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Column | Ionization-assisting substrates DIUTHAME
Development of the ultimate “mass microscope” is becoming a reality thanks to the DIUTHAME technique
Associate Professor Yasuhide Naito
The Graduate School for the Creation of New Photonics Industries
Dr. Yasuhide Naito at the Graduate School for the Creation of New Photonics Industries, who is a co-developer of DIUTHAME, talks about the advantages, features, and possibilities of DIUTHAME - a new ionization-assisting substrate - along with research perspectives.
Looking back on the history of mass spectrometry development one can see that creating new ionization methods has always brought about new trends in mass spectrometry with a constantly repeated evolution that has continued to bring vitality to this field of science for more than a hundred years. Today we are witnessing that mass spectrometry is flourishing than ever with a variety of ionization methods. Inventions of innovative ionization methods yield amazing advances in mass spectrometry and industrial prosperities. I am hoping that DIUTHAME will also give such an impact.
The spatial resolution of imaging mass spectrometry using DIUTHAME can ultimately be expected to reach several hundred nanometers which is on the same scale as the through-hole sizes. Viewed in terms of the current matrix-assisted laser desorption/ionization (MALDI) imaging mass spectrometric performance, this sounds almost too good to be true. However, there are number of development issues to be dealt with, including not only DIUTHAME itself but also instrumentation of mass spectrometers. But making the fullest use of DIUTHAME brings the hope that the ultimate "mass microscope" will become a reality.
Why DIUTHAME did emerge
Before developing DIUTHAME, I had worked on pursuing an ultimate high spatial resolution for imaging mass spectrometry by constructing a stigmatic-mode MALDI imaging mass spectrometer but that approach had its own internal limits.
This is because, in the case of MALDI that is the ionizing method normally used in imaging mass spectrometry, the spatial resolution can never exceed the crystal size of the ionization-assisting material called matrix. This holds true no matter how much the equipment spatial resolution of the imaging mass spectrometer. So, in addition to the instrumentation, I really required an appropriate ionization method that does not in any way rely on a matrix.
Why DIUTHAME has a reduced noise level in a low m/z region compared to MALDI
DIUTHAME uses a through-hole porous alumina thin film instead of the matrix compounds used in MALDI. When using MALDI, low mass ions derived from the matrix appear in the form of strong noise peaks which cause interference of measuring low-molecular-weight components.
DIUTHAME, however, does not use the matrix compounds, so it can provide clear mass spectra without interfering with measurement of low-molecular-weight molecules.
Why DIUTHAME can claimed to be a new technique
DIUTHAME is based on the same principle as a conventional ionization method called surface-assisted laser desorption/ionization (SALDI), for which several types of substrates are available as commercial products. However, the SALDI substrates seen on the current market have no through-hole structure and so are not usable for imaging mass spectrometry. This makes DIUTHAME a new ionizing technique that is clearly different from the conventional SALDI method.
Why DIUTHAME is suitable for imaging mass spectrometry
DIUTHAME was thought to be an ideal candidate for imaging mass spectrometry from the very start of its development because it needs no matrix coating process for the sample. This not only improves the workability of the sample preparations but also demonstrates that high quality data can be captured with good reproducibility．
In addition to imaging applications, measuring samples of which co-crystallization with a matrix compound is less likely to occur due to the characteristics of the ingredient for measurement, or impurities such as salts and additives contained in the sample reach too high of a concentration which impedes crystallization of the matrix, DIUTHAME will prove effective on samples where MALDI is unsatisfactory.
Why development of DIUTHAME will continue
Most biological macromolecules generate positive ions in the form of protonated molecules. In samples with such molecules, DIUTHAME has poor sensitivity compared to MALDI. This is due to the fact that the MALDI matrices function as a proton donner, but the through-hole porous alumina thin film has no such effect.
To use DIUTHAME in a wider range of applications, it must have higher sensitivity, so we will continue our development work to increase its sensitivity. Moreover, the working mechanism is still virtually unknown, so deeper insight into the fundamental aspect of DIUTHAME may lead to achieving higher sensitivity.