## 1 Introduction  
Water quality monitoring at remote field sites often entails the collection of water samples using automated water samplers and periodic retrieval of the water samples for laboratory analysis. Therefore, one of the most important issues for researchers dealing with water monitoring is how to preserve water samples while stored in the automated water sampler while awaiting retrieval. Water quality parameters that have the potential to change over time include nitrate-N and orthophosphate. The United States Environmental Protection Agency (US EPA) suggests methods to help obtain an accurate reading of concentrations in water samples. This includes preserving the sample immediately after on-site collection by placing samples in cool temperatures, adding chemical solutions, or altering the pH of the sample [1]. The US EPA recommended methods for preserving water samples for nitrate-N and orthophosphate are listed in Table 1.  
These preservation methods slow factors that can lead to changes in water quality concentrations. By limiting biological activity, chemical hydrolysis, and absorption effects [1], there can be a longer time delay between collecting the sample and analyzing it. Several chemical and physical interferences have been used as potential preservation methods. The most traditional forms include chemical addition, pH control, freezing, and refrigerating the sample [1]. For example, chloroform has been explored as a preservation but produced erratic results for both nitrate-N and orthophosphate, with large increases in concentrations occurring between eight and 16 days after preservation was initiated [2; 3]. One of the main challenges with preservation methods is length of time when the sample is stabilized, or preserved. There is often only a short period of time before concentrations within the sample begin to change. For example, in a study where water samples were not filtered immediately after collection (10-day delay) and then stored in refrigerated conditions for 107 days, both nitrate-N and orthophosphate showed significant increases, by as much as a 3.7-fold increase for nitrate-N and a 1.8-fold increase for orthophosphate [4]. In other studies [5; 6], it was determined that, while methods such as those listed in Table 1 can be effective for one target parameter, it is difficult, if not impossible, to find a method that is effective for preserving multiple parameters. The variability in concentrations within a water sample over time can be caused by factors like location, chemical make-up of the water, and the microbiology of the water. As a result, the ultimate problem with a universal preservation method is there are too many variables that can influence changes. These variables lead to difficulty in finding a perfect preservation method that holds all water quality levels constant [7]. The US EPA; therefore, recommends preservation methods for different parameters to make testing as reliable as possible. However, even with these techniques, there is still a possibility of fluctuating measurements. Some studies have tested current preservation methods, like freezing, acidification by sulfuric acid, refrigeration, and regular storage-affected nitrogen and phosphorus levels, and then compared the abilities of these methods to maintain concentrations [8; 9; 10; 11; 12; 13; 14; 15]. A study using tile drainage water, river water, and surface runoff water in Indiana found that freezing at subzero temperatures was the best overall method for preserving water samples for orthophosphate and nitrogen (nitrate-N and ammonium) [8]. However, filtering of the water sample may be needed if the sample contains high levels of sediment. In contrast, a study found that while refrigeration and filtration were necessary, acidification was the most effective preservation method for nitrate-N [9].