Atmospheric Pressure Ionization Liquid ChromatographyâMass

Chapter 26

Atmospheric Pressure Ionization Liquid Chromatography—Mass Spectrometry for Environmental Analysis Downloaded by NORTH CAROLINA STATE UNIV on October 11, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1995-0619.ch026

Robert D. Voyksner Analytical and Chemical Sciences, Research Triangle Institute, P.O. Box 12194, Research Triangle Park, NC 27709

Atmospheric Pressure Ionization-Liquid Chromatography/Mass Spectrometry (API-LC/MS) is discussed and reviewed as related to environmental analysis. The chapter covers the use of collision induced decomposition in the API interface to obtain structural information and applications of API-LC/MS for specific classes of environmentally relevant compounds. API-LC/MS determinations are demonstrated for dyes (sulfoninated, azo, cationic, and anthraquinone types), pesticides, herbicides, amines, nitro, hydroxy and carbonyl containing compounds. In particular, keypoints in API-MS operation such as pH, acceptable buffers and derivatizations are covered to achieve optimal sensitivity. Generally, the best sensitivity is achieved when conditions are used which ionize the compound in solution (prior to introduction to the API -MS). Future trends in environmental LC/MS are discussed highlighting new capabilities in MS instrumentation and separations. API ion trap MS or time-of-flight-MS instruments offer capabilities to improve sensitivity, specificity and scanning rates. These capabilities are important when coupling API-MS techniques to chromatographic techniques such as capillary electrophoresis. There are growing concerns over the world's environment The air, water, and foods are highly scrutinized for organic and inorganic contaminants that can lead to health risks. Government legislation and EPA methods have led the way for regulations of wastes, and for establishing water and air quality. Legislation such as the Clean Water Act, Safe Drinking Water Act, Resource Conservation and Recovery Act (RCRA), Clean Air Act and Toxic Substance Control Act has lead to a series of methods using gas chromatography (GC), gas chromatography/mass spectrometry (GC/MS), liquid chromatography (LC) and most recently liquid chromatography/mass spectrometry (LC/MS) to monitor the environment 0097-6156/95/Ό619-0565$12.00/0 © 1996 American Chemical Society

In Biochemical and Biotechnological Applications of Electrospray Ionization Mass Spectrometry; Snyder, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

Downloaded by NORTH CAROLINA STATE UNIV on October 11, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1995-0619.ch026

566

BIOLOGICAL AND BIOTECHNOLOGICAL APPLICATIONS OF ESI-MS

Gas chromatography/mass spectrometry has been particularly important due to its capability to confirm or identify trace residues in complex matrices (1-2). It is routinely used in the identification and measurement of volatiles and semivolatiles in air, food stuffs, water and wastes. Even with intense monitoring programs there are major shortcomings in the characterization of environmental media, and in particular hazardous waste. Often analytical methodology based on GC and GC/MS can only account for a small portion of the carbon mass balance of oxidation products in air, pollutants in water or components in hazardous waste (7). Typically this unaccounted mass is summarized as polar, thermally liable or high molecular weight material unsuitable for GC based methods. While chemical derivatization methods have increased the potential of GC based methods for the detection of polar components, what is really needed is a complement to GC/MS, namely LC/MS. The use of LC to separate nonvolatile, thermally unstable and high molecular weight compounds has been well demonstrated (3-4). Furthermore, the coupling of LC with MS has been reported more than 20 years ago, yet its use for environmental monitoring is still in it's infancy. This is partially due to the complexity, ruggedness and sensitivity limitations of the early interfaces. The development of thermospray and particle beam LC/MS overcame some of the initial interface shortcomings, permitting their use on a more routine basis (5-6). Several methods have been developed to monitor chlorinated phenoxyacid herbicides, azo dyes, nitroso compounds and organophosphorus pesticides using particle beam and/or thermospray LC/MS (7-10). While these interfaces can be used routinely to solve environmental problems they still suffer from several severe drawbacks, which limit the specificity or sensitivity of the approach. For example, thermospray spectra often lack the fragmentation necessary for compound confirmation (77). This weakness can be overcome with the use of tandem MS which adds to the expense and complexity of the method (77-72). Particle beam lacks the ability to ionize nonvolatile compounds and often is insufficiently sensitive to conduct sub-ppb determinations (9-13). Solutions to these problems can possibly be achieved through refinements in these analytical techniques. However, it appears that the atmospheric pressure ionization (API) technique of electrospray and atmospheric pressure chemical ionization (APCI) superseded these developments, offering the required sensitivity and specificity for modern environmental analysis. API-MS API-electrospray is a technique pioneered by Dole and coworkers (14) and combined with mass spectrometry through research by Fenn and coworkers (75-76), has revolutionized mass spectrometry with the ability to ionize high molecular weight molecules and detect femtomole levels of material. Previous chapters in this ACS Symposium Series Volume have described API interfaces with the emphasis on the analysis of biological molecules, particularly high molecular weight moieties. In contrast, environmental analysis involves primarily low molecular weight polar, nonvolatile and thermally unstable compounds typically in the molecular weight range of 100-800. These compounds primarily exhibit single charge molecular ions under API conditions using ion evaporation ionization (17-18) or chemical ionization at atmospheric pressure (APCI) (19-20). While tandem MS can be used to provide structural information on the molecular ion species, the cost and instrument complexity of the technique has limited its routine use in environmental analysis. On the other In Biochemical and Biotechnological Applications of Electrospray Ionization Mass Spectrometry; Snyder, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


