System peak

Eluent dips or system peaks and their causes were first described by Gjerde and Fritz [13]. Stevens et al. [3] described the effect of the system peak in suppressed ion chromatography. Called a carbonate dip, the system peak was said to be the absent peak (from the injection) of the carbonic acid that is retained by the unexhausted portion of suppressor column. 

As an eluent is pumped through a column, the resin becomes equilibrated with the eluent. The desired process is an ion-exchange equilibrium in which the anions on the resin are displaced by the eluent anion. However, a second equilibrium process can occur in which the molecular form of the eluent is sorbed by the resin matrix. A system peak results from a change in this latter equilibrium which is caused by injection of a sample. If the sample pH is more basic than the eluent, then part of the sorbed eluent is ionized and desorbed. The system peak which elutes later is from the readsorption of the eluent hence, in this case it is a decreasing peak of dip. If the sample 

The retention time of the system peak is greater for resins of high surface area and porosity, and when the polarity of the resin matrix and eluent arc more alike. In general, system peak effects arc lowered by choosing a more polar eluent or resin, or by raising the eluent pH. 

The term system peak refers to signals that cannot be attributed to solutes. System peaks are characteristic of ion chromatographic systems that use weak organic acid eluents without a suppressor system. Despite numerous publications regarding this subject [130-134], system peaks were often the cause of misinterpretations. A significant amount of information about the thermodynamic and kinetic processes taking place within the separator column may be 

If this equilibrium is disturbed by injecting a sample, a new equilibrium is established via relaxation that is, the kind of relaxation process depends on the pH value of the sample injected. If the sample pH is lower than the pH value of the mobile phase, benzoate ions in the mobile phase are protonated due to the sample injection. Thus, the concentration of molecular benzoic acid in the mobile phase increases as does the amount adsorbed onto the stationary phase. What is not adsorbed travels through the column and appears as a chromatographic signal the system peak. A qualitatively similar chromatogram is obtained when a sample is injected into the system that contains the solute ions and the corresponding eluent component. However, only the position of the system peak is comparable, not its area and direction. 

Correspondingly, the injection of a sample with a pH higher than that of the mobile phase shifts the dissociation equilibrium of benzoic acid to the ionic side. To maintain the equilibrium, molecular benzoic acid is desorbed from the stationary-phase surface. The resulting deficit in undissociated acid appears as a negative peak in the chromatogram with a characteristic elution time. The 

This explanation is corroborated by the fact that no system peaks occur when fully dissociated eluent ions are used. On the other hand, position of the system peaks can be affected by adding organic solvents. 

As mentioned above, a number of important conclusions can be drawn from the occurrence of system peaks. In this connection, the investigations of Levin and Grushka [135,136] are worth mentioning. From the system peak area, the authors derived the amount of eluent component adsorbed at the stationary phase and calculated the capacity ratio, k. Remarkably, this approach allows the calculation of the capacity ratio without prior knowledge of the colunrn dead time, td. 

Wescan 269-029 eluant 4 mmol/L NaOH + 0.5 mmol/L sodium benzoate flow rate 1.5 mL/min detection indirect conductivity injection volume lOOpL solute concentrations 5 mg/L borate (as B) (1), 10 mg/L silicate (as Si02) (2), 10 mg/L each of formate (3) and sulfide (4), and 20 mg/L each of chloride (5) and cyanide (6) (system peak appears after 28 min)  

The occurrence of system peaks will be discussed, taking an anion exchanger in equilibrium with a benzoic acid (BA) mobile phase as an example. This equilibrium is maintained as long as the chromatographic system is not disturbed. The equilibrium processes of interest are illustrated in Fig. 3-105. They comprise  

An important result of working with mobile phases in spectral regions in which the background absorbance is significant is the potential appearance of large system peaks. System peaks represent depletion and enrichment zones of various mobile 

The best w s to avoid system peaks are (1) to dissolve the sample in the mobile phase vdienever possible, and (2) to work at a wavelength where the sample matrix and the mobile phase have little ot no absorbance. It ould be noted that even if the second condition is satisfied, the refractive index difference between the umnatched san le solution and the solvent system might generate lefiactive index-related peaks or other baseline shifts. These peaks may elute at any time during the elution process and could also be very disruptive, especially when working at sensitive detector settings. 

