Anal. Chem. 1994,66, 276-284
Nondestructive Analysis of Colorants on Paper by Time-of-Flight Secondary Ion Mass Spectrometry Steven J. Pachuta' 3M Corporate Research Laboratories, 20 1-2s- 16 3M Center, St. Paul, Minnesota 55 144 John S. Staral 3M Graphic Research Laboratory, 20 1-4N-0 1 3M Center, St. Paul, Minnesota 55 144
The utility of time-of-flight secondary ion mass spectrometry (TOF-SIMS) for the analysis of colorants on paper surfaces has been investigated. Twenty-one pen inks and 16 printed paper specimens were studied. The use of static analysis conditions has no detectable effect on the paper and makes SIMS suitable for nondestructiveanalyses. In cases where the specimen is too large to fit within the instrument, a single paper fiber can be unobtrusively removed and analyzed separately. SIMS permits rapid differentiationof the colorants and provides qualitative identification of some components. The merits of SIMS vs conventional solvent extraction/ chromatography methods are discussed, and combined thinlayer chromatography/SIMSanalyses of solvent extracts from colored papers are reported. The nondestructive identification of inks, dyes, and pigments on paper is of importance in forensic science and in the study of valuable documents and works of art. In forensics, it is necessary to preserve as large an amount of a fraudulent check, forged document, or other such evidence as possible for use in legal proceedings. In the study of historical documents, rare books, and works of art, the need for nondestructive analytical techniques is self-evident. The constraints placed upon the analyst who deals with these materials makes their analysis an interesting challenge. Several methods have been employed for the nondestructive analysis of colorants on Techniques which have been used for elemental and inorganic detection include scanning electron microscopy/)~ laser microspectral analysis,* particleinduced X-ray emission (PIXE),9s10 and Auger electron spectroscopy." Optical m i c r o ~ c o p yand ~ - ~spectroscopic techniques based on l u m i n e ~ c e n c e , ~ and ~ - ' ~on reflectance and ( I ) Canlu, A. A. Anal. Chem. 1991, 63, 847-54A.
(2) Quigley, M. N.; Qi, H . J. Chem. Educ. 1991, 68, 596-7. (3) Nolan, P. J.; England, M.; Davies, C. Scanning Electron Microsc. 1982, 2, 599-6 10. (4) Drzewinska, E.; Korezynski, A. Coating 1990, 24, 218-23. ( 5 ) Cabelli, D. E.; Orna, M. V.; Mathews, T. F. In Archaeological Chemistry: Lambert, J. B., Ed.; Advances in Chemistry 205; American Chemical Society: Washington, DC, 1984; Vol. 111, Chapter 12. (6) Orna, M. V.; Lang, P. L.; Katon, J. E.; Mathews, T. F.; Nelson, R. S . In Archaeological Chemistry: Allen, R. O., Ed.; Advances in Chemistry 220; American Chemical Society: Washington, DC, 1989; Vol. IV, Chapter 14. (7) Russ, J. C. Ado. X-Ray Anal. 1980, 23, 219-22. (8) Mehrotra, V. K.; Sidhana, S. K. Forensic Sci. 1977, 9, 1-3. (9) Cahill, T. A.; Kusko, B.; Schwab, R. N . Nucl. Instrum. Methods 1981, 181, 205-8.
(IO) Kusko, B. H.; Schwab, R. N. Nucl. Insrrum. Methods Phys. Res. 1987, B22. 401-6.
276
Analytical Chemistry, Vol. 66, No. 2, January 15, 1994
transmittance (including infrared s p e c t r o s ~ o p y ) , ~ J ~are -l~ useful for qualitative comparisons between different materials. Since most colorants are actually mixtures of several materials, perhaps the most widely used technique for colorant analysis is extraction with subsequent chromatographic separation.2q19,20When preservation of material is of importance, small disks (ca. 1-mm diameter) can be punched out of the paper and extracted ex situ, with separation by paper or thinlayer chromatography (TLC).21-23 The paper punch method is well-suited for qualitative comparisons of different colorants-for example, in determining whether more than one type of ink was used on a document2 It is unfortunately semidestructive, and evidence of analysis can be detected upon examination. Recently, highperformance liquid chromatography (HPLC)24and HPLC/ mass ~ p e c t r o m e t r yhave ~ ~ been used to analyze dyes extracted from single textile fibers, thus greatly minimizing sample damage. Paper analysis would seem to be a logical extension of this