Laser photodissociation probe for ion tomography studies in a quadrupole ion-trap mass spectrometer

A focused laser is used to make infrared multiphoton photodissociation (IRMPD) more efficient in a quadrupole ion trap mass spectrometer. Efficient (up to 100%) dissociation at the standard operating pressure of 1 ϫ 10 Ϫ3 Torr can be achieved without any supplemental ion activation and with shorter irradiation times. The axial amplitudes of trapped ion clouds are measured using laser tomography. Laser flux on the ion cloud is increased six times by focusing the laser so that the beam waist approximates the ion cloud size. Unmodified peptide ions from 200 Da to 3 kDa are completely dissociated in 2.5-10 ms at a bath gas pressure of 3.3 ϫ 10 Ϫ4 Torr and in 3-25 ms at 1.0 ϫ 10 Ϫ3 Torr. Sequential dissociation of product ions is increased by focusing the laser and by operating at an increased bath gas pressure to minimize the size of the ion cloud. nfrared multiphoton photodissociation (IRMPD) has been used for tandem mass spectrometry in a quadrupole ion trap mass spectrometer [1][2][3][4][5][6][7] and a Fourier transform ion cyclotron resonance mass spectrometer [8 -11] as an alternative to collision-induced dissociation (CID). Only one m/z ion is selected for excitation in conventional CID, limiting sequential dissociation of product ions into smaller mass product ions. Conversely, in IRMPD all parent and product ions are irradiated simultaneously with a collimated, IR laser providing the capability to generate abundant small mass product ions. IRMPD is typically performed in a quadrupole ion trap mass spectrometer at a smaller low mass cut-off (LMCO) than CID, allowing the observation of a broader m/z range of product ions [2,4], and a higher MS/MS efficiency,where E MS/MS is MS/MS efficiency, F i is the abundance of each product ion, and P 0 is the initial abundance of the parent ion before irradiation. IRMPD is performed in a Fourier transform ion cyclotron resonance mass spectrometer to eliminate the need for a CID target gas to be leaked into and pumped out of the ICR cell [10,11]. The sensitivity of a quadrupole ion trap mass spectrometer is optimized with a bath gas pressure around 1.0 ϫ 10 Ϫ3 Torr. Bath gas collisions reduce the kinetic energy of trapped ions and minimize the size of the ion cloud [12]. However, collisions also remove internal energy gained from IR absorption, necessitating longer periods of irradiation for dissociation and decreasing fragmentation efficiency [13],where E F is fragmentation efficiency, F i is the abundance of each product ion, and P is the abundance of the parent ion remaining after irradiation. IRMPD in a quadrupole ion trap mass spectrometer often is performed at 1 ϫ 10 Ϫ5 to 3 ϫ 10 Ϫ4 Torr to decrease collisional cooling of parent ion internal energy [2-7], [13] and achieve dissociation with shorter irradiation times. However, this reduced bath gas pressure is less effective for trapping ions during injection and reduces sensitivity.Several methods have been developed to increase IRMPD fragmentation efficiency by combining IRMPD with another form of ion activation an...

Section: Laser Power and Overlap With The Ion Cloudsupporting

confidence: 62%

Section: Laser Power and Overlap With The Ion Cloudmentioning

confidence: 99%

Improving IRMPD in a quadrupole ion trap

Newsome

Glish

2009

“…The ion traps are assumed to be operated at the same trapping field frequency and operated such that trapped ions have identical characteristic frequencies of motion in each dimension of the quadrupole field. Using an estimated ion cloud radius of 1.0 mm from ion tomography [29] and an effective cloud length of 30mm, the 2-D trap used in our work should have a 18x higher space charge limit.…”

Section: Space Charge Limitmentioning

confidence: 99%

“…Unlike prior treatments, the ion clouds may be heterogeneous in m/z, and the pseudo-potential approximation is not invoked, as it is rarely applicable at the operating points used for mass analysis. These modifications require that the radii of the trapped ion clouds be estimated from experimental results [29].…”

