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moisture analysis The six common interferences listed above may come into play and cause a bias in the result. Also, something as simple as a single bead of sweat from the hand of the person running the test transferring to the outside of the test pan would be enough to give you 8 mg of mass that will evaporate. Other things may come into play as well. Oils transferred from hands to the pan, or residual oils on the sample pan from its own manufacturing process may be lost during the heating process. The moisture balance is indiscriminate in measuring what evaporates off your pan and sample. Any mass loss is interpreted as being water, whether it is or not. By running fve replicates you will have a good idea of whether your moisture balance has an issue with a biased result. Table 1 shows real data from a pair of moisture balances tested in this way. Instrument A in Table 1 shows a defnite bias that should be reduced or eliminated in order to obtain true results. Instrument B shows a small amount of bias that will provide a more accurate result. The variation in measurements shown by the percent relative standard deviation (% RSD) also indicates that results from Instrument A will have much more variation than results from instrument B. Step 2: Characterize the detection limits. One of the most common methods of determining the method detection limit comes from the U.S. EPA standard, 40 CFR 136, Appendix B. A simplifed version of this method is explained here for illustration. Please refer to the CFR standard when characterizing your instrument. The process involves testing seven replicate samples the same way as in Step 1, except that to each sample a precise, known amount of water is added. The amount added should be around three to fve times the expected detection limit of the instrument. Twenty milligrams is a good place to start for many moisture balances. The 20 mg can either be weighed out with purchased solid standards certifed to contain a certain amount of water or by accurately measuring out 20µl of water in a very accurate syringe. The purpose of this test is to defne the minimum amount of moisture that can be detected, and to identify the minimum amount of water that can be quantifed with a reasonable amount of accuracy. These two quantities are referred to as the Method Detection Limit (MDL) and the Limit of Quantitation (LOQ). Ideally your LOQ should be below the moisture level that you want to dry your materials to, ensuring that you can accurately measure a value below the specifcation limit. The sidebar on p. 37 shows the equations needed to calculate the MDL and LOQ from the seven replicates. Step 3: Verify that you're actually testing water evaporation. This step involves sending a split sample to an outside laboratory to test the amount of water that is given off by the sample. The two instruments that can do this are a Karl Fischer-type titration instrument or a moisture analyzer with a relativehumidity (RH) detector. Both instruments are selective in detecting just the water that is within the sample. Moisture balances only detect changes in weight; because of this, other processes that have nothing to do with the moisture level can bias the result. Chemical reactions occurring at elevated temperatures in air can result in either gaining or losing mass. TAble 1 AvoId 'BIAS' By RunnIng FIve SAmpLeS Sample Replicate Instrument A Weight Loss, mg Instrument B Weight Loss, mg 1 3.6 0.5 2 3.3 0.4 3 3.7 0.5 4 3.3 0.5 5 4.1 0.3 Average 3.6 0.44 % RSD 9.21 0.089 Running fve replicates will give you a good idea of whether your moisture balance has an issue with a biased result. This shows real data from a pair of moisture balances tested in this way. TAble 2 mAteRIAL WeIght LoSS CompARISon: moIStuRe BALAnCe vS. moIStuRe AnALyzeR Sample Replicate moisture Balance, % moisture Analyzer, % 1 0.2990 0.2119 2 0.3030 0.1896 3 0.3120 0.2017 4 0.3050 0.1842 5 0.2960 0.1640 6 0.3010 0.1919 7 0.3070 0.2554 8 0.2940 0.1262 9 0.3020 0.2169 10 0.3100 0.1992 Average 0.3029 0.1941 % RSD 1.91 17.4 This study was conducted with relative humidity detection using the same type of sample. The % relative standard deviation (RSD) is much larger for the moisture analyzer due to the smaller sample size, as it is more susceptible to slight changes in the moisture quantities of the sample. By performing 10 replicates, variations in the samples can be averaged out and a true overall value obtained. Plastics technology february 2013 35