Anal. Chem. 1994,66, 4437-4443
Determination of N=Nitrosodimethylaminein Complex Environmental Matrices by Quadrupole Ion Storage Tandem Mass Spectrometry Enhanced by Unidirectional Ion Ejection Jeffry Blaise Plomley, Carolyn Jean Koester, and Raymond Evans March* Department of Chemistry, Trent University, Peterborough, Ontario, Canada, K9J 788
A gas chromatography (GC)/tandem mass spectrometry method using a quadrupole ion storage mass spectrometer (QISMS, trademark of Varian Associates Inc.) operated in chemical ionization (CI) mode has been developed for the determination of N-nitrosodimethylamine(NDMA) in complex environmental matrices. Using a customized scan function, the method allows for the simultaneous storage of [M HI+ ions of both NDMA and NDMA-ds. Collisionally activated dissociation of both native and deuterated [M + HI+ ions, carried out by consecutive resonant excitation of each isolated ion species, permits daughter ion quantitation based on an internal standard. The method is capable of subpicogram detection limits when unidirectional ejection of stored daughter ions from the ion trap toward the electron multiplier is effected by the superimposition of a dipole field applied across the end-cap electrodes. Enhanced sensitivity was obtained by operating the electron source at a higher than recommended filament emission current, and by setting the electron multiplier to a voltage greater than that required for a gain of lo5. Background interferences were eliminated via the implementation of a low-mass radio frequency (1-9sweep and positive direct current (dc) amplitude application during [M + HI+isolation. In CI mode with automatic reaction control (ARC) disabled, linear calibration plots were obtained over a concentration range of 0.5-128 pg. In contrast, when ARC is enabled, calibration plots over a concentration range 0.5-2000 pg are characterized by polynomial curve-fitting equations. Concentrations of NDMA in aqueous extracts were found to be comparable to those obtained by high-resolution mass spectrometry. Interferencessuch as chlorobenzene, ethylbenzene, and the 0-, m-,and p-xylenes, reported when NDMA concentrations are determined by gas chromatographic separation followed by low-resolution mass spectrometty, were not detected using the GC/QISMS protocol.
+
N-Nitrosodimethylamine (NDMA), Chemical Abstracts Service Registry number 62-75-9, is a well-known car~inogen.~-~ NDMA is both naturally occurring and anthropogenic and may often be formed under acidic conditions by the reaction of amines with (1) Freund, H. A A n n . Int. Med. 1937,10,1144. (2) Magee, P. N.; Barnes, J. M. Adu. Cancer Res. 1967,10,163. (3) L4RC Monogr. Eval. Carcinog. Risk Chem. Man 1972,1,85.
0003-2700/94/0366-4437$04.50/0 0 1994 American Chemical Society
nitrites, particularly in foods such as malt b a r l e ~ ,cured ~.~ meats,+12 and fish products.13 Drinking water14-16has also been found to contain NDMA. Because of its carcinogenicity, NDMA in food products and water is of great concern. The presence of NDMA in water merits careful monitoring because, unlike other food products, water consumption cannot be avoided. For this reason, the Province of Ontario has set a drinking water guideline of 9 ppt for NDMA.I7 As a result of this stringent guideline, the Ontario Ministry of Environment and Energy (MOEE) analyzes some 1000-1500 water samples per year for NDMA. Such an operation requires a specific and sensitive technique to detect trace levels of NDMA. Current methods for NDMA determinations employ GC with either thermal energy analy~ers~-~ or nitrogen/phosphorus detect o r ~ . ' ~ The J ~ use of gas chromatography/mass spectrometry (GC/MS) is preferred because MS increases method specificity while data produced by GC analysis alone are often ambiguous. Several methods for NDMA determinations use GC/low-resolution MS.19-21 However, some situations may arise in which lowresolution MS fails to provide the specificity required to document (and prosecute) NDMA guideline exceedances.22 Hence, high(4) Sen, N. P.;Seaman, S. J-Assoc. Off: Anal. Chem. 1981,64,933-8. (5) Frommberger, R Food Chem. Toxicol. 1989,27,27-9. (6) Scanlan,R A; Barbour, J. F.; Chappel, C. I.]