Characterization of a Hadamard transform time-of-flight mass spectrometer

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Hadamard transform time-of-flight mass spectrometry (HT-TOFMS) is based on the pseudorandom gating of ion packets into a time-of-flight mass-to-charge analyzer. In its typical implementation, the technique is able to monitor continuous ion sources with a 50% duty cycle, independent of all other figures of merit. Recently, we have demonstrated that the duty cycle can be extended to 100% using patterned, two-channel detection. Two-channel HT-TOFMS involves the simultaneous optimization of paired one-channel experiments and imposes more stringent conditions to achieve high-quality spectra. An ion modulation device, known as Bradbury-Nielson Gate (BNG), is central to HT-TOFMS. It is an ideal deflection plate, capable of transmitting or deflecting an ion beam according to a known binary sequence without changing the times-of-flight of the ions. Analytical equations are derived that accurately describe the ion modulation process of the BNG as confirmed by good agreement with SimIon simulations and ion beam imaging experiments. From these expressions, the duty cycle and ion modulation efficiency were calculated for various BNG parameters, ion beam characteristics, and detector dimensions, which permit the optimum conditions to be chosen for the two-channel experiment. We conclude that the outer detector should be three times the maximum deflection angle to detect all deflected ions (100% duty cycle) and that the difference between the modulated ion counts in the sequence elements 0 and 1 should be maximized to achieve high modulation efficiency. This condition is best achieved by tight focusing of the ion beam in the center of the inner detector. When both channels are optimized, the two-channel advantage can be exploited to achieve a further improvement over a single-channel experiment. (J Am Soc Mass Spectrom 2005, 16, 1888 -1901

Section: Understanding and Optimizing 2c-ht-tofmsmentioning

confidence: 70%

Section: Understanding and Optimizing 2c-ht-tofmsmentioning

confidence: 99%

Duty cycle and modulation efficiency of two-channel hadamard transform time-of-flight mass spectrometry

Yoon

Zuleta

Kimmel

et al. 2005

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“…Some other methods include weaving against the threads on nylon bolts [22], etching a thin metal foil [23], and wire-bonding to metal deposited on ceramic frame [24]. Brock et al [14,25] reported a BNG wire spacing of 0.16 mm in 1998 achieved by manually weaving wires onto an etched silicon frame under a microscope. This work represents a significant improvement in reducing the wire spacing, but the procedure is extremely laborious, requiring several days to complete the assembly [26].…”

mentioning

confidence: 99%

Simple template-based method to produce bradbury-nielsen gates

Yoon

Zuleta

Robbins

et al. 2007

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A Bradbury-Nielsen gate (BNG) consists of two interleaved and electrically isolated sets of wires and can transmit or deflect charged particles by applying a varying voltage difference across the two wire sets. We present a simple template-based method to fabricate BNGs with wire spacings as small as 50 m with minimal use of a microscope. The small wire spacing allows modulation rates at tens of megahertz. Using this method, we have fabricated four BNGs with wire spacings of 500, 200, 100, and 50 m using 10 m gold-coated tungsten wires. The performance of the four BNGs is characterized using an imaging detector and compared with theoretical predictions. (J Am Soc Mass

“…Use of these matrices in the deconvolution step led to marked improvements in spectral resolution but require full knowledge of the encoding defects. O ver the past few years, our group has published a series of articles describing Hadamard transform time-of-flight mass spectrometry (HT-TOFMS) [1][2][3][4][5]. Other groups have described the application of Hadamard multiplexing to absorption spectroscopy [6], Raman spectrometry [7], photoacoustic depth profiling [8], and capillary electrophoresis [9 -11].…”

mentioning

confidence: 99%

“…Detailed descriptions of how encoding sequences are applied in HT-TOFMS have been presented previously [2], and only a brief review is given here. Ions are accelerated toward a BNG.…”

mentioning

confidence: 99%

Effects of modulation defects on hadamard transform time-of-flight mass spectrometry (HT-TOFMS)

Kimmel

Fernández

Zare

2003

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In any Hadamard multiplexing technique, discrepancies between the intended and the applied encoding sequences may reduce the intensity of real spectral features and create discrete, artificial signals. In our implementation of Hadamard transform time-of-flight mass spectrometry (HT-TOFMS), the encoding sequence is applied to the ion beam by means of an interleaved comb of wires (Bradbury-Nielson gate), which shutters the ion beam on and off. By isolating and exaggerating individual skewing effects in simulating the HT-TOFMS process, we determined the nature of errors that arise from various defects. In particular, we find that the most damaging defects are: mismatched voltages between the wire sets and the acceleration voltage of the instrument, which cause positive and negative peaks throughout mass spectra; insufficient deflection voltage, which reduces the intensity of real peaks and causes negative peaks that are spread across the entire mass range; and voltage errors as the wire sets return from their deflection voltage to their transmission value, which yield significant reductions in peak intensities, create artificial peaks throughout mass spectra, and broaden real peaks by causing positive peaks to grow in the bins adjacent to them. Because the magnitude of the modulation defects grows as the applied modulation voltage is increased, BradburyNielson gates with finer wire spacing, and hence stronger effective fields for a given applied voltage, were produced and installed. Operating at 10 to 15 V where errors in the electronics are essentially absent, the most finely spaced gate (100 m) yielded signal-to-noise ratios that were more than two times higher than those achieved with more widely spaced gates. As an alternative method for minimizing skewing effects, HT-TOFMS data were post processed using an exact knowledge of the modulation defects. Nonbinary matrices that mimic the actual encoding process were built by measuring voltage versus time traces and then translating these traces to transmission versus time. Use of these matrices in the deconvolution step led to marked improvements in spectral resolution but require full knowledge of the encoding defects. ( , photoacoustic depth profiling [8], and capillary electrophoresis [9 -11]. In each case, the motivation has been the inherent improvement in signal-to-noise ratio (SNR) offered by multiplexing the signal. In addition to its high SNR values, HT-TOFMS coupled to continuous ion sources offers a 50% duty cycle over any mass range with the potential of reaching 100% [3]. As a result of its high SNR and duty cycle, the spectral acquisition rates of HT-TOFMS are superior to conventional instruments.Spectral errors resulting from modulation defects have been described for Hadamard instruments that encode optical signals using machined masks [12] and stationary liquid crystals [7,8]. These authors have also developed mathematical correction schemes to compensate for the effects of systematic errors. Hanley [13] recently published a review describing the so...