Shai Dagan

A direct sample introduction device for mass spectrometry studies and gas chromatography mass spectrometry analyses

A novel direct sample introduction (DSI) device is described, based on sample introduction in a disposable mini-vial (test tube) inside the existing gas chromatography (GC) injector. This DSI device effectively transforms a conventional GC injector, followed by a short capillary column, into a very cost-effective alternative to the standard direct insertion probe and enables a continuous sample flux at the mass spectrometer ion source for mass spectrometry (MS) and MS-MS studies. This same device also enables direct introduction of dirty samples for GC and GC-MS analysis. The dirty sample introduction is based on the thermal extraction of semi-volatile sample compounds inside the GC injector, while the non-volatile residue is retained in the sample vial that is disposed of after the analysis. The major capabilities of this new DSI device are demonstrated and its various features are discussed.

Fast, very fast, and ultra-fast gas chromatography-mass spectrometry of thermally labile steroids, carbamates, and drugs in supersonic molecular beams

Gas chromatography-mass spectrometry (GC-MS) analyses of thermally labile compounds have been studied by using a short column fast gas chromatograph, coupled with fly-through electron ionization in supersonic molecular beams. Thirty-two compounds, which include steroids, carbamate pesticides, antibiotic drugs, and other pharmaceutical compounds, have been analyzed and the details of their GC-MS analysis are provided. The ability to analyze thermally labile compounds is discussed in relation to the speed of analysis. A new term, “speed enhancement factor” (SEF), is defined as the product of column length reduction and the carrier gas linear velocity increase, as compared with normal GC-MS conditions. Fast, very fast, and ultra-fast GC-MS are defined with a SEF in the ranges of 5–30, 30–400, and 400–4000, respectively. Trade-offs in the degree of dissociation, speed, gas chromatograph resolution, and sensitivity were studied and examined with thermally labile molecules. The experimental factors that affect the dissociation are described with emphasis on its reduction. We claim that the use of supersonic molecular beams for sampling and ionization provides the ultimate capability in the GC-MS of thermally labile compounds. The obtained 70-eV electron ionization mass spectra are shown, and an enhanced relative abundance of the molecular ion is demonstrated together with library search capability of these mass spectra, which is better than that reported with particle beam liquid chromatography-mass spectrometry. The performance of fast GC-MS in supersonic molecular beams is compared with other methods of fast GC-MS and with particle beam liquid chromatography-mass spectrometry.

Electron impact mass spectrometry of alkanes in supersonic molecular beams.

The electron impact mass spectrometry of straight chain alkanes C8H18-C40H82, squalane, methylstearate, 1-chlorohexadecane, 1-bromohexadecane, and dioctylphthalate was studied by sampling them with supersonic molecular beams. A fly-through Brink-type electron impact ion source was used, utilizing a vacuum background ion filtration technique based on differences between the kinetic energy of the supersonic beam species and that of thermal molecules. The 70-eV electron impact mass spectra of all the alkanes were characterized by a pronounced or dominant molecular weight peak together with all the fragment ions normally exhibited by the standard thermal 70-eV EI mass spectra. In contrast, the NIST library of most of these molecules did not show any molecular weight peak. By eliminating tile intramolecular thermal vibrational energy we gained control over the degree of molecular ion fragmentation by the electron energy. At an electron energy of 18 eV the molecular ion dissociation was further reduced considerably, with only a small absolute reduction in the peak height by less than a factor of 2. The effect of vibrational cooling increased with the molecular size and number of atoms. Pronounced differences were observed between the mass spectra of the straight chain triacontane and its branched isomer squalane. Similar mass spectra of octacosane (C28H58) achieved with 70-eV EI in a supersonic molecular beam were obtained with a magnetic sector mass spectrometer by using an electron energy of 14 eV and an ion source temperature of 150 °C. However, this ion source temperature precluded the gas chromatography-mass spectrometry (GC-MS) of octacosane. The GC-MS of alkanes was studied with an ion trap gas chromatograph-mass spectrometer at an ion source temperature of 230 °C. Thermal peak tailing was observed for C20H42 and heavier alkanes, whereas for C28H58 and heavier alkanes the severe peak tailing made quantitative GC-MS impractical. In contrast, no peak tailing existed even with C40H82 for GC-MS in supersonic molecular beams. The minimum detected amount of eicosane (C20, H42) was shown to be 60 fg. This was demonstrated by using single ion monitoring with the quadrupole mass analyzer tuned to the molecular weight peak of 282 u. The coupling of electron impact mass spectrometry in supersonic molecular beams with hyperthermal surface ionization and a fast GC-MS inlet is briefly discussed.

