An Evaluation of Unit and ½ Mass Correction Approaches as a Means of Minimizing the False Positives Produced by M+2 species in US EPA Method 200.8 using ICP-MS
Citation:
Smith, S., P. Creed, J. Creed, K. Kubachka, R. Wilson, J. Landero-Figueroa, AND N. Hanks. An Evaluation of Unit and ½ Mass Correction Approaches as a Means of Minimizing the False Positives Produced by M+2 species in US EPA Method 200.8 using ICP-MS. Pittcon 2017, Chicago, IL, March 05 - 09, 2017.
Impact/Purpose:
To evaluate the use of unit and ½ mass correction approaches as a means of addressing M+2 false positives within the context of updating US EPA Method 200.8.
Description:
Rare earth elements (REE) and certain alkaline earths can produce M+2 interferences in ICP-MS because they have sufficiently low second ionization energies. Four REEs (150Sm, 150Nd, 156Gd and 156Dy) produce false positives on 75As and 78Se and 132Ba can produce a false positive on 66Zn. Currently, US EPA Method 200.8 does not address these as sources of false positives. Additionally, these M+2 false positives are typically enhanced if collision cell technology is utilized to reduce polyatomic interferences associated with ICP-MS detection. Correction equations can be formulated using either a unit or ½ mass approach. The ½ mass correction approach does not suffer from the bias generated from polyatomic or end user based contamination at the unit mass but is limited by the abundance sensitivity of the adjacent mass. For instance, the use of m/z 78 in a unit mass correction of 156Gd on m/z 78 can be biased by residual 40Ar38Ar and 78Se while the ½ mass approach can use 77.5 or 78.5 and is limited by the abundance sensitivity issues from mass 77 and 78 or 78 and 79, respectively. This presentation will evaluate the use of both unit and ½ mass correction approaches as a means of addressing M+2 false positives within the context of updating US EPA Method 200.8. This evaluation will include the analysis of As and Se standards near the detection limit in the presence of low (2ppb) and high (50ppb) levels of REE with benchmark concentrations estimated using triple quadrupole ICP-MS.
