By J.H.U. Brown, J.F. Dickson
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Extra resources for Advances in Biomedical Engineering. Volume 4
A spectrum recorded on a photographic plate in a mass spectrometer of M a t t a u c h - H e r z o g geometry is read by means of a photoplate comparator or microdensitometer; the d a t a m a y be recorded either in digital form on magnetic tape, or they can be acquired directly by the computer. I n struments of N i e r - J o h n s o n geometry (such as t h e MS-9) cannot record spectra on photographic plates. Originally used for the mass measurement GAS-PHASE ANALYTICAL METHODS 39 of individual ions by t h e p e a k - m a t c h i n g technique, the addition of circuitry to change the magnetic field with time permits the scanning of a spectrum in which the m a s s / c h a r g e ratio of t h e ions is a known function of time.
Since t h e G C - M S or M S instrument a n d t h e C O M unit are rarely directly compatible, an interface is normally required. 38 Ε. C. HORNING, M. G. HORNING, AND R. Ν. STILLWELL High-resolution mass spectrometry is usually carried out with p h o t o plate recording or by scanning methods. If a photoplate is used, t h e operation is off-line of necessity, but optical comparators are now generally used with on-line or virtual on-line computer acquisition of d a t a . A small computer system can be used in on-line fashion with a highresolution mass spectrometer (resolution about 1:10,000) ; an example of a suitable system is t h a t described by Klimowski et al.
Under these conditions t h e instrument will detect organic compounds (and some inorganic compounds like a m monia and carbon monoxide) with low ionization potential in extremely low concentration. Since organic solvents do not injure t h e source, and m a y serve as a source of r e a c t a n t ions, it m a y be possible t o devise a liquid c h r o m a t o g r a p h - m a s s spectrometer-computer assembly analogous to G C - M S - C O M systems. I t m a y also be possible to carry out direct analyses of extracts of body fluids.