Qualitative analysis interference problems

When a solution is tested, both analyte and solvent absorption bands will be present in the spectrum, and identification, if that is the purpose of the experiment, is hindered. Some solvents have rather simple IR spectra and are thus considered more desirable as solvents for qualitative analysis. Examples are carbon tetrachloride (CC14, only C-Cl bonds), choloroform (CHC13), and methylene chloride (CH2C12). The infrared spectra of carbon tetrachloride and methylene chloride are shown in Figure 8.21. There is a problem with toxicity with these solvents, however. For quantitative analysis, such absorption band interference is less of a problem because one needs only to have a single absorption band of the analyte isolated from the other bands. This one band can be the source of the data for the standard curve since the peak absorption increases with increasing concentration (see Section 8.11 and Experiment 25). See Workplace Scene 8.2. 

Much effort has been made to detect steroids in biological fluids. Even simple TLC methods have been used for qualitative analysis [38], One method that been used for quantification involves an immunoassay, but several problems exist with that method, most notably cross-reactions and interference with other substances [39], On the other hand, a number of chromatographic methods have been developed to overcome these problems. The majority of analytical methods involved GC, which has good detection limits, but requires previous derivatization [40] of the steroids to accomplish volatilization. Many methods have also been reported using HPLC with UV detection or LC-MS [40, 41], Previously used stationary phases for LC was e.g., Sephadex LH-20, Celite and Lipidex, but they could not be operated with high pressure [42], These columns were therefore slow to run and the separation of steroids was very time-consuming [43], Nowadays applications mainly use HPLC as a separation method with both normal-phase and re-versed-phase chromatography. 

It is good experimental practice to run a qualitative scan on any sample submitted for quantitative analysis in order to uncover potential overlap and other interference problems. In order to scan the entire wavelength (0.2-20 A), it is necessary to utilize several crystals and two detectors. The scanning speed [degrees (2 ) per minute] and time constant must be chosen so as not to introduce spectral line shifting... 

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. 

DIES can be used both for qualitative monitoring of chemical reactions in organic materials e.g. curing, drying) and for quantitative measurements e.g. determination of the concentration of polar liquids in materials such as water content in polymers). DIES can be combined with other techniques, such as FTIR, to gain specific molecular information on reactions that take place simultaneously and monitor these reactions. However, only conductivity, a macroscopic property, is measured. Consequently, molecular differentiation between combined reactions cannot be made. A lab-scale experiment in combination with more specific techniques (e.g. FTIR) is necessary to determine quantitatively the specific reactions. Dielectric analysis also measures changes in the properties of a polymer as it is subjected to a periodic field. A general problem in interpretation of dielectric and conductive methods is that they are not specific and are affected by many sources of interferences. These factors may explain the relatively slow introduction of this technique in characterisation of elastomer systems. 

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