FIGURE 1.8 System peaks, (a) A commonly observed void volume refractive index pulse. It is indicative of a solvent mismatch, (b) A system peak that elutes a er the analyte of 

Suppose that we have attained equilibrium between a mobile phase containing sodium benzoate and an anion-exchange column, and nonsuppressed conductivity detection is to be used for a chromatographic mn. When a small volume of an aqueous sample in injected, a zone is formed in the column that has a different conductance from that of the mobile phase. When this zone has moved through the column and passes through the detector cell, a baseline dip or peak is observed in the chromatogram. Most frequently, the sample conductance is lower than that of the eluent and a dip is observed. Eluent dips of this type were described by Gjerde and Fritz [13]. 

Injection of a dilute sample may cause another baseline disturbance that will be observed sometime later in a recorded chromatogram. In our example, the benzoate ion of the eluent is in equilibrium with molecular benzoic acid. 

However, a second equilibrium process can occur in which the molecular form of the eluent solute is adsorbed by the resin matrix. This adsorption process also shifts the acid-base equilibrium so that a greater proportion of the eluent is in the molecular form. When a sample is injected that contains no benzoate or benzoic acid, a new equilibrium is established in which some of the adsorbed benzoic acid passes from the stationary phase into the mobile phase. After the zone occupied by the sample has passed, the resin re-equilibrates with the mobile phase to replace the adsorbed benzoic acid it has lost. This creates a vacancy in the mobile phase that contains a lower total concentration of benzoate plus benzoic acid. In the example described, a decrease in the detector signal is observed when the vacancy passes through the detector. This is commonly called the system peak even if it is in the negative rather than the positive direction. 

This baseline disturbance is more pronounced when a considerable amount of the molecular form of the eluent is adsorbed and when the sample has a more basic pH than the eluent. This pH difference ionizes the adsorbed molecular species and desorbs it into the mobile phase. This action then creates a larger vacancy. In general, system peaks can be minimized by using a more hydrophilic ion exchanger and a more polar eluent. A more basic eluent pH is also helpfijl. 

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SYSTEM PEAKING Normal max. load of short duration and daily starts ... 

Residual SYSTEM PEAKING Normal Max. Load of short duration 1/10 ... 

The use of hexafluoroisopropanol (HFIP) as an SEC eluent has become popular for the analysis of polyesters and polyamides. Conventional PS/DVB-based SEC columns have been widely used for HFIP applications, although the relatively high polarity of HFIP has led to some practical difficulties (1) the SEC calibration curve can exhibit excessive curvature, (2) polydisperse samples can exhibit dislocations or shoulders on the peaks, and (3) low molecular weight resolution can be lost, causing additive/system peaks to coelute with the low molecular weight tail of the polymer distribution... 

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.). 

VII. SEPARATION OF THE OLIGOMER RANGE FROM SYSTEM PEAKS... 

As stated in Section I, columns should be selected so the low molar mass portions of the samples in question can be sufficiently separated from the elution interval of the system peaks. This task cannot always be accomplished, e.g., dimethylacetamide often replaces dimethylformamide as a GPC eluent the analyzed, mostly polar, samples require a neutral salt (e.g., FiBr) (7). The calibration is usually carried out with poly(methylmethacrylate) standards... 

Column manufacturers normally provide basic information about their columns, such as plate count, particle size, exclusion limit, and calibration curve. This information is necessary and fundamental, however, it is not sufficient to allow users to make an intelligent decision about a column for a specific application. For example, separation efficiency, the dependence of separation efficiency on the mobile phase, the ability to separate the system peaks from the polymer peak, the symmetry of the polymer peak, and the possible interaction with polymers are seldom provided. 

Eor low molecular weight polymers the separation of the system peaks from the low molecular weight end of the polymer peak is very critical in obtaining accurate MWD and the percentage of low molecular weight materials in the polymer. The water/methanol mixture is a better solvent for PVP than water. PVP K-15 and K-30 should be better separated from the system peaks in the water/methanol mixture than in water because the difference in hydrodynamic volumes between PVP K-15 or K-30 and system peaks is larger in... 

