determination of n-nitrosodimethylamine in complex environmental matrixes by quadrupole ion storage...

Anal. Chem. 1994,66, 4437-4443

Determination of N=Nitrosodimethylamine in 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 elimi- nated via the implementation of a low-mass radio frequency (1-9 sweep 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. Interferences such 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 water14-16 has 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 detec- t o r ~ . ' ~ J ~ The 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 low- resolution 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,

(7) Poocharoen, B.; Barbour, J. F.; Libbey, L. M.; Scanlan, R A]. Agric. Food

(8) Yoo, L. J.; Barbour, J. F.; Libbey, L. M.; Scanlan, R A]. Agric. Food Chem.

(9) Havery, D. C.; Fazio, T.; Howard, J. W. ].-Assoc. Ojf Anal. Chem. 1978,

442-3.

Chem. 1992, 40, 2216-21.

1992.40, 2222-5.

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

(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.;

(15) Nikaido, M. M.; Dean-Raymond, D.; Francis, A J.; Alexander, M. Water

(16) Kimoto, W. I.; Dooley, C. J.; Carre, J.; Fiddler, W. Water Res. 1981, 15,

(17) Ontario Drinking Water Objectives, 5th ed.; Ministry of Environment and

(18) Scharfe, R R; Mclenaghan, C. C. ].--ASSoc. Ojf Anal. Chem. 1989, 72,

(19) T i i o n s , L.; M u d , E.; Onstot, J.; Brown, R; Cannon, M.; Ameson, D. J

(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 Conference on Mass Spectrometry and Allied Topics; Tucson, AZ, 1990; p 623.

Sci. 1992, 57, 569.

Epstein, S. S. Bull. Enuiron. Contam. Toxicol. 1975, 14, 404-8.

Res. 1977, 11, 1085-7.

1099-106.

Energy: Ontario, Canada, in press.

508-12.

Anal. Toxicol. 1988, 12, 117-21.

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 sensitivity with 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 tetrachlorodibenzo- p-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 under- standing 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 WangZs as 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 significance of 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.; Weber- Grabau, 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 Conference on 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,

(28) Marquette, E.; Wang, M. Proceedings of the 41st ASMS Conference on Mass 372-381.

Spectrometry and Allied Topics; San Francisco, CA 1993; p 698a.

spectrometer will become increasingly important in the future, not only for the analysis of NDMA, which continues to be the subject of extensive toxicological r e~ea rch~*~O and 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 capability has 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.

4438 Analytical Chemistry, Vol. 66, No. 24, December 15, 1994

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 rf vo-PI axial segment length (us) initial rf final modulation waveform

1 40000 80 80 Off Off 2 1000 80 450 on Off 3 2000 450 460 on Off 4 1000 460 211 Off Off 5 20000 211 211 Off CADU 6 1000 211 228 Off Off 7 20000 228 228 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

w, = PzQ/2 (3)

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

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 =

75 ([M + HI+ of NDMA) and 81 ([M + HI+ of NDh%ds), and

(4) (max reaction time) (ionization time)

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-d6 in 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% (cyanopropy- 1phenyl)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 1 pL/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 sensitivity toward 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


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 mass- selective instability mode at an axial modulation voltage of 4.0

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 3 s 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 Prod- ucts (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,3 is given in Figure IA. Figure 1B illustrates the first application of CAD to isolated m/z 75 (scan segment 5 in Table 1) giving predomi- nantly 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 (33) Taguchi,V. Y.; Jenkins, S. W. D.; Wang, D. T.; Palmentier, J.-P. F. P.; Reiner,

VP-V

E. J. Can. /. Appl. Spectrosc., in press.

I 4 I1

t 98 188

Ib

19

1 ie

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

filament emission filament emission m/z current @A) S/N m/z current @A) S/N 44 10 3023 46 10 3887

100 5474 100 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

NDMA ra ratio peak area peak ht concn" (pg/pL) m/z75 m/z81 m/z75 m/z81 m/z75 m/z81

8.03b 36 41 3574 4054 1983 1894 8.03c 64 94 4884 4929 3500 3023

32.11b 59 89 10037 10138 5092 4311 32.1lC 122 162 13685 13486 9566 9182

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 surround- ing 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 approximately linear. Concentra- tions 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


0 . 1 9 ~

m/z 46 -

intensity than the m/z 44 ion, the ionization time and reaction period set by the ARC algorithm will be biased toward ions formed in the trap from the internal standard. As a result, this can affect the detection of the analyte present at a lower concentration than the internal standard with ARC enabled, since ionization time and reaction time vary as a function of ion density in the ion trap. With ARC disabled, both the internal standard and native analyte receive the same length of ionization, thereby negating any "ion count" biases due to GC coelution. Consequently, it would appear that ARC is unsuitable for the detection of NDMA in the low picogram range unless curve-fitting equations are derived.

