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Anal. Chem. 1991, 63,1901-1902
111. In Figure 5 we have reported the chromatograms of (a) 250 p L of 2 mg L-' oxalate, (b) 250 p L of 2 mg L-' oxalate + 0.028 mol L-' sodium chloride (containing 992.7 mg L-' chloride), and (c) 250 pL of 2 mg L-' oxalate + 0.014 mol L-' calcium chloride (containing 992.7 mg L-' chloride). Although the total amount of loaded chloride at 248.2 pg in Figure 5b was slightly higher than 226.9 pg in Figure 4b, the reduction in the peak height of oxalate in Figure 5b at 10.2% was much lower, as compared to 40% in Figure 4b. These results, thereby, indicated that a decrease in the peak height of oxalate in the presence of a large concentration of sodium chloride is not related to the chloride overloading of the separator column. A reduction of 70.5% in the oxalate peak height in the presence of 0.014 mol L-' calcium chloride (containing 992.7 mg L-' chloride), shown in Figure 5c, as compared to 10.2% in the presence of sodium chloride, also containing 992.7 mg L-' chloride, reveals that the peak height of oxalate ion may be affected by the cation of the metal salt, present with oxalate in the sample. The effect of the presence of different salts on the peak height of 2 mg L-' oxalate has been reported in Table 111. It is seen that, at the same anion (chloride) concentration, calcium has exerted a larger change on oxalate peak height than sodium. Similarly, the effect of manganese was much higher as compared to sodium when present with oxalate as their nitrate salts, containing the same amount of nitrate. The experimentalresults presented in this paragraph thus demonstrated that it is the cation that is causing a reduction in the oxalate peak height during its determination by suppressed ion chromatography in the presence of a relatively large concentration of metal salts, such as those reported in Table 111. Although further experiments may be required to put forward an exact mechanism, the accumulation of protons in the suppressor during ion exchange of metal cations (present in the large amounts with the small concentration of determining anion) with protons seems responsible for reducing the peak heights of anions, forming stable ion pairs (such as sulfate, oxalate, arsenate, chromate, etc.). In conclusion, on the basis of the results presented in this paper, we mention that the accurate analysis of a small concentration of sulfate in the presence of a large concentration of metal chlorides in high-salinity subsurface brines by suppressed ion chromatography needs precaution. At lower dilution (lower than 200 times) of subsurface brine Arab-D,
suppressed ion chromatography resulted in lower sulfate results despite a baseline resolution of sulfate peak. The reduction in the sulfate peak s eem to be c a d by the preaence of cations, present in large concentrations in Arab-D brine. This was concluded after studying the effect of different metal salts on the determination of a small concentration of oxalate by suppressed ion chromatography. The reduction in the peak height was found to be related to the concentration of metal salts present with the anions, forming stable ion pairs with protons. The interference of metal salts in the suppressed ion chromatographicdetermination of anions such as sulfate and oxalate can be overcome by reducing the concentration of metal salts in the medium. Therefore, for the accurate analysis of sulfate in high-salinity brines (such as subsurface Arab-D brines) by suppressed ion chromatography, determination at several dilutions, until a constant concentration is obtained (Table I), should be carried out. It is of interest to report that the accuracy of the analysis based on the sharpness and baseline resolution of sulfate peak can be misjudged. For example, the sulfate peak in Figure l a (at 50 times dilution of ArabD water) can be regarded as reasonably sharp, if not compared with the one at 300 times dilution (Figure lb). Registry No. SO4, 14808-79-8;water, 7732-18-5.
LITERATURE CITED Small, H.; Stevens, T. S.; Bauman, W. C. Anal. Chem. 1975. 4 7 , 1801-1809. Smith, F. C.; Cheng, R. C. CRC Rev. Anal. C t " . 1980, 9 , 197-217. Welss, J. Handbook of Ion ChrometosI*ephy; Dlonex Corporation: Sunnyvale, CA, 1986. Smith, F. C., Jr.; Chang, R. C. The Ractice of Ion chrometography; John Wlley and Sons Inc.: New York, 1983. Frltz, J. S.; Qjede, D. T.; Pohhndt, C. I n Ion Chrometogaphy; Hmlg, Alfrsd, Dr., Ed.; Verlag: New York. 1982. Bynum, M. A. 0.;Tyree, S. Y., Jr.; Weiser, W. E. Anal. Chem. 1981, 53, 1935-1936. Lindolf, J. C.; Stoffer, 0. Proceedings of the Society of Petroleum Engineers, Middle East Technical Conference, Bahrain, March 9-12, 1981; SPE 9626, pp 469-472. Lash, R. P.; Hill, C. J. Anal. Chlm. Acta 1979, 708, 405-409. M e r c u l ~ s o n M. , A,; Dasgupta, P. K.; Blakely, D. W.; Johnson. R. L. J. Membrane Sei. 1986, 27. 31-40.
RECEIVED for review November 15,1990. Accepted April 24, 1991. This work is part of KFUPM/RI Project No. 15010 supported by the Research Institute of King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia.
