A Protocol for High-Throughput Drug Mixture Quantitation: Fast LC MS or Flow Injection Analysis MS?

Transcription

1 60 LCGC VOLUME 19 NUMBER 1 JANUARY A Protocol for High-Throughput Drug Mixture Quantitation: Fast LC MS or Flow Injection Analysis MS? The authors demonstrate the pros and cons of fast liquid chromatography mass spectrometry (LC MS) versus flow injection analysis MS. They first developed a fast LC MS method applied to a six-drug model that required positive negative acquisition. They converted the fast LC method from a conventional LC method. By applying fundamental concepts of fast LC, such as using a small column (30 mm 2.1 mm, 2.6- m d p ) at an elevated temperature (40 C) and a higher flow rate (1.0 ml/min), they were able to reduce the LC cycle time from more than 20 min to 2.7 min. They repeated their experiments by taking away the LC column, which reduced the run time to 0.14 min; with this approach, all analytes entered the mass spectrometer at the same time. The authors studied the quantitation limits of this real-world assay model in both LC analysis and flow injection analysis to define and compare the limits of detection and quantitation, linearity, precision, and accuracy for each analyte. Kate Yu and Michael Balogh Waters Corp., 34 Maple Street, Milford, MA Address correspondence to K. Yu. In recent years, substantial efforts have been made to increase the number of lead compounds produced in drug discovery (1). Because of needs for rapid turnaround in discovery projects, the time allowed for method development is sharply decreased. Samples must be analyzed faster and more cheaply. High-throughput screening has become a routine aspect of drug discovery in most large, fully integrated pharmaceutical companies (2). In fact, it is a key component for the pharmaceutical lead identification process (3). As pharmaceutical research and development face increasing pressure to improve productivity and efficiency, high-throughput screening also must increase productivity and efficiency. These demands have resulted in a higher throughput of targets per year and samples per assay (4). This development in the pharmaceutical industry has led to an ever-increasing need in the field of analytical chemistry to develop techniques and methods that can analyze more samples faster and more cheaply (5). Of the diverse analytical techniques available today, mass spectrometry (MS) seems to be the most versatile tool for coping with the analytical demands of drug discovery (6). After years of research and development in analytical instrumentation, MS no longer is the expensive and specialized tool it once was, and it is rapidly becoming the detection method of choice in many areas, especially those in which sensitivity and specificity are important (7). Single-quadrupole MS is the method of choice in many cases because of the instrumentation s low cost and ease of use. The demands of speed in drug discovery combined with the fast detection capability of MS offer a new challenge for sample introduction techniques. Although liquid chromatography (LC), either on-column or flow injection analysis, is the most popular choice among various sample-input techniques and LC MS has been the dominating method of choice for most analysis, traditional LC protocol is insufficient to meet the demands of drug discovery. The current challenge is to separate sample mixtures in the shortest time possible without sacrificing too much of the separation integrity. In addition to the efforts made toward the

2 62 LCGC VOLUME 19 NUMBER 1 JANUARY development of fast LC protocol, flow injection analysis also has gained popularity in terms of coupling with MS because of its versatility, simplicity, reproducibility, high sampling frequency, low sample and reagent consumption, cost, and ease of implementation (8). A common debate centers on whether LC column and analyte separation are necessary at all in both a qualitative and a quantitative sense. MS offers both sensitivity and selectivity, often to a greater degree than UV detection. Therefore, eliminating separation of mixed analytes in time and detecting each by its unique mass should increase sample throughput. In this article, we demonstrate the pros and cons of fast LC MS and flow injection analysis MS in both the qualitative and the quantitative sense. We developed a regular LC MS method and applied it to a six-drug model. The model evaluates analytical diversity, including varying degrees of hydrophobicity and acid, base, and neutral behavior. We later converted this LC method to a fast LC method. By applying fundamental concepts of fast LC, we were able to reduce the cycle time from more than 20 min to 2.5 min. The analytes were detected by a singlequadrupole MS detector in electrospray mode by flow splitting. By applying on-thefly positive and negative polarity switching, all six analytes in the mixture were analyzed within one injection. We repeated the same experiments with flow injection analysis MS. We studied and compared the quantitation limits for both fast LC MS and flow injection analysis MS of this real-world assay model. Experimental We performed three sets of experiments