Characterization of typical chemical background interferences in atmospheric pressure ionization liquid chromatography-mass spectrometry y

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1 RAPID CMMUNICATINS IN MASS SPECTRMETRY Rapid Commun. Mass Spectrom. 2006; 20: Published online in Wiley InterScience ( DI: /rcm.2715 Characterization of typical chemical background interferences in atmospheric pressure ionization liquid chromatography-mass spectrometry y Xinghua Guo 1, Andries P. Bruins 1 * and Thomas R. Covey 2 1 Mass Spectrometry Core Facility, University of Groningen, Antonius Deusinglaan 1, 9713 AV Groningen, The Netherlands 2 MDS Sciex, 71 Four Valley Drive, Concord, L4K 4V8, N, Canada Received 25 July 2006; Revised 24 August 2006; Accepted 24 August 2006 The structures and origins of typical chemical background noise ions in positive atmospheric pressure ionization liquid chromatography/mass spectrometry (API LC/MS) are investigated and summarized in this study. This was done by classifying chemical background ions using precursor and product ion scans on most abundant background ions to draw a family tree of the commonly occurring chemical background ions. The possible structures and the origins of the major chemical background noise are clearly revealed in the family trees. In agreement with some suggestions in the literature, the chemical background ions studied so far can be classified mainly as either ions of contaminants (or their degradation fragments) or cluster-related ones. A significant contribution from the contaminants (airborne, from tubing and/or solvents) from plasticizer additives (phthalates, phenyl phosphates, sebacates and adipates, etc.) and silicones is concluded. These ions of contaminants can also serve as nuclei for the clustering of HPLC solvent or additives, such as water and acetic acid, thereby leading to a second family of background ions. This study explains the persistence of some chemical background noise even under fairly strong declustering conditions in API LC/MS. ne of the other interesting conclusions is that there is a clear difference in structures between the chemical background ions and the protonated analytes generated under atmospheric pressure ionization. This conclusion will contribute to the on-going research efforts to exclusively remove or reduce the interference of chemical background noise in API LC/MS. Copyright # 2006 John Wiley & Sons, Ltd. The innovative atmospheric pressure ionization (API) techniques, such as electrospray ionization (ESI) 1,2 and atmospheric pressure chemical ionization (APCI), 3 have led to today s popular and successful application of liquid chromatography/mass spectrometry (LC/MS). Since the inception of API MS, the interference of chemical background noise has been reported. 1 3 In current quantitative and qualitative studies, the main drawbacks observed include: (a) the contribution to the high and drifting baseline in the total ion current (TIC) chromatogram; (b) the effect on the limit of quantitation/detection (LQ/LD) due to the low signal-to-noise (S/N) ratio and/or isobaric ion interferences; and (c) the poor quality of MS or MS/MS spectra for qualitative studies especially for identification of trace components, such as unknown impurities, low-level metabolites, low abundance peptides/proteins or degradation products. The interference has also been observed even when an improved interface for declustering/desolvation and the *Correspondence to: A. P. Bruins, Mass Spectrometry Core Facility, University of Groningen, Antonius Deusinglaan 1, 9713 AV Groningen, The Netherlands. a.p.bruins@rug.nl y Presented, in part, at the 53rd ASMS Conference, May 2005, San Antonio, TX, USA; poster ThP-232. Contract/grant sponsor: Applied Biosystems/MDS Sciex and the University of Groningen. proper high-purity high-performance liquid chromatography (HPLC) solvents/additives are applied. This problem gets even worse in the case of analysis of biological samples with very low concentrations or with the poor ionization response in complex matrices/extracts, because then these chemical background ions may completely overshadow the appearance of analytes in both chromatograms and MS spectra. Because the API process is rather generic and efficient, any trace of contaminants or stable cluster ions (because the ions survive the declustering conditions) can be ionized and detected in MS as sources of chemical background noise ions. In the literature there have been dramatic efforts reported to improve the hardware for both ionization and transportation of ions into the mass analyzer. rthogonal- and z- spray, off-axis spray, nebulizing-gas assisted spray, ion sources made of stainless steel to prevent contaminants from ion source materials, application of curtain gases, the Q- jet technique, 4 curved collision cells, and high-field asymmetry ion mobility spectrometry (FAIMS) have been implemented into commercial LC/MS systems. In addition to these hardware improvements, some (off-line) software Copyright # 2006 John Wiley & Sons, Ltd.

