Anal. Chem. 1994,66, 3681-3687
Quantitation of Ionic Species in Single Microdroplets by On-Line Laser Desorption/Ionization Bashlr A. Mansoor1,t Murray V. Johnston,'?+and Anthony S. Wexler* Departments of Chemistry and Biochemistry and Mechanical Engineering, University of Delaware, Newark, Delaware 19716
Mass spectra of single micrometer-size glycerol droplets containing dissolved inorganic salts were obtained by on-line laser desorption/ionization. The peak areas of quantitation ions in the mass spectra allowed cation and anion concentrations in solution to be determined. Liquid droplets have several advantages for quantitative work including well-defined morphology, reproducible matrix composition surrounding the analyte, and straightforwardpreparation of calibrationaerosols having known amounts of analyte. Therefore, absolute quantitation can be achieved without an internal standard. Dropletto-droplet variations of the absolute signal intensities ranged in the signal-tofrom 14 to 58%. An improvement of noise ratio was achieved by averaging the spectra of Ndroplets. Calibration plots of signal intensity versus ion concentration in solution were obtained for five test ions: Fe3+, Na+, C1-, NOf, and Sod2-. The plots were linear over at least 2 orders of magnitude in concentration. Minimum detectable concento high M range. At the trations were in the low minimum detectable concentrations, the absolute amounts of analyte in a single droplet were on the order of 10-16-10-18 mol. Potential applications of this methodology to real-time measurements of atmosphericallyrelevant aerosols is discussed. Aerosols play a significant role in virtually all forms of air pollution including indoor toxics and radon, urban and regional smog, acid deposition, stratospheric ozone, and global warming. Although many techniques are now available for singleparticle analysis, real-time quantitation of chemical components in single aerosol particles remains an elusive goal. Recently, we described a new method for on-line analysis of single particles in aerosols based upon laser desorption/ ionization.' The aerosol is sampled directly into a mass spectrometer through a differentially pumped inlet. The resulting particle beam intercepts a continuous helium/neon laser beam in the source region. The scattered radiation from each particle triggers an excimer laser which ablates the particle in-flight, and a time-of-flight mass spectrum is recorded. Each mass spectrum corresponds to a single particle ablated by a single laser pulse. Since the time between sampling and analysis is approximately 1 ms, chemical transformation of the particles by reaction, condensation, evaporation, or phase change is minimized. Similar experiments have been discussed by other researchew2" t Department of Chemistry and Biochemistry. 8 Department of Mechanical Engineering. (1) McKeown, P. J.; Johnston, M. V.; Murphy, D. M. Anal. Chem. 1991, 63,
2069.
(2) Sinha, M. P. Rev. Sei. Imtrum. 1984, 55, 886. (3) Marijnissen, J.; Scarlett, B.; Verheiuen, P. J. Aerosol Sci. 1988, 19, 1307.
0003-2700/94/0366-368 1$04.50/0 0 1994 Amerlcan Chemlcal Soclety
On-line laser desorption/ionization is similar to laser microprobe mass spectrometry (LAMMS), which has been used for over a decade to analyze particulate matter.'-9 With LAMMS, the sample is mounted on a substrate and each particle is successively irradiated with a high-power pulsed laser beam. The laser radiation ablates the particle, and the ejected ions are mass analyzed. LAMMS has several advantages for single-particle analysis including (1) detection of trace metals at the parts-per-million (2) speciation of inorganic materials, especially those containing nitrogen and sulfur,l1-l4 (3) detection of trace organic compounds, particularly aromatic^,'^-^^ and (4) the ability to distinguish the surface versus total volume composition of a part i ~ l e . ' ~ J ~LAMMS, J ~ - ~ ~ however, is an off-line technique and cannot be used for real-time measurements of ambient aerosols. Particles must be collected over a period of time and then evacuated