for our study: traditional HPLC, fast LC, and flow injection. In the traditional LC segment, the experiment was performed using a Waters Alliance 2690 solvent-delivery module with a ZMD 2000 MS detector (both from Waters Corp., Milford, Massachusetts). The 50 mm 2.1 mm, 3.5- m d p LC column was operated at 30 C. The flow rate was 0.3 ml/min. All LC flow went into the MS detector without being split. The system and column equilibration times were incorporated into the gradient table. No precolumn volume was used for the method. In fast LC and flow injection, the work was performed on an Alliance HT 2790 solvent-delivery module with a ZMD 2000 MS detector (both from Waters). In the fast LC experiment, we used a shorter column (20 mm 2.1 mm) with a smaller particle size (2.6- m d p ) at an elevated temperature (40 C). The flow rate was 1 ml/min. This LC flow was split after the column between a model 996 photodiode-array detector (Waters) and the MS detector so that an approximately 200- L/min flow went into the MS detector. The system equilibration was performed with the column off-line at 5 ml/min for 0.2 min. The column equilibration was performed after the system equilibration and parallel with the sample loading. Therefore, the gradient table did not include the time needed for equilibration. We added a 100- L precolumn volume to the method. In the flow injection analysis experiment, we used the same parameters as in the fast LC experiment, except for removing the LC column. As a result of this change, no gradient was required for this experiment, the mobile phase was 50% solvent A, and the experiment was performed isocratically with no equilibration. Reagents: We purchased ammonium acetate and the standards of all six drugs amitriptyline, betamethasone, diphenhydramine, ibuprofen, naproxen, and prednisolone from Sigma Chemical Co. (St. Paul, Illinois). We obtained acetonitrile from J.T. Baker (Phillipsburg, New Jersey). Deionized water was made in our laboratory using a Milli-Q water-purification system (Millipore Corp., Bedford, Massachusetts). Instruments: We used Alliance 2690 and Alliance HT 2790 solvent delivery modules, both of which were equipped with model 996 photodiode-array detectors. The MS detector was a model ZQ single-quadrupole mass spectrometer (Waters). All instruments were controlled by MassLynx software (Micromass Inc., Beverly, Massachusetts). We used a model 11 syringe pump from Harvard Apparatus (Holliston, Massachusetts) for tuning the mass spectrometer. Experiment conditions: The conditions of traditional and fast HPLC and flow injection analysis are listed in Tables I and II. The MS conditions for traditional LC and fast LC are shown in Figures 1 and 2. Table I: HPLC and flow injection analysis conditions Traditional HPLC Fast HPLC Flow injection analysis Column type Waters Symmetry C18 Waters Xterra C18 Not applicable Column dimension 50 mm 2.1 mm 20 mm 2.1 mm Not applicable Particle size ( m) Not applicable Column temperature ( C) Not applicable Mobile phase A 10 mm ammonium acetate 10 mm ammonium acetate 10 mm ammonium acetate in water in water in water Mobile phase B 10 mm ammonium acetate 10 mm ammonium acetate 10 mm ammonium acetate in acetonitrile in acetonitrile in acetonitrile ph Flow rate (ml/min) Sample temperature ( C) Injection volume ( L) Precolumn volume ( L) Table II: Gradient times for traditional and fast HPLC and flow injection analysis Traditional HPLC Fast HPLC Flow injection analysis Gradient time (min) Mobile phase A (%) Mobile phase B (%) Curve number

3 64 LCGC VOLUME 19 NUMBER 1 JANUARY Prednisolone Diphenhydramine Betamethasone Naproxen Amitriptyline Ibuprofen Figure 1: MS conditions for traditional HPLC. Ionization: ESI or ESI ; source temperature: 150 C; desolvation temperature: 300 C; capillary voltage: 2.85 kv. Data were acquired via singleion monitoring. Figure 2: MS conditions for fast HPLC. Ionization: ESI or ESI ; source temperature: 130 C; desolvation temperature: 250 C; capillary voltage: 2.85 kv. Top bar: m/z 200, 361, 256, 393, 278. Bottom bar: m/z 200, 229, 205. Table III: Single-ion recording MS conditions for the six analytes Cone Dwell Time for Dwell Time for MS Detecting Voltage Traditional LC Fast LC Analyte Mode m/z (V) (s) (s) Prednisolone ESI Diphenhydramine ESI Betamethasone ESI Naproxen ESI Amitriptyline ESI Ibuprofen ESI In both sets of experiments depicted in these figures, the data collection mode of the MS detector was single-ion recording. Because all components were well separated in the traditional LC experiment, we set the MS detector so that each component had its own function. For each function, the MS detector was set at the m/z value that corresponded to the (M 1) or (M 1) of our analytes with the cone voltage specifically optimized for that compound. When moving from one function to another, the mass spectrometer would go through polarity switching. In the fast LC and flow injection experiments, the peaks of the components were too close together. Therefore, we set only one function for the MS detector. Within this function, the MS detector was set at six m/z values that corresponded to the (M 1) or (M 1) of our