Impurity peak

The purity of the product was determined by the checkers by GLC analysis using the following column and conditions 3-nm by 1.8-m column, 5% free fatty acid phase (FFAP) on acid-washed chromosorb W (60-80 mesh) treated with dimethyldichlorosilane, 90 C (1 min) then 90 to 200 C (15°C per rain). The chromatogram showed a major peak for methyl 2-methyl-l-cyclohexene-l-carboxylate preceded by two minor peaks for methyl 1-cyclohexene-l-carboxylate and l-acetyl-2-methylcyclohexene. The areas of the two impurity peaks were 5-6% and 0.5-2% that of the major peak. The purity of the product seems to depend upon careful temperature control during the reaction. The total amount of the two impurities was 14-21% in runs conducted at about -15 to -20°C or at temperatures below -23°C. 

The low molecular weight range should not be too narrow often it is very important to sufficiently separate the oligomer range of the sample from the elution area of system peaks (also called impurity peaks , salt peaks , etc.). 

The checkers found that gas chromatographic analysis of one sample using a 305 cm. by 0.3 cm. column packed with 10% SF-96 on Chromosorb P operated at 70° with a 60 ml./minute helium carrier gas flow rate gave five minor impurity peaks, two at shorter retention times, and three at longer retention times. None of these impurities was present in greater than 1.1% total impurities wrere 3%. 

Figure 4.8. Comparison of impurity profiles for the same chemical intermediate from two different suppliers. The impurity peak-areas for each chromatogram were tallied in 0.02 area-% bins for each vendor, the data was normalized by dividing by the number of chromatograms. Vendor A s material has many more peaks in the 0.05-0.2% range, which drives the total impurity level to =5.2% (vs. 1.9 for Vendor B) for <0.2% the number of excess peaks above 0.2% does not appear as dramatic, but greatly adds to the total impurity level = 13.3 v.v. = 2.3% ...

Tetrachlorodiben2o- >-dioxin. Purified 2,4,5-trichlorophenol (50 grams, 0.26 mole) was converted to its potassium salt and dissolved in 100 ml of bEEE. After addition of the copper catalyst and ethylene diacetate, the mixture was transferred to the bottom of a 300-ml sub-limer with chloroform. Sublimation (200°C/2 mm) yielded 14 grams (39% yield) of 2,3,7,8-tetrachlorodibenzo-p-dioxin. Mass spectral analysis revealed trace quantities of pentachlorodibenzo-p-dioxin, tetrachloro-dibenzofuran, and several unidentified substances of similar molecular weight. The combined impurity peaks were estimated to be <1% of the total integrated GLC area. The product was further purified by recrystallizations from o-dichlorobenzene and anisole. The final product had an estimated 260 ppm of trichlorodibenzo-p-dioxin as the only detected impurity. 

Figure 9. HPLC analysis of a monomer, fraction showing retention times and identification (1,2, and 3) represent MM A, styrene, and n-BMA at 200 nm (4 and 5) represent MM A and styrene at 254 nm (6) are THF impurity peaks...

There are several examples in the literature of GFC now being utilized for small molecule analysis (17). However, in this case, attempts to obtain monomer concentrations for kinetic modelling were frustrated by irreproducible impurity peak interference with monomer peaks, time varying refractometer responses and insufficient resolution for utilization of a reference peak. This last point meant that injected concentration would have to be extremely reproducible. 

Each reaction step was monitored qualitatively by TLC using hex-ane ethyl acetate as the developing solvent and quantitatively by GC. Impurity peaks were identified by GC/MS. An HPLC external standard method (Method 2) was developed and used to determine the purity of the final isolated product (RWJ-26240). The following rugged HPLC method was developed to optimize scheme 1, step 6 ... 

It appears that purification of commercially available solvents is sometimes required for the complete elimination of impurity resonances. Occasionally, these impurities may be turned into advantage, as in the case of C2D2CI4 where the (known) C2DHCI4 content may be used as an internal standard for quantitation. Thus, removal of every impurity peak is not always essential for identification and quantitative analysis of stabilisers in PE. Determination of the concentration of additives in a polymer sample can also be accomplished by incorporation of an internal NMR standard to the dissolution prepared for analysis. The internal standard (preferably aromatic) should be stable at the temperature of the NMR experiment, and could be any high-boiling compound which does not generate conflicting NMR resonances, and for which the proton spin-lattice relaxation times are known. 1,3,5-Trichlorobenzene meets the requirements for an internal NMR standard [48]. The concentration should be comparable to that of the analytes to be determined. 

