Instrumentation
R. W. Kondrat and R. G. Cooks Department of Chemistry Purdue University West Lafayette, Ind. 47907
Direct Analysis of Mixtures by Mass Spectrometry Commercial mass spectrometers were developed in the 1940's to ana lyze the complex mixtures encoun tered in oil refining. The mass spectra of complex mixtures are of limited use, and this procedure lost ground to chromatographic methods. The later symbiosis of the two techniques in the form of GC/MS yielded one of the most widely applicable and successful of analytical methods. Nonetheless, the limitations of chromatography and its time-consuming nature have main tained a steady interest in possible al ternatives. Within limited bounds, success is possible by applying pattern recognition to mass spectra (1, 2), by high resolution mass spectrometry (3), or by soft methods of ionization (4). Ionic reactions occur in the analyzer of a mass spectrometer as well as in the ion source; an advantage associ ated with the former is that the reac tion and not simply the reaction prod uct can be specified. This has led to the use of metastable peaks in mixture analysis both on conventional doublefocusing instruments (E/B geometry) and on reversed sector (B/E) instru ments (5-7) (E = electric sector, Β = magnetic sector, Q = quadrupole mass analyzer). Here we describe instrumentation and methodology for the direct analy
sis of mixtures by mass spectrometry. Several closely related procedures fall into this category. Our method is cen tered around the mass-analyzed ion kinetic energy spectrometer (MIKES), a version of mass spectrometer first described in these pages in 1973 (8). Present indications are that the MIKES approach to mixture analysis is generally applicable and has advan tages over currently used methods. The usual mass spectral characteris tics of definitiveness of molecular structure assignment and of high sen sitivity are maintained. The essence of the direct analysis concept is that a mass spectrum (or equivalently a MIKE spectrum) asso ciated with each component of the mixture is obtained. The procedure is nonchromatographic, but it does in volve separation of mixture compo nents—this separation being per formed after rather than before ion ization. A simple analogy can be drawn between this procedure and that met in fluorescence spectroscopy. Scheme 1 develops this analogy to il lustrate the method in the broadest terms: the ionized mixture is passed through a monochromator (which may be operated as a momentum or mass analyzer), and the selected component is dissociated without appreciable
change in velocity. Product ions are then analyzed and detected. The sec ond analyzer can be operated as a mass, momentum, or kinetic energy analyzer since for constant velocity, mass, momentum, and kinetic energy are directly proportional. There are three key requirements for optimization of the method, and each has major consequences in terms of the required instrumentation. These requirements are: (a) soft ion ization, (b) separation of individual ions, and (c) characterization of the separated ions. Thus, the technique is based on (a) the transformation of the components of the mixture into the simplest possible set of ions. For example, using isobutane to effect chemical ionization, a structurally in tact (M + H ) + ion is formed for every component M. (b) The resolution of the ionized mixture into individual components is effected by mass analy sis. One way of accomplishing this is by momentum-to-charge ratio analy sis in a magnetic mass analyzer, (c) Identification of the separated ionized components is by dissociation and re cording of their mass (MIKE) spectra. This can be accomplished, for exam ple, by dissociation with a neutral tar get in a high-energy collision and mea surement of the kinetic energies of the
uorescence Spectroscopy: all hjv
Monochromator
all hi/'
hi>i
Sample
Monochromator
hiV
Detector
(Cuvette) MIKES:
I
Source
allM+
Mi+
Monochromator QorB
Reaction Chamber
allm+
(C