26. VOYKSNER

Atmospheric Pressure Ionization LC-MS

567

hand, the use of collision induced decomposition (CID) in the API transport region can provide structural information. The CID in the API transport region occurs between elements in the interface such as the capillary-skimmer or between two skimmers. The extent of fragmentation is controlled by the voltage placed on these two elements (27). The process has also been called "upfrontCID" or "cone CID". Obtaining CID spectra in the API transport region involves controlling the potential on the capillary exit for the system (Analytica of Branford, Branford, CT) used in our laboratory. An increase in voltage accelerates the ions which undergo multiple collisions with air or nitrogen (present from the counterflow or bath gas) resulting in an increase in internal ion energy. As the voltage is further increased the internal energy exceeds the bond energies resulting in fragmentation (Figure 1). The internal energy imparted into a molecule has been shown to vary linearly with capillary voltage and over 16 eV of internal energy can be introduced into the molecule (27). With typical bond energies of about 1 eV, there is sufficient internal energy to cleave numerous bonds providing the structural information necessary for confirmation. For example, compare the mass spectra for carbofuran at a capillary voltage of 50 V and 120 V (Figure 2). The spectrum at 50 V consists essentially of [M+H] ions. At 120 V the [M+H] ions underwent CID to generate the product ions at m/z 165, 147, 123, 91 and 60. The CID spectrum generated in the API region is qualitatively similar to the product ion spectrum recorded on a tandem MS instrument (Figure 2C). The differences can be accounted for by differences in collision energies between the two techniques. Also the switching from a low (50 V) to a high (120 V) capillary potential can be performed quickly under computer control. This not only permits computer optimization of the CID voltage for better fragmentation, but allows "toggling" of the CID voltage on alternate scans or changing the CID voltage in sequence with the mass scan to achieve the best CID sensitivity for all fragmentations. Furthermore, the CID processes in the API transport is very efficient, with few losses from scatter or neutralization, as depicted by the near constant total ion current over the voltage range evaluated (Figure 1). The use of CID capabilities in the API interface on a single mass analyzer instrument requires the introduction of a pure sample. Since there is no mass separation or isolation as is the case with tandem MS, sample purity relies on chromatography. The presence of coeluting LC peaks will result in a compound spectrum representing all components present and can make interpretation nearly impossible. This limitation seems to be out weighed by the simplicity, cost, ruggedness and sensitivity of LC/electrospray-MS using a single analyzer. This chapter will cover the use of LC/electrospray MS for determination of a variety of compounds of environmental interest including dyes, pesticides, herbicides, amine, hydroxy, and carbonyl compounds and hydrocarbons. In particular, the use of capillary columns, fast analysis times and future techniques employing ion trap mass spectrometry (1TMS) and time-of-flight mass spectrometry (TOF-MS) will be discussed. +

+

Application of LC/API-MS to the Environmental Analysis of Dyes The API techniques of electrospray or pneumatic assisted electrospray (ion spray) achieve the best sensitivity for compounds that are precharged in solution. For example, ionic species or compounds that can be protonated or deprotonated by adjusting pH are well suited for the ion evaporation ionization process in electrospray. In Biochemical and Biotechnological Applications of Electrospray Ionization Mass Spectrometry; Snyder, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


568


0

50

100

150

200

Differential Voltage between Capillary Exit and Skimmer (volts) Figure 1. The relative abundance of the [Μ+ΗΓ and the sum of the CID product ions for aldicarb at various capillary exit potentials.


