Retention times pesticide

A chromatographic analysis for the chlorinated pesticide Dieldrin gives a peak with a retention time of 8.68 min and a baseline width of 0.29 min. How many theoretical plates are involved in this separation Given that the column used in this analysis is 2.0 meters long, what is the height of a theoretical plate ... 

Identification of the pesticides is based on retention time on at least two dissimilar glc columns. A nonpolar and a relatively polar packing are generaUy used, for example, OV-17 and a mixture of QE-1 and DC-200. 

Figure 3.26 Reconstructed ion chromatograms obtained from the ions of m/z 199, 163 and 285 for retention times between 3.95 and 6.65 min in the LC-MS analysis of a pesticide mixture. From applications literature published by Micromass UK Ltd, Manchester, UK, and reproduced with permission.

G.l.c. studies of tributylphosphine, dialkyl phosphites, and dialkyl alkylphosphonates are reported. Tributyl phosphate in nitric acid can be estimated by g.l.c. if a glass column is used. Tetraethyl pyrophosphate has been directly determined on a nanogram scale by g.l.c., whereas it was found most convenient to first convert the tetra-aryl pyrophosphates by methanolysis to diarylmethyl phosphates. Phosphorochloridates were converted by t-butyl alcohol into t-butyl chloride before analysis. G.l.c. studies of pesticides have been reported and the isomeric thiophosphates (138a) and (138b) have quite different retention times. ... 

Multiresidue analysis of 72 pesticides including three diphenyl ethers was carried out by GC/NPD under the following conditions column, 5% DB-5 (30 m x 0.53-mm i.d., 0.53- xm film thickness) temperature, column 100°C(1 min) increased at 5 °Cmin to 280°C (lOmin), inlet and detector 280°C gas flow rates. He 11.2mLmin H2 3.5mLmin air llOuiLmin" injection volume, 2 rL. The retention times... 

The residue levels of 46 pesticides, including oxyfluorfen in soil, were determined using GC/ITDMS as described in S ection 3.2.1. The conditions for GC/ITDMS were as follows column, fused-silica capillary (30 m x 0.25-mm-i.d.) with a0.25- am bonded phase ofDB-5 column temperature, 50 °C (1 min), 30 °Cmin to 130 °C, 5 °C min to 270 °C inlet and transfer temperature, 270 and 220 °C, respectively He gas with column head pressure, 12psi injection method, splitless mode. The retention time and quantitation ion of oxyfluorfen were 23.9 min and mjz 252, respectively. ... 

Polar or thermally labile compounds - many of the more modern pesticides fall into one or other of these categories - are not amenable to GC and therefore LC becomes the separation technique of choice. HPLC columns may be linked to a diode-array detector (DAD) or fluorescence detector if the target analyte(s) contain chromophores or fluorophores. When using a DAD, identification of the analyte(s) is based on the relative retention time and absorption wavelengths. Similarly, with fluorescence detection, retention time and emission and absorption wavelengths are used for identification purposes. Both can be subject to interference caused by co-extractives present in the sample extract(s) and therefore unequivocal confirmation of identity is seldom possible. 

MS detection does not necessarily require as highly resolved GC separations as in the case of selective detectors because the likelihood of an overlapping mass spectral peak among pesticides with the same retention time is less than the likelihood of an overlapping peak from the same element. Unfortunately, this advantage cannot always be optimized because SIM and current gas chromatography/tandem mass spectrometry (GC/MS/MS) methods, it is difficult to devise sequential SIM or MS/MS retention time windows to achieve fast GC separations for approximately > 50 analytes in a single method. 

A polyethylene-coated (PEE) silica column was used with water-methanol eluents to achieve the separation and retention of 27 pesticides.40 The retention times of 33 commercial pesticides were determined on an octadecyl (ODS)-derivatized alumina column using water-methanol eluents and compared with retention properties on an ODS-silica column packing.41 More recently, RP-HPLC was used in combination with diode array detection for the identification and quantification of 77 pesticides (acidic, basic, and neutral) in groundwater samples.42... 

One of the known disadvantages of the use of GC is the need for previous derivatization of some of the most polar pesticides before analysis can be carried out [40]. These derivatization steps might produce low-efficiency results in complex wastewater matrices, which make the analysis rather difficult and cumbersome. However, the reproducibility in retention times when using GC techniques is so precise, that specific identifications of pesticides can be made even in complex environmental samples. 

Interpretation/report The GC retention time of a naphthalene standard and the mass spectrum of this peak confirm its presence. Because of the complexity of the chromatograms of the petroleum products and the pesticide sample, you find it impossible to examine the chromatogram of each. However, a comparison of the GC fingerprints (i.e., the matching of chromatographic peaks and comparison of peak ratios) clearly shows that the sample consists of naphthalene dissolved in kerosene. 

Fig. 21.14. Temperature-programmed capillary GC-MS total ion chromatograms for kerosene (upper trace) and Moth-Knox pesticide (lower trace). Note the similarity in the pattern of peaks, with the exception of the large peak in the pesticide sample (at a retention time of about 12.5 min). The mass spectrum and the retention time of this peak both corresponded to a standard of naphthalene.

By comparing the two adsorbents, it turns out that the non-polar character of the petroleum ether is exploited in the extraction from the strong nonspecific active sites of carbon black. On the other hand, the polar character of acetone makes the extraction from the highly strong specific active sites of the siliceous material of the soil possible. The retention time obtained with the mixture is intermediate between those two separate solvents, and the recovery, in turn, is higher. The proper choice of the solvent mixture plays a very important role on the size of the final volume of solution in which the pesticides are collected. 

