Food analysis sensitivity

In food analysis, sensitivity is not the only requirement for analytical method development. Besides confirmation of the identity of pesticides, the identification of nontarget analytes is also important. One powerful tool is LC/MS, especially when it is combined with appropiate sample-treatment procedures it allows one to obtain detection limits adequate for trace-level analysis. Liquid chromatography-MS has demonstrated that it is an effective way to obtain both qualitative and quantitative information. 

Coupled LC-LC can separate high-boiling petroleum residues into groups of saturates, olefins, aromatics and polar compounds. However, the lack of a suitable mass-sensitive, universal detector in LC makes quantitation difficult SFC-SFC is more suitable for this purpose. Applications of multidimensional HPLC in food analysis are dominated by off-line techniques. MDHPLC has been exploited in trace component analysis (e.g. vitamin assays), in which an adequate separation for quantitation cannot be achieved on a single column [972]. LC-LC-GC-FID was used for the selective isolation of some key components among the irradiation-induced olefinic degradation products in food, e.g. dienes and trienes [946],... 

The Kjeldahl total nitrogen determination method is not very sensitive, hut it suits well for analyzing insoluble samples without preceding disintegration. Automated Kjeldahl protein estimations are used especially in food analysis. 

Mass spectrometry has become an essential analytical tool for a wide variety of biomedical applications such as food chemistry and food analysis. Mass spectrometry is highly sensitive, fast, and selective. By combining mass spectrometry with HPLC, GC, or an additional stage of mass spectrometry (MS/MS), the selectivity increases considerably. As a result, mass spectrometry may be used for quantitative as well as qualitative analyses. In this manual, mass spectrometry is mentioned frequendy, and extensive discussions of mass spectrometry appear, for example, in units describing the analyses of carotenoids (unitfia) and chlorophylls (unit F4.5). In particular, these units include examples of LC/MS and MS/MS and the use of various ionization methods. 

Stability, duration, sensitivity, interference, and availability of substrates to contact enzymes are the criteria for the success of an enzyme sensor. These criteria depend on sources of enzymes, immobilization techniques, and transducers used. Food matrices are much more complicated than the clinical samples, hence, these criteria become extremely important for the application of the enzyme sensor in food analysis. An extensive list of the response time, detection limits, and stability of biosensors was summarized by Wagner (59). 

Table 13.2 summarises the different approaches used to construct enzyme electrochemical biosensors for application to food analysis based on the different types of enzymes available. Generally, the main problems of many of the proposed amperometric devices have been poor selectivity due to high potential values required to monitor the enzyme reaction, and poor sensitivity. Typical interferences in food samples are reducing compounds, such as ascorbic acid, uric acid, bilirubin and acetaminophen. Electrocatalysts, redox mediators or a second enzyme coupled reaction have been used to overcome these problems (see Table 13.2), in order to achieve the required specifications in terms of selectivity and sensitivity. 

Similar to tumor markers discussed above, a number of hazardous proteins are not immediately linked to an accompanying nucleic acid and therefore are prime targets for IPCR. These are interesting examples of how the clinical importance of IPCR is not limited to the diagnosis of diseases. IPCR was reported as a useful tool for the prevention of intoxications or infections because of highly sensitive detection of potentially dangerous compounds, especially in food analysis. 

As sample extraction and sample handling are of general consideration for the more exotic biological matrices often found in food analysis, the application of IPCR in the research project MYCOPLEX [89], founded by the European Union, promises interesting new developments. This project is dedicated to the detection by IPCR of ochra- and aflatoxins in milk and coffee, focusing on sensitivity and simplified antigen extraction by dilution of the samples. 

Chromatography techniques with different detectors followed by skillful sample preparation are usually applied to quantify these antioxidants in natural sources. These techniques offer sensitive and specific analysis methods for most of the antioxidants. This is the first book that particularly covers and summarizes the details of sample preparation procedures and methods developed to identify and quantify various types of natural antioxidants in plants and food products. In the book, the principle of quantification methods for natural antioxidant-rich phytochemicals is introduced and current methods used in the determination of antioxidants in different sources are reviewed and summarized by experts in the field. As a handbook of analysis of natural antioxidant-rich phytochemicals, the book provides useful information for many researchers in this area to learn ideal analysis methods for the antioxidants they are examining. The book may also serve as a lecture resource for courses in food analysis, functional foods, and nutrition. 