26. VOYKSNER


569

222 CH CH Ο - C - NH-CH-


3

q

il

33

Ο 60

Ο

107122

151 165 180 202 M

ι

50

100

239 ι

Ι

250

200

150 m/z

222

165

- 0 = C NH - CHo Loss ^CHQ

^CH

^ 3

123

II

I 50

100

150

200

m/z Figure 2. The electrospray mass spectrum of carbofuran at a capillary voltage of (A) 50 V (no CID); (B) 120 V (CID conditions), and (C) product ion spectrum of the [Μ+ΗΓ ion of carbofuran (30 eV lab) on a triple quadrupole MS system.



570


fjl 0

1 100

200

1 300

1

1

L_

400

500

Figure 3. Positive ion electrospray mass spectra of Basic Yellow 11. (A) NonCID conditions with tlie capillary exit at 120 V. (B) CID conditions with the capillary exit at 200 V.


26. VOYKSNER

571


For this reason some of the earlier environmental work centered on the analysis of sulfonated azo dyes. These dyes were problematic when analyzed by thermospray or particle beam LC/MS, contributing to the need for the development of an LC/MS analysis method. The ion spray or electrospray spectra of the sulfonated dyes consisted of an [M-2Na]" anion with little fragmentation (22-26). Di, tri and up to hexasulfonated azo dyes were analyzed, yielding multiply charged clusters with the highest charge state equal to the number of sulfonated groups present e.g., [M-2Na]" for disulfonated, [M-ôNa]" for hexasulfonated (26). Structural information on these dyes could be generated in the API transport region, resulting in fragments from breaking the azo linkage with the charge remaining on the sulfonate. Also, the [S0 ]" ion at m/z 80 was a common CID fragment for these dyes. Tandem MS provided similar product ion mass spectral information from the molecular anion of these dyes. The flexibility of the API-MS approach enabled coupling of separation techniques covering a wide range of flow rates. Separation performed on a C column at 2 mL/min, to analyze sulfonated azo dyes was accomplished using APCI (22). However, APCI usually resulted in a poorer response for polysulfonated dyes compared to electrospray due to the lack of sample volatility. Ion spray was used to analyze dyes separated on a 1.0 mm column at 40 uL/min flow rate (22). Also, capillary electrophoresis (CE) coupled to electrospray MS was used to separate sulfonated dyes, offering high resolution separation capabilities and high peak concentrations to achieve subpicomole detection limits (25-27). Cationic dyes have also been analyzed using positive ion detection API-MS, with great success (28). These precharged dyes are well suited for ion evaporation ionization analogous to the negative ion formation for sulfonated dyes. The mass spectrum of Basic Yellow Π exhibits an [M] ion at low voltages (non-CID). At a high voltage (200-240 V) on the capillary exit of an Analytica of Branford interface, numerous fragment ion for Base Yellow II are observed (Figure 3). Azo dyes that are not ionic salts have been successfully analyzed by electrospray and APCI-MS (26-28-29). Dyes from the disperse and solvent classes result in optimal ion formation at low pH conditions (e.g., 1% acetic acid or formic acid), which results in sample protonation to form a cation in solution. These dyes exhibited an [M+H] ion at low API-transport voltage (80-100 V), and structurally relevant fragment ions at higher voltages (e.g., capillary voltage of 160-200 V for CID). Several anthraquinone dyes have been analyzed by electrospray and APCI-MS (29). At low pH and CID voltage conditions, optimal [M+H] ion currents could be detected. At higher CID voltage, fragment ions, primarily due to the loss of the alkyl side chains on the anthraquinone ring, were detected. 2


6

3

18

+

+

+

Pesticides/Herbicides. API-MS techniques of electrospray, ion spray and APCI have been used to determine a wide variety of pesticides and herbicides (21,27,29-35). Triazine, organophosphorus, carbamate, and chlorinated acid herbicides or pesticides have been the major classes analyzed by API-MS. The first three classes generate optimal ion signal using positive ion detection and low solution pH. The chlorinated acid herbicides and most organophosphorus compounds can be detected using negative ion detection by increasing the pH using ammonium hydroxide. Generally either mode of ionization yields molecular ions (e.g., [M+H] for positive ion detection or [M-H]" +



Aldicarb Sulfone (mw 222) 148

Ο CH II I JCH=N O C -NH-CH3 Ο Ι 86 (-CH ) CH.