Ripley BD, Braun HE. 1983. Retention time data for organochlorine, organophosphorus, and organonitrogen pesticides on SE-30 capillary column and application of capillary gas chromatography to pesticide residue analysis. J Assoc Off Anal Chem 66(5) 1084-1095. 

Omura M, Hashimoto K, Ohta K, et al. 1990. Relative retention time diagram as a useful tool for gas chromatographic analysis and electron-capture detection of pesticides. J Assoc Off Anal Chem 73(2) 300-306. 

Table III. Relative Retention Times of Various Pesticides or Their Methyl Esters (M.E.) on a SE-54 Capillary Column...

The slope of the lines presented in Figure 5 is defined as k(q/v). The q/v term defines the turnover of the tank contents or what is commonly referred to as the retention time. When q is increased, the liquid contacts the carbon more often and the removal of pesticides should increase, however, the efficiency term, k, can be a function of q. As the waste flow rate is increased, the fluid velocity around each carbon particle increases, thereby increasing system turbulence and compressing the liquid boundary layer. The residence time within the carbon bed is also decreased at higher liquid flow rates, which will reduce the time available for the pesticides to diffuse from the bulk liquid into the liquid boundary layer and into the carbon pores. From inspection of Table II, the pesticide concentration also effects the efficiency factor, k can only be determined experimentally and is valid only for the equipment and conditions tested. 

In 1974, federally recommended procedures were published under authority of the 1972 amendments of the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) that addressed pesticide disposal (2). These recommendations identified an incinerator operating at 1000°C (1832°F) with 2-s retention time in the combustion zone as acceptable for destruction of organic pesticides. Other incinerators, such as those for municipal solid waste capable of effecting complete pesticide destruction, are also acceptable. During this same time frame, i.e., from the early 1970s to date, a number of research and demonstration studies have been conducted involving pesticide incineration. Most of these concern either the identification of incinerator... 

In 1974 Midwest Research Institute operated a pilot-scale multiple chamber incinerator to evaluate for EPA the operational variables for pesticide incineration (8). The system included a. pilot-scale incinerator, a three-stage scrubber system, and a scrubber water treatment system. Nine pesticides (aldrin, atrazine, captan, DDT, malathion, mirex, picloram, toxaphene, and zineb) in 15 liquid and solid formulations were studied. Destruction efficiencies generally exceeded 99.99% over a range of temperatures and retention times ( 950 to 1100°C, 1.2 to 6 s, and 80 to 160% excess air). This study also documented the generation of measurable quantities of cyanide in the incinerator off-gas during the incineration of organonitrogen pesticides. 

Analytical Procedures. The extraction procedure used for solution studies was based on the method supplied by Union Carbide Corporation ( ), and was reported In previous work ( ). Samples were analyzed using a Microtek GC with a Tracer NP detector as described by Lemley and Zhong ( ). Conditions used for the various pesticides analyzed and the retention times obtained are detailed In Table I. In all cases a 4 foot x 4 mm I.D. glass column packed with 1.5% SP-2250/1.95% SP-2401 on 100/120 mesh Supelcoport was used. A glass Injector was used for the analysis of carbofuran and 3-hydroxycarbofuran. Procedures were varied slightly for analysis of methomyl. The first three Inches of the column were packed with 1.5% OV-17 on 100/120 mesh Supelcoport, and a glass Injector was used without glass wool at the end. 

In the pesticide industry, neutralization is provided prior to GAC and resin adsorption, pesticide hydrolysis, and biological treatment. The neutralization basin is sized on the basis of an average retention time of 6 minutes and 70 horsepower per million gallons for mixing requirements [7]. 

The use of two or more columns improves the probability that the identity of an unknown compound is the same as that of a compound with identical retention times however, these data alone are not conclusive proof. The reliability of the identification depends on the efficiency and polarity of the columns used. With efficient columns, the probability of having two or more components under one peak diminishes and the peaks are generally well resolved. Care must be taken in selecting columns to be certain the columns have different selectivities and not just different names. Thus, the McReynold s constants (Chapter 3) must be compared and should be quite different for each column. For example, in the analysis of pesticides, four different liquid phases might be chosen arbitrarily (e.g., OV-1, UCW-98, SE-30, and DC-200). However, the relative retention in this case will be the same for all four columns since they are all methyl silicones and have essentially the same McReynold s constants (2). 

The confirmation of pesticides by GC/MS should be more reliable than that on the GC-ECD using an alternate column. Presence of stray interference peaks, even after sample cleanup, and the retention time shift and coelution problem, often necessitate the use of GC/MS in compounds identification If a quantitative estimation is to be performed, select the primary ion or one of the major characteristic ions of the compounds and compare the area response of this ion to that in the calibration standard. Quantitation, however, is generally done from the GC-ECD analysis, because ECD exhibits a much greater sensitivity than the mass selective detector (MSD). For example, while ECD is sensitive to 0.01 ng dieldrin, the lowest MSD detection for the same compound is in the range of 1 ng. The primary and secondary characteristic ions for qualitative identification and quantitation are presented in Table 2.20.3. The data presented are obtained under MS conditions utilizing 70 V (nominal) electron energy under electron impact ionization mode. 

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