Calibration substances for atmospheric pressure ion sources
A Mixture of substances is provided for precise calibration of the mass scale of mass spectrometers equipped with ion sources that operate at atmospheric pressure (AP) with ionization of the analyte molecules, for example by APCI (chemical ionization), or by APPI (photoionization), often at high temperatures of up to 470° C. Out of physical reasons, the mass scale of any time-of-flight mass spectrometer deviates in the lower mass range from the theoretical relation between charge-related mass m/z and flight time t. A closely spaced arrangement of low mass calibration reference points is necessary if high mass accuracy is to be achieved for substances of low molecular weight. APCI and APPI sources are increasingly used in time-of-flight mass spectrometers with orthogonal ion injection (OTOF MS) for the detection of non-polar and weakly polar compounds, in particular, for analyte molecules with relatively low molecular weights. Calibration substances in solutions that are easily handled, nontoxic, stable at the high temperatures involved, will accept both positive and negative ionization, and supply enough reference points for calibration in the mass range up to 500 daltons. Mixtures of saturated fatty acids dissolved in toluene may be used. For photoionization, also mixtures of phenyl and naphthyl fatty acids.
1. A calibration substance mixture for the mass calibration in a mass range below 500 daltons, for time-of-flight mass spectrometers comprising APCI or APPI ion sources, comprising a series of at least five odd-numbered saturated fatty acids (alkane carboxylic acids), starting from Valeric acid (C5), in a solvent, wherein the solvent comprises a mixture of toluene and chloroform.
2. A calibration substance mixture for the mass calibration of time-of-flight mass spectrometers comprising APPI or APLI ion sources, in the mass range below 500 daltons, comprising at least five phenyl fatty acids or naphthyl fatty acids in a solvent.
3. The calibration substance mixture of claim 2 , wherein the phenyl or naphthyl fatty acids are dissolved in toluene.
4. A method of mass calibrating a mass spectrometer in a mass range below 500 Daltons, comprising:
(a) injecting a calibration substance mixture that includes at least five fatty acids in a solvent into a heating block of the mass spectrometer;
(b) forming ions from the calibration substance mixture using an atmospheric pressure ion source, wherein both aromatic and non-aromatic substances are used as mediator substances for the ionization of the fatty acids;
(c) measuring time-of-flight mass spectra of the ions;
(d) determining coefficients of a calibration curve by comparing known masses of the ions of the calibration substance mixture with their measured flight times using an adaptation algorithm; and
(e) calibrating a mass scale of the time-of-flight mass spectra using the calibration curve .
5. The method of claim 4 , wherein the fatty acids comprise a series of odd-numbered saturated fatty acids.
6. The method of claim 5 , wherein the series of odd-numbered saturated fatty acids starts at Valeric acid (C5).
7. The method of claim 4 , wherein the atmospheric pressure ion source is one of APCI, APPI, and APLI ion source.
8. The method of claim 4 , executed in an automated manner prior to or during analytical measurement runs for compensating for a drifting of the mass scale.
9. A method of mass calibrating a mass spectrometer in a mass range below 500 Daltons, comprising:
(a) injecting a calibration substance mixture that includes at least five fatty acids in a solvent, which comprises a mixture of toluene and chloroform, into a heating block of the mass spectrometer;
(b) forming ions from the calibration substance mixture using an atmospheric pressure ion source;
(c) measuring time-of-flight mass spectra of the ions;
(d) determining coefficients of a calibration curve by comparing known masses of the ions of the calibration substance mixture with their measured flight times using an adaptation algorithm; and
(e) calibrating a mass scale of the time-of-flight mass spectra using the calibration curve.
10. A method of mass calibrating a mass spectrometer in a mass range below 500 Daltons, comprising:
(a) injecting a calibration substance mixture that includes at least five fatty acids, which comprise phenyl fatty acids or naphthyl fatty acids, in a solvent into a heating block of the mass spectrometer;
(b) forming ions from the calibration substance mixture using an APPI or APLI atmospheric pressure ion source;
(c) measuring time-of-flight mass spectra of the ions;
(d) determining coefficients of a calibration curve by comparing known masses of the ions of the calibration substance mixture with their measured flight times using an adaptation algorithm; and
(e) calibrating a mass scale of the time-of-flight mass spectra using the calibration curve.
11. The method of claim 10 , wherein the phenyl or naphthyl fatty acids are dissolved in toluene.
12. A calibration substance mixture for the mass calibration of mass spectrometers comprising APPI or APLI ion sources, in the mass range below 500 daltons, comprising a total number of at least five fatty acids, the fatty acids comprising phenyl fatty acids, naphthyl fatty acids or a mixture thereof, in a solvent.
13. A calibration substance mixture for the mass calibration in a mass range below 500 daltons, for time-of-flight mass spectrometers comprising APCI or APPI ion sources, comprising a series of at least five odd-numbered saturated fatty acids (alkane carboxylic acids), starting from Valeric acid (C5), in a solvent, wherein the solvent comprises a mixture of an aromatic mediator substance and a non-aromatic mediator substance.
14. A method of mass calibrating a mass spectrometer in a mass range below 500 Daltons, comprising:
injecting a mixture of calibration substance capable of framing both positive ions and negative ions in a solvent into a heating block of the mass spectrometer, wherein the calibration substances are at least five saturated odd-numbered fatty acids taken from the range of 5 C-atoms (Valeric acid) to 31 C-atoms;
providing an atmospheric pressure ion source that is configured and arranged to selectively operated in one of a positive mode and a negative mode;
forming ions in one of the positive mode and negative mode from the calibration substance mixture using the atmospheric pressure ion source;
measuring time-of-flight mass spectra of the ions;
determining coefficients of a calibration curve by comparing known masses of the ions of the calibration substance mixture with their measured flight times using an adaptation algorithm; and
calibrating a mass scale of the time-of-flight mass spectra using the calibration curve.