Sample matrix effect

Sources of Error. pH electrodes are subject to fewer iaterfereaces and other types of error than most potentiometric ionic-activity sensors, ie, ion-selective electrodes (see Electro analytical techniques). However, pH electrodes must be used with an awareness of their particular response characteristics, as weU as the potential sources of error that may affect other components of the measurement system, especially the reference electrode. Several common causes of measurement problems are electrode iaterferences and/or fouling of the pH sensor, sample matrix effects, reference electrode iastabiHty, and improper caHbration of the measurement system (12). 

Sample matrix effect Dilute matrix if possible, check pH of matrix, increase the ionic strength of the buffer, re-evaluate matrix... 

J.H. Skerritt and B.E. A. Rani, Detection and removal of sample matrix effects in agrochemical immunoassays, in Immunoassays for Residue Analysis, ed. R.C. Beier and L.H. Stanker, American Chemical Society, Washington, DC, pp. 29 3 (1996). 

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. 

ICP-AES and ICP-MS analyses are hampered in almost all cases by the occurrence of sample matrix effects. The origins of these effects are manifold, and have been traced partly to physical and chemical aerosol modifications inside sample introduction components (nebulisation effects). Matrix effects in ICP-AES may also be attributed to effects in the plasma, resulting from easily ionised elements and spectral background interferences (most important source of systematic errors). Atomic lines are usually more sensitive to matrix effects than are ionic lines. There exist several options to overcome matrix interferences in multi-element analysis by means of ICP-AES/MS, namely ... 

False-positive results with bDNA have been observed with proficiency testing specimens for HTV-1 in the College of American Pathologists HIV-1 viral load survey and HCV in the viral quality control program administered by the Netherlands Red Cross. The reason for the false-positive results with these proficiency testing specimens is not known but may be sample matrix effects. The extent to which this problem occurs with clinical samples has not been determined. However, both the HIV-1 and HCV bDNA assays were designed to have a false-positive rate of 5%. 

Insensitivity to variable sample matrix effects such as particle-size variations. 

Importantly, neither arsenobetaine nor arsenocholine forms an arsine on treatment with borohydride solutions. Consequently, arsenobetaine and arsenocholine may remain undetected in samples, seawater for example, when arsines are generated and determined in an arsenic speciation analysis. The technique HPLC/ICP-MS is most suitable for the analysis of these (non-arsine-forming) compounds (49). Use of the highly sensitive ICP-MS detector allows application of small quantities of material to the chromatography column, thereby obviating possible sample matrix effects previously observed for arsenobetaine (50). 

SFE manifests its best advantages when extracting analytes from solid and semisolid rather than liquid samples. A primary limitation in extracting analytes from liquid sample matrices is the mechanical difficulty of retaining the liquid matrix in the extraction vessel. To extract a liquid sample by SFE successfully, analysts must first mix it with a solid material, such as diatomaceous earth or alumina, so that the sample is no longer free-flowing. Control of sample matrix effects is critical in SFE to limit coextractives, moderate the influence of moisture, and improve the efficiency of the extraction. Recent studies have shown that the addition of both inert and active sorbents to the sample matrix can improve the efficiency of SFE (153). 

Figure 3. Sample matrix effect on reproducibility of internal standard retention times , standards in sample A, standards alone. (Reproduced with permission from reference 26. Copyright 1985 Lewis Publishers.)...

Sources of Error. Several common causes nl measurement problems are electrode interferences and/or fouling of the pH sensor, sample matrix effects, reference electrode instability, and improper calibration of the measurement system. 

The advantage to MHE is that sample matrix effects (which are mainly an issue only with solid samples) are eliminated since the entire amount of analyte is examined. This examination is done by performing consecutive analyses on the same sample vial. With the removal of each sample aliquot from the vial, the partition coefficient K will remain constant however, the total amount of analyte remaining in the sample will decline as each analysis is performed and more of the analyte is driven up into the vial headspace for removal and analysis. Chromatograms of each injection of sample show... 

