Assessing Relative Electrospray Ionization, Atmospheric Pressure

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Assessing relative ESI, APPI, APCI and APPCI ionization efficiencies in MS petroleomic analysis via pools and pairs of selected polar compounds standards Marcos Albieri Pudenzi, and Marcos Nogueira Eberlin Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01403 • Publication Date (Web): 17 Aug 2016 Downloaded from http://pubs.acs.org on August 19, 2016

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Assessing relative ESI, APPI, APCI and APPCI ionization efficiencies in MS petroleomic analysis via pools and pairs of selected polar compounds standards Marcos A. Pudenzi* and Marcos N. Eberlin ThoMSon Mass Spectrometry Laboratory, Institute of Chemistry, State University of Campinas, UNICAMP, Campinas, SP, Brazil

ABSTRACT

Fifteen standard compounds with structures similar to those normally found in crude oils were analyzed using an ultra-high resolution and high accuracy FT-ICR (7.2T LTQ FT Ultra, Thermo Fisher, Bremen, Germany) mass spectrometer. Four different ionization techniques were used: ESI, APCI, APPI and a novel technique that couples APCI and APPI, herein termed APPCI. Relationships between chemical structures and ionization efficiencies were established for these techniques which operate via different ionization mechanisms. Unsaturation level and the position of the double bond was shown to be a key factor on ionization efficiency for all ionization techniques. Comparisons between molecules with similar backbones but with different heteroatoms were also made. For the whole mixture, APPI showed the highest sensitivity for the positive ion mode and ESI for the negative ion mode. APPCI was found to be the most comprehensive ionization technique, whereas as expected ESI preferentially ionized the most polar compounds. APPCI produced however more than one ionic species per molecule, a

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disadvantage in terms of data complexity. Such “splitting” was observed for APPI and APCI. Ions with the same molecular formula formed from different molecules were also detected by APPCI, producing composite abundances that would mislead chemical and geochemical conclusions based on petroleomic approaches. We suggest that although less comprehensive, ESI is overall the most suitable ionization technique for petroleomic studies.

Introduction Mass spectrometry (MS) has been applied for the analysis of crude oils since the early 50’s, especially by the oil companies, which have interest in the quality and composition of sold products. In the early years, volatiles were the main focus and gas chromatography coupled to mass spectrometry (GC-MS)[1,

2]

was used. More recently, much less volatile and heavier

fractions have been refined, making their analysis by GC-MS a great challenge since this technique is restricted to volatiles with boiling points up to ca 350°C[2]. This restriction has been partially solved by the introduction of atmospheric pressure ionization (API) techniques which can handle heavier compounds normally up to 2000 Da. [3, 4] For crude oil analysis, the major API techniques have been ESI,[2, 3, 5-9] APPI,[10-16] APCI,[17-18] and APLI (atmospheric pressure laser ionization).[19, 20] Whereas ESI is simply a way to transfer pre-formed ions from solution to the gas phase,[3] APPI, APCI and APLI relays on gas phase protonation or deprotonation induced by electrical discharges or photons.[12] The contrasting ionization and desorption mechanisms make each of these techniques suitable for different classes of compounds defined mostly in terms of polarities and MW. Since ESI is based on proton exchange in solution, it is more suitable to compounds with either higher basicity or acidity, whereas for the others, volatility is a key factor. APCI and more pronouncedly APPI are