work. While organic dyes can usually be extracted and separated by chromatographic methods, however, inorganic pigments such as metal oxides cannot. Differential solubility must be considered when solvents are being chosen, and both solvent extraction and separation can sometimes induce degradation of dye molecules. Moreover, solvent selection remains largely empirical, and development of a suitable analysis method may require substantially larger sample volumes than would be required for an analysis performed using preoptimized conditions. A method in which colorants are sampled directly in their original matrix would thus have several advantages over extraction-based techniques. ( 1 1) Burns, G.; Wilson-Yang, K. M.; Smeaton, J . E. In Archaeological Chemistry;
Allen, R. O . , Ed.; Advances in Chemistry 2 2 0 American Chemical Society: Washington, DC, 1989; Vol. IV, Chapter 15. (12) Throckmorton, G.J. J. Forensic Sci. 1990, 35, 199-203. ( I 3) Zimmerman, J.; Mooney, D. J. Forensic Sci. 1988, 33, 3 10-8. (14) Tappolet, J. A. J. Forensic Sci. SOC.1986, 26, 293-9. (15) Chowdhry, R.; Gupta, S.K.; Barni, H. L. J . ForensicSci. 1973, 18,418-33. (16) Laing, D. K.; Isaacs, M. D. J. Forensic Sci. SOC.1983, 23, 147-54. (17) Hamman, B. L. J. Forensic Sci. 1968, 13, 544-56. (18) Pfeffferli, P. W. Forensic Sci. I n r . 1983, 23, 129-36. (19) Wallace, M. R.; Milliken, L. T.; Toner, S . D. Tappi 1967, 50, 1 2 1 4 A . (20) Gupta, S . K.; Rohilla, D. R.; Jain, M. K, Can. Soc. Forens. Sci. J . 1981, 14, 23-3 I . (21) Nakamura, G. R.; Shimoda, S. C . J. Crim. Low, Criminol. PoliceSci. 1965, 56, 113-8. (22) Kuranz, R. L. J. Forensic Sci. 1986, 31, 655-7. (23) Breedlove, C. H . J. Chem. Educ. 1989, 66, 170-1. (24) Suzuki, S.-I.; Suzuki, Y.; Higashikawa. Y.; Kishi, T.; Marumo, Y. Anal. Sci. 1991, 7, 117-20. (25) Yinon, J . ; Saar, J. J. Chromatogr. 1991, 586, 73-84.
0003-2700/94/0366-0276$04.50/0 0 1994 American Chemical Society
300 372 200 100
0
l50{ 240
40
30 20 10
0
Flgure 1, Timeof-fllght SIMS spectra of blue ball point pen Inks on paper: (a) Bic Round Stlc medium, (b) Papermate Flexgrip medium, and (c) Pilot BP-S fine. Gs+ primary ion beam, 5-mln acqulsltlon time, 180 X 180 pm analyzed area.
Mass spectrometry is becoming an increasingly useful method for the analysis of colorant^,^^-^^ but there has been surprisingly little forensic work published in this area.25*31 The success of SIMS in the analysis of dyes on metal substrates and fiher ~ a p e r , ~and 2 on textile fibers,33 taken with recent improvements in commercial secondary ion mass spectrometry (SIMS) instrumentation, has suggested the present investigation into the utility of SIMS for forensic/nondestructive applications. SIMS has the ability to detect both atomic and molecular species, and it tends to give molecular ions with a minimum of fragmentation. The latter attribute is a particular advantage in the analysis of complex mixtures such as inks. SIMS and related ionization methods such as laser desorption mass spectrometry may in fact have the widest range of applicability to colorant analysis of any class of analytical technique. Time-of-flight (TOF) SIMS is well-suited to the analysis of minute amounts of material, and TOF-SIMS with a liquid metal ion gun (LMIG) primary ion source has been used to image e l e m e n t ~ 3and ~ s u r f a c t a n t ~ 3on~ paper surfaces. The ~~
(26) Beukelman, T. E. In The Amlyrical Chemistry olSynthetic Dyes; Venkataraman, K., Ed.; Wiley: New York, 1977; Chapter 9. (27) Monaghan, J. J.; Barber, M.; Bordoli, R. S.; Sedgwick, R. D.; Tyler, A. N. Org. Mass Spectrom. 1982,17, 569-74. (28) Yinon, J.; Jones, T. L.; Betowski, L. D. Blomed. Etwiron. Mass Spectrom. 1989, 18, 445-9. (29) Yinon, J.; Jones, T. L.; Betowski, L. D. J . Chromatop. 1989,482, 75-85. (30) Dale, M. J.; Jones, A. C.; Langridge-Smith, P. R. R.; Costello, K. F.;Cummins, P.G. Anal. Chem. 1993,65,793-801. (31) Acampora, A.; Ferranti, P.; Malorni, A.; Milont, A. J. Forensic Sci. 1991, 36, 579-86. (32) Scheifers, S. M.; Verma, S.; Cooks. R. G. Anal. Chem. 1983, 55, 2260-6. (33) Gcochnick, J.; Lipp, M.; Ache, H.-J.; Thomas, H.: Kaufmann, R.; Peters, R.; H&kcr, H. J. Soc. Dyers Colour. 1992, 108, 191-4.
