Single particle (sp) mode analysis has become an established method for the analysis of particle suspensions in the ICP-MS field. In this type of analysis, very short integration times of the mass spectrometer (in most cases less than 5 ms) are used and the suspension (for example nanoparticles) is diluted in such a way that only one particle is detected in this integration window. This approach forms the basis of a new trend currently observed, to measure not only biological and medical cells but also particles, but in this case with a diameter larger than 5 µm. This technique is called single cell analysis. However, unlike in the case of SP-ICP-MS, when adopting this method, it is recommended to transport the sample using a syringe pump rather than a conventional peristaltic pump. This is because cells may be crushed between the walls of the tubing of the peristaltic pump, distorting the results of such a “single cell” analysis. Therefore, an alternative mode of sample delivery by a syringe pump is recommended.
Syringe pumps are already used in many laboratories, especially when nanoliter or microliter volumes need to be metered or transported continuously. Such applications demand a particularly high level of precision, which is usually reflected in the price paid for the acquisition of the relevant technology. Delivery rates of up to 1,000 µL/min, required for ICP-AES or ICP-MS, therefore tend to occur only in exceptional cases. Spetec has therefore developed a cost-effective syringe pump that meets the specific requirements of atomic spectroscopy. The pump was used in a study by a working group led by Gunda Köllensperger at the University of Vienna on the uptake and kinetics of new cytostatic drugs in the treatment of cancer cells. The method involves time-resolved single-cell analysis using a time-of-flight mass spectrometer (icpTOF 2R, TOFWERK AG, Thun, Switzerland) instead of a conventional quadrupole mass spectrometer. The advantage of ICP-TOF-MS is that it allows true multi-element analysis of individual cells, whereas sequential measurement quadrupole or sector field devices only guarantee time-resolved analysis of individual isotopes. In this experiment, yeast cells (Pichia pastoris) were incubated with cisplatin (5 µM, 37°C, 24 h). Afterwards, the cells were washed and loaded into the syringe of a syringe pump. The ejection speed of 10 µL/min was adjusted to match the nebulizer used (Parallel Path PFA 260, AHF Analysentechnik, Tübingen). ICP-TOF-MS simultaneously records data for all isotopes in the mass range m/Z 2 – 257 and, when properly calibrated, allows direct conversion of the measured intensities into femtograms per single cell. In this study, it was shown that it is possible to determine the presence of metals in the low femtogram range in individual cells. The masses determined for elements in individual cells correlated closely with the resolved average values of the results of ICP-MS analyses performed on a large number of cells. However, such a resolution of the results of a multi-cell analysis loses one of the key pieces of information, namely, the heterogeneity of the cellular system. This is because the distribution is not Gaussian, as can be concluded from the fact that individual cells can absorb disproportionately large amounts of cisplatin. However, comparing this distribution of cisplatin with the distributions of other essential elements (Fe, Cu, Zn), we can see that this particular distribution is due to the biological variability of the cells. Overall, this study shows that careful sample delivery by a syringe pump can significantly simplify single-cell analysis using ICP-MS, allowing detection limits in the sub-attogram range for Cu and Zn and in the sub-femtogram range for P and Fe. The method presented here can be used to investigate other cellular systems of medical relevance and to investigate their interactions with new metal-containing cytostatic drugs. Naturally, this type of investigation can also be performed using conventional ICP-MS instruments. However, in this case, the individual isotopes of the target metal must be measured one after the other, significantly increasing the analysis time.The method is applicable not only to pharmaceutical research but also to toxicological investigations that may explain the transport of nanoparticles or toxic heavy metals in cells.
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