Other Types of Mass Spectrometry

The mass spectra described thus far have been low-resolution spectra that is, they report m/z values to the nearest whole number. As a result, the mass of a given molecnlar ion can correspond to many different molecular formulas, as shown in Sample Problan 13.2. 

High-resolution mass spectrometers measure m/z ratios to four (or more) decimal places. This is valuable because except for carbon-12, whose mass is defined as 12.0000, the masses of all other nuclei are very close to— but not exactly—whole numbers. Table 13.1 lists the exact mass values of a few common nuclei. Using these values it is possible to determine the single molecular formula that gives rise to a molecular ion. 

For example, a compound having a molecular ion at m/z = 60 using a low-resolution mass spectrometer could have the following molecular formulas  

If the molecular ion had an exact mass of 60.0578, the compound s molecular formula is CsHgO, because its mass is closest to the observed value. 

The low-resolution mass spectrum of an unknown analgesic X had a molecular ion of 151. Possible molecular formulas Include C7H5NO3, C8H9NO2, and C10H17N. High-resolution mass spectrometry gave an exact mass of 151.0640. What is the molecular formula of X  

Although using the molecular ion to determine the molecular weight of an organic compound is indeed valuable, recent advances have greatly expanded the information obtained from mass spectrometry. 

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Other types of mass spectrometry have also been used to examine ambient particulate samples. One such technique is secondary ion mass spectrometry (SIMS) in which the surface of the sample is bombarded with a beam of ions or neutral atoms that cause ejection of fragments from the surface. The fragments may be neutral atoms or molecules, positively or negatively charged species, electrons, or photons. The... 

Mass bias in ICP/MS is 10 times more severe than in other types of mass spectrometry, on the order of 1%/amu. The observed bias is sensitive to both sample composition and instrument ion optical tuning conditions. The source of this bias is dominated by space charge effects in the ion beam extracted from the ICP (Douglas and Tanner, 1998). 

Other types of mass spectrometry such as FDMS, FABMS, and HRESlET MS that can produce useful information about the molecular weight, elemental composition, and binding sequence of monosaccharide units in the glycoside carbohydrate chain have been broadly applied in addition to ElMS. 

In 1974, Comarisov and Marshall60 developed Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). This technique allows mass spectrometric measurements at ultrahigh mass resolution (R = 100000-1000000), which is higher than that of any other type of mass spectrometer and has the highest mass accuracy at attomole detection limits. FTICR-MS is applied today together with soft ionization techniques, such as nano ESI (electrospray ionization) or MALDI (matrix assisted laser/desorption ionization) sources. 

Understanding the behavior of acids and bases is essential to every branch of science having anything to do with chemistry. In analytical chemistry, we almost always need to account for the effect of pH on analytical reactions involving complex formation or oxidation-reduction. pH can affect molecular charge and shape—factors that help determine which molecules can be separated from others in chromatography and electrophoresis and which molecules will be detected in some types of mass spectrometry. 

They are still the workhorses of coupled mass spectrometric applications, as they are relatively simple to run and service, relatively inexpensive (for a mass spectrometer), and provide unit mass resolution and scanning speeds up to approximately 10,000 amu/s. This even allows for simultaneous scan/ selected ion monitoring (SIM) operation, in which one part of the data acquisition time is used to scan an entire spectrum, whereas the other part is used to record the intensities of selected ions, thus providing both qualitative information and sensitive quantitation. They are thus suitable for many GC-MS and liquid chromatography-mass spectrometry (LC-MS) applications. In contrast to GC-MS with electron impact (El) ionization, however, LC-MS provides only limited structural information as a consequence of the soft ionization techniques commonly used with LC-MS instruments [electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI)]. Because of this limitation, other types of mass spectrometers are increasingly gaining in importance for LC-MS. 

A mass spectrometer is an instrument that produces ions and separates them according to their mass-lo-charge ratios, m/z. Most of the ions we will discuss are singly charged so that the ratio is simply equal to the mass pumber of the ion. Several types of mass spectrometers are currently available from instrument manufacturers. In this chapter, we describe the three types that are used in atomic mass spectrometry the quadra-pole mass spearomeier. the lime-of-ftighl mass spectrometer. and the doubk-foctising mass spectrometer. Other types of mass spectrometers are considered in Chapter 20. which is devoted to molecular mass spectrometry. I he first column in Table I l-l indicates the types of atomic mass spectrometry in which each of the three types of mass spectrometer is usually applied. 

Finally we should keep in mind one very important principle The mass spectrometer is a separating system itself. Thus, a double focusing mass spectrometer is a double separating system one may be used as mass separator and the other one as ion spectrometric device as it is realized in DADI/MIKE spectrometry. Trends for the improvement of this type of mass spectrometry ( MS/MS ) by the addition of a third or even a fourth analyzer are going on Many applications have shown its excellent analytical potential (see Chapter g 67.68,82.83)... 

All of the ion formation processes in SIMS are known for other forms of mass spectrometry. The fimdamental difference is that several ion formation mechanisms may occur simultaneously in competition with each other often one does not have control over which ion formation process will dominate. Also, the dominant ionization process may vary with the parhcular type of polymer involved. One great advantage of SIMS is that both positive and negative ions are formed, often in comparable yields. 

Present day techniques for structure determination in carbohydrate chemistry are sub stantially the same as those for any other type of compound The full range of modern instrumental methods including mass spectrometry and infrared and nuclear magnetic resonance spectroscopy is brought to bear on the problem If the unknown substance is crystalline X ray diffraction can provide precise structural information that m the best cases IS equivalent to taking a three dimensional photograph of the molecule... 

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