Simple Water Flow Control for the Coulson Electrolytic Conductivity Detector J. F. Lawrence and N. P. Sen Food Research Division, Health Protection Branch, Department of National Health and Welfare, Tunney's Pasture, Ottawa, K 1A OL2, Canada
The Coulson electrolytic conductivity detector (ECD) (Tracor Inc., Austin, Texas) is a very useful apparatus for the selective determination of nitrogen-, chlorine-, or sulfur-containing compounds by gas-liquid chromatography (GLC). A number of authors have compared this system to other detectors for the analysis of a variety of compounds (1-6). They considered the ECD more suited for routine analysis since this system was more selective and less influenced by fluctuations in operating parameters. However, there are several features of the apparatus that could be changed to yield improved sensitivity. Patchett ( 7 ) was responsible for several refinements which have been incorporated into the current model. Rhoades and Johnson (8) found that selectivity for N - nitrosamines was greatly improved in the pyrolyt,ic mode when a low (400600 "C) furnace t,emperature was used for conversion to ammonia. Cochrane et al. (9) examined the effect of oven temperature and oxygen flow rate on detector response of chlorine and sulfur-containing compounds. Response increased with rising furnace temperature but decreased with increased oxygen flow. Dolan and Hall ( 1 0 ) studied a number of parameters to optimize ECD response to chlorinated hydrocarbon pesticides. Sensitivity was increased, in some cases, tenfold. The effects of gas flow rates and furnace tctmperature on ECD response to triazine herbicides in the nitrogen (reductive) mode was recently examined (11 ). Water flow rates in the system had significant influence on the ECD response in the reductive mode ( 1 1 ) . Sensitivity increased 2 to %fold when mixing chamber flow rate matched the detector cell flow rate so that no water escaped through the vent tube. Jones and Nickless ( 1 2 ) used the same approach in an ECD apparatus which they described. Lawrence and Moore ( 1 3 ) constructed a conductivity cell with a cooling water-jacket, a 5-inch long mixing chamber, and a flow valve to optimize water flow. The overall sensitivity of their instrument was about 5 times that of the Coulson cell. Water temperature had a significant influence on background conductivity and noise ( 2 3 ) . While most of the parameters mentioned above can be controlled on the present Coulson apparatus, the water flow rate into the mixing chamber cannot. Lawrence ( I 1 j adjusted flow rate by partially filling the water pressure column drain with water to decrease hydrostatic pressure, and thus flow rate, into the mixing chember. This was a somewhat tedious method and flow fluctuations occurred from day-to-day. The use of an adjustable clamp on the water line leading to the cell for water flow control has been examined (14 j. The present authors found that while this does work, the total flow from the pump is so restricted that water turn-over through the system is slow and a greater strain is put on the pump. The present report describes a simple technique which easily optimizes water flow rates to suit each Coulson operator's needs. The method also does not require day-to-day adjustment. The approach of this technique is to decrease water flow by reducing the effective volume of the capillary tubing leading to the mixing chamber. This is simply done by inserting a fine stainless steel wire (W-ilkins Instrument and Research Inc., Walnut Creek, Calif.) into the capillary as shown in Figure 1. The wire was first washed in organic solvents, then with distilled, deionized water before inser-
FURNACE EFFLUENT
HYDROSTATIC PRESSURE COLUMN
I
I TYCiON
WATER FROM ION EXCHANGE RE SIN STAINLESS STEEL
RESERVOIR WATER
Figure 1. Partial diagram of the Coulson ECD cell with wire in place The Tygon tubing is removed and the stainless steel wire is inserted into the capillary. The Tygon tube is replaced and the pump started
I i
Figure 2. Background conductivity for the Coulson as set up with the various diameter wires A Reservoir water level 2 cm above the pump entrance B Reservoir Obtained on the 8 X attenuation at 30-V electrode potential
full
