Matrix effects ionization Suppression

Calibration Solutions (Matrix Free) Clean solutions of the analytical standard (and SIS if appropriate) used to calibrate instrument response use of this clean solutions can lead to failure to correct for variations in extraction efficiency, matrix effects (ionization suppression), instrument response etc. (see Chapter 8). 

Discussion of this phenomenon is more conveniently postponed until the next section, but this observation provides another weapon in the analyst s war against matrix effects (ionization suppression or enhancement). 

Matrix Effects. The matrix effects (ion suppression or ion enhancement) can compromise the selectivity and sensitivity of LC/MS/MS methods for the determination of drug concentrations in biological matrices. One common matrix effect is ion suppression due to co-eluting components that can affect the ionization efficiency of the analyte of interest. The matrix effects are major causes for errors in precision, accuracy, linearity, and reproducibility of the quantitation methods based on LC/MS/MS [104,108,114-118]. It is critical to overcome such effects in quantitative bioanalysis by LC/MS/MS. 

It is well known that electrospray ionization (El) suffers from suppression effects when polar/ionic compounds other than the analyte(s) of interest, such as those originating from the sample matrix, are present, with this phenomenon being attributed to competitive ionization of all of the appropriate species present [33]. Matrix effects can, therefore, be considerable and these have two distinct implications for quantitative procedures, as follows ... 

Atomic absorption spectroscopy is highly specific and there are very few cases of interference due to the similar emission lines from different elements. General interference effects, such as anionic and matrix effects, are very similar to those described under flame emission photometry and generally result in reduced absorbance values being recorded. Similarly, the use of high temperature flames may result in reduced absorbance values due to ionization effects. However, ionization of a test element can often be minimized by incorporating an excess of an ionizable metal, e.g. potassium or caesium, in both the standards and samples. This will suppress the ionization of the test element and in effect increase the number of test atoms in the flame. 

Ionization changes can be efficiently corrected with the use of an isotopically labeled IS, which possesses identical ionization response and fragmentation pattem. Therefore, deuterated IS can be used to correct both the overall method variability (e.g., sample preparation, injection, electrophoretic process, etc.) as well as matrix effects since the amount of suppression from interferents is expected to be similar. However, the total concentration of analyte and IS should be below the saturation of the ionization process. Guidelines to obtain a reproducible CE—MS method were published by Ohnesorge et al. and took into account the use of an isotopically labeled IS. 

Due to the complexity of the food samples, it is possible that the presence of some compounds in the matrix interferes with analyte determination even when working in LC-MS/MS, certain compounds present in the sample can affect the initial ionization of the analyte through what is often called ion suppression/ enhancement or matrix effects. 

One hypothesis is that this phenomenon may be due to a competition between nonvolatile matrix components and analyte ions for access to the droplet surface for transfer to the gas phase. This competition can enhance or suppress the signal depending on the environment in which the ionization and the ion evaporation processes take place therefore, the presence of a matrix can influence ion formation. Chemical-physical properties of the analytes, and in particular polarity, play a role on the degree of matrix effects that have a strong influence on the analytical precision of the method, and in particular, on the sensitivity and the limit of quantification [37]. 

However, some obstacles still have to be removed before HPLC-MS is fully accepted as a routine technique in the forensic labs. First of all the cost of equipment is still high, despite the reduction trend in the last years, as compared to GC-MS. Second, some problems have to be tackled, i.e., the susceptibility of ion sources (and particularly ESI) to matrix effects on analyte s ionization efficiency (suppression or enhancement) and the scarce reproducibility of MS fragmentation. The third obstacle is actually the other side of the coin of versatility the wide choice of technical alternatives makes HPLC-MS still far from being a highly standardized, one-button technique as GC-MS. 

Acceptance. The RSD of the matrix factor for all lots must not deviate <15.0 %. If MF= 1, it indicates that there is no matrix effect. If MF >1, it indicates that there is ionization enhancement. If MF <1, it indicates that there is ionization suppression. 

Matrix effects (ME), caused by co-eluting endogenous and exogenous matrix components, significantly affect the efficiency and reproducibility of the ionization process of target analytes. This phenomenon represents a major concern for LC-MS bioana-lytical method precision, accuracy, sensitivity, and robustness. Amongst the atmospheric pressure ionization interfaces used in LC-MS systems, ESI source is more prone to signal alteration (ion suppression or enhancement) due to matrix. Therefore, careful evaluation and correction for ME must be considered particularly with ESI-MS. 

Suppressing matrix effects on ionization of ipratropium (21 % suppression) were described by Ariffin and Anderson when analysing SPE extracts of whole blood by gradient RP-LC-ESI MS/MS [54], In contrast, only slight ion suppression was described by John et al. for simultaneous measuring of cocaine (4 %), homatropine (4 %), ipratropium (4 %),, S-hyoscyaminc (3 %), littorine (1 %), A-butyl-scopolamine (1 %), and scopolamine (10 %) by gradient RP-LC-ESI MS/MS [50], In this study plasma samples were precipitated with acetonitrile and the supernatant was further diluted prior to injection. 

Although there is usually no need for any chemical derivatization, caution has to be applied when LC-MS/MS data are reviewed. The ionization of analytes might be affected and altered by endogenous compounds, which can be present in the matrix and which might coelute together with the analyte or internal standard. This can lead to ion suppression (predominantly observed with ESI ionization) as well as ion enhancement, which more often is observed when APCI-ionization is used. Matrix effects can lead to false results. 

Pascoe R, Foley JP, Gusev Al (2001) Reduction in Matrix-Related Signal Suppression Effects in Electrospray Ionization Mass Spectrometry Using On-Line Two-Dimensional Liquid Chromatography. Analytical Chemistry 73 6014-6023... 

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