Mass spectrometer sample requirements

The most common HPLC column diameter is 4.6 mm. There is a trend toward narrower columns (2 mm, 1 mm, and capillary columns down to 25 pm) for several reasons. Narrow columns are more compatible with mass spectrometers, which require low solvent flow. Narrow columns require less sample and produce less waste. Heat generated by friction of solvent flow inside the column is more easily dissipated from a narrow column to maintain isothermal conditions. Instruments must be specially designed to accommodate column diameters <2 mm or else band broadening outside the column becomes significant. 

AMS directly measures the number of 14C atoms, and the ratio of 14C to 13C and/ or 12C, using a high-energy accelerator as an inlet to a mass spectrometer. The key characteristics of 14C-AMS are the electron stripping and ion acceleration, which allow 14C to be distinguished from isobars and molecules that would confuse a standard mass spectrometer. AMS requires only a fairly small sample of lOOpg to 1 mg of C. In addition, the measurement only takes minutes per sample. 

With the application of the newly developed continuous-flow isotope-ratio monitoring mass spectrometers, the required seawater sample volume for N2O dual isotope measurements have been drastically reduced and therefore facilitated the determination high resolution depth profiles of the N2O dual isotope signature (Brand, 1996 Yoshinari et al, 1997) Repeated measurements of depth profiles at... 

Mass spectrometry has been utilized for on-line measurements for many years. The technique has been widely used in the petroleum industry as well as the chemical industry where the process stream contains primarily gases. This type of sample is the most compatible with the mass spectrometer and requires little sample preparation before introduction into the instrument. Early instruments designed for process monitoring did not have full scanning capability, but rather had several detectors at fixed position to monitor specific ions. Most of these instruments had the capability to monitor 5-7 components in the gas stream. These instruments were also widely used as leak detectors since they had the requisite sensitivity for low level detection and the speed to detect leaks very quickly. 

Figure 11 - 2 shows schematically the components of a commercial ICPMS system. A critical part of the instrument is the interlace that couples the ICP torch, which operates at atmospheric pressure with the mass spectrometer that requires a pressure of less than lO tnrr. This coupling is accomplished hy a differentially pumped interface coupler that consists of a sampling cone, which is a vsater-coolcd nickel cone with a small... 

The sample introduction system, comprising the peristaltic pump, nebulizer, spray chamber, and drain system, takes the initial abuse from the sample matrix, and as a result, is an area of the ICP mass spectrometer that requires a great deal of attention. The principles of the sample introduction area have been described in great detail in Chapter 3, so let us now examine what kind of routine maintenance it requires. 

Liquids that are sufficiently volatile to be treated as gases (as in GC) are usually not very polar and have little or no hydrogen bonding between molecules. As molecular mass increases and as polar and hydrogen-bonding forces increase, it becomes increasingly difficult to treat a sample as a liquid with inlet systems such as El and chemical ionization (Cl), which require the sample to be in vapor form. Therefore, there is a transition from volatile to nonvolatile liquids, and different inlet systems may be needed. At this point, LC begins to become important for sample preparation and connection to a mass spectrometer. 

Accurate, precise isotope ratio measurements are important in a wide variety of applications, including dating, examination of environmental samples, and studies on drug metabolism. The degree of accuracy and precision required necessitates the use of special isotope mass spectrometers, which mostly use thermal ionization or inductively coupled plasma ionization, often together with multiple ion collectors. 

The choice of mass spectrometer for a particular analysis depends on the namre of the sample and the desired results. For low detection limits, high mass resolution, or stigmatic imaging, a magnetic sector-based instrument should be used. The analysis of dielectric materials (in many cases) or a need for ultrahigh depth resolution requires the use of a quadrupole instrument. 

One of the important advantages of ICPMS in problem solving is the ability to obtain a semiquantitative analysis of most elements in the periodic table in a few minutes. In addition, sub-ppb detection limits may be achieved using only a small amount of sample. This is possible because the response curve of the mass spectrometer over the relatively small mass range required for elemental analysis may be determined easily under a given set of matrix and instrument conditions. This curve can be used in conjunction with an internal or external standard to quantily within the sample. A recent study has found accuracies of 5—20% for this type of analysis. The shape of the response curve is affected by several factors. These include matrix (particularly organic components), voltages within the ion optics, and the temperature of the interffice. 

In the technique developed by Willard Libby in Chicago in the late 1940s, the proportion of carbon-14 in a sample is determined by monitoring the (1 radiation from C02 obtained by burning the sample. This procedure is illustrated in Example 17.4. In the modern version of the technique, which requires only a few milligrams of sample, the carbon atoms are converted into C ions by bombardment of the sample with cesium atoms. The C ions are then accelerated with electric fields, and the carbon isotopes are separated and counted with a mass spectrometer (Fig. 17.19). 

The mass spectrometer should provide structural information that should be reproducible, interpretable and amenable to library matching. Ideally, an electron ionization (El) (see Chapter 3) spectrum should be generated. An interface that fulfils both this requirement and/or the production of molecular weight information, immediately lends itself to use as a more convenient alternative to the conventional solid-sample insertion probe of the mass spectrometer and some of the interfaces which have been developed have been used in this way. 

To enable qnantitative measurements to be made, the analyst requires the ability to determine the areas or heights of the detector responses of analyte(s) and any internal standard that may be present and then, from these figures, to derive the amount(s) of analyte(s) present in the unknown sample. The software provided with the mass spectrometer allows this to be done with a high degree of automation if the analyst so desires. 

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