3.1.8 If the correction routine is operating properly, the determined apparent
analyte(s) concentration from analysis of each interference solution should fall within a
specific concentration range around the calibration blank. The concentration range is
calculated by multiplying the concentration of the interfering element by the value of the
correction factor being tested and divided by 10. If after the subtraction of the calibration
blank the apparent analyte concentration falls outside of this range in either a positive or
negative direction, a change in the correction factor of more than 10% should be suspected.
The cause of the change should be determined and corrected and the correction factor
updated. The interference check solutions should be analyzed more than once to confirm
a change has occurred. Adequate rinse time between solutions and before analysis of the
calibration blank will assist in the confirmation.
3.1.9 When interelement corrections are applied, their accuracy should be verified,
daily, by analyzing spectral interference check solutions. If the correction factors or
multivariate correction matrices tested on a daily basis are found to be within the 20% criteria
for 5 consecutive days, the required verification frequency of those factors in compliance may
be extended to a weekly basis. Also, if the nature of the samples analyzed is such they do
not contain concentrations of the interfering elements at ± one reporting limit from zero, daily
verification is not required. All interelement spectral correction factors or multivariate
correction matrices must be verified and updated every six months or when an
instrumentation change, such as in the torch, nebulizer, injector, or plasma conditions
occurs. Standard solution should be inspected to ensure that there is no contamination that
may be perceived as a spectral interference.
3.1.10 When interelement corrections are not used, verification of absence of
interferences is required.
3.1.10.1 One method is to use a computer software routine for comparing
the determinative data to limits files for notifying the analyst when an interfering
element is detected in the sample at a concentration that will produce either an
apparent false positive concentration, (i.e., greater than) the analyte instrument
detection limit, or false negative analyte concentration, (i.e., less than the lower
control limit of the calibration blank defined for a 99% confidence interval).
3.1.10.2 Another method is to analyze an Interference Check Solution(s)
which contains similar concentrations of the major components of the samples (>10
mg/L) on a continuing basis to verify the absence of effects at the wavelengths
selected. These data must be kept on file with the sample analysis data. If the
check solution confirms an operative interference that is > 20% of the analyte
concentration, the analyte must be determined using (1) analytical and background
correction wavelengths (or spectral regions) free of the interference, (2) by an
alternative wavelength, or (3) by another documented test procedure.
3.2 Physical interferences are effects associated with the sample nebulization and
transport processes. Changes in viscosity and surface tension can cause significant inaccuracies,
especially in samples containing high dissolved solids or high acid concentrations. If physical
interferences are present, they must be reduced by diluting the sample or by using a peristaltic
pump, by using an internal standard or by using a high solids nebulizer. Another problem that can
occur with high dissolved solids is salt buildup at the tip of the nebulizer, affecting aerosol flow rateand causing instrumental drift. The problem can be controlled by wetting the argon prior to
nebulization, using a tip washer, using a high solids nebulizer or diluting the sample. Also, it has
been reported that better control of the argon flow rate, especially to the nebulizer, improves
instrument performance: this may be accomplished with the use of mass flow controllers. The test
described in Section 8.5.1 will help determine if a physical interference is present.
3.3 Chemical interferences include molecular compound formation, ionization effects, and
solute vaporization effects. Normally, these effects are not significant with the ICP technique, but
if observed, can be minimized by careful selection of operating conditions (incident power,
observation position, and so forth), by buffering of the sample, by matrix matching, and by standard
addition procedures. Chemical interferences are highly dependent on matrix type and the specific
analyte element.