Assessing the contamination in a mass-spectrometric MALDI ion source
The invention relates to a method by which the operator of a mass spectrometer with a MALDI ion source, particularly one which operates with delayed extraction of the ions, is provided with a technique for determining the degree of contamination, in particular to determine when the ion source must be cleaned. The method comprises the acquisition of at least one mass spectrum of ions which are generated in the ion source, the recording of at least one characteristic value for each of at least two mass signals in the mass spectrum, and the determination of an indicator number, derived from the characteristic values of at least two mass signals, which shows how urgently the ion source must be cleaned. The invention also relates to a mass spectrometer with a MALDI ion source which can be characterized accordingly.
1. A method for assessing the contamination in a MALDI ion source in a mass spectrometer, comprising the steps:
acquiring at least one mass spectrum of ions which are generated in the ion source;
recording at least one characteristic value for at least two mass signals in the mass spectrum in each case; and
determining a contamination indicator number which uses the characteristic values of the at least two mass signals to indicate how urgent it is that the ion source be cleaned.
2. A method according to claim 1 , wherein the characteristic value of the mass signals is recorded from the group comprising: maximum amplitude, area, width or symmetry.
3. A method according to claim 1 , wherein the mass signals selected originate from matrix ions.
4. A method according to claim 1 , wherein the mass signals selected originate from a special sample preparation.
5. A method according to claim 1 , wherein mass signals are used which originate from the same ion in different charge states or molecular states.
6. A method according to claim 1 , wherein the mass signals used are from a lower range of the mass-to-charge ratios m/z, namely between about 150 and 500 atomic mass units.
7. A method according to claim 1 , wherein the mass signals used differ by around about 100 to 250 atomic mass units.
8. A method according to claim 1 , wherein the contamination indicator number can be determined from a ratio of the characteristic values with the aid of a look-up table which lists the ratio of the values for at least one characteristic as a function of a degree of contamination of the ion source.
9. A method according to claim 8 , wherein the look-up table lists a conversion of the ratios into an indicator number for at least one delay time for an acceleration after ion formation between about 30 and 1000 nanoseconds.
10. A method according to claim 9 , wherein a mass spectrum is acquired at a delay time for which conversion values are recorded in the look-up table.
11. A method according to claim 1 , wherein during the step of determining an optimum leveling voltage for a generation of a largely field-free region in a desorption area of a MALDI sample support is incorporated into the determination of the indicator number.
12. A method according to claim 1 , wherein several individual indicator numbers are determined which are subsequently computed to derive an overall indicator.
13. A method according to claim 1 , wherein, when the contamination indicator number indicates the necessity for cleaning, an automatic cleaning procedure is performed.