Ultramark 1621 as a reference compound for positive and negative ion fast-atom bombardment high-resolution mass spectrometry

Handheld Mass Spectrometer with Intelligent Adaptability for On-Site and Point-of-Care Analysis

The development of miniature mass spectrometry (MS) systems with simple analysis procedures is important for the transition of applying MS analysis outside traditional analytical laboratories. Here, we present Mini 14, a handheld MS instrument with disposable sample cartridges designed based on the ambient ionization concept for intrasurgical tissue analysis and surface analysis. The instrumentation architecture consists of a single-stage vacuum chamber with a discontinuous atmospheric interface and a linear ion trap. A major effort in this study for technical advancement is on making handheld MS systems capable of automatically adapting to complex conditions for in-field analysis. Machine learning is used to establish the model for autocorrecting the mass offsets in the mass scale due to temperature variations and a new strategy is developed to extend the dynamic concentration range for analysis. Mini 14 weighs 12 kg and can operate on battery power for more than 3 h. The mass range exceeds m/z 2000, and the full peak width at half-maximum is Δm/z 0.4 at a scanning speed of 700 Th/s. The direct analysis of human brain tissue for identifying glioma associated with isocitrate dehydrogenase mutations has been achieved and a limit of detection of 5 ng/mL has been obtained for analyzing illicit drugs in blood.

Matrix ions as internal standard for high mass accuracy matrix-assisted laser desorption/ionization mass spectrometry imaging

RATIONALE

High mass accuracy is indispensable for reliable identification in matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) imaging. Ubiquitous matrix ions can serve as reference masses for mass calibration if their sum formula is known. Here we report an overview of ions generated on tissue by 11 common MALDI matrices for use in internal or external mass calibration.

METHODS

Matrices covered in this study were applied onto coronal mouse brain sections using a pneumatic sprayer setup. MALDI imaging was performed on a Q Exactive HF orbital trapping mass spectrometer coupled to an AP-SMALDI 10 source. Measurements were conducted with high mass resolution (240 k full width at half maximum at m/z 200) and high mass accuracy with a root mean square mass error of better than 1.5 ppm achieved via internal mass calibration using matrix ions.

RESULTS

MALDI MS imaging was used to investigate ions generated on tissue by 11 common MALDI matrices. An example of using matrix ions for internal mass calibration in MALDI imaging of drug substances and lipids in murine lung sections is presented. Tables containing the cluster composition, sum formulae, and the measured and theoretical m/z ratios of the identified ions were compiled for each matrix.

CONCLUSION

Using matrix ions as reference masses for internal and external mass calibration in MALDI MS imaging is an effective and elegant way to achieve sub-ppm mass accuracy as it makes use of ubiquitous signals present in every MALDI MS spectrum without the need for an additional calibration standard.

Single Analyzer Neutral Loss Scans in a Linear Quadrupole Ion Trap Using Orthogonal Double Resonance Excitation

In this follow-up paper to our previous work on single analyzer precursor ion scans in a linear quadrupole ion trap (Snyder, D. T.; Cooks, R. G. Single analyzer precursor ion scans in a linear quadrupole ion trap using orthogonal double resonance excitation. J. Am. Soc. Mass Spectrom. 2017, DOI: 10.1007/s13361-017-1707-y), we now report the development of single analyzer neutral loss scans in a linear quadrupole ion trap using orthogonal double resonance excitation. Methodologically, there are three key differences between single analyzer precursor ion scans and neutral loss scans under constant radiofrequency (rf) conditions: (1) in the latter experiment, both excitation and ejection frequencies must be scanned, whereas in the former the ejection frequency is fixed, (2) the need to maintain a constant neutral loss while incrementing both precursor and product ion masses, complicated by the complex relationship between secular frequency and mass, requires use of two simultaneous frequency scans, both linear in mass, and (3) because the ejection frequency is scanned, a third ac signal occurring between the ac excitation and ac ejection frequency scans must also be applied and scanned in order to reject artifact peaks caused by ejection of unfragmented precursor ions. Using this methodology, we demonstrate neutral loss scans on a commercial linear ion trap using mixtures of illicit drugs and acylcarnitines. We also demonstrate neutral loss scanning on a Populus deltoides leaf and on a lignin sample, both significantly more complex mixtures.

