Water clusters, calibrant

Calibrants are required to calibrate the mass scale of any mass spectrometer, and it is important to find reference compounds that are compatible with a particular ion source. Calibrants commonly used in electron ionization (El) and chemical ionization (Cl), such as perfluorocarbons, are not applicable in the ESI mode. The right calibrants for LC-ESI-MS should (1) not give memory effects (2) not cause source contamination through the introduction of nonvolatile material (3) be applicable in both positive- and negative-ion mode. The main calibrants used or still in use to calibrate ESI-MS can be divided into the following categories polymers, perfluoroalkyl triazines, proteins, alkali metal salt clusters, polyethers, water clusters, and acetate salts. 

Water Cluster. Numerous groups have used water clusters successfully as calibration solutions [10,11,20-22]. Water clusters do not produce any source contamination in ESI-MS and provide closely spaced reference peaks with a calibration range up to m lz 1000. In positive-ion mode, protonated water clusters with up to 70 water molecules are observed. In negative ESI singly deprotonated water clusters are observed [OH (HjO), with n > 20], as well as solvated electrons [(H20)m with m > 11]. 

Ultramark 1621, a mixture of fluorinated phosphazenes, was proposed as a reference compound for calibration in the range of m/z 800 to 2000 [28]. It was also recommended for ion-trap instmments from Thermo Finnigan. Ultramaik 1621 strongly adsorbs to surfaces and is difficnlt to remove due to its poor solubility in common solvents [30]. A proprietary mixtnre of fluorinated phosphazenes is recommended as reference compound by Agilent Technologies. Water cluster ions have been proposed for calibration up to m/z 3000 [31]. 

The measured concentration is obtained from the count rate by division by the calibration constant Ccai (see Equation 52.7), which is expressed as the normalized number of counts per part per billion volume (ncps/ ppbv). As noted above, the water cluster H30 "-H20 reacts differently with the trace molecules as the water ion does. A correction factor is introduced to take this difference into account [8] ... 

Results can be compared to all-atom simulations for calibration and refinement of interaction parameters. Further analysis could involve Monte Carlo procedures in order to obtain shape and size distributions of ionomer and water clusters from the structures formed during CGMD simulations. Moreover, stable structures from CGMD studies could form the basis for mobility studies by atomistic MD simulations, using remapping procedures. 

Mixtures of a protein (usually myoglobin) and some smaller peptides can be used polyethylene glycols and polypropylene glycols are also widely used for calibration. Anacleto et al. have summarized different options and have proposed protonated water clusters and salt clusters generated by pneumatically assisted electrospray. Water clusters provided a calibration range up to miz 1000 in the SCIEX API III mass spectrometer. Alkali metal halides (sodium iodide) allow calibration on cluster ions Na (Nal) or I (NaI) up to at least m/z 2000, the fiiU mass range of this instrument. 

Cesium iodide or cesium carbonate cluster ions were proposed [25-26]. A mixture of 2.5% cesium iodide in sodium iodide is recommended by Waters. The use of cluster ions of a variety of cesium salts for calibration up to m/z 10,000... 

Figure 2. Optical properties of hydrated silver clusters. Top Transient absorption spectra of silver oligomers. Bottom Calibrated absorption spectra of Ag , Ag2 and Agj in water obtained by pulse radiolysis. ...

Because the thermospray interface contains a dedicated ion source block, tuning and calibration of the source and analyser parameters is obligatory. Calibration and tuning cannot be performed with common calibrants like perfluorokerosene. Diluted solutions of polyethylene glycols are used in most cases, although a tuning and calibration based on clusters of ammonium acetate, ammonium trifluoro-acetate and even simply water was proposed and used as well. 

---