Sample deposition using solvent

Property measurements of fullerenes are made either on powder samples, films or single crystals. Microcrystalline C6o powder containing small amounts of residual solvent is obtained by vacuum evaporation of the solvent from the solution used in the extraction and separation steps. Pristine Cgo films used for property measurements are typically deposited onto a variety of substrates (< . , a clean silicon (100) surface to achieve lattice matching between the crystalline C60 and the substrate) by sublimation of the Cr,o powder in an inert atmosphere (e.g., Ar) or in vacuum. Single crystals can be grown either from solution using solvents such as CS and toluene, or by vacuum sublimation [16, 17, 18], The sublimation method yields solvent-free crystals, and is the method of choice. 

Compared with the samples typically separated by supercritical fluid chromatography most solvents are significantly more volatile and can be removed from the sample by evaporation in a short open tube or packed precolumn [174-178]. Solventless injection requires only minor modification to a standard rotary injector, either addition of a second valve or tee piece and a precolumn. This allows sequential programming of the independent processes of solvent removal and sample deposition in the precolumn followed by dissolution of the sample in the mobile phase and its transport to the column for refocusing. Quite large solvent volumes (hundreds of p,l) can be handled in this way, but typically smaller volumes are used. The only real limitation to solventless injection is that the volatility difference between the sample and solvent must be sufficient for effective removal of the solvent without loss of sample. In addition, effective sample focusing at the head of the column is required to maintain an acceptable separation... 

In general, spectra from solid samples are used for qualitative identification of the sample, not for quantitative analysis. The spectrum of a solid sample is generally collected when the sample is not soluble in a suitable IR-transparent solvent. There are some problems that can occur with spectra from solid samples. Many organic solids are crystalline materials. The mull and pellet approaches result in random orientation of the finely ground crystals deposition of thin films by evaporation... 

Solvents. Reagent grade toluene and 95% ethanol were used for fractionation without further purification. Spectroscopic grade benzene was used as the solvent in sample-deposition solutions. Inasmuch as the OTMS reacts rapidly with moisture, all traces of water should be removed from the benzene prior to its use for this purpose. To this end, the benzene used for dissolution of samples C, CDS-B-4, D-1, D-2, and D-3 was first refluxed over calcium hydride and distilled to remove any residual water. A less stringent procedure was employed for samples A and B. 

Figure 26. Carousel used for transporting the KCl sample cups used in HPLC/FTIR between the positions used for sample deposition, solvent elimination, and spectral measurement.

In the case of filament pyrolysers, the parameters which pertain specifically to the sample were identified [533]. These factors are method and uniformity of sample deposition, region of sample deposition, sample thickness, sample-to-filament contact, but also catalytic effects, nature of carrier gas, flowrate, pyrolysis chamber temperature and purity of solvents used in sample deposition. Important parameters are also the temperature-time profile (TTP), which depends upon TRT, THT as well as Teq. Reproducibility is enhanced if the entire sample experiences the same TTP and if the primary products... 

Kassalainen and Williams [135] coupled thermal field flow fractionation (ThFFF) and matrix-assisted laser desorption/ionisation time-of-flight mass spectroscopy (MALDI-ToF-MS) to yield a powerful combination of techniques for the analysis of polydisperse PS. ThFFF high selectivity and sensitivity to chemical composition were used to separate polydisperse polymers and polymer mixtures into the narrow polydispersity and homogeneous chemical composition fractions essential for MAT.DT-ToF-MS analyses. On the other hand, because it is possible to measure directly using MALDI-ToF-MS, it alleviates the need for polymer standards for ThFFF. Kassalainen and Williams [135] address the coupling of ThFFF and MALDI-ToF-MS and identify compatibility issues. Optimum conditions were determined and developed to maximise the capabilities of the combined technique. Depending on the polymer and the method of matrix-assisted laser desorption/ionisation (MALDI) sample deposition, fractions from 1-10 ThFFF runs were combined for MALDI-ToF-MS analysis. Binary solvents are used to enhance ThFFF retention and resolution of low (<15 kDa) polymers, and methods developed to allow routine MALDI-ToF-MS analyses of PS polymers up to 575 kDa. Overall, the compatibility of the two techniques was extended from several kilodaltons to several hundred kDa. Polymer... 

A comparison of the low-frequency sensitivity drift is also difficult to do directly, but a reasonable analogy can be drawn. A variation in sensitivity over a series of sample spots is a situation similar to infusion and observation of baseline drift with ESI. Using ideal conditions of sample deposition and crystallization with pure standards and solvents establishes some idea of reproducibility limits. An example is shown in Figure 13.12, where a series of spots from a sample of the dmg Propanolol were created with an electrostatic deposition device." The spots were rastered at a speed such that each spot was traversed in 600 ms. Absolute area reproducibility of approximately 6% was achieved— that is, worse than the infusion data of NanoESI and the high flow techniques, but still reasonable, particularly if referencing the signal to an internal standard (not done here) in the same spot that is ionized at the same time as the analyte. Suffice it to say that good quantitation is possible when an internal standard is cocrystallized with the analyte. ESI or APCI do not require coelution of the internal standard and analyte. 

Decolorisation by Animal Charcoal. It sometimes hap pens (particularly with aromatic and heterocyclic compounds) that a crude product may contain a coloured impurity, which on recrystallisation dissolves in the boiling solvent, but is then partly occluded by crystals as they form and grow in the cooling solution. Sometimes a very tenacious occlusion may thus occur, and repeated and very wasteful recrystallisation may be necessary to eliminate the impurity. Moreover, the amount of the impurity present may be so small that the melting-point and analytical values of the compound are not sensibly affected, yet the appearance of the sample is ruined. Such impurities can usually be readily removed by boiling the substance in solution with a small quantity of finely powdered animal charcoal for a short time, and then filtering the solution while hot. The animal charcoal adsorbs the coloured impurity, and the filtrate is usually almost free from extraneous colour and deposits therefore pure crystals. This decolorisation by animal charcoal occurs most readily in aqueous solution, but can be performed in almost any organic solvent. Care should be taken not to use an excessive quantity... 

---