Section: Space Charge Limitmentioning

confidence: 99%

A two-dimensional quadrupole ion trap mass spectrometer

Schwartz

Senko

Syka

2002

677

718

The use of a linear or two-dimensional (2-D) quadrupole ion trap as a high performance mass spectrometer is demonstrated. Mass analysis is performed by ejecting ions out a slot in one of the rods using the mass selective instability mode of operation. Resonance ejection and excitation are utilized to enhance mass analysis and to allow isolation and activation of ions for MS n capability. Improved trapping efficiency and increased ion capacity are observed relative to a three-dimensional (3-D) ion trap with similar mass range. Mass resolution comparable to 3-D traps is readily achieved, including high resolution at slower scan rates, although adequate mechanical tolerance of the trap structure is a requirement. [8 -11], and standard three-dimensional (3-D) ion trap mass spectrometers [12,13]. Several of the quadrupole based 2-D ion traps are capable of mass selective isolation and activation of ions including MS 3 analysis [7,9 -11]. Syka and Fies have described the theoretical advantages of 2-D versus 3-D quadrupole ion traps for Fourier transform mass spectrometry [14]. These advantages include reduced space charge effects due to the increased ion storage volume, and enhanced sensitivity for externally injected ions due to higher trapping efficiencies. Bier and Syka described several forms of linear and circular 2-D ion traps with larger ion capacity to be used as mass spectrometers [15] using the mass selective instability mode of operation [16] similar to that used in all commercial 3-D quadrupole ion trap instruments.Despite the previously described advantages and the recent progress with 2-D ion traps in hybrid instruments, only a few examples have appeared using these traps as stand-alone mass spectrometers. Senko et al. recently demonstrated image current detection with FT analysis in a 2-D ion trap which utilized independent detection electrodes between the quadrupole rods [17]. Although promising results were presented, space charge effects were found to limit this configuration's ultimate performance as the motion of the ions is largely restricted to one of the symmetry planes of the device. The relative insensitivity of the image current ion detection scheme precluded the use of lower numbers of trapped ions to avoid such space charge effects.Welling et al. demonstrated two variations of mass selective instability in a stand-alone 2-D quadrupole ion trap [18]. The first method, referred to as "q-scanning", exploited the field penetration of the detector between the quadrupole rods to allow for ion extraction using a downward ramp of the RF voltage. This allowed relatively quick scanning of a broad mass range (1 second scan from 20 -1000 m/z), but produced mass spectral resolution of only five to six. The second method, referred to as "secular scanning", used frequency swept parametric excitation to eject ions between the quadrupole rods. Although the scan rate was 100x slower than the q-scan, a resolution of 800 at m/z 130 was obtained.Lammert et al. have presented preliminary results using a toroida...

“…The response distribution of the targeted ions can be experimentally characterized using methods previously described [61,62]. In these methods, a dipolar excitation AC is applied and the response of the ion motion amplitude is measured indirectly while the rf voltage is set to a value corresponding to a secular frequency of the targeted ions.…”

Section: Ion Isolationmentioning

confidence: 99%

Ion trap mass analysis at high pressure: A theoretical view

Song

Smith

et al. 2009

The mass-selective manipulation of ions at elevated pressure, including mass analysis, ion isolation, or excitation, is of great interest for the development of mass spectrometry instrumentation, particularly for systems in which ion traps are employed as mass analyzers or storage devices. While experimental exploration of high-pressure mass analysis is limited by various difficulties, such as ion detection or electrical discharge at high-pressure, theoretical methods have been developed in this work to study ion/neutral collision effects within quadrupole ion traps and to explore their performance at pressures up to 1 Torr. Ion trapping, isolation, excitation, and resonance ejection were investigated over a wide pressure range. The theoretically calculated data were compared with available experimental data for pressures up to 50 mTorr, allowing the prediction of ion trap performance at pressures more than 10 times higher. (J Am Soc Mass