Laser Plasma Ionization

In the past, for the analysis of thin sections of tissues or thin transparent foils a laser ionization ion source in the transmission or reflection mode of laser irradiation (using e.g., the LAMMA 500 or LAMMA 1000, respectively) was employed. The lateral resolution observed in the transmission mode was about 1 (im and in the reflection mode 5-20 gm. 

A high vacuum laser ion source is combined with a dynamic (ToF-MS) or static mass spectrometer (Mattauch-Herzog instrument) for the separation of the ion beams formed and used in LIMS for multi-element major, minor and trace analysis of compact solid samples (bulk analysis).74 

In analogy to the observed behaviour in LIMS, in LA-ICP-MS increasing fractionation effects are observed with decreasing laser power density for I 10 Wcm . These fractionation effects increase significantly if the laser power density is lower than lO Wcm. A stoichiometric laser ablation of sample material is observed at a laser power density between 10 Wcm and 10 °Wcm in the author s laboratory. 

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The technique based on laser-induced breakdown coupled to mass detection, which should thus be designated LIB-MS, is better known as laser plasma ionization mass spectrometry (LI-MS). The earliest uses of the laser-mass spectrometry couple were reported in the late 1960s. Early work included the vaporization of graphite and coal for classifying coals, elemental analyses in metals, isotope ratio measurements and pyrolysis [192]. Later work extended these methods to biological samples, the development of the laser microprobe mass spectrometer, the formation of molecular ions from non-voIatile organic salts and the many multi-photon techniques designed for (mainly) molecular analysis [192]. 

El = electron ionization Cl = chemical ionization ES = electrospray APCI = atmospheric-pressure chemical ionization MALDI = matrix-assisted laser desorption ionization PT = plasma torch (isotope ratios) TI = thermal (surface) ionization (isotope ratios). 

The ablated vapors constitute an aerosol that can be examined using a secondary ionization source. Thus, passing the aerosol into a plasma torch provides an excellent means of ionization, and by such methods isotope patterns or ratios are readily measurable from otherwise intractable materials such as bone or ceramics. If the sample examined is dissolved as a solid solution in a matrix, the rapid expansion of the matrix, often an organic acid, covolatilizes the entrained sample. Proton transfer from the matrix occurs to give protonated molecular ions of the sample. Normally thermally unstable, polar biomolecules such as proteins give good yields of protonated ions. This is the basis of matrix-assisted laser desorption ionization (MALDI). 

Ion extraction. The aspirated or laser ablated sample is transported from the sample introduction system into the center of the torch by a 1 1/min flow of Ar carrier gas where it is immediately dissociated and ionized by energy transfer with the hot -6000 K temperature of the surrounding Ar plasma. Ionization efficiencies are >95% for U and Th (Jarvis et al., 1992). For laser ablation sampling, helium may be employed as the carrier... 

Analytes must be liberated from their associated solvent molecules as well as be ionized to allow mass separation. Several ionization methods enable ion production from the condensed phase and have been used for the coupling of CE to MS. Among them, atmospheric pressure ionization (API) methods, matrix-assisted laser desorption/ionization (MALDI), and inductively coupled plasma (ICP) ionization are mainly used. API techniques are undoubtedly the most widespread ionization sources and cover different analyte polarity ranges. 

The analysis for proteins present in plasma or a cell extract is a challenging task due to their complexity and the great difference between protein concentrations present in the sample. Simple mixtures of intact proteins can be analyzed by infusion with electrospray ionization and more complex ones by matrix assisted laser desorption ionization. MALDI is more suited for complex mixtures because for each protein an [M+H]+ signal is observed while for ESI multiply charged ions are observed. Surface enhanced laser desorption (SEEDI) is a technique for the screening of protein biomarkers based on the mass spectrometric analysis of intact proteins [49]. However in most cases for sensitivity reasons mass spec-... 

