Development of a Simple and Highly Sensitive Microbalance for Measurements of Total Particle Yields in Matrix Assisted L
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DEVELOPMENT OF A SIMPLE AND HIGHLY SENSITIVE MICROBALANCE FOR MEASUREMENTS OF TOTAL PARTICLE YIELDS IN MATRIX ASSISTED LASER DESORPTION T. HUTH-FEHRE, A.P. QUIST, S-0. LINDER AND B.U.R. SUNDQVIST, Division of Ion Physics, Department of Radiation Sciences, Uppsala University, Box 535, S-751 21 Uppsala, Sweden
ABSTRACT
In order to open a new window for getting insight into the process of matrix assisted laser desorption ionization (MALDI) of biopolymers, a highly sensitive quartz crystal microbalance (QCM) was developed to measure desorption yields of neutral particles. For the matrix substances commonly used the mass resolution of 0.95 ng obtained with the balance corresponds to a detection limit of -1012 particles.
INTRODUCTION
In 1988 a novel method for volatilization and ionization of large thermally labile molecules was introduced: Matrix assisted laser desorption ionization (MALDI)[1]. By embedding analyte molecules in a host matrix of a small aromatic species (originally nicotinic acid) and irradiating this solid with a short UV-laser pulse Karas and Hillenkamp showed that it was possible to volatilize and ionize intact proteins with masses above 100000 Dalton [2]. Since then the method has been applied by many groups and is maturing into a regular laboratory tool for the live sciences [3]. In spite of the vigorous progress in methodology, the understanding of underlying processes is lagging behind. Proposed models range from purely phenomenological conciderations [4] to detailed hydrodynamical calculations of material flow [5]. Concepts of pressure pulses [6] and restricted energy flows [7] have also been applied. Total neutral particle yields have proved to be a valuable tool for the understanding of the process of UV-laser ablation of organic materials [8]. For MALDI such data have only been derived indirectly from microscopic observations of sample removal [9] and from considerations about sample exhaustion [10], but have not yet been measured directly. With a theoretical limit for mass resolution of 10-12 g [11] a QCM was our instrument of choice for such a measurement. Unfortunately this resolution can not be reached in experiments due to thermal drifts [12,13]. By applying rigorous active thermal stabilization and electronic compensation resolutions slightly below 1 ng have been reported [12]. The construction of a simple instrument with a resolution of. I ng and first results on total particle yields in MALDI are presented in this work.
EXPERIMENTAL SETUP
The balance used exploits the proportionality between the total mass of a quartz crystal and its resonance frequency [14]. Figure 1 shows the setup schematically: Mat. Res. Soc. Symp. Proc. Vol. 285. @1993 Materials Research Society
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Fig. 1: Layout of the balance: Xm = measurement crystal, Xr = reference crystal, D = crystal driving circuit. An AT-cut quartz crystal (Leybold Inficon, Leybold GmbH) with a resonance frequency of 6 MHz and a gold coated surface is mounted in a rotatable holder, which allows for watercooling under vacuum. This
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