Sensitivity in SIMS

Two approaches are often used to improve the detection limit, including selected ion monitoring (SIM) and multiple reaction monitoring (MRM). In LC/MS studies, it is often desirable to increase detection sensitivity by hmit-ing the mass analyzer scan to just one ion— that is, SIM. In this mode, a single ion of interest is monitored continuously by a mass spectrometer and no other ions are detected. This results in signihcant improvement of signal-to-noise ratio. SIM trades specihcity for sensitivity. In general, the sensitivity in SIM is increased by a factor of 100 to 1000 over full-scan mass spectra. This can be quite useful in detection and quantihcation of specihc compounds at low levels. 

Only scanning instruments like magnetic sector and (triple) quadrupole mass spectrometers show inproved sensitivity in SIM mode. Quadrupole ion traps (QIT and LIT) might be operated as to deliver a SIM output, the time to achieve ion selection is, however, essentially identical to their full scan cycle time, thus cancel-... 

Table 2.53 Dwell times per ion and relative sensitivity in SIM analysis (for beam instruments) at constant scan rates.

Ions are also used to initiate secondary ion mass spectrometry (SIMS) [ ], as described in section BI.25.3. In SIMS, the ions sputtered from the surface are measured with a mass spectrometer. SIMS provides an accurate measure of the surface composition with extremely good sensitivity. SIMS can be collected in the static mode in which the surface is only minimally disrupted, or in the dynamic mode in which material is removed so that the composition can be detemiined as a fiinction of depth below the surface. SIMS has also been used along with a shadow and blocking cone analysis as a probe of surface structure [70]. 

Applications of ISS to polymer analysis can provide some extremely useful and unique information that cannot be obtained by other means. This makes it extremely complementary to use ISS with other techniques, such as XPS and static SIMS. Some particularly important applications include the analysis of oxidation or degradation of polymers, adhesive failures, delaminations, silicone contamination, discolorations, and contamination by both organic or inorganic materials within the very outer layers of a sample. XPS and static SIMS are extremely comple-mentar when used in these studies, although these contaminants often are undetected by XPS and too complex because of interferences in SIMS. The concentration, and especially the thickness, of these thin surfiice layers has been found to have profound affects on adhesion. Besides problems in adhesion, ISS has proven very useful in studies related to printing operations, which are extremely sensitive to surface chemistry in the very outer layers. 

Scanning Auger Electron Spectroscopy (SAM) and SIMS (in microprobe or microscope modes). SAM is the most widespread technique, but generally is considered to be of lesser sensitivity than SIMS, at least for spatial resolutions (defined by primary beam diameter d) of approximately 0.1 im. However, with a field emission electron source, SAM can achieve sensitivities tanging from 0.3% at. to 3% at. for Pranging from 1000 A to 300 A, respectively, which is competitive with the best ion microprobes. Even with competitive sensitivity, though, SAM can be very problematic for insulators and electron-sensitive materials. 

The number of ions chosen has an effect on the sensitivity of the analysis. The advantage of SIM is achieved through spending more time monitoring the ions of interest - the more ions being monitored, then the less time will be spent on each of them and the lower the increase in sensitivity of SIM over fnU scanning. 

Secondary ion mass spectrometry (SIMS) is by far the most sensitive surface technique, but also the most difficult to quantify. When a surface is exposed to a beam of ions (Ar", 0.5-5 keV), energy is deposited in the surface region of the sample by a collisional cascade. Some of the energy will return to the surface and stimulate the ejection (desorption) of atoms, ions, and multi-atomic clusters. In SIMS, positive or negative secondary ions are detected directly with a quadrupole mass spectrometer. 

Tables 6.27 and 6.31 show the main characteristics of ToF-MS. ToF-MS shows an optimum combination of resolution and sensitivity. ToF-MS instruments provide up to 40000 spectra s-1, a mass range exceeding 100000 (in principle unlimited), a resolution of 5000, and peak widths as short as 200 ms. This is better than quadruples and most ion traps can handle. Unlike the quadrupole-type instrument, the detector is detecting every introduced ion (high duty factor). This leads to a 20- to 100-times increase in sensitivity, compared to QMS used in scan mode. The mass range increases quadratically with the time range that is recorded. Only the ion source and detector impose the limits on the mass range. Mass accuracy in ToF-MS is sufficient to gain access to the elemental composition of a molecule. A single point is sufficient for the mass calibration of the instrument. ToF mass spectra are commonly calibrated using two known species, aluminium (27 Da) and coronene (300 Da). ToF is well established in combination with quite different ion sources like in SIMS, MALDI and ESI.

Figure 4.12 Secondary neutral and ion mass spectra of a 1 1 Fe-Ni alloy in the mass regions of monomers lop) and dimers (bottom). The dimer distribution indicates that iron and nickel are atomically mixed, as expected in an Fe-Ni alloy. Note the higher sensitivity of SIMS for iron and the manganese impurity (from ter Veen [36]).

The adduct formation can be largely controlled and directed into the formation of a single selected species by adequate choice of the ionisation mode (possibly at the expense of sensitivity), the eluent composition (buffer addition, pH adjustment, type of organic modifier) and by optimisation of the ion source parameters influencing the stability of individual (adduct) ions. In contrast to the variations in adduct or cluster formation, which principally can be diagnosed by recording more than one (adduct) ion in SIM mode, the occurrence of ion suppression requires more careful diagnosis. 

Koers, J. M. (2008). Selected ion monitoring (SIM) mode data collection using the laser diode thermal desorption (LDTD) source to increase sensitivity. In Proceedings of the 56th ASMS Conference on Mass Spectrometry and Allied Topics. ASMS, Denver, CO. 

It is a remarkable feature of secondary ion mass spectrometry (SIMS) that considerable chemical information is accessible through the procedurally simple physical technique of sputtering. SIMS--espec ia 11 y under low primary ion flux conditions ("static SIMS," a 1 s o known as "molecular SIMS" when applied to compounds)—provides information on molecular weight and molecular structure and allows isotopic analysis. The surface sensitivity of SIMS permits its use in imaging, in monitoring of surface... 

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