Page 78 - Advances in Forensic Applications of Mass Spectrometry - Jehuda Yinon

1522_C02.fm Page 66 Wednesday, November 12, 2003 9:36 AM

only for a limited number of drugs (morphine, codeine, amphetamine, and
benzoylecgonine) and potentially interfering compounds. Additionally, the
matrix effects were not recognized. 10
11
Müller et al. studied the effect of coextracted serum matrix on the signal
of test substances — codeine (as a positive ion) and glafenine (as a negative
ion). Severe ion suppression was observed for both ionization methods after
the injection of serum matrix originating from protein precipitation and SPE.
Less suppression was observed in the case of solvent extraction, while the
combination of protein precipitation with SPE caused no suppression. The
authors stated that the suppression effects were caused by polar, nonretained
matrix components appearing on the beginning of the chromatogram.
Zhou et al. carried out a systematic study on the relation of matrix
12
suppression to the extraction methods, chromatographic conditions, and
concentration of analytes. In this study, blank serum matrix samples were
injected into the HPLC column with the postcolumn infusion of four test
compounds at three concentration levels. The areas of suppression were
located along the whole chromatogram. On the solvent front, salt and other
polar unretained species were present. Other endogenous compounds were
eluted later, sometimes in very high concentrations, causing severe ion sup-
pression which was independent of the analyte concentration.
Avery compared the ion suppression effects caused by extracts of human
and animal (rat, dog, monkey, rabbit, and guinea pig) plasma. The
13
responses for analyte and internal standard were measured in isocratic and
gradient elution, using APCI and ionspray ionization sources. It was stated
that each species showed different suppression. Therefore, the validation
should be performed with samples originating from the same species.
14
Tang et al. studied matrix effects in postcolumn-infusion experiments.
Extracted blanks were injected while the ion transitions of the infused ana-
lytes were monitored. Both suppression and enhancement of ionization was
observed. These phenomena were compensated by changing the ionization
energy, ionization source (ACPI instead of ESI), sample pretreatment
method, or by including matrix ions in acquisition methods.
Bansal and Liang introduced two novel terms to assess the matrix effect
in an LC/MS/MS assay. Matrix factor (MF) is calculated from the ratio of
peak area of pure analyte to the peak area of the analyte injected together
with blank matrix. Extraction uniformity (EU) is the ratio of the extraction
efficiency of analyte to that of internal standard. If the EU was close to 1, it
was possible to perform the validation for the same matrix from different
species. Some authors applied column switching as a method to minimize
15
16
the influence of matrix compounds. Sun et al. applied protein precipitation
in a 96-well format as a sample pretreatment method for the determination
of nicotine and cotinine in plasma with LC/MS/MS. In order to avoid matrix

© 2004 by CRC Press LLC