Laser ratio measurements

Lasers can be used in either pulsed or continuous mode to desorb material from a sample, which can then be examined as such or mixed or dissolved in a matrix. The desorbed (ablated) material contains few or sometimes even no ions, and a second ionization step is frequently needed to improve the yield of ions. The most common methods of providing the second ionization use MALDI to give protonated molecular ions or a plasma torch to give atomic ions for isotope ratio measurement. By adjusting the laser s focus and power, laser desorption can be used for either depth or surface profiling. 

The MC-ICP-MS consists of four main parts 1) a sample introduction system that inlets the sample into the instrument as either a liquid (most common), gas, or solid (e.g., laser ablation), 2) an inductively coupled Ar plasma in which the sample is evaporated, vaporized, atomized, and ionized, 3) an ion transfer mechanism (the mass spectrometer interface) that separates the atmospheric pressure of the plasma from the vacuum of the analyzer, and 4) a mass analyzer that deals with the ion kinetic energy spread and produces a mass spectrum with flat topped peaks suitable for isotope ratio measurements. 

Hirata T, Hayano Y, Ohno T (2003) Improvements in precision of isotopic ratio measurements using laser ablation-multiple collector-ICP-mass spectrometry reduction of changes in measured isotopic ratios. J Anal At Spectrom 18 1283-1288... 

From Eq. (44), to achieve a good S/N ratio measurement, it is necessary and sufficient to lower the fluctuation of /q//. Based on this information, we devised a new method to measure I and /q with a good S/N ratio. We named it the double-pulse method. In the new method, /q is measured by the laser pulse passing just before the laser pulse used to measure 7. Because the frequency of the TirSapphire laser is 81 MHz, the time interval of two laser pulses is 12.3 nsec. The laser pulse for the measurement of 7 passes through the sample 12.3 nsec after the laser pulse for the measurement of 7q. An... 

Park, J., and M. C. Lin, Laser-Initiated NO Reduction by NH, Total Rate Constant and Product Branching Ratio Measurements for the NH2 + NO Reaction, . /. Phys. Chem. A, 101, 5 f3 (f997a). 

Because ICP-MS with different instrumentations and sample introduction systems (besides solution nebulization, also laser ablation or hyphenated methods, such as HPLC, CE, SPME) is today the most frequently used analytical technique for precise and accurate isotope ratio measurements, the following section will mainly focus on this form of mass spectrometry with an inductively coupled plasma source. 

For direct isotope ratio measurements on solid samples, laser ablation ICP-MS (LA-ICP-MS) is applied to an increasing extent in geology, geochronology, biology and environmental research.6... 

An analytical procedure has been proposed for precise uranium isotope ratio measurements in a thin uranium layer on a biological surface by LA-ICP-MS using a cooled laser ablation chamber.125 One drop of uranium isotope standard reference materials NIST, 350, NIST 930, of our isotopic laboratory standard CCLU 500 (20p.l, U concentration 200 ng 1) and of uranium with natural isotopic pattern were deposited on the leaf surface and analyzed by LA-ICP-MS at well defined laser crater diameters of 10, 15, 25 and 50 p.m. A precision for measurements of isotope ratios in the range of 2.1-1.0% for 235U/238U in selected isotope standards was observed whereby the precision and the accuracy of isotope ratios compared to the non-cooled laser ablation chamber was improved.125... 

The effects of raloxifene on the vascular endothelium have been studied in 19 subjects who underwent endothelial function testing at baseline and after treatment with placebo or raloxifene (60 mg/day for 6 weeks) (27). The findings in this small short-term study were entirely positive. Brachial artery diameter change (flow-mediated dilatation) increased 5.0% with placebo and 8.6% with raloxifene in response to a hyperemic stimulus. The ratio of AUC response to AUC reference with the use of laser Doppler measures was 1.18 for placebo and 1.28 for raloxifene. Flow-mediated dilatation and AUC ratio correlated significantly. The authors concluded that raloxifene enhanced endothelial-mediated dilatation in brachial arteries and digital vessels in these women, and they discussed the drug s possible cardioprotective effect. 

Walder, A. J., Abell, I. D., Platzner, I., and Freedman, P. A. (1993a). Lead isotope ratio measurement of NIST 610 glass by laser ablation inductively coupled plasma spectrometry. Spectrochim. Acta 48B, 397-402. 

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