analytes, with polarity switching within the function. At each m/z value, the cone voltage was still set with the optimized value for each compound. The dwell times for each analyte in both methods are listed in Table III. The MS conditions for flow injection analysis were the same as for fast LC, except the run time was shortened from 1.4 min to 0.4 min. For fast LC and flow injection analysis, the flow was split downstream from the analytical column by a 1 16-in. PEEK tee (Upchurch Scientific, Oak Harbor, Washington). The 1.0-mL/min flow was split into two paths: one went into the photodiode-array detector, and the other one went into the mass spectrometer. The in. i.d. PEEK tubing was used for the connection between the splitter and the photodiode-array detector. A 75- m i.d. fused-silica capillary connected the splitter to the mass spectrometer. The length of the capillary was cut to adjust back pressure between the two delivery streams and to ensure that a 0.15-mL/min flow went into the mass spectrometer. Sample preparation: We prepared individual stock solutions of all six compounds at 1.0 mg/ml by dissolving the standard in methanol. These stock solutions were stored at 4 C. We diluted these solutions with water every day before use. Mobile-phase preparation: We prepared a stock solution with 500 mm ammonium acetate in water at ph 5.0 and stored it at room temperature. For mobile phase A, we diluted the solution with water every day before use. For mobile phase B, we diluted the solution with acetonitrile. Quantitation: In the quantitation of the drug mixture, the data were acquired by MS

4 66 LCGC VOLUME 19 NUMBER 1 JANUARY HO CH 2 OH C O OH HO CH 2 OH C O OH 205 [M H] O F H Betamethasone (MW 392) O H H Prednisolone (MW 360) 278 [M H] 279 CH O CH 2 CH 2 N(CH 3 ) [M H] CH 2 CH 2 N(CH 3 ) 2 Amitriptyline (MW 277) H 3 C H 3 C CH H 3 CO CH 3 Naproxen (MW 230) HCH 2 C Diphenhydramine (MW 255) COOH CH COOH CH 3 Ibuprofen (MW 206) [M H] [M H] 361[M H] m/z Figure 3: Structures of model analytes. Figure 4: Full-scan MS spectra of ibuprofen (ESI ), amitriptyline (ESI ), naproxen (ESI ), betamethasone (ESI ), diphenhydramine (ESI ), and prednisolone (ESI ). in single-ion recording mode. Figures 1 and 2 and Table III show the single-ion recording MS conditions for all six analytes. All calibration curves shown in this article were the results of six injections made for three consecutive days. Each day, we made two injections. Each injection came from a separate vial. Two sets of stock solutions were prepared, and they were alternated between points. The limits of detection and quantitation of each analyte were determined by measuring the signal-to-noise ratio (S/N ). For the limit of detection, S/N was 3. For the limit of quantitation, S/N was 10. Quality control samples were injected six times each day and repeated for three consecutive days. The within-day precision was indicated by intra-assay coefficient of variation, which was the average of three days results. The between-day precision was indicated by interassay coefficient of variation, which was determined by taking all 18 injections as one group of data. The between-day accuracy was indicated by the interassay error (%), which was determined by taking all 18 injections as one group of data. Results and Discussion MS behaviors of the model analytes: As shown in Figure 3, the six drug analytes possess different chemical functional groups, which have different behaviors in the mass spectrometer. Naproxen and ibuprofen contain carboxylic acid, so they will favor negative electrospray. Amitriptyline and diphenhydramine have amine functionality, so they will be ionized nicely in positive electrospray; however, betamethasone and prednisolone will not be ionized strongly either way. Even though theory can help in making predictions, the best experimental conditions for each analyte should be determined by tuning the MS detector with each analyte. Figure 4 shows the full-scan mass spectra of the six components. Among the six analytes, four favored electrospray positive and two favored electrospray negative. We were able to detect all six components within one run because the mass spectrometer and MS control software perform on-line polarity switching. Figure 5 shows the extracted mass chromatograms of the six components as a result of traditional HPLC separation protocol. Figure 6 shows the extracted chromatograms of the six components as a result of fast LC protocol. When performing polarity switching within runs, allowing enough time for the signal to be stabilized following the switch is crucial for optional ion detection statistics. For traditional LC (12-min Figure 5: Extracted mass chromatograms from a conventional LC MS analysis. Shown are ibuprofen (ESI, m/z 205), amitriptyline (ESI, m/z 278), naproxen (ESI, m/z 229), betamethasone (ESI, m/z 393.3), diphenhydramine (ESI, m/z 256), and prednisolone (ESI, m/z 361). acquisition), as shown in Figure 5, the peaks were all baseline resolved, which provided enough stabilization time between the switching (Figure 1). For fast LC (1.4-min acquisition), as shown in Figure 6, peaks were more narrow and closer to each other.