Table 9.3 lists some applications of this technique in food analysis. Julshairm et al. [95] developed a method of microwave mineralisation in a closed system for the GF AAS determination of As in seafood. They found the technique to be sufficiently sensitive to determine As at the level of 2.5 mg/kg dry mass. In contrast, Fedorov et al. [96] compared GF AAS with HG AAS in determinations of As and... 

HPLC is a versatile technique applicable to diversified analytes, including labile molecules, ions, organic, and biopolymers. This chapter provides an overview of HPLC applications for the analysis of food, environmental, chemical, polymer, ion-chromatography, and life science samples. In food analysis, HPLC is widely used in product research, quality control, nutritional labeling, and residual testing of contaminants. In environmental testing, HPLC is excellent for the sensitive and specific detection of labile and nonvolatile pollutants... 

Because of its high sensitivity for some compounds, its good selectivity, its ease of operation, the fact that it is non destructive, and that gradient elution is possible, UV-Vis detectors are nearly universal at low wavelength (200 nm). All these factors, taken together, account for the fact that the UV-Visible detector is the one most often used in the food analysis laboratory. 

Liquid membrane electrodes are subject to interferences from ions other than that of prime selectivity. For example, the Ca-ISE is also responsive to Mg2+ and Ba2+, the selectivity coefficients being approximately 0.01 for each ion. This indicates that the electrode is only 100 times more sensitive to Ca than to these ions, and this is normally much more important with respect to Mg than to Ba where food analysis is concerned. There are techniques which can be used to minimise the interference of Mg2+. 

Fluorescence spectroscopy is often used in analytical chemistry, food analysis, environmental analysis etc. It is a very sensitive spectroscopic technique which can be performed nondestructively and provides qualitative and quantitative information of diverse types of chemical analytes [Andersson Arndal 1999, Aubourg et al. 1998, Beltran et al. 1998a, Bright 1995, Bro 1999, Ferreira et al. 1995, Guiteras et al. 1998, Jensen et al. 1989, Jiji etal. 2000, Ross etal. 1991, Wolfbeis Leiner 1985], This application explains an example of estimating relative concentrations and pure analyte spectra from fluorescence measurements of chemical analytes in mixtures. Similar problems also arise frequently in other types of spectroscopy, in chromatography and other areas. Several names are used for this problem unmixing, curve resolution, source separation etc. Specifically, the application... 

Reliable methods are necessary for food analysis. The methods must assure both the best selectivity and the best sensitivity. As discussed earlier, spectrometric methods are used for environmental analysis because of the complexity of the matrix. In food analysis, the ratio between spectrometric methods and electrometric methods is 1 1 because the matrix is less complex. 

In conclusion, for food analysis it is possible to use spectrometric methods as well as electrometric methods. To obtain the most reliable information the complexity of the matrix, the nature of compounds, and the selectivity and sensitivity of the method must be reconciled. There is no single, universal method used for food analysis. Using the same method for food analysis may result in both the best and the poorest analytical information for the same compound depending on the matrix in which the compound is found. Variant results are obtained because of interferences that affect the selectivity of the method as well as its sensitivity. Therefore, the quantity of the compound in the matrix, whether it is a major compound or if it is found in trace amount, is very important. 

Another example connected with the sensitivity, selectivity, and complexity of the matrix is illustrated by the utilization of amperometric biosensors in chemical analysis. It is well known that amperometric biosensors represent the best equilibrium between selectivity and sensitivity needed for an analytical method. Their selectivity can be highly variable in a very complex matrix such as the environment. By using amperometric sensors, the total amount of substances from a certain class are determined. That is the reason these amperometric biosensors cannot assure the accuracy of the analytical methods for analysis of analytes in complex matrices. In food analysis, the complexity of the matrix decreases considerably. Therefore, amperometric biosensors can be used with higher accuracy for the assay of certain compounds. The main field of applicability of amperometric biosensors is clinical analysis, since the matrices in clinical analyses assure for amperometric biosensors the maximum selectivity. 

To select the best analytical method for the analysis of a sample it is necessary to consider the complexity of the matrix, because the complexity affects the selectivity of the measurement and the limits of detection and sensitivities of the methods. Only by adapting the selectivity and sensitivity of the method to the matrix and analyte concentration can the value of uncertainty be as a minimum. In this regard, some methods have been found more suitable for a particular field of analysis (e.g., environmental analysis, food analysis, clinical analysis) than for another. The high complexity of some matrices make analysis difficult, if not impossible, without a separation step. One must keep in mind that each step in the analytical process introduces an uncertainty. It follows that the greater the number of steps in an analytical process, the greater the total value of uncertainty. 

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