2 3146 X. Guo, A. P. Bruins and T. R. Covey approaches have also been developed and intergrated into some commercial systems. However, due to the complex features of their origins and mechanisms of formation, chemical background ions and their interferences are unavoidable 5 and under some circumstances they still become an obstacle 6 to further application of API LC/MS even with the assistance of the above-mentioned techniques. For instance, the trace contaminants in HPLC reagents 7 and ion sources can result in significant contribution to chemical background noise in these studies. Futhermore, it is also encountered that chemical background noise varies very significantly with HPLC mobile phases and MS ionization conditions. In some cases it also contributes to the causes of ghost peaks 8 in HPLC. Although there have been several publications about the interference of individual types of chemical background noise in MS 9 and LC/MS, such as phosphates from the rubber stoppers, 10 polydimethylsiloxanes (or silicones) from the laboratory air, 5,11 plasticizers from -rings, 12 etc., no systematic studies have been reported on the nature of common chemical background noise in advanced API LC/ MS systems after taking precautions to prevent these accidental chemical interferences. The best solution to this problem may rely on the selective removal/reduction of chemical background noise before MS ion detection rather than post-acquisition data processing. Therefore, it is very important to know what structures the chemical background ions indeed have, from what sources they originate, and why they are persistently present in API LC/MS, which may provide further insight into developing some novel methodologies to remove/reduce their interferences. This study summarizes a systematic characterization of typical common chemical background (positive) ions with the focus on finding the origins of the chemical interferences from some commonly used HPLC eluents in API LC/MS by studying their structures using precursor/product ion scans in LC/MS/MS. EXPERIMENTAL API MS and API LC/MS experiments (positive mode) were carried out on triple quadrupole mass spectrometers API365 and API3000 (MDS Sciex, Concord, N, Canada) coupled with a Perkin-Elmer series 200 HPLC system. Some experiments were performed on an API4000 triple quadrupole mass spectrometer (MDS Sciex, Concord, N, Canada) for comparison. Various LC eluents with different organic compositions, additives, elution conditions (gradient and isocratic) and MS ionization and acquisition modes were investigated. MS/MS spectra (i.e. precursor ion scan and product ion scan) of the main chemical background ions were recorded with collision energies of ev. Nitrogen, boil-off from liquid nitrogen without further purification, was used as curtain gas and collision gas and zero air as nebulizer gas. Acetonitrile (ACN) and methanol (gradient grade), formic acid (p.a.) and acetic acid (glacial) were obtained either from Merck (Darmstadt, Germany) or Biosolve (Valkenswaard, The Netherlands). The ultrapure water was supplied by an in-house Elga (High Wycombe, UK) water purification system. RESULTS AND DISCUSSIN Commonly observed chemical background ions in API LC/MS derived from contaminants including plasticizers (phthalates, phenyl phosphates, adipates, etc.) and silicones In practical LC/MS applications, chemical background (ions) interferences may vary under different HPLC solvents, additives and gradients, as well as MS ionization and interface conditions. Although chemical background noise (ions) derived from the solvents/additives from different suppliers may vary slightly, the work reported here focuses on the MS characterization of the typical common chemical background ions rather than the quality characterization of different HPLC solvents and additives. A thorough investigation on the effects of experimental conditions has been done in this study. The in-depth characterizations are focused on a few typical LC/MS conditions, which can represent chemical background interferences in general. Some mobile phases generate less chemical background noise (in the sense of both types and intensities of ions) than others. This is primarily dependent on the contamination of LC/MS