This introduces mass spectrometry in the high-mass region (10-300 k parent region) as a method of characterizing passivated metal nanocrystals in the form of mixtures and purified samples. The methods are illustrated by application to noble metal (Au) nanocrystals passivated by a strongly bound self-assembled monolayer (SAM) of n-alkanethiolate (RS-) surfactant molecules. Neat samples in the form of thick films were irradiated by UV laser (308 nm). The desorbed ions were separated by time-of-flight mass spectrometry (TOFMS), and waveforms are converted to mass spectra, while accounting for ejection velocity. The results are consistant with intact desorption and detection of nanocrystal core, but with extensive, variable loss of the surfactant layer as dialkyldisulfides (RSSR). Aggregation of partially depassivated nanocrystals is detected and can be suppressed by dilution in an organic matrix.

Mass spectra show the size separation of gold nanocrystals from the crude mixture (a), to the separated fractions (b-f). The mass scale is plotted as the cube root of the mass (bottom axis), where k = 1000 amu, and as the cube root of gold atoms (top axis). The size selective precipitation was done on the two phase (Brust, et. al) gold mixture. The inset spectra are the perdicted optimal core structures containing N=459 (b), 314 (c), 225, (d), 140 (e), and 116 (f) gold atoms. The inset frame (e) is a high-resolution mass spectrum plotted on a linear mass scale, with an arrow marking the first peak at 27610 amu, corresponding to nanocrystal ion of 140 gold atoms and one sulfur atom attached. The instrument used to obtain these spectra is a custom built, 1.2 m linear TOF mass spectrometer. The laser desorption and matrix assisted laser desorption was achieved using a Lambda Physiks XeCl excimer at 308 nm.

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