Anal. Chem. 1988, 60,1769-1775
individually derivatized such that, upon injection, known quantities of glycine would be introduced into the system. With the current laser detection system, the minimum detectable amount for the glycine derivative has been determined to be 17 fg a t a signal-to-noise ratio of 3. Further improvements of this detection limit are currently being sought. In summary, we have developed some general syntheses of fluorogenic reagents for the derivatization of primary amines for subsequent separation with high-sensitivity laser-induced fluorescence detection. The reagent of choice at this point, BQCA, has been evaluated on the basis of a number of criteria, including quantum yield and product stability. Our reagent is similar to OPA in that the absorption maximum of the resulting isoindole is well removed from that of the parent molecule. However, BQCA features some advantages over OPA; for example, the isoindole products are substantially more stable than OPA derivatives, facilitating precolumn derivatization. Moreover, the BQCA derivatives display an absorption maximum closely matching a convenient laser line, leading to increased sensitivity with a relatively inexpensive detection system. The fact that the rate of the BQCA reaction is significantly less than that of OPA is of little consequence in our studies. The strict volumetric requirements of capillary LC almost necessitate precolumn derivatization methods. Therefore, an improved stability, not reaction rate, was our primary concern. It is noteworthy that the one quinoline-based reagent we have synthesized (BQCA) has emerged from thisstudy as more acceptable for our purposes than the best of the benzene-based reagents, which suffer primarily from insufficiently high absorption maxima. Design of additional reagents of this type is under way, facilitated by the knowledge we have gained in these early efforts.
1769
LITERATURE CITED (1) Roth, M. And. Chem. 1871, 4 3 , 880-882. (2) Stein, S. Arch. BIochem. Bbphys. 1974, 163, 400-403. (3) DeBemado, S.; Weigele, M.; Toome, V.; Manhart, K.; Leimgruber, W.; Bohlen, P.; Stein, S.; Udenfrlend, S. Arch. Blochem. B/ophys. 1874, 163, 390. (4) Ghosh, P. B.; Whltehouse, M. W. Biochem. J. 1888, 108, 155-156. (5) Lindroth, P.;Mopper, K. Anal. Chem. 1879, 5 1 , 1667-1674. (6) Hodgin, J. C. J. Liq. Chromatogr. 1878, 2 , 1047-1059. (7) Deyl, 2.; Hyanek, J.; Horakova, M. J. Chromatogr. 1986, 379, 177-250. (8) White, J. D.; Mann, M. E. Adv. Heterocycl. Chem. 1989, 1 0 , 113-147. (9) Yeung, E. s.I n Microcoumn SeparanOns; Novotny, M.; Ishii, D., Eds.; Elsevier: Amsterdam, 1985; 135-158. (10) Detectm for LiquM Chromatography; Yeung, E. S., Ed.; Wiley-Interscience: New York, 1986. (11) a r e , R. N. Science (Weshington, D . C . ) 1884, 226, 298-303. (12) Sepaniak. M. J. Clin. Chem. (Winstan-Salem, N.C.) 1885, 3 1 , 671-678. (13) Haddadin, M. J.; Chelhot, N. C. Tetrahedron Lett. 1973, 5185-5186. (14) ktlesics, W.; Anton, T.; Chaykovsky, M.; Toome, V.; Sternbach, L. J. Org. Chem. 1988, 33. 2874-2877. (15) Hauser, C. R.; Tetenbaum, M. T.; Hoffenberg, D. S. J. Org. Chem. 1984, 2 3 , 861-865. (16) Comlns, D. L.; Brown, J. D. J. Org. Chem. 1984, 49, 1078-1083. (17) Beak, P.;Brown, R. A. J. Org. Chem. 1882, 47, 34-46. (18) Haddadin, M. J.; Zahr, G. E.; Rawdah, T. N.; Chelhot, N. C.; Issidorides, C. H. Tetrahedron 1974, 30, 659-668. (19) Cheng, C. C.; Yan. S. J. Org. React. ( N Y ) 1982. 2 8 , 37-201. (20) Stark, 0. Ber. Dtsch. &em. Ges. 1909, 42, 715-719. (21) de Montigny, P.; Stobaugh, J. F.; Givens, R. S.; Carlson, R. G.; Srinivasacher, K.; Sternson, L. A.; Higuchi, T. Anal. Chem. 1887, 5 9 , 1096-1 101. (22) Parker, C. A. Photoluminescence of Solutions ; Elsevier: Amsterdam, Holland, 1968; pp 261-265. (23) Gluckman, J.; Shelly, D.; Novotny, M. J. Chromatogr. 1984, 317, 443-453.
RECEIVED for review June 26,1987. Accepted April 11,1988. This study was supported by Grant No. R 0 1 GM24349 from the Institute of General Medical Sciences, U S . Department of Health and Human Services.
