Direct inlet probe

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). 

Figure 4. Low-pressure El and DCI FTMS spectra of Riboflavin (Vitamin B2). Direct probe inlet, about 210°C.

One of the tenets of normal operation of a mass spectrometer is to introduce as little sanple as possible, while in MS/MS, the sanple sizes are often quite large — up to several mg in trace analyses. Newer instruments are designed for easier source cleaning or utilze ion volumes which are replaceable through the direct probe inlet. 

Another way of sample introduction into the mass spectrometer was performed by Hunt et al. and Onuska et al. who used a direct probe inlet [162,163]. It was claimed that this technique combined with NICI-MS or GC/HREI-MS detection is more precise and ten times faster than GC/MS [19]. However, this technique does not provide structure-specific information. 

The DLI probe can usually be fitted into the direct probe inlet and used with a Cl source within a matter of minutes and operated for extended periods. Plugging of the exit orifice by a particle can reduce the operational time drastically and the need for careful preparation of solvents and cleaning LC tubing can help extend the useful operational range. 

Samples can be introduced into the mass spectrometer through a direct probe inlet or GLC interface (West, 1974). Combined GLC-MS has become increasingly important for oligosaccharide derivatives. However, the resolution of mixtures containing tetra- or larger saccharides may not... 

The purpose of the inlet system is to permit introduction of a representative sample into the ion soutca with minimal loss of vacuum. Most modern mass spectrom-ciers are equipped with several types of inlets to accommodate various kinds of samples these include batch inlets, direct probe inlets, chromatographic inlets, and capillary electrophoretic inlets. 

Mass spectra can be recorded by using any one of several different systems of instrumentation. The inlet system can be either a hot reservoir inlet, a direct-probe inlet, or a gas-liquid chromatography (g.l.c.) inlet. The type of instrumentation and, especially, the inlet system may cause differences in the spectra recorded. However, with modern commercial instruments, these differences are generally small. Combined gas-liquid chromatography-mass spectrometry (g.l.c.-m.s.) has become increasingly important, and is particularly valuable for investigating complex mixtures. Gas-liquid chromatography of carbohydrates and their derivatives is the subject of articles in this Series. "... 

The important complementary technique of mass spectrometry is discussed briefly here, but applications are dealt with at length in later Chapters. Some modern mass spectrometry techniques are not directly compatible with gas chromatographic separations, requiring direct-probe inlet systems, and these are not treated in depth in this book. 

For ionisation techniques that use a direct insertion probe, accurate and precise quantification is difficult to achieve, at variance to the view expressed by Millard [203]. The direct probe inlet has little potential for fractionating samples unless the components differ widely in volatility. I>uring quantitative analysis, the components of interest in mixtures remain largely unseparated from each other and from impurities, causing many background ions. With instruments that allow the direct probe to be heated independently of the ion source, temperature programming achieves some degree of fractionation. 

The introduction of a sample into the ion source may be done in different ways depending upon its physicochemical properties and the purity. A pure solid may be introduced by means of a so-called direct probe inlet system, while a pure gas or liquid may be introduced by means of a so-called gas inlet system The temperature of both the systems can usually be regulated up to several hundred degrees (Celsius). An impure sample may be analysed by using any of these systems... 

Note Sample introduction systems such as reservoir inlets, chromatographs, and various types of direct probes (Chap. 5.3) are of equal importance to other ionization methods. The same holds valid for the concepts of sensitivity, detection limit, and signal-to-noise ratio (Chap. 5.2.4) and finally to all sorts of ion chromatograms (Chap. 5.4). 

For the purpose of sample introduction, any sample introduction system (also sample inlet system or inlet) suitable for the respective compound can be employed. Hence, direct probes, reservoir inlets, gas chromatographs and even liquid chromatographs can be attached to an El ion source. Which of these inlet systems is to be preferred depends on the type of sample going to be analyzed. Whatever type the inlet system may be, it has to manage the same basic task, i.e., the transfer of the analyte from atmospheric conditions into the high vacuum of the El ion source Table 5.1 provides an overview. 

Perhaps the most common problem is that of thermal decomposition of the sample, either in the batch inlet, for which the cure is a lower inlet temperature or use of the direct probe, or in the source itself. A common misconception among mass spectroscopists, often promulgated by manufacturers, is that if the source is kept hot, the decomposition of contaminants is minimized. The ion source should routinely be run no hotter than 180°-200°C. A source at the common temperature of 250°C is much more likely to result in decomposition of sample and contamination of the source, and should be used only rarely. On our AEI MS-30 we run 200 samples per month, many of which are organometallic or inorganic, and we are seldom forced to exceed 200°C more than once a month. If some sample condenses into the source it is far better to sublime it away slowly by carefully raising the temperature than to pyrolyze it. 

