Negative-electron ionization

For a limited range of substances, negative radical anions (M ) can be formed rather than positive ions (Equation 3.3). Negative radical anions can be produced in abundance by methods other than electron ionization. However, since most El mass spectrometry is concerned with positive ions, only they are discussed here. 

Schematic diagram showing the development of a dipolar field and ionization on the surface of a metal filament, (a) As a neutral atom or molecule approaches the surface of the metal, the negative electrons and positive nuclei of the neutral and metal attract each other, causing dipoles to be set up in each, (b) When the neutral particle reaches the surface, it is attracted there by the dipolar field with an energy Q,. (c) If the values of 1 and <() are opposite, an electron can leave the neutral completely and produce an ion on the surface, and the heat of adsorption becomes Q,. Similarly, an ion alighting on the surface can produce a neutral, depending on the values of I and <(), On a hot filament the relative numbers of ions and neutrals that desorb are given by Equation 7.1,which includes the difference, I - <(), and the temperature, T,...

For many years, electron ionization, then more usually known as electron impact, was the only ionization method used in analytical mass spectrometry and the spectra encountered showed exclusively the positively charged species produced during this process. Electron ionization also produces negatively charged ions although these are not usually of interest as they have almost no structural significance. Other ionization techniques, such as Cl, FAB, thermospray, electrospray and APCI, however, can be made to yield negative ions which are of analytical utility. 

Representative mass spectral conditions (negative chemical ionization) ion source temperature, 150°C ionizing current, 0.20 mamp electron energy, 70 eV methane reagent gas (source pressure 0.5 to 1 torr). 

McClure, TD and Liebler, DC, 1995. Electron capture negative chemical ionization mass spectrometry and tandem mass-spectrometry analysis of beta-carotene, alpha-tocopherol and their oxidation products. J Mass Spectrom 30, 1480-1488. 

There are different ways to ionize a molecule (M, Scheme 2.1) extraction of an electron from gas phase molecules (Mg), yielding radical cations [Equation (2.1)], as occurs in electron ionization, or addition of one [Equation (2.2) Cl, MALDI, etc.)] or more protons [Equation (2.3) ESI]. Similarly, molecules can be ionized by the formation of negative ions due to single [Equation (2.4)] or multiple proton abstraction [Equation (2.5)]. 

Qin BH, Yu BB, Zhang Y, Lin XC. Residual analysis of organochlorine pesticides in soil by gas chromatograph-electron capture detector (gc-ecd) and gas chromatograph-negative chemical ionization mass spectrometry (GC-NCI-MS). Environ. Forensics 2009 10 331-335. 

The lncos-50 is a relatively low-cost benchtop instrument as opposed to the research grade instruments discussed earlier. The gas chromatography-mass spectrometer transfer lines allow it to be used with either the Hewlett Packard 5890 or the Varian 3400 gas chromatographs. The Incos 50 provides data system control of the gas chromatography and accessories such as autosampler or liquid sample concentration. It can be used with capillary, wide-bore or packed columns. It performs electron ionization or chemical ionization with positive or negative detection. It also accepts desorption or other solids controls. 

Imazethapyr herbicide has been determined at the pg kg-1 level in soil by microwave assisted extraction using electron capture negative chemical ionization mass spectrometry [176], A soil extract is prepared by extraction with 0.1M ammonium acetate at pHIO or 1M ammonium acetate at pH9.5. 

Microwave-assisted extraction coupled with gas chromatography-electron capture negative chemical ionization mass spectrometry (i. e., MAE-GC-EC-NCI-MS) was described for the simplified determination of imidazolinone herbicides in soil at the ppb level [715]. 

Kim Y-S, Zhang H, Kim H-Y. 2000. Profiling neurosteroids in cerebrospinal fluids and plasma by gas chromatography/ electron capture negative chemical ionization mass spectrometry. Anal Biochem 277 187. 

GC = gas chromatography EC = electrochemical (detector ECD = electron capture (detector HCD = Hall conductivity detector HFBA = heptafluorobutyric anhydride HPLC = high performance liquid chromatography NCI-MS = mass spectrometry in the negative chemical ionization mode NPD = nitrogen-phosphorus detector ppb = parts per billion UV = ultraviolet absorption SPE = solid phase extraction wt wt = weight weight... 

An ion containing unpaired electrons in its ground state Electron impact ionization Positive chemical ionization Negative chemical ionization... 

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