Matrix suppression

Sample preparation can be tailored to suppress excess matrix background peaks by controlling the matrix analyte ratio. Although numerous groups observed this phenomenon soon after the introduction of MALDl, it has only recently been exploited to achieve its fuU potential for smaU-molecule analysis. The matrix analyte molar ratio within the matrix crystals has been found to be critical for MALDl quantification [10]. Critical parameters for a successful execution of the matrix suppression effect (MSE) include sufficient analyte and optimized laser intensity. 

As in the case of FAB-MS, the use of surfactants such as cetyltrimethylammonium bromide (CTAB) can be used to substantially or even completely suppress the matrix-related ion background [81]. Use of the CTAB surfactant also resulted in an improved mass resolution for low-molecular-weight molecules including amino acids, peptides, drugs, and cyclodextrins. This technique has been used successfully for the analysis of caffeine, and the vitamins riboflavin, nicotinamide and pyridoxine in various energy drinks [82]. 

Arguably, small-peptide analysis is one of the largest areas of MALDl appHcation to LMM compounds. Chen et al. [86] initially demonstrated the appHcabUity of 

MALDI to study small peptides (800-1500 Da). A variety of excellent examples of MALDI for the analysis of small peptides can be found in almost every MALDI publication [87]. Furthermore, some small peptides such as angiotensin and substance P have become gold standards for the verification of matrix and instrument performance [4]. The analysis of peptide maps for the characterization of proteins is discussed in detail in Chapter 3. 

One other well-established area of MALDI is that of carbohydrate analysis. It has been shown that MALDI is applicable for the analysis of underivatized and derivatized carbohydrates, and can also provide complementary data to other techniques [88-91]. The MALDI matrices and sample preparation techniques used 

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A triple-quadrupole mass spectrometer with an electrospray interface is recommended for achieving the best sensitivity and selectivity in the quantitative determination of sulfonylurea herbicides. Ion trap mass spectrometers may also be used, but reduced sensitivity may be observed, in addition to more severe matrix suppression due to the increased need for sample concentration or to the space charge effect. Also, we have observed that two parent to daughter transitions cannot be obtained for some of the sulfonylurea compounds when ion traps are used in the MS/MS mode. Most electrospray LC/MS and LC/MS/MS analyses of sulfonylureas have been done in the positive ion mode with acidic HPLC mobile phases. The formation of (M - - H)+ ions in solution and in the gas phase under these conditions is favorable, and fragmentation or formation of undesirable adducts can easily be minimized. Owing to the acid-base nature of these molecules, negative ionization can also be used, with the formation of (M - H) ions at mobile phase pH values of approximately 5-7, but the sensitivity is often reduced as compared with the positive ion mode. 

At Bayer CropScience, the use of a stable isotope IS has become common practice to eliminate the effects of matrix suppression on instrument signals. The stable isotopes are synthesized by deuterium exchange reactions on authentic native standards or the... 

For high-throughput analysis, it is important to increase the specihcity of each bioanalytical method. The enhancement of chromatographic resolution presents various limitations. Better selectivity can be obtained with TOF mass analyzers that routinely provide more than 5000 resolution (full width at half-mass or FWHM). The enhanced selectivity of a TOF MS is very attractive for problems such as matrix suppression and metabolite interference. In one report of quantitative analysis using SRM, TOF appeared less sensitive than triple quadrupole methods but exhibited comparable dynamic range with acceptable precision and accuracy.102... 

McCombie, G. Knochenmuss, R. Small-molecule MALDl using the matrix suppression effect to reduce or eliminate matrix background interferences. Anal. Chem. 2004, 76, 4990-4997. 

Donegan, M. Tomlinson, A. J. Nair, H. Juhasz, P. Controlling matrix suppression for matrix-assisted laser desorption/ionization analysis of... 

Fig. 6.6 Detection of matrix-suppression components at the analyte retention time and at late elution. The chromatogram of the drug analyte is superimposed on the post-column infusion trace of the matrix extract. The chromatography was reversed-phase with a short run time of 3 min. The retention time of the...