Fast, high temperature and thermolabile GC—MS in supersonic molecular beams

Abstract This work describes and evaluates the coupling of a fast gas chromatograph (GC) based on a short column and high carrier gas flow rate to a supersonic molecular beam mass spectrometer (MS). A 50 cm long megabore column serves for fast GC separation and connects the injector to the supersonic nozzle source. Sampling is achieved with a conventional syringe based splitless sample injection. The injector contains no septum and is open to the atmosphere. The linear velocity of the carrier gas is controlled by a by-pass (make-up) gas flow introduced after the column and prior to the supersonic nozzle. The supersonic expansion serves as a jet separator and the skimmed supersonic molecular beam (SMB) is highly enriched with the heavier organic molecules. The supersonic molecular beam constituents are ionized either by electron impact (EI) or hyperthermal surface ionization (HSI) and mass analyzed. A 1 s fast GC—MS of four aromatic molecules in methanol is demonstrated and some fundamental aspects of fast GC—MS with time limit constraints are outlined. The flow control (programming) of the speed of analysis is shown and the analysis of thermolabile and relatively non-volatile molecules is demonstrated and discussed. The tail-free, fast GC—MS of several mixtures is shown and peak tailing of caffeine is compared with that of conventional GC—MS. The improvement of the peak shapes with the SMB—MS is analyzed with the respect to the elimination of thermal vacuum chamber background. The extrapolated minimum detected amount was about 400 ag of anthracence- d 10 , with an elution time which was shorter than 2s. Repetitive injections could be performed within less than 10 s. The fast GC—MS in SMB seems to be ideal for fast target compound analysis even in real world, complex mixtures. The few seconds GC—MS separation and quantification of lead (as tetraethyllead) in gasoline, caffeine in coffee, and codeine in a drug is demonstrated. Controlled HSI selectivity is demonstrated in the range of 10 1 to 10 4 anthracene/decane which helped to simplify the selective analysis of aromatic molecules in gasoline. The contribution of SMB to the operation of the fast GC—MS is summarized and the compatibility with conventional GC having a megabore column is shown. Splitless injections of 100 μL sample solutions for trace level concentration detection is also presented (with a conventional GC).

Megabore versus Microbore as the Optimal Column for Fast Gas Chromatography/Mass Spectrometry:

The use of a short megabore (0.53 mm ID) column for fast gas chromatography/mass spectrometry (GC/MS) was compared with the use of a short microbore (0.1 mm ID) capillary column and demonstrated to be superior. Several aspects and experimental parameters that affect fast GC/MS are demonstrated and discussed. These aspects include the acceptable column flow rate, column internal diameter and length, mass analyzer, GC/MS interface properties, ionization method and the inter-relationship between these features. The microbore column provides, at best, a modest gain in chromatographic resolution, however, the accompanying loss in concentration sensitivity is severe due to greatly reduced sample introduction rate and other effects. We claim that, with fast GC/MS analysis the mass spectrometer and ionization methods play a major role in the overall separation capability and that enhanced mass spectrometric separation is more useful than the modest gain achieved with the microbore column. Thus, the use of a supersonic molecular beam with electron ionization or hyperthermal surface ionization contributes greatly to fast GC/MS. This contribution is further enhanced due to the high flow-rate, fast injection that minimizes the time spent on GC temperature programming and cooling. It is demonstrated that fast GC/MS does not necessarily require time-of-flight mass analysis and that the conjectured problem of mass spectral skewing is negligible even with microbore columns. It is emphasized and demonstrated that compatibility with advanced tools for fast sample preparation is an important consideration for an effective fast GC/MS analysis with real-world samples. Fast GC/MS analysis, without extraction, of the pesticide diazinon in the herb chervil is shown using a novel direct sample introduction device, a short megabore column and electron ionization in a supersonic molecular beam.

High-efficiency surface-induced dissociation on a rhenium oxide surface

We report on the high-efficiency surface-induced dissociation of benzene and cyclohexane polyatomic ions after scattering from a rhenium oxide surface with a kinetic energy of 5–290 eV. Rhenium oxide was prepared by directly heating a rhenium metal foil, under 10−5 mbar partial oxygen pressure, at about 1000 K.Rhenium oxide is characterized by a very high work function of 6.4 eV and thus minimizes ion reneutralization probabilities. The catalytic combustion of surface organic impurities with oxygen ensures good long-term stability.We found that the surface-induced dissociation ion current is 70 times larger on rhenium oxide than on bare rhenium or stainless steel. Absolute scattered ion yields of about 50% were measured. The implications of surface-induced dissociation on mass spectrometry in supersonic molecular beams are mentioned.

