Consider that a reference-quality mass spectrum for a compound is measured when a few "ideal conditions" are fulfilled, or
we assume that they are. For instance, we assume that the instrument measures the spectrum across the entire mass range with
no mass discrimination. Ideally, ions of all masses are analyzed and detected with equal probability and the recorded ion
intensities accurately reflect the extent of the various fragmentation processes. We assume that the resolving power of the
instrument is sufficient to distinguish ions of adjacent masses. There are other assumptions that often "go without saying."
But we are going to say them here because when the assumptions are not true, our measurement data can be skewed. We assume
that the sample for which the reference mass spectrum is recorded is a pure sample, and purity may include enantiomeric composition,
or for larger molecules, higher-order structures. Finally, we assume that the amount of sample in the ionization source of
the mass spectrometer remains constant during the time required to record the mass spectrum. Each of these desiderata is,
of course, an "ideal condition" not usually fully met. Sometimes, the assumed condition is not even slightly achieved. To
achieve at least a measure of sample purity, chromatography is often used to introduce samples into the mass spectrometer.
Many forms of column chromatography, however, elute samples into the mass spectrometer within a varying concentration centered
around a retention time. The elution profile defines the peak shape. Here, we explore the interaction of dynamic chromatography
with the dynamic aspects of mass spectrometry (MS).
Dynamic GC
Figure 1: This symmetrical shape is the idealized representation of a peak eluted from a GC column. The x-axis is time after
injection of the sample onto the column, and the y-axis is a detector response proportional to the amount of sample eluted
from the column.
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Let's start with a leading and hopefully thought-provoking question: If a pure sample is eluted from a gas chromatography
(GC) column operated in its normal fashion, and there is no detector, what is the peak shape at the end of the column? The
dynamic processes of GC are well understood, and we can model the processes of molecular diffusion in the mobile and stationary
phases to arrive at an answer. If the sample is a pure compound and the diffusion processes are unperturbed, then the sample
will be eluted within a symmetrical peak shape as shown in Figure 1, with the x-axis representing time after sample injection at the front of the column and the y-axis representing some parameter proportional to the amount of sample eluted from the column. The time point at which the
mass flow rate from the column reaches a maximum is called the retention time, and this is labeled on the figure. On the same column, using the same GC conditions, the retention time should be reproducible,
and it can be used as a factor in identifying the sample. One analytical principle to keep in mind here is that of "necessary
and sufficient." More precisely, if the retention time of a reference standard is known, then if the unknown sample is the
same compound as the reference it will be eluted at the same retention time. However, if the unknown sample is eluted at the
same retention time, it may be the same compound as the reference. To be sure of our identification we need more information,
and that brings us to information provided by the detector other than just a determination of the retention time. We want
a detector that provides additional and characteristic information, which brings us directly to MS. Although chromatography
coupled with mass spectrometry seems like the marriage made in heaven, we should always be aware of the mutual give-and-take
in the relationship. In this column, we discuss the dynamic characters of our techniques, and we will start very simply.
Detector Characteristics
The characteristics of the detector determine the nature of the y-axis parameter in Figure 1. If the detector operates to produce a response that is directly and linearly proportional to
the mass flow of the sample entering it, and if it operates to produce a measureable signal in a time frame much shorter than
incremental changes in mass flow, then the measured peak shape with this detector is very close to the modeled peak shape.
If the detector produces a response that is concentration-dependent, then the relevant proportionality factor will change
the appearance of the peak, although the peak shape may still be symmetric overall. If the detector operates in such a fashion
that it provides a signal from some types of eluted sample molecules and not others (such as with an electron-capture detector),
then the proportionality factor for the y-axis response changes from sample to sample and peak to peak. Most modern GC detectors produce a near-instantaneous response,
so that the x-axis in Figure 1 remains directly proportional to time. We can foresee that if the detector itself has dynamic characteristics
that have a scale that is similar to the dynamics of the elution of the GC peak, then a proportional factor must be applied
to the x-axis as well.
The quintessential GC detector that provides the wealth of information needed to identify compounds as they are eluted from
the column is the mass spectrometer. Accordingly, we need to evaluate the role of MS as a GC detection method in terms of
its response characteristics that may provide a proportionality factor that changes the y-axis (response) or the x-axis (time) in Figure 1. Many types of ionization methods, mass analyzers, and detectors used in MS have been previously
described in this column. The definitive matrix for MS ionization methods and their mass- or concentration-dependent response
factors across all types of compounds is a construct that will probably never be completed. It cannot be predicted with specificity,
and therefore the calibration curve for the particular sample of interest remains a reliable method to establish the proportionality
on the y-axis. In the remainder of this column, we discuss the dynamic nature of mass analysis using various types of instruments
and describe the effects of that analysis on the measured elution profile of a GC peak (the x-axis).
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