Dimethyl detection limits

Another way in which to gain structural information concerning the N-terminal residue of glycophorins A" and A is to study the N-terminal, mono[ C]methyl derivatives these are produced by using limited amounts of [ C]formaldehyde. There are distinct differences between the N, N -di[ C]methylamino and N -mono[ C]methylamino species (i) a significant, chemical-shift difference exists between the N-terminal dimethyl and monomethyl species (43 and 34 p.p.m.) (li) all of the C resonances of the N-terminal dimethyl species move upheld as the pH is increased (if they move at all), whereas all of the C resonances of the N-terminal, monomethyl species move downfield as the pH is increased and (in) A for the N-terminal monomethyl species tends to be much larger than that for the N-terminal dimethyl species. Point (in) would tend to indicate that it may be more advantageous to study the N-terminal monomethyl species. However, because of allowable protein concentrations, detection limits on available instruments, and technical difficulties, it has thus far... 

Amarnth and Amamth [15] described a specific method for the determination of penicillamine and cysteine. Treatment with 1,1-thiocarbonyl diimidazole converts penicillamine to 5,5-dimethyl-2-thioxothiazolidine-4-carboxylic add. The detection limit is 2 pmol of the drug per injection, and the detector response is linear up to 1 nmol. 

MWNTs favored the detection of insecticide from 1.5 to 80 nM with a detection limit of InM at an inhibition of 10% (Fig. 2.7). Bucur et al. [58] employed two kinds of AChE, wild type Drosophila melanogaster and a mutant E69W, for the pesticide detection using flow injection analysis. Mutant AChE showed lower detection limit (1 X 10-7 M) than the wild type (1 X 10 6 M) for omethoate. An amperometric FIA biosensor was reported by immobilizing OPH on aminopropyl control pore glass beads [27], The amperometric response of the biosensor was linear up to 120 and 140 pM for paraoxon and methyl-parathion, respectively, with a detection limit of 20 nM (for both the pesticides). Neufeld et al. [59] reported a sensitive, rapid, small, and inexpensive amperometric microflow injection electrochemical biosensor for the identification and quantification of dimethyl 2,2 -dichlorovinyl phosphate (DDVP) on the spot. The electrochemical cell was made up of a screen-printed electrode covered with an enzymatic membrane and combined with a flow cell and computer-controlled potentiostat. Potassium hexacyanoferrate (III) was used as mediator to generate very sharp, rapid, and reproducible electric signals. Other reports on pesticide biosensors could be found in review [17],... 

Leek and Bagander [221] determined reduced sulfide compounds (hydrogen sulfide, methyl mercaptan, carbon disulfide, dimethyl sulfide, and dimeth-yldisulfide) in water by gas chromatography using flame detection. Detection limits ranged from 0.2 ng/1 for carbon disulfide to 0.6 ng/1 for methyl mercaptan. Hydrogen sulfide was determined at the 1 ng/1 level. 

Leek and Baagander [311] determined reduced sulfide compounds in seawater by gas chromatography using a flame ionisation detector. Substances determined include methyl mercaptan, dimethyl sulfide, hydrogen sulfide and carbon disulfide. Detection limits range from 0.2ng/l (carbon disulfide) to 0.6 ng/1 (methyl mercapton). 

One of the few zwitterionic surfactant used in CL reactions is /V-dodccyl-MA-dimethyl-ammonium-3-propane-l-sulfonate (SB-12). Particularly, SB-12 has been assayed in the study of the CL reaction of lucigenin with various biological reductants [10]. The results show that SB-12 enhances CL intensity of the luci-genin-glucose and lucigenin-fructose systems by factors of 3.0 and 1.5, respectively, compared to the intensity obtained in aqueous medium. In these conditions detection limits were found for-both analytes of 0.7 X 10 4 and 2.5 X 10 5 M, respectively. 

