Absolute matrix effect

An important issue in the method development for quantitative analysis using LC-MS is the possible occurrence of matrix effects. A matrix effect is an (unexpected) suppression or enhancement of the analyte response due to coeluting matrix constituents. It can be easily detected by comparing responses between a standard solution and a spiked pre-treated sample (post-extraction spike). Detailed studies on matrix effects revealed that the ion suppression or enhancement is frequently accompanied by significant deterioration of the precision of the analytical method [115-116]. Therefore, it can be useful to discriminate the two type of matrix effects. The absolute matrix effect indicates the difference in response between the solvent standard and the post-extraction spike, while the relative matrix effect indicates the difference in response between various lots of post-extraction spiked samples [116]. Unless counteraction is taken, an absolute matrix effect will primarily affect the accuracy... 

An important distinction was drawn (Matuszewski 2003) between an absolute matrix effect, defined as ME above, and a relative matrix effect that refers to the comparison of ME values determined using different sources (batches) of blank matrix (e.g., biofluids such as plasma or urine for bioanalysis, or environmental matrices such as soil or water). Experience has shown that the... 

Absolute matrix effect can be calculated by comparing signal intensity of the analyte in the presence (i.e., spiked into the extracted blank matrix) and absence of the extracted matrix (i.e., spiked into a neat solution). 

The matrix effect is one of the main drawbacks of LC—MS/MS methods, as widely reported in the literature [16,22—26]. It appears as a consequence of fhe influence of coextracted matrix components on analyte ionization in atmospheric pressure ionization (API) interfaces. The matrix effect t)q5ically results in an important loss of sensitivity, due to the ionization suppression from the coextracted components hampering the analyte quantification. Although less frequent, enhancement of the analyte signal can also occur. This imdesirable effect causes a loss of method accuracy, precision, and sensitivity, leading to incorrect quantification and can also cause problems in the confirmation process [16,24,25]. The matrix effect depends on each particular analyte—sample matrix combination and the API source of the instrument, commonly ESI or APCI. As an example. Figure 12.1 shows absolute matrix effects for... 

FIGURE 12.1 Absolute matrix effect (%ME) for selected pesticides in tropical fruits (n = 3) calculated by comparison of their response in matrix (extracts fortified at 25 ng/ ml) and in solvent (reference standards at 25 ng/ml). 

It is critical when performing quantitative GC/MS procedures that appropriate internal standards are employed to account for variations in extraction efficiency, derivatization, injection volume, and matrix effects. For isotope dilution (ID) GC/MS analyses, it is crucial to select an appropriate internal standard. Ideally, the internal standard should have the same physical and chemical properties as the analyte of interest, but will be separated by mass. The best internal standards are nonradioactive stable isotopic analogs of the compounds of interest, differing by at least 3, and preferably by 4 or 5, atomic mass units. The only property that distinguishes the analyte from the internal standard in ID is a very small difference in mass, which is readily discerned by the mass spectrometer. Isotopic dilution procedures are among the most accurate and precise quantitative methods available to analytical chemists. It cannot be emphasized too strongly that internal standards of the same basic structure compensate for matrix effects in MS. Therefore, in the ID method, there is an absolute reference (i.e., the response factors of the analyte and the internal standard are considered to be identical Pickup and McPherson, 1976). 

Interferences have been handled, traditionally, by the use of a matrix compensation response curve. Basically, the system is a series of standard additions to samples of a matrix and the use of these supplementations as the standards in a response curve. Thus, the recoveries of antibiotics, affected positively or negatively, can be corrected for matrix effects over a wide range of concentrations. Absolute recoveries are, of course, determined against standards in buffer. 

The above method can be applied for determining the absolute concentration of one or more than one electroactive species in the sample providing that their electrochemical signals are well-separated in the voltammogram and that no mumal influences (interference effect) and matrix effects due to inert, nonelectroactive components in the sample exist. 

A related matrix effect of considerable analytical interest is the enhancement in absolute ion yield sometimes observed under conditions of high dilution in a solid matrix (22). Comparison of the SIMS spectrum of a neat pyrilium salt with that of the same salt diluted 1000-fold in NH Cl shows that the intact cation signal is observed in about three times greater abundance for the NH Cl-diluted sample. The threefold increase is observed even when the absolute amount of salt analyzed in the dilute sample is one thousand times less than that in the neat sample. An additional aspect of this experiment is the persistence of the enhanced signal. Ion bombardment yields products for one day in the NH Cl matrix, but for only about one hour in the neat sample under identical conditions. Effective desorption of ammonium chloride, which entrains analyte, is one way of accounting for these observations. 

Internal standard calibration can be used to compensate for variation in analyte recovery and absolute peak areas due to matrix effects and GC injection variability. Prior to the extraction, a known quantity of a known additional analyte is added to each sample and standard. This compound is called an internal standard. To prepare a calibration curve, shown in Figure 4.6b, the standards containing the internal standard are chromatographed. The peak areas of the analyte and internal standard are recorded. The ratio of areas of analyte to internal standard is plotted versus the concentrations of the known standards. For the analytes, this ratio is calculated and the actual analyte concentration is determined from the calibration graph. 

