The matrix effect

The very peculiar molar ratio 0.4 DDAB to 0.6 oleate, which gives rise to the narrow size distribution, is really noteworthy. This molar ratio corresponds closely to electroneutrality (this is not at 50 50 molarity, due to the relatively high pK of oleate carboxylate in the bilayer) and suggests that small mixed vesicles with an approximately equal number of positive and negative charges may enjoy particular stability. More detailed studies are needed, and this indicates the richness of the unexplored in the field of vesicles. This is shown in its fullness in the next section on the matrix effect, which is also an unexpected phenomenon and one that may have implications for the origin of early cell. 

Vesicle growth and reproduction caused by addition of fresh surfactant to a solution of pre-existing vesicles has been described previously. Closer investigation of the 

Note also that the kinetic progress depends on the size of the pre-added vesicles. This catalytic effect is present also in a ratio 1 100, or less, between the pre-added and added surfactant, reinforcing the analogy with a catalytic effect. 

It appears, therefore, that the presence of vesicles accelerates the formation of new vesicles. It is not easy to rationalize how and why. What comes to mind is a general observation from the field of surfactants, that pre-organization makes the organization of further material easier. For example, there is no spontaneous vesiculation when POPC is simply added to water, and no significant amount of 

It has also been shown that the matrix effect is not limited to a first addition of fresh surfactant, but can be repeated several times, thus increasing the number of particles of the same size distribution up to an order of magnitude (Rasi et al., 2003  

---

Sample pre-treatment. Novel procedures of electrochemical sample treatment have been proposed to decrease the signal interference with native cholinesterase inhibitors present in fruits and vegetables. Polyphenolic compounds were removed by electrolysis with soluble A1 anode followed by the oxidation of thionic pesticides with electrogenerated chlorine. The procedure proposed makes it possible to decrease the background current and the matrix effect by 80-90%. Thus, the detection limits of about 5 ppb of Pai athion-Methyl and Chloropyrifos-Methyl were obtained in spiked grape juice without any additional sepai ation or pre-concentration stages. 

The usage of the ratio of chai acteristic lines as analytical parameter in the process of formation of the calibration curve provides a significant decrease of the residual error. In Realization of this method simultaneously with the decrease of the matrix effects causes some decrease or even full compensation of the fonu and condition of the measured surface. 

Under Cs bombardment the matrix effect can be significantly reduced by using the MCs" ion signals for quantification of species M. The detection limit is increased, i.e. the detection power deteriorates, by two or more orders of magnitude, but sometimes even standard-free quantification has been reported [3.51]. MCs" ions have high masses this is a disadvantage because many mass interferences occur in this mass range. 

With this labeling, the matrix effectively consists of nine submatrices, d> , i =... 

In many cases when methods involve internal or external standards, the solutions used to construct the calibration graph are made up in pure solvents and the signal intensities obtained will not reflect any interaction of the analyte and internal standard with the matrix found in unknown samples or the effect that the matrix may have on the performance of the mass spectrometer. One way of overcoming this is to make up the calibration standards in solutions thought to reflect the matrix in which the samples are found. The major limitation of this is that the composition of the matrix may well vary widely and there can be no guarantee that the matrix effects found in the sample to be determined are identical to those in the calibration standards. 

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.

The advantages of SIMS are its high sensitivity (detection limit of ppms for certain elements), its ability to detect hydrogen and the emission of molecular fragments that often bear tractable relationships with the parent structure on the surface. Disadvantages are that secondary ion formation is a poorly understood phenomenon and that quantification is often difficult. A major drawback is the matrix effect secondary ion yields of one element can vary tremendously with chemical environment. This matrix effect and the elemental sensitivity variation of five orders of magmtude across the periodic table make quantitative interpretation of SIMS spectra oftechmcal catalysts extremely difficult. 

In Section 8.2.8 we have discussed the standard addition method as a means to quantitate an analyte in the presence of unknown matrix effects cf. Section 13.9). While the matrix effect is corrected for, the presence of other emalytes may still interfere with the analysis. The method can be generalized, however, to the simultaneous analysis of p analytes. Multiple standard additions are applied in order to determine the analytes of interest using many q > p) analytical sensors. It... 

In trace organic analysis there is usually an extraction or clean-up process, rather than a sample dissolution. Here not only must the matrix effect be considered, but also the recovery yield of the extraction. Frequently an external spike standard is added, but there is often no way of knowing if the recovery of the spike standard matches the analyte in question. There is considerable evidence that the U S E P A method for VOA analysis (Minnich 1993) is subject to such error, as reported by Schumacher and Ward (7997). The analyst must always consider the possibility of such an error, especially when using CRMs to control methods that are applied in routine mode. 

