Retention times absolute

Sample handling high flexibility regarding sample concentration, solvent, and column temperature optimal reproducibility of absolute retention times demand for analyses requiring high accuracy high risk of systematic errors. 

Produces relatively accurate results for volatile solutes problems with quantitation of high-boiling solutes (matrix effects ) requires re-concentration of the initial bands by cold trapping or solvent effects, which often forces cooling of the column for the injection. This is time-consuming and causes problems with absolute retention times. 

In most cases, absolute retention times are used for the identification. Modem GCs have high precision and accuracy of retention times within 0.05 %, or better can be obtained in subsequent runs during the same day. When the analysis is repeated by another instrument, the deviation may be of several percent. The reproducibility of absolute retention times is strongly dependent on the proper adjustment of all chromatographic parameters. In addition, the column properties are not exactly the same even when similar columns from the same manufacturer are used. On the other hand, absolute retention times are the most useful identification parameters if a few chemicals are to be monitored and the background is low. This method requires frequent calibration because even small changes in chromatographic conditions will influence the absolute retention times. 

Conditions 6 ft X 4 mm I.D. glass column 15% Apiezon L on Gas-Chiom P, 100-120 mesh 120°C carrier gas (nitrogen) flow-rate, 60 ml/min. Retention times are given relative to hexadecanol and its derivatives and phenol derivatives, respectively. Values in parentheses are absolute retention times... 

Chromatographic retention times are characteristic of the compounds they represent but are not unique. Coincidence of retention times of a test and a reference substance can be used as a feature in construction of an identity profile but is insufficient on its own to establish identity. Absolute retention times of a given compound vary from one chromatogram to the next. Comparisons are normally made in terms of relative retention, which is calculated by the equation... 

Gas chromatographic analyses were performed on a Hewlett Packard Model 5880 system equipped with a thermal conductivity detector with an injector temperature of 200°C, detector temperature of 250°C and an oven temperature of 35°C. All runs were isothermal. The columns used for the separation were 20 foot AgNOj-ethylene glycol columns. Absolute retention times were 1.15 min (trans 2-pentene), A.65 min (1-pentene), and 5.81 min (cis 2-pentene). Methods of automatic peak integration that were used during the analyses were peak height, peak area, area %, and internal standard experiments. 

For all calibration solutions, the retention times of the isomers must fall within the appropriate retention time windows established by the window defining mix analysis. In addition, the absolute retention times of the recovery standards, 13Ci2-1,2,3,4-TCDD and 13Ci2-1,2,3,6,7,8-HxCDD, shall not change by more than 10 seconds between the initial CC3 analysis and die analysis of any other standard. 

In order to make a positive identification of the 2,3,7,8-substituted isomers for which an isotopically labeled internal or recovery standard is present in the sample extract, the absolute retention time (RT) at the maximum peak height of the analyte must be within 1 to 3 seconds of the retention time of the corresponding labeled standard. 

In order to assure that retention time shifts do not adversely affect the identification of PCDDs/PCDFs, the absolute retention times of the two recovery standards added to every sample extract immediately prior to analysis may not shift by more than 10 seconds from their retention times in the continuing calibration standard (see Paragraph 17.1.4). 

Because the elution appears as a broadened zone, the retention time tR must be measured when the analyte reaches the maximum of its Gaussian elution peak. The absolute retention time tp is the sum of the (average) time spent by a component in the mobile phase (U) and the (average) time spent in the stationary phase (tR-) ... 

The objective of a chromatographic analysis is to resolve the components in a mixture into separate bands. The resolution R of two neighboring peaks is defined as the quotient of the difference in the absolute retention times and the arithmetic mean of their peak widths w at the respective peak base. 

In qualitative analysis, the goal is to establish the identity of unknown components in the sample.2,4 5 In HPLC, peaks are identified by its retention time as compared with those of the known analytes in the standard solution. Since absolute retention times are affected by many parameters, relative retention times (RRT) (retention time ratio against a reference component) are often used in more complex samples. Spiking the sample with known analytes is another useful technique to establish peak identity. Other techniques include ... 