5 68 LCGC VOLUME 19 NUMBER 1 JANUARY For this reason, we designed the method to switch polarity for every scan throughout the whole LC run (Figure 2). To allow time for the changed polarity to stabilize, we added a 200-Da dummy mass to both modes. We also assigned a 0.01-s (for electrospray positive) or 0.02-s (for electrospray negative) dwell time for each analyte mass. This dwell time allowed the collection of enough data points for each peak. Development of fast LC protocol: The goal for developing a fast LC method is to shorten the LC cycle time that is, the time required from injection to injection as much as possible without sacrificing the separation integrity. Chromatographers must consider many factors and parameters to shorten the LC cycle time. A detailed discussion regarding the theory and fundamental principles of fast LC can be found elsewhere (9). It is always easier to shorten an existing LC method than it is to develop a fast LC method from scratch. Briefly, here s what we did to shorten the cycle time for our separation. Reduced the size of the LC column to reduce the LC run time and the column equilibration time: We used a shorter LC column with a smaller inner diameter packed with smaller particles. We switched from a 50 mm 2.1 mm column packed with 3.5- m particles to a 20 mm 2.1 mm column packed with 2.6- m particles. This change effectively decreased the run time by a factor of 10. Raised the column temperature: In LC separation, viscosity plays a major role. With Figure 6: Extracted mass chromatograms from a fast LC MS analysis. Shown are ibuprofen (ESI, m/z 205), amitriptyline (ESI, m/z 278), naproxen (ESI, m/z 229), betamethasone (ESI, m/z 392), diphenhydramine (ESI, m/z 256), and prednisolone (ESI, m/z 361). increased temperature, the viscosity decreases and subsequently analyte diffusion increases. In our case, we raised the column temperature from 30 C to 40 C. Increased the flow rate: It is obvious that higher flow rates will speed a separation, in keeping with the Knox equation (10). This effect is less significant than the other factors, however. In addition, system back pressure increases drastically, therefore shortening the column life significantly. Even though we were able to perform our separation at a 1.4-mL/min flow rate with more than 4000 psi back pressure, we chose 1.0 ml/min as the flow rate in our final protocol. This flow rate gave us sufficient results and allowed the system to work at much lower back pressure (approximately 2400 psi). Shortened the equilibration time: Allowing a system to be fully equilibrated to initial conditions after each injection is crucial in gradient LC. The system equilibration includes instrument equilibration (swept volume) and column (dimensional) equilibration. Common practice includes flushing an LC system with a mobilephase volume that is threefold the instrument volume and fivefold the column volume to fully equilibrate the system (9). Therefore, reducing LC column dimensions and increasing the flow rate reduces the column equilibration time significantly. In our case, the column equilibration time was reduced from 3.0 min to 0.34 min. Our solvent delivery module had an additional columnswitching valve that allowed the column to be switched on and off the flow path easily. Switching the column off the flow path and increasing the flow rate to 5 ml/min (from an analytical flow of 1 ml/min) caused the system to equilibrate in less than 0.2 min. Altogether, the system equilibration time was reduced from 9.2 min in traditional LC to 0.52 min in fast LC. Reduced the LC cycle time using parallel processing: In LC analysis, a full analysis cycle includes sample injection, needle wash, the actual gradient, system and column equilibration, and sample loading. The LC cycle time could be shortened if two or more of the above processes could be performed in parallel. We were able to perform injection and sample loading during the time taken for equilibration. The needle wash and sample draw were performed during the time the gradient was processed. The optimum target cycle time depends upon the purpose of the analysis. Shorter acquisition run times do not necessarily result in shorter total cycle times. The phrase faster is not necessarily better is especially true for quantitative work. An appropriate wash cycle is necessary to ensure minimal carryover. It takes time to wash the syringe needle, the injector valves, and the LC columns. It also takes time for samples to be drawn and loaded. These actions are performed simultaneously with the gradient in the parallel mode in the solvent delivery module. Therefore, gradient times must be long enough for the procedures to be completed. Otherwise, the instrument will halt all other activities to wait for these actions to finish, which yields no time-saving benefit. With our fast LC method, we were able to obtain separation within 1 min at 60 C