micrometer-regime spot size of the LMIG provides sufficient spatial resolution to limit the analysis area to a region smaller than the width of a single paper fiber (on the order of 30 pm or less). The time-of-flight mass analyzer gives the high sensitivity needed for static SIMS and has a mass range far superior to that found in most quadrupole and sector SIMS instrument^.^^*^^ The extended mass range of TOF-SIMS is important in the study of colorants, since many colorants are mixtures of molecules with molecular weights of several hundred daltons. Static SIMS as performed in the present work is sufficiently nondestructive as to leave no detectable evidence of the analysis-that is, no sputter craters or “ion burn” is visible under an optical microscope. Limitations on the sample size accepted by most SIMS instruments necessitate a compromise, however, in cases where large pieces of paper must be studied and cannot be folded or cut. In these cases, a portion of a single paper fiber can be removed from the surface and analyzed separately. Similar procedures have been employed by Orna and co-workers to sample pigments on medieval manuscripts,5.6and by Suzuki and c0-workers2~and Yinon and Saar25to sample textile dyes. While this procedure is nominally destructive, experience has shown that even under an optical microscope it is usually impossible to tell that the surface has been in any way disturbed. This is due to the inherent heterogeneity of paper surfaces, in which the loss of (34) Brinen, J. S.;Greenhouse, S.;DunlopJones, N. Nord. PulpPap. Res. J. 1991, 6, 47-52, 94. (35) Brinen, J. S.; Proverb, R. J. Nord. Pulp. Pap. Res. J. 1991, 6, 177-83. (36) Benninghwen, A.; Rtidmauer, F. G.; Werner, H. W. Secondary Ion Mass Spectrometry; Wiley: New York, 1987.
AnaMicaJChernistty, Vol. 66, No. 2, January 15, 1994
277
Table 1. Fingerprint Ions in the LMIG TOF-SIMS Spectra of Pen Inks on Paper
pen identification
pen type
fingerprint ions ( r n / z ) O
Bic Round Stic medium (black) Bic Round Stic medium (blue)
ball point ball point
Bic Round Stic medium (blue),a different lot number Express0 Extra Fine (blue) FaberCastell Wonderiter (purple) Hewlett Packard plotter pen, 3 mm (blue) Hewlett Packard plotter pen, 3 mm (green) Hewlett Packard plotter pen, 3 mm (purple) Hewlett Packard plotter pen, 3 mm (red) Papermate fine ball point (blue) Papermate Flexgrip medium (black) Papermate Flexgrip medium (blue) Papermate Flexgrip medium (red) Papermate Metal Roller micro (blue)
ball point fiber tip plastic tip fiber tip fiber tip fiber tip fiber tip ball point ball point ball point ball point ball point
Papermate 860 Series (blue) Papermate 860 Series (blue)b Pentel Superball fine (blue) Pilot BP-S fine (blue) Sanford Sharpie fine (black) Sanford Sharpie fine (red) Sheaffer Skrip Cartridge I (black) disappearing ink (pink), visible disappearing ink (pink), aged 3 weeks, invisible
fiber tip fiber tip ball point ball point fiber tip fiber tip fountain fiber tip fiber tip
Cut, 328,340,356,372 Cut, 106,120, Pb+, 212,226, 240, 254, 268, 274,288,302, 316, 330,356,372 Cut, 106,120, Pb+, 212, 226, 240, 254, 268, 302,316, 356, 372 Na+, 298,324, 326, 340 Na+, 45, 356,368,384,400, 554,570, 711,740 Mg+, Si+, 59,559,581 Na+, Mg+, Si+, 59,329 Na+, Si+, 59, 309, 559,581,603,619 Naf, Si+,Cr+, 59 Cut, 106, Pb+, 240,302,358,372,407,434,448,478, 541,645 Cut, 240, 268, 344,358, 372, 504,518, 766,792 Cut, Pb+, 212, 240, 268,470,484,498 Cut, Pb+, 268, 415,443 Na+, 559, 581,603,621,665,687,709, 731,753,775,797,819, 841,863,907,951 Na+, 227,356,372 80,170,199,227,255,508 Na+, 340,354,356,372 Pb+, 212,240,247,261,275,358,372,478 Si+, Fe+, 73, 147, 207,325,541,556,632,646 Sic, Cr+, 73, 147, 355,369,415, 427, 443 Lit, Na+, 150, 156 Na+, 118,129,142,172,194,217,261,277,305,349,367,393,437 Na+, 172,217,261,305,345,349,367,393,435,488,510
Positive ion spectra, unless otherwise indicated. In cases where isotopic abundances and/or exact masses indicate a specific element, the element is listed, rather than its m/z ratio. Negative ion spectrum.
