Carefully controlled bench-scale and on-site experiments demonstrated that cyanide can form in the treated drinking water sample container during preservation and storage. In the bench-scale experiment, treated tap water samples were collected on 20 days over six months. The tap water samples were split and some of the splits were spiked with formaldehyde, a known ozone disinfection byproduct, held for three hours and tested for cyanide. Then they were preserved and held for 2-10 days. None of the 69 initial samples had cyanide detects, but 22 of 49 formaldehyde-spiked samples and three of the 20 unspiked samples developed detectable cyanide concentrations during storage. In the on-site experiment, six samples were collected at a finished water tap at an ozone/chloramination treatment plant over three days. Each sample was split, and a portion was spiked with formaldehyde. Each portion was analyzed in triplicate after three different procedures: (1) immediately distilled on-site, (2) stabilized on-site in a distillation tube and distilled back at the laboratory several days later, or (3) following the conventional procedure of preserving the sample to pH > 12 in a container and distilling the sample back at the laboratory. Only the samples handled in the conventional way had detectable amounts of cyanide. Both experiments demonstrated that cyanide can form during conventional preservation and storage, and it is likely that the cyanide detected for this treated drinking water was formed in the sample container as a consequence of the preservation and storage conditions.

Reliable determination of cyanide in water samples is important for both wastewater and drinking water treatment plant operators and regulatory agencies. However, as a result of cyanide's diverse chemistry, obtaining reliable results has been challenging because several chemical mechanisms can form or destroy cyanide—and some of these can occur within the sample container or during laboratory pretreatment and analysis, leading to biased results. Also, sample matrix constituents or preservation chemicals can interfere with the analytical determination. The US Environmental Protection Agency acknowledged this difficulty for wastewater samples in the 2012 Methods Update Rule by revising the footnote for cyanide preservation to indicate that some interferences may not be mitigated and any technique for removal or suppression of interferences can be used as long as quality control measures are used to demonstrate that the technique worked. Many of the same concerns inherent in testing wastewater apply to testing drinking water. In this study, the effects of holding time, preservation, and on‐line digestion and distillation on cyanide results for wastewater and drinking water were examined, including the use of field dilution as a treatment for interferences and field spikes as a means to gauge whether sample integrity was maintained.

Obtaining accurate and precise results for total cyanide concentrations in wastewater samples is fraught with positive and negative interferences. Even the United States Environmental Protection Agency has acknowledged that it may be difficult or impossible to adequately mitigate all interferences. We demonstrated that a field spike of complex cyanide can be successfully used to demonstrate when sampling, preservation, pre-treatment, and analysis techniques are working adequately to retain any cyanide present in the sample without causing false positives or false negatives. For 257 industrial wastewater effluent samples collected at a wide variety of Greater Boston industries, 237 (92.2%) had usable field spike recoveries, averaging 86.2% recovery. Field spike recoveries for problematic industries that had very high or very low field spike recoveries were useful to show when alternative preservations and field dilutions were successfully preserving total cyanide. The field spike approach is general and should also work in a similar manner for raw and treated drinking water samples.

Easily detectable amounts of free cyanide (FCN) were formed when deionized water was treated like drinking water and preserved and tested for FCN. This occurred when either ascorbic acid or thiosulfate was used to dechlorinate, though higher FCN concentrations were observed with ascorbic acid. The amount of FCN observed was up to 50–60 µg/L but strongly depended on the concentration of ascorbic acid used. The amount of FCN observed was less dependent on the amount of thiosulfate used. The FCN was observed immediately after the samples were preserved, tended to increase—primarily during the first 24 h—and persisted for at least five days. This demonstrates the potential to get false positive FCN results on drinking water samples that a US public water system would be required to report in its annual Consumer Confidence Report. Since drinking water sampling, preservation, and testing is prescriptive, there are few available ways to avoid these false positives.