Atmospheric small-molecule MALDI

MALDI has also been used to quantify small molecules. This aspect has been demonstrated by the analysis of lysergic acid diethyl amide (LSD) with atmospheric pressure (AP) MALDI [20]. LSD is a hallucinogen. Its quantification is highly important from forensic and clinical point of views. In this method, LSD was extracted and precleaned from urine samples by SPE and quantified by recording the ion current due to m/z 327 226 and 324 223 transitions. The standard curves were plotted by analyzing several LSD standard solutions, each of which was spiked with LSD-iis as an IS. A linear calibration curve was obtained over the concentration range 1 to 100 ng mL with = 0.9917. The results were comparable with those of an existing HPLC/ESI-MS method. 

Although the majority of MALDI-MS experiments have followed the originators (Tanaka 1988 Karas 1987, 1988) by inserting the matrix-analyte mixture into the high vacuum chamber of the mass spectrometer before irradiation, development of MALDI sources that operate at atmospheric pressure has been described (Laiko 2000 Moyer 2002). Such methods obviously require efficient interfaces between the atmospheric pressure ion source and the mass spectrometer vacuum as for the more usual API sources (Section 5.3.3). Currently this approach does not appear to have been exploited for trace quantitative analyses of small molecules. 

Complications of drug analysis were successfully resolved by the combination of TLC with MALDI-MS. TLC combined with MALDI-MS was used for analysis of psychotropic drugs (3,4-methylenedioxy methamphetamine, 4-hydroxy-3-methoxy methamphetamine, 3,4-methylenedioxy amphetamine, methamphetamine, p-hydroxy methamphetamine, amphetamine, ketamine, caffeine, chlorpromazine, triazolam, and morphine) in biological samples [42]. This technique was able to analyze 3,4-methylenedioxy methamphetamine (MDMA) and its metabolites in urine samples without sample dilution, and the detection limit of the MDMA spot was 0.05 ng/ spot. Crecelius and coworkers described the use of TLC with MALDI-MS/MS for the structural analysis of small drug molecules [43]. This method was successfully applied to analyze two representatives of nonsteroidal antiinflammatory drugs (tenoxi-cam and piroxicam), and pharmaceutically active compound UK-137,457 and one of its related substances UK-124,912. The feasibility of UTLC-atmospheric pressure (AP)-MALDI-MS was described for the analysis of small molecules (triazole, midazolam, verapamil, and metaprolol) [44]. The authors compared the selectivity and sensitivity between UTLC- and HPTLC-AP-MALDI-MS. It was observed that UTLC plates provided 10-100 times better sensitivity in MALDI analysis than the conventional... 

Haapala M, Pol J, Saarela V, Arvola V, Kotiaho T, Ketola RA, Franssila S, Kauppila TJ, Kostiainen R (2007) Desorption atmospheric pressure photoionization. Anal Chem 79 7867-7872 Hsieh Y, Li F-B, Korfmacher WA (2010) Mapping pharmaceuticals in rat brain sections using MALDI imaging mass spectrometry. Methods Mol Biol 656 147-158 Hu LG, Xu SY, Pan CS, Zou HF, Jiang GB (2007) Preparation of a biochip on porous sihcon and application for label-free detection of small molecule-protein interactions. Rapid Commun Mass Spectrom 21 1277-1281... 

MAI,PI was introduced in the late 1980s and is one of the most successfully developed MS soft ionization techniques that uses the matrix assists laser ablation of sample-coated target to vaporize gas-phase ions for injection into a mass spectrometer. The advantage of MALDI is its gentleness compared with ESI and Atmospheric Pressure Chemical Ionization (APCI) and its ability to analyze the polar, nonvolatile, and large molecules. It has been very successfully used for the analysis of both biopolymers compounds and small molecular organic compounds (<1,500 Da). 

A secmid popular soft ionization method is MALDI. In this technique, analytes are co-crystallized with a matrix (typically a small, acidic, organic molecule), with an absorption maximum close to the wavelength of a laser used to irradiate the substrate. This process is typically performed in vacuum (although it has been shown to be feasible at atmospheric pressure) after the analytes have been desorbed and ionized (in MALDI, it is believed that fast heating caused by the laser pulse desorbs analytes into gas phase however, the process by which... 

Electrospray ionization and atmospheric pressure chemical ionization are popular as ionization techniques, for qualitative and quantitative LC—MS analysis of lipids [63—65]. Based on flieir ionization mechanisms, ESI is more suitable for ionization of polar and ionic compounds and is capable of ionizing both small and large biomolecules. APCI can ionize less polar and neutral compounds more efficiently than ESI. Consequently, APCI—MS coupled to LC is the most used tool for TAG identification, because of the full compatibility with common NARP LC conditions, easy ionization of nonpolar TAGs, and the attainment of both protonated molecules [M + H]+ and fragment ions [M - - H — RzCOOH]. On the other hand, ESI is usually employed for the more-polar phospholipids. However, ESI or matrix-assisted laser desorption—ionization (MALDI) have been used for TAGs, as well [66,67]. 

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