. Agric. Food Chem. 1990,38, 442-3. (7) Poocharoen, B.; Barbour, J. F.; Libbey, L. M.; Scanlan, R A]. Agric. Food Chem. 1992,40,2216-21. (8) Yoo, L. J.; Barbour, J. F.; Libbey, L. M.; Scanlan, R A]. Agric. Food Chem. 1992.40,2222-5. (9) Havery, D. C.; Fazio, T.; Howard, J. W. ].-Assoc. Ojf Anal. Chem. 1978, 61,1374-8. (10) IR4C Sci. Public 1980,31,361-76. (11) Fiddler, W.; Pensabene, J. W.; Gates, R k;Hale, M.; Jahncke, M. ]. Food Sci. 1992,57,569. (12) Pensabene, J. W.; Fiddler, W.J Food Saf 1993,13,125-32. (13) Takatsuki, K; Kikuchi, T. ]. Chromatop. 1990,508,357-62. (14) Fine, D. H.; Rounbehler, D. P.; Huffman, F.; Garrison, A W.; Wolfe, N. L.; Epstein, S. S. Bull. Enuiron. Contam. Toxicol. 1975,14, 404-8. (15) Nikaido, M. M.; Dean-Raymond, D.; Francis, A J.; Alexander, M. Water Res. 1977,11, 1085-7. (16) Kimoto, W. I.; Dooley, C. J.; Carre, J.; Fiddler, W. Water Res. 1981,15, 1099-106. (17) Ontario Drinking Water Objectives, 5th ed.; Ministry of Environment and Energy: Ontario, Canada, in press. (18) Scharfe, R R; Mclenaghan, C. C. ].--ASSoc. Ojf Anal. Chem. 1989,72, 508-12. (19) T i i o n s , L.; M u d , E.; Onstot, J.; Brown, R; Cannon, M.; Ameson, D. J Anal. Toxicol. 1988,12,117-21. (20) Lesage, S. Fresenius]. Anal. Chem. 1991,339,516-27. (21) Vo, A/. High Resolut. Chromatogr. 1992,15,552-4. (22) Taguchi, V. Y.; Reiner, E. J.; Jenkins, S. W. D.; Wang, D. T.; Palmentier, J. P.; Robinson, D.; Kleins, R J.; Ngo, IC P. Proceedings ofthe 38th ASMS Conferenceon Mass Spectrometry and Allied Topics;Tucson, AZ,1990; p 623.
Analytical Chemistry, Vol. 66, No. 24, December 15, 1994 4437
resolution MS has been adopted by the MOEE as the standard method for NDMA determination~.~2,~~ While the quality of data produced by high-resolution MS is excellent, the use of this technique for NDMA analysis is disadvantageous for several reasons. First, high-resolution MS instruments are costly to purchase, to maintain, and to staff. In addition, as the resolution required to separate NDMA from interferents is only 7000,22,23 the capabilities of the high-resolution MS are underutilized. Fortunately, high-resolution MS is not the only option available for obtaining analytical specificity;MS/MS also greatly increases analyte specificity. In addition, chemical ionization (CI) yields greater specificity than does electron impact (EI)ionization and often provides higher sensitivity. For this reason, we have chosen to combine MS/MS with CI and to use a relatively inexpensive quadrupole ion storage mass spectrometer (QISMS, trademark of Varian Associates Inc.) for NDMA determinations. Recent developments in QISMS technology, for example, the implementation of a waveboard generator for the construction of userdelined scan functions,24-26 have improved operational sensitivitywith the result that the QISMS is a powerful instrument for environmental analysis. Previously, we have shown that MS/MS by QISMS can be used to quantify subpicogram quantities of tetrachlorodibenzop-dioxin in environmental samples.27 However, in this previous study we were unable to use conventional 13C-labeled internal standards for quantitation. We have since overcome this limitation and have used successfully a customized scan function to store simultaneously protonated NDMA and NDMA$G and to perform collision-activated dissociation (CAD) of both isolated analytes via consecutive resonant excitation. Furthermore, enhanced sensitivity toward the target analyte is realized upon the superimposition of a radio frequency ($-generated dipole field, applied across both end-cap electrodes. A rather simplistic approach to our understanding of this behavior is that the superimposed dipole field has the effect of moving the ion cloud coherently from the center of the ion trap toward the end-cap electrode nearer the detector. The effect of the dipole field on the trapping potential well is to reduce the well potential in the direction of this electrode. While such a procedure has been reported recently by Marquette and WangZsas a means of achieving unidirectional ion ejection, this is the first account of the application of this technique in an analytical protocol as a means of increasing signal-to-noise (S/N) ratios at