Laser desorption mass spectrometry of squaric acid and its salts

The laser desorption mass spectrometry of the oxocarbon squaric acid (3,4-dihydroxy-3-cyclobutene-1,2-dione) and its salts of the form A₂C₄O₄ (A = cation) is described. Both positive and negative ion spectra were obtained. The positive ion spectrum of the acid is characterized by an ion corresponding to loss of CO from [M + H]⁺. The negative ion spectrum shows an intense [M ? H]^? peak in addition to a dimer species. The alkali salt spectra contain [M + A]⁺ in the positive mode and [M ? A]^? and an intense [C₄HO₄]^? in the negative mode. The smaller alkali salts also have an [M + H]⁺ adduct ion. Unlike the alkali squarates, the ammonium salt shows ions corresponding to losses of neutrals from the molecular adduct in the positive ion spectrum and a dimer species in the negative ion spectrum. Molecular weight information was obtained in all cases. A (bis) dicyanomethylene derivative of potassium squarate was also studied. Some field desorption mass spectrometry results are presented for comparison.