Optimizing the Analysis of Volatile Organic Compounds

34
Figure 35.
The quantification ion for
chlorobenzene-d5 can be changed
from ion 117 to ion 82, to eliminate the
need for chromatographic resolution from
1,1,1,2-tetrachloroethane.
Ion 117
Ion 82
Figure 36.
A slower oven temperature program
eliminates the need to change the internal
standard or the quantification ion.
ion 117
Ion 131
chlorobenzene-d5 }chlorobenzene-d5
ethylbenzene
chlorobenzene
1,1,1,2-tetrachloroethene
ion 117
Extracted ion chromatogram (black)
overlayed on total ion chromatogram (green)
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Identifying Target Analytes
Qualitative identification of a target compound is based on retention time (± 0.06 minutes) and
on comparison of the sample mass spectrum to a reference mass spectrum. Compounds are
identified from three ions of the greatest intensity. The quantification ion, usually the highest
m/z fragment, is used for determining the concentration of a particular analyte. When using
any column for GC/MS, attention must be given to coeluting compounds to determine if
acceptable quantification ions can be found. It is important that there be no coelution between
compounds sharing ions used for quantification. As long as unique ions can be selected for
quantifying compounds that share retention time, chromatographic coelution is acceptable.
Reducing analysis time without carefully checking for coelutions can lead to problems. For
example, internal standard chlorobenzene-d5 and analyte 1,1,1,2-tetrachloroethane, which
share quantification ion 117, can coelute from a “VMS,” “624,” or “VRX” stationary phase.
Many laboratories eliminate the need for chromatographic resolution by changing the quantification
ion for chlorobenzene-d5 from 117 to 82 (Figure 35). An alternative solution is to
replace chlorobenzene-d5 with another internal standard that elutes in the same region of the
chromatogram, such as 4-bromofluorobenzene. In performance-based measurement systems
(PBMS), surrogates and internal standards may be changed, as long as the analyst can show
that the performance will be equivalent or better. Auditors for state or local regulatory agencies
might not allow these changes, however. An additional option, the suggested surrogates can be
used and the analysis performed using a slower GC oven temperature program that resolves
the coelution (Figure 36). This option eliminates the need to change either the internal standard
or the quantification ion, but prolongs analysis time. An analysis time of less than 10 minutes,
with chromatographic resolution of these compounds, is possible with a 20m x 0.18mm x
1.0µm df Rtx ® -VMS column (see Application section, page 51).
Identifying Non-Target Analytes
For samples containing analytes that do not match retention time and/or mass spectra for the
target compounds, a library search can be used in an attempt to match the unknown spectra
with known spectra. Unknown compounds in the sample, referred to as tentatively identified
compounds (TICs), should be reported only as estimates.
Mass Spectral Clues for Identifying TICs: The MS provides three valuable clues to identifying
TICs: parent ion, isotopic composition, and common fragmentation ions. The parent
ion (also known as the molecular weight ion) is the ionized form of the neutral compound,
but not all compounds are stable enough to produce a molecular weight ion. Most environmental
contaminants, except compounds that contain nitrogen, will have an even number
mass for a parent ion. The parent ion reveals information about the elemental composition
and the distribution of isotopes. The term “isotope” is used to describe atoms of an element
with differing numbers of neutrons. Most elements have isotopes in a particular distribution
to each other. For example, carbon occurs primarily in two forms, 12C and 13C. 13C is at an
abundance of 1.1% relative to 12C. 12 This information can contribute to determining the number
of carbons present in the fragmented ion. Carbon is considered an A+1 element because
its isotopes vary by 1amu. Compounds such as oxygen, sulfur, silicon, chlorine, and bromine
are A+2 elements because their isotopes vary by 2amu or more (Figure 37, page 35).
Fragmentation ions also can offer clues to compound composition (Table V).
Leak-Free Column/MS Installation Using an Injection Port Connection
The most common problem associated with volatiles analyses by GC/MS is the presence of
leaks. The following procedure will help ensure optimum performance. Do not use this procedure
with columns with IDs larger than 0.25mm, because the amount of oxygen that
would be introduced into the MS source during the last step will oxidize the metal parts and
reduce sensitivity.
12. F.W. McLafferty and F. Turecek. Interpretation of Mass Spectra, University Science Books, Mill
Valley, 4th edition, 1993, pp. 283-291.