subpicogram levels of analyte. In addition to S/N enhancement via unidirectional ejection, we demonstrate further the utility of CI-MS/MS on QISMS for the analysis of NDMA. The significanceof this work is that it provides a simple, sensitive, and selective detection scheme for NDMA. Thus, in our opinion, tandem mass spectrometric operation of the ion trap mass (23) The Determination of N-Nitrosodimethylamine (NDMR, in Drinking Water and in Aqueous Samples by Gas Chromatogra$hy/High Resolution Mass Spectrometry; LSB Method MSABN-E3291A; Ministry of Environment and Energy: Etobicoke, ON, Canada, 1993. (24) Shaffer, €3. A: Kamicky, J.; Buthill, S. E. Jr. Proceedings of the 4Ist ASMS Conference on Mass Spectrometry andANied Topics; San Francisco, CA, 1993; p 468a. (25) Tucker, D. B.; Hameister, C. H.; Bradshaw, S. C.; Hoekman, D. J.; WeberGrabau, M. Proceedings ofthe 36th ASMS Conference on Mass Spectrometry and Allied Topics; San Francisco, CA 1988 p 628. (26) Schachterie, S.; Brittain, R Proceedings of the 4lstASMS Conferenceon Mass Spectrometry and Allied Topics; San Francisco, CA, 1993; p 638a. (27) Plomley, J. B.: Koester, C. J.; March, R E. Om.Moss Spectrom. 1994,29, 372-381. (28) Marquette, E.; Wang, M. Proceedings of the 41st ASMS Conference on Mass Spectrometry and Allied Topics; San Francisco, CA 1993; p 698a.
4438 Analytical Chemistry, Vol. 66, No. 24, December 15, 1994
spectrometer will become increasingly important in the future, not only for the analysis of NDMA, which continues to be the subject of extensive toxicologicalr e ~ e a r c h ~and *~O environmental concern, but of other environmental contaminants as well. It is of interest to point out that, up to the present time, ion trap MS/MS capabilityhas been available commercially only with the research version of the ion trap, that is, with the Fmnigan MAT ion trap mass spectrometer (ITMS). Normally, such instruments do not use a gas chromatograph as the inlet. The novelty of the contribution of the QISMS instrument is that it combines a gas chromatographic inlet together with appropriate software, which has permitted operation of the ion trap as a tandem mass spectrometer, and the entire instrument is under computer control. The advantage of having both the GC and the MS/MS under common computer control becomes very clear when multiple scan functions are used within the same GC run. In this manner, quantitation based on tandem mass spectrometric production of daughter ions is possible when one is using an internal standard that coelutes with the native analyte. EXPERIMENTAL SECTION Instrumentation. All experiments were carried out on a modified Varian Saturn 111 GC/QISMS equipped with a Varian waveform generator, Varian 8200 autosampler, and Varian septum programable injector (SPI) with a high-performance insert. Unidirectional ion ejection was achieved by the application of a superimposed rf dipole field across both endcap The dipole field operates at the same frequency as the trapping field. Saturn Revision C software was used for data acquisition in CI mode, while a prototrpic software package (QISMS version 1.0) was employed for the construction of custom scan functions required for ion isolation and CAD of target analytes. Scan function segments were inserted between ionization and the automatic reaction control (ARC) algorithm of the Saturn software. The ion trap was operated in mode 1131 (Le., with the end-cap electrodes grounded) with dipolar resonance ejection at a fixed frequency of 485 kHz (which the superimposed dipole field does not affect24 to give mass-selective ejection at qz x 0.89. Under such conditions ion motion can be described by second-order differential equations, the stable solutions to which map an ion’s trajectory in (uz, 41) space.32 The dimensionless parameters uz and qn are equated as follows:
and
where Uand Vrepresent the amplitudes of the dc and rf potentials applied to the ring electrode, respectively, r, is the radius of the ring electrode (10.00 mm), z, is half the separation of the end(29) Peto, R; Gray, R; Brantom, P.; Grasso, P. CancerRes. 1991,51,6415-51. (30) Stohrer, G. Cancer Res. 1993,53,4107. (31) Bonner, R F. Int. 1.Mass Spectrom. Ion Phys. 1977,23, 249.