A high-speed sensor for the assay of dimethyl sulfide in the marine troposphere based on its CL reaction with F2 was recently reported [18]. Sample air and F2 in He were introduced at opposite ends of a reaction cell with a window at one end. The production of vibrationally excited HF and electronically excited fluorohydrocarbon (FHC) produced CL emission in the wavelength range 450-650 nm, which was monitored via photon counting. Dimethyl sulfide could be determined in the 0-1200 pptv (parts per trillion by volume) concentration range, with a 4-pptv detection limit. 

For the cationic surfactants, the available HPLC detection methods involve direct UV (for cationics with chromophores, such as benzylalkyl-dimethyl ammonium salts) or for compounds that lack UV absorbance, indirect photometry in conjunction with a post-column addition of bromophenol blue or other anionic dye [49], refractive index [50,51], conductivity detection [47,52] and fluorescence combined with postcolumn addition of the ion-pair [53] were used. These modes of detection, limited to isocratic elution, are not totally satisfactory for the separation of quaternary compounds with a wide range of molecular weights. Thus, to overcome the limitation of other detection systems, the ELS detector has been introduced as a universal detector compatible with gradient elution [45]. 

Dabsyl Chloride (4-dimethyl-aminoazobenzene-4-sulfonyl chloride) Aabs = 420 nm. Derivatives are very stable (days) and can be formed from both primary and secondary amino acids. Detection is by absorption only. Detection limits are in the high picomole range. Reaction time is typically around 10 minutes at 70°C. Completeness of reaction can be adversely affected by the presence of high levels of various salts. Because reaction efficiency is highly matrix dependent and variable for different amino acids, standard amino acid solution should be derivatized under similar conditions/matrix for accurate calibration. Commercial systems available uti-... 

Bockhardt et al. (78) derived a manual extraction and automated [4-(4-dimethyl-aminophenylazo)benzensulfonyl] derivatization procedure from the method previously proposed by Krause et al. (44). Reversed-phase HPLC was carried out on a Spherisorb ODS-2 column at 50°C with gradient elution and detection at 436 nm, reaching detection limits (DLs) of 0.3-0.8 pmol. 

A sensor for organic chloride-containing compounds was constructed by immobilization of luminol or tris (2,2 bipyridyl) ruthenium (III) between a PMT and a poly (tetrafluoro) ethylene (PTFE) membrane [15], through which a stream of air was sampled by diffusion. A heated Pt filament incorporated in the gas line leading to the CL cell was used to oxidize the analytes prior to diffusion across the PTFE membrane. Detection limits for CC14, CHC13, and CH2C12 were 1.2-4 ppm. A similar device could also be used for the determination of hydrazine and its monomethyl and dimethyl derivatives or NH3 vapor. The detection limit for hydrazine was only 0.42 ppb [16]. 

Another interesting application of MDGC is in the rapid determination of isoprene (the most reactive hydrocarbon species) and dimethyl sulfide (DMS) (the major source of sulfur in the marine troposphere and a precursor to cloud formation) in the atmosphere (16). The detection limits were 5 and 25 ng F1, respectively. 

The insecticide acephate, applied as a water spray has a very short life in terns of biological activity and for this, as well as economic reasons, it is not used on large scale operations in Canada (12). Residue analysis of acephate foliar deposits (13) showed that more than half of the insecticide was lost within one day of spraying, and that by 32 days post spray, the amount of insecticide had decreased to less than 0.01 ppm (the detection limit for 20 gm of substrate). The metabolite of acephate known as Ortho 9006 (0,S-dimethyl phosphoramidothioate) was also assessed in this study but was found to be present only in very small amounts. At 2 hours post spray the average acephate concentration on spruce foliage was 55.15 ppm, and the average concentration of the Ortho 9006 was 0.12 ppm. There was no increase with time in the amount of the metabolite. The rapid... 

Verwej et al. [175] have described a procedure for the determination of PH3-containing insecticides in surface water. In this procedure the insecticide is hydrolysed to methylphosphonic acid, and the acid is concentrated by anion exchange and converted to the dimethyl ester. After clean-up on a microsilica gel column the ester is analysed by gas chromatography using a thermionic phosphorus-specific detector. Detection limit is lnmol L 1. 