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unique since they start via electron abstraction hence the final product of the ionization process can be either the ionized or (de)protonated molecule, or both. Even leading to a MS data with higher spectral complexity, APCI and APPI expand the petroleomic screening to less polar compounds such as high MW polyaromatic hydrocarbons (PAHs), polyaromatic sulfur-containing hydrocarbons (PASHs) and aliphatic hydrocarbons, and this feature has made them the technique of choice for some target crude oil analysis. [10-18] The spectral complexity comes from detecting a molecule as both the protonated or ionized molecule, and this feature becomes extremely problematic when considering that crude oil is already a highly complex mixture containing hundreds of thousands of constituents[22]. This increased complexity has called for the use of mass analyzers with extremely high resolving power (> 1 million) and accuracy (lower than 1 ppm).[13] In this work, to investigate what would be the best ionization technique for petroleomic studies focused on different applications, such as those aiming to sulfur or nitrogen compounds, we first chose a set of 15 molecules that could serve as a representative test case for the classes of compounds normally found in crude oils. Then, we tested ESI, APCI, APPI or APPI + APCI (that is APPCI) in both the positive and negative ion modes for their ability to ionize such set of standard molecules, comparing ionization efficiencies (IEff) as well as the diversity of ionized species produced from each molecule. An overview of the suitability of such techniques for petroleomic studies was therefore established. Experimental Section Fifteen commercial standards for compounds similar to those found in crude oil[23] were purchased (Table S1 and Figure 1) (Sigma-Aldrich, St. Louis MO, USA). The standards were selected for molecules with MW ranging between 100 and 200Da, so the tuning of the equipment


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would have minimum influence in the abundances of their ions. We tried to select also molecules from different classes (O1, N1O1, O2, N1, S1) with different DBE (double bond equivalent, from 2 to 10), Carbon numbers (from C6 to C13), polarities and acidities and basicities, with structures similar to those found in crude oil. Analogue molecules with the same backbone were also selected to evaluate the influence of the specific heteroatom. Solutions of each pure standard as well as an equimolar solution of the whole mixture were prepared at 7.10-5 mol L-1 concentration, in methanol:toluene (1:1,v/v) and infused directly into the respective ion source connected to an FT-ICR MS (7.2T LTQ FT Ultra, Thermo Fisher, Bremen, Germany) with the aid of a syringe pump. When APPI was used in combination with APCI, the technique was termed APPCI (atmospheric pressure photo and chemical ionization). Analysis using ESI were done with additive (0.05% v/v of formic acid for the positive ion mode and ammonium hydroxide for the negative ion mode). Spectra were acquired averaging 100 transients per scan, with resolution of 100,000@400, for the 100-200Da mass range, with flow rate of 5 µL min-1 for ESI and 20 µL min-1 for APCI, APPI e APPCI. Table S2 summarizes the operation conditions. Data acquisition was made using Xcalibur 2.0 (Thermo Scientific, Bremen, Germany). External mass calibration was performed using LTQ FT calibration mix prepared as recommended in the equipment manual.

Results and Discussions Figures 2 and 3 compare the ESI, APCI, APPI and APPCI mass spectra for the mixture of the 15 different standards in either the positive (Figure 2) and negative (Figure 3) ion modes. All techniques may detect such molecules (M) as [M + H]+ or [M – H]- ions, whereas APCI, APPI


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and APPCI can also eventually produce M+., M-., [M – H]+ and other ionic species discussed further below. The differences in spectra complexity (Figures 2 and 3), overall sensitivity (Figure 4) and comprehensiveness (“detectability”, Table S3) for the four techniques tested were outstanding. In terms of total ion current (TIC), clearly no technique displays a far best performance when considering both ionization modes. In the positive ion mode, APPI(+) was far the best, but was worse than ESI(-) in the negative ion mode. ESI(+) was the least efficient but ESI(-) displayed the highest TIC. APCI(+) performs quite well, but APCI(-) displayed the lowest TIC whereas APPCI performs well in both ionization modes. Overall, for best TIC the order was APPI(+) > APPCI(+) > APCI(+) > ESI(+)and ESI(-) > APPI(-) > APPCI(-) > APCI(-). But TIC alone is not the best criteria and other figures of merit should be considered for best petroleomic coverage. To evaluate spectral complexity, Figure 5 displays the number of detected ions for the mixture of 15 standards as well the ratio of ions per detected standard. In general, note the much simpler spectra for ESI in both ionization modes. APCI(-) produced as little absolute number of ions as ESI(-), but this limited number of ions results mainly from the lack of sensitivity as measured by TIC (Figure 4) . It seems therefore that much of the TIC from APCI, APPI and APPCI results not from their ability to ionize a greater number of standards present in the mixture, therefore, suffering less from ionic suppression, but reflects a weakness coming from detecting a single standard molecule as various ionic species. This “ion splitting” is a serious drawback for petroleomic studies due to the extreme complexity of the sample and therefore the inevitable superposition of isobaric species. To check for this drawback, each standard was individually analyzed for all four techniques (Table S3). Figure 6 shows two representative examples of such


