Both Method 1311 and Method 1312 have language specifying the use of an internal calibration quantitation method for each metallic component if the spike recovery for the element is < 50% and the concentration of the contaminant falls in the range from 80 to 100% of the regulatory level. Both of these methods then specify that “the method of standard additions shall be employed as the internal calibration quantitation method for each metallic component.”
For Methods 1311 and 1312, is it an acceptable alternative to use internal standards for all leachates to meet the requirement of an internal calibration quantitation method rather than using methods of standard additions?
The purpose of the language in Section 8.4 of the methods is to remove the effects of matrix suppression that might cause a sample extract within 20% of the regulatory limit to appear to be below the regulatory limit. When 1311 and 1312 were written, atomic absorption techniques were most commonly used and could be strongly affected by matrix effects. Since you are free to choose the analytical method for TCLP and SPLP extracts, choosing an inductively coupled plasma (ICP) method, which is far less prone to matrix effects, will make the situation described in Section 8.4 of Methods 1311 and 1312 much less likely.
Use of dilution or internal standards, or both, increases the likelihood of a passing matrix spike recovery above 50%, removing the need for the method of standard additions (MSA). If dilution and internal standards do not work and spike recoveries are still below 50%, then MSA may be the only option left. This would be very unlikely if ICP analysis is used.
Internal standards are not mentioned anywhere in either Method 1311 or 1312. As mentioned in Section 8.4.1 of both methods, "The method of standard additions shall be employed as the internal calibration quantitation method for each metallic contaminant." Use of an analytical method that utilizes internal standards (whether ICP-MS or ICP‐AES) does not eliminate the need for MSA when spike recoveries are below 50%. Section 8.2.3 allows you to use "other internal calibration methods, modification of the analytical methods, or use of alternate analytical methods to accurately measure the analyte concentration in the [TCLP or 1312] extract when the recovery of the matrix spike is below the expected analytical method performance." "Accurately measure" in that statement means acceptable spike recovery.
Other Category: QA/QC