Coded Apertures in Mass Spectrometry

Abstract

The use of coded apertures in mass spectrometry can break the trade-off between throughput and resolution that has historically plagued conventional instruments. Despite their very early stage of development, coded apertures have been shown to increase throughput by more than one order of magnitude, with no loss in resolution in a simple 90-degree magnetic sector. This enhanced throughput can increase the signal level with respect to the underlying noise, thereby significantly improving sensitivity to low concentrations of analyte. Simultaneous resolution can be maintained, preventing any decrease in selectivity. Both one- and two-dimensional (2D) codes have been demonstrated. A 2D code can provide increased measurement diversity and therefore improved numerical conditioning of the mass spectrum that is reconstructed from the coded signal. This review discusses the state of development, the applications where coding is expected to provide added value, and the various instrument modifications necessary to implement coded apertures in mass spectrometers.

Improving the Performance of a Cycloidal Coded-Aperture Miniature Mass Spectrometer

“…In a conventional sector mass spectrometer with a position-sensitive array detector, a narrow slit is used to define the position of ion input into the mass analyzer and make the measurement matrix an identity matrix. 11 This ensures that there is only one set of detector array elements (pixels) associated with each mass-to-charge (m/z), and a simple calibration of detector position (pixel number) to m/z is all that is needed to convert the detector signal to a mass spectrum. However, as the instrument size shrinks, to maintain resolution, the slit must also shrink leading to severe throughput limitations at resolutions required for most applications.…”

“…The measurement matrix then contains off-diagonal elements representative of multiplexing and there is no longer a single set of detector elements for each m/z. After determination of the measurement matrix H from the physics and architecture of the system, a mass spectrum can be reconstructed from the coded spectrum for an unknown compound or compounds by solving the inverse problem S ̂= H −1 g, where S ̂represents an estimate of the real spectrum S. 11 Previous work demonstrated the ability of aperture coding to increase the throughput without sacrificing resolution in a 90°m agnetic sector mass analyzer, 21,23 a Mattauch−Herzog mass analyzer, 22 and a cycloidal mass analyzer, 20 all of which were coupled with a position sensitive array detector. For the 90°a nd Mattauch−Herzog instruments, a distorted image of the aperture is projected on the detector that varies for each m/z due to the physics and architecture of the mass analyzer.…”

“…Miniaturization of mass spectrometers is desirable for many applications including trace detection of explosives, pesticides, or narcotics; − point-of-care diagnostics; environmental monitoring; − and space exploration. , Barriers toward miniaturization persist in the form of size, weight, and power constraints; limited detector technologies; and vacuum requirements. Additionally, miniaturization of mass spectrometers must contend with a trade-off between throughput and resolution that limits performance relative to traditional, full-scale laboratory instruments . Most commonly used miniature mass spectrometers are based on ion trap, quadrupole, or time-of-flight mass analyzers. − Recently, there has been a renewed interest in miniature sector mass spectrometers due to the development of high-performance neodymium–iron–boron (Nd–Fe–B) permanent magnets that reduce the mass analyzer size and weight, and ion array detectors that offer simultaneous detection of ions over a wide mass range. − Furthermore, the use of spatially coded apertures can break the trade-off between throughput and resolution in a sector mass spectrometer and increase throughput without sacrificing resolution. ,− …”

“…Spatial aperture coding is a computational sensing technique that has been historically used in optical , and X-ray imaging and more recently in sector mass spectrometry. − In an analytical instrument like a sector mass spectrometer, the measurements, g , are related to the mass spectrum through the equation g = HS + n , where S is the input mass spectrum, n is noise, and H is the measurement matrix that incorporates the physics and architecture of the system and maps the input of the mass analyzer to the output of the mass analyzer at the detector. In a conventional sector mass spectrometer with a position-sensitive array detector, a narrow slit is used to define the position of ion input into the mass analyzer and make the measurement matrix an identity matrix . This ensures that there is only one set of detector array elements (pixels) associated with each mass-to-charge ( m / z ), and a simple calibration of detector position (pixel number) to m / z is all that is needed to convert the detector signal to a mass spectrum.…”

“…The measurement matrix then contains off-diagonal elements representative of multiplexing and there is no longer a single set of detector elements for each m / z . After determination of the measurement matrix H from the physics and architecture of the system, a mass spectrum can be reconstructed from the coded spectrum for an unknown compound or compounds by solving the inverse problem Ŝ = H –1 g , where Ŝ represents an estimate of the real spectrum S …”