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splitting for selected standards. Note that whereas ESI ideally produce a single ion for each selected standard, the other technique splits analyte detection in several different ions. But the ultimate test in terms of spectra quality would require data treatment normally applied to crude oil samples in classical petroleomic studies in order to attribute detected ions to different heteroatomic classes. For that, a series of steps are taken, initiated by isopotologue subtraction, internal calibration based on common homologue series of ions and formula attribution based on exact mass measurements. Figure 7 summarizes the results in terms of class attribution. Great variation on class coverage was expected due to major differences in ionization mechanisms. As mentioned, ESI relies on analyte ionization in solution, depending primarily on the acidity or basicity of the molecule in solution (pKa and pKb, respectively), whereas for the other techniques, ionization occurs in the gas phase, being greatly affected by desorption efficiency and gas phase reactions involving mainly proton and electron transfers. Overall, it is clear that ESI is the worst in both ion modes for class attribution, clearly favoring the most acidic/basic analytes whereas a quite comprehensive class coverage was obtained for the other techniques. Both ESI(+) and ESI(-) failed to detected the S class standards (benzo and dibenzothiophene) since such molecules possess very low acidities (pKa values around 22 [24]) and basicities. But APPI APCI and, most particularly, APPCI are quite efficient in detecting the S class, mainly as M+. from gas phase electron abstraction. Higher sensitivities for the O class standards were also observed for APCI, APPI and APPCI than for ESI, both in the positive and negative ion modes. The N class was detected with low sensitivity by ESI(-), likely due to their much lower pKa as compared to the O2 class. The APCI, APPI and APPCI techniques present, therefore, a much more balanced ionization performance for all detected classes, most particularly in the negative ion mode. For the positive ion mode, the far greatest basicity of nitrogen compounds make them


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predominant regardless protonation taking place either in solution [ESI(+)] or in the gas phase [APCI(+), APPI(+) and APPCI(+)]. Tables S4 and S5 reveals another interesting trend, that is, for all four techniques IEff increases as a function of DBE, which reflects the unsaturation level, namely, the π electron density. For instance, the APPCI(+) response for the M+. ion of dibenzothiophene (5) with DBE = 9 is ca. eight times higher than that from benzothiophene (4) with DBE = 6. Another drastic change in IEff was noted for the O2 class in APCI(-), in which 5,6,7,8-tetrahydronaphthalene-2-carboxylic acid (15) with DBE = 6 yielded a ca. fourteen times more abundant [M – H]- as that for cyclohexanecarboxylic acid (14) with DBE = 2.

In all so far, as judged by spectra quality, ESI was the least comprehensive in terms of class attribution, but displays the best performance in terms of “splitting”, an important feature for petroleomic studies. That is, ESI in both modes produces a single ion, either [M + H]+ or [M – H]- for each detected standard. For overall sensitivity, APPI was slightly superior, but no clear preference for a technique was noted. The “ion splitting” phenomena in APCI(+) has been rationalized previously. Gao et. al.[25] have shown that, despite both the [M + H]+ and M+. are commonly seen in APCI(+)-MS, [M – H]+ ions are also formed due to hydride loss from M+.. Purcell et. al.[11] then rationalized that [M – H2]+. and [M – H – H2]+. can also be observed due to fragmentation of “hot” molecular ions. To test further this hypothesis, the standard mixture was analyzed varying the energy available for in-source CID (tube lens voltage tuning). Figure 8 shows that, for the indole (9) and 2propylpyridine (2) standards, the relative abundances of H2 loss species of m/z 120.0807 indeed


