posted on 2020-03-05, 23:29authored byAlexander Knodel, Ulrich Marggraf, Norman Ahlmann, Sebastian Brandt, Daniel Foest, Bienvenida Gilbert-López, Joachim Franzke
This
study introduced sandwich-structured copper-glass substrates
for standardization of laser desorption and plasma ionization. For
standardized quantitative analysis, cavities were constructed which
allow better reproducibility in droplet deposition and for laser application.
Applying the diode laser, molten substrate material is incorporated
into the glass, being trapped inside. Therefore, this method can be
separated from laser ablation, achieving high ion signals without
ablating material from the surface. Flexible microtube plasma (FμTP)
was selected as the ionization source, this being the first time that
laser desorption and FμTP ionization are coupled. This laser–plasma
interface was applied to the detection of cholesterol, which showed
a significantly improved limit of detection of 0.46 ng and linear
dynamic range of 3 orders of magnitude in positive ion mode compared
to other (ambient air mass spectrometry) methods. The main reason
was the change of phase on the copper surface. The dehydrated molecule
[M-H2O+H]+ was the base peak of the spectrum
and no further dissociation or fragmentation was observed. Blood plasma
was spiked with cholesterol. In a 1:100 chloroform dilution, the presence
of the plasma was neglectable and led to the same detection limits
and linear dynamic range as in the cholesterol standard. No sample
preparation or internal standards were needed for calibration. The
physical effects of the surface modification were investigated, including
the calculation of the laser beam waist to simplify the comparison
and reproducibility of results.