before analysis, thus permitting chemical transformations to occur. One would expect the desorption/ionization processes to be similar for the off-line and on-line experiments since the mounting substrate used in LAMMS, typically an electron microscopy grid, has little effect on the mass spectrum. Of course, no mounting substrate is used in the on-line experiment. Indeed, we have found that, for a given substance, the ions produced by each method are similar. Therefore, on-line laser desorption/ionization should exhibit analytical characteristics similar to LAMMS for single-particle analysis. A unique advantage of on-line laser desorption/ionization, however, is the ability to interrogate individual microdroplets. The short (4) Thompson, D. S.;Murphy, D. M. Appl. Opr. 1993, 32, 6818. ( 5 ) Prather, K. A.; Nordmeyer, T.; Salt, K. Anal. Chem. 1994, 66, 1403. (6) Hinz, K.-P.; Kaufmann, R.; Spengler, B. Anal. Chem. 1994,615, 2071. (7) Physical and Chemical Characrerizarion of Individual Airborne Parricles; Spurny, K. R., Ed.; Ellis HorwoodLimit& Chichester, West Sussex, England, 1986; Chapters 13-15. (8) Wouters, L.; Artaxo, P.; Van Grieken, R. Int. J. Enuiron. Anal. Chem. 1990,
38, 427. (9) Michiels, E.; Van Vaeck, L.; Gijbels, R. Scanning Electron Microsc., Part III 1984, 1111. (10) Bruynseels, F.; Storms, H.; Van Grieken, R. Atmos. Enuiron. 1988,22,2593. (1 1) Otten, Ph.; Bruynseels, F.; Van Grieken, R. Anal. Chim. Acra 1987,195,117. (12) Bruynseels, F. J.; Van Grieken, R. E. Anal. Chem. 1984, 56, 871. (13) Bruynseels, F.; Otten, Ph.; Van Grieken, R. J. Anal. Atom. Spectrosc. 1988, 3, 237. (14) Ro, C.-U.; Musselman, I. H.; Linton, R. W. Anal. Chim. Acra 1991,243,139. (15) De Waele, J. K.;Gjbels, J. J.; Vansant, E. F.; Adams, F. C. Anal. Chem. 1983, 55, 2255.
(16) De Waele, J. K.; Vansant, E. F.;Van Epsen, P.; Adams, F. C . Anal. Chem. 1983, 55, 67 1. (17) Mauney, T.; Adams, F. Sci. Total Enuiron. 1984, 36, 215. (18) Niessner,R.;Klockow,D.;Bruynscels,F. J.;VanGrieken,R.E.;Int.J.Enuiron. Anal. Chem. 1985, 22, 281. (19) Bruynseels, F.; Van Grieken, R. Armos. Emiron. 1985, 19, 1969. (20) Bruynseels, F.; Van Grieken, R. Int. J. Ma$$ Spectrom. Ion Processes 1986, 74, 161. (21) Wouters, L. C.; Van Grieken, R. E.; Linton, R. W.; Bauer, C. F. Anal. Chem. 1988, 60, 22 18.
Analytical Chemistty, Vol. 66,No. 21, November 1, 1994 3001
transit time between sampling and analysis prevents evaporation of semivolatile solvents. Volatile solvents may undergo partial evaporation, but evaporative cooling will inhibit drying on the time scale of the experiment. Liquid droplets are difficult to handle with LAMMS since cross contamination and complete evaporation of the solvent may occur during the sample collection and preparation steps. Furthermore, morphological distortion of the droplet on the electron microscopy grid is likely. On-line laser desorption/ionization is less susceptible to these problems and has the advantage that successive droplets in the aerosol can be detected and ablated at a rapid rate. The ability of on-line laser desorption/ionization to make quantitative measurements is questionable given the intrinsic difficulty of quantitation by LAMMS. Problems associated with relating ion signal intensity to concentration have been the subject of numerous investigation^.^^-^^ These problems arise from both instrumental factors and sample characteristics. Instrumental factors include the effects of extraction and lens potentials,22-26detector nonlinearity and dynamic range,26-28and pulse-to-pulse variations in the laser energy and focusing conditions (laser beam profile; overlap between the laser beam and p a r t i ~ l e ) . ~Variations ~ ? ~ ~ , ~in~ the signal intensity caused by the ion optics and detector characteristics are usually predictable and can be controlled or compensated by the analytical procedure. Pulse-to-pulse variations in the laser energy and beam profile, however, are substantial and inevitably cause large fluctuations in the ion yield. Sample characteristics include matrix composition of the particle, particle size, and m 0 r