and 1.4 ml/min, but we eventually chose 40 C and 1.0 ml/min as our final conditions, which provided a 1.4-min run time, because both protocols gave the same cycle time (2.7 min). The final conditions offered sufficient time for parallel processing to happen, produced less stress for the LC system, and resulted in separations with slightly better resolution. Quantitation was crucial to our successful method development, and minimizing carryover was a must. However, in the pursuit of fast LC speed, chromatographers may encounter potential problems that hamper both the speed of method development and the quality of the assays. Carryover from previous high-concentration injections affects sample measurements at lower concentrations. The sources of carryover arise from injector needles, injector valves, and HPLC columns. Using a needle wash with an appropriate solvent during injection can minimize the carryover from the needle (11). It also is important to incorporate a high-concentration organic solvent wash into the gradient to ensure that sample impurities do not accumulate on the column head. In our fast LC protocol, we have incorporated approximately 0.7 min of 100% organic solvent in the gradient table. Figure 7 and Table IV show examples of calibration curves and the quantitation results of the assay. As Figure 8 shows, we observed no carryover in using this protocol. In this case, the samples were running in the following sequence: a blank, a series of standards from low concentration to high concentration (1 ng/ml to 10 g/ml), and a blank. Figure 8 compares the lowest signal detectable for each analyte with the blank

6 70 LCGC VOLUME 19 NUMBER 1 JANUARY Table IV: Fast LC MS quantitation results* Limit of Limit of Intra-Assay Intra-Assay Intra-Assay Detection Quantitation Coefficient of Variation Coefficient of Variation Error (pg) (pg) R 2 (%) n 6 (%) n 18 (%) n 18 Prednisolone Diphenhydramine Betamethasone Amitriptyline Naproxen Ibuprofen * All precision and accuracy data were obtained at a low quality control level (3 limit of quantitation). The limits of detection and quantitation were calculated based on total injection amount. The flow was split in an approximate 1:7 ratio before the MS detector n 6 n Concentration (pg/ L) Concentration (pg/ L) Figure 7: Calibration curves from fast LC MS positive negative on-the-fly acquisition for amitriptyline (ESI ; correlation coefficient ; calibration curve: y x ; response type: external standard, area; curve type: linear, origin: exclude, weighting: null, axis trans: none) and naproxen (ESI ; correlation coefficient ; calibration curve: y x ; response type: external standard, area; curve type: linear, origin: exclude, weighting: null, axis trans: none). injected after the highest concentration of sample. Comparing fast LC MS with flow injection analysis MS for high-throughput quantitation: Figure 9 and Table V show the extracted mass chromatograms and the quantitation results of the flow injection analysis MS for the drug mixture model. Only five analytes were detected, even though the sample mixture contained six components. This outcome may be a result of ion suppression. Using fast LC MS, ionization-suppression reactions during the ionization of mixture are minimized, whereas in flow injection analysis MS, all components reach the MS detector at virtually the same time. The consequence is that certain compounds will be ionized preferentially because of competing proton affinities. Some compounds may not appear at all (8). Our drug mixture contained two weakly ionized compounds: prednisolone and betamethasone. In the flow injection analysis MS experiments, prednisolone produced extremely high limits of detection and quantitation compared with those of fast LC MS. Betamethasone never appeared in these experiments. Every quantitation parameter is better in fast LC MS analysis than in flow injection analysis MS analysis. If the task is compound identification or confirmation, even for qualitative screening, it is possible that some compounds will not appear in flow injection analysis MS. For quantitative analysis, fast LC MS provided much better results compared with flow injection analysis MS (Figure 10). Fast LC MS provided similar quantitative results when compared with conventional separation intended for UV detection Sample on column Blank Figure 8: The effect of carryover in fast LC MS for samples run in the following sequence: blank, series of standards from low concentration to high concentration (1 ng/ml to 10 g/ml), and blank. Shown are the lowest signal detectable for each analyte and a blank injection (injected after the highest concentration of sample) for ibuprofen (m/z 205, 10 pg on column), amitriptyline (m/z 278, 10 pg on column), naproxen (m/z 229, 10 pg on column), betamethasone (m/z 393, 50 pg on column), diphenhydramine (m/z 256, 50 pg on column), and prednisolone (m/z 361, 50 pg on column).

