When using SW-846, are we required to use the internal standards listed in Method 8260 or may we select other internal standards? We are using Method 8260 to analyze some samples for only the BTEX compounds and some of the internal standards are not relevant to our analytes of interest.
SW-846 does not require the use of the internal standards listed in the 8000 series methods for analyses. SW-846 only recommends the use of the internal standards listed in the methods. Normally, the internal standards listed in a particular method are those used to validate the procedure. However, all of the internal standards mentioned in the current version of Method 8260 may not be pertinent for a project where the analytes of interest are a subset of the analytes listed in the method. When the list of target analytes is short, the analyst may not need to use multiple internal standards if one or two will suffice to cover the range of retention times of the target analytes. As noted in the current version of Method 8000, the target analytes should have relative retention times in the range of 0.80 to 1.20 compared to the internal standard. In the case of BTEX analyses, the analyst need only use those internal standards that are relevant to the BTEX compounds, e.g., having similar chemical and chromatographic behaviors and retention times.
The same considerations also apply to Method 8270, or any other method that employs internal standards.