Sample introduction direct insertion probe

Direct-inlet probe. A shaft or tube having a sample holder at one end that is inserted into the vacuum system of a mass spectrometer through a vacuum lock to place the sample near to, at the entrance of, or within the ion source. The sample is vaporized by heat from the ion source, by heat applied from an external source, or by exposure to ion or atom bombardment. Direct-inlet probe, direct-introduction probe, and direct-insertion probe are synonymous terms. The use of DIP as an abbreviation for these terms is not recommended. 

Section 6.4 deals with other EI-MS analyses of samples, i.e. analyses using direct introduction methods (reservoir or reference inlet system and direct insertion probe). Applications of hyphenated electron impact mass-spectrometric techniques for poly-mer/additive analysis are described elsewhere GC-MS (Section 7.3.1.2), LC-PB-MS (Section 7.3.3.2), SFC-MS (Section 13.2.2) and TLC-MS (Section 7.3.5.4). 

Cl and El are both limited to materials that can be transferred to the ion source of a mass spectrometer without significant degradation prior to ionisation. This is accomplished either directly in the high vacuum of the mass spectrometer, or with heating of the material in the high vacuum. Sample introduction into the Cl source thus may take place by a direct insertion probe (including those of the desorption chemical ionisation type) for solid samples a GC interface for reasonably volatile samples in solution a reference inlet for calibration materials or a particle-beam interface for more polar organic molecules. This is not unlike the options for El operation. 

Direct introduction of a sample, either in solid or liquid state, in the ion source of a mass spectrometer may be achieved through two procedures the first one is based on the use of a direct insertion probe (DIP) the second one necessitates a direct exposure probe (DEP). Direct introduction followed by heating of the sample in the ion source of the mass spectrometer is also known as direct temperature resolved mass spectrometry (DTMS). 

Fig. 11.1. Conceptual diagrams of a mass spectrometer showing the various functional components. The top diagram represents instruments that employ conventional modes of ionization such as El or Cl. In such instruments, the sample introduction process (for example, direct insertion probe) bridges the atmospheric pressure/high-vacuum interface. The bottom diagram represents instruments that employ the recently developed API techniques such as ESI. Ions are formed outside the vacuum envelope of the instrument and transported into the instrument through the API interface.

The fast atom bombardment ionization (FAB) technique is a soft ionization method, typically requiring the use of a direct insertion probe for sample introduction in which a high energy beam of Xe atoms, Cs+ ions, or massive glycerol-NH4+ clusters sputter the sample and matrix from the probe surface (Figure 8). 

Vaporize compounds of varying volatility. This is accomplished in the inlet system. Introduction of the sample is done by direct insertion probe, reservoir inlet, or following a chromatographic separation (GC, HPLC, and CE). As mentioned earlier, to introduce the LC flow to the mass spectrometer on-line, we need an appropriate interface. Development of appropriate interfaces was the utmost for evolution of the LC-MS coupling. 

In pyrolysis-mass spectrometry (Py-MS) the pyrolysate is directly transferred to a mass spectrometer and analyzed, generating a complex spectrum. The sample introduction can be done using various techniques. One simple technique is the direct insertion probe (DIP) where the sample is deposited on an insert that has the capability of heating the sample and of introducing the pyrolysate directly into the ion source of the mass spectrometer (see e.g. [1]). Another technique is the Curie point Py-MS where an attachment to the mass spectrometer allows the sample to be placed in a radio frequency (RF) region continued by an expansion chamber connected to the ion source. The sample is pyrolyzed and the pyrolysate ionized and analyzed in the MS instrument. A schematic diagram of a Curie point Py-MS system is shown in Figure 3.3.2. 

A direct insertion probe is used for introduction of liquids with high boiling points and solids with sufficiently high vapor pressure. The sample is put into a glass capillary that fits into the tip of the probe shown in Fig. 9.5. The probe is inserted into the ionization source of the mass spectrometer and is heated electrically, vaporizing sample into the electron beam where ionization occurs. A problem with this type of sample introduction is that the mass spectrometer can be contaminated because of the volume of sample ionized. 