p h o l o g y . ~ ~ , ~ 6These , ~ ~ . ~factors 0 strongly influence the mechanistic details of the desorption/ionization process including energy transfer from the laser beam to the p a r t i ~ l e , ~ vaporization ~ , ~ ~ , ~ ' of the particle, and ion formation in the vaporized p l ~ m e . ~ ~Since * ~ ~the q ~chemical ~ and morphological characteristics of a particle are often ill-defined and the roles these factors play in the desorption/ionization process are complex, their influence on ion yield is frequently indeterminant. Quantitative determinations by LAMMS usually involve the relative sensitivity factor a p p r o a ~ h ~ where ~ , ~ ~the , ~signal ~v~~ intensities of the analytes are measured relative to a standard and relative sensitivity factors are used to determine the analyte concentrations. Under optimum conditions, the precision for analysis of a single particle is on the order of 20-40%.22 Accuracy is determined by how well the matrix composition and morphology of the standard used to obtain the relative sensitivity factors can be matched to the unknown.33 (22) Musselman, 1. H.; Simons, D. S.; Linton, R. W. Int. J . Mass Spectrom. Zon Processes 1992, 112, 19. (23) De Wolf, M.; Mauney, T.;Michiels. E.;Gijbels, R. ScanningEIectron Microsc. Part I1 1986, 799. (24) Harris, A.; Wallach, E. R. Microbeam Anal., 2lst 1986, 464. (25) Michiels, E.; De Wolf, M.; Gijbels, R. Scanning Elecfron Microsc. Part II 1985, 947. (26) Surkym, P.; Adams, E. J . Trace Microprobe Tech. 1982, I , 79. (27) Kaufmann, R.; Wieser, P.; Wurster, R. Scanning Electron Microsc. Part II 1980, 607. (28) Simons, D. S. Int. J . Mass Specfrom. Ion Processes 1983/1984, 55, 15. (29) Housden, J.; Hutt, K. W.; Leake, J. A.; Wallach, E. R. Microbeam Anal. 23rd 1988, 371. (30) Wurster, R.; Haas, U.;Wieser, P. Fresenius Z . Anal. Chem. 1981,308, 206. (31) Kaurmann, R.; Hillenkamp, F.; Wechsung, R.; Heinen, H. J.; Schurmann, M. Scanning Electron Microsc. Part I1 1979, 279. (32) Hutt, K. W.; Wallach, E. R. Microbeam Anal.. 23rd 1988, 374. (33) Bate,D. J.;Leake, J. A.;Matthews,L. J.; Wallach,E. R.Int. J . MassSpectrom. Ion Processes 1993, 127, 85.
3682
Analytical Chemlstty, Vol. 66,No. 21, November 1, 1994
On-line laser desorption/ionization should be similarly constrained when solid particles are analyzed. The only functional difference relative to LAMMS is the laser focusing condition. In LAMMS, overlap of the laser beam with the particle is performed manually with the aid of a microscope illumination system. The laser beam is on the order of the particle size, so achieving reproducible alignment can be difficult. With on-line laser desorption/ionization, the laser beams are aligned such that a particle detected by light scattering of the helium/neon laser beam is located in the excimer laser beam path when it fires. The particle beam entering the source region is 1 cm in diameter. The laser beams are focused to relatively large spot sizes (-100 pm diameter) so that a significant fraction of particles entering the source region can be analyzed. Thus, the excimer laser beam is significantly larger than an individual particle, and the location of each particle in the beam is different. In this work, we consider the ability of on-line laser desorption/ionization to make quantitative measurements of ionic species in liquid microdroplets. Liquid droplets have several advantages for quantitative work including well-defined morphology (spherical; homogeneous; narrow size distribution if a monodisperse aerosol is generated), constant matrix composition surrounding the analyte, and straightforward preparation of calibration aerosols having known amounts of analyte. Therefore, many of the problems inherent to solid particle analysis by laser desorption/ionization may be averted. In this study, glycerol microdroplets are investigated because of the the ease of preparing and sampling monodisperseaerosols containing dissolved inorganic salts. Applications of on-line laser desorption/ionization to real-time measurements of atmospherically relevant aerosols will be discussed.
