Sample transfer efficiencies

The dimensions of concentric-tube nebulizers have been reduced to give microconcentric nebulizers (MCN), which can also be made from acid-resistant material. Sample uptake with these microbore sprayers is only about 50 xl/min, yet they provide such good sample-transfer efficiencies that they have a performance comparable with other pneumatic nebulizers, which consume about 1 ml/min of sample. Careful alignment of the ends of the concentric capillary tubes (the nozzle)... 

Atomic absorption spectrometry requires that the species under investigation prevails in the gaseous and atomic state so that absorption of free atoms can be observed. The two most common methods for the production of atoms in the gas phase make use of thermal energy to vaporise and atomise the analyte. The sample transfer efficiency, i. e. how much of the sample is reaching the actual atomisation zone. 

The transfer efficiencies for ultrasonic nebulizers (USN) are about 20% at a sample uptake of about 1 ml/min. Almost 100% transfer efficiency can be attained at lower sample uptakes of about 5-20 pl/min. With ultrasonic nebulizers, carrier gas flows to the plasma flame can be lower than for pneumatic nebulizers because they transfer sample at a much higher rate. Furthermore, reduction in the carrier-gas flow means that the sample remains in the mass measurement system for a longer period of time which provides much better detection limits. 

There should be high sample transfer to the mass spectrometer or, if this takes place in the interface, ionization efficiency. This is of particular imporfance when frace-level componenfs are of inferesf or when polar and/or labile analytes are involved. 

The residue is removed from the leaf surface by shaking the leaf punch sample in an aqueous surfactant solution. This allows for removal of test substance residue from the leaf surface. It does not remove residue absorbed on the plant matrix that extraction and maceration in organic solvents would release. Generally, the extraction with aqueous surfactant is performed using a mechanical shaker for a 10-min interval and is repeated to increase transfer efficiency. 

Pyrolysis spectra become distorted with respect to their diagnostic features for two major sets of reasons. The first is variations in instrument operation (e.g., heat transfer efficiency from wire to sample, ion source temperature, MAB gas identity, analyzer calibration, tuning, and ion transmission discrimination attributable to contaminated optics). Most of these factors can be controlled... 

Check transfer efficiency by staining the gel after transfer, or by staining a second blot with a total protein stain, such as coomassie blue or ponceau red. Alternatively, use commercially available prestained protein standards that are run along the samples of interest and that are visible during both the separation electrophoresis and on the membrane after transfer... 

Allow efficient and precise sample transfer from the LC into the MS with little destrnction or loss of analytes. 

Although electrothermal vaporization has been widely accepted as an extension of atomic absorption, its use in inductively coupled plasma spectroscopy is fairly recent. In this technique the requirement for the vaporizer is somewhat different—the electrothermal vaporizer does not have to double as the atom cell. In fact, it is only needed to effect efficient and reproducible sample transfer from the rod, or a similar device, into the plasma. 

The data obtained from a set of samples by both MALDI-MS/MS and LC-ESI-MS/MS are shown in Fig. 11.12. ESI provides the opportunity to consume a comparatively large volume of sample, whereas with MALDI the sample consumption is small and relatively fixed. Typically, sample spots represent a pipetted volume of 0.25 xL, and measuring across the sample only consumes about a 25-nL equivalent. In this example, the ESI injection volume was 25 iL. Even though the ESI analysis consumed 1000 times more sample, the analyte area produced was only 10 times greater, suggesting that the MALDI-MS/MS was 100 times more efficient in terms of ionization efficiency and/or ion transfer efficiency. Discrimination was not possible between the two, but this example suggests that assay performance equality between the two techniques could be possible if the MALDI samples were further concentrated during the sample cleanup step. 

The collected samples must be identified with unique sample numbers efficiently tracked in the field stored in a secure location for the preservation of sample integrity and transferred to the laboratory with the information on their identification (ID) and the requested analysis. The provisions for sample numbering, labelling, storage in the field tracking and transfer to the laboratory are set forth in the SAP. 

Limited sample clean-up could overload the analytical column, and residual matrix components can accumulate on the column after multiple injections. The residual matrix components can also solidify and deposit over a period of time in the LC-MS ionization source or vacuum interface, resulting in a decrease in ion transfer efficiency. The decrease in instrumentation performance (i. e., signal intensity) can be monitored by the signals of system-suitability samples dispersed within an analytical batch. The practice of replacing the pre-column in every run and scrubbing the analytical column periodically with a cleaning mobile phase will help to maintain instrument performance. 

The silylation of all glassware that contacts the plant extract has proven to effectively reduce adsorption losses. As diagrammed in Figure 8, the hydroxyl adsorption sites on the silica surface can be coated with dichlordimethyl silane. The unreacted chloride groups are then displaced with methanol in a substitution reaction. A secondary advantage of the silyation process is that water will not adhere to the glass surface. Aqueous residues bead together, which allows more efficient sample transfers. 

Splitless injection high carrier gas flow rates improve the efficiency of the sample transfer below 2.5 ml/min sample transfer becomes unsatisfactory (below 1.5 ml/min if solvent recondensation accelerates the sample transfer). 

Inaccurate calibration of relative counting efficiencies. In the use of a dual sample transfer system where both sample and standard are counted simultaneously with different counters it is important that the relative efficiencies of the two counting systems be accurately determined and that these efficiencies do not vary with time. These errors may be partly compensated for by irradiating the same sample and standard several times and alternately reversing the sample and standard counting positions. 

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