Ο II — S-

CHΙο 3

q

II

3

ο

143


A 100 Γ

86

c φ

1

50

58 ι

Φ

I

CC

100

. 143 125 ι

1 0

ι

J»

Ί

.

*

»• i i

80

B

'67

ΊΓ 160 m/z

L

I— 200

~τ~240

58

100

148

86 -

1 f i 7

50

J ii ί.ι h 109

ça

Φ

DC

1

143

11.

I 120

80

+

[M+H] 223

A

_l

Ί 160 m/z

I 200

240 240 +

[M+NH4]

>> 100 Γ CO C

+

7,

[M+H] 223

50 h

I— 120

80

200

240

-

86

50

[M+H]+ 223

148 1

59 OC

— Γ "

76

100

Φ

Τ Ι 60 m/z

103

J .L. J ..I.

80

ii

·• J .

120

..i

166 .1 „ 160 m/z

...,.1 1 1, 200

240

Figure 4. Comparison of mass spectra for aldicarb obtained by (A) 100 ng injected using particle beam EI, (B) 100 ng injected using particle beam methane positive CI, (C) 20 ng injected using thermospray and (D) 100 pg injected using electrospray at a capillary voltage of 90 V.



26. VOYKSNER


573

for negative ion detection). Structural information can be achieved by using a higher CID voltage in the transport region of the interface (Figure 2). The analysis of these classes of pesticides have been previously accomplished by particle beam and thermospray LC/MS, therefore, the implementation of electrospray was to seek enhancements in sensitivity and/or specificity. A comparison of the particle beam electron ionization (EI) and chemical ionization (CI), thermospray and electrospray spectra in Figure 4 for aldicarb sulfone demonstrate the improvements from electrospray. The EI spectra from particle beam exhibits primarily fragment ions at m/z 86 and 143, which are common to aldicarb and its metabolites. The lability of the NO bond limits the usefulness of gas phase ionization techniques. On the other hand, thermospray lacks the structural information necessary for compound identification, showing only [M+H] and [M+NH ] ions. Electrospray can provide the molecular ion and structurally relevant ions for confirmation by proper choice of API transport CID conditions. Furthermore, sensitivity of electrospray is far superior to these other interfaces, considering 100 pg was used to generate the electrospray spectrum compared to 20 ng for thermospray and 100 ng for particle beam spectra. On-line LC-MS determinations of pesticides and herbicides have been reported using APCI, pneumatic electrospray and electrospray. Under APCI conditions, separation of organophosphorus compounds could be done using a 3.9 χ 300 mm column at 1 mL/min (30). Pneumatic-assisted electrospray LC/MS has been demonstrated for the separation of selected carbamates on a 1 χ 150 mm column or 0.32 χ 150 mm perfusion column operating at a flow rate in the range of 50-100 uL/min (32) and for the separation of chlorophenoxy acid herbicides on a 3 χ 125 mm column operating at 0.25 mL/min (34). Electrospray LC/MS was demonstrated to separate several carbamate pesticides using a capillary C column (0.32 χ 150 mm) at 4 uL/min (29). Capillary electrophoresis - API-MS has been used to separate and detect chlorophenoxy acid herbicides (23) and sulfonylurea herbicides (33). The advantages of API-MS (APCI and ion spray MS) compared to particle beam and thermospray were compared for the analysis of carbamate pesticides (35). The comparison showed APCI resulted in the best detection limits (0.5-1 ng full scan) for the carbamates evaluated. Ion spray detection limits were 0.3-1.5 ng. Thermospray detection limits were 2-4 ng and particle beam detection limits were 100-240 ng (35). +

+

4

18

Amines/Nitro Compounds. Electrospray or pneumatic assisted electrospray analysis of volatile and nonvolatile amines is best achieved at low pH solution conditions to ensure sample protonation. Mobile phase pH of 2-3 (acetic or formic acid) results in the best electrospray sensitivity, with the spectra primarily exhibiting [M+H] ions under non-CID conditions in the ion transport region. APCI approaches also work well for amines that can be vaporized for gas phase ionization. The implementation of capillary LC combined with electrospray-MS was used to separate a mixture of 28 amines that are representative of the reduction products of azo dyes. Figure 5 shows the total ion current chromatogram for the separation of the amine mixture on a 0.32 χ 150 mm C column operated at a flow rate of 6 uL/min in about 35 minutes. The chromatogram also indicates the variance in response factors for the amines, which were each present at the 32 ng/|iL level (0.5 uL injected). The separation was sufficient to isolate most amines from one another so as to obtain CID +

