ABSTRACT
The laser formation of positive and negative ions on a nanostructured metal surface is observed at laser fluence below the plasma formation threshold. The laser radiation energy dependences of the yield of positive and negative Au ions and charged clusters as well as electrons from the laser-induced nanostructures on the surface of gold are obtained at laser fluence below the plasma formation threshold using a pulsed laser with a wavelength of 355 nm and a pulse duration of 0.37 ns. It is shown that the ratio of the signals of positive and negative ions is constant over the entire range of the laser radiation energies, while the ion signal dependence on the laser radiation energy is described by a power function with an exponent of 9. The role of gold nanoparticles with a size of less than 5 nm in the formation of Au ions and charged Au clusters is discussed.
ABSTRACT
Surface layers of molybdenum oxide MoO3 and tungsten oxide WO3 produced by thermal oxidation of molybdenum and tungsten plates in the air were studied for the first time as a platform for laser-induced electron transfer desorption/ionization. High analytical performance of such layers for the determination of metal complexes with dithiocarbamates, such as tetramethylthiuram disulfide, tetraethylthiuram disulfide, and diethyldithiocarbamate, has been demonstrated. All studied complexes are detected as radical cations, with no fragment ions. The ion yields from MoO3 and WO3 surfaces were found to be more than two orders of magnitude higher than those from nanocrystalline silicon surfaces. A novel method has been developed for the determination of trace amounts of dithiocarbamates based on the complexation of analytes with gold ions, followed by laser-induced electron transfer desorption/ionization. The limits of detection of dithiocarbamates were estimated to be about 1 ng/mL. The proposed method was successfully applied to the rapid screening of tetramethylthiuram disulfide residues in juice.
Subject(s)
Electrons , Molybdenum , Ions/chemistry , Lasers , Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization/methodsABSTRACT
The approach to quantitative analysis by silicon Surface Assisted Laser Desorption Ionization Mass Spectrometry (Si-SALDI) is proposed. The approach is based on the new method for forming an active surface layer on a silicon substrate by exposing to laser radiation directly in the ion source of a mass spectrometer. The method can be used repeatedly on the same substrate, providing high reproducibility of its surface ionization properties and high ionization efficiency of organic compounds. Within the proposed approach, the methods of improvement of signal reproducibility are also considered, including continuous monitoring of the silicon surface ionization properties using a Knudsen effusion cell; scanning the surface of a silicon substrate with a laser beam; selecting the optimal value of laser fluence and using a reproducible sample introduction technique. It is demonstrated that this approach can be successfully applied to quantify clinically relevant concentrations of pharmaceutical drugs in extracts of blood.
ABSTRACT
A rotating ball interface for surface-assisted laser desorption/ionization (SALDI) mass spectrometry was designed and tested. One side of the ball was exposed to atmospheric pressure and the other to the vacuum in a time-of-flight mass spectrometer. Analytes (arginine, atenolol, reserpine, tofisopam, and chloropyramine) were applied using electrospray to a silicon substrate on the atmospheric side, the ball was rotated 180°, and the analyte was desorbed on the vacuum side using a pulsed, 200 Hz, 355 nm laser. In order to increase the desorption area, the laser focus was scanned over the substrate in a raster pattern repeated once every second. The design allows for rapid sample throughout with a sample turn-around time as short as 5 s. Newly produced porous silicon substrates initially yielded very low ion signals, and they required several hundred laser shots to attain maximum sensitivity. In contrast, amorphous silicon did not require such 'activation'. Quantitative analysis showed a sample-to-sample reproducibility of about 10%. The sensitivities with model analytes were in the 1000 to 10,000 ions/fmole range and detection limits in the low fg range.
