. G. Fild and J. E. Gilbert, Anal. Chem., 38, 628 (1966). (9) I. L. Butaeva and A. G. Vitenberg, “Chromatographic Analysis in the Wood Chemistry”, Coll. of papers, Zinatne Riga, 1975, p 294. (IO) H. Williams and F. E. Murray, Pulp Pap. Mag. Can., 347 (1966). (11) B. Kolb, “Angewandte Gas-chromatographie”, Bdenseewerk Perkin-Elmer Co., GmbH, Uberlingen, 11-1 1E (1968). (12) L. A. Alferova and A. A. Alekseev, “Chemical Treatment of Effluents in the Production of Sulfate Pulp”, Moscow, Lesn. Prom., 1966. (13) R. V. Golovnya and Yu. N. Arsen’ev, lzv. AkadNauk SSSR, Ser. Khim., 1402 (1972). (14) A. G. Vitenberg. I. L. Butaeva. and 2 . St. Dimitrova, Chromatographia, 8, 693 (1975). (15 ) R. Kaiser, Int. Congr. “Chromatography 1972”, Montreus, Switzerland, 9-13 October 1972. (16) A. G. Vitenberg, B. V. loffe, 2. St. Dimitrova, and I. L. Butaeva, J. Chromatogr., 112, 319 (1975).
of 1-0.01 ppm, at contents of about 1ppm, the analytical error does not exceed 8%for the flame ionization detector and 12% for the microcoulometric detector, of the given amount of the compound. When analyzing solutions with concentrations of not more than 0.1 ppm, the analytical error in the region of the highest sensitivity reaches 15%and 20%for the flame ionization and the microcoulometric detector, respectively. This is apparently associated with the large error of the detecting device which operates near its sensitivity threshold at such concentrations.
CONCLUSION A possibility is shown of determining sulfur compounds in industrial effluents in the concentration range lO-6-lO-7% in the presence of organic and mineral compounds. In the concentration range specified, the error in the determination of the above compounds does not exceed 12% when using the flame ionization detector, and 20%with the microcoulometric detector. An analysis takes 20 min, including the time needed to prepare the sample. LTTERATURE CITED
RECEIVEDfor review January 8, 1976. Accepted May 24, 1976.
(1) “Chemical Analysis. Determinationof Organic Compounds: Methods and
Procedures”, Vol. 32, F. T. Weiss, Ed., Interscience, New York, 1972.
Evidence for Solute-Brush Interactions on Nonpolar Chemically Bonded Stationary Phases in Gas Chromatography Joseph J. Pesek” and Jeffrey A. Graham Department of Chemistry, Northern Illinois University, DeKalb, 111. 60 1 15
Sample size studles, thermodynamic parameters, and comparlsons to a conventionally coated column are used to show that nonpolar solutes interact primarily wlth the nonpolar bonded material. Data obtained on sllanlzed columns indicate that even more polar or hydrogen bonding solutes can interact with the bonded material. In all cases the data appear to be consistent with an adsorption mechanlsm rather than solvation of the solute wlth the bonded material behaving as a liquid.
Chemically bonded stationary phases are being used more frequently in a variety of separation problems. Unlike conventionally coated phases which are held to the chromatographic support via physical forces only, chemically bonded phases are attached by a chemical bond. The result is, in general, a higher thermal stability than comparable conventionally coated phases ( 1 ) . In addition, chemically bonded phases are more efficient in mass transfer than conventionally coated phases (2-4). This is due to the fact that a uniform coating (usually a monolayer) exists for the bonded phase while the conventional phase generally exists in pools (quite often many molecular layers deep) on the surface. The faster rate of mass transfer results in smaller theoretical plate heights. Thus, resolution is increased and higher gas velocities may be employed with little loss in column efficiency. Many researchers have investigated the separating abilities and efficiencies of some bonded phases (2,3,5-23).Much of this work has been done in high messure liauid chromatoeraphy (9,10,13,19,21,23).Achdugh bonded phases may bk structurally identical in gas and liquid chromatography, differences in properties exist due to mobile phase solvation in
HPLC. Therefore, separation mechanisms for the same bonded phase may be different in these two techniques. There are very few reports in which the retention mechanism(s) for various solutes on bonded phases has been studied. Karger and Sibley (6) have investigated retention characteristics on nonpolar bonded phases by varying the length and branching of the hydrocarbon brushes and by silanization of the support. They concluded that unreacted surface hydroxyl groups were the main source of retention. The extent of this retention is a function of the size and polarity of the solutes. Pesek and Daniels ( 5 ) have investigated the retention mechanism of various bonded brushes containing functional groups. They concluded that retention was controlled by adsorption on the brush, adsorption on surface hydroxyls, or a combination of both mechanisms depending on the solute. This study involves gas chromatographic investigation of the retention mechanism for nonpolar bonded phases. These phases should be selective for relatively nonpolar solutes. Several methods are employed to help elucidate the mechanism. First, sample size studies are used to indicate the type of solute isotherm (24).Thermodynamic parameters are used to compare the properties of the brush, support surface, and conventionally coated columns. Finally, varying amounts of bonded brush are employed as well as different types of support-brush bonds. The latter is important when silanizing the surface because the Si-0-R bond can be quite easily cleaved. This combination of techniques should be sufficient to allow elucidation of all possible solute interactions.
EXPERIMENTAL Instrumentation and Equipment. The gas chromatograph emANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
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