Direct insertion probes

The oldest but obviously old-fashioned way to introduce solids into a mass spectrometer ion source is by directly placing them inside an ion source which is mounted to the instrument thereafter and heated. [37] The use of a micro sample vial or crucible containing some 0.1-2 pg of the analyte that can be directly 

To prevent more volatile samples from sudden evaporation a circulation water cooling is often incorporated in the DIP, and refrigerated probes for more volatile samples have also been developed. [43] Sometimes, glass wool is placed into the sample vial to increase the surface for adsorption, and thus to slow down evaporation of the sample. Sudden evaporation causes distorted spectra and may even result in a temporary breakdown of the high vacuum. 

Note Disposable vials are free of memory effects. Glass vials bear the additional advantage of showing how much sample is placed at which position in the vial. Afterwards, it is also possible whether the analyte decomposed during the measurement to yield some black residue. 

A DIP consists of a stainless steel shaft with a tip suitable to insert the sample vial. The shaft is polished to allow installation an effective vacuum seal against the O-rings of a vacuum lock. Being transferred into the high vacuum of the ion source housing through the vacuum lock [26], the probe is pushed in, eventually until it contacts the ion source block (Fig. 5.7). Spectra are then acquired while the 

Special high-temperature probes reaching up to 1000°C are also available. Modem direct probes have temperature programed heaters [27] allowing to set rates of 5-150°C min or are equipped with ion current-controlled heaters [28]. 

---

If samples are largely pure, single substances, then the sample inlet can be quite simple, as with a direct insertion probe or a gas inlet. However, most analyses require assessment of the number of components, their relative proportions, and their chemical structures. This level... 

For solids, there is now a very wide range of inlet and ionization opportunities, so most types of solids can be examined, either neat or in solution. However, the inlet/ionization methods are often not simply interchangeable, even if they use the same mass analyzer. Thus a direct-insertion probe will normally be used with El or Cl (and desorption chemical ionization, DCl) methods of ionization. An LC is used with ES or APCI for solutions, and nebulizers can be used with plasma torches for other solutions. MALDI or laser ablation are used for direct analysis of solids. 

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. 

Thermally labile compounds may also be studied - for example, the El specffa from the condensation products from the reaction between dimedone and substituted phenylbenzopyrans obtained via a particle-beam interface show less thermal degradation than do the mass spectra obtained using a direct-insertion probe [10]. 

The direct insertion probe consists of a metal sample holder drilled to accept standard melting point capillaries up to 1 inch in length. This is inserted into the ion source through a vacuum lock and may be heated to 250°C at varying rates. 

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. 

The DCI probe is particularly attractive for samples that are susceptible to thermal decomposition, although it can equally well be used as a general means of introducing samples into the ionisation source, i.e. as an alternative to the direct insertion probe. The types of sample which benefit most from DCI probing are higher-molecular-weight, less-volatile compounds, organometallics, and any thermally sensitive compounds [40,67]. DCI is considered to be a soft ionisation technique. 

Quantitative analysis using FAB is not straightforward, as with all ionisation techniques that use a direct insertion probe. While the goal of the exercise is to determine the bulk concentration of the analyte in the FAB matrix, FAB is instead measuring the concentration of the analyte in the surface of the matrix. The analyte surface concentration is not only a function of bulk analyte concentration, but is also affected by such factors as temperature, pressure, ionic strength, pH, FAB matrix, and sample matrix. With FAB and FTB/LSIMS the sample signal often dies away when the matrix, rather than the sample, is consumed therefore, one cannot be sure that the ion signal obtained represents the entire sample. External standard FAB quantitation methods are of questionable accuracy, and even simple internal standard methods can be trusted only where the analyte is found in a well-controlled sample matrix or is separated from its sample matrix prior to FAB analysis. Therefore, labelled internal standards and isotope dilution methods have become the norm for FAB quantitation. 

In direct insertion techniques, reproducibility is the main obstacle in developing a reliable analytical technique. One of the many variables to take into account is sample shape. A compact sample with minimal surface area is ideal [64]. Direct mass-spectrometric characterisation in the direct insertion probe is not very quantitative, and, even under optimised conditions, mass discrimination in the analysis of polydisperse polymers and specific oligomer discrimination may occur. For nonvolatile additives that do not evaporate up to 350 °C, direct quantitative analysis by thermal desorption is not possible (e.g. Hostanox 03, MW 794). Good quantitation is also prevented by contamination of the ion source by pyrolysis products of the polymeric matrix. For polymer-based calibration standards, the homogeneity of the samples is of great importance. Hyphenated techniques such as LC-ESI-ToFMS and LC-MALDI-ToFMS have been developed for polymer analyses in which the reliable quantitative features of LC are combined with the identification power and structure analysis of MS. 

