RU2542722C2 - Time-of-flight mass-analysis method and apparatus therefor - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention relates to mass spectrometry and can be used to broaden analytical capabilities of time-of-flight mass analysers. Bunches of ions at each input cycle are time-distributed according to a pseudo-random law, which is selected such that a periodic autocorrelation sequence function has zero side lobes and the value of the principal maximum is equal to the number of units in the sequence. Upon detection, signals corresponding to pulses of output ion current of the time-of-flight mass analyser are processed in a matched filter which operates according to the principle of summation of input and shifted sequences with plus and minus signs according to distribution of symbols "1" and "0" in the pseudo-random sequence. A time-of-flight mass analysis apparatus comprises a pseudo-random sequence generator and a matched filter, which are respectively connected to ion sources and detectors.

EFFECT: high sensitivity and wider dynamic range of time-of-flight mass spectrometers by increasing average values of currents of analysed ions.

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Description

The invention relates to the field of mass spectrometry and can be used to expand the analytical capabilities of mass analyzers of the time-of-flight type.

для времяпролетных масс-анализаторов [ВПА] со статическими полями, Tmin~m_max для ВПА с радиочастотными полями), а максимальное число ионов в пакетах p_max ограничено действием пространственного заряда во время дрейфа заряженных частиц, среднее значение вводимых во времяпролетные масс-анализаторы ионных токов I_cp=P_max е/Т, где е - заряд иона, по сравнению с масс-анализаторами с непрерывным вводом ионов оказываются существенно меньшим. Поэтому ВПМС по чувствительности и динамическому диапазону значительно уступают масс-спектрометрам со статическими анализаторами и квадрупольными анализаторами типа фильтра масс. За прототип приняты ВПМС со статическими [1-3] и радиочастотными [4] полями с периодическим вводом одиночных пакетов ионов.Known methods of mass separation of ions by time of flight suggest a time-of-flight mass transit spectrometer [IMS] of a single, with a duration of τ, ion packets [1-4] periodically with a period of T during K cycles. Since the minimum duration of the repetition periods of ion packets T _{min is} limited by the largest mass of the analyzed range m _max ( $$T\min \sim \sqrt{m_{\max }}$$

for time-of-flight mass analyzers [VPA] with static fields, Tmin ~ m _max for VPA with radio-frequency fields), and the maximum number of ions in packets p _{max is} limited by the action of the space charge during the drift of charged particles, the average value introduced into the time-of-flight ion mass analyzers currents I _cp = P _max e / T, where e is the ion charge, in comparison with mass analyzers with continuous input of ions, are much smaller. Therefore, the VPMS in sensitivity and dynamic range are significantly inferior to mass spectrometers with static analyzers and quadrupole analyzers such as a mass filter. VPMS with static [1-3] and radio-frequency [4] fields with periodic input of single ion packets were taken as a prototype.

The technical task of the invention is to increase the sensitivity and expand the dynamic range of the IMSC by increasing the average currents 1, p of the analyzed ions. This is achieved by periodically introducing cycles of 2 ≤n≤T / 2τ + 1 ion packets, each of which of duration τ << T, consists of p ions, periodically with a period T during K cycles of input during mass transit analyzers. Ion packets at intervals (i-1) T <t <iT, where i = 1, 2, 3 ... K are distributed in time in accordance with the positions of n characters “1” in pseudo-random sequences [PSP] of length N = 2n-l [ 5]. Pseudorandom sequences can be:

- the maximum length (M - sequence) at N = 2 ^k -1, or Legendre at N = 4 ^k +3, where k is an integer, N is a prime;

- Hall at N = 4k ² +27, where k is an integer;

- Jacobi for N = k (k + 2), where k, (k + 2) are primes.

During detection, a coordinated processing of the periodic sequence of pulses of the VPA output ion current is carried out, which consists in calculating, in accordance with the pseudo-random law, its periodic autocorrelation function (ACF).

From the properties of the PSP, it follows that the magnitude of the main maximum of its periodic autocorrelation function with a period of N = 2n-1 is n, and there are no side lobes.

Figure 1 shows a periodic M-sequence with N = 7, n = 4 and its periodic ACF. The autocorrelation functions of the SRP are calculated using matched filters (SF), working on the principle of summation with weighting factors -1 or +1 of the input and N-1 sequences shifted by the intervals τ, 2τ, 3τ ... (N-1) τ. Weighting factors -1 or +1 are selected in accordance with the values of 0 or 1 characters in pseudo-random sequences [5].