PVP K-15 and K-30 peaks are symmetric in water and water/methanol, except for the TSK GM-PWxl column in water. This suggests an interaction between PVP K-15 and K-30 with the TSK GM-PWxl column in water. System peaks overlap with the low molecular weight tails of the PVP K-15 and K-30 peaks for all four columns in water. In water/methanol the separation of the system peaks from the polymer peaks is much better for all four columns. 

Figure 14.11 Typical cln omatogram obtained by using the aromatics analyser system. Peak identification is as follows 1, non-aromatics 2, benzene IS, internal standard (MEK) 3, ethylbenzene 4, p-and m-xylenes.

In parallel with the new approaches to generating capacity additions, the utilities, with encouragement from regulators, introduced incentives during the 1980s for reducing load demand. Since the system peak hour load provided the inertia for capacity requirement definition, shaving of the peak became the focus of these incentives. 

Settlement/storage areas for effluent need to be sized not just for average flow but also for peak periods. Where production is based on a shift system, peak flows created during holiday periods (shutdown, major maintenance, etc.) should be considered. 

Flgvire 4.5 The influence of endcapping on peak shape and retention of soee PTH-anino acids using a reversed-phase separation system. Peak identification 1 PTH-histidine, 2 PTH-arginine and 3 PTH-valine. (Reproduced with permission from ref. 71. Copyright Preston Publications, Inc.)... 

Mizrotsky, N. and Grushka, E., Use of system peaks in liquid chromatography for continuous on-line monitoring of chemical reactions, Anal. Chem., 67,1737,... 

The solvent in which a sample is dissolved can play a very important role in HPLC analysis. Immiscibility, precipitation, decomposition, and system peaks are all problems potentially caused by a sample solvent incompatible with the analysis. Ideally, the mobile phase should be identical to the reaction solvent. The addition of an internal standard permits a kinetic analysis to be conducted. 

Experimental non-ideality at pH extremes in isotachophoresis has been compared with theoretical models.56 The model was able to predict phenomena that are usually regarded as artifactual, including system peaks, diffuse... 

Analytical Techniques. Sessile drop contact angles were measured with a NRL C.A. Goniometer (Rame -Hart, Inc.) using triply distilled water. The contact angles reported are averages of 2-8 identically treated samples with at least three measurements taken on each sample. ESCA spectra were obtained on a Kratos ES-300 X-ray Photoelectron Spectrometer under the control of a DS-300 Data System. Peak area measurements and band resolutions were performed with a DuPont 310 Curve Resolver. 

In the method described by Willie et al. [167] atomic absorption measurements were made with a Perkin-Elmer 5000 spectrometer fitted with a Model HGA 500 graphite furnace and Zeeman effect background correction system. Peak absorbance signals were recorded with a Perkin-Elmer PRS-10 printer-sequencer. A selenium electrodeless lamp (Perkin-Elmer Corp.) operated at 6W was used as the source. Absorption was measured at the 196.0nm line. The spectral band-pass was 0.7nm. Standard Perkin-Elmer pyrolytic graphite-coated tubes were used in all studies. 

The variation of flame speed with equivalence ratio follows the variation with temperature. Since flame temperatures for hydrocarbon-air systems peak slightly on the fuel-rich side of stoichiometric (as discussed in Chapter 1), so do the flame speeds. In the case of hydrogen-air systems, the maximum SL falls well on the fuel-rich side of stoichiometric, since excess hydrogen increases the thermal diffusivity substantially. Hydrogen gas with a maximum value of 325 cm/s has the highest flame speed in air of any other fuel. 

FIGURE 5 (a) Peptide digest run on a 4.8-gm particle on a traditional HPLC system. Peak count is 70, and peak capacity is 143. (b) Peptide digest run on a 1.7-gm particle on the ACQUITY UPLC system. Peak count is 168, and peak capacity is 360. (Courtesy of Waters Corp.)... 

Thermal effects may also cause baseline disturbances and system peaks. These can be different from instrument to instrument as well. 

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