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Laser microprobe mass spectrometry: The past, present, and future
MICROCHEMICAL
JOURNAL
38, 3-23 (1988)
Laser Microprobe
Mass Spectrometry: and Future’
The Past, Present,
DAVID M. HERCULES Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Received February 4, 1988; accepted February 4, 1988 Development of the laser mass spectrometer has been a major step forward in solid state mass spectrometry. This technique has allowed routine acquisition of mass spectra of solids in the middle mass range. Although the volatilization/ionization mechanism(s) is not well understood, laser mass spectrometry is clearly a “soft” ionization technique. Even though power densities in the range lo’-lo9 W/cm* are used, quasimolecular ions are frequently observed even from labile molecules. Laser mass spectra show approximately equal intensities for both positive and negative ions. Examples are cited to illustrate the versatility of laser mass spectrometry applied to a variety of problems. These include quantitative analysis, polymer chemistry, mapping of organic surface impurities, and obtaining spectra under atmospheric pressure. A survey is presented of the types of spectra observed in laser mass spectrometry; of particular importance are ions produced by either proton transfer or cationization. The future of laser mass spectrometry is addressed. Of particular importance is how the technique can develop for technical analyses. An assessment is given of how laser mass spectrometry compares with other microprobe techniques and what potentially limits its application to the analysis of smaller domains in solids. 0 1988 Academic PXSS, 1~.
INTRODUCTION
Mass spectrometry has been one of the most successful analytical methods developed in recent times, having profound impact on diverse fields. There is an ever-increasing trend toward obtaining spectra from materials that have high molecular weight, have low volatility, are thermally unstable, or some combination of the three. Over the past several decades, a number of solid-state ion sources have been developed which have largely solved the problems associated with obtaining meaningful mass spectra from technical materials. However, with increased emphasis on microdomains, the need for a mass spectral microprobe is readily apparent. The current paper reviews developments to date in laser microprobe mass spectrometry and gives some projections for this technique for the future. The concept of using a laser to directly ionize a solid material is not new. In 1963 Honig and Woolston published the first report of laser mass spectrometry (LMS) (I). A few years later Vastola and co-workers published attempts to apply laser mass spectrometry to the analysis of coal, organic, and biological compounds (2). The results of this early work were largely limited by both mass analyzers and laser technology. However, these early studies clearly showed the feasibility of the method. ’ This article is based on the Benedetti-Pichler Award Address presented by Professor Hercules at the 26th Eastern Analytical Symposium, September, 18, 1987. 3 0026-265X/88 $1.50 Copyright 0 lY88 by Academic Press, Inc. All rights of reproduction in any form reserved.
4
DAVID M. HERCULES
Table 1 designates the milestones in the development of laser mass spectrometry. Nearly a decade after the work of Vastola et al., Kaufman, Hillenkamp, and Wechsung (3) developed the LAMMA-500, the first commercial laser microprobe mass spectrometer. Independently, the application of laser desorption to large molecules by Kistemaker et al. (4) showed the potential of the technique for compounds of this sort. In subsequent years extensive application of LMS to organic compounds was studied by many groups, particularly by Heinen et al. (5); the initial application of LMS to polymers came from our laboratory (6). A block diagram of the LAMMA(the first commercially developed laser microprobe mass spectrometer) is shown in Fig. 1. A beam from the pulsed laser is focused on the sample through a microscope, and the pulse from the laser simultaneously triggers a transient recorder by a signal from a photodiode. This starts a clock which times the flight of the ions down the tube of the mass spectrometer; the ions are accelerated into the flight tube as they leave the sample. An electrostatic reflector is used to correct the spread of ion kinetic energies; this helps to narrow the spectral lines. When operating as a microprobe, the pilot laser (He-Ne) indicates where the main laser beam will strike the sample. The microscope focuses both laser beams on the sample. The bottom of Fig. 1 shows the sample arrangement in the LAMMA-500. Note that the microscope introduces the laser beam through an oil immersion lens system and that the laser shot must penetrate the sample for ion production. The instrument features interchangeable illumination source and ion lens systems which permit reproducible positioning of the laser on the sample. Most of the early work with the LAMMAwas on thin sections; the primary purpose of its development was to study metal ions in biological materials. Figure 2 shows a spectrum of a thin resin section doped with alkali ions; spectral lines from each element are clearly evident. The detection limits for metal ions are very impressive, on the order of IO-l9 to 10e2’ g (7). Even on a relative (ppm) basis, the detection limits correspond to la. 1 ppm. The concentration detection limit is TABLE 1 Milestones in the Development of Laser Microprobe Mass Spectrometry Year 1963 1968 1977 1978 1979 1980 1981
First report of laser mass spectrometry (Honig and Woolston) First attempts to apply LMS to analysis of coal, organic (Vastola et al.) Development of LAMMAJOO (Kaufmann, Hillenkamp, and Wechsung) Application of laser desorption to large molecules (Kistemaker et al.) Organic compounds studied with LMS (Heinen et al.) Polymers studied with LMS (Gardella and Hercules) Development of LAMMA(Leybold-Heraeus)
LASER
MICROPROBE
5
MASS SPECTROMETRY
2+~!kE$$E>~L
ILLUMINATION
ION
LENS
I
;
ANALYSIS
@----4
FIG.
1 1
SAMPLE
TRANSrn
1. Block diagram of LAMMA-SOO laser microprobe mass spectrometer.
in the ppm range because of the small volume sampled by the laser. Studies of metal ions were very successful using the LAMMA, for example, use of %a2+ to study the calcium uptake by retinal tissue (8). One of the problems with the LAMMAgeometry was that solids had to be studied using grazing incidence, as illustrated in Fig. 3. This shows the spectrum 3
Rb’
Spurr
resin
0.3
m thick,
doped
with
1 mM
Li. Na,
K, Rb.
CS’
Cs
FIG. 2. LAMMAJOO spectrum of thin resin section doped with alkali salts (Leybold-Heraeus, with permission).
6
DAVID
I
LAMMA
Mass
highly alloyed
Spectrum steel,
. % 5 % Lii 2
analyzed
M. HERCULES
with grazing
Fe (70%) Ni (10%) Cr (16%)
Fe* rl-
56
incidence
I-
FIG. 3. Obtaining grazing incidence spectra with the LAMMAsion) .
(Leybold-Heraeus, with permis-
of a high alloy steel obtained by striking the edge of the sample with the laser beam. Using this technique, along with thin sections, it was possible to obtain meaningful spectra from a variety of samples such as hair, blood platelets, and metalized plastics. A sectioning technique was used to analyze pinewood treated with an aqueous solution to detect F, Cr, and Cu (9). In 1981 Leybold-Heraeus developed the LAMMA-IOOO (20) which differed from the LAMMAin that it permits front surface radiation of the sample. This is shown in Fig. 4. Thus two microprobe capabilities are possible, front surface and transmission. Both can be combined in one instrument such as the LIMA, manufactured by Cambridge Instruments (II). Beam diameters are on the order of a few micrometers, making LAMMA/LIMA a true microprobe technique. LAMMA
&”
-500
0 LAMMA
ion optics
-1000 -2 / ion optics
FIG. 4. Geometries of the LAMMA-SOO and LAMMA-1000.
7
LASER MICROPROBE MASS SPECTROMETRY
IONIZATION PROCESS The mechanisms by which ions are generated using a high intensity laser to impact a solid are not clearly understood. Figure 5 shows a conceptual diagram to indicate the major processes possible in ion formation. Region (1) is where the laser directly impacts the surface. In this region it is likely that direct ion production is limited to small molecular fragments. However, the impact of the laser generates a secondary ionization Region (2) which is probably a semiliquid and is subjected to high thermal gradients and to forces from the shock wave created by laser impact. This produces a surface (3) which is similar to a field ionization source. Ions are probably emitted directly from this surface. There is also a “gas phase” Region (4) directly over the surface in which reactions may occur. The significance of Region (4) is somewhat in question because of the very high pressure gradient between the surface of the sample (several atmospheres) and the vacuum (lop4 Tot-r) less than a micrometer away. A 10 to 20-ns laser pulse is used to produce the ions directly in Region (1). Ions from Regions (2) and (3) will also be emitted as a pulse, slightly time-delayed from the initial laser pulse, although only by a few nanoseconds. Neutrals will be generated from the surfaces as well; the time duration of neutral generation is several orders of magnitude longer than that of ions. Thus one has the situation in which the laser creates a narrow pulse of ions, an ideal situation for mass measurement by a time-of-flight mass spectrometer. The time resolution of the instrument will be limited by the pulse width of the laser and the width of the corresponding ion pulse. A frequency quadrupled Nd-YAG laser is used for ionization, providing photons of 265 nm (4.68 eV, 107.9 kcah’mol). Clearly two or more photons must be involved in the formation of ions for organic compounds. One can represent the overall process according to Solid + hv + Positive ions + Negative ions Neutral Neutral Overall Neutral
.
(1)
Because ion generation involves a neutral solid and a neutral photon beam, positive and negative particles must be produced in equal abundance. Although electron generation occurs in some cases, it is not large, making the abundance of positive and negative ions approximately equal. Although high power densities
Laser
beam
Reqlan of “‘qar-phase” reactmns (4 ) Surface
10n1zat10n i 3)
Direct ionization by laser ( I I
FIG. 5. Ion formation processes in laser mass spectrometry (17).