18



574

BIOLOGICAL AND BIOTECHNOLOGICAL APPUCATIONS OF ESI-MS

3500000 3000000 2500000

ί g 2000000 Έ 1500000 1000000 500000 5.00

10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00 Time (min)

Figure 5. Capillary LC/electrospray-MS total ion current chromatogram for the separation of a mixture of aromatic amines. (Conditions: 0.32 χ 150 mm C column, 5 um panicles, using a gradient of 40-90% acetonitrile (1% acetic acid) in 30 minutes at aflowrate of 6 μΐ/min. 16ngof each amine was injected. The identities of the peaks are as follows: (1) aniline, (2) phenol, (3) m-phenylenediamine, (4) 2-fluoroaniline, (5) 4-fluoroaniline, (6) benzidine, (7) 2,4dinitroaniline, (8) 3,3'-dimethoxybenzidine, (9) 2-methylaniline, (10) 2-methoxyaniline, (11) 3£'-dimethylbenzidine, (12) 4-fluoro-2-methykniline, (13) 4-nitroaniline, (14) ethylene dianiline, (15) 2,4-dimethylaniline, (16) 4amino-3-nitrobenzonitrile, (17) 3$,'£J5'-tetramethylbenzidine, (18) Ν,Ν,Ν',Ν'tetramethylbenzidine, (19) methyl-mercaptoaniline, (20) thiochroman-4-ol, (21) 1naphthylamine, (22) 4,4'-difluorobiphenyl, (23) 4-chloro-2-methylaniline, (24) 4,5difluoro-2-nitroaniline, (25) 32'-dichlorobenzidine, (26) 2,6-dichloro-4-nitroanline, (27) 4,4'-diaminooctafluorobiphenyl, (28) diphenylamine. 18


26. VOYKSNER


575


spectra in the API transport region for confirmation. Nitro compounds also have been successfully analyzed with pneumatic assisted electrospray (36). The chlorinated nitroaromatic compounds showed the best sensitivity using negative ion detection, with structural information obtained by tandem MS. Carbonyl and Hydroxy Compounds. The determination of oxygenated compounds of environmental interest proves a bit more challenging due to the relative difficulty for protonation or deprotonation to form ions in solution compared to the amines, acids or ionic dyes previously discussed. Hydroxy aromatic compounds have been analyzed by electrospray with positive and negative ion detection (37). These hydroxy aromatics exhibited [M+H] and [M-H]" for the respective modes of detection. Often APCI is performed for the analysis of the compounds that can be transferred into the gas phase since the proton affinity of hydroxy compounds is sufficiently high to enable gas phase protonation with H 0 (water CI). Carbonyl compounds often lack a site for protonation or deprotonation to be well suited for electrospray. In this case, APCI approaches may prove superior for volatile and thermally stable molecules. Work in our laboratory to detect both volatile and nonvolatile carbonyl compounds has involved derivatization with 2,4dinitrophenylhydrazine (DNPH) (38). This derivatization processes served several purposes: (1) The formation of the DNPH derivatives helped trap volatile aldehydes and ketones which could not be sampled on Tenax cartridges. (2) The DNPH derivatives provided a UV chromophore for LC/UV detection. (3) DNPH has sites of protonation and deprotonation, so the addition of this moiety to a carbonyl compound would greatly improve electrospray MS response for this class of compounds. The electrospray LC/MS ion chromatograms for the analysis of the DNPH derivatives of various aldehydes and ketones are shown in Figure 6. The [M-H]' ion for each derivative under non-CID conditions is shown. Negative ion detection (at pH 8) proved superior in sensitivity compared to positive on detection (pH 3) for these derivatives. The electrospray mass spectra of the standard aldehydes and ketones in Figure 6 exhibited the [M-H]" ion for the derivative [m/z = molecular weight of organic compound - 16 (for oxygen) + 196 (for molecular weight of DNPH) -1 (for the loss of H to form the anion)] at a low capillary voltage (120 V). A spectrum for the DNPH derivative of acetone is shown in Figure 7, as a representative spectrum for the DNPH derivative of aldehydes and ketones. At a high capillary exit voltage (140 V) several common fragment ions were detected at m/z 181 and m/z 169 (data not shown). These fragments were postulated to be [(N0 ) C H N]" and [(N0 ) C H ]" from the DNPH derivative. No ions corresponding to the aldehyde or ketone were detected, therefore the fragment ion information was of limited use, although these ions can serve to screen for compounds that have undergone derivatization. +