*
a piece of fiber typically 30 X 500 pm is not detectable without a priori knowledge of its exact location. The advantage of SIMS over TLC becomes obvious if the total removed volumes are calculated: assuming a typical paper thickness of 100 pm (0.004 in.), removal of a single l-mm plug for TLC produces a loss of -0.08 mm3 of material; removal of a 30 X 500 pm fiber produces a loss of -0.0004 mm3, or a factor of 200 less. In this paper, TOF-SIMS will be applied to the analysis of several different pen inks and printed patterns on paper. The principal advantages of SIMS over other techniques are its high specificity (elements can be identified, molecular weights can be obtained), its speed, and its low sample consumption (nondestructivity). The principal disadvantage of the technique occurs in the case where the colorant does not lie at the surfaces of the paper fibers. This might be the result of a coating on the paper, a surface-active component in the ink which preferentially migrates to the surface, or absorption of colorant into the paper fibers. In these cases, it may be possible to combine TLC and SIMS to gain two dimensions of information, albeit with some sample destruction. Direct SIMS analysis of separated paper extract components on TLC plates will be reported. A second disadvantage is the use of colorant systems which do not produce characteristic ions in SIMS. Most dyes and pigments contain functional groups and elements which lend themselves well to SIMS analysis, however. EXPERIMENTAL SECTION Sample Preparation. A total of 20 pens of various colors were gathered-primarily ball point pens, but also several fiber tip pens, one fountain pen, and one “disappearing ink” sewing pen. A few characters were written with each pen on standard 20-1b white bond paper, and the ink was allowed to dry for 24 h. Reference SIMS spectra were obtained directly from the traces on paper; the paper was cut to allow mounting in the sample holder. In those instances where single paper
-
278
Analytical Chemistry, Vol. 66, No. 2, January 15, 1994
fibers were analyzed, the fibers were removed using a fine needle and a pair of sharp Dumont No. 3 tweezers. Under an optical microscope, the needle was used to pry a fiber loose from the paper surface, and the tweezers were used to remove it and transfer it to double-stick tape mounted on a metal substrate. A similar procedure was followed for a set of proprietary printed patterns on paper, and for two United States postage stamps. The proprietary printed patterns consisted of red and blue lines of varying width on white and pale blue backgrounds. The postage stamps were a 29-cent “White House” stamp with red, blue, and black printing on a white background, and a 29-cent “Grace Kelly” stamp with blue and black printing on a white background. Reference SIMS spectra were obtained directly from cut pieces. Single fibers were removed as described above. SIMS Analysis. All analyses were performed on a Charles Evans & Associates TFS time-of-flight SIMS instrument equipped with an FEI monoisotopic Ga+ liquid metal ion gun, a Perkin-Elmer Physical Electronics Cs+ ion gun, and a microchannel plate detector. The LMIG was operated at 25-keV beam voltage with a continuous current of 1.1 nA; the pulse rate was 11.3 kHz, with a pulse width of 30 ns. The Cs+ gun was operated at 1 l-keV beam voltage with a continuous current of 3.3 nA; the pulse rate was 10.0 kHz, with a pulse width of 2 ns. The sample potential was fixed at 3.0 kV in all cases. Charge neutralization was provided by a pulsed electron gun, cycled every fifth primary ion pulse. A detector postacceleration voltage of 10 kV was applied to enhance detection of high-mass ions. The analyzed area was typically in the range 100 X 100 to 200 X 200 pm. Under these conditions, the total primary ion dose for a 5-min analysis was 1 X 10l2ions/cm2,or 1 order of magnitude below the static limit.37 In all but one case, positive ion spectra were obtained. In general, negative ion spectra were found to be less useful for colorant identification than were positive ion spectra.
-
Thin-layerChromat~pby.All TLC workwas performed on the proprietary printed patternson paper. Forconvenience, relatively large samples ofpaper consisting ofmultiplesections approximately 0.5 X 0.5 cm containing the colored printing ofinterest werecut out andextracted with methanol. Extracts were separated on Merck 60F254 plates (coating thickness 0.25 mm) using the followingthreecombinations of stationary phases and elutants: (1) silica gel plate, acetone elutant; (2) silica gel plate, 30%aqueous ammonium hydroxide/methanol (1:9 by volume) elutant; (3) aluminum oxide plate, acetone/ chloroform (l:3 by volume) elutant. Multiple elution cycles were sometimes performed to improve resolution. After separation, selected spots were analyzed by SIMS. It was necessary to analyze the spots as scan as possible after separation, as the isolated dyes were frequently observed to fade on the TLC plate within a few days as a result of instabilities induced by the separation process and materials. Due to sample size limitations, the plates could not be loaded directly into the instrument. Rather, portions of the isolated dyes on the TLC plate coating were removed with a spatula, mounted on double-stick tape, and then placed into the instrument. Spotson silicagel plates werequite easily removed intact, but spots on aluminum oxide plates had a tendency to crumble and required careful handling.
RESULTS AND DISCUSSION Pen Inks on Paper. The analysis of inks on paper entails the study of one complex mixture in another. In order to determine whether ions in the SIMS spectra of inks are due to the inks or to the underlying paper, it is necessary to obtain a background spectrum of the paper. The paper used in the present work gives aliphatic and aromatic hydrocarbon ions below m/z 200, as well as an abundant AI+ ion and a weak Na+ ion. Above m/z 200, the only ions which stand significantly above the noise are weak ions a t m/z 257 and 239, whicharethoughtto beduerespectivelytoprotonated palmitic acid and loss of water from this ion. Representativespectraofthree ballpoint pen inkson paper are illustrated in Figure 1. Spectra were taken from an area less than the width of a single pen trace; after removal from the instrument, no evidence of sputtering could be detected witheither thenaked eyeor a high-pweredoptical microscope. The mass range 20S500 contains nearly all of the molecular information needed to differentiate the inks, and fortuitously there is little interference from the paper in this range. Most of the elemental information is found, of course, below m/z 200. In Figure 1 it may be seen that each of the inks contains Pb (m/z 206-208) and numerous organic components. The ink in Figure l a appears to contain at least three homologous series of dyes: the first begins at m/z 212 and extends to m/z 268 with separations of 14 mass units, the second begins at mlz 274 and extends to m/z 316 with separations of 14 mass units, and the third contains ions of m/z 356 and 372 with a separation of 16 mass units. It is thought that the majority of these ions are due to distinct molecules and that they are not fragment ions; some evidence for this assumption is given byCcaksandco-workers?*andadditionalevidenceisprovided by the TLC separations discussed below. (37) Brig@. D.:H a m . M. J. Vocuum 1986.36, IWS-IO.