Atomization Characteristics and Direct Determination of Manganese and Magnesium in Biological Samples Using a Magnetically Altered Thin-Film Plasma Stephen W. Brewer, Jr.* Department of Chemistry, Eastern Michigan University, Ypsilanti, Michigan 48197 Richard D. Sacks Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109 A magnetic field with peak value of 3.7 kG is used to improve the atomization characteristics of an electrically vaporized thln-film plasma for the direct determination of Mg and Mn in solid biological materials, Plasmas are generated by highcurrent capacitive discharges through 350-pg Ag or Au thin films formed on polypropylene substrates. Radiation intensity vs time plots are compared with and without the magnetic field for the NBS materials bovine liver, oyster tissue, orchard leaves, citrus leaves, tomato leaves, and pine needles. Analyticai standards for Mg are prepared from suspensions of MgO powder, and standards for Mn are prepared from aqueous solutions of Mn(NO,), or MnSO,. Analytical accuracy usually Is improved with the presence of the magnetic field.
In a recent feature article in this journal, Scheeline and Coleman (1) pointed out the desirability of direct analysis of
solids without prior dissolution, which is often a time-consuming process that can introduce contaminants. Numerous attempts have been made to perform direct and near-direct elemental analysis of solids by sensitive atomic spectrometric techniques. Mohamed et al. (2) reported analysis of coal slurries by flame atomic absorption, with 51% losses of the slurries in transport. Hoenig and Van Hoeyweghen (3) determined P b and Cd in biological samples by using slurries to which a matrix modifier was added for platform electrothermal atomic absorption spectrometry. Rettberg and Holcombe (4) analyzed a variety of solid NBS reference materials with platform techniques without sample pretreatment or matrix modifier addition. McCurdy et al. (5) successfully analyzed NBS coal by introduction of slurries into a direct current plasma (DCP) after reduction of the coal to roughly 5.7-1m particles. Considerable work has been done on the introduction of solids into inductively coupled plasma (ICP) torches. Sugimae
0003-2700/88/0360-1769$01.50/0 Q 1988 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 17, SEPTEMBER 1, 1988
and Barnes (6) have used HF dissolution of samples of particulate material trapped on glass filters, with subsequent electrothermal vaporization from a microboat. Shao and Horlick (7) have used a moving cup for direct insertion of solution residues into the ICP. Cobbold (8) has recently reported on using a special nebulizer for insertion of high solid matrices. Electrothermal vaporization for sample introduction with solids has been explored at least since 1974 (9, IO). Ono et al. (11)used a spark discharge to generate ultrafine particles from alloy samples for direct analysis by ICP. In 1982, Goldberg and Sacks (12)reported on a novel and simple method for the direct emission spectroscopic analysis of a variety of solid matrices. In their method, a capacitive discharge with about 1-kJ energy is used to generate a hightemperature plasma by vaporizing a thin silver film on which powder samples no larger than 1mg have been placed. Atomic emission intensities from the short-lived, luminous plasma thus generated are used to determine metals in the powder sample. Large sample particles, up to 30 pm in diameter, are vaporized completely when high (ca. 1200 p H ) inductances are used. Intensity integration must be delayed until about 1.8 ms after the start of the discharge to mnimize particle size effects. Sample vapor loss during the delay may be significant. Albers and Sacks (13,14) later reported use of a magnetic-field-tailoringtechnique to increase interaction of electrically vaporized thin-film plasmas with solid powder samples. In their work, a modest magnetic field is generated by the discharge current in an inductor wound around the discharge chamber so that the magnetic field-is n_ormal to the electric field of the plasma. The resultant E X B drift motion of ions and electrons drives the plasma toward the film support, with the result that shorter vaporization times are needed, necessary integration delays are shorter, and line-to-background intensity ratios may be improved. The analytical utility of the method was demonstrated by the analysis of a variety of NBS reference materials, of widely varying volatility and compositions, for a number of trace elements (14). Sample matrix preparation typically involved only a few minutes of grinding, matrix effects are minimal, and aqueous solution standards could be used for the direct determination of microgram per gram metal concentrations. In this study, comparisons are made of the determinations of a minor element, Mg, and a trace element, Mn, in a variety of NBS biological reference materials by using nontailored and magnetically tailored, electrically vaporized thin-film plasma techniques. Samples were chosen to represent a wide variety of plant and animal tissues and metal concentrations. Magnesium concentractions range from 0.06 to 0.7% in bovine liver (SRM 1577a), oyster tissue (SRM 1566), citrus leaves (SRM 1572), orchard leaves (SRM 1571), and tomato leaves (SRM 1573). Manganese concentrations range from 91 to 675 pg/g in orchard leaves, tomato leaves, and pine needles (SRM 1575). Improved accuracy is shown to be a result of using magnetically tailored discharges with no sacrifice in sample preparation time.