systems and the ease of the evaporation and clustering ability of the LC solvents and additives. Some typical chemical background mass spectra obtained under two different ESI LC/MS conditions are given in Fig. 1. If only the contribution of the major ions is taken into account, the chemical background ions appearing in the mass spectrum in Fig. 1(a), obtained when the mobile phase ACN/H 2 /(formic acid) (50:50:0.1%) was used, are in fact the typical common chemical interference ions in all positive API LC/MS including both APCI (the mass spectra are not shown) and ESI modes. When other LC solvents and additives are used, some more specific interference ions appear on top of the common chemical background ions in Fig. 1(a). Furthermore, the relative intensity of the common chemical interference ions and the solvent/additive-specific background ions may vary under different conditions. An Figure 1. Typical ESI LC/MS spectra obtained when different LC solvents/additives were used: (a) ACN/H 2 /formic acid (50:50:0.1%) and (b) MeH/H 2 /acetic acid (50:50:0.1%), respectively. Both MS spectra were obtained under turboionspray ionization (the nebulizing gas at 4008C) and at relatively high up-front collision energies. DI: /rcm

3 Characterization of chemical background ions in API LC/MS 3147 example is shown in Fig. 1(b) obtained when the mobile phase MeH/H 2 /(acetic acid) (50:50:0.1%) was applied, where the additional chemical background ions such as m/z 143, 159 and 219 (their identities will be discussed in the text below) were observed. We are interested in studying the possible structures/ types of and the relationship among the chemical background ions in order to characterize them by grouping them according to their origins. This is realized by performing tandem mass spectrometry (MS/MS) studies on the major chemical background ions under a given LC/MS mobile phase and ionization condition. As an example, the collisioninduced dissociation (CID) product ion scan and precursor ion scan spectra of the ions at m/z 149, a well-known identifier of phthalates, are shown in Figs. 2(a) and 2(b). The product ion spectrum (Fig. 2(a)), showing the consecutive loss of three neutral molecules of 28 (C), confirms that the identity of the ions at m/z 149 is indeed protonated phthalic acid anhydride C 8 H 5 þ 3 derived from phthalate esters. Furthermore, the precursor ion scan MS spectrum of the ions at m/z 149 in Fig. 2(b) shows abundant ions at m/z 205 (m/z 149 þ C 4 H 8 ), 279 (protonated dibutyl or octyl (or ethylhexyl) phthalate esters) and 391 (protonated dioctyl (or ethylhexyl) phthalate ester) 13,14 as the major precursors of m/z 149 along with some minor contribution from ions at m/z 167 (protonated phthalic acid), 177, 241 and 315, etc., respectively. However, the product ion scan study of the ions at m/z 315 given in Fig. 2(c) finds the abundant fragment ions at m/z 185 and 203 (m/z 185 þ H 2, or loss of 2 C 4 H 8 from m/z 315), m/z 241 (loss of C 4 H from m/z 315) and 259 (loss of C 4 H 8 from m/z 315), as well as m/z 273, which indicates the presence of sebacates, such as dibutyl sebacate. n the other Figure 2. (a) Collision-induced dissociation (product ion scan) and (b) precursor ion scan mass spectra of the chemical background ions at m/z 149, a fragment ion from phthalates; (c) product ion spectrum of the chemical background ions at m/z 315. hand, the observation of the fragment ions at m/z 121, 149 and 167 in the same CID mass spectrum indicates that a mixture of isobaric ions resulting from phthalates and sebacates may co-exist in this case. All other abundant product and precursor ions were also selected and studied in the similar way as for m/z 149 and 315 ions to draw a possible chemical/ structure relationship among them. As a result, a family tree of the phthalate- or sebacate-related ions in the API LC/MS chemical background studied above can be summarized in Scheme 1. Through this simple classification, it is sufficient to distinguish these ions from the others. ther chemical interference ions observed in the MS spectrum in Fig. 1 but not included in the family tree of the phthalate/sebacate-related ions can be grouped as similar series of background ions. For instance, starting with the relatively