+

3

2

2

6

3

2

2

6

5

Hydrocarbons. Hydrocarbons and polynuclear aromatic hydrocarbons (PAHs) prove the most difficult to be ionized by electrospray due to the lack of sites of protonation or deprotonation. Furthermore, they have a low proton affinity limiting their ability to form ions by APCI. While most hydrocarbons and PAH's are volatile,


576


Ion 209.00 , Formaldehyde

Ion 223.00 Acetaldehyde

Ion 235.00


Acrolein

Ion 237.00 Proplonaldehyde

Acetone

Ion 249.00 Crotonaldehyde

Ion 251.00 2-Butanone

A

Butyraldehyde

Ion 285.00

Benzaldehyde

Ion 265.00 Valeraldehyde

Ion 299.00 m-Tolualdehyde

Ion 279.00 Hexanol

T i l

5.00

10.00

I "

"'

'

15.00 Time

I

' 25.00

20.00

30.00

(min)

Figure 6. Ion chromatogram for the [M-HT ion of the DNPH derivatives of various aldehydes and ketones, including formaldehyde (m/z 209), acetaldehyde (m/z 223), acetone and propionaldehyde (m/z 237), acrolein (m/z 235), crotonaldehyde (m/z 249), and 2-butanone and butyraldehyde (m/z 251). The LC/MS separation was performed using a Nova Pak C 3.9 χ 150 mm column with a gradient from 30% acetonitrile:10% THF in water (hold 3 min) to 60% acetonitrile in water in 10 min at 1.5 mh/min. A post-column 10:1 split reduced the flow of the electrospray interface which was mixed with 200 mM ammonium hydroxide at 0.1 mh/min to increase the pH. A 10 uL sample injection of a 60 ng/uh standard was performed. The electrospray system was operated in the negative ion mode. 18


26. VOYKSNER


577

237


[M-H]'

NO, CH

3

I C=N—Ν I Η CH

Ο

N0

9

3

DNPH Derivative of Acetone MW238

ι ι Γι

40

j

80

ι I ι

i ι \ ι ι 11Ί

120

ι 11

111

160

11

ι ι \\

11 200 m/z ι

ι 11

ι ι"ι ι ! ι 1

240

I

Ί

Λ|

I I I

280

l |'l

I I I

[ ι ι 11

ι

11 ι ι

ι ι"ι 11

320

Figure 7. Electrospray negative ion mass spectrum of the DNPH derivative of acetone at a capillary exit voltage of 120 V (non-CID condition in the API transport). The ion at m/z 237 is the [M-H]' ion for the acetone derivative and the signab below m/z 80 are mobile phase background ions. (See Figure 6 for LC/MS conditions.)


11

I I


578


Table 1 Comparison of the capabilities of thermospray, particle beam and electrospray MS to provide the combination of sensitivity and specificity for the determination of various compounds classes of environmental interest

Compound Class Phenyl urea Triazines Organophosphorus Chlorinated acids Azo dyes Sulfonated azo dyes

Ο

Anthraquinone dyes

(J

Alcohols, ketones, aldehydes

Ο

Nitro compounds

Ο

Aromatic amines and nitro compounds Hydrocarbons, PAH

•• •• • Ο • Ο • Ο • • • • ο

•• •

Thermospray

Particle Beam

Electrospray

3 3

Ο

Increasing sensitivity and specificity


26. VOYKSNER


579

certain species from these classes do not elute through a GC column due to limited volatility and thus require LC/MS. Electrospray has been demonstrated to ionize these non-polar species by charge exchange with solutions such as acetonitrile. For example, negative ion detection of fullerenes have been reported using electrospray-MS (39).


