Anal. Chem. 1982, 54, 2603-2604
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Table 11. Comparisory of a Spectrophotometric and LC Method for the Determination of NO,- in Well Water NO,- in well water, ppm
spectrophotometric method'
LC method
21.0 34.0 47.0 50.0 55.0 67.0 85.0 110 135
%
difference +0.5 -2.9 t 1.7 + 2.0 0 -6.9 -1.2 t 1.8 -0.7
21.1 33.0 47.8 51.0 55.0 62.4 84.0 112 134
0
'Analyses done at The State (Iowa) Hygienic Labora-
Analyses done by LC using the conditions outtory. lined in Figure 1. A 1.0-pL aliquot of each water sample was used. Table 111. Data for thLe Determination of NO,- and NO,- in Several Samples' analyte
sample baby apple juice baby creamed cornC bacon oven cleanerC tap waterb'd distilled water b beer l b
NO,NO,NO,NO,NO,NO,NO,NO;-
NO,~ d
NO,NO;BrNO,-
amt found: ppm 14 6.9 370 94 150 24 0 1200 160 18 2 2 10 6
'Chromatographic conditions are given in Figure 1;sample volume injected WPII usually 1 0 pL. ppm is calculated from weight of analyte vs. volume of sample. ppm is calculated from weight of analyte vs. weight of sample. Water samples were taken at University of Iowa, Chemistry Building. e All analyses are rounded off to two significant figures. The NO3- in these same water samples was determined by the LC procedure outlined here. A comparison of the results found by the two methmds is shown in Table 11. In general, they differ by not more than 2% and indicate that this LC method is an excellent method for NOy- water analysis. The amount of NO3- and/or NO2- in several other real samples was determined by this LC procedure. In general, the LC conditions used for the separation are the same as those used in Figure 1. Since sample pretreatment involved only dilution or homogenization followed by filtration when necessary and repeated analyses were not done, the data shown in Table 111, which are a summary of these determinations, are generally semiquantitative. Samples other than
6 12 volume, ml
Figure 9. A typlcal chromatogram for the Separation of a beer sample on PRP-1. See Figure 1 for column conditions.
those listed in Table I11 were also examined; these include nine different commercial beers and several different baby foods, processed meats, and types of treated waters. From these experiments it is clear that accurate, reproducible NO2and NO3- analyses of a wide variety of samples are possible by this LC procedure. Chromatograms for the NO;-NOf separation for the Table I11 samples are similar to Figure 1; the only difference is that the dead volume peak is much larger and several other peaks are obtained prior to the N0f-N03- peaks. A typical chromatogram (a beer sample) illustrating this is shown in Figure 3. Apparently, the beer samples also contain a significant amount of Br- and thus, the amount of Br- present was also estimated. For the nine different beer samples tested the amount of NOz-,Br-, and NO3- present covered the ranges 2 to 17, 4 to 23, and 4 to 19 ppm, respectively. For all samples peak identification was confirmed by comparison to standards and by standard addition. Also, all data are reported in terms of NO2- or NO3- levels.
LITERATURE CITED (1) "Standard Methods for the Examination of Water and Wastes"; Report No. EPA/625/116/77-003; U S . Envlronmental Protectlon Agency: Washington, DC, 1974; pp 197-205. (2) "Standard Methods for the Examlnatlon of Water and Waste Water", 14th ed.; American Public Health Association, American Water Works Associatlon, Water Pollutlon Control Federation: Washington, DC, 1976; pp 418-436. (3) "Official Methods of Analysis of the Associatlon of Offlclal Analytical Chemlsts", 12th ed.;Associatlon of Offlcial Anlytical Chemists: Washington, DC, 1975. (4) Cox, R. D. Anal. Chem. 1980, 5 2 , 332-335. ( 5 ) Iskandaranl, 2.; Pietrzyk, D. J. Anal. Chem. 1982, 5 4 , 1065. ( 6 ) Iskandarani, 2.; Pletrzyk, D. J. Anal. Chem. 1982, 5 4 , 2427. (7) Small, H.; Stevens, T. S.; Bauman, W. C. Anal. Chem. 1975, 4 7 , 1801-1809.
RECEIVED for review June 7, 1982. Accepted September 14, 1982. This work was presented at the 183rd National Meeting of the American Chemical Society, Las Vegas, NV, March 28, 1982. This investigation was supported by Grant CHE 7913203 awarded by The National Science Foundation.
