The accuracy of the ion flight time measurement in the time-of-flight mass spectrometer is critical to achieving high resolution. The pulse amplitude variation of the detector pulses leads to the registration time spread at a given pulse detection threshold. This time spread can be eliminated by determining the position of the pulse apex. To determine the position of the pulse apex, the output of the ion detector is fed simultaneously to the two channels of the time-to-digital converter. In this case, the first channel is set to register the leading edge, and the second channel is set to register the trailing edge of the pulse. Using a simple processing of the received data, the position of the pulse tip is determined. Thus, the dependence of the temporal position of the peak on the pulse amplitude is largely eliminated. Examples are given, and the efficiency of using this algorithm to increase the resolution of time-of-flight mass spectral peak registration is demonstrated.

We report the results of a study of the effect of a supersonic gas jet on the formation and extraction of analyzed ions from a liquid supplied through a capillary to an RF quadrupole connected to a time-of-flight mass spectrometer with orthogonal ion injection. The supersonic gas jet is formed inside a cylindrical channel and passes through an electron ionization ion source. The results confirm the prospects of using a supersonic jet to creating a high-performance ion source from a liquid supplied to the vacuum region under the pressure of ~10–4 Torr.

В статье описываются результаты исследования воздействия сверхзвуковой газовой струи, сформированной внутри цилиндрического канала и пропущенной через источник электронной ионизации, на образование и экстракцию анализируемых ионов из жидкости, подаваемой через капилляр внутрь радиочастотного квадруполя, который сопряжён с времяпролётным масс-спектрометром с ортогональным вводом ионов. Полученные результаты подтверждают перспективность использования сверхзвуковой струи для создания высокоэффективного источника ионов из жидкости, подаваемой в область вакуума 10-4 Торр.

The experimental results of a mass spectral analysis of volatile organic compounds in a gaseous sample, obtained using an original design of an ion source based on the Penning ionization of a gas sample by excited metastable inert gas atoms, are presented. Using ANSYS software, a gas-dynamic simulation of reagent gas flow from discharge zone to ionization region was carried out to analyze the effect of gas flow profile on the transport of metastable atoms and ionization efficiency. The n -octane and toluene samples diluted with helium at 100 ppb mole concentrations were used for our experiments. The resulting mass spectra of n -octane and toluene samples containe far more intensive molecular ions in comparison to n -octane and toluene electron ionization mass spectra from the NIST database. The sensitivity of 5 ions per 1 pg and 130 ions per 1 pg was achieved for n -octane and toluene molecular ions using the developed ion source combined with our mass spectrometer. The corresponding detection limits are 2.3 pg s –1 for n-octane molecular ions and 0.08 pg s –1 for toluene molecular ions. The detection limit for the reported ion source was considered theoretically.

Generation of an ion beam and its transmission into a mass analyzer is one of central problems in mass spectrometry. The use of a narrowly directed supersonic gas jet has a number of advantages in comparison with other sampling methods. The aim of this work was to confirm the declared earlier properties of the jet formed at the outlet of a cylindrical channel when the free path length of gaseous atoms at the beginning of the channel is comparable with the channel diameter. The paper describes the ability of such a supersonic jet to conserve an additional energy of jet gas atoms. A significant influence of the temperature of the gas flow on the yield of cyclohexane fragment ions was found, cyclohexane being an admixture in the noble gas jet passing through an electron ionization ion source. A possibility of obtaining a flow of metastable electronically excited atoms inside the jet is also shown. The results of the work confirm the availability of the supersonic gas jet for the design of a high efficiency ion source inside the radio-frequency quadrupole at the input of the mass analyzer.

Transport properties of gramicidin S, apamin, melittin and cytochrome C ions [M + nH] n+ were experimentally measured at conditions: T = 300 K, p = 1.9 Pa and in the E/N range from 40 up to 400 Td using gas filled segmented RF-quadrupole with rotational excitation of ions. Modified theoretical model of ion deceleration in collisions with gas molecules was used for the data analysis. The model is valid for hard sphere collisions and for ion mass much higher than mass of gas molecule. Using this model it is possible to simulate ion motion in gas under influence of electric fields in wide ion velocity range. Collision cross sections of the ions were calculated from the experimentally measured drift times. Calculated cross sections for most of the ions showed significant dependence on the ion drift velocity. It was supposed that long-range (polarization) ion-molecular interaction plays an essential role in the observed transport properties of the gas phase peptide ions with molecular weight in the range of 1000–3000 Da and m/z values up to 1000 Th.

