Residue analysis absorbed

Limits of detection become a problem in capillary electrophoresis because the amounts of analyte that can be loaded into a capillary are extremely small. In a 20 p.m capillary, for example, there is 0.03 P-L/cm capillary length. This is 1/100 to 1/1000 of the volume typically loaded onto polyacrylamide or agarose gels. For trace analysis, a very small number of molecules may actually exist in the capillary after loading. To detect these small amounts of components, some on-line detectors have been developed which use conductivity, laser Doppler effects, or narrowly focused lasers (qv) to detect either absorbance or duorescence (47,48). The conductivity detector claims detection limits down to lO molecules. The laser absorbance detector has been used to measure some of the components in a single human cell (see Trace AND RESIDUE ANALYSIS). 

A variety of formats and options for different types of applications are possible in CE, such as micellar electrokinetic chromatography (MEKC), isotachophoresis (ITP), and capillary gel electrophoresis (CGE). The main applications for CE concern biochemical applications, but CE can also be useful in pesticide methods. The main problem with CE for residue analysis of small molecules has been the low sensitivity of detection in the narrow capillary used in the separation. With the development of extended detection pathlengths and special optics, absorbance detection can give reasonably low detection limits in clean samples. However, complex samples can be very difficult to analyze using capillary electrophoresis/ultraviolet detection (CE/UV). CE with laser-induced fluorescence detection can provide an extraordinarily low LOQ, but the analytes must be fluorescent with excitation peaks at common laser wavelengths for this approach to work. Derivatization of the analytes with appropriate fluorescent labels may be possible, as is done in biochemical applications, but pesticide analysis has not been such an important application to utilize such an approach. 

Detection of peptides in HPLC can be achieved by measuring natural absorbance of peptide bonds at 200-220 nm. Unfortunately at these wavelengths a lot of food components and also the solvents used for analysis absorb, demanding an intensive sample pretreatment and clean-up [129]. Peptides with aromatic residues can be detected at 254 nm (phenylalanine, tyrosine, and tryptophan) or 280 nm (tyrosine and tryptophan). Taking advantage of the natural fluorescence shown by some amino acids (tyrosine and tryptophan), detection by fluorescence can also be used for peptides containing these amino acids [106]. 

The single most useful and versatile physicochemical detectors in drug residue analysis are probably those based on ultraviolet-visible (UV-Vis) spectrophotometry. These detectors allow a wide selection of detection wavelengths, thus offering high sensitivity for analytes that exhibit absorbance in either the ultraviolet or the visible region of the electromagnetic radiation. 

The most commonly used detector for pesticide residue analysis by LC is UV-VIS. It includes fixed wavelength and variable wavelength. Most OCPs and OPPs absorb appreciably only at wavelengths below 250 nm, the same spectral region where many solvents, solvent impurities, and matrix-derived interferences absorb. Analysis of these compounds by HPLC is still possible with very clean environmental substrates such as water. 

An Instron tensile machine can be used to measure stress and strain in the automotive lab. However, incoming inspection at a supplier is generally where this type of analysis takes place. More often, an automotive chemist at an OEM will be required to analyze a sample for failure analysis of that particular part. A thermogravimetric analyzer (TGA) will measure the change in mass of an elastomer in an inert (nitrogen) atmosphere as the temperature is waived. These instruments are very precise and will give information such as degradation temperature, filler content solvent residue, and absorbed moisture content. 

In addition to laboratoiy glassware and equipment necessary fOT cleanup of the extract, traditional pesticide residue methods require expensive chromatogrsqihic instrumentation for identification and quantitation of residues. EIA methods require minimal amounts of glassware, disposable plasticware, or other supplies. Quantitative EIAs often make use of 96-well microtiter plates fOT multiple simultaneous assays of about a dozen extracts and associated reference standard. Major equipment consists of a plate reader, which automatically measures the absorbance of each well. Plate readers can be used alone or in conjunction with a personal computer, which can correlate replicate measurements, construct the calibration curve, calculate results, and provide a complete statistical analysis. Such an EIA workstation can be obtained for roughly half the cost of the GC or HPLC system typically used for pesticide residue analysis. 