These extra lines are often mistakenly thought to be impurity peaks. An in-depth understanding of how they may arise is not really necessary for the purpose of interpretation. What is important is that you instantly recognise the appearance of such spin systems. Check that the system integrates correctly and check that the two halves of the system are symmetrical. Note This phenomenon has nothing whatsoever... 

No impurity peak that has a retention time greater than Lindane will have a peak height greater than that produced by 10 pg/mL Heptachlorepoxide (ECD) and 5 pg/mL Parathion (NPD). Hydrocarbon analysis... 

The EDX spectrum (Fig. 11.8) shows the main surface scale impurity peaks of silica, aluminium, sodium, chloride and iron. If this EDX is compared to that of a new, clean membrane surface (Fig. 11.9), the clean surface shows sulphur, carbon and oxygen, which is typical of a porous polysulphone support. It was concluded that the scale is amorphous, composed of aluminosilicate and silicate. These compounds are normally found in trace amounts in brine solutions. Analysis showed that the surface could be cleaned with hydrochloric acid and analysis of the dissolved scale was similar to the EDX spectrum analysis. Review of the plant operation determined that the precipitation was the result of high pH in combination with high silica concentrations in the brine. 

Unspecified impurities are those that are typically found at levels below the reporting threshold. These are typically reported by the summation of all the impurity peaks. 

FIGURE 15 ESI/MS/MS daughter ion mass spectrum of the impurity peak at RT = 3.8 min.The impurity is a weak acidic compound, and LC/ESI MS was operated at acidic condition. 

FIGURE 16 (a) UV spectrum, (b) parent ion and (c) daughter ion mass spectra of an impurity peak can be built into an LC/MS database. 

Figure 16. Quantitative Li MAS NMR spectra of an-LL-VOPO4 acquired during the first and second electrochemical cycles at Bq = 7.1 T vr = 8.0 kHz). The shifts of the isotropic resonances are marked next to the peaks. The circles and asterisks indicate extra impurity peaks and sidebands, respectively.

Figure 6.36 500 MHz H NMR spectra obtained during a stop-flow LC-NMR experiment on a 1 mg injection of a crude sample of a drug compound, (a) LC chromatogram, (b) spectram corresponding to the parent bulk drug compound acquhed for 64 transients and (c) to the impurity peak RRT 0.87 (—3% by area), acquhed for 1024 transients. NOESY-type presaturation was used to suppress the solvent resonances. Bruker DRX500 H/ C 4-mm z-gradient probe with a 120 pi active cell volume.

Gradient elution places special demands on solvent purity. Only carefully purified solvents should be used, and, it is recommended that prior to use they be passed over activated alumina or silica (7). The column acts as a collector of impurities which may elute as sharp peaks at a certain eluent composition and can be mistaken for sample components. It is therefore advisable to run the gradient first without injecting the sample in order to recognize the impurity peaks. 

In Figure 5-1, impurity peak 3 is not completely resolved from the cefotaxime. In this case, another reasonable criterion for specificity might be that unresolved impurities at their maximum expected concentration will not affect the assay of cefotaxime by more than 0.5%. If we were trying to measure impurities, as opposed to assaying cefotaxime, a reasonable criterion for specificity is that all impurity peaks having >0.1% of the area in the electropherogram are baseline separated from cefotaxime. 

One way to avoid the nonlinear problem is to dilute the standard and unknown with a compatible solvent that is fully resolved chromatographically from all of the sample components. These dilutions need not be accurately made or need not be the same for the standard and unknown. Good practice dictates that they be approximately the same for each. This is merely a technique for injecting a smaller amount of the standard and sample into the chromatograph. Since the calculations do not involve sample size, this dilution is not a factor the solvent and any solvent impurity peaks are not measured and are not to be considered in the calculations. 

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