P
i.
?
-4.8
mm-
Flgurb 2. Optical micrographsof the hanaprinted letter " m "before and atter fibers have been removed; blue ink from PapermateFlexgrip medium pen: (a) letter as written wnh no fibers removed. (b) slngle fiber removed. and (c) a second fiber removed.
The spectra of the pen inks in Figure lb,c contain some of the same ions found in Figure la. They show significant differences, however. For example, while m/z 240 occurs in each spectrum, the m/z 254 homologue is absent in Figure Ib,c. Figure lbcontainsauniqueionofm/z484,whileFigure IC contains a unique ion of m/z 478. On this basis alone the inks can be distinguished. The spectra taken as a whole represent a powerful method for distinguishing the inks. Reproducibility i s q u i t e g d , with littlevariation in the spectra when the same ink is analyzed a t different spots over a handwriting specimen. No attempt was made in this work to identify the individual dyes which comprise the inks. This should in many cases be possible with suitable reference spectra, combined with the high mass resolution capability of TOF-SIMS. In Figure I C it may be speculated that the ion of m/z 372 is crystal violet and that the ion of m/z 358 is A n a W a i Chemistry, Vol. 66, No. 2. January 75. 1994
279
80d
~
484
400.
:
io4
I
I
212
'
I
221 281
I
1
60
T
281
240
4Oh 20
484
200
250
300
350
460
440
5hO
&Z
Figure 3. Time-of-flight SIMS spectra of a single paper fiber on double-stick tape (see Figure 2c): (a) analyzed area limitedto the fiber, (b)analyzed area limlted to the area of tape immediately adjacent to the fiber, and (c) analyzed area including the fiber and the surrounding area of tape. (a and b) Ga+ primary ion beam, 10-min acquisition time, 130 X 130 pm analyzed area with selective electronic gating of secondary ions. (c) Cs+ primary ion beam, 10-min acquisition time, 100 X 100 pm analyzed area.
methyl violet,32but corroboration by other means-particularly chromatography-is needed. Characteristic ions from all of the pen inks are summarized in Table 1. Ions listed are those which either are not found on the virgin paper or are present at much higher levels in the ink than on the paper. An example of the latter case is Na+, which occurs with considerable intensity in the spectra of some inks (possibly indicating the use of surfactants). In light of this example, it should be stressed that the ions reported in the table come exclusively from the inks, but they cannot be ascribed exclusively to colorants. An additional example of this can be found in the spectrum of the black Sanford Sharpie ink, where ions of m/z 28, 13, 147, and 201 are obvious indicators of a silicone oil. It is also possible that some metallic ions such as Cu+ and Pb+ come from mechanical components of the pens or (see below) printing presses. For the purposes ofqualitative inkcomparison this is rather an advantage, since discrimination based on the sum of all inkcomponents is likely to be more reliable than that based only on the colorants. Some general comments on the relative ease of ink discrimination by SIMS are in order. In nearly every case in which a ball point pen ink was analyzed, an excellent spectrum was obtained, with abundant colorant ions and good signal/noise ratio (typically on the order of 100; see Figure 1). Most of the fiber tip pens gave good spectra as well, but on average with less colorant ion intensity. The ease of obtaining SIMS spectra from ball point pen inks on paper is attributed to the tendency of these inks to reside primarily on the paper's surface.23 In contrast, inks from fiber tip pens 280
Analytical Chemistry, Vol. 66,No. 2, January 15, 1994
and fountain pens are more readily absorbed into the paper fibers, thus lowering the surface concentration and making detection more difficult. The most intractable inks were found to be those in the fiber tip Hewlett Packard plotter pens. In this case, only the elemental ions were abundant (Na+, Mg+, Si+),and higher mass organic ions were of very low abundance. A wicking effect could explain these results, although alternatively the inks might contain dye systems which are not amenable to SIMS analysis. The difficulties encountered with some inks do not preclude the utility of SIMS in making comparisons of writing on paper, however. If one needs to distinguish between a blue Hewlett Packard plotter pen and any other blue pen in the table, this can still be readily done. In no case was an ink totally "transparent" to SIMS; that is, ions could always be found which enabled differentiation of the ink and the paper substrate. The forensic applicability of SIMS to colorant analysis is well-illustrated by the spectra of disappearing ink. While still visible, the ink gives a large assortment of ions which are readily detected by SIMS. After its disappearance, the ink continues to give many of the same ions. Thus it should be possible not only to determine that an invisible ink is present on a piece of paper but also to identify the source of the ink. Single Paper Fiber: Pen Ink. While the ideal analytical situation for an ink on paper would be to insert the entire sample intact into the instrument, this is not always possible due to the fixed size of the sample handling system. The Charles Evans & Associates instrument can accommodate samples up to about 55 X 60 X 10 mm. Presumably, the
3
470
I
372
2 1
0
100
330
'i' 1 I
I
470
358
I
372
3?