Conclusions and Future Trends In conclusion, API-MS has the combination of sensitivity and specificity needed in the detection of nonvolatile and thermally unstable compounds of environmental concern (Table I). This combination is not achieved to the same degree using particle beam or thermospray LC/MS. While the electrospray ionization process is best suited to polar species or molecules with an acidic or basic site, less polar compounds (e.g., hydrocarbons, ethers, aldehydes, etc.) can be addressed using either APCI, derivatization with an easily ionizable functionality (e.g., DNPH) or solvent initiated charge exchange (e.g., fullerenes). Particle beam LC/MS is still valuable in the analysis of some of these less polar relatively volatile molecules. Furthermore, the versatility of API-MS for handling various column flow rates and mobile phases (using electrospray, pneumatic electrospray and APCI) makes it especially suitable for capillary LC and microsampling techniques and for using 2.1 or 4.6 mm i.d. capillary columns. These capabilities demonstrate that electrospray LC/MS will play a rapidly increasing role in monitoring our environment Future techniques in LC/MS will address improvements in sensitivity, specificity, analysis time and instrument cost In particular, time-of-flight MS (TOF-MS) (40-44) and ion trap MS (ITMS) (45-49) show particular promise, especially when coupled to the rapid and high resolution separation technique of capillary electrophoresis (CE) (50,51). Both TOF and ITMS offer the ability to sample a higher percentage of the ions generated (higher duty cycle), therefore improving signal levels. TOF-MS instruments have employed orthogonal storage devices to improve the duty cycle to greater than 20-30%. Ion traps can accumulate ions in the analyzer to achieve similar duty cycles. These duty cycles are far superior to the 0.1% duty cycle achieved in a quadrupole MS scanning from 0-1000 a.m.u. This translates into the ability to obtain full scan spectra by ITMS or TOF-MS at levels that were obtained only by selected ion monitoring on a quadrupole instrument Secondly, the ITMS offers the ability to obtain CID spectra on a mass selected ion (MS/MS). For example, the ITMS product ion mass spectrum of the [M] ion at m/z 337 for Basic Yellow 11 (Figure 8) exhibits several intense product ions, specific for this dye. While many of these product ions were observed for the CID of this dye in the API transport region (Figure 3B), additional specificity is achieved by mass selection prior to CID in the ITMS. This removes the need to completely resolve components with chromatography prior to generation of the product ion mass spectra which is not the case with CID in the API transport. Furthermore, tandem mass spectrometry capabilities can be achieved using post source decay on a TOF-MS (52). Thirdly, scan times can be greatly increased, enabling a full scan mass spectrum to be acquired in 50 ps for a TOF-MS instrument or in 100 ms for an ITMS. These cycle times become increasingly important when considering rapid and high resolution chromatography. High resolution techniques like CE can only become viable with +



580


Figure 8. Electrospray-ITMS product ion mass spectrum of the [ΜΓ ion for Basic Yellow 11. The ion at mlz 337 was isolated by a combination ofRfand dc voltages on tlie ring electrode and that ion was subjected to helium CID using a tickle voltage of 1.7 V for 30 ms at a q of 0.3. z

sensitive detectors which can acquire a sufficient number of data points (mass spectra) to define a 1-3 s wide peak. To obtain 20 samples across a 1 s wide CE peak will require 50 ms cycletimeswhich can easily be achieved by TOF-MS. Finally, the cost of these instruments are nearly equivalent and potentially lower than quadrupole based MS analyzers. Obviously, the capabilities of the TOF-MS and ITMS combined with LC can offer significant advantages in monitoring the environment and could play a significant role in future environmental methods. Acknowledgments Although the research described in this article has been funded in part by the U.S. Environmental Protection Agency through Cooperative Agreement Number CR-819555 and Contract Number 68-02-4544 to Research Triangle Institute, it has not been subjected to agency review. Therefore, it does not necessarily reflect the views of the Agency. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.

Literature Cited 1. Lesage, S. J. Chromatogr., 1993, 642, 65-74. 2. Holland, P. T. Pure Appl. Chem., 1990, 62, 317-336. 3. Barceló, D. Chromatographia, 1988, 25, 928-936. 4. Grosser, Ζ. Α.; Wren, J.F.; and Dong, M. W. J. Chromatogr., 1993, 642, 75-87. 5. Winkler, P. C.; Perkins, D. D.; Williams, W. K.; and Browner, R. F. Anal. Chem., 1988, 60, 489.


26. VOYKSNER Atmospheric Pressure Ionization LC-MS 581 6. 7. 8. 9. Downloaded by NORTH CAROLINA STATE UNIV on October 11, 2012 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1995-0619.ch026

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12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.

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