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What are the benefits of ICP-oTOF (orthogonal time of flight) MS?

For more information about the Optimass 9500 ICP-oTOF MS, Click Here

Introduction

While ICP-MS has rapidly attained acceptance as the choice in trace metal analysis, most commercially available instruments are equipped with quadrupole based mass analyzers. Quadrupole mass spectrometers do provide rapid analysis compared to alternative single and multi-elemental techniques (e.g., graphite furnace atomic absorption and ICP-OES); however, they do suffer limitations in terms of speed of spectral acquisition, precision at high speed, precision of isotopic ratios and multi-element transient signal capability.

Interest in using a time-of-flight mass spectrometer to further advance the ICP-MS technique and expand laboratory capabilities has grown in recent years. The true and greatest advantage of a time-of-flight mass spectrometer is simultaneous sampling mass analysis. The simultaneity allows for the most precise internal standard and isotope ratio correction, transient signal multi-element analysis, elemental fingerprinting and advance retrospective analysis techniques. This page will illustrate the benefits of the GBC Scientific Equipment Optimass 9500 ICP- oTOFMS. Examples of design, routine data from commercial laboratories, isotopic ratio results and transient signal application will be presented.

How does Time of Flight (TOF) work?

The time-of-flight mass analyzer is a pseudo-simultaneous detection type device, which can accomplish a full mass spectrum (2-260 amu) in approximately 30µs. In this technique, ions are subjected to a pulsed electric field which, ideally, imparts the same kinetic energy (KE) on all ions in the packet. These ions are then directed to a field free region (i.e., drift tube) where the differences in velocities spatially separate ions of differing m/z. The KE attained by each ion will be product of the ion's charge (q), the electric field (E), and the distance (sa) under which the ion is subject to the electric field. This expression can be equated to the expression for KE based on ion mass (m), and ion velocity (v) to yield:

Rearranging Equation 1, the time for an ion to traverse the drift tube, td, can be determined by Equation 2:

where D is the distance traversed before striking the detector.

Two spatial focus points exist in the GBC Optimass 9500 ICP-oTOFMS. The first spatial focus occurs at the SMARTGATE ion blanker. At this point, any UNWANTED ions can be rejected from the flight path by deflection plates in the SMARTGATE, while ions of interest are passed on for detection at the 2nd spatial focus (e.g., detector). The reflectron increases resolution by simply extending the path length. Resolution at mid mass range is approximately 1500.

Orthogonal acceleration scheme utilizes an ion beam of much lower energy spread; the higher order spatial focusing will also provide better peak shapes (resolution) and greater abundance sensitivity.

Orthogonal Acceleration + SMARTGATE + REFLECTRON

Resolution and ion blanking result in:

Effective Elimination of 10 ppm Na23 from the Mg24 peak

TOF Applications - Simultaneous Sampling Mass Analyses

Soil sample mass spectrum showing Pb isotopes. Users can determine quickly if interferences are present at any mass:

  1. Sample peaks can be compared to the natural abundant sensitivity (red lines) real time. When the isotopic peaks do not compare with natural abundances, an interference may be present.
  2. The OptiMass is simultaneous; all isotopic values can be generated and stored for the entire spectrum. A suspect result from the user selected isotope can be compared to a result from another isotope for the same element.

The simultaneous nature of the instrument eliminates noise associated with instrumental drift and plasma flicker noise. As a result, very precise isotopic ratio data can be obtained.

The fingerprint capability can include the possibility of using an internal standard; therefore, one can take into account any daily drift in sensitivity. Furthermore, the fingerprint feature can be selectively set for specific elements of interest in the sample. In this way a known contaminant to the sample can be avoided and comparison can be made.

Due to the TOF's ability to easily collect the full mass spectrum, a statistical algorithm can be used in order to quickly analyze samples. For example, a forensic standard with known elemental content can be prepared, analyzed and the spectrum stored. Subsequent unknown samples can be analyzed as needed and compared to the stored spectrum. If the result is close to 1, the unknown is said to match the known. If the number is approaching 0, the unknown is said not to match. Quick go/no go analyses can aid technicians in making educated decisions on how to proceed with sample analysis.

The ability for retrospective analysis is easily available as complete spectral information is stored for each solution measured. This is a great asset for deconvolution of spectral interferences. Peak hopping is not necessary as a full peak shape is measured for each mass across the spectrum. By using Relative Sensitivity Factors (RSF) to define detector response to an unknown concentration, a retrospective semi-quantitative analysis can be performed. Therefore, all data from mass 1 through 260 AMU is available for future examination and quantification for masses not previously calibrated.

For more information about the Optimass 9500 ICP-oTOF MS, Click Here

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