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Analysts interested in determining what compounds are in a sample or quantifying known compounds based on absorbance should consider using a UV-VIS spectrophotometer. There are four spectrophotometers in the engineering laboratories: Cary 100, Cary 4000, Hach DR5000, and Hach DR6000. Spectrophotometers in the laboratory are used for both broad spectrum scans and selective scans. A broad spectrum scan is generally used for qualitative data to gain a better understanding of what compounds are in a sample and where the compounds’ peak absorbance occurs. A broad spectrum scan shows the spectrum of a sample over a range of wavelengths in the ultraviolet and visible spectrums, e.g. 200-800nm. Each peak in the spectrum represents a different compound or group of compounds absorbing at similar wavelengths. A selective scan is used for quantitative analyses. In a selective scan, the user chooses a specific wavelength(s) at the peak of a curve where the highest absorbance value of an analyte occurs often due to a colorimetric reaction. If the user knows an analyte’s attenuation coefficient, the concentration can be determined with the Beer-Lambert law,Ìý A=ε l c. If the attenuation coefficient is unknown, the concentration of a set of standards can be analyzed and plotted against each standard’s absorbance creating a linear fit. The sample’s concentration is then determined by plugging its absorbance into the linear fit formula. While the Cary 100 and Cary 4000 spectrophotometers are able to read the absorbance one order of magnitude lower than the Hach DR5000 and DR6000 spectrophotometers, the applicable concentration range is dependent on the linearity of the calibration curve and must be assessed for all tests.ÌýThe Hach DR5000 and DR6000 spectrophotometers may also be used in tandem with Hach tests kits creating colormetric reactions to determine concentrations of different forms of nitrogen, phosphorus, chlorine and moreÌýin water samples.Ìý