Analyte suppression

Total or partial ion suppression is a well-known LC-MS effect, which is induced by coeluting matrix components that can have a dramatic effect on the intensity of the analyte signal. As can be observed in Fig. 1, analyte suppression occurs as a consequence of the different matrix interferences present in waste-water samples, making the identification and/or quantification process difficult or unfeasible. Even when working under selection ion monitoring (SIM) conditions, these matrix effects can cause ion suppression in the detection of some analytes that are present at low levels of concentration, as seen in this figure. Several papers have reported this effect [30-32] and different alternatives to overcome these problems, such as the inclusion of a size-exclusion step [33] or sequential SPE [28], have been applied for the determination of pesticides in... 

Although sodium adducts were preferably formed in both instruments, even in the absence of added electrolyte, in order to avoid the possible reduction in ionisation due to insufficient concentration of metal ions in solution, it is recommended that sample extracts are fortified with sodium ions prior to injection. However, the addition of higher concentrations can induce system instability and analyte suppression. Therefore, the concentration of added electrolyte should... 

While a hydrophobic ion-pair is retained on hydrophobic stationary phases better than an ionized analyte, the retention of the duplex on normal phases is easily predicted to be lower than that of the ionized analyte because polar interactions are reduced. Actually the trend of k versus IPR concentration under normal phase IPC is the opposite of reversed phase IPC [34]. An aminopropyl, a cyanoethyl, and a silica stationary phase were compared for the analysis of alcohol denaturants. The cyanoethyl phase was selected and anionic IPRs were used to reduce retention of cationic analyte, suppressing their interactions with negatively charged silanols... 

Figure 9.7 Filtered MALDI images of samples containing a mixture of yohimbine and caffeine analytes with matrices CHCA (a) and DHB (b). The ratio of yohimbine to caffeine to matrix was 4 1 36. In the absence of analyte suppression effects, the ratio of yohimbine to caffeine signals is expected to be 4. The yohimbine MSE score was used as a filter. The ratio of caffeine to yohimbine...

Even more important for present purposes than the effect of ionization suppression of the MALDI matrix is the suppression of one analyte by another examples are shown in Figure 5.7. This effect is intrinsically more complex than ionization suppression of the matrix it may be the result of competition among analytes for the same or different primary matrix ions, or via direct reactions among anal5hes, so the concentration dependence can become highly comphcated with serious implications for quantitation experiments. Inter-analyte suppression has been observed (Knochenmuss 2000) for both similar (e.g., both protonated) and dissimilar (e.g., protonated vs sodi-ated) analytes (Figure 5.7) again, the dissimilar case... 

Figure 5.7 Examples of MALDI suppression of one analyte by another. Left MALDI mass spectra of gramicidin S and of a mixture of gramicidin S and substance P in DHB matrix (mole ratio 1 2 2000), illustrating the similar analyte suppression effect (both analytes appear as protonated molecules). The gramicidin S concentration was the same in both spectra, and the spectra were recorded under identical conditions. When sufficient substance P is present, the gramicidin S signal disappears. Right MALDI mass spectra of valinomycin and of a mixture of valinomycin and substance P in DHB matrix (mole ratio 1 2.5 1000), illustrating the dissimilar analyte suppression effect (one analyte appears as the Na adduct, the other as the protonated molecule). The valinomycin concentration was the same in both spectra, and the spectra were taken under identical conditions. When sufficient substance P is present forming abundant protonated molecules, the valinomycin Na adduct signal almost completely disappears. Reproduced from Knochenmuss, /. Mass Spectrom. 35, 1237 (2000), with permission of John Wiley Sons, Ltd.

Whether due to matrix-analyte or analyte-analyte reactions, the analyte with the most favorable charge transfer thermodynamics will appear with greatest intensity in the mass spectrum. Since the thermodynamic picture of secondary plume reactions is not specific to any charge transfer reaction type, the same kind of suppression phenomena are expected for protonation, cationization, and electron transfer. As for the MSE, an increase in primary ions due to higher laser intensity reduces inter-analyte suppression since the matrix-analyte reactions are forced to the right. Also parallel to the MSE, ASE is favored by high analyte concentration. It is therefore normally preceded and accompanied by MSE. An example of both effects is shown in Figure 5.8. Because MSE and ASE are connected, suppression of one type of analyte ion by another can occur, and has been observed. ... 

Figure 5.8. Positive-mode MALDI spectra versus matrix-analyte mole ratio (DCTB matrix) for an equimolar five-component mixture. A, M-T data ionization potential (IP), 6.04 eV (CAS number 124729-98-2) B, TTB IP, 6.28 eV (76185-65-4) C, NPB IP, 6.45eV (123847-85-8) D, rubrene IP, 6.50eV (104751-29-9) E, D2NA IP, 7.06 eV (122648-99-1). The molar mixing ratios of matrix to analyte arc indicated for each spectrum. TTiese analytes are observed exclusively as radical cations, and they exhibit matrix and analyte suppression effects analogous to those known from proton or cation transfer secondary reactions. Low ionization potential (IP) analytes suppress high IP analytes and matrix. (Adapted from Ref. 32.)...

