Mass spectrometry sample quantities required

From the practical standpoint one of the most appealing features of mass spectrometry is the requirement of minute sample quantities. Yet a great deal of information can be retrieved from a mass spectrum, even by the nonexpert with only a qualitative knowledge of mass spectrometry. Because the mass spectrum shows the mass of the molecule (molecular ion, Mt) and the masses of the fragments derived from it, the identification of relatively simple molecules is easier than by other spectroscopic methods. The combination of mass spectrometry with other spectral techniques provides a powerful and indispensable complement to the existing and constantly improving chemical methods of structure elucidation. 

To assure consistency and speed in multidisciplinary structure analysis of low-MW compounds involving various techniques (IR, NMR, MS, etc.) most industrial laboratories use a Standard Operating Procedure (SOP). In such schemes IR analysis is frequently used as a cheap filter for a quick starting control and as a means for verification. As IR detects only structural units identification of an unknown compound on the basis of IR is difficult. Mass spectrometry is used as the prime identification tool and is especially important in the determination of the exact mass and gross formulae. While structural prognostication on the basis of MS is difficult for the non-expert, a posteriori interpretation is quite feasible. H NMR is both easy and cheap, however requires greater sample quantities than either... 

Gas-source mass spectrometry. Work on Se stable isotopes has a long history, dating back to the Ph.D. work of H. Roy Krouse around 1960. From 1960 to 1990, analyses were done by gas-source mass spectrometry using SeF (Krouse and Thode 1962). The sample Se was converted to Se(0), then reacted with Fj gas to produce SeF. This method required large quantities (e.g., tens of micrograms) of Se for measurements and thus was not widely applied. Recent continuous flow mass spectrometry methods could enable gas-source measurements on much smaller quantities, but will still use too much sample to compete with TIMS and MC-ICP-MS methods. 

Many methods have been used to quantify steroidal compounds. These include RIA, gas chromatogra-phy-mass spectrometry (GC/MS), high-performance liquid chromatography (HPLC), and liquid chroma-tography-mass spectrometry (LC/MS). Although these techniques are successful in the analysis of steroids, it has been difficult to achieve quantitative analysis of small samples of neurosteroids because of their low concentrations in nervous tissues. Highly specific analytical methods are required to analyze small quantities of neurosteroids and their sulfates. Only with extremely sensitive methods of analysis is it possible to discover whether neurosteroids are synthesized in nervous tissues in quantities sufficient to affect neuronal activity, and whether these neurosteroids are distributed uniformly in brain. 

Mass spectrometry is a sensitive analytical technique which is able to quantify known analytes and to identify unknown molecules at the picomoles or femto-moles level. A fundamental requirement is that atoms or molecules are ionized and analyzed as gas phase ions which are characterized by their mass (m) and charge (z). A mass spectrometer is an instrument which measures precisely the abundance of molecules which have been converted to ions. In a mass spectrum m/z is used as the dimensionless quantity that is an independent variable. There is still some ambiguity how the x-axis of the mass spectrum should be defined. Mass to charge ratio should not lo longer be used because the quantity measured is not the quotient of the ion s mass to its electric charge. Also, the use of the Thomson unit (Th) is considered obsolete [15, 16]. Typically, a mass spectrometer is formed by the following components (i) a sample introduction device (direct probe inlet, liquid interface), (ii) a source to produce ions, (iii) one or several mass analyzers, (iv) a detector to measure the abundance of ions, (v) a computerized system for data treatment (Fig. 1.1). 

This paper is the only one in the liquid chromatography portion of this symposium which will attempt to deal with chromatography specifically from the viewpoint of the pesticide metabolism chemist. A residue analyst knows what compound he must analyze for, and develops his method with the properties of that substance in mind. On the other hand, the pesticide metabolism chemist has a different problem. At the conclusion of the treatment, exposure, and harvest phases of a radiolabeled metabolism study, he divides his material into appropriate samples, and extracts each sample with selected solvents to obtain the radioactive materials in soluble form. Typically these extracts consist of low levels (ppm) of carbon-14 labeled metabolites in a complicated mixture of normal natural products from the plant, animal, or soil source. The identity of each metabolite is unknown, and each must be isolated from the natural background and from other labeled metabolites in sufficient quantity and in adequate purity for identification studies, usually by mass spectrometry. The situation is rather like looking for the proverbial "needle in the haystack" when one does not know the size, shape,or composition of the needle, or even how many needles there are in the stack. At this point a separation technique must be selected with certain important requirements in mind. 

Isotope dilution mass spectrometry is a powerful method for determining the quantity of an element or an associated compound in a sample. It requires that a spike of the same element but with an isotopic composition different from that of the sample be introduced to it in a controlled manner. The mass spectrum of the mixture of spike and sample is then used to determine the concentration of the target element in the original sample. In most cases, a single ratio is used, one that incorporates the major isotope in the spike and the major isotope in the sample these must be different isotopes. The difference in the value of this ratio in the sample and in the mixture of sample and spike is proportional to the amount of the target element in the sample. 

Because fatty acids derived from natural sources are present in a mixture, an ideal analysis method for these molecules should be applicable to mixtures without requiring a prior separation or derivatization. Mass spectrometry is an excellent tool for determining the structure of fatty acids present in a mixture. It is possible to determine not only the molecular weight and thus the elemental composition but also, in most cases, the nature and position of the branching and the other substituents on the carbon chain. [268,269] Furthermore, such an analysis requires low quantities ranging from 10 pg to 100 ng of total lipid, depending upon the analysed sample, the ionization method used and the configuration of the spectrometer. [270,271]... 

The low concentrations of Pb found in Greenland and Antarctic snow and ice makes reliable concentration and isotopic composition measurements difficult to determine. Contamination with anthropogenic Pb during sample collection or drilling must be minimised, then extreme precautions must be taken to access a contamination-free sample (12, 28). Sensitive analytical methods which can analyse pg quantities of Pb are also required. A number of different methods meet this requirement however, discussion in this chapter will be limited to Thermal Ionisation Mass Spectrometry (TIMS) because this is the only technique, to date, to be successfully used to measure isotope abundances in polar ice. IDMS is an integral part of the technique used to measure the isotopic composition of the samples. 

Unlike other techniques of mass spectrometry, field desorption does not require vaporization of the sample prior to ionization. The field desorption technique has been suggested as a method to determine the molecular weights of sub-milligram quantities of involatile substances such as sugar phosphates without derivatization. 

Similarly the concentrations of individual components in the exit stream as analyzed by mass spectrometry may or may not be correctly represented by the intensity of the parent peak of the said component. Two steps are required to transform the raw data to kinetically useful quantities. The first step stems from the fact that each mass peak may contain contributions from a number of components in the effluent. To resolve this difficulty, a mathematical procedure called deconvolution is applied to the observed total mass spectrum of the exit stream from the reactor. Deconvolution allows us to separate the total intensities at individual mass numbers into their several components, each of which is due to a contribution arising from the spectrum of an individual constituent present in the effluent sample. The deconvoluted spectrum then reports the mol fractions of the individual components in the effluent. With this in hand, the next step is to transform the output mol fractions to concentrations so they can be used in rate expressions to correlate reaction rates. 

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