Ion isolation and multigenerational collision-induced dissociation using the inverse Mathieu q scan

RATIONALE

In a bid to develop a mass spectrometer using ac frequency scanning for ion isolation, ion activation, and ion ejection, we have developed scan functions for each process using the inverse Mathieu q scan.

METHODS

Ion isolation is accomplished by frequency hopping, that is, by skipping past the ranges of frequencies corresponding to the ions to be isolated during the frequency sweep. Multigenerational collision-induced dissociation is demonstrated by scanning the frequency of excitation from low to high so that multiple generations of product ions can be observed in the product ion mass spectra. Because the excitation frequency is scanned quickly across a large range, fragmentation of some precursor ions can be too limited. However, by first fixing the excitation frequency on the precursor ion and then scanning the frequency using the inverse Mathieu q scan, a higher abundance of product ions can be obtained.

RESULTS

Isolation of a single mass-to-charge (m/z) as well as nonadjacent m/z ions is demonstrated with isolation efficiency greater than 70%. Fragmentation of caffeine and noroxycodone is demonstrated, the latter of which shows multiple generations of product ions.

CONCLUSIONS

The results demonstrated here provide strong evidence that an ion trap mass spectrometer can be operated without using an rf amplitude ramp for any operation, and instead ac frequency scanning can be used for all mass-selective operations. Copyright © 2016 John Wiley & Sons, Ltd.

Linear mass scans in quadrupole ion traps using the inverse Mathieu q scan

RATIONALE

Secular frequency scanning is a method of mass selectively scanning ions out of a quadrupole ion trap by linearly ramping the frequency of the resonance ejection signal through ion secular frequencies at constant rf amplitude and frequency. The method is electronically much simpler than resonance ejection but it requires a complex nonlinear calibration procedure to correlate mass-to-charge with time.

METHODS

A method of secular frequency scanning in quadrupole ion traps is described in which mass-to-charge is linear with time. This method, termed an "inverse Mathieu q scan", contrasts with linear frequency sweeping which requires a complex nonlinear mass calibration procedure. In the current method, mass scans are forced to be linear with time by scanning the frequency of the supplementary ac so that there is an inverse relationship between the ejected ion's Mathieu q parameter and time.

RESULTS

In all cases, excellent mass spectral linearity is observed. The rf amplitude is shown to control both the scan range and the scan rate, whereas the ac amplitude and scan rate influence the mass resolution. The scan rate depends linearly on the rf amplitude, a unique feature of this scan. Although changes in either rf or ac amplitude affect the positions of peaks in time, they do not change the mass calibration procedure since this only requires a simple linear fit of m/z vs time. Space charge effects are shown to give rise to significant changes in resolution as well as to mass shifts.

CONCLUSIONS

A method of secular frequency scanning which provides a linear mass scale has been demonstrated. The inverse Mathieu q scan offers a significant increase in mass range and power savings while maintaining access to linearity, paving the way for a mass spectrometer based completely on ac waveforms for ion isolation, ion activation, and ion ejection. Copyright © 2016 John Wiley & Sons, Ltd.

Alternative mass reference standards for direct analysis in real time mass spectrometry

RATIONALE

Mass spectra were acquired with the Direct Analysis in Real Time (DART®) ion source for an amine-terminated polyether used as positive-ion mass reference standards and for several fluorinated materials commonly used as negative-ion reference standards for mass spectrometry.

METHODS

A commercial time-of-flight mass spectrometer equipped with a DART ion source was used for all measurements. Mass reference standards deposited onto the sealed end of a glass melting point tube were suspended in the DART gas stream for analysis.

