The SI mass spectra for more than 600 organic substances have been obtained so far in the systematic study on the SIOMS. This study will be extended to various classes of organic compounds: (1) N-containing compounds [16,18], (2) hydrocarbon compounds and halogenated hydrocarbon compounds [33], (3) O- containing compounds [34], (4) organometallic compounds [35–39], and (5) bio- logically important compounds [40]. The mass spectral report should indicate the predominant ions in the SI spectrum, the possible ion structures, the sensitivity (A/torr), and so on. Available IE and AP data from the literature [17] are also desirable (cf. Table 1). The following subsection describes briefly the SI mass spectral features of the various classes of organic compounds. This is useful for the interpretation of SI mass spectra from the GC–MS. Examples of three applica- tions are then presented to provide a broader understanding of the potential of SI in GC–MS.
5.1. Mass Spectral Features
5.1.1. Nitrogen-Containing Compounds
SIOMS is (particularly) well suited to the analysis of alkylamine, aminoalcohols, quaternary ammonium salts, and hydrazines, which exhibit an intense molecular ion, together with several diagnostic ions for structural determination. In particu- lar, alkylamine and hydrazines show sensitivities in SI that are significantly higher than for EI. The appearance of the ion in the spectrum of tetramethylam- monium chloride provides evidence for the evaporation of intact salt molecules.
Aminoalcohols provide a characteristic set of peaks similar to that for methyl- amines. The spectra show very intense [M⫺H]⫹ions and many diagnostic ions, such as [M⫺ 3H]⫹, and [M⫹ H]⫹. This is in contrast to the EI spectra [41], which show relatively weak (or no) molecular ions and very extensive fragmenta- tion. Several 5-membered and 6-membered ring N,N-heterocyclic amine com- pounds were studied. Pyrrole, indole, and carbazole exclusively provide an in- tense molecular ion, while pyridine, quinoline, and acridine invariably exhibit [M⫹H]⫹ions with great intensity.
5.1.2. Hydrocarbons and Halogenated Hydrocarbons
Most of the alkane, alkene, and alkyne compounds tested do not have SI sensitivi- ties, except a few species such as cyclohexene and 1,3,5-cycloheptatriene. Aro- matic-aliphatic hydrocarbons generate intense DSI ions with a small number of peaks, whereas polycyclic aromatic hydrocarbons (PAHs) give dominant M⫹•
Surface Ionization 45
ions. Other suitable compounds are terpenoids, which form many peaks of sig- nificant intensity.
5.1.3. Oxygen-Containing Compounds
In general, many oxygen-containing organic compounds show lower SI sensitivi- ties than analogous nitrogen-containing compounds, but the SI mode gives greater output currents for three compounds (benzyl alcohol, 4-methoxytoluene, and benzaldehyde) than does the EI mode.
5.1.4. Organometallic Compounds
Organometallic compounds are virtually complexes and are an interesting class of compounds. The relatively high thermal stability and small IE values of some of these compounds suggest the possible formation of ions in SI mode. Metallo- cenes of five transition metals (iron, nickel, cobalt, zirconium, and titanium) are efficiently surface ionized [35,38] in either the molecular or radical form. The carbonyls [36] can provide an efficient method for producing ligand-free metal ion in the gas phase, while [M-acac]⫹types of ions have been produced by all metal-acetylacetonate (acac) complexes examined [36]. Surface ionization has tremendous potential for elucidating certain aspects of complex organometallic chemistry and, hence, SIOMS should become important in this field.
5.1.5. Biologically Important Compounds
The SI techniques are apparently useful for amino acids, purine bases, and alka- loids, but not for steroids and carbohydrates. Some of the SI ions observed can simply be assigned and used for structure determination, while a very intense species can be valuable for trace analysis. However, some of the biomolecules give many more unidentified fragment peaks than other compounds.
5.1.6. Tabular Correlation of Surface-Ionization Ionic Species
Since the reference mass spectra of known compounds have been run previously for a number of years, correlations of SI mass spectra with structure can be made for many of the common classes of organic compounds. Most of these correla- tions emphasize the spectral pattern or simple decomposition pathways to be expected for a particular molecular structure. This has led to the tabulation of mass spectral correlations to provide empirical and structural formulas of ions that might be found at a particular m/z in a mass spectrum, plus an indication of how each such ion might have arisen. Such a table has been reported else- where [10].