abundant ions at m/z 99, the product ion scan and precursor ion scan mass spectra (Figs. 3(a) and 3(b)) and the product ion scan mass spectrum of m/z 327 (Fig. 3(c)) indicates that m/z 99 is probably protonated phosphoric acid H 4 P þ 4 and most chemical background ions observed in these experiments are derived from phosphate esters such as triphenyl phosphate P(C 6 H 5 ) 3 (m/z 327 of the protonated molecule [MþH] þ ) one of the typical plasticizers present in plastic materials. Similarly, all other major chemical background ions are investigated and they are also summarized in Scheme 1 as the specific family trees of a series of ions, which can be found in the product ion scan and precursor scan experiments. They are adipates- 15 and silicone-related chemical interference ions in API LC/MS (Scheme 1). The five groups of ions, i.e. from phthalates, phenyl phosphates, adipates, sebacates and silicones, respectively, summarized in Scheme 1 still make the major contribution to chemical background noise in positive API LC/MS. In most cases, they cannot be avoided, especially the first three types of ions. However, as mentioned above, their relative abundances vary somewhat with the LC/MS conditions. Phthalates, phenyl phosphates and adipates are among the commonly applied plasticizer additives present in tubing, in materials for some parts of the ion sources, LC pumps and accessories, and in disposables used for sample preparation. They can also come from the LC mobile phase, i.e. present in solvents, additives and storage bottles, etc., due to contamination during manufacture, extended period of improper storage or handling. The summary of the family trees in Scheme 1 clearly illustrates the presence of a certain type of contaminant that contributes to chemical background noise. The identities of chemical background ions can assist us in the prevention of chemical interference during API LC/MS method development for trace analysis. Interestingly, the APCI study of the laboratory air (obtained without infusion or LC flow; the MS spectrum is not shown) results in an almost identical chemical background mass spectrum as the one shown in Fig. 1(a). This indicates that some chemical interference ions may result from air-borne contaminants, which may originate either from the ion source (such as the tubing, the sprayer, or the corona-discharge needle, etc.) or even from materials present in laboratory air. 16 Finally, it should also be noted that, within a family tree in Scheme 1, there are some ions (as indicated by the grey arrows) not directly related to that type of contaminants. These include: DI: /rcm

4 3148 X. Guo, A. P. Bruins and T. R. Covey CH n = 4 n = 6 n = 8 n = 8 CH 2 CH 2 H CH 2 CH CH 2 CH CH CH n n 2 n H 2 n C C (CH 2 ) 4 H C H 2 H H H C H H H 273 H H 3 3 H H H 279 H H C 4 93 C H 4 C 8 H m/z 149 C 4 H 205 H C=H C 4 H [390Na] [278Na] 135 C 6 H 5 phthalates related ions H H P H P H H m/z m/z H 129 CH phenyl phosphates related ions silicones related ions 221/239 3 Si Annotation: (1) Product ion scan; (2) Precursor ion scan; (3) Product / precursor ion scan; (4) Grey arrows: possible contribution of isobaric ions. sebacates related ions 419 m/z 73 CH3 Si or CH 3 Si CH 2 Scheme 1. Ions from contaminants adipates related ions Si 6 m/z (1) water adducts (clusters) to the basic ions of that contaminant, (2) sodiated species, and (3) some ions that are simply isobaric to the m/z value of ions in that group. Examples are m/z 157 (water þ m/z 139), 259 (water þ m/z 241), and 413 (sodiated dioctyl (or ethylhexyl) phthalate ester), etc., in the phthalates-related family tree; m/z 215 and 233 (related to m/z 251), 291 and 309 (related to m/z 327), etc., in the phenyl phosphate-related family tree. The identities of some of these ions are not investigated further in this study. They are included in Scheme 1 simply to indicate a kind of chemical relationship with the major background ions in LC/MS. Clusters as chemical background ions in API LC/MS solvent- and additive-specific chemical interferences Besides the common chemical background ions derived from the contaminants summarized in Scheme 1, there are some other chemical interferences observed when a combination of the specific HPLC solvents/additives is