Agriculture Reference
In-Depth Information
commonly used methods to evaluate matrix effects are (i) postcolumn infusion for
qualitative evaluation [52] and (ii) postextraction spikes for quantitative evalua-
tion [53]. To perform postcolumn infusion, an infusion pump delivers a constant
amount of analyte into the LC stream entering the ion source of the mass spectrometer.
Ablank sample extract is injected under the same conditions as those for the assay. Any
variation in ESI response of the infused analyte caused by an endogenous compound
that elutes from the column is seen as a matrix effect. If no matrix effect exists, a steady
ion response is obtained as a function of time because the analyte is infused into theMS
at a constant
ow (concentration). In the postextraction spikemethod, matrix effects are
quantitatively accessed by comparing the response of an analyte in neat solution with
the response of the analyte spiked into a blank matrix sample that has been carried
through the sample preparation process [54].
Both the postcolumn infusion method and the postextraction spike method are not
suitable for studies in which a representative blank sample matrix is not available. In
these situations, recovery studies can be performed to evaluate the matrix effects [31].
To perform a recovery study, known amounts of analytes are added to a sample before
the sample preparation, and the same procedure as that for a unspiked sample is
carried out to prepare the spiked sample. If the recovery is lower than 70% or greater
than 130%, the sample matrix is considered to cause serious ion suppression or
enhancement, respectively. Because some samples contain analytes and some
samples lack analytes, the recoveries for these two types of samples are calculated
using different formulas. For samples lacking analytes, recoveries are calculated by
the following formula (Eq. 2.3):
detected amount
spiked amount
Recovery
%
100
(2.3)
For samples containing analytes, the recoveries are calculated as follows (Eq. 2.4):
total amount
amount in sample
spiked amount
Recovery
%
100
(2.4)
There are various techniques used to minimize matrix effects for different matrices.
The use of stable isotopically labeled analogs is a preferred technique to compensate
for matrix effects because the analogs have very similar properties and almost the
same retention times as the analytes due to their identical structures. However, the
labeled analogues are not always commercially available and may be costly. Dilution
is the simplest way to minimize matrix effects if the concentrations of the samples are
high enough to withstand the dilution [31]. Other common sample preparation
techniques such as QuEChERS,
liquid extraction, solid-phase extraction
(SPE), and solid-phase microextraction (SPME) have also been applied to minimize
matrix effects. In addition to the use of the method of standard additions, other options
include smaller injection volumes, using structurally similar unlabeled compounds
that elute close to the compounds of interest as internal standards, and modi
liquid
-
cation of