-
EXPERIMENTAL SECTION Monodisperse aerosols were produced with a vibrating orifice aerosol generator (Model 3450; TSI, St. Paul, MN). Primary droplets were generated from a 0.2% by weight solution of glycerol in a 50/50 (v/v) mixture of methanol and water. The size distribution of these droplets was determined by the experimental conditions (liquid and gas flow rates, orifice diameter, piezoelectric vibration frequency) used for aerosol generation. The primary droplets were dried to remove the volatile solvents, leaving a monodisperse aerosol of glycerol microdroplets. The salt concentrations in the final glycerol droplets were determined from concentrations in the original glycerol/methanol/water solution and were kept well below saturation. The size distribution of the glycerol droplets was independently monitored with an aerodynamic particle sizer (Model 3310; TSI, St. Paul, MN). In most cases, a lognormal distribution of droplet sizes was obtained with u = 0.6 pm. The droplet diameters given in later sections represent the mean droplet diameter measured during the experiment with the particle sizer. Aerosols were sampled into the mass spectrometer through a two-stage momentum separator. The resulting particle beam passed through the center of the source region where it intercepted a continuous helium/neon laser beam. The transit time between the inlet and source region was -0.5 ms. Scattered radiation from individual particles in the helium/ neon laser beam was detected with a photomultiplier, and the
output pulse triggered an excimer laser (Model PSX-100; MPB Technologies, Dorval, Quebec) operating at 248 nm. The excimer laser pulse energy was periodically monitored by inserting an energy meter (Model J25LP-2; Molectron, Portland, OR) into the beam path. In these experiments, no attempt was made to record pulse-to-pulse and hence particleto-particle fluctuations of the excimer laser pulse energy. The two laser beams were aligned such that particles detected by the first laser were ablated by the second laser. Since the delay time between particle detection and ablation was less than 2 ps, only minor displacement of the helium/neon and excimer laser beams, -200 pm, was required. Ions ejected by the ablation event were accelerated into a reflecting field time-of-flight mass analyzer. The reflecting field was found to significantly improve resolution relative to the linear flight path used in our previous work.' The ions were detected with a dual microchannel plate detector, and the output signal was sampled with a 200 Mhz transient digitizer (Model 9845; Precision Instruments, Knoxville, TN) mounted in a personal computer. The maximum signal-to-noise ratio of digitization was experimentally determined to be 6.5 bits. Each raw spectrum was stored directly on a hard disk and later converted from the time domain to the mass domain. No additional processing (smoothing, background subtraction, etc.) was performed other than spectral averaging as discussed later. The number density of particles in the aerosol and the flow rate of aerosol into the mass spectrometer were adjusted so that only a few particles per second were detected by light scattering. Since the residence time of a particle in the excimer laser beam path was on the order of 1 ps, the probability of having two particles in the beam when the laser fired was very low. Therefore, each mass spectrum was assumed to originate from a single particle. Since different droplets were located at slightly different positions in the excimer laser beam, the time-to-mass conversions of the corresponding spectra were also different. For example, the centroid of the Na+ peak in a series of NaCl single-particle mass spectra was found to have a mean flight time of 11 770 ns with a standard deviation of 30 ns. This standard deviation was consistent with the experimentally determined excimer laser width of 100 pm. Thus, a laser pulse energy of 1 mJ corresponded to an irradiance of -4 X lo9W/cm2. The irradianceof the excimer laser was essentially flat across the beam, so particles located at different positions in the beam nominally experienced the same irradiance. However, it is likely that the beamcontained "hot" spots whose location and intensity varied from pulse to pulse.
-
RESULTS AND DISCUSSION Figure 1 shows the mass spectrum of a single glycerol droplet containing several metal salts. Three types of ions are observed: matrix ions derived from glycerol and/or traces of the volatile solvents used to generate the aerosol (water, methanol), bare atomic metal ions, and metal cluster ions. This spectrum was taken with a high excimer laser pulse energy. With lower pulse energies, the matrix fragment ions became smaller relative to the atomic metal ions and metal cluster ions. For a given nominal laser pulse energy, the absolute peak areas were fairly constant. Table 1 gives the coefficients of variation (CV) of the absolute peak areas for
iC
25
r3
55
85
70
33
li5
'33
~ 4 5 162
m/z
Figure 1. Laser desorption mass spectrumof a single 3.0 pm diameter glycerol droplet Containing VOZ2+(0.66 M), Fe3+ (0.19 M), Ni2+(0.15 M), Cu2+ (0.06 M), and Cs+ (0.03 M). The laser pulse energy was 2.1 mJ. ~
~~
Table 1. Coefflciento of Varlatlon for Peak Area Yeaourmento m/Z assignment coeff of variation'
45 51 54 56 51 58 60 63 65 61
13 15 93 133 155
CzHsO+ V+ s4Fe+ 56Fe+ 57Fe+b 58Ni+b @Ni+ 63CU+ 65CU+
vo+
s6FeOH+ 58NiOH+ GH+C cs+ 63CuG+C
38 19 30 14 49 40 23 23 31 29 29 58 41 43 41
a Based u n 18 single-particlespectra. Droplet composition is given in Figure 2. Gragment ion current from glycerol is also observed at this m / z . G = glycerol, C ~ H ~ O S .
