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3.20 SELECTED ION MONITORING (SIM) - A mass spectrometric technique where only
one or a few ions are monitored to improve sensitivity.
3.21 STOCK STANDARD SOLUTION (SSS) - A concentrated solution containing the
method analyte that is prepared in the laboratory using assayed reference materials or
purchased from a reputable commercial source.
4. INTERFERENCES
4.1 Method interferences may be caused by contaminants in solvents, reagents (including
reagent water), sample bottles and caps, and other sample processing hardware that lead
to discrete artifacts and/or elevated baselines in the chromatograms. All items such as
these must be routinely demonstrated to be free from interferences (less than 1/3 the
MRL for the target analyte) under the conditions of the analysis by analyzing LRBs as
described in Section 9.3.1. Subtracting blank values from sample results is not
permitted.
NOTE: The use of low or high density polyethylene plastic is recommended in place of
glass when possible. If glassware is used, it should be washed with detergent and tap
water and rinsed thoroughly with reagent water since ClO 4- was found in common lab
detergent during method development.
4.2 In anion chromatography, cations are not retained on the analytical column and, in
theory, pass through in the void volume. The anions are separated by charge, size and
polarizability. As a large polarizable molecule, ClO 4- elutes later than the common
inorganic anions (Cl - , SO 4
2-
, CO 3
2-
, and HCO 3- ). Separation of ClO 4
-
from the matrix
ions combined with the specificity of mass spectrometry has resulted in a method that
minimizes interferences for drinking water matrices. There are, however, the following
known conditions or contaminants that, if present, could result in positive or negative
bias in the reporting of ClO 4- .
4.2.1 Direct Chromatographic Co-elution of Contaminants: At sufficiently high
concentration, direct chromatographic co-elution of a contaminant with ClO 4
-
could result in ionization suppression of one or more of the ions of interest
(m/z 99, 101, and/or 107). Alternatively, the contaminant could have the same
m/z as ClO 4- , or in-source collisionally induced dissociation of a co-eluting
contaminant in the ESI interface could produce a fragment ion with the same m/z
as ClO 4- . Any of these conditions could lead to a positive or negative bias of
ClO 4
-
depending on the affected ion.
If a contaminant is present at a concentration detectable by conductivity, a full
mass scan on a replicate analysis may reveal the presence and m/z of the coeluting
contaminant. Direct chromatographic coelution problems or
concentration dependent coelution problems may be solved by achieving
adequate chromatographic separation. This may be done by modifying the
332.0-6