8
DAVID M. HERCULES
are used in LMS, it is essentially a “soft” ionization technique. This means that ions are emitted having low internal energy, and frequently molecular ions (or quasimolecular ions) are observed. However, if the laser power density is raised beyond a certain critical threshold level, small fragment ions will be produced at the expense of larger ions. For example, a 0.4~p,Jpulse on a solid sample of coronene gave a spectrum containing only M+’ at m/z 300. For the same geometry, when the power level was raised to 14 ~.LJ,the major ions in the spectrum corresponded to C+, C3+, CsH+, Cd+, CSf, C,H+, CT+, and C7H+ (12). Ionization processes for organic compounds in LMS involve four major mechanisms. These are shown in Table 2. Gain or loss of electrons is observed for some organic compounds but is not the dominant process. This distinguishes LMS from electron impact mass spectrometry. Figure 6 shows the positive ion LMS of coronene and the negative ion LMS of TCNQ as examples. An important ionization process is the gain or loss of protons. This is illustrated by Fig. 7, showing the positive and negative ions LMS of phenylalanine. Note that the major ions in the positive ion spectrum correspond to (M + H)+ and (M + H - H2C02)+ and in the negative ion spectrum to (M - H)- . Another very important process in organic LMS is ion attachment; here a molecule attaches to a small (usually inorganic) positive or negative ion. This ionization mechanism is particularly important for organic compounds containing multiple oxygen atoms. Figure 8 shows a spectrum of the polysaccharide, stachyose (5); note that the main peak is at m/z 689, corresponding to (M + Na)+ . It is also possible to observe attachment of negative ions (13). If an organic material exists as a salt, the laser energy can be used to overcome lattice forces and produce both positive and negative ions. An example, shown in Fig. 9, is the negative ion spectrum of sodium valerate. The peak at m/z 101 corresponds to the valerate anion. Notice also the peaks at m/z 113, 169, and 225, corresponding to cluster ions; cluster ions are common in the spectra of both organic and inorganic salts. TABLE 2 Ionization Processes in Laser Mass Spectrometry Gain or loss of electrons nhv M-+ M+’ + eM + e- + M-’ Gain or loss of protons MH+B+M-+BH+ M+BH+MH++BIonization of salts nhv M+X-+M+
+ X-
Ion-attachment reactions M + C+ + MC+ M-k A--+MA-
9
LASER MICROPROBE MASS SPECTROMETRY 100 POSITIVE
Mf’
ION LDMS
I
MW:300 CORONENE z
50.
i
TCNQ MW:204
I
i I
FIG. 6. (A) Positive-ion laser mass spectrum of coronene. (B) Negative-ion laser mass spectrum of TCNQ.
ORGANIC LASER MASS SPECTROMETRY How large nonvolatile molecules behave in LMS. Vitamin B,, and its derivatives represent a good example of large labile molecules investigated by LMS (14). Both positive and negative ion spectra were obtained. In the positive ion spectrum, ions were observed in three regions: the vicinity of (M + H)+ and (M - H)- , small fragment ions below m/z 200, and a series of peaks in the range m/z 400-500. All compounds studied showed (M + H)+ and (M-H)peaks in the range m/z 1300-1600. The LMS at m/z 400-500 showed a series of six peaks which shifted, depending on the structure of the B,, derivative studied. It was concluded that these peaks arise from the corrin ring system, with a group such as CN, OH, or CH, attached; the specific group depends on the nature of the original functionality.
10
DAVID M. HERCULES 100
0/ \ ?
POSITIVE ,
ION
LDMS
d+H-C02H2+
M’H+
CH+t+c-0~
NH:! PHENYLALANINE
H$=CH-COpH
NEGATIVE 100
ION
LDMS
J I
M-H
aQ 50
FIG. 7. Laser mass spectra of phenylalanine. Top, positive-ion spectrum. Bottom, negative-ion spectrum.
We also investigated a series of zwitterionic compounds known as the ammonio alkanoates (1) and the sultanes (2) (13). Structures of these compounds are shown below.
CH, +I
H(CH,), - N - C,H,,COOI CH, 1
CH, +I
H(CH,), - N - C,H,SO, I CH, 2
The number of CH, groups on the chain ranged from 1 to 24. Characteristic features of the positive ion LMS of the alkanoates were prominent peaks due to the negative ion spectra showed (M+CH,)+, (M+H)+, and (M+H-CO,)+;
11
LASER MICROPROBE MASS SPECTROMETRY
I
627
2 50& .c 2 ‘y
365 314 I
j 200
I 300
I
LAMMA LO spectrum of a
429 b,l
mixture
(I 400
II‘I
-
II”, 500
of stachyme and NaCl (5:l by
weight.
-A 600
700
macrosco&xlly).
FIG. 8. Positive-ion laser mass spectrum of stachyose (5).
(M - CH,)-. The sultanes also showed (M + CH3)+ and (M +H)+, along with (M + H - SO,) + and (M + CH, - SOJf . The major peak in the negative ion spectrum was also (M - CH,)- . It was possible to distinguish among these materials structurally on the basis of their LMS, showing that LMS has the same value for structure determination as does normal EI mass spectrometry. Ion-molecule reactions were observed in the negative ion spectra of nitro compounds (16). For example, o-dinitrobenzene showed two major peaks in its 50 NEG.
CH3CH2CH2CH$OO-Na’ SODIUM
ION
VALEAATE
(HC~;)~N~’ 113 HCO;Nim02CCH,CH2CH2CH3 169
20
40 FIG.
50
60
100
150
200
9. Negative-ion laser mass spectrum of sodium valerate.
300
12
DAVID
M. HERCULES
negative ion LMS at m/z 138and 183. The peak at 138corresponds to (M-NO) and 183 corresponds to (M+ 15)-. It was determined that the (M+ 15)- peak arises from substitution of an oxygen atom for a hydrogen on the aromatic ring. The most probable mechanism is ArH + NO*- + ArO- + HNO.
(2)
It was also discovered that organic nitrocompounds mixed with aromatic hydrocarbons transferred an NO, group to the hydrocarbon. For example, a mixture of 1,3,5-trinitrobenzene and coronene gave an LMS peak at m/z 315, corresponding to (M+ 15)- for the aromatic hydrocarbon. It was also discovered that NaN03 and NaNO, could induce the same kind of behavior. For example, 1,2,3,4dibenzanthracene mixed with NaN03 showed its major peak at m/z 293. All of these results are consistent with the mechanism of nucleophilic NO2 substitution. Other examples of ion molecule reactions were discovered. For example, hexachlorobenzene mixed with phenyl-P-glucopyranose showed a pair of peaks at m/z 291 and 293, corresponding to chlorine substitution on the aromatic ring. Not all ion-molecule reactions are desirable. One of the great disappointments of laser mass spectrometry is its inability to obtain meaningful spectra for many transition metal compounds (27). Figure 10 shows the negative ion LMS of Co(en),I,. Note that peaks are observed up to m/z 1252. Upon close inspection it is clear that these peaks correspond to combinations of Co, I, and CN and have no structural relationship to the original compound. The CN ion is typically present in the negative LMS of compounds containing carbon and nitrogen. For example, the peak at 212 is CoICN; the peak at 313 is CoI, and is the base peak in the spectrum. This does not mean that LMS is unable to obtain meaningful spectra from transition metal compounds but that extreme caution must be used when results for unknown compounds are interpreted. It is far more common (18) to see fragmentation than molecular ion formation. LMS OF POLYMERS
A major class of compounds which are nonvolatile and potentially labile are polymers. Thus, the application of LMS to polymers is a natural. Three types of information can be obtained from LMS for polymers: oligomer distributions, structural identification, and information concerning the sequence of units in the polymer chain. Figure 11 shows the oligomer distribution obtained from a polyethylene glycol of average molecular weight (M,,) of 400 (19). Note that the intense peaks in the spectrum correspond to cationization of oligomers with sodium. Table 3 shows a comparison of average molecular weight determinations for some glycols between LMS and other methods. Agreement is quite good, even up to average molecular weights of M, = 1000. Beyond this value low values were observed by LMS due to decreased detector response in the high mass range. Fragment ion peaks are observed for the polyethylene glycols, corresponding to cleavage around the ether bond with hydrogen transfer. Fragment peaks in the lower mass range can clearly be distinguished from the oligomers. Oligomer dis-
13
LASER MICROPROBE MASS SPECTROMETRY POSITIVE
IONS
LDMS
1 COL 44
COI
co2 118
166
"Q
cop 102
IL
-
NEQATIVE
Co21[CN]
I
271
245
TRIS [ ETHYLENEDIAMINE
] COBALT
306
COP’
-
II
IONS
IODIDE
2
372 !