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changed as a function of in-source CID energy, but such H2 loss fragment ions are present in every tube lens condition. Apparently therefore, there is no efficient way to eliminate “ion splitting” by controlling the source energy and that this ion splitting by M+. dissociation is an intrinsic drawback of APCI as well as APPI and APPCI. Running the whole set of selected standards (Figure 1) was useful for the general comparisons so far discussed, but specific pairs or trios would also provide useful indications for IEff comparisons. Figure 9 summarizes the IEff as measured by the relative abundances (as to the total abundance of all standards) for either the positive (a) and negative (b) ion modes and for all standards that were found interesting for such comparison. For instance, lets us first focus on the 5, 10 and 11 trio, which provides an interesting case for comparing a S-class dibenzothiophene (5), a N-class carbazole (10), and an O-class benzoketone (11) as a set of analogues of the same backbone structure. Note first that 11 was predominantly detected by all four techniques as [M + H]+, whereas 5 and 10 were predominantly detected as M+.. Due to extremely low basicities, the three standards were expected to display poor IEff in ESI(+), since this technique is based on protonation in solution, and indeed 11 was the only one detected by ESI(+)-MS but with a quite low relative abundance. Protonation at the carbonyl oxygen of 11 would place a positive charge on a delocalized π-system, which should compensate for the low basicity of ketones in general. For APPI(+), APCI(+) and APPCI(+), which are based on initial electron abstraction, the heteroaromatics 5 and 10 were much more efficiently ionized than 11. For the S-class, there was a clear preference for the photon-based APPI(+) and APPCI(+) techniques. In a similar trend, ESI(+) failed to detected the S-class analogs 4 and 5 due to their low basicity, but APCI(+), APPI(+) and APPCI(+) were able to detected both of them, but most particularly the dibenzo analogue 5 as M+., again likely due to the lower


8

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ionization energy of 5. The same analogy as for 4 and 5 can also be drawn and a very similar trend was clearly observed for the N-class analogues 9 and 10. Note also for the analog pair formed by the NO-class 8 and the O-class 12 a much greater ESI(+) efficiency for the basic pyridine-derivative 8, whereas 12 is not detected at all by ESI(+). For APCI(+), APPI(+) and APPCI(+), both standards display similar IEff, but again 8 is predominantly detected as [M + H]+, whereas 12 is mainly detected as M+.. For the pair of analogs formed by the N-class 3 and NO-class 8, very similar trends when moving from ESI(+) to APCI(+), APPI(+) and then to APPCI(+) were observed, but overall the IEff of the NO-class 8 was inferior likely due to its lower basicity since 8 is detected in all four techniques as [M + H]+. Some standard pairs would also be interesting to compare in the negative ion mode. The pair of analogs formed by the N-class carbazole 10 and the O-class benzoketone 11 bear the same fluorene backbone, but display drastic different behavior upon ionization. The considerably acidic carbazole was barely detected by ESI(-) predominantly as [M - H]-, but gas phase deprotonation upon APCI(-), APPI(-) and APPCI(-) was far more efficient.

The analog

benzoketone 11 was not detected at all by ESI(-) since it would require solution deprotonation of a virtually non-basic molecule. On the opposite, all three APCI(-), APPI(-) and APPCI(-) techniques detected 11, but predominantly as its molecular anion M-.. The O2-class trio formed by the naphthenic acids 13, 14 and 15 also provides an interesting case for comparison. The two aliphatic acids 12 and 13 display nearly the same IEff in all four techniques. But the aromatic acid 15 display by far the best IEff within the trio and ESI(-) was quite more efficient than the other three techniques. Such acidic standards were always detected as [M – H]-. The reason for the far best IEff of 15 is unclear.