There are different types of instruments that can be attached to the El source for MS analysis. The most commonly used technique consists of the analysis of a gaseous sample obtained from the gas chromatograph. However, other introduction systems, such as reservoir inlets and direct insertion probes, are also frequently used. 

The sample introduction system used will depend on the type of sample and whether a chromatographic separation is required. The usual technique for separated and relatively pure samples is the direct insertion probe which carries a small amount of solid sample through an air lock into the high vacuum of the mass spectrometer. The sample would then be vapourised by applying gentle heat, and the vapours ionised in an ionisation source. Samples already in the gas phase, such as vapours or the eluent from gas chromatography, can be introduced directly into the ionisation source at low flows ( 1 mLmin ). Usually two ionisation techniques are used for these samples, electron ionisation (El) or chemical ionisation (Cl). 

Identification or quantification of molecules up to 1 kDa represents the majority of mass spectrometric analyses. The simplest analysis is probably that of a single product from a synthetic chemical reaction where the required information is only the molecular mass. Such samples used to be introduced using a direct insertion probe. Nonpolar compounds are now analyzed by GC-MS with El or Cl. For polar compounds the usual method of introduction is in a liqnid flow, e.g., in methanol (without an LC column), that then is vaporized and the analyte ionized using ESI or APCl. This approach is termed flow injection analysis (FIA). While quadrupole analyzers are applicable for ether GC-MS or FIA, TOF and FT instruments provide the added dimension of accurate mass determination. 

Analysis of chemical samples and biological substances of nonproteomic origin requires a rather different approach than that implemented for the study of proteins and nucleic acids. Sample collection and introduction may be achieved via aerosol particle collection for volatile compounds and air-bome pathogens or via a direct insertion probe for solid, nonvolatile samples. Alternatively, SPME may be used to extract and preconcentrate volatile compounds present in air, water, or... 

Samples analyzed by El mass spectrometry must be converted to gas phase. For pure gases or volatile liquids the samples may be introduced directly through a small orifice that allows an appropriate amount of material into the vacuum chamber. A small amount of a solid sample can be placed in a melting point capillary tube and inserted into the mass spectrometer at the end of a metal rod, called a direct insertion probe (DIP).The temperature at the tip of the probe can be varied to promote sublimation of the sample. Another common method of sample introduction is gas chromatography, which is the ideal choice for samples that are impure. 

Note The below-mentioned sample introduction systems (reservoir inlets, various direct insertion probes, and chromatographs) are of equal importance to other ionization methods. 

As was discussed in detail in Chapter 4, sample preparation is crucial, especially for samples of biological/biochemical origia Samples can be introduced via a direct inlet, a GC, or an HPLC. Direct introduction may include a heated reservob (for volatile compounds that are bquids at room temperature), a direct insertion probe (for relatively pure, synthesized sobd organic compounds (El) or fast-atom bombardment (FAB) and biomolecules (MALDI), and a direct infusion or flow injection for electtospray ionization (ESI) or atmospheric pressure chemical ionization (APCI, see the following text). GC and HPLC are strongly recommended and routinely used for the analysis of complex mixtures. (These separation techniques will be discussed briefly in Section 3, and has already been discussed in somewhat more detail in Chapter 5.)... 

The first mass spectra of TATP were recorded by introduction of the sample with a direct-insertion probe [48,49]. The El mass spectrum was very uninformative and contained its major ions in the low-mass region, with a very low-abundance molecular ion. It had some resemblance to the spectrum of acetone. The Cl mass spectra, using methane [49] and isobutane [48,49] contained a distinct [M + H] ion at m/z 223. A GC-MS analysis of TATP was later performed, using El [51-53] and Cl-methane [51]. The resulting mass spectra were similar to those obtained by the solid-probe technique. 