Produces relatively accurate results for volatile solutes problems with quantitation of high-boiling solutes (matrix effects ) requires re-concentration of the initial bands by cold trapping or solvent effects, which often forces cooling of the column for the injection. This is time-consuming and causes problems with absolute retention times. 

One of the shortcomings of LIBS, particularly in relation to quantitative elemental analysis, arises from the instability of the laser-induced plasma emission resulting from laser intensity fluctuations (1-5%) the amount of scattered light present depends on local matrix effects and on physical and chemical properties of the target material. The most common way of compensating for signal fluctuations in LIBS is by calculating the ratio of the spectral peak intensity to that of a reference intensity. However, this internal calibration method provides relative rather than absolute concentrations. 

In practice, the absolute %ME has limited relevance. In bioanalytical practice, it is more important to demonstrate the absence of relative matrix effects, i.e., between different sources of the biofiuid [70]. Alternatively, relative matrix effects may be evaluated by a comparison of precision expressed as %RSD in repetitive injection of standards and post-extraction spiked samples. If significant differences in the %RSD exist, a relative matrix effect is present between the different sample batches. 

High recovery of the analyte(s) from the matrix is a desirable outcome of sample preparation, and is therefore an important characteristic of the extraction procedure. The absolute recovery is the ratio of the response measured for a spiked sample (in matrix) treated according to the whole analytical procedure to that of a non-biological sample spiked (in aqueous solution) with the same quantity of the analyte substance and directly injected into the chromatographic system [81], The relative recovery is the ratio of the responses between extracted spiked samples (in matrix) and extracted spiked pure samples (in aqueous solution). The relative recovery can be used, together with the absolute recovery, to reveal whether sample losses in the extraction are due to matrix effects or to bad extraction. 

In spark ablation, a spark at constant density is obtained in a matter of seconds, and thus, particularly in the case of small spark chambers, preburn times are accordingly low. In plasma emission as well as in plasma mass spectrometry a linear dynamic range of more than 4 orders of magnitude can be obtained and RSDs are a few percent in the case of absolute measurements. However, as shown by the results in Table 6, they can easily fall to below 1%, when using an internal standard element (Fe in the case of steel samples). The matrix effects from the sampling... 

HS-SPME is a very useful tool in polymer analysis and can be applied for absolute and semi-quantitative determination of the volatile content in polymers, for degradation studies, in the assessment of polymer durabihty, for screening tests and for quality control of recycled materials. For quantitative determination of volatiles in polymers, SPME can be combined with multiple headspace extraction to remove the matrix effects. If the hnearity of the MHS-SPME plot has been verified, the number of extractions can be reduced to two, which considerably reduces the total analysis time. Advantages of MHS-SPME compared to MAE are its higher sensitivity, the small sample amount required, solvent free nature and if an autosampler is used a low demand of labor time. In addition, if the matrix effects are absent, the recovery will always be 100%. This is valuable compared to other techniques for extracting volatiles in polymers in which the recovery should be calculated from the extraction of spiked samples, which are very difficult to produce in the case of polymeric materials. 

There are many frequently measured quantities that are related to the absolute strength of an absorption line or band (Pugh and Rao, 1976). The fundamental quantity to which the others are related is the integrated cross section, <7°-, which has units of area times frequency (often cm2 cm-1 molecule-1 or, equivalently, cm/molecule). The integrated cross section is more fundamental than the peak cross section, 0), because it is insensitive to environmental factors (e.g., temperature, pressure, solvent or matrix effects), the effects of which are typically manifested in the width of the transition, Az/y. 

The matrix in oils consists primarily of low atomic number elements, which have a low absorption for the x-ray energies of interest. This causes the primary beam to penetrate much deeper in the specimen than usually anticipated. Also, the characteristic radiation of the analyte elements experience low absolute absorption. Between specimens, however, the relative differences can still be significant, leading to matrix effects. The low absolute absorption causes the photons to travel a long way — and jrrst like flie photons of the primary beam, geometry becomes an issue as indicated in Fig. 4. The critical depdi, d, is given b ... 

Depending on the X-ray source and the spectral modification devices, the LD are in the pg range for 2—3 kW X-ray tubes and in the fg range with excitation by means of synchrotron radiation. Figure 11.15 shows a typical TXRF spectrum the absolute detection limit values of typical TXRF instruments are shown in Fig. 11.10. Thus, TXRF permits to simultaneously determine trace elements in samples of small volume. Additional advantages are insensitivity to matrix effects, easy cahbration, fast analysis times and low cost. In practice, the method is in particular apphed for multi-element determinations in water samples of various nature and for the routine analysis of Si-wafer surfaces employed in the microelectronics industry. 

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