The author replies to Cruz:

In our paper (1) we described an undergraduate laboratory that uses capillary electrophoresis (CE) for the determination of nitrate and nitrite in water samples. As stated in this paper, there are alternative methods for nitrate and nitrite determinations based on such techniques as colorimetric measurements, some of which provide lower limits of detection than our CE method. However, our main goal in this work was to demonstrate the possible use of CE in analytical measurements and in the simultaneous determination of multiple analytes, not to develop an improved method for the routine testing of nitrate and nitrite at trace levels.

In his letter, Dr. Cruz suggests that there may be a systematic error in this method because no appreciable levels of nitrite were seen in some of our samples. He then suggests that the internal standard (periodate) was responsible for this. However, there is no experimental evidence to indicate that this was the case. We have checked for possible changes in our samples over time and have found that periodate does not cause the type of decrease in nitrite levels that he suggests. For example, if our nitrate and nitrite standards are combined with periodate as described in the paper, we see no significant change (i.e., a random variation of only 2–3%) in the levels of nitrate, nitrite, and periodate over a period of several hours. If the same samples are stored at room temperature for 2–3 days and reanalyzed, there is a 15–18% decrease in the size of the nitrite peak, but this same level of decrease occurs whether or not periodate has been added to the sample. Although it is true that periodate is an oxidizing agent, it is also true that the periodate concentration, reaction time, pH, and nature of the other reactants play an important role in determining the extent and selectivity of this oxidation process (2). Our evidence indicates that if this reaction is occurring in the samples, it is only taking place on a timescale that is not significant compared to the analysis time of our CE method.

Even if periodate were found to cause problems with some samples, we have found that other compounds can be used in place of periodate as an internal standard in our assay method. For example, pyromellitic acid (PMA, or 1,2,4,5-benzene tetracarboxylic acid) works nicely for this purpose. This agent requires the use of a slightly longer run time than periodate on the CE instrument (a total run time of 6 min), but PMA can be used at the same concentration and at the same detection wavelength as periodate and gives a comparably sized signal (3). An added advantage of using PMA is that is does not have the potential oxidative ability of periodate. Thus, this compound is attractive for use an alternative internal standard in the CE analysis of nitrate and nitrite.

Literature Cited

1. Hage, D. S.; Chattopadhyay, A.; Wolfe, C. A. C.; Grundman, J.; Kelter, P. B. J. Chem. Educ. 1998, 75, 1588–1590.
2. Dryhurst, G. Periodate Oxidation of Diol and Other Functional Groups: Analytical and Structural Applications; Pergamon Press: New York, 1970.
3. Chattopadhyay, A.; Hage, D. S. J. Chromatogr., A 1997, 758, 255–261.