applied. They are mainly background ions resulting from solvation/clustering reactions of solvents and additives. Although the total contribution of this type of chemical interferences can be reduced by applying stronger desolvation by means of upfront CID or heated capillary, etc., or simply prevented by choosing a different mobile phase, they are present persistently and show up clearly in mass spectra of trace components. The contribution of these cluster-type ions to the chemical background in API LC/MS varies significantly with the mobile phase when compared with common chemical background ions derived from the contaminants discussed above. As a typical example, Fig. 1(b) shows the chemical background ESI LC/MS spectrum when the HPLC mobile phase MeH/H 2 /AcH (50:50:0.1%) was applied. The precursor ion scan mass spectra of the ions at m/z 99 and 159 are given in Fig. 4, which indicate that the chemical background ions at m/z 159 and 219, m/z 177 and 237, etc., are associated with protonated phosphoric acid H 4 P þ 4 (m/z 99) by formation of cluster ions with acetic acid (molecular weight of 60). The origin of H 3 P 4 is not clear. It may be present as an inorganic contaminant, or it may have been formed by degradation of phosphate plasticizers. In fact, many chemical interference ions in Fig. 1(b) are related to the acetic acid clusters, such as m/z, 143 and 203, m/z 101 and 161, m/z 121, 181 and 241, etc., and so on. A summary of these cluster ions is given in Scheme 2 to highlight some of these examples. It is shown that up to four neutral acetic acid molecules can be involved in the formation of a cluster DI: /rcm

5 V Characterization of chemical background ions in API LC/MS 3149 Figure 4. Precursor ion scan mass spectra of the chemical background ions (a) at m/z 99 and (b) at m/z 159, which indicate that these chemical background ions are related to phosphoric acid (m/z 99 of [MþH] þ ), by formation of cluster ions with acetic acid (molecular weight of 60) and water. Figure 3. (a) Collision-induced dissociation (product ion scan) and (b) precursor ion scan mass spectra of the chemical background ions at m/z 99and(c)theproductionscanmassspectrum of m/z 327, which indicate that these chemical background ions are derived from triphenyl phosphate P(C 6 H 5 ) 3. interference ion. n top of these cluster ions with acetic acid molecules, further clustering with neutral water molecules has also been observed. The pure-water cluster ions are often observed as low abundance chemical interferences. However, for the vast majority of the cluster ions studied here, the involved primary ions, which are as a nucleus for the cluster formation, are mainly those derived from contaminants, such as m/z 99, 85, and 73, etc. It is also shown that there is some contribution from the pure clusters of protonated/sodiated solvent/additive molecules, such as [nachþmh 2 þna] þ and/or [nacnþmh 2 þh] þ, depending on the solvents and additives used. In addition, there is evidence that some cluster-type background ions are actually formed by association of ions/neutral species crossing the groups of ions summarized in Schemes 1 and 2. Finally, it should be noted that there are clear differences in structures between the majority of the chemical background ions studied here and those conventional protonated molecules encountered in most API LC/MS applications, such as small molecular pharmaceuticals, environmental pollutants, food ingredients/flavors, as well as peptides, [H 4 P 4 nach mh 2 ] H H P H H m/z [99n*60mH 2 ], n = 2, 3; m = 1, 2 [nach mh 2 Na] [nach Na] [197n*60], n = 1, 2 [119n*60mH 2 ], n = 2, 3; m = 1, 2 m/z 73 or Si Si CH 2 nach mh Annotation: (1) AcH involved; 85 (2) H 2 involved; (3) Primary ions as a nucleus for clustering [115n*60mH 2 ], n = 2, 3; m = 1, [17360] 133 [73n*60mH 2 ], n = 2, 3 ; m = 1, 2 [115nH 2 ], n = 2, [121n*60], n = 2, 3, Scheme 2. Cluster ions involving CH and water. DI: /rcm