The relative retention time should fulfil the criteria. Only if step 1 is positive can step 2 be made. For GC-analyses these criteria can be very strict. For instance, the relative or absolute retention time measured in the sample should not differ by more than 0.2% (from the relative retention time in the last measured external standard solution). In HPLC analyses, a higher deviation has to be accepted. 

The relative retention time of a compound compared to another compound on a given column is of more value than the absolute retention time, because the absolute retention time is hard to reproduce from one laboratory to another. 

For quantitative interpretations of the chromatogram the detected compounds must be identified, and response factors have to be apphed to transform peak areas into weights. Automatic identification of peaks may be accomplished by numbering the peaks, on the basis of absolute retention time bands or by retention times relative to an internal time standard. Since retention times relative to one standard are not constant enough for reliable identification especially in long chromatograms, multistandard... 

This particular stationary phase gave a retention time difference between the enantiomers of 7 minutes at an absolute retention time of about 50 minutes. 

The LC/MS decoding chromatography is isocratic and low resolution. The specificity of the method is primarily due to the mass selection of the selected ions. The retention time window in the primary Agilent peak identification protocol is left relatively wide to allow for normal isocratic chromatographic drift across a set of analyses. In some cases interferences are seen in the data. A tighter retention time discrimination can be applied during the Access analysis of Decode by hitting the Use RT Delta radio button. An absolute retention time window is then entered and the Decode software will exclude identifications outside that window. 

The ortho-, meta-, and w/a-isomers of chloro-, bromo-, iodo-, nitro-, methyl-, hydroxyl-, and aminophenols were studied on a silica column (A = 270 nm) with various mixtures of 1-butanol (1-2.5%) in hexane or heptane [603]. The elution order in terms of substituent and increasing retention times was Cl < CH3 authors noted that reversals in solute elution times occurred as the 1-butanol level changed. Absolute retention times and substituent-related a. 

The different absolute retention times will be actual reaction times of a possible hydrogenation reaction in the column. The peak area of propyne was normalized to a saturated hydrocarbon, propane, and listed in Table 7-1. From the results it is easy to see that for unsaturated compounds no hydrogenation occurs at temperatures of 200 °C. On using hydrogen as the carrier gas hydrocarbons can be separated up to n-decane. 

Recent developments in capillary GC, i.e., electronic pneumatic control (EPC) of the carrier gas, improved oven temperature stability, and excellent reproducibility in column making have led to the concept of retention time locking (RTL) [77], [78]. With retention time locked data bases, absolute retention times instead of retention indices can be used to elucidate the structures of eluting solutes. Moreover, retention time locking can be u.sed in combination with different injectors and detectors. Exact scaling of capillary GC-FID, capillary GC-MS, and capillary GC-AED chromatograms is feasible. 

Inherently, two important pieces of data can be obtained from a gas chromatograph. The output of the detector is either processed electronically or placed on a simple strip chart recorder. The first piece of data obtained is simply the time it took for a given component to travel through the column. This is the time from the point of injection to the maximum of the peak as it passes through the detector. This time is referred to as the absolute retention time tR. It is this retention time information that is not used in qualitative analysis. The second important piece of information that is obtained is simply the size of the peak. Size data is discussed in Part 2 of this chapter, Quantitative Analysis. A third piece of information that can be obtained from the chromatograph is the shape... 

Attempts to compare retention times on two different columns of the same type can be difficult at best. Differences in packing density, liquid loading, activity of the support, age and previous use of the packing, and variations in the comparison of the column wall can lead to large differences in retention time measurement between the two columns. Thus tabulations of absolute retention times are not... 

An example of one method to control the robustness and reproducibility of a method is to set retention time windows for the target compounds. Absolute retention times are used for compound identification. Retention time windows can be established to compensate for minor shifts in absolute retention times as a result of sample loadings and normal chromatographic variability. The width of the retention time window should be carefully established to minimize the occurrence of both false positive and false-negative results. This can be accomplished by multiple injections of standard solutions over the course of a time period (days or weeks) and then calculating the mean and standard deviation of the retention time. 

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