Complex sample matrices

Spectrofluorometry presents sensitivity and selectivity greater than the absorbance spectroscopy, being more suitable for chlorophyll estimates in the nmol range and for residual amounts of derivatives in food products. Absorbance spectroscopy is satisfactory for concentrations > 1 xMP Spectrofluorometry is also more accurate for a wide range of chlorophyll a-to-chlorophyll b ratios, but it is less accurate when applied to complex sample matrices because of unpredictable quenching effects. 

Specifically for triazines in water, multi-residue methods incorporating SPE and LC/MS/MS will soon be available that are capable of measuring numerous parent compounds and all their relevant degradates (including the hydroxytriazines) in one analysis. Continued increases in liquid chromatography/atmospheric pressure ionization tandem mass spectrometry (LC/API-MS/MS) sensitivity will lead to methods requiring no aqueous sample preparation at all, and portions of water samples will be injected directly into the LC column. The use of SPE and GC or LC coupled with MS and MS/MS systems will also be applied routinely to the analysis of more complex sample matrices such as soil and crop and animal tissues. However, the analyte(s) must first be removed from the sample matrix, and additional research is needed to develop more efficient extraction procedures. Increased selectivity during extraction also simplifies the sample purification requirements prior to injection. Certainly, miniaturization of all aspects of the analysis (sample extraction, purification, and instrumentation) will continue, and some of this may involve SEE, subcritical and microwave extraction, sonication, others or even combinations of these techniques for the initial isolation of the analyte(s) from the bulk of the sample matrix. 

The main limitation of TLC is its restricted separation efficiency. The separating efficiency (in terms of plates per metre) decreases rapidly over long development distances. That is, highest efficiencies are only achievable within a development distance of approximately 4-7 cm. Therefore, the total number of theoretical plates achievable on an HPTLC plate is limited (about 5000) and inferior to long LC or GC columns. Consequently, complex separations of many compounds are usually not achievable by means of HPTLC. This method is most useful for quantitating only a few components in simple or complex sample matrices. The efficiencies can also be reduced if the plate is overloaded, in an attempt to detect very trace components in a sample. 

More elaborate sample preparation is often needed for complex sample matrices, e.g., lotions and creams. Many newer SP technologies such as solid-phase extraction (SPE), supercritical fluid extraction (SFE), pressurized fluid extraction, accelerated solvent extraction (ASE), and robotics are frequently utilized (see Ref. [2]). Dosage forms such as suppositories, lotions, or creams containing a preponderance of hydrophobic matrices might require more elaborate SP and sample cleanup, such as SPE or liquid-liquid extraction. 

Stevens, K. A., and Jaykus, L. A. (2004). Bacterial separation and concentration from complex sample matrices A review. Crit. Rev. Microbiol. 30, 7-24. 

Delaunay, N., Pichon, V, and Hennion, M. C., Immunoaffinity solid-phase extraction for the trace-analysis of low-molecular-mass analytes in complex sample matrices. Journal of Chromatography. B, Biomedical Sciences and Applications 745(1), 15-37, 2000. 

The most useful chemical species in the analysis of arsenic is the volatile hydride, namely arsine (AsH3, bp -55°C). Analytical methods based on the formation of volatile arsines are generally referred to as hydride, or arsine, generation techniques. Arsenite is readily reduced to arsine, which is easily separated from complex sample matrices before its detection, usually by atomic absorption spectrometry (33). A solution of sodium borohydride is the most commonly used reductant. Because arsenate does not form a hydride directly, arsenite can be analyzed selectively in its presence (34). Specific analysis of As(III) in the presence of As(V) can also be effected by selective extraction methods (35). 

The compounds MMA, DMA, and TMAO are reduced in acidic aqueous media by borohydride solutions to methylarsine (MeAsH2, bp 2°C), dimethylarsine (Me2AsH, bp 35°C), and trimethylarsine (Me3As, bp 55°C), respectively. These products are useful derivatives for speciation analysis of arsenic because they are readily separated from complex sample matrices and may be further separated from each other by distillation (41) or by gas chromatography (42) prior to their determination by element-specific detectors. Consequently, arsine generation techniques are the most commonly used methods for determining MMA, DMA, and TMAO in marine samples. 

The technique of voltammetric stripping analysis is one of the most sensitive techniques available for the determination of metal ions in complex sample matrices. Traditionally such techniques have been undertaken at Hg electrodes, but in recent years, a large number of reports have focused on the use of SPCEs in this area. Reports prior to 2003 have been reviewed recently by the present authors [3]. 

Although composed of weak and overlapping spectral features, near-infrared spectra can be used to extract analytical information from complex sample matrices. Chemical sensing with in-line near-infrared spectroscopy is a general technique that can be used to quantify multiple analytes in complex matrices, often without reagents or sample pretreatment.7-9 Applications are widespread in the food sciences, agricultural industry, petroleum refining, and process analytical chemistry.10-13 These activities demonstrate that near-infrared spectroscopy can provide selective and accurate quantitative measurements both rapidly and nondestructively. 

The use of ICP-MS for the analysis of foods has been reviewed recently [270]. Food analysis can provide information on potentially toxic elements, nutrient elements, or geographical origin of the food. The application of ICP-MS to experimental nutrition has recently been reviewed [271]. The importance of quality control for multielement analysis of complex sample matrices like foods by ICP-MS was shown [272]. 

GC/MS/MS also allows the determination of scheduled chemicals in complex sample matrices with great selectivity and at low detection levels. Information by MS/MS can be obtained either by recording product (parent-daughter) ion spectra of a selected mass or by performing single reaction monitoring (SRM) or multiple reaction monitoring... 

Usually, in LS AAS, the most sensitive analytical line is used for the determination of an element, because AAS is mostly applied for trace and ultra-trace analysis, which obviously requires the highest sensitivity. Another reason for using the most sensitive line is that it makes it possible to apply higher dilution in case of complex sample matrices, and hence avoid potential interferences. On occasions, however, the most sensitive line is not recommended in LS AAS, as it does not provide the best SNR, as in the case with the 217.001 nm Pb line. Another reason might be a strongly nonlinear working curve due to the presence of other lines in the lamp spectrum that cannot be excluded even with a 0.2 nm bandwidth [3]. 

With the availability of less expensive and more dependable commercial instruments, liquid chromatography coupled to mass spectrometry is quickly becoming the industry standard. However, the role of electrochemistry in pharmaceutical analysis has been well defined, and will likely continue to be preferentially employed in applications where low analyte concentrations, small sample volumes, or complex sample matrices requiring high specificity challenge the analytical method. 

Sample clean-up Column chromatography, HPLC, GC, TLC, GPC To fractionate and enrich analytes from complex samples matrices... 

The next chapter introduces chromatographic techniques for analyzing complex samples, whereby multiple analytes are separated on a column and detected as they emerge from the column. But very often, samples need to be cleaned up prior to introduction into the chromatographic colunm. The techniques of solvent extraction and solid-phase extraction and related techniques are very useful for isolating analytes from complex sample matrices prior to chromatographic analysis. Solvent extraction is also useful for spectrophotometric determination. 

For extremely complex sample matrices, it may not be possible to make external standards with a similar matrix. In this case, the MSA should be used for quantitative analysis. The use of standard additions can correct for some types of interference but not for spectral interference. 

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