Preparation of gelatine calibration standards for LA-ICP-MS bioimaging with 266 nm laser ablation systems.

Gelatine is the external standard matrix of choice for quantitative biomaging of elements and metal tags in tissue. Its ablation characteristics closely match that of tissue when using 193 and 213 nm lasers, but this has not been demonstrated at 266 nm. With the interest in 266 nm laser ablation systems growing due to the selective ablation of tissue over glass substrates, this gap needed to be investigated. Optical profilometry was used to map the line crater volumes of gelatine and murine brain and quadriceps tissues ablated with a 266 nm laser. Raw gelatine did not ablate in a similar manner to either tissue type and ultra-violet absorbent chemical additives were required to match ablation volumes. Addition of either 4 g L ^-1 l-tryptophan or 3 g L ^-1 gallic acid was used to match the ablation volume of murine quadriceps. Murine brain tissue had an increased ablation volume over the quadriceps tissue (1400 ± 130 versus 1100 ± 160 μm ³  at 1.5 J cm ^-2 ), and the addition of 5 g L ^-1 gallic acid and the use of low laser energy (≤1.5 J cm ^-2 ) was required to match the ablation of brain tissue. The modified standards were tested on a 213 nm laser, and the addition of either additive to gelatine did not affect ablation volume. The effect of additives on fractionation and two-phase sample transport was investigated, no fractionation was observed, and a decrease in two-phase sample transport of up to 60 % was obtained with the modified gelatine. This decrease was caused by the reduced laser energy required for ablation. Finally, the potential uses of optical profilometry as a standardisation tool are discussed.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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