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5.2. Determination of Trimethylamine in Air
The occurrence and determination of aliphatic amines have received a great deal of attention in recent years [42–44]. These foul-smelling compounds have been found in a number of ambient environments [45–47] and become a source of serious social and psychological problems. They are also involved in nitrosamine synthesis in air [48], because methylamines react with nitrogen oxides (NOx) and O2.
The necessity of determining these compounds at low levels in complex matrices has stimulated a study [49] on the applicability of GC–SIOMS, based upon the findings of a surprisingly high sensitivity for trimethylamine (TMA) in SI. This compound yields much higher ionization efficiency in the SI mode than in the conventional EI mode.
The analytical procedure [49] employs the direct gas sample inlet without complicated sample preparation and selective ion monitoring (SIM). It saves time and minimizes the risk of sample contamination and sample loss during manipu- lation. Ambient air analyses were performed as follows. A 1-ml sample was in- jected into a gas chromatograph with a 1-ml glass syringe. The SI spectra of
Figure 4 SIM chromatograms of aliphatic amines in a 1-ml air sample: spectrum of a 1-ml standard gas sample in EI mode (left), spectrum of a 1-ml standard gas sample in SI mode (center), and spectrum of an air sample from garbage site in the SI mode (right).
All measuring ranges of ion currents are 1⫻10⫺10A (full scale), except that of ion currents at m/z 58 in the SI mode, which are 5⫻10⫺9A (full scale).
Surface Ionization 47
monomethylamine (MMA), dimethylamine (DMA), and TMA are dominated by only a few peaks [20], showing that the [M⫺H]⫹intensity is highest for these compounds. The mass spectrometer was set to monitor m/z 30 for MMA and TMA, m/z 44 for DMA, and m/z 58 for TMA. The effectiveness of the technique is demonstrated by a comparison of single ion chromatograms obtained with SI and EI of a known mixture of MMA, DMA, and TMA (Fig. 4). The much greater sensitivity of SI for TMA than that of EI is clearly demonstrated. The sensitivities of SI and EI for MMA are comparable. Other experiments with TMA gave a peak with a signal-to-noise ratio (S/N) of 6 for a 225-pg/L (0.8 ppb by volume) sample, which could not be detected in a single ion trace using EI. These results are consistent with previous observations of a very high sensitivity for TMA with a SI detector [50].
A 333-fold improvement in the detection limit was achieved by the use of the SI ion source. Estimated from recovery study, the relative standard deviation (RSD) of peak area of TMA was 6.8% at a concentration of 225 pg/L. The method offers a few advantages over the flame ionization detector, which is com- monly used. Because no preconcentration is needed, the analysis time can be less than 10 minutes, which is much shorter than a GC method including a preconcen- tration procedure. A second advantage is a high degree of specificity due to the characteristics of SI. Therefore, the method may be useful in routine analysis for a large number of air samples.
5.3. Determination of Lidocaine in Serum
Lidocaine is used extensively to treat ventricular cardiac arrhythmias, especially in patients who have had cardiac surgery or sustained an acute myocardial in- farction [51]. Among many methods for its determination, the use of capillary GC–EI-MS in SIM mode may be the best [52]. It is applied in many clinical studies.
Gas chromatography–surface-ionization organic mass spectrometry also seems to be a promising method for the detection of various drugs, such as lido- caine. This drug was selected because it can be expected that the amine radical moiety has the potential to be surface ionized efficiently [53] and the previous SI detection studies [50] showed promising results.
Figure 5 shows the mass spectra of lidocaine obtained by SI and EI along with total ion chromatograms (TICs) [54]. The samples were 10-ml drug-free serum extracts spiked with lidocaine in concentrations of 40 mg/ml and 400 mg/
ml. Interestingly, the TIC in the SI mode shows no interference from the com- pounds associated with the blood. The lidocaine peaks in both TIC traces yield easily identified mass spectra, which are, as expected, almost identical to that obtained from the direct introduction of the pure lidocaine sample.
