Quantitative matrix effects

More complicated quantitation (matrix effects except for MHE)... 

Ema data can be quantitated to provide elemental concentrations, but several corrections are necessary to account for matrix effects adequately. One weU-known method for matrix correction is the 2af method (7,31). This approach is based on calculated corrections for major matrix-dependent effects which alter the intensity of x-rays observed at a particular energy after being emitted from the corresponding atoms. The 2af method corrects for differences between elements in electron stopping power and backscattering (the correction), self-absorption of x-rays by the matrix (the a correction), and the excitation of x-rays from one element by x-rays emitted from a different element, or in other words, secondary fluorescence (the f correction). 

Nuclear reaction analysis (NRA) is used to determine the concentration and depth distribution of light elements in the near sur ce (the first few lm) of solids. Because this method relies on nuclear reactions, it is insensitive to solid state matrix effects. Hence, it is easily made quantitative without reference to standard samples. NRA is isotope specific, making it ideal for isotopic tracer experiments. This characteristic also makes NRA less vulnerable than some other methods to interference effects that may overwhelm signals from low abundance elements. In addition, measurements are rapid and nondestructive. 

In the discussion so far we have considered the typical LEIS experiment only, i.e. large angles of incidence of exit relative to the surface plane. Under these conditions, in general, quantitative composition analysis is possible, because the ion-target interaction can be considered as a binary collision, because of the absence of matrix effects (see below). 

Quantitative analysis by SIMS is difficult because of matrix effects. The yield of secondary ions depends strongly on the chemical and electronic characteristics of the sample. For example, the yield of Al, Cr, and V are about 10 greater for... 

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 5.59 Molecular structures of the diarrhetic shellfish poisons (a) pectenotoxin-6 (PTX6) (b) okadaic acid (OA) (c) dinophysistoxin-1 (DTXl) (d) yessotoxin (YTX). Reprinted from J. Chromatogr., A, 943, Matrix effect and correction by standard addition in quantitative liquid chromatographic-mass spectrometric analysis of diarrhetic shellfish poisoning toxins , Ito, S. and Tsukada, K., 39-46, Copyright (2002), 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... 

The method using GC/MS with selected ion monitoring (SIM) in the electron ionization (El) mode can determine concentrations of alachlor, acetochlor, and metolachlor and other major corn herbicides in raw and finished surface water and groundwater samples. This GC/MS method eliminates interferences and provides similar sensitivity and superior specificity compared with conventional methods such as GC/ECD or GC/NPD, eliminating the need for a confirmatory method by collection of data on numerous ions simultaneously. If there are interferences with the quantitation ion, a confirmation ion is substituted for quantitation purposes. Deuterated analogs of each analyte may be used as internal standards, which compensate for matrix effects and allow for the correction of losses that occur during the analytical procedure. A known amount of the deuterium-labeled compound, which is an ideal internal standard because its chemical and physical properties are essentially identical with those of the unlabeled compound, is carried through the analytical procedure. SPE is required to concentrate the water samples before analysis to determine concentrations reliably at or below 0.05 qg (ppb) and to recover/extract the various analytes from the water samples into a suitable solvent for GC analysis. 

Electrospray ionization (ESI) and APCI are the two popular API techniques that will be discussed here. The applications to the analysis of pesticides that will be discussed include imidazolinone herbicides, phenoxy acid herbicides, and A-methyl carbamate insecticides. Matrix effects with respect to quantitation also will be discussed. Eor the... 

HPLC/MS and HPLC/MS/MS analyses are susceptible to matrix effects, either signal enhancement or suppression, and are often encountered when the cleanup process is not sufficient. To assess whether matrix effects influence the recovery of analytes, a post-extraction fortified sample (fortified extract of control sample that is purified and prepared in the same manner as with the other samples) should be included in each analytical set. The response of the post-extraction fortified sample is assessed against that of standards and samples. Matrix effects can be reduced or corrected for by dilution of samples, additional cleanup, or using calibration standards in the sample matrix for quantitation. 

Desorption/ionisation techniques such as LSIMS are quite practical, as they give abundant molecular ion signals and fragmentation for structural information. In the conditions of Jackson et al. [96], all the molecular ion and/or protonated molecule ion species were observed in the LSIMS spectrum when only 1 pmol of each additive was placed on the probe tip. However, as mentioned above, in LSIMS/MS experiments the choice of the matrix (e.g. NBA, m-nitrobenzylalcohol) is very important. Matrix effects can lead to suppression of the generation of molecular ions for some additives. LSIMS is not ideal for the quantitative detection of polymer additives, as matrix effects are very important [96]. 

Table 6.18 lists the main characteristics of FD-MS. FD is a superior ionisation technique for quantitative analysis, as there are no matrix effects as in LSIMS or MALDI which might suppress the generation of ions from certain additives. However, the technique has some serious drawbacks. The primary difficulty is that FD produces only short-lived, highly variable currents of analyte ions. These analyte ion currents are also very... 

Quantitative XRF analysis has developed from specific to universal methods. At the time of poor computational facilities, methods were limited to the determination of few elements in well-defined concentration ranges by statistical treatment of experimental data from reference material (linear or second order curves), or by compensation methods (dilution, internal standards, etc.). Later, semi-empirical influence coefficient methods were introduced. Universality came about by the development of fundamental parameter approaches for the correction of total matrix effects... 

Conventional XRF analysis uses calibration by regression, which is quite feasible for known matrices. Both single and multi-element standards are in use, prepared for example by vacuum evaporation of elements or compounds on a thin Mylar film. Comparing the X-ray intensities of the sample with those of a standard, allows quantitative analysis. Depending on the degree of similarity between sample and standard, a small or large correction for matrix effects is required. Calibration standards and samples must be carefully prepared standards must be checked frequently because of polymer degradation from continued exposure to X-rays. For trace-element determination, a standard very close in composition to the sample is required. This may be a certified reference material or a sample analysed by a primary technique (e.g. NAA). Standard reference material for rubber samples is not commercially available. Use can also be made of an internal standard,... 

A correction for matrix effects is usually required. Difficulties in PIXE quantitation may be relieved by complementary information from RBS [290], or FTIR and elastic backscattering (EBS) analysis [291], FTIR can give a rough estimation of the elemental composition, while EBS or RBS can deliver information on the major-element composition. 

All available methods (TG-MS, PyGC-MS and LDI-MS) suffer from difficult quantitation, although for different reasons. In TG-MS, selective volatilisation may not reflect the composition in the solid the quantitation problem of PyGC-MS requires assessment of the importance of matrix effects. Laser ablation methods cannot easily be calibrated. Quantitation is simplified in case of dual detection (MS for identification, FID for quantitation). A general drawback of many direct methods, which allow only small sampling volumes, is granule-to-granule variations. 

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