Жылдам атомдық бомбардылау және массалық спектрометрия техникасы

Fast atom bombardment (FAB) is an ionization technique used in mass spectrometry in which a beam of high energy atoms strikes a surface to create ions. It was developed by Michael Barber at the University of Manchester in 1980. When a beam of high energy ions is used instead of atoms (as in secondary ion mass spectrometry), the method is known as liquid secondary ion mass spectrometry (LSIMS). In FAB and LSIMS, the material to be analyzed is mixed with a non volatile chemical protection environment, called a matrix, and is bombarded under vacuum with a high energy (4000 to 10,000 electron volts) atomic beam. The atoms are typically from an inert gas such as argon or xenon. Common matrices include glycerol, thioglycerol, 3 nitrobenzyl alcohol (3 NBA), 18 crown 6 ether, 2 nitrophenyloctyl ether, sulfolane, diethanolamine, and triethanolamine. This technique is similar to secondary ion mass spectrometry and plasma desorption mass spectrometry.

FAB is a relatively low fragmentation (soft) ionization technique and produces primarily intact protonated molecules denoted as [M + H]+ and deprotonated molecules such as [M H]−. Radical cations can also be observed in a FAB spectrum in rare cases. FAB was designed as an improved version of SIMS that allowed for the primary beam to no longer cause damaging effects to the sample. The major difference between the two techniques is the difference in the nature of the primary beam used; ions vs atoms. Xe atoms are used because they tend to be more sensitive than Argon atoms due to their larger masses and more momentum. For the molecules to be ionized by FAB, first the slow moving atoms (Xe or Ar) are ionized by colliding electrons. Those slow moving atoms are then ionized and accelerated to a certain potential where they develop into fast moving ions that become neutral in a dense cloud of excess natural gas atoms that make a flowing stream of high translational energy atoms. and chemical ionization.

As previously stated, in FAB the samples are mixed with a non volatile environment (matrix) in order to be analyzed. FAB uses a liquid matrix that is mixed with the sample in order to provide a sample ion current that is sustained, reduces damages made to the sample by absorbing the impact of the primary beam, and keeps the sample molecules form aggregating. (CF) FAB was developed to minimize the problem of poor detection sensitivity that is caused by an excess of the matrix background that results in a high matrix to sample ratio. Samples can be introduced by flow injection, microdialysis, or by coupling with liquid chromatography. Flow rates are typically between 1 and 20 μL/min.

The first example of the practical application of this FAB was the elucidation of the amino acid sequence of the oligopeptide efrapeptin D. This contained a variety of very unusual amino acid residues. The sequence was shown to be: N acetyl L pip AIB L pip AIB AIB L leu beta ala gly AIB AIB L pip AIB gly L leu L iva AIB X. PIP = pipecolic acid, AIB = alpha amino isobutyric acid, leu = leucine, iva = isovaline, gly = glycine. This is a potent inhibitor of mitochondrial ATPase activity. Another application of FAB includes its original use for the analysis of condensed phase samples. FAB can be use for measurements of the molecular weight of samples below 5000 Da, as well as their structural characteristics. FAB can be paired with various mass spectrometers for data analysis, such as with a quadrupole mass analyzer, liquid chromatography–mass spectrometry, and more.

In 1983 a paper was published describing the use of fast atom bombardment mass spectrometry (FAB MS) to analyze isotopes of calcium. Glycerol was not used; samples in aqueous solution were deposited on the sample target and dried prior to analysis. The technique was effectively secondary ion mass spectrometry using a neutral primary beam. This was a welcomed development for biomedical researchers studying the nutrition and metabolism of essential minerals but lacking access to inorganic mass spectrometry instrumentation such as thermal ionization mass spectrometry or inductively coupled plasma mass spectrometry (ICP MS). In contrast, FAB mass spectrometers were widely found in biomedical research institutions. Multiple laboratories adopted this technique, using FAB MS to measure isotope ratios in isotope tracer studies of calcium, iron, magnesium and zinc. The analysis of metals required minimal modification of the mass spectrometers, e. g. replacing the stainless steel sample targets with pure silver ones to eliminate background from ionization of stainless steel components. Signal acquisition systems were sometimes modified to perform peak jumping instead of scanning and to do ion counting detection. While satisfactory precision and accuracy were attained with FAB MS, the technique was labor intensive with a very low sample through put rate due in part to the absence of auto sampling options. By the early 2000s this severe sampling rate limitation had motivated users of FAB MS for mineral isotope analysis to switch to conventional inorganic mass spectrometers, usually ICP MS which also exhibited improved affordability and isotope ratio analysis performance by that time.