[III]
COL21
c02L213 610
-__
-
LDMS
‘3
I CN 2s
co12 313
127 I
I
I
COBALT
[III]IODIDE
CO216
I
Col [CN] 212
i
TRIS [ETHYLENEDIAMINE] co13 440 753
I
1233
1,”
c02l3 499 I
c02’4 628 I
co315
c031s 039
co317 1066 ,
-41s
FIG. 10. Laser mass spectra of Tris(ethylenediamine) cobalt(III) iodine (17). Top, positive-ion spectrum. Bottom, negative-ion spectrum.
tributions for polydimethylsiloxanes have been obtained with peaks observed above m/z 3000 (20). It is also possible to distinguish between polymers on the basis of low massrange spectra (21). Figure 12 shows the positive LMS of polyacrylamide and polystyrene as an example. It is clear from inspection that the two are different; the major peaks in the spectra can be correlated with structural features of the polymers. The use of both positive and negative ion spectra for polymer characterization is also important. This is illustrated by the difference between poly(benzylmethacrylate) and poly(phenylmethacrylate). For the polybenzyl isomer the major peak in the positive ion spectrum is at m/z 91, corresponding to the benzyl ion. The negative ion spectrum is uninstructive. In the case of poly(phenylmethacrylate), the positive ion spectrum gives no structural information, but the negative ion spectrum shows a major peak at m/z 93, corresponding to the phenoxide ion (6).
14
DAVID
M. HERCULES I
[M i+Na] [h4 i+~]+
Na
250
Na Na
Na
Na
Na
]
r”, / ,“/,“<
Na
H 1
H I
500
400
300
Na
H 1
4 600
700
600
M/Z
FIG. 11. Positive-ion laser mass spectrum of polyethylene glycol (PEG) 400 (19).
Another example of polymer characterization using LMS is measurement of forward versus backward addition in polyvinylidine fluoride (PVF,); the structure is 6%
- CW, 3
The LMS of PVF, shows a large number of peaks in the mass range m/z < 200. Four important peaks are at m/z 59, 77, 95, and 113, corresponding to the threecarbon fragment ions shown in Fig. 13 (22). A polyvinylidine fluoride chain resulting from normal addition can produce only two three-carbon fragments of m/z 59 and 113. For the case of head-to-head and tail-to-tail addition, three-carbon fragments are seen at m/z 95 and 77, respectively. Thus, by measuring the combined intensities of m/z 95 and 77 relative to the total of all three-carbon fragTABLE 3 M, Determinations for Poly(ethylene glycols) (19) PEG sample
End-group titration
EHDMS
FDMS
LMS
400 600 1000 1400
406 605 1041 1396
406 (1.05) 572 (1.06) 962 (1.05) 1365 (1.02)
444 (1.05) 601 (1.05) 1010 (1.03) 1360 (1.03)
420 (1.05) 618 (1.03) 1007 (1.02) 1266 (1.03)
Note. M JMn values are in parentheses.
15
LASER MICROPROBE MASS SPECTROMETRY 72
39
165
4
94
143
II
II 100
80
II111
I 140
1 120
160
180
91
39
77 115 63 51
1
103
I
I I
I
165 I
I I
27
I 20
1 40
60
1 80
100
rll(It 1I 120
140
160
189
180
FIG. 12. Positive-ion laser mass spectra of polymers (21). (A) polyacrylamide. (B) Polystyrene.
ments, one can estimate the percentage of backward addition in the polymer chain. When LMS results were compared with a similar analysis done by 19F NMR for three polymer samples, the percentages of backward addition measured by LAMMA were 2.4, 4.3, and 4.8, while the results by NMR for the same samples were 2.5, 4.7, and 4.8. Although the precision of the LAMMA analysis (+20%) was not as good as that of NMR (+S%), the LAMMA analyses could be performed in 10 min on the undissolved sample, while the NMR analyses required sample dissolution and long data acquisition times. LMS UNDER ATMOSPHERIC
CONDITIONS
Two problems associated with obtaining laser mass spectra of solids are possible loss of volatile materials in the vacuum and extensive degassing from the
16
DAVID Head
M. HERCULES -To-Tall
(Normal
Addlilon Add!i!on)
- CH,CF,CH,kF,CH,CF,~
-7-Y 0 CH,-CF m/z
0 CF,-
= CH,
Head
CH = CF2
m/z = 113
=59 -To
- Head
Addltmn
- CH,CH,CF,CF,CH,CF,-
77 CF2
CFz = CF-CH,O m/z
Tall
-To-Toll
m/z
A = CF, = 77
CF-CH,O = 35
Addlion
- CH,CF,‘CH,CH,CF,‘CH,-
0 CH,-CH
q
m/z
= 35
I \ CH,m/z
CH = CF2 = 77
FIG. 13. Three-carbon fragment ions from polyvinylidene fluoride (22).
sample, both of which may overtax the spectrometer pumping system. To overcome these problems we developed a technique for obtaining laser mass spectra at atmospheric pressure; a specific example was obtaining LMS of a pesticide on a leaf (23). Figure 14 shows how the specimen was obtained. A very thin Formvar film was deposited from solution on a glass slide and floated on water. The sample from which a spectrum was to be obtained was placed with the front surface on the Formvar film and a zinc foil touched to the Formvar film so that it sealed the sample. The laser then penetrated the Formvar film, and ions from the sample surface were extracted into the laser mass spectrometer. When LMS was obtained from a leaf under Formvar with no pesticide added, the positive ion spectrum was amazingly clean. Only Na+, K+, and Caf ions showed any significant intensity. Although several peaks were observed in the negative ion spectrum, no significant background peaks were observed above m/z 200. Figure 15 shows a spectrum of the commercial preparation Dursban sprayed onto grass. The active ingredient in Dursban is chlorpyriphos, the structure of which is Cl cl
The spectrum at m/z 352 corresponds to (M + H)+ from Dursban; peaks at 370 and
LASER MICROPROBE MASS SPECTROMETRY
17
------- ~.--;;_; Deionized
Water
Leaf
Section
Formvar Film -------________________ ________________________________________-----------------------
VACUUM
VACUUM
zinc foil leaf section formvar film atmospheric
pressure
FIG. 14. Method for obtaining laser mass spectra from a leaf surface. (23).
388 correspond to addition of water. The main peaks in the spectrum correspond to known fragments of chlorpyriphos (24). The peaks for Dursban accurately reflect the chlorine isotope distribution for chlorpyriphos. Also, the spectrum for Dursban on grass accurately reflects the spectrum obtained for chlorpyriphos on zinc in an atmosphere saturated with water vapor. LMS MAPPING
MICROPROBE
The laser microprobe was modified by adding stepper controller motors so that it was possible to obtain a map of a surface for a particular peak in the laser mass spectrum. The minimum possible step size was 1.25 km, the width of the beam is about 2.5 pm, limiting the spatial resolution to ca. 3 km. The scanning microprobe has been described in detail elsewhere (25). To test the performance of the scanning microprobe, a series of cationic dyes were deposited in a grid pattern. Typical dyes were Gentian Violet (5) and Brilliant Green (6), as shown.
18
DAVID M. HERCULES 206
260
169
262
(M-Z)+
206
(Z+Ns+H)+
262
W+H-2EtO)+
260
(M+H-2EtO+H20)+
352
(M+H)+
370
(M+H+H20)+
380
(M+H+2H20)+
352
S
Cl
N I
II
Chlorpyriphos
I,
(EtO)2i-6
349 Cl
m Cl ’
III
II ,“:“388h I * I II..,
III,
I,,,
200
250
III,,’
II .
I’,“” 350
300
400
450
II lrrl(I
&
500
m/z
FIG. 15. Laser mass spectrum of Dursban on grass (23).
5
6
TEM grids of various mesh sizes were laid on nitrocellulose and a dye solution electrosprayed onto the composite. Therefore, the sample consisted of a series of square dye spots on nitrocellulose, along with uncovered nitrocellulose areas. Figure 16 shows a map of the molecular ion of Gentian Violet on nitrocellulose obtained for a sample prepared with a loo-mesh grid. The map was obtained with m/z 372; the map area is 750 x 750 pm. Each step corresponds to 25 p.m. An identical map was obtained at m/z 355 using a fragment ion peak of Gentian Violet. The square pattern of the grid work was maintained. Similar maps were obtained for 400-mesh and lOOO-meshsamples; the latter corresponds to an area of 85 x 85 pm, with a minimum step size of 2.5 pm. For the 1000-mesh grid the squares became elongated into rectangles because of the elliptical shape of the laser beam impacting the surface at 45”. The results represent the first successful map of an organic material on an organic surface and open the possibility for obtaining maps of organic species similar to the way in which Auger spectroscopy has been used to obtain elemental maps from surfaces. Maps were also obtained from inclusions in coal known as macerals. For exam-
LASER MICROPROBE MASS SPECTROMETRY
19
FIG. 16. Laser microprobe map of Gentian Violet on nitrocellulose (25).
ple, the map of FeS, from a coal maceral is shown in Fig. 17. This map was obtained at mass 120 (FeS,-); the area map is 350 km square, with a step size of 10 pm. It is clear that FeS,- arises from an hourglass-type structure within the coal maceral. WHAT OF THE FUTURE
It is clear from the above that the laser microprobe mass spectrometer permits one to obtain mass spectra from involatile materials and to do so for microscopic inclusions. It is possible to study fragile molecules and high molecular weight materials. The general assets of laser microprobe mass spectrometry are summarized in Table 4. It is applicable to both organic and inorganic materials. It is a soft ionization method and thus provides structural information. Although the mass range has been generally limited to below m/z 1000, this is not a limit of the time-of-flight analyzers used. It is also clear that the laser mass spectrometer can be used as an analytical microprobe and that mapping capabilities are possible. Of particular importance is the ability to map organic materials on organic substrates. The method is semiquantitative. Under the best of circumstances precision is probably no better than +lO%. In general, sample charging is not a problem as with many surface techniques. Given the above, what are the current limitations of laser microprobe mass spectrometry? These are summarized in Table 5. First, it is not a high resolution technique. The mass analyzers used have M/AM equal to 1000 or less, and thus exact mass measurements are not possible. A second problem is that spatial
20
DAVID M. HERCULES
FIG. 17. Laser microprobe map of a coal mace&.
resolution is generally limited to a few micrometers because of the characteristics of the laser beam. Yet many important microprobe problems involve an order of magnitude smaller domains. Third, the technique is not particularly selective. Spectra are generated by nonspecific excitation processes; photon selection has not been used. Thus a current limitation is the need for predictable ionization probabilities for different compounds. Another current limitation is sensitivity. Although the absolute number of ions detected is often very small, it would be desirable to increase this by one or two orders of magnitude. This is particularly important as one seeks to measure smaller and smaller domains. Mixtures and matrix effects are problematic. Interactions between compounds occur, and equimolar mixtures of materials having vastly different molar absorbtivities at 265 nm will produce vastly different numbers of ions. And finally, it really helps to un-
TABLE 4 Assets of Laser Microprobe Mass Spectrometry 1. 2. 3. 4. 5. 6. 7.