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Trying to understand such behavior, pairs of isomeric carboxylic acids (Figures 1 and S1) were analyzed from pure solutions by ESI(-)-MS (Figure 10a) or as a poll (Figures 10b and c). Figure 10a shows similar IEff for two isomeric aliphatic acids 13 and 16, as expected, but the most interesting case is offered by 15 and 17, which differ only by the relative position of the aromatic and “aliphatic” rings, having the carboxylate group as the reference. When the ring system and, therefore, the level of unsaturation (DBE) is the same, the IEff by ESI(-) followed the pKa trend, that is, it was ca. 5 times higher for the “aliphatic” acid 17 as compared to the “benzoic acid” analog 15. But as Figures 10b and 10c clearly indicate, the IEff was directly proportional to DBE and increases from 14 to 20. The fully saturated 14 (Figure 10b) displays an IEff via ESI(-) as much as ca. 20 times lower than its aromatic analog 20, whereas increased DBE led to increased IEff in 19 reaching the highest IEff for 20. The same trend can be seen when moving from 17 to 18. This IEff x DBE trend shows that IEff was far more influenced by DBE than by pKa.

Conclusion In all, we can summarize the results by concluding that all three gas phase ionization techniques based primarily on electron abstraction, that is APCI, APPI and APPCI, as compared to the solution-based ESI technique, display much better coverages in terms of number of standards detected, when data is summed from both the positive and negative ion modes. But note that in fact such better performance in both modes for APCI, APPI and APPCI as compared to ESI could only be achieved for a set of quite few standards (15), but a myriad of components is present in real crude oil samples. For petroleomic studies therefore, APCI, APPI and APPCI


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would suffer from a serious limitation related to “ion splitting”, and components would frequently be detected by various species depending upon their specific acidic/basicity or IE values. Such “ion splitting” would greatly increase spectral complexity and the potential for different molecules producing the same ionic species leading to ion superposition and, therefore, unrealistic peak intensities would mislead geochemical interpretations. Although ESI coverage is the narrowest, it greatly benefits from producing a single ion per analyte, either [M + H]+ for ESI(+) or [M – H]- for ESI(-). This “one ion per analyte” direct relationship is, in all, the most desirable feature in the analysis of a highly complex mixtures such as those of crude oils. We propose, therefore, that ESI should be taken as the gold standard technique for petroleomic analysis due to its ideal “one ion per analyte” figure of merit. Note also that the narrowest coverage of ESI(+) plus ESI(-) is not an intractable deficiency, since its reduced IEff for less acidic or less basic analytes can be solved via simple derivatization protocols, such as those already applied when targeting for the S-class compounds.[12] The factors affecting IEff such as DBE and pKa/pKb for both ESI(+) and ESI(-)for the most common polar components present in crude oils should therefore be fully investigated, and this work using a standard of such molecules have already highlighted some important, perhaps major trends.

FIGURES


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Figure 1. Structural formula for the standard molecules used to compare the IEff for ESI x APCI x APPI x APPCI in both the positive and negative ion modes.

Figure 2. Mass spectra for the mixture of standards using either (a) ESI(+); (b) APCI(+); (c) APPI(+) or (d) APPCI(+). Most abundant ions are highligthed: bold numbers refer to protonated molecules and italic numbers to radical cations.


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Figure 3. Spectra for the mixture of standards using either (a) ESI(-); (b) APCI(-); (c) APPI(-) or (d) APPCI(-). The most abundant ions were deprotonated molecules, and are shown in bold numbers.

3.00E+07 2.50E+07

Total ion current

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2.00E+07 1.50E+07 1.00E+07 5.00E+06 0.00E+00 ESI

APCI

APPI

POS

APPCI

ESI

APCI

APPI

APPCI

NEG (x10)

Figure 4. TIC of the standard mixture for each ionization technique in both the negative and positive ion modes.


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(a)

28 25

25

3.50

27 20

20 15

17

18

11

10 5

5 0

Number of detected ionic species/detected molecules

30

Number of detected ionic species

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(b) 3.11

3.00

3.00

2.50

2.50

2.50 2.13

2.25

2.00 1.50

1.22

1.00

1.00 0.50 0.00

ESI

APCI

APPI APPCI

POS

ESI

APCI

APPI APPCI

NEG

ESI

APCI APPI APPCI ESI POS

APCI APPI APPCI NEG

Figure 5. (a) Total number of ions produced by each ionization technique for the whole mixture of standards. (b) “Ion splitting” as revealed by the average number of ions per detected standard.


