Major vacuum ionization techniques

Negative ions are also formed though in smaller quantity and by different mechanisms. Thus the technique is usually used in the positive ion mode, but the negative ions can be also studied by inverting the polarity of the accelerating 

Chemical ionization results from the gas-phase collision between the molecules of analyte M and the species obtained through the electron bombardment of a reagent gas, such as methane, ammonia or isobutene introduced concomitantly, at a pressure of a few hundred pascals, with the compound into the ion source of 

However, for compounds of type RH, the ion R+ is also observed though the chemical ionization always leaves a doubt concerning the value of the molecular mass of the compound studied. 

In ammonia chemical ionization, NH4]+ behaves as GHj] . With isobutane, the main reactive ion is (CH3)3C]+, which leads to the formation of the stable ion (CH3)3C M]+, which leads to the ion (M+ 1)]+, and to isobutene. 

This technique of ionization and those described in the following sections are reserved for polar or non-volatile compounds under vacuum conditions. 

---

A mass spectrometer analyzes ions that are created from molecules. The overall working scheme can be viewed as a combination of three major processes ionization, sorting, and detection. (a) The first step is introduction of sample into the instrument. Samples can be introduced as either a solid, liquid, or vapor into a vacuum chamber through an inlet. Depending on the type of inlet and ionization techniques used, the sample may already exist as preformed ions or it may be imiized in the ion source. Several ionization modes [1] are commercially available with electrospray ionization (ESI) [2] and matrix-assisted laser desorption/ionization (MALDl) [3, 4] being the most widely used nowadays. ESI uses an electrical field to produce a droplet spray the sample in this case is typically in a volatile solvent. Irrespective of the original... 

Currently, high-performance liquid chromatography (HPLC) combined with atmospheric pressure ionization (API) triple-quadrupole mass spectrometry (MS) is the predominate quantitative technique used in modem pharmaceutical bioanalysis. The key technological achievement in API-MS was the efficient ionization in a liquid stream and transference of ions from atmosphere to vacuum. Of the API approaches developed, electrospray ionization (ESI) is the most commonly used. ESI provides an efficient means of soft ionization amenable to most molecules encountered in a dmg discovery setting. An alternative soft ionization approach is the use of desorption ionization (DI) techniques. The major distinguishing feature of DI techniques is that ions are typically produced from dried samples. 

The direct detection of radiation induced crosslinks in polyethylene has been a major goal of radiation chemists for many years. It was recognized as early as 1967 that solution 13c nuclear magnetic resonance (NMR) spectroscopy could be used to detect structures produced in polymers from ionizing radiation. Fischer and Langbein(l) reported the first direct detection of radiation induced crosslinks (H-links) in polyoxymethylene using 13c NMR. Bennett et al.(2) used 13c NMR to detect radiation induced crosslinks in n-alkanes irradiated in vacuum in the molten state. Bovey et al.(3) used this technique to identify both radiation induced H-links and long chain branches (Y-links) in n-alkanes... 

The major problem with all API techniques for mass spectrometry concerns the transfer of the ions from the atmospheric pressure ion source into the vacuum required for operation of the miz analyzer itself, a pressure drop by a factor 10 Such a transfer involves a sudden expansion of the gas at some stage and this tends to enhance the condensation of solvent molecules (particularly water) on the ions to produce clusters of various sizes that redistribute the total ion current among several species thus compUcatmg the spectra and reducing S/B values. An interface between any atmospheric pressure ionization (API) source and a mass spectrometer must be able to deal with the pressure ratio (pumping speed is a crucial factor here. Section 6.6.1) and the de-clustering of the analyte ions before m z analysis and detection. 

---