(c)lZO100
443
I
415
312
327 I,
341
_,.,,.
355
..I,..
387
369 , I11,.
.,..I.
358
..,11.
,I,I,
I,
4bO
' 3 $ 0 " " 35
429
399 '
'
'
11. '
, ,
4$0
'
'
'
'
560
385
0
&
Flgurr 4. Tlmeof-flight SIMS spectra of paper sample F with printed patterns: (a) pale blue background, (b) blue pattern, (c) red pattern, and (d) single flber from blue pattern. (a-c) Ga+ primary ion beam, 5-min acquisition time, 180 X 180 pm analyzed area. (d) Ga+ primary ion beam, 10-min acqulsitlon time, 260 X 260 pm analyzed area. Table 2. Flng.rprlnt
Ions In the LMIG TOF-SIYS Spectra of Prlnted Patters on Paper
paper
description of printing
G G 'Whib House" stamp 'Grace Kelly" stamp
blue pattern on white paper blue pattern on white paper blue pattern on white paper blue pattern on white paper pale blue paper blue pattern on pale blue paper red pattern on pale blue paper blue line on pale blue paper pale blue paper blue pattern on pale blue paper red pattern on pale blue paper blue line on pale blue paper red pattern on white paper blue line on white paper blue lines on white paper blue-black limes on white paper
A B C D E E E E F F F F
fingerprint ions (m/z)a
316,330,344,358,372,428,442,456,410 428,442,456,410 415,428,443,456,410 363,391,442,456,410 none detected
456,410 415,429,443 442,456,410 358,372,456,470 330,344,358,312,385,456,410 415,429,443 330,344,358,312,385,442,456,410 Pb+, 415,429,443 450,418 cu+ Zn+, 355
a Positive ion spectra. In cases where isotopic abundances and/or exact masses indicate a specific element, the element is listed, rather than ita mlz ratio.
sample chamber could be redesigned to allow the analysis of larger samples, but at some point this would become impractical. In our experience, the removal of a single paper fiber (or even several fibers) can be accomplished with relative ease, leaving little or no evidence of tampering. Even in the worst-case scenario in which sample damage is visible, the damaged area is still much smaller than that produced by plug removal with a syringe needle. In our numerous (several dozen) attempts at removing single fibers, only twice was there visible evidence of fiber removal. In both cases the damage was avoidable and due to the careless removal of large mats of fibers, rather than a single fiber. It is therefore
recommended that considerable caution be exercised when this procedure is performed. Removal of single fibers is shown in the optical micrographs of the hand-printed letter "m" in Figure 2. In Figure 2b the area from which a fiber was removed can be detected upon careful examination. This is due to the injudicious choice of a fiber at an edge of the central vertical ink trace. A second fiber was removed from the middle part of an ink trace (Figure 2c), and this is not detectable, even with prior knowledge of where it was removed. In any case, detection of alteration in a given area is virtually impossible without an optical micrograph of that area prior to removal of a fiber, such as Ana&tical Chemistty, Voi. 00, No. 2, Janu8ry 15, 1994
201
in Figure 2a. Detection of tampering-particularly by the naked eye-is extremely unlikely if care is taken to remove a minimum of material from a suitable area. The fiber removed to produce Figure 2c was analyzed by SIMS using both Ga+ and Cs+ primary ion beams. Figure 3a shows a Ga+ spectrum of the fiber, obtained by electronically gating the secondary ions to select only those coming from the fiber. Figure 3b is a Ga+ spectrum of the double-stick tape on which the fiber was mounted, obtained by a similar gating technique. Ions of m/z 207,221, and 281 are due to a silicone release agent on the tape. The ink-derived ions of m/z 21 2, 240, 268, 470, 484, and 498 are all clearly detectable in the spectrum of the single fiber. The presence of silicone on the fiber indicates that there has been some cross-contamination between the fiber and the tape substrate, but this has little effect on the ability to discriminate ink-related ions. Figure 3c shows the Cs+ spectrum of an area which, due to the large spot size of the Cs+ ion beam, necessarily includes both the fiber and the substrate. Here, too, the characteristic ink ions are readily observed, although with somewhat lower relative abundances than were observed using Cia+. Both ink spectra compare favorably to that of the intact ink on paper (Figure lb). I n fact, the intensities of the colorant ions in the Ga+ spectrum are only a factor of -2 lower on the single fiber than they are on the intact paper.
