Peaks fronting

Figure 1, Two GC-TEA tracings of experiments with analysis by our method. The left tracing shows the results of an experiment, similar to that in Table /, Experiment 4, in which nitrite was added to the diet and the hexane wash of the column (which reduced the solvent-front peak) was omitted. The right tracing shows one of the positive experiments (including the hexane wash) on the homogenate of a mouse exposed to morpholine and NOt (Table II, Experiment 3),...

Figure 7.6 Effect of pH on the stability of AIC after incubation at 30°C for 12 h in citrate buffer with the pH adjusted to 5 (panel A), 6 (panel B), and 7.4 (panel C). The peak with a retention time >10 min is due to a buffer component. Panel C consist mainly of monomer, and the front peak observed in panels A and B is due to aggregation/unfolding of the conjugate. Analysis conditions as described in Figure 7.5.

With the chromatographic system employed, nonpolar fats and detergents present in the dyes can be expected to have little or no retention and therefore should elute with the solvent front. Peak 1 in Figure 6 might partly consist of such substances. Most... 

FigurB 25-26 Application of the method development triangle to the separation of seven aromatic compounds by HPLC. Column 0.46 x 25 cm Hypersil ODS (C)e on 5-(j.m silica) at ambient temperature ( 22°C). Elution rate was 1.0 mL/min with the following solvents (A) 30 vol% acetonitrile/70 vol% buffer (B) 40% methanol/60% buffer (C) 32% tetrahydrofuran/68% buffer. The aqueous buffer contained 25 mM KH2P04 plus 0.1 g/L NaN3 adjusted to pH 3.5 with HCI. Points D, E, and F are midway between the vertices (D) 15% acetonitrile/20% methanol/65% buffer (E) 15% acetonitrile/16% tetrahydrofuran/69% buffer (F) 20% methanol/16% tetrahydrofuran/64% buffer. Point G at the center of the triangle is an equal blend of A, B, and C with the composition 10% acetonitrile/13% methanol/11% tetrahydro-furan/66% buffer. The negative dip in C between peaks 3 and 1 is associated with the solvent front. Peak identities were tracked with a photodiode array ultraviolet spectrophotometer (1) benzyl alcohol (2) phenol (3) 3, 4 -dimethoxyacetophenone (4) m-dinitrobenzene (5) p-dinitrobenzene ...

The ID has a direct influence on retention, efficiency, and capacity of the column. The on-column injection technique requires an ID of at least 0.30 mm. A narrow-bore column with an ID of 0.20 mm provides good resolving power with a minimum bleed. It is a good choice for MS analysis as it facilitates a proper adjustment of the carrier gas flow. Narrow-bore columns of limited capacity, however, may be a disadvantage for identification due to a fronting peak shape of overloaded peaks. Columns of an ID between 0.25-0.33 mm can be considered equal for the CWC-related chemicals. Columns of an ID 0.53 mm are useful if the sample contains a limited number of chemicals in widely different concentrations. 

Ion-pair chromatography (IPC) is more susceptible to peak fronting than other modes in LC. Column temperature problems can cause fronting peaks in IPC. Figure 1 shows the separation of an antibiotic amine at ambient temperature. Repeating the... 

The use of a sample solvent other than the mobile phase is another cause of fronting peaks in IPC. In this case, the sample should only be injected as a solution in the mobile phase. No more than 25-50 /rl of sample should be injected, if possible. 

Broad peaks, ghost peaks, pseudo peaks, negative peaks, peak doubhng, peak fronting, peak tailing, spikes, no peaks. The major causes and their solutions are tabulated in Table 1. 

Peak Fronting (Peak Asymmetry Factor < 0.9). This indicates that a small band is eluting before a large band, a wrong pH value of the mobile phase is used, an overloaded column, a void volume at the inlet, or that the sample solvent is incompatible with the mobile phase. 

In both cases T> 1.0 when peaks are tailed and T< 1.0 with fronting peaks. A peak with T= 1.0 is symmetrical. 

Mikkers et al. [1] concluded that symmetrical peaks are obtained only when the mobility of the carrier co-ion closely matches that of the analyte ion. If mobility of the analyte ion is higher, fronted peaks will result. 

An ideal HPLC curve is a linear isotherm leading to symmetrical Gaussian peaks as depicted in Figure 1. Quite often, tailing peaks are encountered in a chromatogram they are due to the difference in active chromatographic sites on the column. Fronting peaks are also encountered but to a much lesser... 

A severely fronted peak usually means too much sample. 

What is the possible problem if you have a severely fronted peak ... 

Less often, the interactions between solute molecules may be strong relative to those between solute and stationary phase, in which case initial uptake of solute molecules by the stationary phase is slow but increases as the first solute molecules to be adsorbed draw up additional ones. In such a case, the peak has a shallow front and a sharp tail and is said to be a fronting peak (Fig. 9). Fronting and tailing can be a problem because they tend to lead to overlap of peaks. Dealing with the sorts of problems associated with such phenomena is discussed more fully in Chapter 6. 

Anti-Langmuir type isotherms are more common in partition systems where solute-stationary phase interactions are relatively weak compared with solute-solute interactions or where column overload occurs as a result of large sample sizes. In this case, analyte molecules already sorbed to the stationary phase facilitate sorption of additional analyte. Thus, at increasing analyte concentration the distribution constant for the sorption of the analyte by the stationary phase increases due to increased sorption of analyte molecules by those analyte molecules already sorbed by the stationary phase. The resulting peak has a diffuse front and a sharp tail, and is described as a fronting peak. 

The actual amount of stationary phase per unit column length on a capillary column is much less than that supported on the particles of the typical packed column. Therefore the capacity for analytes of the capillary is much less than that of the packed column. When the capacity is exceeded, the analytes will spread out over the front of the column after injection, and the improved resolution of the capillary will be lost. This results in the phenomenon of leading or fronting peak shape described in Section 11.4. In capillary GC, one must decrease the concentrations of the injected analytes, often by dilution with solvent to levels which are at concentrations of only the part per thousand or ppm level. For this to be practical one must have detectors orders of magnitude more sensitive than the TCD described in Section 12.1. 

ELSD has much better signal-to-noise response to three sugars, and better baseline stability than the RI detector. Note that the ELSD shows no solvent front peak, since the solvent droplets evaporate completely leaving no particle behind to scatter light [Fig. 13.9(3)]. 

---