Biomedical Engineering Reference
In-Depth Information
1
Introduction
Mass spectroscopy is an extremely powerful technique to analyze gases and
vaporized liquids or solids both with respect to mass as well as concentration.
In combination with gas chromatography such systems are the core of the most
selective and sensitive microanalytical systems. Therefore, their application ranges
from sophisticated laboratory experiments with extreme resolution in mass and con-
centration to rather robust applications, e.g., in process monitoring. Furthermore,
they could be powerful analytical systems in patient monitoring, healthcare, or bio-
technology, and harsh environments like under water, space, and environmental
analysis. In spite of this wide range of possible useful and valuable applications in
reality their use is still rather restricted due to the fact that present systems need
extended resources with respect to volume, power, infrastructure, and specialist
knowledge and are rather sensitive in handling. Standard systems due to their mostly
extended dimensions require low pressures in the 10
−3
Pa range and a correspond-
ingly extensive vacuum equipment. Thus, the application of mass spectrometers is
presently restricted to a laboratory environment and the few portable devices are
still rather clumsy.
In recent years a variety of attempts were undertaken to generate miniaturized
microanalytical systems like gas chromatographs [
1,
2
] , fl ame ionization detec-
tors [
3,
4
], chemical and gas sensors [
5
], and the like. Using the very powerful
technologies of microfabrication, which offer a number of well-established, reli-
able, low cost, and mass production proven processes, very sophisticated, com-
plex, and highly diverse structures can be fabricated in large volume. Typical
examples of this technology are pressure and acceleration sensors, positioning
sensors, or electronic compasses, e.g., for automotive and mobile phone applica-
tions. In this context also several approaches have been undertaken to generate
mass spectrometers as a microsystem. This research mostly, however, is concen-
trated on the generation and optimization of one or the other of the various
components of a standard mass spectrometer, i.e., chambers to ionize the sample,
ionization sources—commonly accelerated electrons emitted from an electron
source—ion focusing and acceleration sections, mass separators, and lastly detec-
tors, in many cases containing a multiplier stage and a Faraday cup. Different
types of mass separators have been miniaturized, e.g., ion traps [
6
] , time of fl ight
separators [
7,
8
], and several designs of quadrupole mass separators [
9,
10
] and
also miniaturized electron sources like thermal emitters [
11
] and carbon nanotube
cold emitters [
12
] have been proposed. Mostly these devices are just miniaturized
versions of their macro sized counterparts. Such isolated microcomponents are
then usually introduced into a standard mass spectrometer environment, i.e., a real
micro-mass spectrometer is not realized this way.
A closer look into the origin of this limited success of microtechnology in this
field is, that either technologies like LIGA (German acronym for Lithographie,
Galvanoformung, Abformung—Lithography, Electroplating, and Molding [
13
] ) or
rather simple silicon bulk micromachining technologies are used, which signi fi cantly