Applicable to inorganic and organic materials. Soft ionization method-provides structural information. Large potential mass range-not analyzer limited. Functions well as an analytical microprobe-mapping possible. Can do maps of organics on organics. Method is semiquantitative at worst. Charging of sample is not a general problem.
LASER MICROPROBE MASS SPECTROMETRY
21
TABLE 5 Current Limitations of Laser Microprobe Mass Spectrometry 1. Not a high-resolution technique. M/AM = looo. 2. Spatial resolution is 5 Wm. Important problems are 0.1 km. 3. Need for selectivity or at least predictable ionization probabilities, 4. Improved sensitivity needed (S/N). 5. Mixtures and matrix effects are problematic. 6. Fundamentals-it helps to understand what you are doing.
derstand what one is doing. The fundamentals of the interaction of laser beams with organic solids are not well understood. The future development of laser mass spectrometry then, as shown in Table 6, relates to the problems above. The laser spot size is approaching the diffraction limit. With decreases in optical aberration, one can perhaps reduce the laser spot size to the vicinity of 0.1 to 0.2 km. The signal-to-noise ratio is limited by background in the spectrum which is primarily chemical noise. Thus research is necessary on the origin or chemical noise in LMS and how this relates to noise from the detector electronics. Third is the use of neutrals. The possibility of using postionization of neutrals has the potential for generating larger numbers of ions, since the number of neutrals exceeds the number of ions generated by several orders of magnitude. However, all is not necessarily sweetness and light. Figure 18 illustrates some problems in the postionization of neutrals. In Fig. 18 the ions are formed by a narrow laser pulse at t = 0; the neutrals are emitted over a much longer time. If one uses a second narrow laser pulse (R = 1000) at the maximum of neutral emission to ionize neutrals, one will utilize only a small fraction of the total neutrals emitted. If one uses a much broader postionizing pulse (R = 33), a much greater fraction of the neutrals can be utilized. However, the broader pulse will significantly degrade the resolution of the spectrometer. The R = 1000 pulse has a half-width equal to that of the initial laser pulse, and the overall spectrometer resolution of M/AA4 = 1000 is maintained. The increased width of the R = 33 TABLE 6 Future Development for Laser Microprobe Mass Spectrometry Current problem 1. Laser spot size 2. S/N ratio 3. Use of neutrals 4. Selectivity and resolution
5. Mass range
Solution Diffraction limit Reduce background chemical noise R-E-S-E-A-R-C-H Postionization MS-MS methods CID Resonance ionization TOF M/AM > 10000 How big an ion can we make? R-E-S-E-A-R-C-H
22
DAVID
M. HERCULES
Neutrals Emitted Post Ionizing
Time
FIG. 18. Time dependence of ion and neutral emission in laser mass spectrometry.
postionizing pulse decreases the overall spectrometer resolution to 33. Thus effective utilization of more neutrals to improve sensitivity results in loss of resolution by several orders of magnitude. There are several possibilities for improving the selectivity and resolution of laser mass spectrometry. Time-of-flight analyzers have resolutions of M/AM > 10,000 (26). Therefore, the possibilities of exact mass measurements do exist. Selectivity also can be improved by incorporating the kind of experiments which have been used in conventional mass spectrometry. For example, MS/MS methods and CID methods allow for identification of ions by performing mass spectrometry on an initially formed ion by some secondary process, either before or after collision. So far only a few of these types of experiments have been tried for LMS. Finally, the important question really is how large an ion can we make? From experiments on the same polymers using both LMS and secondary ion mass spectrometry (SIMS) (27), it is clear that in the SIMS experiment, larger ions are detected. Thus an understanding of the ion formation process of large molecules by lasers is an important area of basic research for the future. In summary laser mass spectrometry has much potential for many practical problems. However, it also has current limitations. It is clear that studies on ion formation processes and the nature of a chemical noise are fundamentally important to the future development of the technique. ACKNOWLEDGMENTS I gratefully acknowledge the support of our LMS research by the Office of Naval Research and the National Science Foundation. I also acknowledge assistance in our program by Leybold-Heraeus and wish to thank the Alexander von Humboldt Foundation for a Fellowship. Finally I want to thank the members of my research group who have contributed significantly to our development of the laser microprobe mass spectrometer and to the understanding of the technique.
REFERENCES 1. Honig, R. E.; Woolston, J. R. Appl. Phys. L&t., 1963, 2, 138. 2. Vastola, F. J.; Pirone, A. J. Adv. Mass Spectrom., 1968, 4, 107-l 11. 3. Nitsche, R.; Kaufmann, R.; Hillenkamp, F.; Unsold, E.; Vogt, H.; Wechsung, R. Zsr. J. Chem., 1978, 17, 181-184.
LASER MICROPROBE MASS SPECTROMETRY
23
4. Posthumus, M. A.; Kistemaker, P. G.; Meuzelaar, H. L. C.; Ten Noever de Brauw, M. C. Anal. Chem., 1978, 50, 985-991. 5. Heinen, H. J.; Meier, S.; Vogt, H.; Wechsung, R. Adv. Mass Spectrom., 1980, 8A, 942-953. 6. Gardella, J. A.; Hercules, D. M.; Heinen, H. J. Spectrosc. Lett., 1980, 13, 347-360. 7. Heinen, H. J.; Hillenkamp, F.; Kaufmann, R.; Schroder, W.; Wechsung, R. Proc. 6th Int. Symp. Mass Spectrosc. Biochem. Med., Venice, 1979. 8. Schriider, W. H. Z. Anal. Chem., 1981, 308, 212-217. 9. Klein, P.; Bauch, J. Z. Anal. Chem., 1981, 308, 283-286. 10. Heinen, H. J.; Meier, S.; Vogt, H.; Wechsung, R. Znt. J. Mass Spectrom. Ion Phys., 1983, 47, 19-22. 11. Dingle, T.; Griffrths, B. W. Microbeam Anal., 1984, 19, 23-26. 12. Denoyer, E.; vanGrieken, R.; Adams, F.; Natusch, D. F. S. Anal. Chem., 1982, 54, 26A. 13. Balasanmugam, K.; Dang, T. A.; Day, R. J.; Hercules, D. M. Anal. Chem., 1981,53,2296-2298. 14. Graham, S. W.; Dowd, P.; Hercules, D. M. Anal. Chem., 1982, 54, 649-654. 15. Balasanmugan, K.; Hercules, D. M. Anal. Chim. Acta., 1984, 166, l-26. 16. Balasanmugan, K.; Viswanadham, S. K.; Hercules, D. M. Anal. Chem., 1983, 58, 2424-2426. 17. Hercules, D. M.; Day, R. J.; Balasanmugam, K.; Dang, T. A.; Li C. P. Anal. Chem., 1982, 54, 280A. 18. Balasanmugam, K.; Day, R. J.; Hercules, D. M. Znorg. Chem., 1985, 24, 4477-4483. 19. Mattem, D. E.; Hercules, D. M. Anal. Chem., 1985, 57, 2041-2046. 20. Holm, R.; Karas, M.; Vogt, H. Anal. Chem., 1987, 59, 373-376. 21. Gardella, J. A.; Hercules, D. M. Z. Anal. Chem., 1981, 308, 297. 22. Mattem, D. E.; Lin, F. T.; Hercules, D. M. Anal. Chem., 1984, 56, 2762-2769. 23. Morelli, J. J.; Hercules, D. M. Anal. Chem., 1986, 58, 12961298. 24. Morelli, J. J.; Viswanadham, S. K.; Sharkey, A. G., Jr.; Hercules, D. M. Int. J. Environ. Anal. Chem., 1987, 31, 295-323. 25. Wilk, Z. A.; Hercules, D. M. Anal. Chem., 1987, 59, 1819-1825. 26. Niehuis, E.; Heller, T.; Feld, H.; Benninghoven, A. J. Vat. Sci. Technol. A, 1987, 5, 1243-1246. 27. Bletsos, I. V.; Hercules, D. M.; vanleyen, D.; Benninghoven, A. Macromolecules, 1987, 20, 407-413.