Sometimes, even the slight separation between an analyte and its deuterated internal standard could cause significant quantitation errors due to differential ion suppression towards the analyte and its deuterated internal standard [34, 35], As shown in Fig. 6, the elution of carvedilol and its internal standard (D5-carvedilol) overlaps with the declining edge of a matrix suppression region in a matrix lot. This resulted in more pronounced ion suppression for the slightly later eluted... 

Longerich, H.P. (1989) The effect of nitric acid, acetic acid and ethanol on inductively coupled plasma-mass spectrometric ion signals as a function of nebulizer gas flow, with implications on matrix suppression and enhancements. J. Anal. Atomic Spectroscopy 4, 665-677. 

Stable isotopes of carotenoids (usually labeled with or H), or of compounds which react with carotenoids, are often used with MS studies. We previously mentioned the use of stable isotopes as internal standards for carotenoid quantitation (especially important when matrix suppression of signal is observed), but there are also many other applications. 

Tanner, S.D. Characterization of ionization and matrix suppression in inductively coupled cold plasma mass spectrometry. J. Anal. At. Spectrom. 10, 905-921 (1995)... 

The potential application of APPI includes analysis of compounds (nonpolar and neutral analytes) that are not effectively ionized by ESI or APCI. APPI appears to be less influenced by matrix suppression as seen in ESI or APCI. It can serve as a complementary ionization source to ESI/APCI. 

Knochenmuss, R., Dubois, F., Dale, M.J. and Zenobi, R. (1996) The matrix suppression effect and ionization mechanisms in matrix-assisted laser desorption/ionization. Rapid Conunun. Mass Spectram. 10, 871-877. 

Figure 1 shows a standard curve for aqueous molybdate standards (circles). The line through the points that are indicated by triangles is a standard additions experiment on a serum sample containing about 5 /xg/L Mo. The serum matrix interference reduces the sensitivity markedly. The difference in slopes between the aqueous standards and the sample illustrates the matrix suppression effect. Figures 2 and 3 show... 

As discussed in Ch. 6.6.3, some mobile-phase additives are also known to suppress or enhance analyte response. It appears useful to discriminate between effects due to the mobile-phase composition and effects due to the actual analyte matrix. Some additives were found to reduce the matrix suppression [119]. 

Different mobile-phase additives may influence the extent of matrix suppression in a particular method [119, 121]. 

As mentioned above, in a competitive ionization process, molecules with the lowest ionization potentials will be preferentially ionized and it is quite possible that this competition, in addition to matrix suppression, will result in the relative abundance of sample metabolites not being reflected in the MS data. Any reduction in the number of analyte ions available for analysis will have an impact on the assay with loss of sensitivity (higher limits of detection and quantitation). 

The same scheme has been adopted for the determination of several antibacterials (quinolones and erythromycin A), fungicides, and parasiticides in salmon tissue. In this case, samples were extracted with acidic ACN, and then dSPE was carried out with a Bondesil NH2 sorbent, followed by LC-TOE-MS determination. Excellent recoveries were obtained with the exception of enrofloxacin (40%), although matrix suppression effects were observed for most of target compounds. ... 

All LC-MS techniques tend to be subject to matrix effects, especially suppression, although enhancement effects may also be observed. A procedure has been suggested to systematically investigate matrix effects when developing and validating methods using LC-MS or LC-MS/MS for detection. First, run pure standards to determine the analyte response in the absence of matrices. Next, either prepare standards in a matrix extract or infuse standards in the presence of matrix extract into the mass spectrometer and determine whether the response differs from that observed for pure standards. Differences in response may be attributed to matrix suppression (or enhancement) effects. Finally, fortify blank tissue with standards, perform the extraction and clean-up steps of the method, and then determine the detector response. The difference between the response observed for fortification into matrix extract and fortification into matrix prior to extraction and clean-up is attributed to method recovery. The evaluation of matrix effects is discussed in detail in Chapter 6. 

It is interesting to note that the secondary lines for Cr, Mg, Mo, and Si, chosen to avoid iron interference, do alleviate or reduce the effects of Fe line overlap. For Cr and Mo, standard deviation of the background for the secondary lines is lower for the Fe matrix blank than for the dilute acid blank. This is probably caused by matrix suppression of the calibration curve as described by Mr. Midkiff."... 

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