This study provides the most comprehensive and up-to-date set of international sex-specific and age-specific 20mSRT norms for children and youth, which have utility for health and fitness screening, profiling, monitoring and surveillance.

A combination of liquid matrix and graphite particulates (2 μm) has been proposed as a method suitable for the laser desorption/ionization mass spectrometry of peptides and proteins (Sunner, J.; et al. Anal. Chem. 1995, 67, 4335). Here we demonstrate the potential of this approach as a straightforward, and very general, method of achieving the ultraviolet laser desorption/ionization of a broad range of intermediate weight analytes. The desorption/ionization mechanism, the influence of preparative procedures, and the breadth of application of this methodology have been investigated. A simple and robust preparative procedure is presented for the analysis of proteins, oligosaccharides, and synthetic polymers. Detection sensitivities are in the femtomole region for lower molecular weight peptides and oligosaccharides. The graphite acts as an energy transfer medium by absorbing the UV radiation, leading to thermal desorption of the liquid matrix and analyte. The liquid matrix was observed to fulfill several important roles. In the case of peptides and proteins, which preferentially form protonated molecular ions, it acts as a protonating agent. It also enhances the signal intensities of cationized species (e.g., polysaccharides and polar polymers) by assisting their desorption. An excess of liquid matrix serves to cool the analyte during the desorption step and minimize decomposition. The presence of liquid matrices increases the sample lifetime at a particular desorption spot, minimizing the time-consuming search for "hot spots". The addition of cationizing salts has been shown to improve the quality of mass spectra obtained for polar polymers and extend the range of materials that can be investigated to include apolar synthetic polymers.

At appropriate matrix:analyte mixing ratios, small to moderate sized analyte ions (lOOO-20OOOu) can fully suppress positively charged matrix ions in matrix-assisted laser desorptiodionization (MALDI) mass spectra. This is true for all matrix species, including radical cations and adducts with protons or alkali-metal ions. Full matrix suppression is also observed, regardless of the preferred d y t e ion form, be it protonated or an alkali adduct. These facts lead us to propose a mechanism for prompt, primary (not secondary gas-phase) MALDI ionization in which excited matrix molecules are the key species. At least two such excited molecules are believed to be necessary for free ion generation. This model is found to be consistent with the available data, as well as making several predictions which are confinned by new observations. The model also predicts that the matrix suppression effect will not be observable with heavy analytes because their large excluded volume precludes desorption at the neceSSary mixing ratios.Although it is not widely discussed in the literature, practitioners of matrix-assisted laser desorptionlionization (MALDI) have often noted a remarkable phenomenon: when analytes of moderate size (1000-20 OOO u) are mixed with matrix in relatively high molar ratio (1O:l to about 2OOO:l matrix-to-analyte ratio), the appearance of positive matrix ions in the mass spectrum can be completely suppressed.'.' Along with the fact that analyte fragmentation is typically weak in MALDI, this leads to nearly ideal mass spectra: strong analyte molecular ions but no other signals of any kind. Since the effect has been observed with a number of matrices'.' including nicotinic acid, a-cyanoChydroxycinnamic acid (CCA) and 2,5 dihydroxybenzoic acid (DHB), it seems to be a general phenomenon in MALDI. It would clearly be advantageous to understand the effect, so as to extend it to the widest possible range of analytes. At the same time we can also expect to learn something about the molecular-level mechanisms of the MALDI method.In their paper describing CCA as a MALDI matrix, Beavis et aL3 presented a spectrum of substance P (1 347 u) in which the matrix peaks were nearly absent. They did not specifically note this partial matrix suppression effect, nor did they study the matrix-to-analyte mixing ratio dependence of the matrix signals, but they did propose a disproportionation ionization mechanism involving nonionized matrix as an intermediate. They also noted the possibility of proton generation from the matrix as a first step, followed by efficient transfer to the analyte, but did not go on to verifj these hypotheses.Matrix-ion suppression was explicitly studied by Chan er aL,' using nicotinic acid as the matrix, and insulin (5807 u), lysozyme (-14 300 u) and myoglobin (-18 OOO u) as the analytes. They investigated matrix-to-analyte ratios of about 600 to 6OO0, and found that larger analytes required larger ratios for good suppression. They tentatively proposed a suppression model based on the competition between ma...

The aim of this study was to determine the accuracy of dual‐energy X‐ray absorptiometry (DXA)‐derived percentage fat estimates in obese adults by using four‐compartment (4C) values as criterion measures. Differences between methods were also investigated in relation to the influence of fat‐free mass (FFM) hydration and various anthropometric measurements. Six women and eight men (age 22–54 years, BMI 28.7–39.9 kg/m2, 4C percent body fat (%BF) 31.3–52.6%) had relative body fat (%BF) determined via DXA and a 4C method that incorporated measures of body density (BD), total body water (TBW), and bone mineral mass (BMM) via underwater weighing, deuterium dilution, and DXA, respectively. Anthropometric measurements were also undertaken: height, waist and gluteal girth, and anterior‐posterior (A‐P) chest depth. Values for both methods were significantly correlated (r2 = 0.894) and no significant difference (P = 0.57) was detected between the means (DXA = 41.1%BF, 4C = 41.5%BF). The slope and intercept for the regression line were not significantly different (P > 0.05) from 1 and 0, respectively. Although both methods were significantly correlated, intraindividual differences between the methods were sizable (4C‐DXA, range = −3.04 to 4.01%BF) and significantly correlated with tissue thickness (chest depth) or most surrogates of tissue thickness (body mass, BMI, waist girth) but not FFM hydration and gluteal girth. DXA provided cross‐sectional %BF data for obese adults without bias. However, individual data are associated with large prediction errors (±4.2%BF). This error appears to be associated with tissue thickness indicating that the DXA device used may not be able to accurately account for beam hardening in obese cohorts.

The light-induced aging of natural triterpenes used in varnishes was studied using graphite-assisted laser desorption/ionization mass spectrometry. This method was found to be superior to conventional matrix-assisted laser desorption/ionization for these materials because of higher signal levels and less chemical noise from the matrix. The dammar and mastic raw materials were found to be partially oxidized compared to their nominal composition, with up to six oxygens incorporated. The light-induced aging process leads initially to polymerization, up to trimers. Further aging leads to a decrease in average molecular weight. These observations are supplemented by FTIR and 13 C NMR spectra which show complete