The correlation with experimental phenomena is good. The matrix and analyte suppression effects are well reproduced, including all the characteristics noted above such as dependence on secondary reaction exothermicities, concentration, and laser flu-The model shows that analyte intensity ratios can approach the original concentration ratios if more primary ions are created at higher laser fluence, but this ratio does not reach the correct values at any fluence. Even analytes of similar reactivity but different molecular weight may exhibit different intensities, because of the size-dependent collision probabilities. ... 

Sodium and chloride may be measured using ion-selective electrodes (see Electro analytical techniques). On-line monitors exist for these ions. Sihca and phosphate may be monitored colorimetricaHy. Iron is usually monitored by analysis of filters that have had a measured amount of water flow through them. Chloride, sulfate, phosphate, and other anions may be monitored by ion chromatography using chemical suppression. On-line ion chromatography is used at many nuclear power plants. 

Another type of interference in ICPMS is suppression of the formation of ions from trace constituents when a large amount of analyte is present. This effect depends on the mass of the analyte The heavier the mass the worse the suppression. This, in addition to orifice blockage from excessive dissolved solids, is usually the limiting factor in the analysis of dissolved materials. 

Generally, the interaction of polar analytes with the packing is rather weak due to the hydrophilic polyhydroxy functions on the surface of the packing. However, small amounts of acidic functional groups are present on the surface of the packing. The influence of these functional groups can be suppressed easily with the use of salts in the mobile phase. 

Electrochemical analytical techniques are a class of titration methods which in turn can be subdivided into potentiometric titrations using ion-selective electrodes and polarographic methods. Polarographic methods are based on the suppression of the overpotential associated with oxygen or other species in the polarographic cell caused by surfactants or on the effect of surfactants on the capacitance of the electrode. One example of this latter case is the method based on the interference of anionic surfactants with cationic surfactants, or vice versa, on the capacitance of a mercury drop electrode. This interference can be used in the one-phase titration of sulfates without indicator to determine the endpoint... 

A major difference between MALDI and FAB is that a solid rather than a liquid matrix is used and a mixture of the analyte and matrix is dried on the laser target. For this reason, the effective combination of HPLC with MALDI is not as readily achieved although, since MALDI is largely free of the suppression effects experienced with FAB, it is able to provide useful analytical data directly from mixtures. 

Ions at m/z 55, 60, 214 and 236 are observed but do some or all of these arise from the background and are present throughout the analysis, or are they present in only a few scans, i.e. are they from a component with insufficient overall intensity to appear as a discrete peak in the TIC trace An examination of reconstructed ion chromatograms (RICs) from these ions generated by the data system may enable the analyst to resolve this dilemma. The TIC shows the variation, with time, of the total number of ions being detected by the mass spectrometer, while an RIC shows the variation, with time, of a single ion with a chosen m/z value. The RICs for the four ions noted above are shown in Figure 3.15. These ions have similar profiles and show a reduction in intensity as analytes elute from the column. The reduction in intensity is a suppression effect. 

The pressure difference between the source of the mass spectrometer and the laboratory environment may be used to draw a solution, containing analyte and matrix material, through the probe via a piece of capillary tubing. When an adequate spectrum of the first analyte has been obtained, the capillary is simply placed in a reservoir containing another analyte (and matrix material) and the process repeated. This may therefore be used as a more convenient alternative to the conventional static FAB probe and this mode of operation may also benefit from the reduction in suppression effects if the analyte is one component of a mixture. 

The electrospray process is susceptible to competition/suppression effects. All polar/ionic species in the solution being sprayed, whether derived from the analyte or not, e.g. buffer, additives, etc., are potentially capable of being ionized. The best analytical sensitivity will therefore be obtained from a solution containing a single analyte, when competition is not possible, at the lowest flow rate (see Section 4.7.1 above) and with the narrowest diameter electrospray capillary. 

Suppression effects may be observed and the direct analysis of mixtnres is not always possible. This has potential implications for co-eluting analytes in LC-MS. 

A potential problem encountered in these determinations is the ion suppression encountered in the presence of polar/ionic interfering materials which compete with the analyte(s) for ionization (see Section 4.7.2 earlier). Many of these analyses therefore involve some degree of off-line purification and/or an appropriate form of chromatography. Since it is not unusual to encounter closely related compounds that are not easily separated, it is also not unusual to employ both of these approaches, as in the following example. This illustrates the use of HPLC as a method of purification and demonstrates that highly efficient separations are not always required for valuable analytical information to be obtained. 

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 ... 

Figure 5.56 Structures of the three analytes pesticides used in an investigation of the matrix effects observed in LC-MS-MS. Reprinted from J. Chromatogr., A, 907, Choi, B. K., Hercnles, D. M. and Gnsev, A. I., Effect of liquid chromatography separation of complex matrices on liqnid chromatography-tandem mass spectrometry signal suppression , 337-342, Copyright (2001), with permission from Elsevier Science.

Suppression effects The decrease in analyte signal intensity brought about by the presence of extraneous materials in the sample. 

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