Temperature Control of Liquid Chromatographic Columns and Enzyme Reactors by Using a Proportional-Integral-Dlff erentlal Regulator in Conjunction with a Thermoelectric Element Niklas Tyrefors Uppsala University, Institute of Chemistry, P.O. Box 531, S-751 21 Uppsala, Sweden
The use of a thermoelectric device for temperature control of liquid chromatography columns and its usefulness have ~ involved recently been reported (1).This laboratory h a been in the determination of acetylcholine (ACh) and choline (Ch) in microdialysis samples using liquid chromatography with postcolumn enzymatic derivatization and electrochemical detection (2,3). The postcolumn immobilized enzyme reactor (IMER) used in determination of ACh and Ch shows optimum activity at 30-35 "C, but shelf life is significantly improved by storing at 4 "C. Further, the background current of the 0003-2700/9 1/0383-190 1$02.50/0
electrochemicaldetector is temperature dependent, making temperature control necessary to reduce baseline drift (4,5). These circumstances have prompted the development of a device providing efficient and useful means for temperature control. Proportional-integral-differential (PID) regulators are well-known by engineers and frequently used for temperature control of industrial processes. There are numerous commercially available models with varying degrees of sophistication. These regulators usually operate with an on/off time Q I991 American Chemical Society
Anal. Chem. lQS1, 63, 1902-1904
1902
Flgure 1. Typical timing diagram for a PID regulator.
B
A
(min)
-
Time
Flgure 2. (A) Original on/off relay. (6)Replacement, which changes the polarity of the output when activated.
cycle, with power turned on during a fraction of the cycle. The quotient b/a in Figure 1is determined by the difference (AT) between the target temperature and the measured temperature as well as the time integral of AT and the time derivative of AT. There are several methods to fine tuning the influence of the above terms, resulting in very smooth temperature control without oscillations and overshots. A commercially available PID regulator (FQV 261-00600 from ERO Electronic, Milan, Italy) was modified to change current directions, instead of switching the current on/off, by replacing an internal relay (Figure 2). The replacement was purchased from a local electronics warehouse. When the regulator output is in the off state, the thermoelement acts as a cooler and in the on state it acts as a heater. The regulator will treat the system as if the temperature of the environment was the same as the equilibrium temperature reached upon polonged cooling. Target temperatures close to or at room temperature are thus maintained with the same precision as other temperatures within the regulatory range. The target temperature is set in 0.1 "C increments by using two thumbwheels on the regulator, and measured temperature is digitally displayed with 0.1 "C resolution. A Pt-100 thermoresistive element is used as the temperature sensor. The
20
40
60
80
Figure 3. Temperature vs time for the column holder block. Arrows Indicate changes in the target temperatwe settings.
column block assembly is very similar to the one described by Sander and Craft (I), with a thermoelectric element sandwiched between a machined aluminium column holder block and a circulating water heat exchanger. The thermoelectric element described here (Supercool AB, Goteborg, Sweden) has a capacity to transport 70 W from the cold to the warm side, with an efficiency of 50%. In this particular application the element was fed with 35 W of electric power, resulting in the transport of 17 W. This was sufficient to maintain temperatures between 4 and 40 "C. Operation of an IMER is significantly simplified with this thermostat. The temperature is set to 20,30, or 37 "C or any temperature of choice for normal operation and 1,4,or 6 "C or any other temperature of choice for storage. Previously, it has been necessary to remove the IMER for storage in a refrigerator when not in use. Figure 3 shows temperature vs time response when the target temperature is changed. The PID regulator provides smooth envelopes, without oscillations or overshots. The regulator maintains the measured temperature at the target temperature within the 0.1 OC resolution of the display.
ACKNOWLEDGMENT I express my gratitude to Rolf Danielsson for introducing PID regulators in the daily life in the laboratory. LITERATURE CITED (1) Sander, L. C.; Craft, N. E. Anel. Chem. 1990, 62, 1545-7. (2) Potter, P. E.; Meek. J. L.; Neff. N. H. J . Neufochem. 1983, 41, 100-94. (3) Tyrefors, N.; Carlsson, A. J . Chmmerog. 1990, 502, 337-49. (4) Weber, S. G.; Long, J. T. Anel. U".1988, 60, 903A-13A. (5) Kissinger, P. T.; Shoup, R. E. J . Newoscl. Methods 1990, 34. 3-10.
RECEIVED for review February 26,1991. Accepted May 22, 1991.
@-CyclodextrinSolubility in Reversed-Phase High-Performance Llquid Chromatographic Eluents Mehdi Taghvaei and George H. Stewart* Department of Chemistry and Physics, Texas Woman's University, Denton, Texas 76204
INTRODUCTION One advantage of high-performance liquid chromatography (HPLC) over gas chromatography (GC) is the possibility of 0003-2700/91/0363-1902$02.50/0
incorporating special selectivitiesinto the mobile phase of the system. This reduces the number of special columns a laboratory needs. The cyclodextrins represent an agent that provides selectivity for aromatic positional isomers and for @ 1991 American Chemical Society