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Figure 5 Comparison of TICs (left) and mass spectra (right) of lidocaine in a spiked serum sample obtained by (A) using the SI mode and (B) using the conventional EI mode.
On the assumption that the extraction efficiency is 100%, the lidocaine peak of the TIC profile in the SI mode corresponds to 0.8 mg, while the peak in the EI mode corresponds to 8 mg. Mass spectral intensity is plotted as the normalized percentage scale.
Comparison with the EI mode reveals that GC–MS in SI mode does not give rise to peak broadening, tailing, and baseline drift in the TIC profiles, owing to its fast response characteristics. Thus, the SI is compatible with capillary col- umn techniques.
Another important aspect of SIOMS is its high sensitivity, which is demon- strated by a comparison of the TIC traces obtained with the SI and EI of a spiked serum sample at different concentrations. If a spiked sample of 500-àl serum was extracted and a 10-àl injection of the extract was made, the lidocaine detec-
Surface Ionization 49
tion limit in serum was 42 ng/ml (S/N 3) from TIC profiles obtained with SI.
Certainly, a SIM procedure for the m/z 86 base peak would allow the determina- tion of much lower concentrations, i.e., at or below the clinically important con- centrations.
The accuracy and precision of this assay were determined by measuring five different plasma samples spiked with lidocaine. The RSD was 2.5%. The good reproducibility of the method is due to the reliability of the GC–MS opera- tional mode and to the minimal handling of samples. The recovery from five analyses ranged 101.0⫾ 27%.
Hence, GC–SIOMS appears to have great potential for use in the routine biomedical analysis of drugs, many of which possess nitrogen heteroatoms. A preliminary study showed that a routine determination of the antidepressant drug imipramine in serum at concentrations as low as 1.5 ng/ml (S/N 3) can be reliably made.
Gas chromatography–surface-ionization organic mass spectrometry can be also used successfully for sensitive and selective detection of imipramine in se- rum [55]. In this respect, the GC–SIOMS method can result in new opportunities in the field of pharmacology.
5.4. Determination ofs-Triazine Herbicides in Water
s-Triazine derivatives are important compounds in agriculture and industry be- cause of their herbicidal properties. Triazine herbicides are some of the most widely applied pesticides in the United States and Europe. The most common member of this class, atrazine, was the most heavily used pesticide in recent years.
The triazines represent a class of environmental chemicals that are continuously causing severe poisoning [56]. Analysis is often needed for low-level concentra- tions in the environmental media. Recent studies indicate a relationship between water levels of these pesticides and environmental response [57]. Water levels can be very low, even in fatal impact.
Some research was performed fors-triazines in the aqueous environments [58]. Figure 6 shows the GC–SIOMS results obtained by TIC and by SIM (m/z 213). For the TIC profile, a 10-àl river water extract was injected (concentrated 100-fold, spiked with the model compounds [see caption] at concentrations of 0.8 mg/ml each). The peaks yield easily identified mass spectra that are almost identical to those obtained from the direct introduction of the pures-triazines.
Interestingly, both chromatograms in the SI mode show little interference from other compounds in the river water.
Certainly, the SIM detection, using for instance the m/z 213 ion of sime- tryne, allows the determination of much lower concentrations. If a spiked sample of water was treated (again concentrated 100-fold) and a 10-àl injection of the
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Figure 6 Gas chromatograms of a spiked river water sample obtained by full scan (left) and SIM (right). Peaks: (1) atrazine, (2) simazine, (3) prometryne, (4) simetryne, and (5) ametryne. On the assumption that the extraction efficiency is 100%, each peak of the profiles in the TIC corresponds to 0.8 mg, while the simetryne peak (4) in the SIM corre- sponds to 28 ng.
water extract was made, the simetryne detection limit in the water was calculated to be 22 pg/ml (S/N 3). This value is at or below that typically needed in the environmental analysis.
This is the first study on GC–SIOMS for the characterization of some pesti- cides as an alternative, or at least as complementary, to conventional EI-MS.
It yielded a better understanding of the possibilities and limitations of the SI technique.