After converting the nitrate into a purple dye, and measuring the absorbance of the purple dye on a spectrophotometer, a standard curve is used to convert the absorbance into concentration. to make a standard curve, samples with known concentrations of no3- are run on the spectrophotometer. the samples with known concentrations are called standards. a linear regression is then performed to relate the concentration of no3- to measured absorbance values. here is a link to a spreadsheet containing a simulated data set. there are standards and their related absorbance values, and there are samples from two sites that were diluted, prior to processing and measuring their absorbances. the groundwater originates from the upslope site, and the hope is that the microbes in the soil are removing the no3- from the groundwater before it reaches the downslope site. using the given data create a standard curve in excel, and use trendline to add a linear regression with the equation. then use the standard curve and the dilutions to determine the concentration of no3- in all the samples. using the data analysis tool pack, perform the appropriate t-test to deduce if the nitrate concentration upslope is less than or greater than the nitrate concentration downslope. when performing a t-test using the data analysis tool pack, the output will include the means for both groups.