Printed Patternson Paper. In addition to pen inks, printing inks on paper represent another category of materials which are amenable to nondestructive S I M S analysis. The range of printed materials is, of course, overwhelmingly large, and the applicability of S I M S throughout the entire range will not be addressed. Most of the spectra reported in this section were obtained from a series of proprietary papers which happened to be under study in our laboratories. These papers contain red and blue printing, and although they come from different sources, the colors show a high degree of similarity. In addition, two United States postage stamps were examined as a class of materials in which nondestructive forensic analysis might be useful. Seven different sets of printed patterns were studied, labeled A-G. Figure 4 shows S I M S spectra obtained from printed patterns on paper sample F. The pale blue background (Figure 4a) yields ions of m/z 358,372,456,470, and 478. The darker blue printed pattern (Figure 4b) contains most of these ions, and additional ions of m/z 314, 330, 344, 385, and 442. The red pattern (Figure 4c) contains very intense ions of m/z 41 5 , 429, and 443. A single fiber pulled from the blue printed pattern gives a spectrum (Figure 4d) of quality comparable to that obt.iined from the intact paper (Figure 4b), again demonstrating the viability of single fiber analysis as a nondestructive method. Characteristic ions from printed patterns A-G, and from the postage stamps, are reported in Table 2, with selection criteria similar to those used for the pen inks. It may be speculated that the ion of m/z 470 detected in many of the blue areas is the molecular ion of the common dye Victoria Blue B and that ions of m/z 456. 442, and 428 are lower homologues. Similarly, ions of m/z 443, 429, and 41 5 might bedue to Rhodamine Band lower homologues. As mentioned earlier, unequivocal identifications would probably require additional analyses with appropriate confirmatory techniques, 282
AnalyticalChemistty, Vol. 66,No. 2, January 15, 1994
Total positive ions
mlz 415
m/z 443
I
35 pm
I
35 prn Flgure 5. SIMS images of a single fiber from paper sample E, redprinted area, on double-stick tape: (a) total positive ion image, (b)m/z 415 image, and (c) m/z 443 image. Images are two-tone threshold images, with ions indicated in black. Gaf primary ion beam, 10-min acquisition time, 180 X 180 ,urn analyzed area.
but the potential of SIMS as a qualitative technique for in situ dye analysis is apparent. The postage stamps proved to be rather difficult materials to analyze using TOF-SIMS,giving very few ink-specific ions. As was the case with the plotter pen inks (see Table l ) , elemental ions are the primary colorant indicators. Some possible reasons for the lack of unique ions have already been listed. Once again, this difficulty does not rule out S I M S as a useful method for analysis. It would be quite easy, for example, to distinguish between a counterfeit stamp printed with Victoria Blue B dye and an authentic stamp printed with Cu-containing ink. The ability to image secondary ions in SIMS when a focused liquid metal ion source is used adds an additional dimension to paper a n a l y s i ~The . ~ ~distributions ~~~ of colorants on paper surfaces can, of course, be mapped. Mapping of single fibers pulled from paper surfaces has been found useful as well, particularly in the present nondestructive work. Figure 5 shows S I M S images of a single fiber pulled from printed pattern E and mounted on double-stick tape. The total ion image shows the fiber and the mounting medium. Images of m/z 415 and 443, however, show these ions originating only from the fiber. With appropriate software, it is possible to acquire a total ion
(a) ""g
470
2501 2w 15 10 5
0 456
I
358
4
2
0
Flgurr 8. Timeof-fllght SIMS spectra of separated spots on a TLC plate: sample F, methanol extract of blue printing, eluted wlth acetone/ chloroform ( 1 9 by volume) on an aluminum oxide plate: (a) blue spot, (b) blue spot with lower R, than spot a, and (c) violet spot with lower R, than spot b. Ga+ prlmary Ion beam, 5-mln acqulsttlon time, 180 X 180 pm analyzed area.