The structural diagram of a time-of-flight mass analyzer with periodic input of a series of n ion packets distributed in time according to a pseudo-random law is shown in Fig. 2, and explanatory timing diagrams in Fig. 3.

The symbol repetition rate in the SRP f = 1 / τ is set by the clock generator. Under the action of clock pulses in the generator of pseudorandom sequences, periodic signals with the period T≥Nτ are generated that control the operation of the ion source. The ion source in accordance with the control signals forms a periodic series of ion packets of n packets in each series. Packets of ions of duration τ, p ions in each packet, are introduced into the drift space of the VPA. When the amplitude of the current pulses in the ion packets I _m = pe / τ, the average ion current introduced into the VPA is I _cp = pre / T. This is n times larger than in the case of introducing periodic single ion packets.

In VPA, the ions of all packets are separated in time in accordance with their masses. Since the packets in the series are distributed in time according to the pseudo-random law, the output of the VPA ion current will be a superposition of n pulse sequences shifted relative to each other, each of which is the result of the passage of individual ion packets through the analyzer drift space. In this case, at the output of the VPA, a complex periodic sequence of current pulses is formed in which ions of different masses are not separated in time and can overlap each other (Figure 3, d). The periodic series of pulsed signals converted and amplified in wind turbines and silos enter a matched filter, which, in accordance with the algorithm for calculating periodic ACFs, converts them into signals that, up to scale factors, coincide with sequences of pulses of the output ion current of the VPA when periodic single packets of ions pass through it . In this case, the signal at the detector output is n times larger than in the case of mass analysis of periodic single ion packets.

The advantage of the proposed method of time-of-flight mass analysis of periodic series of n ion packets and a device for its implementation is to increase n times the average number of analyzed ions in comparison with the known prototypes. This allows n times to increase the sensitivity and expand the dynamic range of time-of-flight mass spectrometers.

Figure 1 a) - periodic with a period of N = 7, the number of unit characters n = 4 M - sequence; b) - periodic autocorrelation function of the periodic M - sequence.

Figure 2 Structural diagram of a time-of-flight mass analyzer with periodic input of a series of ion packets. GTI is a clock pulse generator, GPSP is a pseudo-random sequence generator, AI is an ion source, VPA is a time-of-flight analyzer, a wind turbine is a secondary electron multiplier, a silo is a broadband amplifier, an SF matched filter, and SN is a summing mass spectral storage device.

Figure 3 Timing diagrams VPA with the introduction of periodic series of ion packets, a) - clock pulses, b) and c) - ion currents at the input and output of the mass analyzer, d) and e) - ion currents at the input and output of the mass analyzer , e) - signal at the output of the matched filter.

Literature

1. A.E. Cameron, D.F. Eggers An Ion "Velocitron" // Review Scientific Instruments - 1948 - v.19, p.605.

2. N.I. Ionov, B.A. Mamyrin. TITLE // ZhTF - 1953 - v.23, p.2101.

3. B.A. Mamyrin. Copyright certificate No. 1980346 1966; Bulletin of inventions No. 13, 1967, p. 148.

4. E.V. Mammoth, B.C. Gurov, I.V. Filippov, R.N. Woodpeckers. RF patent No. 2293396 issued February 10, 2007.

5. L.E. Varakin. Communication systems with noise-like signals. M .: Radio and communications, 1985, p. 49-68.

Claims (2)

1. The method of time-of-flight mass analysis, which consists in dividing by time of flight in accordance with the masses of m ions, periodically with a period T for N cycles introduced into the drift space of the mass analyzer by pulsed packets of duration τ, with a current amplitude in ion packets I _m , characterized in that on each mass analysis cycle T, a series of n pulse ion packets is introduced into the analyzer drift space, where 2≤n≤T / 2τ + 1 distributed over the intervals (i-1) T <t <iT, where i = 1, 2, 3 ... N, according to the pseudo-random law, and the pseudo-random law is knocked out so that for zero side lobes of the periodic autocorrelation function of periodic series of ion packets, its main maximum is nI _m , and when detecting periodically with period T, the output series of ion current pulses of the time-of-flight mass analyzer are processed in accordance with the pseudo-random law. 2. A device for time-of-flight mass analysis, comprising a control signal generator, a pulsed ion source, an analyzer with a drift space for mass separation of ions by flight time and an ion detector, characterized in that the pseudo-random sequence generator is connected to the control signal generator, the input of which is connected with the output of the clock generator, and the output with the input of the pulsed ion source, and the filter is matched to the pseudorandom sequences in the ion detector, whose input th output is connected to a broadband amplifier, and an output - to an input of the summing accumulator mass spectra.

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