Infrared laser desorption techniques

Ideally, scientists would like to be able to perform laser desorption and analysis directly, but typical laser wavelengths cause fragmentation of bacteria and other particles. Due to the low energy produced by infrared lasers, however, bacterial fingerprints can indeed be obtained as shown by researchers at Lawrence Livermore National Laboratory. It is also possible to detect much larger species, an impossible task with earlier technology. Infrared laser desorption techniques are undergoing constant improvement. 

For infrared laser desorption Fourier transform mass spectrometry (LD-FTMS), all den-drimer samples were prepared by dissolving ca. 1 mg of sample in CH2CI2, followed by deposition upon stainless steel probe tips by the aerosol spray technique described previously [37]. Dendrimer samples 4 and 5 were deposited directly onto a stainless steel probe tip. The sample 6 was prepared by first spraying 50 mL of a saturated silver nitrate/ethanol solution (containing ca. 3 mg of silver nitrate) onto the rotating probe tip, prior to dendrimer deposition. Samples were introduced into the vacuum system and the source cell pressure reduced to 2.2 X 10 Torr and the analyzer cell pressure to 2.0 x 10 Torr, before analysis. 

LD-FTMS is a valuable technique for characterisation of industrial materials. Simonsick et al. [215] analysed novel dispersants, fluorinated surfactants, and natural oils with masses in the 500-3000 Da range. Wyplosz [219] has carried out a laser desorption mass spectrometric study of artists organic pigments. The considerable potential of infrared laser desorption has yet to be fully exploited. 

Identification of the forms thus obtained using complementary molecule-specific techniques (nuclear magnetic resonance infrared [IR] matrix-assisted laser desorption/ionization electrospray ionization [ESl]/atmospheric pressure chemical ionization mass spectrometry [MS])... 

Elucidation of degradation pathways and identification of transformation products (TPs) is of crucial importance in understanding their fate in the environment and requires the employment of advanced instrumental techniques. Analytical methods that can be used for this purpose include Uquid chromatography with diode array or fluorescence detector (LC-DAD/FL), nuclear magnetic resonance (NMR), infrared spectroscopy (IR), matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS), gas... 

Impurity inclusions and surface defects are a cause of many difficulties to the polymer producer and user. Equipment used for studying these phenomena discussed in Chapter 4 include electron microprobe x-ray emission/spectroscopy, NMR micro-imaging, various forms of surface infrared spectroscopy, e.g., diffusion reflection FTIR, ATR, also photoacoustic spectroscopy and x-ray diffraction - infrared microscopy of individual polymer fibres. Newer techniques such as scanning electron microscopy (SECM), transmission electron microscopy, time of flight secondary ion mass spectrometry (TOFSIMS), laser induced photoelectron ionisation with laser desorption, atomic force microscopy and microthermal analysis are discussed. 

Frequently, in work carried out to attempt to elucidate the microstructure of polymers and copolymers, it is found necessary to employ not one physical analytical technique, but a range of such techniques. The literature abounds in examples of this need. Judging by the number of papers published, three of the most useful and commonly used techniques are combinations of Fourier transform infrared spectroscopy (FTIR) or infrared (IR) spectroscopy and matrix assisted laser desorption-ionisation time-of-flight (MALDI-TOF) mass spectrometry with nuclear magnetic resonance spectroscopy or photon magnetic resonance spectroscopy. 

A relatively unknown technique (laser desorption fourier transform ion cyclotron resonance mass spectroscopy) has been used (397) to identify dyes in plastics such as polymethylmethacrylate. A detection limit of 0.1% was obtained, which compared with a limit of 1-2% using an ATR infrared spectroscopy technique. 

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