18 successive single-particle spectra. The CVs range from 14 to 58%. These values are comparable to those obtained from peak areas measured relative to an internal standard by LAMMS of glass micro sphere^.^^^^^ In general, the lowintensity peaks exhibited larger CVs than the high-intensity peaks owing to a larger contribution from digitization noise. Matrix and cluster ions tended to give larger CVs than atomic metal ions. For any given ion, the CV was found to depend strongly upon the laser irradiance, analyte concentration, and transient digitizer settings used in the experiment. Atomic metal ions rarely exhibited CVs above 50% under any conditions. Figure 2a shows an expanded portion of the mass spectrum of a single glycerol droplet containing iron(II1) chloride at a concentration of 4.3 X M. At this concentration and laser irradiance, only the bare metal ion is observed. The baseline noise in this spectrum arises from digitization noise in the A/D converter rather than background ion current from the detector. Both software and hardware approaches to reducing the effect of digitization noise have been discussed elsewhere.' (34) The glass microspheres in the LAMMS study and the glycerol microdroplets in this work are comparable in that individual particles have similar matrix composition and morphology. In the more general case where the matrix composition and morphology vary from particle to particle, the CVs obtained from bath methods are significantly larger.
Analytical Chemistry, Vol. 66, No. 21, November 1, 1994
3683
0
7
0
10
20
30
40 50 Particle Number
€4
70
80
Flgure 3. Absolute peak area of 5eFe+for 80 droplets taken over a period of 1 h. The experimental conditions were the same as In Figure
2. I
ci
d
55
63
m/z
Flgure 2. (a) Laser desorption mass spectrum of a single 3.8 pm diameter glycerol droplet containing 4.3 X lo-*M Feci3. (b) Averaged spectrum of 35 droplets. The laser pulse energy was 0.75 mJ.
Figure 2b shows an averaged spectrumof 35 droplets. Two important differences from Figure 2a are observed. First, the baseline noise, as expected, is decreased by " I 2 . Second, the peak profiles are broadened owing to small changes in the ion flight times caused by varying location of the droplets in the excimer laser beam. In this experiment, the raw spectra (ion current versus flight time) were averaged and then the time axis was converted to mass-to-charge ratio. This type of peak broadening can be eliminated by mass calibrating the individual spectra first and then averaging the converted spectra. Since mass calibrating each spectrum can be time consuming, this approach is not used unless high mass resolution is required. Although the baseline noise of the averaged spectrum is smaller, it is not necessarily true that the absolute peak area of the averaged spectrum is a more accurate and precise measure of the true peak area. If the effective laser irradiance or droplet morphology changes over time, then the absolute peak areas in the individual spectra may drift and therefore exhibit time-dependent systematic error when they are averaged. To examine this possibility, we performed stability tests by measuring the absolute peak areas of spectra from successive droplets. Figure 3 shows the results of one such test covering 80 droplets that were taken over a period of 1 h. No discernible time-dependent changes in the mean or variance of the peak area were detected over the course of the experiment. Indeed, we have found that the stability is such that absolute peak areas can be kept constant for a period of days without extensive retuning. Therefore, this system can be regarded as both stationary and stable,35 and averaging the spectra of N particles should increase the precision of peak area measurement by N I P . Elimination of time3684
Analytical Chemlstry, Vol. 66, No. 21, November 1, 1994
__ Flgure 4. Laser desorption mass spectrum of a single 3.6 pm dlameter glycerol droplet containing 4.3 X lo-' M FeCI3.The laser pulse energy was 1.25 mJ.