JOURNAL
38, 3-23 (1988)
Laser Microprobe
Mass Spectrometry: and Future’
The Past, Present,
DAVID M. HERCULES Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Received February 4, 1988; accepted February 4, 1988 Development of the laser mass spectrometer has been a major step forward in solid state mass spectrometry. This technique has allowed routine acquisition of mass spectra of solids in the middle mass range. Although the volatilization/ionization mechanism(s) is not well understood, laser mass spectrometry is clearly a “soft” ionization technique. Even though power densities in the range lo’-lo9 W/cm* are used, quasimolecular ions are frequently observed even from labile molecules. Laser mass spectra show approximately equal intensities for both positive and negative ions. Examples are cited to illustrate the versatility of laser mass spectrometry applied to a variety of problems. These include quantitative analysis, polymer chemistry, mapping of organic surface impurities, and obtaining spectra under atmospheric pressure. A survey is presented of the types of spectra observed in laser mass spectrometry; of particular importance are ions produced by either proton transfer or cationization. The future of laser mass spectrometry is addressed. Of particular importance is how the technique can develop for technical analyses. An assessment is given of how laser mass spectrometry compares with other microprobe techniques and what potentially limits its application to the analysis of smaller domains in solids. 0 1988 Academic PXSS, 1~.
INTRODUCTION
Mass spectrometry has been one of the most successful analytical methods developed in recent times, having profound impact on diverse fields. There is an ever-increasing trend toward obtaining spectra from materials that have high molecular weight, have low volatility, are thermally unstable, or some combination of the three. Over the past several decades, a number of solid-state ion sources have been developed which have largely solved the problems associated with obtaining meaningful mass spectra from technical materials. However, with increased emphasis on microdomains, the need for a mass spectral microprobe is readily apparent. The current paper reviews developments to date in laser microprobe mass spectrometry and gives some projections for this technique for the future. The concept of using a laser to directly ionize a solid material is not new. In 1963 Honig and Woolston published the first report of laser mass spectrometry (LMS) (I). A few years later Vastola and co-workers published attempts to apply laser mass spectrometry to the analysis of coal, organic, and biological compounds (2). The results of this early work were largely limited by both mass analyzers and laser technology. However, these early studies clearly showed the feasibility of the method. ’ This article is based on the Benedetti-Pichler Award Address presented by Professor Hercules at the 26th Eastern Analytical Symposium, September, 18, 1987. 3 0026-265X/88 $1.50 Copyright 0 lY88 by Academic Press, Inc. All rights of reproduction in any form reserved.
4
DAVID M. HERCULES
Table 1 designates the milestones in the development of laser mass spectrometry. Nearly a decade after the work of Vastola et al., Kaufman, Hillenkamp, and Wechsung (3) developed the LAMMA-500, the first commercial laser microprobe mass spectrometer. Independently, the application of laser desorption to large molecules by Kistemaker et al. (4) showed the potential of the technique for compounds of this sort. In subsequent years extensive application of LMS to organic compounds was studied by many groups, particularly by Heinen et al. (5); the initial application of LMS to polymers came from our laboratory (6). A block diagram of the LAMMA(the first commercially developed laser microprobe mass spectrometer) is shown in Fig. 1. A beam from the pulsed laser is focused on the sample through a microscope, and the pulse from the laser simultaneously triggers a transient recorder by a signal from a photodiode. This starts a clock which times the flight of the ions down the tube of the mass spectrometer; the ions are accelerated into the flight tube as they leave the sample. An electrostatic reflector is used to correct the spread of ion kinetic energies; this helps to narrow the spectral lines. When operating as a microprobe, the pilot laser (He-Ne) indicates where the main laser beam will strike the sample. The microscope focuses both laser beams on the sample. The bottom of Fig. 1 shows the sample arrangement in the LAMMA-500. Note that the microscope introduces the laser beam through an oil immersion lens system and that the laser shot must penetrate the sample for ion production. The instrument features interchangeable illumination source and ion lens systems which permit reproducible positioning of the laser on the sample. Most of the early work with the LAMMAwas on thin sections; the primary purpose of its development was to study metal ions in biological materials. Figure 2 shows a spectrum of a thin resin section doped with alkali ions; spectral lines from each element are clearly evident. The detection limits for metal ions are very impressive, on the order of IO-l9 to 10e2’ g (7). Even on a relative (ppm) basis, the detection limits correspond to la. 1 ppm. The concentration detection limit is TABLE 1 Milestones in the Development of Laser Microprobe Mass Spectrometry Year 1963 1968 1977 1978 1979 1980 1981
First report of laser mass spectrometry (Honig and Woolston) First attempts to apply LMS to analysis of coal, organic (Vastola et al.) Development of LAMMAJOO (Kaufmann, Hillenkamp, and Wechsung) Application of laser desorption to large molecules (Kistemaker et al.) Organic compounds studied with LMS (Heinen et al.) Polymers studied with LMS (Gardella and Hercules) Development of LAMMA(Leybold-Heraeus)
LASER
MICROPROBE
5
MASS SPECTROMETRY
2+~!kE$$E>~L
ILLUMINATION
ION
LENS
I
;
ANALYSIS
@----4
FIG.
1 1
SAMPLE
TRANSrn
1. Block diagram of LAMMA-SOO laser microprobe mass spectrometer.
in the ppm range because of the small volume sampled by the laser. Studies of metal ions were very successful using the LAMMA, for example, use of %a2+ to study the calcium uptake by retinal tissue (8). One of the problems with the LAMMAgeometry was that solids had to be studied using grazing incidence, as illustrated in Fig. 3. This shows the spectrum 3
Rb’
Spurr
resin
0.3
m thick,
doped
with
1 mM
Li. Na,
K, Rb.
CS’
Cs
FIG. 2. LAMMAJOO spectrum of thin resin section doped with alkali salts (Leybold-Heraeus, with permission).
6
DAVID
I
LAMMA
Mass
highly alloyed
Spectrum steel,
. % 5 % Lii 2
analyzed
M. HERCULES
with grazing
Fe (70%) Ni (10%) Cr (16%)
Fe* rl-
56
incidence
I-
FIG. 3. Obtaining grazing incidence spectra with the LAMMAsion) .
(Leybold-Heraeus, with permis-
of a high alloy steel obtained by striking the edge of the sample with the laser beam. Using this technique, along with thin sections, it was possible to obtain meaningful spectra from a variety of samples such as hair, blood platelets, and metalized plastics. A sectioning technique was used to analyze pinewood treated with an aqueous solution to detect F, Cr, and Cu (9). In 1981 Leybold-Heraeus developed the LAMMA-IOOO (20) which differed from the LAMMAin that it permits front surface radiation of the sample. This is shown in Fig. 4. Thus two microprobe capabilities are possible, front surface and transmission. Both can be combined in one instrument such as the LIMA, manufactured by Cambridge Instruments (II). Beam diameters are on the order of a few micrometers, making LAMMA/LIMA a true microprobe technique. LAMMA
&”
-500
0 LAMMA
ion optics
-1000 -2 / ion optics
FIG. 4. Geometries of the LAMMA-SOO and LAMMA-1000.
7
LASER MICROPROBE MASS SPECTROMETRY
IONIZATION PROCESS The mechanisms by which ions are generated using a high intensity laser to impact a solid are not clearly understood. Figure 5 shows a conceptual diagram to indicate the major processes possible in ion formation. Region (1) is where the laser directly impacts the surface. In this region it is likely that direct ion production is limited to small molecular fragments. However, the impact of the laser generates a secondary ionization Region (2) which is probably a semiliquid and is subjected to high thermal gradients and to forces from the shock wave created by laser impact. This produces a surface (3) which is similar to a field ionization source. Ions are probably emitted directly from this surface. There is also a “gas phase” Region (4) directly over the surface in which reactions may occur. The significance of Region (4) is somewhat in question because of the very high pressure gradient between the surface of the sample (several atmospheres) and the vacuum (lop4 Tot-r) less than a micrometer away. A 10 to 20-ns laser pulse is used to produce the ions directly in Region (1). Ions from Regions (2) and (3) will also be emitted as a pulse, slightly time-delayed from the initial laser pulse, although only by a few nanoseconds. Neutrals will be generated from the surfaces as well; the time duration of neutral generation is several orders of magnitude longer than that of ions. Thus one has the situation in which the laser creates a narrow pulse of ions, an ideal situation for mass measurement by a time-of-flight mass spectrometer. The time resolution of the instrument will be limited by the pulse width of the laser and the width of the corresponding ion pulse. A frequency quadrupled Nd-YAG laser is used for ionization, providing photons of 265 nm (4.68 eV, 107.9 kcah’mol). Clearly two or more photons must be involved in the formation of ions for organic compounds. One can represent the overall process according to Solid + hv + Positive ions + Negative ions Neutral Neutral Overall Neutral
.
(1)
Because ion generation involves a neutral solid and a neutral photon beam, positive and negative particles must be produced in equal abundance. Although electron generation occurs in some cases, it is not large, making the abundance of positive and negative ions approximately equal. Although high power densities
Laser
beam
Reqlan of “‘qar-phase” reactmns (4 ) Surface
10n1zat10n i 3)
Direct ionization by laser ( I I
FIG. 5. Ion formation processes in laser mass spectrometry (17).