7 72 LCGC VOLUME 19 NUMBER 1 JANUARY For quantitation purposes, thorough system washing was necessary after each injection; therefore, even though flow injection analysis yielded shorter run times, its cycle time was not necessarily much shorter than that of fast LC (2.0 min in flow injection analysis versus 2.5 min in fast LC). The tradeoff is significant for the limited shorter cycle time Conclusion We were able to reduce the LC cycle time for a drug mixture from more than 20 min to 2.7 min by using a solvent delivery system and applying and coordinating the fast LC principles. We observed no carryover with this protocol. Quantitation results were obtained for both fast LC MS and flow injection analysis MS. With all conditions being equal, fast LC yielded better results, both qualitatively and quantitatively, than flow injection analysis MS. Acknowledgment The authors wish to thank Bonnie Alden of Waters Corp. for providing the Xterra columns. References (1) B.D. Dulery, J. Verne-Mismer, E. Wolf, C. Kugel, and L. Van Hijfte, J. Chromatogr. B 725, (1999). (2) R.M. Eglen, J. Biomol. Screen. 4, (1999). (3) M. Beggs, H. Blok, and A. Diels, J. Biomol. Screen. 4, (1999). (4) W. Stahl, J. Biomol. Screen. 4, (1999). (5) J. Zweigenbaum, K. Heinig, S. Steinborner, T. Wachs, and J. Henion, Anal. Chem. 71, (1999). (6) R.D. Submuth and G. June, J. Chromatogr. B 725, (1999). (7) D. Temesi, and B. Law, LCGC 17(7), (1999). (8) H.F. Schroder, Trends Anal. Chem. 15, (1996). (9) J. Li and J. Morawski, LCGC 16(5) (1998). (10) J.H. Knox, J. Chromatogr. Sci. 15, (1977). (11) Z. Liang, P. Weigl, T. Nieuwenhuis, D. Chang, and S. Bansal, Issues Covering Rapid Bioanalytical Method Development and Sample Analysis, paper number 1427, presented at the 46th ASMS Conference on Mass Spectrometry and Allied Topics, Orlando, Florida, 31 May 4 June Limit of detection (pg) Not detectable Limit of detection of fast LC MS Limit of detection of flow injection analysis MS Figure 9: Extracted chromatograms obtained using flow injection analysis MS for ibuprofen (ESI, m/z 205), naproxen (ESI, m/z 229), amitriptyline (ESI, m/z 278), diphenhydramine (ESI, m/z 256), betamethasone (ESI, m/z 393), and prednisolone (ESI, m/z 361). 0 Prednisolone Diphenhydramine Betamethasone Amitriptyline Figure 10: Fast LC MS versus flow injection analysis MS. Naproxen Ibuprofen Table V: Flow injection analysis MS quantitation results* Limit of Limit of Intra-Assay Intra-Assay Intra-Assay Detection Quantitation Coefficient of Variation Coefficient of Variation Error (pg) (pg) R 2 (%) n 6 (%) n 18 (%) n 18 Prednisolone Diphenhydramine Betamethasone Amitriptyline Naproxen Ibuprofen * All precision and accuracy data were obtained at a low quality control level (3 limit of quantitation). The limits of detection and quantitation were calculated based on total injection amount. The flow was split in an approximate 1:7 ratio before the MS detector.