The internal standard method of qualitative analysis is used to help identify components in a mixture when the reproducibility of retention times of the components in the sample versus the standard mixture is inadequate. The change in retention times from the standard mixture to the sample mixture is usually due to sample matrix effects that are difficult to mimic in the standard mixture. Thus, a small amount of a compound that is known to be absent from the sample is added to both the standard mixture and the sample prior to analysis. 

Ahnoff M, Wurzer A et al. (2003) Characterization of serum albumin and lysoPCs as major contributors to plasma sample matrix effects on electrospray ionization efficiency, Presented at the 2009 ASMS conference, Montreal, Canada... 

Physical and chemical effects can be combined for identification as sample matrix effects. Matrix effects alter the slope of calibration curves, while spectral interferences cause parallel shifts in the calibration curve. The water-methanol data set contains matrix effects stemming from chemical interferences. As already noted in Section 5.2, using the univariate calibration defined in Equation 5.4 requires an interference-free wavelength. Going to multivariate models can correct for spectral interferences and some matrix effects. The standard addition method described in Section 5.7 can be used in some cases to correct for matrix effects. Severe matrix effects can cause nonlinear responses requiring a nonlinear modeling method. 

In Sections 5.2.1 and 5.2.2, it was stated that the samples must be matrix-effect-free for univariate models, e.g., inter- and intramolecular interactions must not be present. The standard addition method can be used to correct sample matrix effects. It should be noted that most descriptions of the standard addition method in the literature use a model form, where the instrument response signifies the dependent variable, and... 

In summary, requirements for the univariate standard addition methods are that (1) the response for the analyte should be zero when the concentration equals zero (as well as for the matrix), (2) the response is a linear function of the analyte concentration, (3) sample matrix effects are independent of the ratio of the analyte and matrix, and perhaps most importantly, (4) a standard solution of only the analyte is available for the additions. 

The selection of an analytical technique and the development of procedures and operating conditions are only one part of the process for achieving reliable measurements. Among the first steps of the validation figure the assessment of the method precision this parameter must be assessed between analysts and days. Appropriate precision, although necessary for reliable work, is not sufficient to guarantee accurate results Evaluation of sample matrix effect, development, and analysis of spiked samples, analysis of Certified Reference Materials (CRM) of similar composition for which the Pb concentration has been established by other laboratories and comparison of results between laboratories (proficiency testing) are equally important [13]. 

In practice an instrumental detection limit is of limited use because in analytical chemistry it is rare that no other procedural steps are involved. Normally a limit of detection for the whole analytical method is required. The terminology used in this area is confusing. In general, limit of detection and detection limit are synonymous. The detection limit will encompass factors such as (a) sample matrix effects (b) loss of the analyte during sample preparation etc. The detection limit for the analytical procedure is defined as The minimum single result which, with a stated prohahility, can be distinguished from a suitable blank value . ... 

It is noted that the apparent dynamic range can be expanded at the high-concentration end via sample dilution provided that the dilution itself does not materially impact the response (e.g., via sample matrix effects). Additionally, a sample preparation process that accomplishes the concentration of the analyte may decrease a method s hmit of detectability. 

Unlike HPLC, sample preparation in CE requires careful thinking and strategy to obtain a good analysis. There is a relationship between the sample matrix and the separation buffer. Based on how the sample is prepared and how the separation buffer is selected, sample matrix effects can be both favorable and detrimental to the analysis. Matrix effects are different in capillary zone electrophoresis (CZE) from those observed in micellar electrokinetic capillary chromatography (MEKC). Understanding sample matrix effects is the first step in sample preparation in order to obtain a good separation by CE [1]. 

Another important aspect is that the sample matrix effect should ideally be maintained during the implementation of SAM. To this end, strategies involving a constant sample dilution during all of the measurement steps are preferred. 

SAM has been more widely exploited with detection techniques involving nebulisation (e.g., FAAS, ICP-OES and ICP-MS), as this step tends to be particularly affected by sample matrix effects, mainly salinity and viscosity. Relevant examples of SAM exploitation with spectrophotometry are given below. 

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