(32) March, R E.; Hughes, R J. Quadrupole Storage Mass Spectrometry: Chemical Analysis Series 102; John Wiley and Sons: New York, 1989.
Table 1. Custom Scan Function for the Isolation of the [M H]+ Ions of NDMWDMA-4 Followed by Consecutive CAD of [M H]+ Ions
+
+
scan segment 1 2 3 4 5 6
rf vo-PI
40000 1000 2000 1000 20000 1000 20000
7
axial
length (us) initial rf final 80 80 450 460 211 211 228
modulation waveform
80 450 460 211 211 228 228
Off
on on Off Off
Off Off Off Off
CADU
Off
Off
Off
CAD"
The CAD waveform is a singlefrequency sine waveform of 153.5 kHz with an amplitude of 0.8 Vp-p.
cap electrodes (7.83 mm), Q is the angular frequency of the rf drive potential (1.05 MHz), e is the electronic charge, and m is the mass of the ion. The fundamental axial secular frequency, oz,of ion motion within the ion trap is described by
(3)
w, = PzQ/2
where pz is a function of the stability parameters a, and q,.32 Ion Isolation and CAD. The individual component segments that make up the scan function for the initial ion isolation of m/z 75 ([M HI+ of NDMA) and 81 ([M HI+ of NDh%ds), and subsequent consecutive CAD to m/z 43 and 44, and m/z 46 and 49 from m/z 75 and 81, respectively, are shown in Table 1. The rf amplitude was set so as to store all reagent ions formed during ionization; that is, the low-mass cutoff was 13 amu. The duration of the first scan function segment corresponds to the optimal reaction time when the acquisition occurs with ARC disabled. With ARC on, the 40 ms time segment corresponds to the maximum reaction time set in the Saturn CI/ARC parameter menu. The ARC algorithm is based on the relationship expressed in eq 4.
+
+
actual reaction time = (max reaction time) (ionization time) (4) max ionization time Clearly, when ARC is disabled and the maximum ionization time is set equal to the actual ionization time, the input value for the maximum reaction time becomes the actual reaction time the ARC algorithm will use. The optimum reaction time with ARC disabled was found to be 40 ms. In scan segments 2 and 3 (Table l),the rf potential is ramped to sweep out low-mass ions up to and including m/z 73 while applying an axial modulation voltage of 4.0 Vp-p The rf amplitude is then decreased to 211 Vo-p, thus moving the working point for m/z 75 to q, x 0.40 whereupon a singlefrequency waveform, CAD, of 153.5 kHz, with an amplitude of 0.80 Vp-p,is applied for 20 ms; 153.5 kHz corresponds to the fundamental axial secular frequency of m/z 75 at qz x 0.40. The CAD waveform is then applied for a further 20 ms in scan segment 7, but at an rf storage amplitude of 229 Vo-p,by which the working point for m/z 81 is moved to q, = 0.40. The scan function was optimized by using a 175 pg/pL solution each of NDMA and NDMA-d6in methylene chloride (see NDMA Calibration section below). For the analysis of NDMA in environmental matrices, a positive dc voltage was inserted in scan
segments 2 and 3 (Table 1) so as to eject axially high-mass matrix ions of m/z > 85 at the p, = 0 stability boundary. In segment 2, the dc voltage was increased from 0 to 3 V for 1000 ps while the rf amplitude was maintained at 80 Vo-p In segment 3, the dc voltage was further increased to 3.2 V for 2000 ps and then returned to 0 V over a period of 1000 ps; the rf amplitude was maintained at 80 Vo-p throughout. Application of the positive dc voltage was observed to have no effect on the conversion efficiencies for CAD of NDMA and NDMA-ds. Gas Chromatography. A 30 m DB-1701 14% (cyanopropy1phenyl)methylpolysiloxane (l& W Scientific, Folsom, CA) column with a 0.25 mm i.d. and 0.25 pm film thickness was used for all analyses. Helium flow was adjusted to give a column head pressure, in conventional units, of 13.5 psi. For acquisitions of NDMA calibration data, the GC was held at 38 "C for 1 min and then ramped to 120 "C at 15 "C/min. The transfer line temperature from the gas chromatograph to the QISMS was held at 220 "C while the QISMS manifold temperature was 170 "C. The SPI injector was used in the solvent flush injection mode with a 2 pL methylene chloride solvent plug, 0.5 pL upper air gap, and 0.8 pL lower air gap. A 2 pL aliquot of standard analyte solution containing equimolar concentrations of NDMA and NDMA-ds in methylene chloride was injected at a rate of 1pL/s. The injector was held at 25 "C for 0.1 min, ramped to 250 "C at 200 "C/min, and then held at 250 "C for 5 min. For the determination of NDMA concentrations in environmental matrices, the GC