Detection in food analysis

Table 7.2. Summary of HPLC Modes and Detection in Food Analysis...

M. Cai eii and A. Mangia, Multidimensional detection methods for sepai ations and their application in food analysis . Trends Anal. Chem. 15 538-550 (1996). 

On-line coupled LC-GC methods have been developed in food analysis for several reasons, i.e. lower detection limits can be reached, the clean-up is more efficient, and large numbers of samples can be analysed with a minimum of manual sample preparation in shorter times. 

Mirex residues were detected in food samples analyzed as part of the FDA Pesticide Residue Monitoring Studies conducted from 1978--1982 of 49,877 food samples and from 1982-1986 of 49,055 food samples however, the frequency of detection was unspecified but was <1 and 2% respectively (Yess et al. 1991a, 1991b). A similar 1985 analysis of foods grown in Ontario, Canada, failed to detect any mirex or photomirex in any of the vegetable, fruit, milk, egg, or meat products tested (Davies 1988). Mirex was also detected in the FDA Pesticide Residue Monitoring Study from 1986-1987 however, the frequency of detection was unspecified but less than 1% (FDA 1988). 

Vitamin D2 and D3 exhibit identical UV absorption spectra and they do not possess fluorescence. Electrochemical detection is limited and only few methods are applied in food analysis [530,533], MS detection has been applied achieving satisfactory detection limit (10 mol/mL) [534,535],... 

The limit of quantification is more relevant than the limit of detection in the analysis of drug residues in foods. In these applications, the limit of quantification can be more practically defined as the lowest drug concentration in food samples that can be measured with a desired level of accuracy and precision. It is usually determined by reducing the analyte concentration until a level is reached where the precision of the assay becomes unacceptable. If the required precision of the method at the limit of quantification has been specified, a number of samples with decreasing amounts of the analyte are analyzed 6 times at minimum, and the calculated RSD% of the precision is plotted against the analyte amount the amount that corresponds to the previously defined required precision is equal to the limit of quantification. 

Caboni, M.F. and Rodriguez-Estrada, M.T. 1997. High-performance liquid chromatography coupled to evaporative light scattering detection in lipid analysis Some application. Seminars in Food Analysis 2 159-169. 

Detection of amino acids is typically by UV absorption after postcolumn reaction with nin-hydrin. Precolumn derivatization with ninhydrin is not possible, because the amino acids do not actually form an adduct with the ninhydrin. Rather, the reaction of all primary amino acids results in the formation of a chromophoric compound named Ruhemann s purple. This chro-mophore has an absorption maximum at 570 nm. The secondary amino acid, proline, is not able to react in the same fashion and results in an intermediate reaction product with an absorption maximum at 440 nm. See Fig. 5. Detection limits afforded by postcolumn reaction with ninhydrin are typically in the range of over 100 picomoles injected. Lower detection limits can be realized with postcolumn reaction with fluorescamine (115) or o-phthalaldehyde (OPA) (116). Detection limits down to 5 picomoles are possible. However, the detection limits afforded by ninhydrin are sufficient for the overwhelming majority of applications in food analysis. 

In general it has to be stated that molecular species analysis of phospholipids is not frequently applied in food analysis most of the studies involving molecular species are instead found in the fields of biochemistry and nutrition. Thus, in the recent reviews by Bell and by Olsson and Salem, special emphasis has been given to the characterization of biological tissue samples (83,84). However, the molecular species composition has been shown to affect the accuracy of the quantification of phospholipid classes and hence is important in food analysis too (47,52). In the vast majority of published methods, isocratic elution has been used. In our opinion, this should be ascribed mainly to the fact that the traditional UV detector remains. Keeping account of the inherent problems associated with UV detection of underivatized phospholipids, it is astonishing that ELSD has hardly been exploited in this subdomain. As far as the stationary phase is concerned, nearly all methods prefer octadecyl-coated stationary phases. 

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. 

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). 

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. 

Nielsen, I.L.F., Nielsen, S.E., Ravn-Haren, G., and Dragsted, L.O., Detection, stability and redox effects of black currant anthocyanin glycosides in vivo positive identification by mass spectrometry, in Biologically-Active Phytochemicals in Food Analysis, Metabolism, Bioavailability and Function, Pfannhauser, W., Fenwick, G.R., Khokhar, S., Eds., Royal Society of Chemistry, Cambridge, U.K., 2001, pp. 389-393. 

Refractive index monitors are used in food analysis, for detecting carbohydrates, alcohols, and other substances with weak or no UV absorption. 

Ito, Y. Ikai, Y. Oka, H. Matsumoto, H. Kagami, T. Takeha, K. Application of ion-exchange cartridge clean-up in food analysis, in. Determination of henzylpenicillin, phenox3methylpenicilhn, oxacillin, cloxacillin, nafcillin, and dicloxacillin in hovine hver and kidney hy liquid chromatography with ultraviolet detection. J. Chromatogr., A 2000, 880, 85-91. 

Methods for the determination of fonnaldehyde in drinking water are available and they utilize the same detection methods as those utilized for the analysis of formaldehyde in air, with LODs reported to be 20 ppb (Tomkins et al. 1989) and 8.1 ppb (EPA 1992b). The MRL for chronic oral exposure to formaldehyde is 0.2 mg/kg/day. If a 70-kg person is assumed, the maximum intake is 14 mg/day. If a daily intake of 2 L of water or 2 kg/day of food per day is assumed, then any analytical method must have an LOD of less than 7 mg/L for water or 7 mg/kg (ppm) for food. The cited methods for detecting formaldehyde in water have LODs far below the needed value and are sensitive enough to measure background levels in the environment no additional methods for formaldehyde detection in water are required. Other than for milk (Kaminski et al. 1993b, LOD=9 ppb), no methods for formaldehyde detection in food were found. Additional methods for detection of formaldehyde in foods are needed. Methods for the detection of formaldehyde in soil are not adequately described in the available literature. 

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