Design and performance of a plasma-source mass spectrograph

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An ambient desorption/ionization (ADI) source, known as the flowing atmospheric pressure afterglow (FAPA), has been coupled to a Mattauch-Herzog mass spectrograph (MHMS) equipped with a focal plane camera (FPC) array detector. The FAPA ionization source enables direct mass spectral analysis of solids, liquids, and gases through either positive or negative ionization modes. In either case, spectra are generally simple with dominant peaks being the molecular ions or protonated molecular ions. Use of the FAPA source with the MHMS allows the FPC detector to be characterized for the determination of molecular species, whereas previously only atomic mass spectrometry (MS) has been demonstrated. Furthermore, the FPC is shown to be sensitive to negative ions without the need to change any detector parameters. The FAPA ionization source consists of a glow discharge in helium using a pin-to-plate configuration. A hole at the center of the plate electrode enables the flow of helium ions and metastables (He*) from the discharge cell into the open atmosphere. These energetic species ionize atmospheric constituents, including water, oxygen, and nitrogen, which in turn can ionize, through chemical ionization [1], analytes from samples placed in the afterglow region. As with other ambient desorption/ionization (ADI) sources, solids, liquids, or gases can be analyzed with the FAPA source, and spectra are generally very simple [1][2][3]. Additionally, positive and negative ions can be generated with the ionization source.The FPC utilizes a linear arrangement of hundreds of Faraday strips. Traditionally, Faraday-based detectors have been inferior in sensitivity to ion-multiplying devices because of their lack of direct amplification. However, the use of Faraday detectors continued because of the superior precision they offer. With the FPC, high precision is retained while sensitivity is boosted by incorporating electronic amplification within each detection channel. Furthermore, because the Faraday array detector is based exclusively on charged particle collection, the mass-bias effects observed with energybased detectors is not a problem.Previously, the FPC has been used exclusively for atomic mass spectrometry [4 -10]. However, three notable FPC traits make it attractive for molecular analyses. First, as just stated, mass bias should not be problematic with the Faraday-based FPC. Second, since the FPC merely integrates charge, enhanced sensitivity should be observed for analytes that carry multiple charges. This advantage would be most significant through the use of an ESI source, which often generates multiply charged species. A final trait of the FPC is its ability Address reprint requests to Dr.

Section: Mhmsmentioning

confidence: 99%

Detection of positive and negative ions from a flowing atmospheric pressure afterglow using a mattauch-herzog mass spectrograph equipped with a faraday-strip array detector

Schilling

Shelley

Barnes³

et al. 2010

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“…The spatial focusing on the focal plane is proportional to the width of the opening slit (S) located at the analyzer entrance and to the ratio R m /R e (magnetic and electric radius described by the ion). Several types of detectors have been used with the MH-MS, such as: photographic plate [6], ion collectors coupled to electrometers [7], electro-optical imaging detector (EOID) [8,9], Spiraltron matrix coupled to a resistive anode [10], collector cups for linear mixture separation and material deposition (soft-landing) using a nonhomogeneous magnetic field [11,12], delta-doped CCD, [13] faraday-strip array detector [14][15][16][17][18][19][20][21][22][23], CMOS based pixilated array detector [24], and of course the modified CCD array detector (IonCCD) in early work [1]. Using an ESI source, we present results for the IonCCD coupled with a miniature MH-MS showing simultaneous biological mixture separation for potential micro-array ion deposition applications [25].…”

Section: Introductionmentioning

confidence: 99%

IonCCD™ for Direct Position-Sensitive Charged-Particle Detection: from Electrons and keV Ions to Hyperthermal Biomolecular Ions