In principle, any type of magnetic or quadrupole mass spectrometer can be utilized for the analytical pyrolysis of organic materials, if a direct introduction system capable of producing a desired tempera-ture/time profile is available. For example, direct insertion probes (DIPs) and direct exposure probes (DEPs) are Avidely used for sample introduction and such probes are supplied with control units that allow heating and temperature programming of the sample up to 500-800°C. Therefore, such modules should be considered as the most readily available probes for Py-MS studies. 

The analyte introduction systan is a gas chromatograph. Direct introduction of the sample into the source is rare nowadays because of the difficulty of insertion due to the different pressures between the inside of the source and the exterior. Furthermore, direct introduction is theoretically confined to the analysis of pure compounds that are rare in analytical chemistry. Direct insertion probes are not presented in this book because they do not fall within the scope of GC-MS coupling. 

Acids in the free form (with the exception of some low-molecular-weight acids) are too polar to permit GC introduction into the MS, and most spectra of free compounds have been obtained from samples introduced from a reservoir inlet or a direct insertion probe. This has limited their study to pure samples or very simple mixtures. Nevertheless a surprising number of acids have sufficient vapour pressure and thermal stability to be volatilized and, when coupled with a purification procedure where necessary, MS analysis may be satisfactory. 

The sample must be introduced into the ionization source so that vacuum inside the instrument remains unchanged. Samples are often introduced without compromising the vacuum using direct infusion or direct insertion methods. For direct infusion, a capillary is employed to introduce the sample as a gas or a solution. For direct insertion, the sample is placed on a probe, a plate or a target that is then inserted into the source through a vacuum interlock. For the sources that work at atmospheric pressure and are known as atmospheric pressure ionization (API) sources, introduction of the sample is easy because the complicated procedure for sample introduction into the high vacuum of the mass spectrometer is removed. 

FIGURE 20-12 Schemalic of (a) an external sample-introduction system—note that the various parts are not to scale—and (b) a sample probe for inserting a sample directly into the ion source. (From G. A. Eadon. in TreaWse on Anatyticat Chemistry, 2nd ed., J. D, Winefordner, M. M, Bursey. and L M. Kotthoff, eds.. Part t, Vol, 11. p. 9. New York Wiley. 1989, Reprinted by permission of John Wiley Sons, inc.)... 

Numerous ambient direct ionization methods have been introduced for use with mass spectrometry over the last several years.< - °) A major advantage of these methods is speed of analysis, which is achieved not only by the fast insertion and ionization of the sample, but by the elimination of most sample preparation and chromatographic separations. However, this presents a problem in materials analysis and for mixtures in general because of the complexity of the mass spectra that result from direct analysis of complex mixtures. The atmospheric solids analysis probe (ASAP)< > mass spectrometry (MS) method offers some separation related to volatility by control of the heated gas used to effect vaporization, but this is not sufficient for many mixtures. Ion mobility spectrometry (IMS) offers rapid gas-phase separation of ions based on differences in charge state and collision cross section (CCS) (size/shape). Here we explore the utility of a commercial IMS/MS instrument with ASAP sample introduction for analysis of complex mixtures. 

The most straightforward tool for the introduction of a sample into a mass spectrometer is called the direct inlet system. It consists of a metal probe (sample rod) with a heater on its tip. The sample is inserted into a cmcible made of glass, metal, or silica, which is secured at the heated tip. The probe is introduced into the ion source through a vacuum lock. Since the pressure in the ion source is 10-5 to 10-6 torr, while the sample may be heated up to 400°C, quite a lot of organic compounds may be vaporized and analyzed. Very often there is no need to heat the sample, as the vapor pressure of an analyte in a vacuum is sufficient to record a reasonable mass spectrum. If an analyte is too volatile the cmcible may be cooled rather than heated. There are two main disadvantages of this system. If a sample contains more than one compound with close volatilities, the recorded spectrum will be a superposition of spectra of individual compounds. This phenomenon may significantly complicate the identification (both manual and computerized). Another drawback deals with the possibility of introducing too much sample. This may lead to a drop in pressure, ion-molecule reactions, poor quality of spectra, and source contamination. 

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