Sample solution effects

In many applications in mass spectrometry (MS), the sample to be analyzed is present as a solution in a solvent, such as methanol or acetonitrile, or an aqueous one, as with body fluids. The solution may be an effluent from a liquid chromatography (LC) column. In any case, a solution flows into the front end of a mass spectrometer, but before it can provide a mass spectrum, the bulk of the solvent must be removed without losing the sample (solute). If the solvent is not removed, then its vaporization as it enters the ion source would produce a large increase in pressure and stop the spectrometer from working. At the same time that the solvent is removed, the dissolved sample must be retained so that its mass spectrum can be measured. There are several means of effecting this differentiation between carrier solvent and the solute of interest, and thermospray is just one of them. Plasmaspray is a variant of thermospray in which the basic method of solvent removal is the same, but the number of ions obtained is enhanced (see below). 

The drop in pressure when a stream of gas or liquid flows over a surface can be estimated from the given approximate formula if viscosity effects are ignored. The example calculation reveals that, with the sorts of gas flows common in a concentric-tube nebulizer, the liquid (the sample solution) at the end of the innermost tube is subjected to a partial vacuum of about 0.3 atm. This vacuum causes the liquid to lift out of the capillary, where it meets the flowing gas stream and is broken into an aerosol. For cross-flow nebulizers, the vacuum created depends critically on the alignment of the gas and liquid flows but, as a maximum, it can be estimated from the given formula. 

Ensure if possible that standard and sample solutions are of similar bulk composition to eliminate matrix effects (matrix matching). 

A pharmaceutical specialty is produced in three dosage strengths (major component A) A and a second component B are controlled by HPLC for batch release purposes. It is decided to replace the manual injection of the sample solution by an automatic one. It is expected the means will remain the same but the standard deviations will be smaller for the automatic injection. Cross-validation of the methods is effected by running both methods on each of 10 samples. The mean and the standard deviation for each series of 10 measurements is given in Table 4.19. 

The half-cell potentials of the two reference electrodes are constant sample solution conditions can often be controlled so that E,j is effectively constant and the composition of the internal solution can be maintained so that (ai)i , ai is fixed. Consequently Eq. (3) can be simplified to give... 

A second source of error may be in the detector. Detector linearity is an idealization useful over a certain concentration range. While UV detectors are usually linear from a few milliabsorbance units (MAU) to 1 or 2 absorbance units (AU), permitting quantitation in the parts per thousand level, many detectors are linear over only one or two decades of operation. One approach in extending the effective linear range of a detector is high-low injection.58 In this approach, an accurate dilution of a stock sample solution is prepared. The area of the major peak is estimated with the dilution, and the area of the minor peak is estimated with the concentrated stock. This method, of course, relies on linear recovery from the column. Another detector-related source of error that is a particular source of frustration in communicating... 

On the other hand, if 02 existed in the reaction system, the reaction mechanism would be affected by the reactions with 02 the reaction mechanism is dependent on the types of dissolved gases in the sample solution. The details for the effects of various parameters on the reduction of metal ions and formation of metal nanoparticles are described in the following sections. 

In CL measurements many factors that influence the intensity of the CL signal should be taken into account. The CL signal may depend on the geometry of the sample. Internal refraction and reflection at the air-solution interfaces are important factors in determining the measured CL intensity, and should be taken into account, for example, when a CL cocktail is placed over a sample. The effect of sample geometry can be evaluated using model systems, such as enzymes... 

Internal standardization involves adding a chemical standard to the sample solution so that standard and sample are effectively measured at the same time. Internal chemical standards can be either the actual analyte, an isotopically labelled analyte or a related substance. The last one is usually chosen as something expected to be absent from the sample yet expected to behave towards the measurement process in a way similar to the analyte. There are a number of different ways of using internal standards and they sometimes serve a different purpose. 

Electrodes responding to other halides, sulphide, cyanide, silver, lead, copper and cadmium are made using membranes fabricated from pure or doped silver sulphide (Ag2S). The membrane potential is affected by the movement of Ag+ ions between cationic lattice sites which in turn is determined by the activities of the Ag+ ion in the internal and sample solutions. As the activity of the former is fixed, that of the latter alone influences the membrane potential. The electrode will also respond to the presence of S2- ions because of their effect on the Ag+ ion activity via the solubility product expression ... 

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