A new ionization method called desorption electrospray ionization (DESI) was described by Cooks and his co-workers in 2004 [86]. This direct probe exposure method based on ESI can be used on samples under ambient conditions with no preparation. The principle is illustrated in Figure 1.36. An ionized stream of solvent that is produced by an ESI source is sprayed on the surface of the analysed sample. The exact mechanism is not yet established, but it seems that the charged droplets and ions of solvent desorb and extract some sample material and bounce to the inlet capillary of an atmospheric pressure interface of a mass spectrometer. The fact is that samples of peptides or proteins produce multiply charged ions, strongly suggesting dissolution of the analyte in the charged droplet. Furthermore, the solution that is sprayed can be selected to optimize the signal or selectively to ionize particular compounds. 

In the third method, Chiu and Beattie (148) used an interface constructed from a T-shaped glass tube and a constant volume sampler, such as is shown shown in Figure 8.18. One arm of the tee was connected to the furnace tube of the thermobalance, whereas the other was welded to the stopcock of the sampler. The third arm was either vented into the atmosphere or connected to a vacuum pump. The sampler was connected to the heated inlet of the mass spectrometer. A 3-L gas reservoir and a gold leak tube were placed between the inlet and the ion source. To achieve the highest sensitivity, one can directly connect the sampler to the ion source through a direct probe attachment. 

Direct Probe Analysis. In the direct inlet (direct probe or solids probe) MS technique, the sample is placed in a small cup and inserted into the ion source a few millimeters from the ionizing electron beam (Figure 4). The cup is heated so that solids or liquids of low volatility can be directly vaporized into the electron beam. Only 10" to 10" ... 

Field ionization involves the removal of electrons from a species by quantum mechanical tunneling in a high electric field. In practice, FI-MS refers to the technique in which the sample to be analyzed is introduced as a vapor using a heatable direct probe, heated batch inlet, or GC/MS interface. 

Many mass spectrometers, especially larger, research-oriented units (as opposed to GC detectors), are equipped with a solids probe inlet port. This is a vacuum locking inlet that permits the insertion of a solid sample on a heatable probe directly into... 

The use of a direct insertion probe for pyrolysis-MS requires that the mass spectrometer be equipped with an inlet for a solids probe. Most mass spectrometers used in analytical labs are configured as detectors for gas chromatographs and are relatively simple and inexpensive, but are rarely equipped with a probe inlet. The only way for a sample compound to enter a mass spectrometric detector is via the capillary column inlet, configured to accept a piece of fused silica. Nevertheless, if the capillary column is removed and replaced with a piece of fused silica sufficiently restrictive to limit the flow into the mass spectrometer, pyrolysis-MS data may be... 

FIGURE 27.6 An example of in-line MIMS system used in kinetic studies. This system consists of a constant temperature and well-mixed batch reactor, a high performance hquid chromatography (HPLC) piston pump (Acuflow Series I, Lab Alliance Inc., State College, PA), a direct membrane inlet probe (MIMS Technology), and a Saturn 4D ion trap (Varian) or Hewlett-Packet 5972 quadrupole mass spectrometer (MS) (Agilent Technologies, Santa Clara, CA). The MS operated in the electron impact (El) mode. Reprinted fromNa and Olson [16] with the permission of the American Chemical Society. 

Relatively few descriptions of direct mass spectral analysis of plastics compounds have appeared in the literature. In a rather early report, additives in PP compounds were thermally desorbed into a heated reservoir inlet for mass spectral analysis [58]. It was found that numerous stabilisers could be identified via 80 eV EI-MS. Thermal, desorption of additives via direct probe introduction of PP compounds was described in a later report [59]. A more recent paper considered the mass spectral analysis of both rubber and plastic compounds. This report was an overview, without much detail. Analysis of additives in PP compounds via direct thermal desorption CI-MS has also been described [45]. 

Bombick et al. [3] presented a simple, low cost method for producing thermal potassium metal ions for use as Cl reagents. All studies were performed on a commercial gas chromatography-mass spectrometiy (GC-MS) system. Thermionic emitters of a mixture of silica gel and potassium salts were mounted on a fabricated probe assembly and inserted into the Cl volume of the ion source through the direct insertion probe inlet. Since adduct ions (also referred to as cationized molecular ions or pseudomolecular ion ) of the type (M + K)+ have been observed, molecular weight information is easily obtained. The method is adaptable to any mass spectrometer with a Cl source and direct inlet probe (DIP). In addition, the technique is compatible with chromatographic inlet systems, i.e., GC-MS modes, which will provide additional dimensions of mass spectral information. 

Direct Inlet Probe (DIP) The DIP is one of the earliest techniques used to introduce nonvolatile or highly insoluble samples into mass spectrometers. This technique is still popular today because it provides a means to perform rapid sample analysis with minimal or no sample preparation [27, 28]. Applications of the direct-probe technique include quality-control analyses and the screening of drug, polymer, and synthesis samples. It has also been used to study the temperature programmed decomposition of synthetic polymers and inorganic materials, to characterize the molecular properties of materials under thermal degradation conditions. The same is also true of the studies with lAMS (see Sect. 6.4). 

---