tion into the capillary. Water level in the reservoir was 3/4 from the top. A second water level, 2 cm above the pump entrance was also examined. A 14-turn glass cooling coil was inserted into the reservoir and cold tap water circulated to maintain a water temperature of 10 "C. GLC conditions were: helium carrier, 60 ml/min; helium sweep, 60 ml/min; hydrogen, 50 ml/min; furnace temperature, 800 "C; transfer unit, 210 "C. The 6-ft X 6-mm i.d. glass column consisted of 4% SE30/6% QF 1 on 80-100 mesh Chromosorb WHP. Column temperature was 185 "C. The test compound was atrazine (2-chloro-4-ethylamino-6-isopropylamino-s- triazine). Figure 2 illustrates the changes in background conductivity obtained by insertion of different diameter wires into the capillary. The lower values ( A ) were obtained a t the lower (second) reservoir water level examined. These values were obtained after equilibration for at least :?0 minutes a t which time the background remained stable. The observed increase in background conductivity was small for the 0.003- and 0.004-in. wires while the 0.005-in. caused a significant jump. The noise level with the 0 005-in. wire also increased. This was due to the fact that the water flow
ANALYTICAL CHEMISTRY, VOL. 47, NO. 2, FEBRUARY 1975
e
367
005'
1
I
5i
5 i
I
I
003" WIRE
NO WIRE
w 0
g
0
2
4
6
2
4
6
8 2 4 MINUTES
6
8
I
2 4 6 8 1 0
Figure 3A. Chromatographic results obtained using different diameter wires inserted into the capillary 9 ng atrazine injected. 8 X attenuation. 30 V. Reservoir y4 full
i ,
,
0
2
,
,
4
6
,
2 ,
,
8 0 2 MINUTES
4
I
,
6
8
J
Figure 38. Chromatographic results obtained at a reservoir water level of 2 cm above pump entrance
4 ng of atrazine injected. 2 X attenuation. 30 V
into the mixing chamber was, a t this point, less than that going through the cell arm to the electrodes. Thus, the detector cell flow fluctuated greatly and air bubbles were frequently drawn into the cell arm causing irregular response. Figure 3A shows the response of the Coulson to 9 ng of atrazine with different diameter wires inserted. These were obtained with the reservoir 3/4full. Insertion of the wires increased sensitivity 2 - to %fold compared to the system with no wire. Background noise remained the same for all flow rates except when the 0.005-in. wire was inserted. Also, significant tailing of peaks occurred even without the wire (see Figure 3A). This was due to the slow flow rate of water through the cell arm a t high reservoir water levels (11 ). At faster flow rates (water level 2 cm above the pump entrance), peak shape improved significantly. Figure 3B depicts the results obtained a t the lower reservoir water level. The peaks were sharper and much less tailing occurred. In this case the 0.003-in. wire caused a >2-fold increase in sensitivity. The 0.004-in. diameter wire did not further increase the sensitivity. While the literature reports one-half full-scale deflection for 5 ng ( 1 5 ) and 7 ng (16) of atrazine, we have been consistently obtaining values of less than 2 ng when the flow was optimized by the appropriate wire. Where resolution of peaks and peak height are important, as opposed to peak
368
area, the conditions described in Figure 3B with the 0.003in. wire were considered optimum. The flow rates may also be further adjusted by changing the distance the wire enters the capillary.
LITERATURE CITED (1)R . Purkayastha and W. P. Cochrane, J. Agr. f o o d Chem., 21, 93 (1973). (2)H. Y. Youngand A. Chu, J. Agr. FoodChem., 21,711 (1973). (3)D. M. Coulson, J. Gas Chromatogr., 3, 134 (1965). (4)D. M. Coulson, L. A . Cavenagh, J. E. devries. and 6.Walther, J. Agr. f o o d Chem., 8,399 (1960). (5)J. F. Palframan, J. MacNab, and N. T. Crosby, J. Chromatogr., 76,307 (1973). (6)R. Greenhalgh and W. P. Cochrane, J. Chromatogr.. 70,37 (1972). (7)G. G.Patchett, J. Chromatogr. Sci., 8, 155 (1970). (8)J. W. Rhoades and D. E. Johnson, J. Chromatogr. Sci.. 8,616 (1970). (9)W. P. Cochrane, B. P. Wilson, and R . Greenhalgh, J. Chromafogr., 75, 207 (1973). (10)J. W. Dolan and R. C. Hall, Anal. Chem., 45,2198 (1973). (1 1) J. F. Lawrence, J. Chromatogr., 87,333 (1973). (12) P. Jones and G. Nickless, J. Chromatogr., 73, 19 (1972). (13)J. F. Lawrence and A. H. Moore, Anal. Chem., 46,755 (1974). (14)0. H. Fullmer, FMC Corp., private communication. (15) R . R . Laski and R. R . Watts, J. Ass. Offic. Anal. Chem., 56,328 (1973). (16)W. P. Cochrane and 6.P. Wilson, J. Chromatogr., 63,364 (1971).
RECEIVEDfor review September 3,1974. Accepted November 18, 1974.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 2, F E B R U A R Y 1975
