Mass Spectrometry
© L. Spenst

Spectra Interpretation

We try to ensure that no signals from previously measured substances ("carryover", "memory effects") are visible in the spectra. Unexpected signals in a spectrum most likely stem from your sample. If you have any doubts, do not hesitate to ask us.

We print calculated isotope patterns and calculate elemental compositions that match an accurate mass determination on request.
Another option are online programs for calculating an isotope pattern or the maximum of an undissolved isotope pattern. We recommend ChemCalc.

A very nice representation of the isotope patterns of all elements can be found here.

Good to know when evaluating the intensity of a (molecular) ion:
To show low-intensity ions in addition to the base peak, we sometimes magnify signal intensities in a selected mass range or starting from a certain m/z value. This is indicated in the spectrum by a hook or arrow and the note *X. The magnification factor X is usually 10, 20, 50 or 100.

Useful, freely accessible spectra databases: NIST and SDBS

Autosampler_Schläuche
© L. Spenst

Frequently used matrix substances:

Elemental
composition
Mass
meta-nitrobenzylalcohol (mNBA) C7H7NO3 153  mNBA spectrum
nitrophenyloctylether (NPOE) C14H21NO3 251  NPOE spectrum

Elemental
composition
Mass
DCTB C17H18N2 250
dithranol C14H10O3 226
dihydroxybenzoic acid (DHB) C7H6O4 154
HCCA C10H7NO3 189
2,5-dihydroxyacetophenon (DHAP) C8H8O3 152

With DCTB, some substances produce [M]+● instead of [M+H]+. DCTB can form adducts. For example, a signal at a distance of 250 Da from the singly-charged molecular ion [M+Na]+ can be interpreted as [M+Na+DCTB]+.

Accurate masses

Frequently asked questions:

Possibility 1:

There is no molecular ion nor a characteristic fragment in the spectrum.

Possibility 2:

The substance is heavily contaminated. We only measure accurate masses for pure substances.

Possibility 3:

The accurate mass of this substance has already been determined in a previous measurement.

Possibility 1:

The intensity of the molecular ion is too low for an accurate mass determination (please pay attention to magnifications!). Therefore, a characteristic fragment of the molecule was selected,
e. g. [M - CH3]+, [M - C4H9]+, [M - 3 CO]+●.

Possibility 2:

ESI does not always lead to [M+H]+. [M+NH4]+ or [M+Na]+ can be used for characterization as well. Occasionally, adducts with K+ or Li+ can occur, especially when salts of these ions have been used in the synthesis (e. g. BuLi, KMnO4). In the negative mode, [M-H]- is not the only ion that can be formed, anion adducts such as [M+Cl]- are also possible.

Possibility 3:

We determine the accurate mass of the lightest still measurable peak of an isotope patternn in order to avoid distortion of the accurately determined mass due to overlap with other isotopes. Some frequently needed isotope masses can be found below.
For the same reason, the fragment [M-H]+ is used for characterization if it is very intense instead of [M]+●.

            ×     Frequent Signals

Frequently measured - rarely desired - signals

HäufigeMSSignale.png
© ME

More detailed lists of typical MS background signals can be found e.g.

Silicone grease

For silicone grease, peak series are observed with a distance of 74 Da,
which in EI often start with m/z 207 or 281, respectively.

Sodium formate

Signals of the calibration substance sodium formate sometimes are visible in the ESI spectra measured at the Q/TOF.
Sodium formate forms ion clusters during ESI. The most important signals are:

Natriumformiat.png
© ME

Isotope patterns

Useful isotope masses

If possible, we select the following isotopes to determine the exact molecular mass:

Isotope exact mass Isotope exact mass

1H

6Li

10B

12C

14N

16O

19F

23Na

24Mg

28Si

31P

32S

35Cl

39K

40Ca

46Ti

52Cr

55Mn

56Fe

58Ni

63Cu

64Zn

79Br

92Mo

96Ru

103Rh

104Pd

107Ag

127I

182W

194Pt

1.007825

6.0151232

10.0239380

12.0000000

14.003074008

15.99491464

18.99840325

22.9897697

23.9850450

27.9769284

30.9737634

31.9720718

34.968852729

38.9637079

39.9625907

45.9526327

51.9405097

54.9380463

55.9349393

57.9353471

62.9295992

63.9291454

78.9183361

91.906809

95.907596

102.905503

103.904026

106.905095

126.904477

181.948225

193.962679

-

7Li

11B

-

-

-

-

-

-

-

-

-

-

-

-

48Ti

-

-

-

-

-

-

81Br

98Mo

102Ru

-

106Pd

-

-

184W

195Pt

-

7.0160045

11.0093053

-

-

-

-

-

-

-

-

-

-

-

-

47.9479467

-

-

-

-

-

-

80.916290

97.9054050

101.9043475

-

105.903475

-

-

183.950953

194.964785

Contact