RESULTS

A polyetheramine (Jeffamine® M-600) produced intense peaks corresponding to protonated molecules. Perfluorotributylamine (PFTBA), and perfluorotripentylamine, gave useful reference spectra for different m/z ranges. DART mass spectra of Ultramark 1621® resembled those previously reported for Fast Atom Bombardment (FAB) and Electrospray Ionization (ESI). Fomblin®Y, a fluorinated ether, was the most useful negative-ion reference standard of the materials tested. The material is commercially available, inexpensive, and provides reference peaks covering the m/z range 85 to >3000.

CONCLUSIONS

Jeffamine-M600 was found to be a convenient alternative to polyethers such as polyethylene glycol (PEG) for DART positive-ion mass calibration. Fomblin Y was suitable for use as a negative-ion reference standard. Copyright © 2016 John Wiley & Sons, Ltd.

Effective methods for the measurement of CsI cluster ions using MALDI-MS with suitable solvent combinations and additives

A method to measure CsI cluster ions ((CsI)(n)Cs(+), (CsI)(n)I(-)) from CsI samples in matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) was developed with a 2-[(2E)-3-(4-tert-butylphenyl)-2-methylprop-2-enylidene] malononitrile (DCTB) matrix and additives. Solvent combinations in which the CsI and DCTB solutions were miscible were effective in detecting CsI cluster ions at a mass range of over m/z 2000 and are associated with a characteristic spread of DCTB within the CsI/DCTB mixture. The addition of saccharides or sugar alcohols to the CsI/DCTB mixture improved the DCTB distribution and widened the mass distribution of CsI cluster ions up to m/z 10,000 in the linear mode.

Wide-range mass calibration for negative-ion DART mass spectrometry

A method for accurate mass calibration covering a wide m/z range in negative-ion direct analysis in real time mass spectrometry (DART-MS) is described. It is based on a commercially available tuning solution (Agilent ES tune mix) normally applied in electrospray ionization. This acetonitrile solution is composed of ammonium trifluoroacetate, betaine, 2,4,6-tris(heptafluoropropyl)-1,3,5-triazine, and various symmetrical hexakis-(fluoroalkoxy)-phosphazenes. Upon addition of 2% (v/vl trifluoroacetic acid in negative-ion DART-MS, this mixture yields negative ions covering a range of m/z 69 to m/z 2834. Validity of the calibration procedure is demonstrated on a Fourier transform ion cyclotron resonance mass spectrometer by application to polyethylene glycol diacid and [60]fullerene. Experimental conditions and a mass reference list are provided.

Dual polarity accurate mass calibration for electrospray ionization and matrix-assisted laser desorption/ionization mass spectrometry using maltooligosaccharides

In view of the fact that memory effects associated with instrument calibration hinder the use of many mass-to-charge (m/z) ratios and tuning standards, identification of robust, comprehensive, inexpensive, and memory-free calibration standards is of particular interest to the mass spectrometry community. Glucose and its isomers are known to have a residue mass of 162.05282Da; therefore, both linear and branched forms of polyhexose oligosaccharides possess well-defined masses, making them ideal candidates for mass calibration. Using a wide range of maltooligosaccharides (MOSs) derived from commercially available beers, ions with m/z ratios from approximately 500 to 2500Da or more have been observed using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and time-of-flight mass spectrometry (TOF-MS). The MOS mixtures were further characterized using infrared multiphoton dissociation (IRMPD) and nano-liquid chromatography/mass spectrometry (nano-LC/MS). In addition to providing well-defined series of positive and negative calibrant ions using either electrospray ionization (ESI) or matrix-assisted laser desorption/ionization (MALDI), the MOSs are not encumbered by memory effects and, thus, are well-suited mass calibration and instrument tuning standards for carbohydrate analysis.

Novel calibrants for accurate mass measurement in ESI-MS

We discovered novel calibrants, several 2,2,6,6-tetraalkylpiperidinyl derivatives, for accurate mass measurements by electrospray ionization mass spectrometry (ESI). These compounds showed excellent performance as internal calibrants for the accurate mass measurement in ESI-MS, without problems arising from differences in the ionization efficiencies between the analytes and calibrants.