Sample concentrations, picomole

Considering sample concentrations in the picomole to femtomole range on the target and the ablated sample volume during MALDI, it can be estimated that only a few attomoles of the analyte are needed for informative MS analysis. 

For both ESI and MALDI, the concentration of the sample and the complexity of the contaminants play important roles in both the sensitivity and the mass accuracy. Biological samples most often are diluted solutions of peptides or proteins containing a great number of contaminants. These two problems, dilution and contaminants, are not easy to handle, especially when the total amount of sample is low, such as picomoles. 

In comparison with NMR, mass spectrometry is more sensitive and, thus, can be used for compounds of lower concentration. While it is easily possible to measure picomoles of compounds, detection limits at the attomole levels can be reached. Mass spectrometry also has the ability to identify compounds through elucidation of their chemical structure by MS/MS and determination of their exact masses. This is true at least for compounds below 500 Da, the limit at which very high-resolution mass spectrometry can unambiguously determine the elemental composition. In 2005, this could only be done by FTICR. Orbitrap appears to be a good alternative, with a more limited mass range but a better signal-to-noise ratio. Furthermore, mass spectrometry allows relative concentration determinations to be made between samples with a dynamic range of about 10000. Absolute quantification is also possible but needs reference compounds to be used. It should be mentioned that if mass spectrometry is an important technique for metabolome analysis, another key tool is specific software to manipulate, summarize and analyse the complex multivariant data obtained. 

In recent years, several membranes have been discovered to be compatible with MALDI. Preliminary work has focused on eliminating minor impurities from low picomole concentrations of peptides and proteins. Mock et. al.(5) were able to resolve 10 pmole quantities of peptides and proteins in the presence of small concentrations of contaminants by spotting samples on an electrosprayed nitrocellulose surface. Zaluzec et. al. (6) were able to wash water soluble contaminants from samples immobilized on nylon membranes in a method analogous to washing samples immobilized on stainless steel surfaces. Both MALDI MS of samples electroblotted onto... 

Using masspectrometric detection in SIM mode for ions with m/z 77 and/or 92, detection and determination of picomol concentrations of F is possible in different analytical samples. 

If the mass ratio between the two matched peak maxima differed at any time during the evaporation of the sample by more than 40 ppm (due to drift of the instrument, sample contamination, or electrical interference), the sample was disregarded. The calibration functions calculated by linear regression for 12 calibration samples containing known picomole quantities of each compound had the correlation coefficients 0.9920, 0.9914 and 0.9865, for cadaverine, monoacetylcadaverine and monopropionylcadaverine, respectively. The reported concentrations were measured as quantities more than three times higher than their blanks. They were not corrected for losses during extraction and thin-layer chromatography. 

Professionals working with GC analyze the content of a chemical product, for example in assuring the quality of products in the chemical industry or measuring toxic substances in soil, air or water. GC is very accurate if used properly and can measure picomoles of a substance in a 1 ml liquid sample, or parts-per-billion concentrations in gaseous samples. 

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