8
DAVID M. HERCULES
are used in LMS, it is essentially a “soft” ionization technique. This means that ions are emitted having low internal energy, and frequently molecular ions (or quasimolecular ions) are observed. However, if the laser power density is raised beyond a certain critical threshold level, small fragment ions will be produced at the expense of larger ions. For example, a 0.4~p,Jpulse on a solid sample of coronene gave a spectrum containing only M+’ at m/z 300. For the same geometry, when the power level was raised to 14 ~.LJ,the major ions in the spectrum corresponded to C+, C3+, CsH+, Cd+, CSf, C,H+, CT+, and C7H+ (12). Ionization processes for organic compounds in LMS involve four major mechanisms. These are shown in Table 2. Gain or loss of electrons is observed for some organic compounds but is not the dominant process. This distinguishes LMS from electron impact mass spectrometry. Figure 6 shows the positive ion LMS of coronene and the negative ion LMS of TCNQ as examples. An important ionization process is the gain or loss of protons. This is illustrated by Fig. 7, showing the positive and negative ions LMS of phenylalanine. Note that the major ions in the positive ion spectrum correspond to (M + H)+ and (M + H - H2C02)+ and in the negative ion spectrum to (M - H)- . Another very important process in organic LMS is ion attachment; here a molecule attaches to a small (usually inorganic) positive or negative ion. This ionization mechanism is particularly important for organic compounds containing multiple oxygen atoms. Figure 8 shows a spectrum of the polysaccharide, stachyose (5); note that the main peak is at m/z 689, corresponding to (M + Na)+ . It is also possible to observe attachment of negative ions (13). If an organic material exists as a salt, the laser energy can be used to overcome lattice forces and produce both positive and negative ions. An example, shown in Fig. 9, is the negative ion spectrum of sodium valerate. The peak at m/z 101 corresponds to the valerate anion. Notice also the peaks at m/z 113, 169, and 225, corresponding to cluster ions; cluster ions are common in the spectra of both organic and inorganic salts. TABLE 2 Ionization Processes in Laser Mass Spectrometry Gain or loss of electrons nhv M-+ M+’ + eM + e- + M-’ Gain or loss of protons MH+B+M-+BH+ M+BH+MH++BIonization of salts nhv M+X-+M+
+ X-
Ion-attachment reactions M + C+ + MC+ M-k A--+MA-
9
LASER MICROPROBE MASS SPECTROMETRY 100 POSITIVE
Mf’
ION LDMS
I
MW:300 CORONENE z
50.
i
TCNQ MW:204
I
i I
FIG. 6. (A) Positive-ion laser mass spectrum of coronene. (B) Negative-ion laser mass spectrum of TCNQ.
ORGANIC LASER MASS SPECTROMETRY How large nonvolatile molecules behave in LMS. Vitamin B,, and its derivatives represent a good example of large labile molecules investigated by LMS (14). Both positive and negative ion spectra were obtained. In the positive ion spectrum, ions were observed in three regions: the vicinity of (M + H)+ and (M - H)- , small fragment ions below m/z 200, and a series of peaks in the range m/z 400-500. All compounds studied showed (M + H)+ and (M-H)peaks in the range m/z 1300-1600. The LMS at m/z 400-500 showed a series of six peaks which shifted, depending on the structure of the B,, derivative studied. It was concluded that these peaks arise from the corrin ring system, with a group such as CN, OH, or CH, attached; the specific group depends on the nature of the original functionality.
10
DAVID M. HERCULES 100
0/ \ ?
POSITIVE ,
ION
LDMS
d+H-C02H2+
M’H+
CH+t+c-0~
NH:! PHENYLALANINE
H$=CH-COpH
NEGATIVE 100
ION
LDMS
J I
M-H
aQ 50
FIG. 7. Laser mass spectra of phenylalanine. Top, positive-ion spectrum. Bottom, negative-ion spectrum.
We also investigated a series of zwitterionic compounds known as the ammonio alkanoates (1) and the sultanes (2) (13). Structures of these compounds are shown below.
CH, +I
H(CH,), - N - C,H,,COOI CH, 1
CH, +I
H(CH,), - N - C,H,SO, I CH, 2
The number of CH, groups on the chain ranged from 1 to 24. Characteristic features of the positive ion LMS of the alkanoates were prominent peaks due to the negative ion spectra showed (M+CH,)+, (M+H)+, and (M+H-CO,)+;
11
LASER MICROPROBE MASS SPECTROMETRY
I
627
2 50& .c 2 ‘y
365 314 I
j 200
I 300
I
LAMMA LO spectrum of a
429 b,l
mixture
(I 400
II‘I
-
II”, 500
of stachyme and NaCl (5:l by
weight.
-A 600
700
macrosco&xlly).
FIG. 8. Positive-ion laser mass spectrum of stachyose (5).
(M - CH,)-. The sultanes also showed (M + CH3)+ and (M +H)+, along with (M + H - SO,) + and (M + CH, - SOJf . The major peak in the negative ion spectrum was also (M - CH,)- . It was possible to distinguish among these materials structurally on the basis of their LMS, showing that LMS has the same value for structure determination as does normal EI mass spectrometry. Ion-molecule reactions were observed in the negative ion spectra of nitro compounds (16). For example, o-dinitrobenzene showed two major peaks in its 50 NEG.
CH3CH2CH2CH$OO-Na’ SODIUM
ION
VALEAATE
(HC~;)~N~’ 113 HCO;Nim02CCH,CH2CH2CH3 169
20
40 FIG.
50
60
100
150
200
9. Negative-ion laser mass spectrum of sodium valerate.
300
12
DAVID
M. HERCULES
negative ion LMS at m/z 138and 183. The peak at 138corresponds to (M-NO) and 183 corresponds to (M+ 15)-. It was determined that the (M+ 15)- peak arises from substitution of an oxygen atom for a hydrogen on the aromatic ring. The most probable mechanism is ArH + NO*- + ArO- + HNO.
(2)
It was also discovered that organic nitrocompounds mixed with aromatic hydrocarbons transferred an NO, group to the hydrocarbon. For example, a mixture of 1,3,5-trinitrobenzene and coronene gave an LMS peak at m/z 315, corresponding to (M+ 15)- for the aromatic hydrocarbon. It was also discovered that NaN03 and NaNO, could induce the same kind of behavior. For example, 1,2,3,4dibenzanthracene mixed with NaN03 showed its major peak at m/z 293. All of these results are consistent with the mechanism of nucleophilic NO2 substitution. Other examples of ion molecule reactions were discovered. For example, hexachlorobenzene mixed with phenyl-P-glucopyranose showed a pair of peaks at m/z 291 and 293, corresponding to chlorine substitution on the aromatic ring. Not all ion-molecule reactions are desirable. One of the great disappointments of laser mass spectrometry is its inability to obtain meaningful spectra for many transition metal compounds (27). Figure 10 shows the negative ion LMS of Co(en),I,. Note that peaks are observed up to m/z 1252. Upon close inspection it is clear that these peaks correspond to combinations of Co, I, and CN and have no structural relationship to the original compound. The CN ion is typically present in the negative LMS of compounds containing carbon and nitrogen. For example, the peak at 212 is CoICN; the peak at 313 is CoI, and is the base peak in the spectrum. This does not mean that LMS is unable to obtain meaningful spectra from transition metal compounds but that extreme caution must be used when results for unknown compounds are interpreted. It is far more common (18) to see fragmentation than molecular ion formation. LMS OF POLYMERS
A major class of compounds which are nonvolatile and potentially labile are polymers. Thus, the application of LMS to polymers is a natural. Three types of information can be obtained from LMS for polymers: oligomer distributions, structural identification, and information concerning the sequence of units in the polymer chain. Figure 11 shows the oligomer distribution obtained from a polyethylene glycol of average molecular weight (M,,) of 400 (19). Note that the intense peaks in the spectrum correspond to cationization of oligomers with sodium. Table 3 shows a comparison of average molecular weight determinations for some glycols between LMS and other methods. Agreement is quite good, even up to average molecular weights of M, = 1000. Beyond this value low values were observed by LMS due to decreased detector response in the high mass range. Fragment ion peaks are observed for the polyethylene glycols, corresponding to cleavage around the ether bond with hydrogen transfer. Fragment peaks in the lower mass range can clearly be distinguished from the oligomers. Oligomer dis-
13
LASER MICROPROBE MASS SPECTROMETRY POSITIVE
IONS
LDMS
1 COL 44
COI
co2 118
166
"Q
cop 102
IL
-
NEQATIVE
Co21[CN]
I
271
245
TRIS [ ETHYLENEDIAMINE
] COBALT
306
COP’
-
II
IONS
IODIDE
2
372 !
