Think of your old manual Spectronic 20, or your direct reading spectrophotometer that you use in your lab. You line up your samples in a row. In front of them, you place some small sample cups or maybe even a series of cuvettes, and you pipette a known amount of sample into each cup. You then add a reagent and somehow mix the reagent and sample. You do this for each sample. You may have more reagents to add so you repeat the whole process until all reagents are added. Then you start a timer. When the timer beeps you know you have a certain “time window” to read the absorbance (or concentration) of your samples. You read by manually transferring the color-developed sample to a spectrometer cuvette, by using a peristaltic pump to transfer the sample to a flow cell already in the spectrometer, or by inserting the tube or cuvette that you used to develop the sample color in. Then, you press a button to send the reading to a printer, a computer program, or you manually record the reading onto a laboratory worksheet.
Did you shake and mix every sample exactly the same way every time? Will you mix them the same way every day? Will every analyst run them exactly the same way you have?
Is there color or turbidity in the samples? Should you zero your instrument with each sample, or only with reagent water blanks?
Is the exact time you read the final absorbance critical?
The process described is what you are automating by using a discrete analyzer. Instead of lining up samples, you are pouring aliquots into sample cups that are placed on an auto sampler tray. Instead of transferring a known amount of sample to a cuvette, the discrete analyzer does. Instead of adding reagents and mixing, the discrete analyzer does. Instead of starting a timer, the discrete analyzer does. Instead of reading the absorbance, recording the reading, and calculating a result the discrete analyzer does.
The analyzer has automated almost all the simple discrete variable colorimetric methods for you. Sample volume is measured and dispensed exactly the same way, every time. Reagents are added and mixed exactly the same way every time. The timer is set and absorbance is measured exactly the same way every time. Results are calculated exactly the same way every time.
The discrete analyzer pipettes, dilutes, adds reagents, mixes, calibrates, measures, calculates, and reports all for you. You select a method by keyboard. There is no hardware to manually change, no cartridge to rinse out, no baselines to monitor, no wavelength filters to change. Sample and reagent volumes are determined by a selection in a computer program, not by the internal diameter of a peristaltic pump tube.
The discrete analyzer has done a lot for you but it cannot control nor do everything. It cannot accurately prepare the stock calibration standard for you, even though it can accurately dilute it. It cannot guarantee the standards and samples were placed on the auto sampler tray in the right order. It cannot prepare the reagents for you or guarantee they were placed in the right order; however, it can monitor their purity and remind you where they are supposed to go. It cannot make sure you’ve entered the proper sample ID for each sample position, however, it can guarantee that the result obtained for that sample position is traceable to the ID you entered. It cannot know the sample lot ID for each standard or reagent, but if you enter those ID’s into the software, it can guarantee traceability of those reagents with your sample sets.