I 'I I i ,, - ..._ I ' , f ' * , , ..'5,, I<$, ,,, ;.+ \ , I,; .":, ;~.>,p*. I

L

1

,) 1 I 1

i" .,-.

Nonlinearity observed when ARC is enabled may also be due to shifts in the fundamental axial secular frequencies as the abundance of ions in the trap increases. Such spacecharge effects have the tendency to decrease the conversion efficiency of CAD, making the tandem mass spectrometric process nonlinear with respect to daughter ion turnover.

One apparent peculiarity which has arisen is that the instrumental response is tremendously greater when ARC is used. For example, when 500 fg of NDMA was injected, the response was calculated as 162 area counts/pg when ARC was disabled, whereas with ARC active, the response was 6086 area counts/pg. However,


Table 4. Comparlson of NDMA Concentrations from Aqueous Extracts Analyzed Uslng GC/QISMS and GC/HRMS

concentration of NDMA (ng/L) QISMS

aqueous extra@ ARC disabled ARC enabled HRMS 1 9.95 11.6 17 2 55.4 44.8 49 3 8.60 8.73 7 4 45.3 39.2 34 5 15.0 10.4 13

a Aqueous extracts were spiked with 12.6 ng of N D M - & prior to extraction and represent a 0.8 L water sample.

the S/N ratios in each case are virtually identical at the low picogram levels, and the average response factors of 0.77 (f0.065) when ARC is on and 0.76 (f0.079) when ARC is disabled (based on triplicate injections at each concentration) indicate little ambiguity between the two alternate modes of operation. The observed differences in instrumental response (but not in response factor), dependent upon whether or not ARC is disabled, are due to a scaling factor in the QISMS software which is based upon the relationship between analyte ion intensity, analyte concentration, and reaction time. Thus, when ARC is active, the analyte ion intensity is proportional to the product of the analyte concentration and the square of the reaction time. Such a scaling factor is omitted when ARC is disabled, and therefore, instrumental response values are lower than those obtained when ARC is enabled.

In order to evaluate the applicability of the GC/QISMS protocol toward the analysis of NDMA in aqueous extracts, five separate samples previously analyzed at the MOEE by GC/HRMS were examined; Table 4 summarizes the concentrations of NDMA found in each extract by GC/QISMS (with and without ARC) and by GC/HRMS. In most instances, concentrations of NDMA determined using GC/QISMS are in good agreement with those obtained by GC/HRMS. Clearly, concentrations of NDMA vary slightly, depending upon whether or not ARC is enabled. For the most part, concentrations of NDMA determined when ARC was enabled more closely approximate those obtained using GCI

HRMS, which may reflect the greater ease by which peak integration is performed when ARC is active. Gas chromatographic peak shapes were found to be of Gaussian form when acquisitions were obtained with ARC enabled. In contrast, when ARC was disabled, peaks often demonstrated a sloping front or back face. The noted improvement in GC peak shape to a Gaussian form when ARC is enabled may be credited to the computer algorithm scaling factor, which is absent when ARC is disabled.

Potential Interferences. Taguchi et al.n reported chlorobenzene, ethylbenzene, and the 0-, m-, and p-xylenes as potential interferents when NDMA is analyzed by LRMS in E1 mode. Analysis of each interferent by GC/QISMS at the 200 pg level failed to produce any m/z 43,44,46, or 49. As expected, operation of the QISMS in a tandem mass spectrometric mode under CI conditions gives a degree of selectivity unavailable when a mass selective detector is used.

CONCLUSIONS Operation of a quadrupole ion trap in tandem mass spectro-

metric mode and in conjunction with a gas chromatograph offers an alternative technique of comparable sensitivity to high-resolution mass spectrometry and one of much greater specificity than gas chromatography/mass spectrometry. The fundamental limita- tions of the ion trap when used in conjunction with a gas chromatograph are twofold: first, the pulsed mode of operation, in that all operations take place sequentially (tandem in time) within the ion trap (localized in space) hence only a small fraction of the eluent gas stream is subjected to ionization and, second, the efficiency of ionization during the ionization period is low.

ACKNOWLEDGMENT We thank Dr. Vince Taguchi of the MOEE for providing

environmental samples and for helpful discussions. We also thank Varian Associates Inc., the MOEE, and the Natural Sciences and Engineering Council of Canada for financial and material support. We are grateful to Varian Associates Inc. for the loan of a Saturn I11 GC/QISMS and supporting software.

Received for review May 13, 1994. Accepted September 22, 1994.@

@ Abstract published in Advance ACS Abstracts, November 1, 1994,

Analytical Chemistty, Vol. 66, No. 24, December 15, 1994 4443

determination of n-nitrosodimethylamine in complex environmental matrixes by quadrupole ion storage...

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