[III]
COL21
c02L213 610
-__
-
LDMS
‘3
I CN 2s
co12 313
127 I
I
I
COBALT
[III]IODIDE
CO216
I
Col [CN] 212
i
TRIS [ETHYLENEDIAMINE] co13 440 753
I
1233
1,”
c02l3 499 I
c02’4 628 I
co315
c031s 039
co317 1066 ,
-41s
FIG. 10. Laser mass spectra of Tris(ethylenediamine) cobalt(III) iodine (17). Top, positive-ion spectrum. Bottom, negative-ion spectrum.
tributions for polydimethylsiloxanes have been obtained with peaks observed above m/z 3000 (20). It is also possible to distinguish between polymers on the basis of low massrange spectra (21). Figure 12 shows the positive LMS of polyacrylamide and polystyrene as an example. It is clear from inspection that the two are different; the major peaks in the spectra can be correlated with structural features of the polymers. The use of both positive and negative ion spectra for polymer characterization is also important. This is illustrated by the difference between poly(benzylmethacrylate) and poly(phenylmethacrylate). For the polybenzyl isomer the major peak in the positive ion spectrum is at m/z 91, corresponding to the benzyl ion. The negative ion spectrum is uninstructive. In the case of poly(phenylmethacrylate), the positive ion spectrum gives no structural information, but the negative ion spectrum shows a major peak at m/z 93, corresponding to the phenoxide ion (6).
14
DAVID
M. HERCULES I
[M i+Na] [h4 i+~]+
Na
250
Na Na
Na
Na
Na
]
r”, / ,“/,“<
Na
H 1
H I
500
400
300
Na
H 1
4 600
700
600
M/Z
FIG. 11. Positive-ion laser mass spectrum of polyethylene glycol (PEG) 400 (19).
Another example of polymer characterization using LMS is measurement of forward versus backward addition in polyvinylidine fluoride (PVF,); the structure is 6%
- CW, 3
The LMS of PVF, shows a large number of peaks in the mass range m/z < 200. Four important peaks are at m/z 59, 77, 95, and 113, corresponding to the threecarbon fragment ions shown in Fig. 13 (22). A polyvinylidine fluoride chain resulting from normal addition can produce only two three-carbon fragments of m/z 59 and 113. For the case of head-to-head and tail-to-tail addition, three-carbon fragments are seen at m/z 95 and 77, respectively. Thus, by measuring the combined intensities of m/z 95 and 77 relative to the total of all three-carbon fragTABLE 3 M, Determinations for Poly(ethylene glycols) (19) PEG sample
End-group titration
EHDMS
FDMS
LMS
400 600 1000 1400
406 605 1041 1396
406 (1.05) 572 (1.06) 962 (1.05) 1365 (1.02)
444 (1.05) 601 (1.05) 1010 (1.03) 1360 (1.03)
420 (1.05) 618 (1.03) 1007 (1.02) 1266 (1.03)
Note. M JMn values are in parentheses.
15
LASER MICROPROBE MASS SPECTROMETRY 72
39
165
4
94
143
II
II 100
80
II111
I 140
1 120
160
180
91
39
77 115 63 51
1
103
I
I I
I
165 I
I I
27
I 20
1 40
60
1 80
100
rll(It 1I 120
140
160
189
180
FIG. 12. Positive-ion laser mass spectra of polymers (21). (A) polyacrylamide. (B) Polystyrene.
ments, one can estimate the percentage of backward addition in the polymer chain. When LMS results were compared with a similar analysis done by 19F NMR for three polymer samples, the percentages of backward addition measured by LAMMA were 2.4, 4.3, and 4.8, while the results by NMR for the same samples were 2.5, 4.7, and 4.8. Although the precision of the LAMMA analysis (+20%) was not as good as that of NMR (+S%), the LAMMA analyses could be performed in 10 min on the undissolved sample, while the NMR analyses required sample dissolution and long data acquisition times. LMS UNDER ATMOSPHERIC
CONDITIONS
Two problems associated with obtaining laser mass spectra of solids are possible loss of volatile materials in the vacuum and extensive degassing from the
16
DAVID Head
M. HERCULES -To-Tall
(Normal
Addlilon Add!i!on)
- CH,CF,CH,kF,CH,CF,~
-7-Y 0 CH,-CF m/z
0 CF,-
= CH,
Head
CH = CF2
m/z = 113
=59 -To
- Head
Addltmn
- CH,CH,CF,CF,CH,CF,-
77 CF2
CFz = CF-CH,O m/z
Tall
-To-Toll
m/z
A = CF, = 77
CF-CH,O = 35
Addlion
- CH,CF,‘CH,CH,CF,‘CH,-
0 CH,-CH
q
m/z
= 35
I \ CH,m/z
CH = CF2 = 77
FIG. 13. Three-carbon fragment ions from polyvinylidene fluoride (22).
sample, both of which may overtax the spectrometer pumping system. To overcome these problems we developed a technique for obtaining laser mass spectra at atmospheric pressure; a specific example was obtaining LMS of a pesticide on a leaf (23). Figure 14 shows how the specimen was obtained. A very thin Formvar film was deposited from solution on a glass slide and floated on water. The sample from which a spectrum was to be obtained was placed with the front surface on the Formvar film and a zinc foil touched to the Formvar film so that it sealed the sample. The laser then penetrated the Formvar film, and ions from the sample surface were extracted into the laser mass spectrometer. When LMS was obtained from a leaf under Formvar with no pesticide added, the positive ion spectrum was amazingly clean. Only Na+, K+, and Caf ions showed any significant intensity. Although several peaks were observed in the negative ion spectrum, no significant background peaks were observed above m/z 200. Figure 15 shows a spectrum of the commercial preparation Dursban sprayed onto grass. The active ingredient in Dursban is chlorpyriphos, the structure of which is Cl cl
The spectrum at m/z 352 corresponds to (M + H)+ from Dursban; peaks at 370 and
LASER MICROPROBE MASS SPECTROMETRY
17
------- ~.--;;_; Deionized
Water
Leaf
Section
Formvar Film -------________________ ________________________________________-----------------------
VACUUM
VACUUM
zinc foil leaf section formvar film atmospheric
pressure
FIG. 14. Method for obtaining laser mass spectra from a leaf surface. (23).
388 correspond to addition of water. The main peaks in the spectrum correspond to known fragments of chlorpyriphos (24). The peaks for Dursban accurately reflect the chlorine isotope distribution for chlorpyriphos. Also, the spectrum for Dursban on grass accurately reflects the spectrum obtained for chlorpyriphos on zinc in an atmosphere saturated with water vapor. LMS MAPPING
MICROPROBE
The laser microprobe was modified by adding stepper controller motors so that it was possible to obtain a map of a surface for a particular peak in the laser mass spectrum. The minimum possible step size was 1.25 km, the width of the beam is about 2.5 pm, limiting the spatial resolution to ca. 3 km. The scanning microprobe has been described in detail elsewhere (25). To test the performance of the scanning microprobe, a series of cationic dyes were deposited in a grid pattern. Typical dyes were Gentian Violet (5) and Brilliant Green (6), as shown.
18
DAVID M. HERCULES 206
260
169
262
(M-Z)+
206
(Z+Ns+H)+
262
W+H-2EtO)+
260
(M+H-2EtO+H20)+
352
(M+H)+
370
(M+H+H20)+
380
(M+H+2H20)+
352
S
Cl
N I
II
Chlorpyriphos
I,
(EtO)2i-6
349 Cl
m Cl ’
III
II ,“:“388h I * I II..,
III,
I,,,
200
250
III,,’
II .
I’,“” 350
300
400
450
II lrrl(I
&
500
m/z
FIG. 15. Laser mass spectrum of Dursban on grass (23).
5
6
TEM grids of various mesh sizes were laid on nitrocellulose and a dye solution electrosprayed onto the composite. Therefore, the sample consisted of a series of square dye spots on nitrocellulose, along with uncovered nitrocellulose areas. Figure 16 shows a map of the molecular ion of Gentian Violet on nitrocellulose obtained for a sample prepared with a loo-mesh grid. The map was obtained with m/z 372; the map area is 750 x 750 pm. Each step corresponds to 25 p.m. An identical map was obtained at m/z 355 using a fragment ion peak of Gentian Violet. The square pattern of the grid work was maintained. Similar maps were obtained for 400-mesh and lOOO-meshsamples; the latter corresponds to an area of 85 x 85 pm, with a minimum step size of 2.5 pm. For the 1000-mesh grid the squares became elongated into rectangles because of the elliptical shape of the laser beam impacting the surface at 45”. The results represent the first successful map of an organic material on an organic surface and open the possibility for obtaining maps of organic species similar to the way in which Auger spectroscopy has been used to obtain elemental maps from surfaces. Maps were also obtained from inclusions in coal known as macerals. For exam-
LASER MICROPROBE MASS SPECTROMETRY
19
FIG. 16. Laser microprobe map of Gentian Violet on nitrocellulose (25).
ple, the map of FeS, from a coal maceral is shown in Fig. 17. This map was obtained at mass 120 (FeS,-); the area map is 350 km square, with a step size of 10 pm. It is clear that FeS,- arises from an hourglass-type structure within the coal maceral. WHAT OF THE FUTURE
It is clear from the above that the laser microprobe mass spectrometer permits one to obtain mass spectra from involatile materials and to do so for microscopic inclusions. It is possible to study fragile molecules and high molecular weight materials. The general assets of laser microprobe mass spectrometry are summarized in Table 4. It is applicable to both organic and inorganic materials. It is a soft ionization method and thus provides structural information. Although the mass range has been generally limited to below m/z 1000, this is not a limit of the time-of-flight analyzers used. It is also clear that the laser mass spectrometer can be used as an analytical microprobe and that mapping capabilities are possible. Of particular importance is the ability to map organic materials on organic substrates. The method is semiquantitative. Under the best of circumstances precision is probably no better than +lO%. In general, sample charging is not a problem as with many surface techniques. Given the above, what are the current limitations of laser microprobe mass spectrometry? These are summarized in Table 5. First, it is not a high resolution technique. The mass analyzers used have M/AM equal to 1000 or less, and thus exact mass measurements are not possible. A second problem is that spatial
20
DAVID M. HERCULES
FIG. 17. Laser microprobe map of a coal mace&.
resolution is generally limited to a few micrometers because of the characteristics of the laser beam. Yet many important microprobe problems involve an order of magnitude smaller domains. Third, the technique is not particularly selective. Spectra are generated by nonspecific excitation processes; photon selection has not been used. Thus a current limitation is the need for predictable ionization probabilities for different compounds. Another current limitation is sensitivity. Although the absolute number of ions detected is often very small, it would be desirable to increase this by one or two orders of magnitude. This is particularly important as one seeks to measure smaller and smaller domains. Mixtures and matrix effects are problematic. Interactions between compounds occur, and equimolar mixtures of materials having vastly different molar absorbtivities at 265 nm will produce vastly different numbers of ions. And finally, it really helps to un-
TABLE 4 Assets of Laser Microprobe Mass Spectrometry 1. 2. 3. 4. 5. 6. 7.
