Page 259 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Mass Spectrometry 147

electrically to 400 C. Automation of a direct probe sample different from performing the same specification at levels
◦
introduction system has recently been described (Manura of 0.01%. With increased resolution, chromatography is
and Manura, 2000). With the automated system, samples better able to isolate components present in mixtures at
can be introduced to the mass spectrometer at the rate of lower levels. With increased sensitivity, mass spectrome-
15–20 samples per hour. This is a faster rate of sample try is better able to identify such components. Finally, with
analysis than is normally possible than with chromatogra- increased emphasis on the measurement of analytical in-
phy coupled with mass spectrometry, in which the column formation for both screening and regulatory purposes, the
separation time accounts essentially for all of the time of demand for trace analyses has increased significantly.
the analysis.
a. Gas chromatography. The combination of gas
chromatography with mass spectrometry (GC/MS), itself
2. Chromatographic Columns
the topic of an excellent text by McFadden in 1973, is
The ability of mass spectrometry to identify a sample com- now realized within an integrated, low-cost, widely avail-
pound is maximized when the sample is pure; the combi- able analytical instrument. There is no longer any need to
nation of mass spectrometry with a chromatographic inlet review the history of development of GC/MS instrumen-
system has therefore become a mainstay of instrumental tation, nor the transition from packed-column to capillary
analysis. Chromatography separates sample components column GC. Instead, it is relevant to consider the simple
in time, passes them into the mass spectrometer, and each physical interface between the methods, and then con-
is then characterized via measurement and interpretation straints on the operation of the methods in the combined
of its mass spectrum. The first gas chromatography/mass GC/MS instrument, and finally, to preview the information
spectrometry (GC/MS) instruments of the middle 1970s available to the analyst from the GC/MS combination.
used packed columns, a 5-mm outer diameter, 1-m long In the modern GC/MS instrument, there is no interface
glass tube filled with the packing material (silica or di- per se between the capillary column and the ionization
atomaceous earth) onto which the stationary phase was source of the mass spectrometer. The influx of helium
coated. An enrichment device was needed to separate the carrier gas is of such low amount that it can be accommo-
molecules of the sample from the great excess of the he- dated without difficulty by the vacuum pump connected
lium carrier gas flow. The resolution achieved with such to the source, while still maintaining high vacuum in the
columns was relatively low; peak widths could be tens of ionization source and the mass analyzer. The capillary col-
seconds wide, and the occurrence of overlapping peaks in umn is terminated by direct connection to the ionization
the separation of a complex mixture common. If we were source, with all sample from the column passed directly
still using packed columns for the separation of complex into the source. Termination of the column at the pres-
mixtures, mass spectrometry would be limited to analyz- sure of the mass spectrometer rather than the usual at-
ing mixtures of 10–20 nonoverlapped components, all ex- mospheric pressure of many other GC detectors does not
hibiting the proper thermal stability and volatility for GC change the retention time significantly, since the pressure
characterization. drop occurs in only the last few cm of the column. It is
Clearly, the capabilities of modern chromatographic important that a bonded, stable stationary phase be used
techniques have been vastly improved. Packed column in the capillary column to minimize the amount of column
GC has been replaced by capillary column GC. Similarly, bleed.Columnbleediselutionofthestationaryphaseitself
the large columns of normal-phase liquid chromatogra- into the ionization source of the mass spectrometer. Sen-
phy (LC) are replaced by microcolumn reverse phase LC sitivity is compromised, as is unambiguous identification
columns. Capillary electrophoresis (CE) is an entirely new of compounds due to the presence of extraneous ions in
means of separating small amounts of more complex, and the mass spectrum formed from the stationary phase. Al-
charged sample molecules, and has evolved into several though background subtraction can remove or reduce the
distinct forms with unique capabilities. Mass spectrom- contribution of these bleed ions, this requires additional
etry coupled with different forms of chromatography is spectral processing time.
now applied to the analysis of many mixtures, of higher As eluting peaks from a GC become narrower (as sep-
complexity, and more disparate sample types. aration resolution increases), the need for faster scanning
It is appropriate here to revisit the meaning of “mixture of the mass analyzer becomes more stringent. Even for
analysis.” A mixture becomes increasingly complex as GC peaks only a few seconds wide, at least a few com-
measured component levels decreases, and as the mixture plete mass spectra should be recorded so that they can be
is examined with increasingly sensitive and sophisticated averaged together to form an approximation of the mass
methods. Specification of mixture components with levels spectrum measured with a steady sample concentration in
greater than 1% provides a snapshot of composition vastly the ionization source. Modern mass analyzers, including