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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.
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3.4 Memory interferences result when analytes in a previous sample contribute to the
signals measured in a new sample. Memory effects can result from sample deposition on the uptake
tubing to the nebulizer and from the build up of sample material in the plasma torch and spray
chamber. The site where these effects occur is dependent on the element and can be minimized
by flushing the system with a rinse blank between samples. The possibility of memory interferences
should be recognized within an analytical run and suitable rinse times should be used to reduce
them. The rinse times necessary for a particular element must be estimated prior to analysis. This
may be achieved by aspirating a standard containing elements at a concentration ten times the usual
amount or at the top of the linear dynamic range. The aspiration time for this sample should be the
same as a normal sample analysis period, followed by analysis of the rinse blank at designated
intervals. The length of time required to reduce analyte signals to within a factor of two of the
method detection limit should be noted. Until the required rinse time is established, this method
suggests a rinse period of at least 60 seconds between samples and standards. If a memory
interference is suspected, the sample must be reanalyzed after a rinse period of sufficient length.
Alternate rinse times may be established by the analyst based upon their DQOs.
3.5 Users are advised that high salt concentrations can cause analyte signal
suppressions and confuse interference tests. If the instrument does not display negative values,
fortify the interference check solution with the elements of interest at 0.5 to 1 mg/L and measure the
added standard concentration accordingly. Concentrations should be within 20% of the true spiked
concentration or dilution of the samples will be necessary. In the absence of measurable analyte,
overcorrection could go undetected if a negative value is reported as zero.
3.6 The dashes in Table 2 indicate that no measurable interferences were observed even
at higher interferant concentrations. Generally, interferences were discernible if they produced
peaks, or background shifts, corresponding to 2 to 5% of the peaks generated by the analyte
concentrations.
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