Attempts to develop a GC/MS method for this metabolite were unsuccessful, no doubt because of thermal instability. An LC/MS/MS method using thermospray ionization, after derivatization to the dimethyl ester, gave a modest detection limit of 25 ng/ml, again probably due to poor thermal stability, in this case, in the thermospray ion source (12). A substantial improvement has recently been achieved (detection limit 1 ng/ml) using LC/ESI/MS/MS without derivatization (31). Concentration from acidified urine was achieved on... 

We have described elsewhere the nature of the MI FTIR spectra of PAHs and their derivatives (1, 12, 13, 1J>, 18, 21-24) the following points are especially significant. First, FTIR spectra devoid of rotational structure and having individual bandwidths on the order of 2-7 cm are obtained both for PAHs and for polar derivatives thereof (such as nitrogen heterocycles) These spectra are sufficiently characteristic to enable identification of individual isomers to be made in mixtures [e.g., the six methylchrysenes (12) and the various mono- and dimethyl naphthalenes and biphenyls (24)]. Second, detection limits for individual PAHs can be as low as 50 ng, if special "micro samp ling 1 deposition apparatus is used (23) Third, Beer s law plots typically are linear over 1.5-2 decades in PAH concentration by the complementary use of two different deposition cells, linearity over 3 decades in PAH concentration for Beer s law plots can be approached (23). Finally, for both MI FTIR and MI fluorescence spectrometry, analytical precision of ca. 3-7 % relative standard deviation can be achieved. 

Other workers [358] carried out the acylation of estrogens in acetone and reported the following conditions as optimal for the preparation of tris-HFB-estriol 0.1—0.3 jul of acetone per 1 jug of the substrate and 0.05 ml of HFB anhydride at room temperature for 10 min. The use of a larger amount of another solvent (benzene, methylene chloride, dimethyl sulphoxide, diethyl ether, dioxane) was said to result in the formation of a number of by-products. Poole and Morgan [359], however, stated that the HFB derivatives of some steroids are not thermally stable and that only the decomposition product is detected, e.g., cholesta-3,5-diene is produced from cholesterol. This leads to a considerably lower ECD response, so that the detection limit, which under favourable circumstances can be as low as 0.005 ng, is usually not achieved. As steroids that form unstable HFB derivatives they reported cholesterol, lumisterol, ergosterol, estradiol, pregnanetriol and others. 

Quantification. Gas Chromatography. In plasma nifedipine and dimethyl 2,6-dimethyl-4-(2-nitrophenyl)pyridine-3,5-dicar-boxylate, detection limit 1 ng/ml, ECD—J. Dokladalova et al, J. Chromat., 1982,231, Biomed. AppL, 20,451-458. 

In an important study on alcoholic drinks presented to the OIV, Bertoli et al. (2004) studied different phthalates (dimethyl, diethyl, diisobutyl, dibutyl, benzyl, butyl, bi 2-ethylhexyl, dioctyl). They detected the constant presence of these compounds in alcoholic drinks due to environmental contamination and recommend that special care be taken when choosing contact materials (for pipes, tanks, etc.). In any case, the levels present in wines rarely exceed tens of p.g/L for dibutyl phtha-late they are lower than 10 p.g/L and for the other phthalates are often below the detection limit. 

A fluorodensitometric assay was developed by Montague and co-workers 140) to analyze cultures from Ochrosia elliptica for ellipticine (1), 9-methoxy-ellipticine (2), and 9-hydroxyellipticine (3) by thin-layer chromatography (TLC) without the need for prior purification. Using silica gel impregnated with dimethyl sulfoxide and a mobile phase of EtOAc-water-l-octanol (17 2 2), these workers were able to achieve good separation of these alkaloids and to assay the resulting chromatograms by fluorodensitometry (40-300 fmol detection limits of alkaloid). 

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