dependent systematic error is crucial for achieving a stationary and stable system so that absolute concentrations can be obtained from absolute peak area measurements. When the concentration of the analyte ion decreases, overlap with matrix ions may become important. Figure 4 shows the mass spectrum of a single glycerol droplet containing iron(II1) chloride at a concentration of 4.3 X lo4 M. The matrix ion signal intensities are comparable to that of Fe+ ( m l z 56), but no isobaric interference occurs. Matrix ions are also observed in negative ion mass spectra. Figure 5 is a spectrum of a single glycerol droplet containing 1.6 X M ammonium sulfate. The presence of the sulfate ion in solution is indicated by the ions SO1-, HSO3-, and HS04- in the mass spectrum. Again, the matrix ion signal intensities are comparable to those of the analyte. In this case, however, the signal at m l z 97 arises from both HS04- and the glycerol matrix. Under the conditions of Figure 5 , the matrix ion signal is only 14% of the total signal intensity. To a certain extent, the overlap problem can be reduced by decreasing the excimer pulse energy since the matrix ion signal intensities decrease more rapidly than the analyte ion signal intensities (35) Liteanu, C.; Rica, I. Srarisfical Theory and Methodology of Trace Analysis; Ellis Horwood Limited: Chichester, England, 1980; Chapter 4.
3.0
30
20
40
60
50
7C
50
80
00
110
0.0
T / Z
Flgurel. Laser desorption mass spectrum of a single 4.7 pm diameter glycerol droplet containing 1.6 X 10" M (NH4)2S04. The laser pulse energy was 1.45 mJ.
Y
m
100-
a " .
+
u
m-
' Bo-
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
Concentration (M) x 103
Figure 6. Absolute peak area of 5eFe+versus concentration of Fe3+ In 3.6 pm diameter glycerol droplets. Each data point Is the average of at least 25 spectra. The error bars indicate the standard error of the mean. The laser pulse energy was 1.25 mJ. Also shown is a least-squares fit (correlation coefficient 0.995).
(see earlier discussion). In the quantitative work described below, the matrix ion signal intensity from droplets containing no sulfate in solution was subtracted from the m / z 97 signal intensities of droplets containing sulfate. Quantitation of ionic species in glycerol droplets is possible if isobaric interferences are minimized and the spectra of many droplets are averaged. Examples of cation and anion quantitation are given in Figures 6 and 7. Figure 6 shows a calibration plot of the Fe+ absolute peak area versus iron(II1) concentration between 2 X lo4 and 7 X M. For each concentration, the Fe+ peakarea was determined by averaging the spectra of at least 25 particles. A linear relationship was observed with a correlation coefficient of 99%. The concentration range in Figure 6 was chosen so that the signal intensities fit within the dynamic range of the transient digitizer. Higher concentrations, up to 5 X lo-* M, were studied by lowering the laser pulse energy and/or changing the vertical sensitivity of the digitizer. In each case, a linear relationship between signal intensity and concentration was observed over the range of signal intensities that could be digitized. Although changing the laser pulse energy and/or vertical sensitivity of the digitizer permits quantitation over
1
1
0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
2.25
2.50
2.75
3.W
Concentration (M) x 103
Flgure 7. Absolute peak area of HS04- (mlz 97) versus concentration of S042- In 4.7 fim diameter glycerol droplets. Each data polnt Is the average of at least 25 spectra. The error bars Indicate the standard error of the mean. The laser pulse energy was 1.45 mJ. Also shown is a least-squares fit (correlation coefficient 1.OOO).
several orders of magnitude, prior knowledge of the concentration range is required to set the appropriate parameters. This is a limitation of the current data system of our instrument rather than a fundamental limitation of the desorption/ ionization process. Several possibilities exist for improving the data system in the future. A logarithmic amplifier could be used to expand the effective dynamic range of the data system,' but the precision of peak area measurement would be reduced. Another approach would be to employ several digitizers operating in parallel with different vertical expansion sensitivities.22 Microchannel plate detectors are easily saturated and have a limited linear working range. This problem could be eliminated by using an electron multiplier, but the detector response time would be longer. The ability to quantitate anions is demonstrated in Figure 7 where the absolute peak area of HS04-( m / z 97) is plotted as a function of the sulfate concentration in the droplet. Again, a linear relationship is observed. The data points in this plot have already been corrected for the matrix signal intensity at m / z 97. The uncorrected plot is also linear, but shifted to higher signal intensity by 0.4 unit on the y-axis. To test the applicability of on-line laser desorption/ionization to other ions, we performed similar experiments with glycerol droplets containing Na+, C1-, and NO3-. In each case, linear relationships between the absolute peak area of the quantitation ion and concentration of the ionic species in solution were obtained. No isobaric interferences were observed, except for sulfate as discussed previously. The nonzero (positive) y-intercept of the plot in Figure 7 is characteristic of all of the anions studied. Significant positive intercepts were also observed for the cations, but only with low (
