ICP_Operations_Guide_2016

Calibration Curves C Both the accuracy and precision of ICP measurements is dependent, in part, upon the calibration technique used. This section is focused upon errors (both fixed and random) that can be introduced through the use of different calibration techniques using accurate calibration standards, samples that have been prepared accurately to within defined error limits, and an instrument that has been ‘set-up’ correctly using a procedure programmed where there are no spectral/mass interferences that include background correction. You may believe that if the above errors have been confined to within acceptable and known limits that there is nothing else to worry about. Unfortunately, this is not the case. The most common calibration technique options for ICP measurements are calibration curve and standard additions. In addition, the option of using internal standardization is available for the calibration curve technique and the ability of matrix matching may also be available. ICP-MS has the added option of using an internal standard that is an enhanced isotope of the element being measured (i.e., isotope dilution ICP-MS). This discussion will be limited to the above approaches, to the introduction of the analyte as a nebulized solution, and to the use of Ar as the plasma gas. Basic Considerations Before reading ahead, it may be helpful to restate the assumptions made above and make some additional considerations: t *$1 JT B A$PNQBSBUJWF .FUIPE XIFSF UIF NFBTVSFNFOU PG BO VOLOPXO TBNQMF JT CBTFE VQPO DIFNJDBM TUBOEBSET J F UIF measurement is a comparison process. t *U JT OPU BTTVNFE UIBU UIF DBMJCSBUJPO TUBOEBSET BOE TBNQMFT IBWF JEFOUJDBM NBUSJDFT t *U JT BTTVNFE UIBU UIF DBMJCSBUJPO JT MJOFBS ćJT NFBOT UIBU UIF TUBOEBSE BOE TBNQMF FMFNFOUBM DPODFOUSBUJPOT HJWF BO instrumental response that is described by the equation for a straight line. t *U JT BTTVNFE UIBU UIF BOBMZTU IBT QSFQBSFE UIF DIFNJDBM TUBOEBSET BDDVSBUFMZ UP XJUIJO EFĕOFE FSSPS MJNJUT J F UIF uncertainty of the prepared standard solution is known and has been calculated). t *U JT BTTVNFE UIBU UIF TUBCJMJUZ PG UIF TUBOEBSET IPXFWFS BOE CZ XIPFWFS QSFQBSFE JT LOPXO BOE BSF POMZ VTFE XJUIJO UIFTF defined limits of time, matrix, concentration, temperature/humidity, and container material(s). t ćF VODFSUBJOUZ PG UIF NFBTVSFNFOU PG BO VOLOPXO DBO POMZ CF XPSTF HSFBUFS UIBO UIF VODFSUBJOUZ PG UIF DBMJCSBUJPO t *U JT BTTVNFE UIBU UIFSF BSF OP TQFDUSBM NBTT JOUFSGFSFODFT ćJT PG DPVSTF JT BO BSFB PG HSFBU DPODFSO BOE FČPSU PO UIF QBSU of the analyst. This assumption is made to allow us to focus completely upon the potential errors involved with the calibration process. t *U JT BTTVNFE UIBU UIF TBNQMF QSFQBSFE GPS BOBMZTJT JOWPMWFT OP QPTJUJWF PS OFHBUJWF DPOUBNJOBUJPO FSSPST BOE OP TBNQMJOH errors. It is therefore assumed that the uncertainty in preparation can be described by the random and known sampling, weighing and volume dilution errors. Again, this is an assumption that is often not the case but is made to allow us to focus completely upon the potential errors involved with the calibration process. Calibrations Standards ICP is a matrix-dependant technique. Based upon the above assumptions and the fact that ICP is a comparative method, the prime concern is the availability and use of appropriate calibration standards. The problem analysts face is that ICP (ICP-OES and ICP-MS) is extremely matrix-dependent. Therefore, the ideal situation is that the matrices of the standards and samples be identical. Recommendations This section lists several recommendations. Discussions relating to these recommendations are provided in the next section for the reader who would like more detail. Recommendation (a) - Match the acid content of your calibration standards and samples in both the type of acid used and the concentration of the acid. Recommendation (b) - Match the elemental matrix components of your calibration standards and samples to the greatest extent possible. In this situation, the analyst who knows the composition of the sample has this capability. 10 alibration Techniques

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