was held at 38 "C for 1 min, ramped to 150 "C at 15 "C/min, then ramped to 250 "C at 30 "C/min, and held at 250 "C for 20 min. The injector was held at 25 "C for 0.1 min, ramped to 250 "C at 200 "C/min, and held at 250 "C for 30 min. QISMS Operating Parameters. The ion trap was calibrated using FC-43 (perfluorotributylamine) in E1 mode. Methane reagent gas (Matheson ultrahigh purity, Whitby, ON) was introduced into the ion trap via a needle valve such that the ratio of the signal intensities of m/z 17 (CHs+) to 16 was 1O:l and that for m/z 17 to 29 (C2H5+) was 1:l. With this procedure, the pressure of methane in the ion trap is approximately (1-2) x Torr. Since reagent ion density is proportional to reagent gas pressure, this tuning procedure was adhered to strictly. In ARC mode, the maximum ionization time was set to 2000 ps and the maximum reaction time to 40 ms. The ARC prescan target was set to 5000 ions. During ionization, the low mass cutoff was 5 amu; this value was raised to 13 amu for the reaction period between reagent gas ions and target analyte. The reagent ion ejection parameter (i.e., that mass value which is greater than the mass of the largest reagent ion produced by the selected reagent gas, such that all masses below the set value are ejected after the reaction period) was 45 amu. With ARC disabled, the actual ionization and maximum ionization times were set to 2000 ps, while the reaction time, set via the maximum reaction time parameter, was 40 ms. All other operating conditions were identical to those employed when ARC was used. The filament emission current was set to an optimal 100 pA while an electron multiplier voltage of 1850 V was required to achieve a gain of lo5 during E1 tuning. The electron multiplier voltage was increased by a further 200 V while operating the ion trap in CI mode, so as to increase instrumental sensitivitytoward subpicogram quantities of analyte. All acquisitions were obtained over a scan range of 35-90 amu at a scan rate of 1.00 s/scan. Each scan is composed Analytical Chemistry, Vol. 66, No. 24, December 15, 1994
4439
of eight microscans and represents that sum average. A filament/ multiplier delay (i.e., the time delay between analyte injection onto the column and the simultaneous activation of the filament and multiplier voltages) of 240 s prevented the detection of solvent. For the analytical scan, the QISMS was operated in the massselective instability mode at an axial modulation voltage of 4.0 VP-V
NDMA Calibration. All NDMA and NDMA$G standards and sample extracts were obtained from the MOEE. Seven calibration standards of 0.50,2.01,8.03,32.11,128.44,513.75,and 2055 pg/2 p L in methylene chloride, each containing equimolar concentrations of NDMA and NDIvlA-d,3,were used to construct calibration curves following the method of Taguchi et The main use of these standards was to determine the shapes of the calibration curves for both NDMA and NDMA-d6. All environmental samples were spiked with 12.6 ng of NDMA-~G and represented a 0.8 L water sample. When dilution of a sample was necessary, the aliquot was always spiked with the same amount of internal standard so as to obviate the necessity for further demonstration of linearity of response between analyte and internal standard. The criteria for positive NDMA identification in environmental matrices were as follows: (1) retention times to within ~ t s3 of that obtained with standard NDMA solutions, (2) detection of m/z 44 and 46 formed via the loss of hydrazine and/or methanol, and loss of HNO and/or NOH, respectively, and (3) the S/N ratio of the m/z 44 daughter ion to exceed 3:l. The m/z 46 daughter ion of the ds analogue served as an internal standard for NDMA analysis. Less abundant daughter ions of m/z 43 (from m/z 75) and 49 (from m/z 81) were used as secondary contimatory ions. All peak integrations were performed manually. Finally, to ensure that no carry-over of analyte had occurred between acquisitions, methylene chloride was injected after each sample had been analyzed. In these blank runs, no NDMA carryaver was detected. Interferences. Solutions containing 200 pg/pL each of chlorobenzene, ethylbenzene, and the 0-, m-, and pxylenes in methanol were purchased from Supelco Chromatography Products (Toronto, ON). A 1 pL (200 pg) sample of each solution was injected under the operating conditions described above when ARC was disabled. RESULTS AND DISCUSSION Ion Isolation and CAD. The isolation efficiency of