Cluster chemical ionization and deuterium exchange mass spectrometry in supersonic molecular Beams

A cluster-based chemical ionization method has been developed that produces protonated molecular ions from molecules introduced through a supersonic molecular beam interface. Mixed clusters of the analyte and a clustering agent (water or methanol) are produced in the expansion region of the beam, and are subsequently ionized by “fly through” electron impact (EI) ionization, which results in a mass spectrum that is a combination of protonated molecular ion peaks together with the conventional EI fragmentation pattern. The technique is presented and discussed as a tool complementary to electron impact ionization in supersonic molecular beams. Surface-induced dissociation on a rhenium oxide surface is also applied to simplify the mass spectra of clusters and reveal the analyte spectrum. The high gas flow rates involved with the supersonic molecular beam interface that enable the easy introduction of the clustering agents also have been used to introduce deuterating agents. An easy-to-use, fast, and routine on-line deuterium exchange method was developed to exchange active hydrogens (NH, OH). This method, combined with electron impact ionization, is demonstrated and discussed in terms of the unique information available through the EI fragmentation patterns, its ability to help in isomer identification, and possible applications with fast gas chromatography-mass spectrometry in supersonic molecular beams.

Laser desorption fast gas chromatography–Mass spectrometry in supersonic molecular beams

A novel method for fast analysis is presented. It is based on laser desorption injection followed by fast gas chromatography-mass spectrometry (GC-MS) in supersonic molecular beams. The sample was placed in an open air or purged laser desorption compartment, held at atmospheric pressure and near room temperature conditions. Desorption was performed with a XeCl Excimer pulsed laser with pulse energy of typically 3 mJ on the surface. About 20 pulses at 50 Hz were applied for sample injection, resulting in about 0.4 s injection time and one or a few micrograms sample vapor or small particles. The laser desorbed sample was further thermally vaporized at a heated frit glass filter located at the fast GC inlet. Ultrafast GC separation and quantification was achieved with a 50-cm-long megabore column operated with a high carrier gas flow rate of up to 240 mL/min. The high carrier gas flow rate provided effective and efficient entrainment of the laser desorbed species in the sweeping gas. Following the fast GC separation, the sample was analyzed by mass spectrometry in supersonic molecular beams. Both electron ionization and hyperthermal surface ionization were employed for enhanced selectivity and sensitivity. Typical laser desorption analysis time was under 10 s. The laser desorption fast GC-MS was studied and demonstrated with the following sample/matrices combinations, all without sample preparation or extraction: (a) traces of dioctylphthalate plasticizer oil on stainless steel surface and the efficiency of its cleaning; (b) the detection of methylparathion and aldicarb pesticides on orange leaves; (c) water surface analysis for the presence of methylparathion pesticide; (d) caffeine analysis in regular and decaffeinated coffee powder; (e) paracetamol and codeine drug analysis in pain relieving drug tablets; (f) caffeine trace analysis in raw urine; (g) blood analysis for the presence of 1 ppm lidocaine drug. The features and advantages of the laser desorption fast GC-MS are demonstrated and discussed.

Collisionally-activated dissociation in hyperthermal surface ionization of cholesterol

Abstract Cholesterol in a hydrogen-seeded supersonic molecular beam was scattered from a continuously oxidized rhenium foil. The hyperthermal surface scattering exhibited efficient molecular ionization with a controlled amount of molecular ion dissociation. At 5.3 eV incident molecular kinetic energy the hyperthermal surface ionization mass spectrum was dominated by the parent molecular ion. Upon the increase of the molecular kinetic energy, a gradual increase in the degree of ion dissociation was observed. At 22eV incident kinetic energy the parent ion was completely dissociated and the mass spectrum was dominated by an extensive consecutive fragmentation. An efficient kinetic-vibrational energy transfer was observed, and it is extimated to be over 18% of the available incident kinetic energy. The implication for surface collisionally-activated dissociation of polyatomic ions is discussed. Rhenium oxide is suggested as an optimal surface for this purpose, as well as for the hyperthermal surface ionization of neutral species.

Surface ionization mass spectrometry of drugs in the thermal and hyperthermal energy range — a comparative study

Abstract Thermal and hyperthermal surface ionization (SI) mass spectra of nicotine, caffeine and lidocaine were obtained using a rhenium oxide surface. Thermal surface ionization was studied on an oxidized surface positioned inside an electron impact ion source, while hyperthermal surface ionization (HSI) was obtained upon seeding the compounds into a hydrogen or helium supersonic molecular beam that scattered from the rhenium oxide surface. Both HSI and SI provide rich, informative and complementary mass spectral information. The results indicate that SI follows thermal dissociation processes on the surface prior to the desorption of the ion, while in HSI no thermal equilibrium is established and the ionization process is impulsive, followed by mostly unimolecular ion dissociation. HSI mass spectra are similar to electron impact mass spectra in the fragment ion masses, but the observed relative intensities are different. HSI is a softer ionization method compared to SI, and enables the degree of ion fragmentation to be tuned so that it can be minimized to a low level at low molecular kinetic energy. In SI, limited control over the degree of fragmentation is possible through the surface temperature. The analytical mass spectrometric applications of SI and HSI are briefly mentioned.