Effects of Sample Pretreatment on Calculations

The concentration obtained from the standard curve is rarely the final answer in a real-world instrumental analysis. In most procedures, the sample has undergone some form of preanalysis treatment prior to the actual measurement. In some cases, the sample must be diluted prior to the measurement, as mentioned in Workplace Scene 6.3. In other cases, a chemical must be added prior to the measurement, possibly changing the analyte s concentration. In still other cases, the sample is a solid and must be dissolved or extracted prior to the measurement. 

The instrument measurement is the measurement of the solution tested, and the concentration found is the concentration in that solution. What the concentration is in the original, untreated solution or sample must then be calculated based on what the pretreatment involved. Often this is merely a dilution factor. It may also be a calculation of the grams of the constituent from the molar concentration of the solution, or the calculation of the parts per million in a solid material based on the weight of the solid taken and the volume of extraction solution used and whether or not the extract was diluted to the mark of a volumetric flask. Some examples of this follow. Remember that parts per million for a solute in dilute water solutions is in milligrams per liter, while for an analyte in solid samples it is in milligrams per kilogram. Review Chapter 5 for more information about the parts per million unit. 

A water sample was tested for iron content, but was diluted prior to obtaining the instrument reading. This dilution involved taking 10.00 mL of the sample and diluting it to 100.00 mL. If the instrument reading gave a concentration of 0.891 ppm for this diluted sample, what is the concentration in the undiluted sample  

In order to determine chemical elements in soil, samples of the soil must undergo a solid-liquid extraction. Sometimes the extracts resulting from this procedure have analyte concentrations that are too high to be measured accurately by the chosen method. Therefore, they must be diluted. At the Natural Resources Conservation Service (NRCS) Soil Survey Laboratory in Lincoln, Nebraska, an automated diluting device is used. Using this device, the analyst accurately transfers aliquots of the extract and a certain volume of extraction solution to the same container. This dilutor may also be used to pipet standards and prepare serial dilutions. 

Patty Jones, a physical science technician at the NRCS Soil Survey Laboratory, prepares extracts for phosphorus analysis using a diluting device. 

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