Quantitative MALDI-MSⁿanalysis of cocaine in the autopsied brain of a human cocaine user employing a wide isolation window and internal standards

Abstract

Detection of drugs in tissue typically requires extensive sample preparation in which the tissue is first homogenized, followed by drug extraction, before the extracts are finally analyzed by LC/MS. Directly analyzing drugs in intact tissue would eliminate any complications introduced by sample pretreatment. A matrix-assisted laser desorption/ionization tandem mass spectrometry (MALDI-MS n ) method as been developed for the quantification of cocaine present in postmortem brain tissue of a chronic human cocaine user. It is shown that tandem mass spectrometry (MS 2 and MS 3 increase selectivity, which is critical for differentiating analyte ions from background ions such as matrix clusters and endogenous compounds found in brain tissue. It is also shown that the use of internal standards corrects for signal variability during quantitative MALDI, which can be caused by inhomogeneous crystal formation, inconsistent sample preparation, and laser shot-to-shot variability. The MALDI-MS n method developed allows for a single MS 3 experiment that uses a wide isolation window to isolate both analyte and internal standard target ions. This method is shown to provide improved precision [ϳ10 -20 times reduction in percent relative standard deviation (%RSD)] for quantitative analysis compared to using two alternating MS 3 experiments that separately isolate the target analyte and internal standard ions. (J Am Soc Mass Spectrom 2010, 21, 564 -571)

Quantification in MALDI-MS imaging: what can we learn from MALDI-selected reaction monitoring and what can we expect for imaging?

“…Among the different normalization methods, the use of labeled internal standards (ISTDs) showed the best linear regression fit [24,25]. ISTD can be a labeled version of the analyte [26][27][28]23], a structural analogue [29], or an endogenous signal (e.g., a peptide in the same mass range as the targeted analyte [30] or a lipid species [31]). In Q-MSI, ISTD normalization compensates for signal variations resulting in difference in analyte extraction, co-crystallization, and ionization [32], assuming that the ISTD is homogenously distributed over the region of interest.…”

Imaging Mass Spectrometry for Drugs and Metabolites

“…Several researchers have been working on a quantitative approach for MALDI imaging, but a well-tested and validated method has yet to be delivered; however, it should be highlighted that the semi-quantitative nature of MALDI imaging data can serve as an invaluable complement to pharmacokinetic data [118,119]. Richard Caprioli's lab (Vanderbilt university) has illustrated that imaging data correlate very well with traditional pharmacokinetic data approaches [120].…”

“…Labs such as Richard yost's (university of Florida) are continuously working on a solution for the direct quantitation of drugs from imaging data. Early results illustrate that with special sample preparation steps and incorporation of deuterated internal standards, the level of inherent MALDI variability can be controlled [119,121]. These procedures significantly reduce the % relative standard deviation (RSD) and increase the confidence of quantitation data.…”

Analysis of Drugs of Abuse in Biological Specimens

“…On the other hand, Reich et al 70) pointed out the difficulty of detecting drugs in brain tissue, which typically requires extensive specimen preparation followed by drug extraction before the extracts are finally analyzed by LC-MS. Therefore, in order to eliminate any complications introduced by specimen pretreatment, a method for directly analyzing drugs in intact tissue was developed.…”

“…70) Adipose Tissue: Adipose tissue can be used to detect absorbed chemicals in living organisms, provided that the chemicals are highly lipophilic and bind to it. 71) In mammals, an increased storage of chemicals in adipose tissue indicates a longer exposure of the tissue to the chemicals even after the cessation of external exposure; that is, chemicals that are adsorbed by the body tend to persist in adipose tissue on the basis of their lipophilicity.…”