Mass‐scale calibration of TOF‐SIMS spectra using quaternary ammonium ions

Abstract

A novel method with quaternary ammonium salts as internal additives has been applied to the mass-scale calibration of timeof-flight secondary ion mass spectrometry (TOF-SIMS). Five kinds of quaternary ammonium salts, octyltrimethylammonium bromide (C8TMA), tetradecyltrimethylammonium chloride (C14TMA), octadecyltrimethylammonium chloride (C18TMA), cetylpyridinium chloride (CPC) and benzyldimethyltetradecylammonium chloride (Bzl) were diluted with NH 3 aqueous solution and were mixed in equal quantities. The mixed solution was covered on a Si wafer and was measured by TOF-SIMS. When molecular ions of C8TMA, C14TMA, and C18TMA were used for the mass-scale calibration, the relative mass accuracy of lowmass C X H Y fragment ions indicated negative values and depended on the carbon chain length and the degree of unsaturation. This result means that the conventional mass-scale calibration with frequently employed low-mass C X H Y fragment ions leads to degradation of the mass accuracy of high-mass molecular ions. To improve the mass accuracy of molecular ions of CPC and Bzl, ions should be selected for the mass-scale calibration in the following two ways: One way is for ions to consist only of molecular ions of C8TMA, C14TMA, and C18TMA, and the other way is for ions to include one of the molecular ions: C8TMA, C14TMA, and C18TMA, along with the C X H Y fragment ions. This novel mass-scale calibration method is promising for realizing the correct identification of unknown high-mass molecular ions. Those quaternary ammonium salts are powerful candidates of internal additives.

Mass scale calibration of TOF‐SIMS spectra with molecular ions of internal additives

“…The kinetic energy distribution has a direct effect on the accuracy of the mass scale calibration in TOF‐SIMS. The deterioration in the relative mass accuracy is considered to be caused by a large kinetic energy distribution in low‐mass C X H Y fragment ions . On the other hand, the relative mass accuracy of target molecular ions tends to be converged near 0 ppm by using combinations A2–C2 and D2–G2 included the molecular ions of the internal additives.…”

“…The relative mass accuracy of Tinuvin 770 is approximately 46 ppm on average of the combinations D2–G2. Figure (d) shows the result of our previous report which provides the average values of relative mass accuracy. The target molecule is Tinuvin 770, and the internal additive is a solution containing five kinds of quaternary ammonium salts with different molecular weights: octyltrimethylammonium bromide (C8TMA + Br − ), cetylpyridinium chloride (CPC + Cl − ), benzyldimethyltetradecylammonium chloride (Bzl + Cl − ), C14TMA + Cl − and C18TMA + Cl − .…”

“…The relative mass accuracy ensured by the internal additive method is less than approximately ±46 ppm. The relative mass accuracy of the target molecular ions decreases by including the molecular ions of the internal additives as mass calibration peaks because the kinetic energy of the molecular ion is significantly low …”

“…Mass scale calibration has been discussed in the TOF‐SIMS working group (WG) of Surface Analysis Society of Japan established in 2007. A mass scale calibration method using molecular ions of internal additives has been proposed from the WG . ISO recommendations are met by the use of internal additives that can be detected as molecular ions of mass m A such as quaternary ammonium salts.…”

High Spatial Resolution Laser Desorption/Ionization Mass Spectrometry Imaging of Organic Layers in an Organic Light-Emitting Diode

“…4) Matrix-assisted laser desorption/ionization (MALDI) is a so ionization mass spectrometric technique that results in less molecule fragmentation. When organic layers contain UV absorbers and electrically conductive materials, laser desorption/ionization (LDI) can be directly employed as a so ionization technique without the use of matrices.…”