ICP_Operations_Guide_2016

Accuracy, Precision, Mean and Standard Deviation B There are certain basic concepts in analytical chemistry that are helpful to the analyst when treating analytical data. This section will address accuracy, precision, mean, and deviation as related to chemical measurements in the general field of analytical chemistry. • Method validation using a CRM containing Cr in the suspected or known chemical form(s) is vital. The importance of CRMs prepared from ‘real world’ materials is critical (i.e., synthetic CRMs are likely to contain easily dissolved compounds). • Avoid mixing water-soluble hexavalent chrome with Ba or Pb to avoid loss of Cr Pb and Ba as the insoluble chromates. Cr elemental data* 14 asic Calculations (Cl, F, Br, and I), thiosulfate, arsenate, and sulfide to name the most common. However, the major problem with trace Pb analysis is contamination from the apparatus and atmosphere. Pb is used in industry in plumbing (pipes), solder, gasoline (significantly curtailed), drying agent for oils, glass, plumber’s cement, covering of steel to prevent rust, as a pigment in paint (significantly curtailed), hair dye and as a pigment in plastics. • Environmental contamination from airborne particulates is still a major concern in certain regions/laboratories depending upon location and age. When tetraethyl lead was widely used as an octane booster it was impossible to avoid environmental contamination in an open digestion apparatus. Open digestions in hoods where large volumes of air pass over the apparatus are of most concern. Closed container digestions or clean rooms / hoods are suggested to avoid this source of contamination. • Avoid the use of any type of glass in sample preparations for Pb. Use quartz or fused silica and perform a sufficient number of blanks to define the degree of contamination. • Avoid the use of any plastic with an inorganic pigment. Here Pb is only one of many concerns. • Teflon containers should be carefully leached with dilute nitric acid before use and blanks performed to confirm freedom from Pb contamination. Be particularly suspicious of Teflon that has been used in sample preparations where Pb was a major, minor or trace component. Pb elemental data* Chromium (Cr) The major difficulty that I have experienced with Cr is that it often exists in forms that are difficult to put into solution. Chromite (FeO.Cr 2 O 3 ), chromic oxide, pigments, stainless steel and ferro-chrome all present a challenge but the hexavalent chromium oxides are the most difficult. If the oxide has been ignited (pigments) the refractory nature is such that an analyst confronted with the task of bringing about solution will never forget the experience. The most common approach is to perform a fusion. Fusions that have been used include but are not limited to potassium and sodium bisulfate, carbonate (sodium or potassium), sodium peroxide, NaOH / KNO 3 , and NaOH / Na 2 O 2 . In addition, the fusion will not be complete unless the chrome is finely divided and mixed with the flux. • Know you sample to the fullest extent possible. The possible chemical forms of Cr should influence the sample preparation technique employed. • If your sample is an inorganic pigment containing Cr then you know that you have an extremely refractory material to dissolve. • If you are unfamiliar with your sample type a literature search is strongly suggested.

* Visit inorganicventures.com/tech/icp-operations/ for additional information from this link

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