Hadjar

Johnson

Laskin

et al. 2011

A novel, low-cost, pixel-based detector array (described elsewhere Sinha and Wadsworth (76 (2), 1) is examined using different charged particles, from electrons to hyperthermal (G100 eV) large biomolecular positive and negative ions, including keV small atomic and molecular ions. With this in mind, it is used in instrumentation design (beam profiling), mass spectrometry, and electron spectroscopy. The array detector is a modified light-sensitive charge-coupled device (CCD) that was engineered for direct charged-particle detection by replacing the semiconductor part of the CCD pixel with a conductor Sinha and Wadsworth (76(2), 1). The device is referred to as the IonCCD. For the first time, we show the direct detection of 250-eV electrons, providing linearity response of the IonCCD to the electron beam current. We demonstrate that the IonCCD detection efficiency is virtually independent from the particle energy (250 eV, 1250 eV), impact angle (45 o , 90 o ) and flux. By combining the IonCCD with a double-focusing sector field mass spectrometer (MS) of Mattauch-Herzog geometry (MH-MS), we demonstrate fast data acquisition. Detection of hyperthermal biomolecular ions produced using an electrospray ionization source (ESI) is also presented. In addition, the IonCCD was used as a beam profiler to characterize the beam shape and intensity of 15 eV protonated and deprotonated biomolecular ions at the exit of an rf-only collisional quadrupole. This demonstrates an ionbeam profiling application for instrument design. Finally, we present simultaneous detection of 140 eV doubly protonated biomolecular ions when the IonCCD is combined with the MH-MS. This demonstrates the possibility of simultaneous separation and micro-array deposition of biological material using a miniature MH-MS.

“…Although our MCP-phosphor assembly has proven adequate for demonstrating DOFMS principles, substantial enhancements are expected by employing a flat detector with better sensitivity and spatial resolution. Recently, discrete charge-detector arrays were developed and coupled to sector-field focal plane mass spectrometers [18][19][20]. These detection systems consist of a linear array of charge-collecting micro-scale Faraday strips with operational amplifiers to integrate and amplify charge resulting from ion strikes, with all components fabricated on a single silicon monolith [21].…”

Section: Discussionmentioning

confidence: 99%

First Distance-of-Flight Instrument: Opening a New Paradigm in Mass Spectrometry

Graham

Ray

Enke

et al. 2011

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A new instrumental concept, distance-of-flight mass spectrometry (DOFMS), is demonstrated experimentally. In DOFMS the mass-to-charge ratio of ions is determined by the distance each ion travels during a fixed time period; the mass spectrum is then recorded with a positionsensitive detector. The DOF approach provides a new way to separate and quantify components of complex samples. Initial results are demonstrated with a glow discharge ion source and a microchannel plate-phosphor screen detector assembly for atomic ion determination. This detection system demonstrated mass spectral peak widths of approximately 0.65 mm, corresponding to resolving powers of approximately 400-600 for a number of elemental samples.Key words: Mass spectrometry, Instrumentation, Glow discharge S ince the development of the first mass spectrograph over 90 years ago [1], numerous methods and instruments have been developed to separate and determine ions of varying mass-to-charge ratio (m/z). Currently, mass spectrometry (MS) can be accomplished by electrostatic and magnetic dispersion (sector-field MS), radio-frequency stability and filtering (quadrupoles and ion traps), resonance frequency determination (Fourier-transform ion cyclotron resonance and Orbitrap® MS), or velocity-based separation (time-of-flight MS). These mature MS technologies are routinely employed in a range of applications from biomolecule analysis to elemental isotope determination, and are often coupled with each other to achieve tandem MS analysis [2,3]. In the present paper, we describe the first implementation of a new form of MS, termed distance-offlight mass spectrometry (DOFMS), and suggest potential benefits of this new mass separation technique.Distance-of-flight mass spectrometry is akin to time-offlight mass spectrometry (TOFMS) in that both techniques separate ions of different m/z based upon an imparted m/zdependent velocity. In TOFMS, each ion is given the same energy (thus achieving a m/z-dependent velocity), and the m/z of each ion is calculated from the time required for it to traverse a known distance to a single detector. Conversely, in DOFMS the m/z of an ion is measured based on the spatial location of each ion at a specific time after the initial acceleration. As a useful analogy, DOFMS is to TOFMS as thin-layer chromatography is to elution chromatography (e.g., LC). TOFMS measures ions as they come off the "column", whereas DOFMS measures how far the ions travel after a specific separation time. Whereas TOFMS disperses ions in time, DOFMS disperses ions in space.The principle behind DOFMS is illustrated in Figure 1. To implement DOFMS, the primary ion beam is introduced into an orthogonal extraction region where ions are subjected to a brief electrostatic field, imparting identical momentum to each ion. This constant-momentum acceleration (CMA) is accomplished by limiting the temporal width and electrostatic field