The objective of this work is the study of the ion mobility dependence of electrospray Gramicidin S ions [M + 2H]2+ on the ion bunch drift velocity under variation of the specific electrical field strength at certain gas number density in the range from 0 to 300 Td. The ion mobility measurements were carried out under rotational excitation of ions using custom built gas filled segmented radio frequency quadrupole with the nitrogen buffer gas pressure of 1.9 Pa. The resulting ion mobility dependence of [M + 2H]2+ Gramicidin S ions upon the bunch drift velocity is essentially weaker, than it could be expected from the hard sphere collision model. By comparing the acquired ion mobility dependencies with the theoretical ones derived from the hard sphere collision model we suppose that the attractive polarization potential takes an essential part into the ion-molecular interaction even for such big organic ions as Gramicidin S.

One of the possible methods for 3D protein structure investigation is the study of gas-phase hydrogen/deuterium (H/D) exchange reaction between protein ions and a D-containing reactant gas, e.g., D2O or ND3. A segmented radio frequency quadrupole (RFQ) was used as a molecule-ion reactor to study gasphase H/D-exchange of protonated ions of three different peptides. The ions were produced in an electrospray ion source. The RFQ is a part of ion transport interface of a high resolution orthogonal time-of-fiight mass spectrometer (O-TOF MS). The RFQ was modified for a linear ion trap (LIT) mode of operation to increase a dwell time of target ions inside the RFQ. Phase-sensitive operation of the LIT and the O-TOF MS was controlled by custom developed PC executive program. The reaction mixture of N2 and ND3 was injected into the reactor, while keeping its partial pressure in the range of 10−3–10−2 mbar, and corresponding ND3 concentration in the range of 1013–1014 cm−3. It was possible to vary the ion dwell time in the reactor between 30 ms and 1 s. H/D-exchange was studied for leucine enkephalin, gramicidin S and apamin. The data analysis based on statistical approach has shown a principal possibility to distinguish different mobile H-atoms of peptides, taking part in H/D-exchange, according to reaction rates.

The de novo sequence of a new peptide from venom of the East-European hornet Vespa orientalis L. was determined by mass spectrometry. It was used as an example to show that time-of-flight mass spectrometry with an electrospray ion source can be used effectively to determine the amino-acid sequence of microquantities of peptides.

A new method to study the kinetics of ion decay reactions in a radio frequency quadrupole (RFQ) with resonance rotational excitation is described. The decay reaction occurs while ions move through the RFQ and the resulting precursor-ion consumption, as well as the appearance of fragment ions, are monitored using high-resolution orthogonal time-of-flight mass spectrometry (o-ToF MS). The developed method enables one to extract absolute values of the decay rate constants using the ion drift motion under the uniform DC electrical field along the RFQ created by the segmented structure. The ion dwell time, as well as its internal temperature are calculated using the ion mobility measured under the same experimental conditions as the kinetic investigations. The values of the decay rate constants, activation energies and preexponential factors for three substances under investigation are reported.

An ion rotating excitation mode of operation of a segmented gas-filled radio-frequency quadrupole (RFQ) ion guide for a high-resolution orthogonal time-of-flight (TOF) mass spectrometer is described. It is shown theoretically, by computer simulation and experimentally, that ion rotating excitation in a gas-filled RFQ has several advantages over other types of ion oscillation excitation. The main advantages are an approximately twofold increase in average ion kinetic energy for the same maximal deviation from the RFQ axis and therefore an increase of about this factor of average internal excitation energy of ions, and the extended mass range of fragment ions that can be observed. The new method of ion decomposition by ion rotation around the axis of an RFQ ion guide was experimentally implemented and tested using a home-built ‘SIN-COS’ generator to supply the excitation voltage. This generator enables control of phase shift and amplitude of excitation voltages applied to quadrupole rods smoothly from the data acquisition program running on a PC. Copyright © 2001 John Wiley & Sons, Ltd.

A new effective and robust approach to the detection of incompletely resolved peaks, and evaluation of their parameters in high-resolution time-of-flight mass spectra for time-to-digital convertor (TDC) data acquisition mode, is described. The method is based on fast construction of a smoothed continuous curve that approximates the initial data (transformed to a constant relative width of time intervals for ion counting) with respect to precision of measurements. The first derivative of this curve is used for correction of skewness of the peak shape as far as possible. A contribution of the second derivative is subtracted from the smoothed curve for better resolution of partially resolved peaks. The comparison of local maxima of this resulting final curve with those for the initial smoothed curve allows reliable detection of the peaks and to test whether or not they are spoiled by overlapping. Ion counting performed by TDC gives an opportunity to estimate standard deviations of peak locations and their intensities. These values proved to be close to theoretically minimal standard deviations for these parameters for single fully resolved peaks. Thus, estimates of the main parameters of mass peaks by the described method are close to statistically efficient estimators for these parameters.