Inlet discrimination

Upon injection, the less volatile sample components will not vaporize as rapidly as the more volatile sample components. Immediately following injection, the vaporized sample has a greater proportion of the more volatile compounds than the less volatile compounds. Therefore, more of the volatile compounds are introduced into the column. This effect is called inlet discrimination. The peaks for the less volatile compounds will be smaller than the more volatile compounds. The longer the sample spends in the heated injector, the less severe this type of discrimination is experienced by the sample. If inlet discrimination needs to be eliminated, an on-column injector should be used. 

There are four major eapillary injection techniques split, splitless, on-column, and megabore or direct injection. Nearly every standard capillary injector is capable of split and splitless injections. On-column injections require a dedicated capillary on-column, injector and is a required injector for high-temperature gas chromatography and to minimize inlet discrimination [1,2]. 

Another characteristic similar to A/ 100 is the Distribution Octane Number (DON) proposed by Mobil Corporation and described in ASTM 2886. The idea is to measure the heaviest fractions of the fuel at the inlet manifold to the CFR engine. For this method the CFR has a cooled separation chamber placed between the carburetor and the inlet manifold. Some of the less volatile components are separated and collected in the chamber. This procedure is probably the most realistic but less discriminating than that of the AJ 100 likewise, it is now only of historical interest. 

TOF mass spectrometers are very robust and usable with a wide variety of ion sources and inlet systems. Having only simple electrostatic and no magnetic fields, their construction, maintenance, and calibration are usually straightforward. There is no upper theoretical mass limitation all ions can be made to proceed from source to detector. In practice, there is a mass limitation in that it becomes increasingly difficult to discriminate between times of arrival at the detector as the m/z value becomes large. This effect, coupled with the spread in arrival times for any one m/z value, means that discrimination between unit masses becomes difficult at about m/z 3000. At m/z 50,000, overlap of 50 mass units is more typical i.e., mass accuracy is no better than about 50-100 mass... 

The choice of a mass spectrometer to fulfill any particular task must take into account the nature of the substances to be examined, the degree of separation required for mixtures, the types of ion source and inlet systems, and the types of mass analyzer. Once these individual requirements have been defined, it is much easier to discriminate among the numerous commercial instruments that are available. Once suitable mass spectrometers have been identified, it is then often a case of balancing capital and running costs, reUability, ea.se of routine use, after-sales service, and manufacturer reputation. 

Cold on-column injection differs from the vaporization techniques discussed above in that the sample is introduced directly as a liquid into the column inlet, where it is subsequently vaporized. In this way, discrimination is virtually... 

As for all trace-level analyses, sample preparation and handling are of crucial importance. In addition to all the usual problems of GC-MS, measurements of isotope ratios must ensure that none of these steps introduce any isotope discrimination. Any chemical reactions, including conversion of the organic sample molecules to the simple gases which are those actually analyzed, must be quantitative (100% conversion) to avoid kinetic isotope effects [627]. Until relatively recently, all gas IRMS experiments employed a dual-inlet system to permit switching between sample and standard C02 contained in two bellows containers. The pressures in the two bellows are adjusted to be equal and,... 

In direct injection, all injected material is carried onto the column by the mobile phase. This eliminates the possibility of sample discrimination in the inlet, but can overload capillary columns and so is more commonly used with packed columns and wide-bore capillary columns. 

FIGURE 4.5 Sea turtle orientation, (a) Bird eye s view of apparatus to test whether sea turtles discriminate sea water samples from different locations. Sea water flows from water inlet toward the standpipe (arrows). The turtle is placed near the standpipe. Phototransistors record when the animals move into different compartments. The water washes of sand come from Padre Island (P) and Galveston (G), and unscented water in two compartments serves as control (C). (b) Turtles spent more time in water extract of sand from Padre Island than in that from Galveston. (From Grassman etal., 1984.)... 

Mass spectrometry should be used discriminately where a maximum amount of information can be expected. The heavier nonvolatile feedstocks are for practical purposes, beyond the useful range of routine mass spectrometry. At the elevated temperatures necessary to encourage volatility, thermal decomposition will occur in the inlet and any subsequent analysis would be biased to the low molecular weight end and to the lower molecular products produced by the thermal decomposition. 

Cold on-column injections allow the inlet and/or inlet section of the column to be cooled or maintained at lower temperatures than the oven. Cold on-column injection suffers from the possibility of sample zone band broading due to excessive solvent flooding. The advantage of cold on-column injection is that it reduces sample discrimination or sample loss, due to syringe needle heating during injection. 

Initial reaction rates obtained with a pure feed in which only reactants are present can be used for the discrimination between rival kinetic models, i.e. to identify whether adsorption, desorption, or surface reactions are the rate-determining steps. When pure A is fed to an integral reactor, for example, initial rates are observed at the inlet, where the product concentration is still zero. Comparing possible rate equations, which are often simpler in case of absence of products, with experimental data obtained at different concentrations of A, helps to reveal the appropriate [33,35]. 

---