Temperature Fluicctuatlons in Nonsuppressed Ion Chromatography Dennis R. Jenke" anid Gordon K. Pagenkopf" Department of Chemistry, Montana State Unlverslty, Bozeman, Montana 597 17
Ion chromatography, first described by Small and associates in 1975 (I),has evolved in a relatively short time into a widely used analytical method fOr the determination of anion content
of aqueous samples. The utilization of conductivity detection, with its universal nature and relatively simple design requirements, contributes much to the techniques's utility and
0003-2700/82/0354-2603$01.25/00 1982 Amerlcan Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
n
Table I. Operating Characteristics of the Insulated and Noninsulated Systems
species
detection limit: mg/L noninsu- insulated lated
reproducibilityb noninsu- insulated lated
NO,-
10
1.0
6.3
CiBr-
5 10 20
0.5
7.1
1.0 1.0 1.0
5.9 5.2 3.4
so,2s,o,
2-
10
5.2 3.3 4.7 2.8 3.4
a 100 fiL sample. % RSD of six replicate analyses at analyte concentration 1 0 times the detection limit. _ I _ -
S042-,and Sz03z-by dissolution of their sodium salt and for Brby dissolution of KBr in distilled water. All salts were reagent grade. Standard solutions were prepared for analysis by dilution of the appropriate stock solution with the eluent solution to avoid the appearance of the solvent dip in the chromatogram. t
0
1
2
3
r.t
4
,
5
6
min
Flgure 1. Typical chromatograms for the determination of NO,-: chromatogram A produced with noninsulated system, chromatogram B produced with the insulated system. The first peak represents the sample's cation content while the second peak represents NO3- elution. NO3- concentration = 1 X M; total analysis time = 6.0 min.
popularity; however, this type of detection is not without its drawbacks. It is observed that temperature of conductivity measurements [temperature coefficient of conductivity is approximately 2%/"C (Z)]requires very close control if background noise is to be minimized and sensitivity maximized. The accurate control of temperature is especially important in ion chromatographic systems that use relatively high conductivity and nonsuppressed eluents. In such cases, the change in the background conductivity due to the presence of the analyte must be distinguishable from the high intrinsic conductivity of the ionic eluent. This report documents the effect of fluctuating laboratory temperature on the performance of a nonsuppressed ion chromatographic system and describes a simple means for producing a constant temperature environment.
EXPERIMENTAL SECTION Instrumentation. The chromatographic system employed in this research consisted of a Perkin-Elmer Series 3B liquid chromatograph, a Vydac Model 3021 c4.6 anion separator column, a Vydac Model 6000 CD conductivity detector, and a Sargent Welch XKR strip chart recorder. Sample introduction loop volume was 0.1 mL and samples of 0.5 mL were introduced into the loop with a Hamilton Co. Model 750 microliter syringe. Eluent pump rate was 2 mL/min and laboratory temperature was 22.5 0.2 OC. Both the detector and column were mounted outside of the pump housing. Reagents. The eluent, 2 X M potassium hydrogen phthalate, pH 5.9, was prepared by dissolution of the reagent grade phthalate salt in 900 mL of doubly distilled water, addition of 25 mL of 0.1 M KOH, and dilution to a final volume of 1.0 L with the distilled water. Final eluent pH was measured before utilization but after degassing with suction. The column and detector system were flushed with eluent at a flow rate of 2.5 mL/min for a period in excess of 45 min prior to sample analysis. Stock analyte solutions were prepared at 0.1 M concentrations for Cl-, NO3-,
*
RESULTS AND DISCUSSION A typical chromatogram for 1 X M NaN03 obtained by using the chromatographic system described above is shown in Figure 1. One readily notes the relatively large peak to peak background noise signal which adversely affects analytical sensitivity. While short-term temperature variations were not initially considered as the background source, other possibilities which included pump and outside electronic noise were rapidly eliminated. The extreme temperature sensitivity of the exposed column/detector system was confirmed when placement of fingers on the steel tubing connecting the two was observed to cause a measurable change in eluent conductivity. The magnitude of this effect was comparable to the instrumental response for 1 X M NaN03. In hopes of providing thermal protection for the detector, column, and support tubing, the column was encased in a Styrofoam box whose walls were 2.5 cm thick while the tubing after the injector loop was wrapped with 1 cm thick foam rubber. As shown in Figure 1,utilization of the enclosed system removes the background fluctuations, thereby firmly establishing that the source of the background noise was temperature related. Detection limits obtainable by using both systems are shown in Table I. Detection limits reported for the unwrapped system are near those reported for typical nonsuppressed systems, and it is noted that utilization of the insolated system results in an increase in the analytical sensitivity of a full order of magnitude. The detection limit as used herein refers to the analyte concentration equivalent to two times the peak to peak base line noise level on the recorded chromatogram. It is also observed that the reproducibility of the analysis of replicate samples which contain the analyte in concentrations an order of magnitude higher than the detection limit is improved for the thermally controlled system. This improvement is a direct result of the more accurate identification of the true base line. Thermal isolation of the active components of a chromatographic system from minor and rapid temperature gradients is thus necessary to produce maximum sensitivity and precision. LITERATURE CITED (1) Small, H.; Stevens, T.; Bauman, W. Anal. Chem. 1975, 4 7 , 1081-1809. (2) Pohl, C. A,; Johnson, E. L. J . Chromatogr. Sci. 1980, 18, 442-452.
RECEIVED for review June 17,1982. Accepted August 18,1982. This work was supported in part by the Environmental Protection Agency under Grant No. R-805935.
