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abundance for any one permutation formula is larger than that for the original formula, the mass of the permutation formula is computed, and a check is made to determine the existence of an ion corresponding to that mass in the spectrum. If the ion corresponding to the more abundant permutation formula is not found, the less abundant original formula is rejected. In other words, if any one of the permutations which are more abundant is not found, the formula is unsatisfactory.The final printed output is shown in Table V for the same trichloroaniline spectrum as in Tables I and III.The columns give the observed mass of the ion, its intensity in per cent of tallest peak, the number of rings and double bonds for the formula, the ppm error between the observed mass and the mass calculated for the formula, and the formula. The ppm error averages 5-10 ppm, but worst-case error runs as high as 30-40 ppm. It is expected that this will improve when vibration is eliminated from the instrument, but a tolerance of 50 ppm between the formula and the observed mass is being used until then. It should be noted that even with a 50-ppm tolerance in the mass, most of the ions correspond to only one formula. Only one formula is given for the molecular ion at mass 194.93779. That formula is H4C6NCI3 which is correct for the sample, trichloroaniline. Peaks can be observed at 159.96703 corresponding to p-Cl; at 125.00142 corresponding to p-2Cl; and at 90.03779 corresponding to p-3Cl. The structure of the compound is well represented in the formulas derived as a final result of the data reduction. ACKNOWLEDGMENTWe wish to thank Paul Bender and his staff for their efforts and cooperation in the development of this work and Milt Levenberg of Abbott Laboratories, Chicago, 111., for his valuable advice and comments on various aspects of the approach to obtaining meaningful data.

The ATLAS liquid argon calorimeter has been operating continuously since August 2006. At this time, only part of the calorimeter was readout, but since the beginning of 2008, all calorimeter cells have been connected to the ATLAS readout system in preparation for LHC collisions. This paper gives an overview of the liquid argon calorimeter performance measured in situ with random triggers, calibration data, cosmic muons, and LHC beam splash events. Results on the detector operation, timing perfore-mail: atlas.secretariat@cern.ch mance, electronics noise, and gain stability are presented. High energy deposits from radiative cosmic muons and beam splash events allow to check the intrinsic constant term of the energy resolution. The uniformity of the electromagnetic barrel calorimeter response along η (averaged over φ) is measured at the percent level using minimum ionizing cosmic muons. Finally, studies of electromagnetic showers from radiative muons have been used to cross-check the Monte Carlo simulation. The performance results obtained using the ATLAS readout, data acquisition, and reconstruction software indicate that the liquid argon calorimeter is well-prepared for collisions at the dawn of the LHC era.

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