Lipids detectors

In practice a few iodine crystals are usually placed on the bottom of a dry, closed trough chamber. After the chamber has become saturated with violet iodine vapor the solvent-free plates are placed in the chamber for 30 s to a few minutes. The iodine vapor condenses on the TLC layers and is enriched in the chromatogram zones. Iodine vapor is a universal detector, there are examples of its application for all types of substances, e.g. amino acids, indoles, alkaloids, steroids, psychoactive substances, lipids (a tabular compilation would be too voluminous to include in this section). 

A method which uses supercritical fluid/solid phase extraction/supercritical fluid chromatography (SE/SPE/SEC) has been developed for the analysis of trace constituents in complex matrices (67). By using this technique, extraction and clean-up are accomplished in one step using unmodified SC CO2. This step is monitored by a photodiode-array detector which allows fractionation. Eigure 10.14 shows a schematic representation of the SE/SPE/SEC set-up. This system allowed selective retention of the sample matrices while eluting and depositing the analytes of interest in the cryogenic trap. Application to the analysis of pesticides from lipid sample matrices have been reported. In this case, the lipids were completely separated from the pesticides. 

The second most widely used detector in HPLC is the differential refractometer (RI). Being a bulk property detector, the RI responds to all substances. As noted in Table 3 the detection limits are several orders of magnitude higher than obtained with the UV detector. Thus, one turns to the RI detector in those cases in which substances are non-UV active, e.g. lipids, prostaglandins. In addition, the RI detector finds use in preparative scale operation. Finally, relative to the UV detector, the RI is significantly more temperature and flow sensitive and cannot be used in gradient elution. 

Biological, Chemical Reconnaissance System (JSLNBCRS), 5 832 Joint Service Standoff Chemical Agent Detector (JSLSCAD), 5 832 Joint ventures, 15 639-640 Joint Warning and Reporting Network (JWARN), 5 832 Jojoba oil, 26 212-213 Jojoba wax, cosmetically useful lipid, 7 833t... 

The sample is extracted with a mixture of hexane, acetone and water. After separation, the hexane phase is reduced in volume and divided into two aliquots, one of which is first shaken with 7% fuming sulphuric acid to remove lipids, and then with cyanide to eliminate interference by elemental sulphur. The other aliquot is evaporated to dryness and heated with ethanolic potassium hydroxide. The two aliquots are injected into a gas chromatograph fitted with a glass capillary column and an electron capture detector. Hexabromobenzene is used as an internal standard. Polychlorinated biphenyls are determined quantitatively by comparing the peaks of the sample with those of Clophen A... 

Gross contamination can overload the HPLC or GC columns with obvious and usually rapid deterioration of chromatographic performance. This can occur with so called rapid techniques where the detector is used as a filter, e.g., selected ion monitoring (SIM) MS, or where the clean-up method has been overloaded (e.g., excess of lipid). This problem can be overcome by using and monitoring more selective clean-up techniques. 

Evaporative light scattering is gaining popularity due to its ability to detect analytes on a nonse-lective basis. Basically, this detector works by nebulizing the column effluent, forming an aerosol that is further converted into a droplet cloud for detection by light scattering. This type of detector has been applied to studies of small molecule combinatorial libraries [13,14], carbohydrates [15], and lipids [16,17]. 

Lipid hydroperoxides are also generated in singlet molecular oxygen mediated oxidations and by the action of enzymes such as lipoxygenases and cyclooxygenases. Chemiluminescence (CL) arising from lipid peroxidation has been used as a sensitive detector of oxidative stress both in vitro and in vivo . Several authors have attributed ultra-weak CL associated with lipid peroxidation to the radiative deactivation of O2 and to triplet-excited carbonyls (63, 72) (equations 35 and 36) " . It has been proposed that the latter emitters arise from the thermolysis of dioxetane intermediates (61, 62) (equation 35), endoperoxide (73) (equation 37) and annihilation of aUtoxyl, as well as peroxyl radicals ... 

Selective detectors tend to be employed where the analyte is present in small amounts in a complex matrix such as in bioanalytical procedures where components extracted from the biological matrix along with the analyte can cause interference. Some formulated compounds have only very poor chromophores - these include sugars, lipids, surfactants, amino acids and some classes of drugs, e.g. a number of anticholinergic drugs lack chromophores. In these cases an alternative to UV detection has to be employed. 

Determination of CholesteroL For meat extraction, the procedures for determining the cholesterol of extracted lipid samples were described Chao et al. (2i). For edible beef tallow extraction, the preparation of samples for cholesterol content was based on the AOAC (22) method Section 28.110. The prepared sample was then injected into a Supelco SPB-1 fused silica capillary column of 30 meters x 032 mm i.d. in a Varian Model 3700 gas chromatograph equipped with dual flame ionization detectors. The initial holdup time was 4 min at 270°C and then programmed to a temperature of 300°C at a ramp rate of 10°C/min. Helium flow rate and split ratio were 13 ml and 50 1, respectively, while the injector/detector temperature was 310°C. 

The application of high-sensitivity ICP-MS detectors coupled to HPLC has enabled the detection of trace arsenic compounds present in marine animals. Thus, arsenocholine has been reported as a trace constituent (<0.1% of the total arsenic) in fish, molluscs, and crustaceans (37) and was found to be present in appreciable quantities (up to 15%) in some tissues of a marine turtle (110). Earlier reports (46,47) of appreciable concentrations of arsenocholine in some marine animals appear to have been in error (32). Phosphatidylarsenocholine 45 was identified as a trace constituent of lobster digestive gland following hydrolysis of the lipids and detection of GPAC in the hydrolysate by HPLC/ICP-MS analysis (70). It might result from the substitution of choline with arsenocholine in enzyme systems for the biogenesis of phosphatidylcholine (111). 

This book provides a practical guide to various aspects of lipid analysis, covering topics from sample preparation (extraction, fractionation, and deri-vatization) to CC analysis. Various derivatization methods are discussed and specific procedures are given for each of them. The book provides a comprehensive overview of GC technology including instrumentation (i.e., column, oven, carrier gas, injector, and detector) and data collection. 

Thin-layer chromatography (TLC) on silica gel is well known for its separation power for lipids and related compounds. The flame ionization detector (FID) is a universal analytical instrument that offers high sensitivity and linearity for carbon-containing organic compounds. The combination of TLC and FID led to the wide use of the Iatroscan TLC-FID for the analysis of lipid classes. The adoption of the Iatroscan TLC-FID in both academia and industry has generated sufficient data to indicate that TLC-FID is currently one of the most efficient tools for the quantitation of lipids classes (Ackman et al., 1990 Hammond, 1993). 

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