Citrus fruits were also the test materials in biphenyl residue analysis. The official AOAC method for determination of fungicide biphenyls in citrus fruits is based on steam distillation, TLC separation, and determination by UV spectrophotometry. Using a simpler method, biphenyls can be determined after extraction by UV absorbance on HPTLC plates. The reproducibility and specificity are comparable with that of the former method. The HPll C silica gel plates gave better results when they were prewashed with methanol-chloroform (1 1) solution. Less diffuse and darker zones were observed when biphenyl was analyzed on conventional silica gel or on C g chemically bonded reversed-phase layers (101) (Table 8). The average percentage of recovery results ranged between 92% and 98%. 

In the DPD colorimetric method for the free chlorine residual, which is reported as parts per million of CI2, the oxidizing power of free chlorine converts the colorless amine N,N-diethyl-p-phenylenediamine to a colored dye that absorbs strongly over the wavelength range of 440-580 nm. Analysis of a set of calibration standards gave the following results... 

Chemical Analysis. The presence of siUcones in a sample can be ascertained quaUtatively by burning a small amount of the sample on the tip of a spatula. SiUcones bum with a characteristic sparkly flame and emit a white sooty smoke on combustion. A white ashen residue is often deposited as well. If this residue dissolves and becomes volatile when heated with hydrofluoric acid, it is most likely a siUceous residue (437). Quantitative measurement of total sihcon in a sample is often accompHshed indirectly, by converting the species to siUca or siUcate, followed by deterrnination of the heteropoly blue sihcomolybdate, which absorbs at 800 nm, using atomic spectroscopy or uv spectroscopy (438—443). Pyrolysis gc followed by mass spectroscopic detection of the pyrolysate is a particularly sensitive tool for identifying siUcones (442,443). This technique rehes on the pyrolytic conversion of siUcones to cycHcs, predominantly to [541-05-9] which is readily detected and quantified (eq. 37). 

Residual aromatic ether concentrations are determined from the absorbance at 278 mfi of the crude reduction products in methanol solution. Steroidal ether concentrations of 1 mg/ml are employed. The content of 1,4-dihydro compound is determined, when possible, by hydrolysis to the a, -unsaturated ketone followed by ultraviolet analysis. A solution of the crude reaction product (usually 0.01 mg/ml cone) in methanol containing about 1/15 its volume of water and concentrated hydrochloric acid respectively is kept at room temperature for 2 to 4 hr. The absorbance at ca. 240 mfi is measured and, from this, the content of 1,4-dihydro compound can be calculated. Longer hydrolysis times do not increase the absorbance at 240 mp.. 

Calculation of ID using biological monitoring techniques requires the knowledge of the pharmacokinetics of the parent pesticide in laboratory animals. This will allow the use of the parent or its urine metabolite(s) to calculate the total amount of the parent that had been absorbed through the skin of the test subject. The amount of the residue in the urine should be corrected for any molecular weight differences between the parent and its urine metabolite(s) and also corrected for daily urine excretion volumes based on creatinine analysis of the urine samples. 

Area analysis of chromatograms is familiar to the chromatographer. The assumption is made that the area of a peak is proportional to the mass of the component present. The spectrophotometric detector is set at about 280 nm to detect the aromatic residues or at 210 to 230 nm to detect the peptide bond. The absorbance at 280 may be influenced by peptide composition, while the absorbance near 220 nm is more closely correlated to mass. 

Polymer/additive analysis then usually proceeds by separation of polymer and additives (cf. Scheme 2.12) using one out of many solvent extraction techniques (cf. Chapter 3). After extraction the residue is pressed into a thin film to verify that all extractables have been removed. UV spectroscopy is used for verification of the presence of components with a chromophoric moiety (phenolic antioxidants and/or UV absorbers) and IR spectroscopy to verify the absence of IR bands extraneous to the polymer. The XRF results before and after extraction are compared, especially when the elemental analysis does not comply with the preliminary indications of the nature of the additive package. This may occur for example in PA6/PA6.6 blends where... 

GC is extensively used to determine phenolic and amine antioxidants, UV light absorbers, stabilisers and organic peroxide residues, in particular in polyolefins, polystyrene and rubbers (cf. Table 61 of Crompton [158]). Ostromow [159] has described the quantitative determination of stabilisers and AOs in acetone or methanol extracts of rubbers and elastomers by means of GC. The method is restricted to analytes which volatilise between 160 °C and 300 °C without decomposition. A selection of 47 reports on GC analysis of AOs in elastomers (period 1959-1982) has been published... 

---