Discontinuity between the physical form of the sample and reference material used can lead to error. This is another manifestation of the matrix effect, but one which has to be considered when analyzing biological and environmental samples. There is no easy answer to the relationship between partide size and homogeneity. It is a popular assumption that the smaller the partide size the less the degree of heterogeneity. In some cases this may be true but there are a number of considerations. 

It is important to note that the matrix effects, interferences, and variability in method efficiency are to be factored in when determining the MDL. If this was not done then only the background noise (see Figure 2, peak 13) would be considered in the definition of the MDL. In real-life samples there is a good possibility that matrix component peaks would either co-elute or elute at retention times close to... 

Because of the possibility that the herbicide alachlor could adulterate food if either poultry or livestock consumed contaminated materials, Lehotay and Miller evaluated three commercial immunoassays in milk and urine samples from a cow dosed with alachlor. They found that milk samples needed to be diluted with appropriate solvents (1 2, v/v) to eliminate the matrix effect. One assay kit (selected based on cost) was also evaluated for use with eggs and liver samples from chickens. Egg and liver samples were blended with acetonitrile, filtered, and diluted with water. Linear calibration curves prepared from fortified egg and liver samples were identical... 

Experiments with aldicarb sulfone in ground beef involved simple extraction with acetonitrile during tissue homogenization and resulted in a definite immunoassay response at the tolerance level of 10 qg kg A moderate, but rather consistent, matrix effect was observed. A more severe matrix effect was observed in bovine milk, blood, and urine. For the liquid matrices, sample dilution was not a satisfactory strategy, because the assay variability increased at lower concentrations, negating any benefit of reducing the matrix effect. This work clearly demonsttated that matrix effects are... 

Many additives fragment quite extensively with FAB. The observation of molecular ion and many fragment ions for each additive makes the determination of the number of additives in the extract difficult if not impossible. Moreover, due to the matrix effect , not all the additives in polymer extracts are detected under FAB. 

It has to be stressed that selectivity of both separation methods depends on the matrix effect and the additives used, which can strongly influence chromatographic or electrophoretic equilibria. For that reason proper sample preparation plays a crucial role in the analytical process. 

Taylor and Zeitlin [43] described an X-ray fluorescence procedure for the determination of total sulfur in seawater. They studied the matrix effects of sodium chloride, sodium tetraborate, and lithium chloride and show that the X-ray fluorescence of sulfur in seawater experiences an enhancement by chloride and a suppression by sodium that fortuitously almost cancel out. The use of soft scattered radiation as an internal standard is ineffective in compensating for matrix effects but does diminish the effects of instrument variations and sample inhomogeneity. 

Neither external nor internal standardization techniques make allowance for the different behaviour of samples and chemical standards, due to the matrix effect in the samples or due to the different state of the analyte in the samples and... 

Matrix effect is a phrase normally used to describe the effect of some portion of a sample matrix that causes erroneous assay results if care is not taken to avoid the problem or correct for it by some mechanism. The most common matrix effects are those that result in ion suppression and subsequent false negative results. Ion enhancement may lead to false positive results.126 127 Several reports about matrix effects include suggestions on what can cause them and how to avoid them.126-147 While various ways to detect matrix effects have been reported, Matuszewski et al.140 described a clear way to measure the matrix effect (ME) for an analyte, recovery (RE) from the extraction procedure, and overall process efficiency (PE) of a procedure. Their method is to prepare three sets of samples and assay them using the planned HPLC/MS/MS method. The first set is the neat solution standards diluted into the mobile phase before injection to obtain the A results. The second set is the analyte spiked into the blank plasma extract (after extraction) to obtain the B results. The third set is the analyte spiked into the blank plasma before the extraction step (C results) these samples are extracted and assayed along with the two other sets. The three data sets allow for the following calculations ... 

The ionization probabilities It vary over some five decades across the elements in the periodic table. In addition, they vary also with the chemical environment of the element. This effect, usually referred to as the matrix effect, makes quantitation of SIMS spectra extremely difficult. As illustrated in Table 4.1, positive secondary ion yields from metal oxides are typically two orders of magnitude higher than those of the corresponding metals. A similar increase in yields from metals is observed after adsorption of gases such as oxygen or carbon monoxide. 

In SNMS, sputtered neutrals are post-ionized before they enter the mass spectrometer. In contrast to SIMS, SNMS does not suffer from the matrix effects associated with the ionization probability of sputtered particles. Here, the sensitivity for a cer-... 

---