6 3150 X. Guo, A. P. Bruins and T. R. Covey proteins, etc. Two significant differences are concluded for the chemical background ions. First of all, there are the clusters (including sodiated molecules) with HPLC solvents, additives and/or between contaminants that are often involved in the contribution of some persistently observed chemical interference ions. This indicates that some weak interaction such as non-covalent bonding is involved in the structure of some chemical background ions. Secondly, the corresponding neutral molecules of the vast majority of the background ions resulting from the contaminants are chemically unstable (reactive) or contain non-conventional chemical bonds compared to protonated molecules. Examples are m/z 121, 149, 177 and m/z 205 from phthalates; and m/z 111, 129 and 241 from adipates, etc. Both of these differences in structures make the chemical background ions uniquely distinguishable where the relationship between the chemical reactivity and their structures is concerned. It has been indeed proven in our on-going study that major chemical background ions react with a reactive collision gas (chemical reagent) in mass spectrometry, while most analyte ions generated from pharmaceuticals are unreactive. 17 CNCLUSINS By drawing family trees of the chemical background ions in API LC/MS, the possible structures and origins of some typical chemical background cations are proposed. In agreement with the suggestions in the literature, the chemical background ions studied so far can be classified mainly as either stable (degradation fragments of) contaminants or cluster-related ions. Phthalates, phenyl phosphates, adipates and sebacates, as well as silicones, respectively, make the major contribution to chemical background noise in positive API LC/MS. These persistent contaminants may come from solvents, additives, columns, LC systems (pumps, tubing etc.), ion sources, and/or air-borne species in the laboratory atmosphere. These ions of contaminants can also serve as nuclei for the clustering of HPLC solvents or additives, such as water and acetic acid, thereby leading to a second family of background ions. Although this study does not permit a comprehensive summary of all chemical interference ions in API LC/MS, it provides useful information about what types of ions make a contribution to the chemical background in API LC/MS. This may provide information on how to prevent/reduce background ions during LC/MS analysis. In practical application for proteomics studies, chemical interferences can be avoided by starting MS acquisition from a high m/z value. However, this is not applicable to LC/MS of small molecules because the typical common chemical interference ions cover the range of mass-to-charge ratio up to ne interesting conclusion is the significant difference in structure between the major chemical background ions and the protonated analytes generated by electrospray ionization, which may provide a way of reducing chemical background noise. Acknowledgments This study is the result of a research contract between Applied Biosystems/MDS Sciex (Concord, N, Canada) and the University of Groningen. REFERENCES 1. Yamashita M, Fenn JB. J. Phys. Chem. 1984; 88: DI: /j150664a Fenn JB, Mann M, Meng CK, Wong SF, Whitehouse CM. Science 1989; 246: Covey TR, Lee ED, Bruins AP, Henion JD. Anal. Chem. 1986; 58: 1451A. 4. Product brochure: API 5000 TM LC-MS/MS system, Applied Biosystems/MDS Sciex: Foster City, CA, USA, Publication 114BR15-02, April lsen JV, de Godoy LMF, Li G, Macek B, Mortensen P, Pesch R, Makarov A, Lange, Horning S, Mann M. Mol. Cell. Proteomics 2005; 4: DI: /mcp.T MCP Marquet P. Therap. Drug Monit. 2002; 24: Mabic S, Regnault C, Krol J. LCGC Eur. 2005; 18: Williams S. J. Chromatogr. A 2004; 1052: 1. DI: / j.chroma Ende M, Spiteller G. Mass Spectrom. Rev. 1982; 1: 29. DI: /mas Zhang XK, Dutky RC, Fales HM. Anal. Chem. 1996; 68: DI: /ac960245n. 11. Schlosser A, Volkmer-Engert R. J. Mass Spectrom. 2003; 38: 523. DI: /jms Verge KM, Agnes GR. J. Am. Soc. Mass Spectrom. 2002; 13: 901. DI: /S (02) Kumar R. Am. Lab. 1999; 11: Yinon J. rg. Mass Spectrom. 1988; 23: 755. DI: / oms Guisto R, Smith SR, Stuart JD, Huball J. J. Chromatogr. Sci. 1993; 31: Emmett MR, Caprioli RM. J. Am. Soc. Mass Spectrom. 1994; 5: 605. DI: /S (03) Guo X, Bruins AP, Covey TR. Reduction of chemical background interferences in mass spectrometry (API LC-MS) using exclusive reactions with reagent gases. Poster presented at the 54th ASMS Conference on Mass Spectrometry and Allied Topics, Seattle, WA, May 28 June 1, DI: /rcm