m/z 75 ([M HI+ of NDMA) and 81 ([M HI+ of NDMAdd, as defined by the percentage of ion intensity following isolation relative to that prior to isolation, was some 98%. A typical mass spectrum of the isolated [M HI+ ions each of NDMA and NDh4A-d,3is given in Figure IA.Figure 1B illustrates the first application of CAD to isolated m/z 75 (scan segment 5 in Table 1) giving predominantly m/z 43 and 44 daughter ions; only 1% of m/z 75 remains undissociated and the signal intensity of m/z 81 is unperturbed. This latter mass spectrum was obtained using a filament emission current of 10 pA; the relatively low conversion efficiency is discussed below. Figure 1C represents the situation in which CAD is applied consecutively to, initially, m/z 75,and then to m/z 81. Clearly, m/z 46 is the more abundant of the two predominant daughter ions (Le., m/z 46 and 49) arising from CAD of m/z 81, and therefore, this ionwas used for quantitation. The conversion efficiency for consecutive CAD (with a filament emission current of 100 pA, an electron multiplier voltage of 2050
+
98
t
188
I4 I1
Ib
1
ie
19
+
+
(33)Taguchi,V.Y.; Jenkins, S. W. D.; Wang,D. T.; Palmentier, J.-P.F. P.; Reiner, E. J. Can./. Appl. Spectrosc., in press.
4440 Analytical Chemistry, Vol. 66, No. 24, December 15, 1994
B
Figure I.(A) Representative mass spectrum generated in chemical ionization mode using a customized scan function (segments 1-3 in Table 1) for the isolation of the [M H]+ ions of NDMA (m/z75)and NDMA-4 (m/z 81). (B)Generation of daughter ion fragments, m/z 43 and 44,from m/z 75 via resonant excitation at 153.5 kHz (0.80 Vp-p). (C) Consecutive resonant excitation of m/z 75 and 81 at qr = 0.40.
+
V, an ionization time of ZOO0 ,us, and a reaction period of 40 ms) of the isolated parent ions was 67% for the process m/z 75 m/z 43 m/z 44,and 81% for the dissociation m/z 81 m/z 46 m/z 49. In contrast, conversion efficiencies of only 54% for m/z 75 m/z 43 m/z 44,and 70% for m/z 81 m/z 46 m/z 49
- -
+
-
+
-
+
+
Table 2. Comparison of S/N Ratios Obtalned from Extracted Daughter Ion Mass Chromatograms of NDMA and NDMA-& at Filament Emission Currents of 10 and 100 pA
m/z
filament emission current @A)
S/N
m/z
10 100
3023 5474
46
44
+
+
NDMA concn" (pg/pL)
filament emission current @A)
S/N
10 100
3887 7333
were observed when the recommended filament emission current of 10 pA was used. These differences in conversion efficiencies may be attributed to space charge effects that occur at higher concentrations of analyte. To explain, as the concentration of [M HI+ ions in the trap increases as a function of the filament emission current (10-100 PA), the fundamental axial secular frequency shifts. As a result, the applied irradiating resonant frequency may not adequately match the ion axial secular frequency. Such frequency mismatches may account for the differences observed in the conversion efficiencies when the filament emission current is set to 10 p A as opposed to 100 pA Notably, the CAD waveform (Table 1) was optimized by using a 175 pg/pL solution each of NDMA and NDMA-ds. The innuence of the filament emission current on the enhancement of instrumental sensitivity can be appreciated when the S/N ratios in Table 2 are compared for the extracted daughter ion chromatograms from a mixture of NDMA and NDMA-ds. With a filament emission current of 10 pA, the S/N ratios for m/z 44 and 46 are 3023 and 3887, respectively. When the filament emission current was augmented by 9OpA however, the S/N ratios for m/z 44 and 46 increased to values of 5474 and 7333, respectively. These latter results were obtained with ARC disabled, a reaction time of 40 ms, and an ionization time of 2000 ps. Interestingly, when ARC is employed with a filament emission current of 100 pA, an ionization time of only 165 ps is used, which appears to indicate that an ionization time of 2000 ps with ARC disabled is excessively long. However, such a long period of ionization is required to detect 500 fg of target analyte, and since the ionization time cannot be changed continuously between acquisitions of differing concentrations of analyte when ARC is disabled, 2000 ps is the minimum ionization time that must be used if low concentrations of NDMA and NDMA-ds are to be detected. When ARC is active, both the actual ionization time and the reaction time itself are diminished at low NDMA concentrations, more so than would otherwise be required for trace detection. Since there are ions in the trap other than