Description

Introduction

TSTSpec is a program for quantitative analysis of time-of-flight (TOF) mass spectra mainly in secondary ion mass spectrometry (SIMS). It has been developed continuously since 1991. Starting as several minor tools written in C and as Visual Basic macros for Microsoft Excel, TSTSpec is now a stand-alone program written in Microsoft Visual Basic .NET.

The major idea after TSTSpec is to have a program that allows extracting reliable quantitative data on elemental and isotopic compositions from mass spectra especially from geo- and cosmochemical samples. It has been developed for mass spectra from TOF-SIMS instruments built at the University of Münster, but TSTSpec can also be used for all TOF mass spectra in general. It is also certainly not limited to mineralogical samples and has even been used for organic molecules.

Some of the general aspects realized in the program have been described in the literature (Stephan, 2001).

Basic Features

A prerequisite for all data evaluation is that TSTSpec can read time-of-flight spectra and display them with a time scale and, after a proper mass calibration, also with a mass scale. Mass calibration is very easy with TSTSpec. A practically unlimited number of well-identified peaks are selected for mass calibration. With a minimum of two identified peaks a proper calibration is possible. Further selected peaks reduce statistical variations. In the mass spectra peak intervals can be easily selected as well as intervals for background subtraction. The program makes intelligent suggestions for the peak intervals that can be confirmed or rejected by the user.


Spectrum overview (click on image to get larger view)	Detailed view (click on image to get larger view)

General Approach for Element Analysis

There are two major problems in TOF-SIMS to evaluate element abundances. First of all, elemental mass peaks have to be separated from all possible mass interferences. Second, adequate standards have to be used to determine secondary ion sensitivities.

For a proper separation of mass interferences a sufficient mass resolution is necessary. Often in TOF-SIMS this is not the achievable. Isobaric interferences from isotopes from neighboring elements are not resolvable. These interferences have to be considered but are not crucial. They can be corrected by using other isotope peaks from the respective element. Hydride peaks that are always present in surface analysis are also difficult to separate and similar correction procedures have to be used taking other – non-disturbed – isotopes of the respective elements into account. Oxide and hydroxide peaks are also possible candidates for mass interferences. For masses above 100 amu they are typically also not resolvable. Only pure hydrocarbon peaks represent in most cases no serious problem. Their mass peaks are well separated from pure element peaks. However, with increasing mass the number of possible interferences for any peak increases drastically. To achieve reliable data for several element peaks mathematical correction methods have to be applied assuming normal, i.e., terrestrial or solar, isotopic ratios (for non-solar isotopic ratios cf. the next chapter). A proper calculation of these corrections, including a correct computation of the statistical error, is realized in TSTSpec taking into account the actual mass resolution achieved in the respective spectrum.

Obviously, the same calculations have to be applied to the analysis of standards in order to obtain relative sensitivity factors for all elements. Element ratios normalized to an appropriate element, often silicon, are the typical output of TSTSpec. These values can of course be normalized to any standard element ratio like the composition of carbonaceous chondrites. To increase the user’s possibilities it is possible to export the results via copy and paste, e.g., into Microsoft Excel for further calculations or any data presentation software.

Isotope Analysis

For isotope analysis high mass resolution is even more important. Here the above-described correction methods can only be applied to interferences that have well-known isotopic ratios. In cosmochemistry, e.g., ²⁶Mg enrichment from the decay of extinct ²⁶Al plays often an important role. To separate ²⁵Mg¹H from ²⁶Mg a normal ²⁵Mg/²⁴Mg-ratio within typical error limits can in most cases be assumed. Then the MgH⁺/Mg⁺-ratio can be calculated from the surplus at mass 25 amu

MgH⁺ / Mg⁺	=	I₂₅	–	0.127

		I₂₄

where I_x is the intensity at x amu. With this it is possible to calculate the ²⁶Mg/²⁴Mg-ratio

²⁶Mg / ²⁴Mg	=	I₂₆	–	0.127	·	MgH⁺

		I₂₄				Mg⁺

and ²⁷Al⁺ that is influenced by ²⁶Mg¹H⁺

²⁷Al⁺	=	I₂₇ – I₂₄ ·		²⁶Mg	·	MgH⁺
			I27
				²⁴Mg		Mg⁺

This example has only be chosen to demonstrate the principles of the mathematical correction made by TSTSpec. Often the necessary correction steps are even more complex.

In other cases where such correction procedures are not applicable, e.g., for the separation of ¹²C¹H from ¹³C, and a sufficient mass resolution cannot be achieved only peak separation techniques can be used. This is also possible with TSTSpec. Since peak shapes in TOF-SIMS depend on the respective ion species and are typically not Gaussian and even not symmetric, it is difficult to predict the proper peak shape. In this case it is possible to use the shape of a non-disturbed isotope to predict the shape of the respective peak. E.g., if one uses the ¹²C peak to fit the low-mass edge of the peak at mass 13 amu, the scaling factor for ¹²C gives directly the isotopic ratio ¹³C/¹²C. The same peak deconvolution technique can be used to separate ²⁵Mg from ²⁴Mg¹H if one assumes that the ²⁴Mg-peak is not disturbed by any interference. Further details are given in the literature (Stephan, 2001).

References

Stephan T. (2001) TOF-SIMS in cosmochemistry. Planet. Space Sci. 49, 859–906. [Link to PDF]

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