Applicable to inorganic and organic materials. Soft ionization method-provides structural information. Large potential mass range-not analyzer limited. Functions well as an analytical microprobe-mapping possible. Can do maps of organics on organics. Method is semiquantitative at worst. Charging of sample is not a general problem.
LASER MICROPROBE MASS SPECTROMETRY
21
TABLE 5 Current Limitations of Laser Microprobe Mass Spectrometry 1. Not a high-resolution technique. M/AM = looo. 2. Spatial resolution is 5 Wm. Important problems are 0.1 km. 3. Need for selectivity or at least predictable ionization probabilities, 4. Improved sensitivity needed (S/N). 5. Mixtures and matrix effects are problematic. 6. Fundamentals-it helps to understand what you are doing.
derstand what one is doing. The fundamentals of the interaction of laser beams with organic solids are not well understood. The future development of laser mass spectrometry then, as shown in Table 6, relates to the problems above. The laser spot size is approaching the diffraction limit. With decreases in optical aberration, one can perhaps reduce the laser spot size to the vicinity of 0.1 to 0.2 km. The signal-to-noise ratio is limited by background in the spectrum which is primarily chemical noise. Thus research is necessary on the origin or chemical noise in LMS and how this relates to noise from the detector electronics. Third is the use of neutrals. The possibility of using postionization of neutrals has the potential for generating larger numbers of ions, since the number of neutrals exceeds the number of ions generated by several orders of magnitude. However, all is not necessarily sweetness and light. Figure 18 illustrates some problems in the postionization of neutrals. In Fig. 18 the ions are formed by a narrow laser pulse at t = 0; the neutrals are emitted over a much longer time. If one uses a second narrow laser pulse (R = 1000) at the maximum of neutral emission to ionize neutrals, one will utilize only a small fraction of the total neutrals emitted. If one uses a much broader postionizing pulse (R = 33), a much greater fraction of the neutrals can be utilized. However, the broader pulse will significantly degrade the resolution of the spectrometer. The R = 1000 pulse has a half-width equal to that of the initial laser pulse, and the overall spectrometer resolution of M/AA4 = 1000 is maintained. The increased width of the R = 33 TABLE 6 Future Development for Laser Microprobe Mass Spectrometry Current problem 1. Laser spot size 2. S/N ratio 3. Use of neutrals 4. Selectivity and resolution
5. Mass range
Solution Diffraction limit Reduce background chemical noise R-E-S-E-A-R-C-H Postionization MS-MS methods CID Resonance ionization TOF M/AM > 10000 How big an ion can we make? R-E-S-E-A-R-C-H
22
DAVID
M. HERCULES
Neutrals Emitted Post Ionizing
Time
FIG. 18. Time dependence of ion and neutral emission in laser mass spectrometry.
postionizing pulse decreases the overall spectrometer resolution to 33. Thus effective utilization of more neutrals to improve sensitivity results in loss of resolution by several orders of magnitude. There are several possibilities for improving the selectivity and resolution of laser mass spectrometry. Time-of-flight analyzers have resolutions of M/AM > 10,000 (26). Therefore, the possibilities of exact mass measurements do exist. Selectivity also can be improved by incorporating the kind of experiments which have been used in conventional mass spectrometry. For example, MS/MS methods and CID methods allow for identification of ions by performing mass spectrometry on an initially formed ion by some secondary process, either before or after collision. So far only a few of these types of experiments have been tried for LMS. Finally, the important question really is how large an ion can we make? From experiments on the same polymers using both LMS and secondary ion mass spectrometry (SIMS) (27), it is clear that in the SIMS experiment, larger ions are detected. Thus an understanding of the ion formation process of large molecules by lasers is an important area of basic research for the future. In summary laser mass spectrometry has much potential for many practical problems. However, it also has current limitations. It is clear that studies on ion formation processes and the nature of a chemical noise are fundamentally important to the future development of the technique. ACKNOWLEDGMENTS I gratefully acknowledge the support of our LMS research by the Office of Naval Research and the National Science Foundation. I also acknowledge assistance in our program by Leybold-Heraeus and wish to thank the Alexander von Humboldt Foundation for a Fellowship. Finally I want to thank the members of my research group who have contributed significantly to our development of the laser microprobe mass spectrometer and to the understanding of the technique.
REFERENCES 1. Honig, R. E.; Woolston, J. R. Appl. Phys. L&t., 1963, 2, 138. 2. Vastola, F. J.; Pirone, A. J. Adv. Mass Spectrom., 1968, 4, 107-l 11. 3. Nitsche, R.; Kaufmann, R.; Hillenkamp, F.; Unsold, E.; Vogt, H.; Wechsung, R. Zsr. J. Chem., 1978, 17, 181-184.
LASER MICROPROBE MASS SPECTROMETRY
23
4. Posthumus, M. A.; Kistemaker, P. G.; Meuzelaar, H. L. C.; Ten Noever de Brauw, M. C. Anal. Chem., 1978, 50, 985-991. 5. Heinen, H. J.; Meier, S.; Vogt, H.; Wechsung, R. Adv. Mass Spectrom., 1980, 8A, 942-953. 6. Gardella, J. A.; Hercules, D. M.; Heinen, H. J. Spectrosc. Lett., 1980, 13, 347-360. 7. Heinen, H. J.; Hillenkamp, F.; Kaufmann, R.; Schroder, W.; Wechsung, R. Proc. 6th Int. Symp. Mass Spectrosc. Biochem. Med., Venice, 1979. 8. Schriider, W. H. Z. Anal. Chem., 1981, 308, 212-217. 9. Klein, P.; Bauch, J. Z. Anal. Chem., 1981, 308, 283-286. 10. Heinen, H. J.; Meier, S.; Vogt, H.; Wechsung, R. Znt. J. Mass Spectrom. Ion Phys., 1983, 47, 19-22. 11. Dingle, T.; Griffrths, B. W. Microbeam Anal., 1984, 19, 23-26. 12. Denoyer, E.; vanGrieken, R.; Adams, F.; Natusch, D. F. S. Anal. Chem., 1982, 54, 26A. 13. Balasanmugam, K.; Dang, T. A.; Day, R. J.; Hercules, D. M. Anal. Chem., 1981,53,2296-2298. 14. Graham, S. W.; Dowd, P.; Hercules, D. M. Anal. Chem., 1982, 54, 649-654. 15. Balasanmugan, K.; Hercules, D. M. Anal. Chim. Acta., 1984, 166, l-26. 16. Balasanmugan, K.; Viswanadham, S. K.; Hercules, D. M. Anal. Chem., 1983, 58, 2424-2426. 17. Hercules, D. M.; Day, R. J.; Balasanmugam, K.; Dang, T. A.; Li C. P. Anal. Chem., 1982, 54, 280A. 18. Balasanmugam, K.; Day, R. J.; Hercules, D. M. Znorg. Chem., 1985, 24, 4477-4483. 19. Mattem, D. E.; Hercules, D. M. Anal. Chem., 1985, 57, 2041-2046. 20. Holm, R.; Karas, M.; Vogt, H. Anal. Chem., 1987, 59, 373-376. 21. Gardella, J. A.; Hercules, D. M. Z. Anal. Chem., 1981, 308, 297. 22. Mattem, D. E.; Lin, F. T.; Hercules, D. M. Anal. Chem., 1984, 56, 2762-2769. 23. Morelli, J. J.; Hercules, D. M. Anal. Chem., 1986, 58, 12961298. 24. Morelli, J. J.; Viswanadham, S. K.; Sharkey, A. G., Jr.; Hercules, D. M. Int. J. Environ. Anal. Chem., 1987, 31, 295-323. 25. Wilk, Z. A.; Hercules, D. M. Anal. Chem., 1987, 59, 1819-1825. 26. Niehuis, E.; Heller, T.; Feld, H.; Benninghoven, A. J. Vat. Sci. Technol. A, 1987, 5, 1243-1246. 27. Bletsos, I. V.; Hercules, D. M.; vanleyen, D.; Benninghoven, A. Macromolecules, 1987, 20, 407-413.