Dobanol Ethoxy late [443], At least 16 Triton units with mass 910 were observed. A study of the reactions of amines and amine derivatives with scC02 using cSFC-MS was also reported [448], Both cSFC-APCI-MS and cSFC-ESI-MS of PEG 600 and PPG 425 were described [416]. Direct insertion probe (DIP) methodology was used for the structure analysis of the antistatic agent V,fV-bis-(2-hydroxyethyl)alkylamine. When analysed by SFC-MS coupling, the same sample could be separated into six components. The alkyl chains consist of saturated Cn, Ci4, C16 and C18 chains and of Cig chains with one double bond where 18 1 and 16 0 chains dominate. 

Wide use of TLC-MS is hampered by the lack of commercially available interfaces. This also restricts automation and high throughput. Commercial direct insertion probes for scanning TLC-MS are available [811]. Compared with on-line LC-MS operation, TLC-MS hyphenation is much less highly developed. 

Pyrolysis mass spectroscopy was conducted with a Hewlett-Packard model 5985B gas chromatograph/quadrupole mass spectrometer, operated at sslO- Torr and 70eV electron-impact ionization energy. Samples were introduced into the mass spectrometer via a glass lined direct insertion probe (DIP). The samples were decomposed in the DIP to a nominal temperature of 300°C at a heating rate of 30°C/min. 

Direct insertion probe pyrolysis mass spectrometry (DPMS) utilises a device for introducing a single sample of a solid or liquid, usually contained in a quartz or other non-reactive sample holder, into a mass spectrometer ion source. A direct insertion probe consists of a shaft having a sample holder at one end [70] the probe is inserted through a vacuum lock to place the sample holder near to the ion source of the mass spectrometer. The sample is vaporized by heat from the ion source or by heat from a separate heater that surrounds the sample holder. Sample molecules are evaporated into the ion source where they are then ionized as gas-phase molecules. In a recent study, Uyar et al. [74] used such a device for studying the thermal stability of coalesced polymers of polycarbonate, PMMA and polylvinyl acetate) (PVAc) [75] and their binary and ternary blends [74] obtained from their preparation as inclusion compounds in cyclodextrins. 

In DTMS the sample suspended in a suitable solvent is applied to a platinum/rhodium filament (Pt/Rh 9 1) of the direct insertion probe. The probe is inductively heated at a rate of 0.5 1 A min 1 to a maximum temperature of 800 °C. This means that the temperature is linearly increased from ambient to 800 °C in 1 2 min. 

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.

Fig. 11.2. Diagram of the components of an El source. Gaseous samples (gases or vaporized liquids or solids) are introduced into the ionization chamber using a reservoir with molecular leak, direct insertion probe or as an eluent from a GC column. The collimated stream of 70 eV electrons interacts with the neutral analyte molecules to generate stable radical cations (M+ ) and unstable radical cations (M+ ) that undergo dissociation reactions to form the characteristic fragment ions observed in many El mass spectra.

How are low vapor pressure solid samples introduced into the source of a mass spectrometer (direct insertion probe). 

A second method uses permethylation of the dephosphated (48% aqueous HF, 48 h, 4°C) and 2H-reduced fipid A. This approach allowed the assignment of amide-bound fatty acids linked to GlcN(I) and GlcN(II), as well as the identification of the backbone structure as a HexpN disaccharide (85). Mass-spectrometric analysis of the products was performed by using either a short g.l.c. column (0.3 X 5 cm) or by direct insertion-probe analysis (87). In the case of C. violaceum (85), the mass spectra obtained from the permethyl-ated HexpN disaccharide bearing attached TV-methylacyl residues revealed unequivocally that both amino groups carried 12 0(3-OH). 

Mass-spectral (MS) Analysis. Mass spectral analyses were obtained using a computerized Finnigan Model 1015 gas chromatograph-mass spectrometer (GC-MS) operated at 70 eV. Samples were introduced via direct insertion probe or by utilizing the GC-MS combination, both operated over a programmed temperature range. 

Highly volatile samples cannot be introduced into the ion source by means of a direct insertion probe even when cooling is applied. A reference inlet system or reservoir inlet system is better suited for that purpose. [59] The name of this type of... 

Kankare, J.J. Simple Temperature Programmer for a Mass Spectrometer Direct Insertion Probe. Anal. Chem. 1974, 46, 966-967. 

Sawdo, R.M. Blumer, M. Refrigerated Direct Insertion Probe for Mass Spectrometry. Anal. Chem. 1976, 48, 790-791. 

In Cl, the analyte is introduced into the ion source the same way as described for El, i.e., via direct insertion probe (DIP), direct exposure probe (DEP), gas chromatograph (GC), or reservoir inlet (Chap. 5.3). 

---