Injection system peak

SPME has been utilized for deterrnination of pollutants in aqueous solution by the adsorption of analyte onto stationary-phase coated fused-siUca fibers, followed by thermal desorption in the injection system of a capillary gas chromatograph (34). EuU automation can be achieved using an autosampler. Eiber coated with 7- and 100-p.m film thickness and a nitrogen—phosphoms flame thermionic detector were used to evaluate the adsorption and desorption of four j -triazines. The gc peaks resulting from desorption of fibers were shown to be comparable to those obtained using manual injection. 

Figure 13.7 Electropherograms showing the simultaneous measurement of low- and high-energy explosives as recorded with the (a) conductivity and (b) amperometric detectors. Analytes, ammonium (1), methylammonium (2), sodium (3), TNB (4), TNT (5), 2,4-DNB (6), and 2-Am-4,6-DNB (7), system peak (SP). Explosive concentration, 2 mM (1,2,3) and 15 ppm (4,5,6,7). Conditions MES/His buffer (20 mM, pH 6.1) containing 15 mM lithium dodecyl sulfate as the run buffer separation field strength, +250 V/cm injection field strength, +250 V/cm for 2 s detection at 200 kHz, (a) 5 Vp p and at (b) —0.5 V. (Reprinted in part with permission from [34]. Copyright 2002 American Chemical Society.)...

New accessories are continually made available. This unit discusses the basics necessary to conduct SPME analysis for flavor analysis. An automated sampling and injection system is available from Varian. Supelco offers a manual sampling stand setup. Injection liners are available that reduce the injection port volume to presumably produce sharper peaks. Predrilled septa for the GC are available to reduce septum coring. 

The injection of a sample in a chromatographic column may result in more peaks than there are components in the mixture if the mobile phase contains one or several additives. These additional peaks result from the perturbation of the additive equilibrium between the two phases caused by the injection of a sample. It may be assumed that there is a competitive equilibrium of the sample and the modifier. Solutes enter the column, moving with the velocity of the mobile phase and not with the equilibrium velocities dictated by the equilibrium between mobile and stationary phases. System peaks are visualized with an appropriate detector, particularly a refractive index detector. This may cause trouble for the analyst, since the system peak may exhibit k values more than 1 (37). 

The application of indirect methods in biomedical analysis has been reviewed by Schill and Arvidsson.23 Reversed-phase HPLC is the main field of application for indirect detection, and both charged and uncharged species can be visualized, although sensitivity is better for ionic solutes. With indirect UV detection for reversed-phase HPLC, the eluent contains a chromophoric group (probe), and peaks are obtained for injected solutes as well as for the mobile phase additives (system peaks). For solutes that... 

Finally, AMDIS displays on the PC screen its Result Window, which contains - only for chemicals defined as detected - the chemical name, retention time, RI, and some additional QA (Qualitative Analysis) parameters. The operator will consider the run as valid only if HCB was detected with QA parameters fulfilling the acceptance criteria. There are other QA parameters that are reported by AMDIS after each run. They are peak width, peak tailing, solvent tailing, total ion background, and background for ion 207. Even in blinded mode when chromatogram is not visible, the values of these parameters allow operator to assess the quality of GC system, accept or reject the particular run, or take proper preventive action (bake or change the column or maintain injection system). 

Yao et al. reported a flow injection analytical system for the simultaneous determination of acetylcholine and choline that made use of immobilized enzyme reactors and enzyme electrodes [25]. Acetylcholineesterase-choline oxidase and choline oxidase were separately immobilized by reaction with glutaraldehyde onto alkylamino-bonded silica, and incorporated in parallel as the enzyme reactors in a flow injection system. The sample containing acetylcholine and choline in 0.1 M phosphate buffer (pH 8.3) carrier solution was injected into the system. The flow was split to pass through the two reactors, recombined, and mixed with 0.3 mM K4Fe(CN)6 reagent solution before reaching a peroxidase immobilized electrode. Because each channel had a different residence time, two peaks were obtained for choline and total acetylcholine and choline. Response was linear for 5 pM-0.5 mM choline, and for 5 pM 1 mM acetylcholine plus choline. The detection limits were 0.4 pM for choline and 2 pM for acetylcholine. 

Note that it is a common practice to dissolve the analytes to be separated in the mobile phase. In fact, the injection of the sample in a solution whose composition differs from that of the eluent would create a disturbance at the top of the column that will migrate down the column. The lack of detectable eluent component in the injected solution would create a negative system, perturbation, or ghost peak because the concentration of that component is lower compared to the initial conditions under which the baseline was recorded. Obviously the retention time of the system peak is that of the component under the experimental conditions used in the chromatographic run [28,29]. System peaks are also very useful for indirect detection [30], as will be explained in Chapter 13. 

Gennaro, M.C. Ghost, vacancy, dip, or system peaks A contribution in investigations about injection and system peaks in liquid chromatography. J. Liq. Chromatogr. Rel. Technol. 1987, 10, 3347-3375. 

The mass sensitivity of a chromatographic system (including column, sample valve or injection system, connecting tubes and detector) is defined as the mass of solute (m ) that will provide a peak with a height equivalent to twice the noise level. 

System peaks are most often recognized as a pair of peaks, one positive and the other negative, which represent enrichment and depletion zones eluted from the column. They may vary in retention time and size, depending on the sample matrix, injection volume, mobile phase composition, and the stationary phase. Dissolving samples in the mobile phase is the best way to minimize... 

Precision of peak areas or heights is important because they are used for calculating amounts during quantification. It is also the most important performance criterion for an LC injection system. Precision should be deter-... 

The mechanism of PCP and ketamine action involves antagonism of a subset of receptors for the excitatory amino acid glutamate (Balazs et al., 2006). PCP is absorbed rapidly after smoking or injection, with peak blood concentrations noted 5-15 minutes after smoking. In contrast, peak concentrations are reached wo hours after oral administration. The drug remains in the system unmetabolized for more than nvo days, and PCP is detectable in urine for several weeks after a single use (Hawks Cliiang, 1986). 

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