brownish to blue to violet; at least three additional, more weakly image in less than 1 min and then select a region of interest-in colored spots could also be discerned. Spectra of three of the this case, a mounted fiber-from which to acquire the spots are shown in Figure 6. It is clear from the figure that spectrum. This minimizes any background interference from the ions of m/z 470 and 456 arise from separate molecules and the mounting medium. that m/z 456 is not simply a fragment ion. This is also the Printed Patterns on Paper: TLC/SIMS. SIMS has been used as a TLC detection technique for a number of y e a r ~ , ~ * ~case ~ ~ for the ion of m/z 442, which is assumed to be a member of the same homologous series. It may be noted that these and TLC/SIMS has been successfully applied to the analysis ions cannot be the result of degradation during TLC, since of food dyes.m SIMS in combination with TLC is thus they are observed in the unseparated ink on paper. Although potentially a powerful method for the identification of colorants the separations are generally clean, evidence of incomplete on paper. To investigate the utility of TLC/SIMS in this separation is found in the presence of weak ions of m/z 470 application, several of the printed patterns A - G were analyzed in Figure 6b and m/z 344 in Figure 6c. by TLC/SIMS. It was found convenient to extract relatively large (50-80 cm2) paper samples in order to produce easily One of the limitations of extraction-based methods for visible separations on the plates. No efforts were made to colorant identification was encountered during TLC/SIMS define the minimum sample size required for optical visualanalyses of paper sample G. Direct SIMS analysis of the red ization of the separated dye components. With automated pattern on white paper (see Table 2) gave Pb+, in addition to TLC/SIMS,41*42 however, optical visualization is less critical, organic dye ions. Pb was not seen in any of the TLC spots and it is likely that much smaller amounts of material would sampled by SIMS, nor was it detected in the initial spot on be required. Further optimization might be achieved with in the plate. This inability to obtain complete extraction of ink situ extraction39and condensationm techniques. components is an unfortunate weakness in cases such as this. The blue ink on sample F provides a representative example With TLC alone it is not possible to distinguish between the of TLC/SIMS of a paper extract (see Figure 4b,d for spectra red pattern on sample E and on sample G. The detection of of this ink on paper). After separation, five distinct strongly Pb in sample G is the key to differentiating the inks. colored spots could be discerned on the plate, ranging from Despite the inherent disadvantages of extraction with subsequent chromatographic separation, this combination (38) Ungcr, S.E.; Vinczc, A.; Cooks,R. G.; Chrisman, R.; Rothrnan, L. D. Anal. remains extremely powerful. In situations where colorants Chem. 1981,53,97681. (39) Busch, K. L. Trends Anal. Chem. 1992.11, 31424. do not lie at the surface of a paper fiber, direct analysis by (40) Masuda, K.; Harada, K.-I.; Sumki, M.; Oka,H.; Kawarnura, N.; Yamada, SIMS would be futile. Extraction and separation followed M. Org. M a s Spectrom. 1989, 24, 7 4 5 . (41) Fiola, J. W.; DiDonato, G . C.; Busch, K. L. Reu. Sci. Insrrum. 1986, 57, by SIMS, or by other mass spectrometric methods,25on the 2294302. other hand, has the advantage of providing molecular weight (42) Flurcr, R . A.; Busch, K. L. A n a l . I n s t r u m . 1988, 1 7 , 255-76. AnaWIcal Chemistry, Vot. 66,No. 2, January 15, 1994
203
information in addition to retention times, which should in many cases allow the unambiguous identification of dyes. This additional information is probably not essential if the goal of the analysis is simply to differentiate two inks, but it could be very useful if qualitative identification is needed.
CONCLUSIONS Time-of-flight SIMS, with its extended mass range and high sensitivity, can be a useful tool for the nondestructive analysis of colorants on paper. Ball point pen inks are very readily analyzed by this technique, as are most other types of pen inks. Many machine-printed inks are also amenable to analysis by SIMS. The use of static analysis conditions produces no detectable alteration of the paper samples after analysis. Large samples which do not fit into the instrument can be analyzed in a quasi-nondestructive manner by carefully removing a single fiber from the region of interest and analyzing the fiber. TOF-SIMS presents many advantages for this type of analysis, in addition to its nondestructivity. It is quite fast in comparison to extraction/chromatography, with typical pumpdown times of 2 min and acquisition times of 5-10 min. Even allowing for single fiber selection and mounting, 0.5 h or less is the time typically needed to analyze one colorant on a new sample. This speed advantage is complemented by the ability of S I M S to detect every element in the periodic table and to give molecular weight information, and to do so on samples with microscopic dimensions. The limits of detection of colorants on paper using TOF-SIMS-and therefore the minimum amount of sample required-have not been fully explored but must be quite low, based upon our numerous successful analyses of single fibers.
284
Analytical Chemistry, Vol. 66,No. 2, January 75, 1994
The analysis of colorants on paper by S I M S is not without limits, however. The requirement that the colorants lie on the surfaces of the paper fibers is a disadvantage, although fortunately they often do lie a t the surface. There are also likely to be colorant systems which do not produce abundant ions in SIMS. Nevertheless, the time investment required for S I M S analysis is not large, and S I M S might be a good preliminary screening method in many situations. In the worst case, if a SIMS analysis is unsuccessful it could be followed with little cost in time by another method such as extraction/ chromatography. In the case where SIMS does in fact give abundant colorant ions and confirmatory identifications are needed, the S I M S information could potentially be used to guide selection of the extraction solvent and a set of appropriate chromatographic conditions. Future efforts in this area might include the determination of detection limits, both for direct analysis of paper fibers and for components separated by TLC/SIMS. A systematicSIMS study of inks, dyes, and papers might provide assistance in predicting the probability of success in the analysis of specific printed media by SIMS. This methodology should be extendible to other systems where nondestructivity is important-for example, forensic analysis of textile fibers.
ACKNOWLEDGMENT W e thank J. C. Alexander for his assistance in the TLC separations, and J. F. Eisele and P. A. Philip for support and encouragement of this work. Received for review August 26, 1993. Accepted November 1, 1993. e
Abstract published in Advance ACS Abstracts, December 1, 1 9 9 3 .