target ions, the 100 ps ARC prescan sets a larger than required peak area. The result is an ionization time and reaction time based on the entire ion ensemble and not only the target ions. Furthermore, there is difficulty with nonlinearity when constructing calibration plots from data obtained in CI/ARC mode (see below). Besides the ability to increase the S/N ratio of target analyte by increasing both the filament emission current and the electron multiplier voltage (the latter to a value exceeding that required to give a gain of lo5), we have found that the superimposition of a dipole field on the already-existing quadrupolar field increases the S/N ratios of the [M HI+ ions from NDMA and NDMA-&, at all analyte masses injected including, most importantly, the 500 fg level. Initially, signal enhancement was investigated by use of the [M HI+ ions, m/z 75 and 81. Table 3 illustrates the effects
+
Table 3. Effects of Unidirectional Ejection on S/N Ratio, Peak Area, and Peak Height for ndz 75 and 81
8.03b 8.03c 32.11b 32.1lC
ra ratio
peak area
peak ht
m/z75
m/z81
m/z75
m/z81
m/z75
m/z81
36
41 94 89 162
3574 4884 10037 13685
4054 4929 10138 13486
1983 3500 5092 9566
1894 3023 4311 9182
64 59 122
Results are based on triplicate injections at each concentration. Data were acquired under those conditions described when ARC is disabled. Only the first three segments from Table 1 were inserted between the ionization period and the ARC-algorithm. Bidirectional ejection. Unidirectional ejection.
of unidirectional ejection upon S/N, peak height, and peak area of the parent ion extracted chromatographic peaks, at two easily detected concentration levels. Although the application of the superimposed dipole field increases peak height and S/N, it has a less marked effect on peak area. The fact that S/N ratios were shown to increase when the [M HI+ ions were examined under conditions of unidirectional ejection makes it unlikely that increases in detected daughter ion intensity upon application of CAD (Table 1) are due to some other artifact. Hence, Figure 2A demonstrates the S/N ratio obtained on a 500 fg injection each of NDMA and NDMA-ds without unidirectional ejection, while Figure 2B, in contrast, shows data acquired with the superimposed dipole field. It is evident that the increase in the S/N ratio observed upon application of the dipole field more clearly delineates extracted daughter ion peak shapes from the surrounding background, making manual integration more simple. Calibration Plots. Calibration plots comprising a mass range of 0.5-2055 pg were constructed with ARC disabled and were found to exhibit a plateau effect when the mass of analyte injected exceeded 500 pg. However, calibration curves obtained for the mass range of 0.5-128 pg were approximatelylinear. Concentrations of NDMA found in environmental matrices usually fall into this latter concentration range. For concentrations of NDMA in environmental matrices that fall outside of the linear portions of the calibration plot, a simple dilution is all that would be required. The plateau effect exhibited at high concentrations of analyte can readily be explained when one considers the fact that, with ARC disabled, the ionization time is fixed at 2000 ps (the minimum time required to detect 500 fg with ARC disabled). Therefore, as the concentration of analyte increases, the point is reached where the complete consumption of a fixed number of reagent ions occurs and a plateau is observed in the calibration curve. When calibration plots were constructed over a mass range of 0.5-2055 pg with ARC enabled, deviations from linearity were observed between 0.5 and 128 pg. However, since the ionization time and reaction time vary with concentration when ARC is used, the plateau effect exhibited at high concentrations when ARC was disabled was not observed. Although active sites present in the chromatographic system may contribute to some degree toward nonlinearity, particularly at low concentrations of analyte, this effect is not observed when ARC is disabled. The source of nonlinearity is, therefore, likely due to detector response. Since the internal standard coelutes with the native analyte, this may effect the degree of nonlinearity. To explain, because the internal standard is observed to undergo CAD with a greater efficiency than the native analyte, and because the m/. 46 ion has a greater
+
Analytical Chemistry, Vol. 66, No. 24, December 15, 1994
4441
I
L
'I
I i
,
I