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Figure 6. Mass spectra using (a) ESI(+)and (b) APCI(+)for carbazole (10), and (c) ESI(-)and (d) APPI(-) for 5,6,7,8-tetrahydronaphtalene-2-carboxylic acid (15). Note in (a) and (b) the protonated molecule of m/z 108 and in (c) and (d) the deprotonated molecule of m/z 175.

Figure 7. Relative percentage distribution for the classes of crude oil polar compounds in either the (a) positive and (b) negative ion modes for all four ionization techniques. Note that the S (positive ion mode) and NO (negative ion mode) classes could be detected when using APCI, APPI and APPCI, but not by ESI.


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Figure 8. Mass spectra using APCI(+)for the whole standard mixture zoomed at m/z 116-124 obtained using different in-source CID energies: (a) 0 V, (b) 40 V, (c) 100 V; and (d) 160V


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Figure 9. Relative abundances for selected standards with contrasting DBE or heteroatom classes for all four ionization techniques, both in the (a) positive and (b) negative ion modes.

(a) 100 90

Relative intensity (%)

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80 70 60 50 40 30 20 10 0 C8 H13 O2

C8 H13 O2

C11 H11 O2

C11 H11 O2

Compound


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(b) 100

Relative intensity (%)

90 80 70 60 50 40 30 20 10 0 C7 H11 O2 (x10)

C7 H9 O2 (x10)

C7 H5 O2 (x10) Compound

C11 H11 O2

T: FTMS - p ESI Full ms [100.00-200.00]

C11 H7 O2

18

x8

171.04515 C 11 H 7 O 2

(c) 100

20 121.02952 C 7 H5 O2

90

17

80

175.07645 C 11 H 11 O 2

70 Relative Abundance

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60 50

19

40

125.06086 C 7 H9 O2

30 20

14

10

127.07652 C 7 H 11 O 2

0 100

110

120

130

176.07984 197.08205 140

150 m/z

160

170

180

190

200

Figure 10: (a) and (b) Relative abundances for the [M – H]- ions detected by ESI(-)-MS for selected carboxylic acids and (c) the actual mass spectra using ESI(-) for the whole equimolar mixture of 14, 17, 18, 19 and 20. Note that the spectrum was amplified 8 times from m/z 110 to m/z 135 for clarity.

Supporting Information Available: Standard commercial information (Table S1), Experimental conditions for analysis (Table S2), Number of species (Table S3) and relative abundance for each standard (Tables S4 and S5) and additional acid standard structures (Figure S1).


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AUTHOR INFORMATION Corresponding Author *[email protected] ACKNOWLEDGMENT The authors thank Petróleo Brasileiro (PETROBRAS) for the financial funding and Dr. Clécio F. Klitzke and MSc. Vanessa G. Santos helpful discussions. ABBREVIATIONS ESI – electrospray ionization, APPI – atmospheric pressure photoionization, APCI – atmospheric pressure chemical ionization, APPCI – atmospheric pressure photo and chemical ionization, FTICR MS – Fourier transform ion cyclotron resonance mass spectrometry, IEff- Ionization Efficiency REFERENCES 1. Grayson, M.A., Measuring mass - from positive rays to proteins. Philadelphia, PA: Chemical Heritage Foundation, 2002, 149 pages. 2. McKenna, A.M., Nelson, R. K., Reddy, C. M., Savory, J. J., Kaiser, N. K., Fitzsimmons, J. E., Marshall, A. G., Rodgers, R. P., Environmental Science & Technology, 2013, 47(13): 7530-7539. 3. Zhan, D.L. and Fenn, J.B., International Journal of Mass Spectrometry, 2000, 194(2-3), 197-208. 4. Rodgers, R.P., T.M. Schaub, and A.G. Marshall, Analytical Chemistry, 2005 77(1), 20a-27a. 5. Schaub, T.M., Hendrickson, C. L., Horning, S., Quinn, J. P., Senko, M. W., Marshall, A. G., Analytical Chemistry, 2008, 80(11), 3985-3990.


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