CONTENTS Preface, Joseph Sneddon. Analyte Excitation Mechanisms in the Inductively Coupled Plasma, Kuang-Pang Li and J.D. Winefordner. Laser-Induced Ionization Spectrometry, Robert B. Green and Michael D. Seltzer. Sample Introduction in Atomic Spectroscopy, Joseph Sneddon. Background Correction Techniques in Atomic Absorption Spectrometry, G. Delude. Flow Injection Techniques for Atomic Spectrometry, Julian F. Tyson. 

The dominating method of ion formation in metabolic flux analysis is electron impact. It might be supplemented in the future by novel methods, such as matrix assisted laser desorption and electrospray. Additional techniques such as chemical ionization, fast atom bombardment or inductively coupled plasma ionization are only of minor importance and not further discussed in this context. 

The Nickel Producers Environmental Research Association (NiPERA) is sponsoring research on the application of inductively coupled plasma-mass spectroscopy (ICP-MS) to isotopic analysis of nickel in biological samples, on the development of sampling instrumentation for assessing workers exposure to nickel in the nickel industry, and on methods for utilizing newly developed analytical methods, such as laser beam ionization mass spectrometry, for the identification and speciation of nickel compounds in powders and dusts with particular reference to nickel refining. 

Emission spectroscopy utilizes the characteristic line emission from atoms as their electrons drop from the excited to the ground state. The earliest version of emission spectroscopy as applied to chemistry was the flame test, where samples of elements placed in a Bunsen burner will change the flame to different colors (sodium turns the flame yellow calcium turns it red, copper turns it green). The modem version of emission spectroscopy for the chemistry laboratory is ICP-AES. In this technique rocks are dissolved in acid or vaporized with a laser, and the sample liquid or gas is mixed with argon gas and turned into a plasma (ionized gas) by a radio frequency generator. The excited atoms in the plasma emit characteristic energies that are measured either sequentially with a monochromator and photomultiplier tube, or simultaneously with a polychrometer. The technique can analyze 60 elements in minutes. 

There are many different ionization techniques available to produce charged molecules in the gas phase, ranging from simple electron (impact) ionization (El) and chemical ionization (Cl) to a variety of desorption ionization techniques with acronyms such as fast atom bombardment (FAB), plasma desorption (PD), electrospray (ES), and matrix-assisted laser desorption ionization (MALDI) (Mano and Goto 2003). 

Laser ablation and inductively coupled plasma ionization... 

In contrast, the LA-ICP-MS (in comparison to laser ionization mass spectrometry (LIMS) where the ion source operates under high vacuum conditions) at present, in spite of the disadvantage of a higher polyatomic ion formation rate, uses an argon plasma ionization at normal pressure - a promising inorganic mass spectrometric technique for trace, isotope and surface analysis which will... 

In atomic spectroscopy, a substance is decomposed into atoms in a flame, furnace, or plasma. (A plasma is a gas that is hot enough to contain ions and free electrons.) Each element is measured by absorption or emission of ultraviolet or visible radiation by the gaseous atoms. To measure trace elements in a tooth, tiny portions of the tooth are vaporized (ablated) by a laser pulse1 and swept into a plasma. The plasma ionizes some of the atoms, which pass into a mass spectrometer that separates ions by their mass and measures their quantity. 

Elements are incorporated into teeth from the diet or by inhalation. The figure shows trace element profiles measured by laser ablation—plasma ionization-mass spectrometry of the dentine of teeth from a modem person and one who lived in Scandinavia about a.d. 1800. The contrast is striking. The old tooth contains significant amounts of tin and bismuth, which are nearly absent in the modern tooth. The old tooth contains more lead and antimony than the modem tooth. Tin and lead are constituents of pewter, which was used for cooking vessels and utensils. Bismuth and antimony also might come from pewter. 

A. Tsarbopoulos, M. Karas, K. Strupat, B. N. Pramanlk, T. L. Nagabhushan, and F. Hillen-kamp, Comparative mapping of recombinant proteins and glycoproteins by plasma desorption and matrix-assisted laser desorption/ionization mass-spectrometry, Anal. Chem., 66 (1994) 2062-2070. 

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