EXPERIMENTAL SECTION Experiment Design. Discharge circuits for both unconfined and magnetically confined plasma generation have been described in detail (12,14),as have coil and chamber design and orientation (13,14). Table I shows conditions for high-inductance nontailored and low-inductance magnetically tailored discharges. Both Ag and Au films were used. Their production has been described (15, 16). Silver films were used for Mg determinations. Gold films were used for Mn determinations, where acid erosion of the Ag films by aqueous standards occurred. Film characteristics also are shown in Table I. Optical and Electrical Monitoring. The chamber, with a plane quartz viewing window, has been described (13, 14). The center of the plasma was located about 6 3 cm from the entrance
Table I. Discharge Conditions and Thin-Film Properties
Discharge Conditions
high inductance, no magnetic field
support gas pressure capacitance, fiF charging voltage, kV inductance, pH ringing freq, kHz energy, J peak current, kA peak magnetic field, kG
60140 A r l o ,
atmospheric
low inductance, with magnetic field 60140 Arlo2 atmospheric
30 8.0 1200 0.83
30
960 1.0
540
6.0 105 2.8 2.8 3.7
Thin-Film Properties Au
Ag
substrate material substrate dimens, cm film dimens, cm thickness, %, film mass vaporized, Kg resistance,
polypropylene polypropylene 7.3 X 1.6 X 0.05 7.3 X 1.6 X 0.05 7.3 X 1.6 7.3 X 1.6 400
350 5-10
230 350
16-40
slit of the 1.0-m Czerny-Turner plane grating spectrometer using a 1200 line/” grating and an 80-pm entrance slit width. No ancillary optics were used, as the plasma intensity was great enough to allow sufficient throughput. Exploratory photographic work was done with Kodak SA No. 1 emulsion, processed by standard procedures. A 1P28 photomultiplier tube, biased between 500 and 700 V, with a l-kQ load was used for photoelectric monitoring. Oscilloscope traces of discharge current and line intensity versus time were obtained by the methods previously described (13,14). Delayed integrated intensities were measured with the gated integrator described by Swan and Sacks (17). Experimental Procedures. For Mg determinations, standards were prepared from MgO powder (Baker and Adamson Reagent), which was ground in a light-duty mill and sieved on a sonic sifter. The fraction passing through a 10-pm screen was used for standards. Powder standards were weighed on a microbalance and then suspended in 5.00 mL of reagent grade 2-propanol, from which 50-pL aliquots were deposited on thin films and dried with a heat lamp. Manganese standards were prepared from aqueous solutions, since the low Mn concentrations in the samples studied would not allow good sampling statistics if 10-pm particles were used. Two aqueous standards were used: 999 mg/L Mn dissolved as Mn(N0J2 (Sigma Chemical Co., St. Louis, MO) and solutions made from solid MnS04 (Spex Industries, Metuchen, NJ). Manganese concentrations of stock solutions made from MnS04 were checked against the Mn(NO,), standard by flame atomic absorption, as the hydration number of the sulfate salt was uncertain. Fresh dilutions were made daily from stock solutions. Standards were applied to film surfaces as 50-pL aliquots in eight roughly equal drops and dried with a heat lamp. Standard reference materials from NBS were dried according to instructions. Portions of NBS reference materials of roughly 2 g were ground with a stainless steel mortar in a light-duty laboratory mill for 15 min. Diameters of ground sample particles were determined from scanning electron microscopy to be mostly less than 25 pm. Samples of 10 mg were weighed and suspended in 5 mL of 2propanol. Sample tubes containing the suspensions were placed in an ultrasonic bath for 1 min and then transferred to a vortex mixer. An aliquot of 50-pL volume, containing a 100-pg mass of suspended sample, was transferred to each heated film surface and dried with a heat lamp. Silver films were prepared from 99.999% Ag needles (Alfa Products, Thiokol Ventron Division, Danvers, MA). Gold films were prepared from surplus gold used in a previous study (16). Trace quantities of Ag and Cu may be present in the gold films. Experiments all were conducted at atmospheric pressure in a gas mixture of 60/40 A r / 0 2 flowing
ANALYTICAL CHEMISTRY, VOL. 60, NO. 17, SEPTEMBER 1, 1988
1771
Time, ms
Figure 1. Radiation intensity-time profiles for 105-pH discharges with no magnetic field (a) and with a magnetic field (b). The Mg I277.9-nm line was monitored from 100-pg samples of bovine liver (solid lines) and MgO powder standards containing the same amount of Mg (broken lines). Dashed lines are background intensity. The discharge current waveform (c) is shown as a time reference. I
,
21
I
0
I
I
0.2
0.4
0.6 T i m e , ms
0.0
1.0
Figure 3. Radiation intensity-time profiles for 105-pH discharges with no magnetic field (a)and with a magnetic field (b). The Mg I277.9-nm line was monitored from 100-pg samples of citrus leaves (solid lines) and MgO powder standards containing the same amount of Mg (broken lines). Dashed lines are background intensity. The discharge current waveform (c) is shown as a time reference.
r
0
0.2
0.4
0.6
0.0
1.0
Time, ms
Figure 2. Radiation intensity-time profiles for 105-pH discharges with no magnetic field (a) and with a magnetic field (b). The Mg I277.9-nm line was monitored from 1OO-hg samples of oyster tissue (solid lines) and MgO powder standards containing the same amount of Mg (broken lines). Dashed lines are background intensity. The discharge current waveform (c) is shown as a time reference.
through the chamber at 5 Llmin. Intensity-time traces were made from the sum of four replicate traces less the sum of four prerecorded background traces at the analytical wavelength. For quantitative work, average intensity values were obtained from five replicate determinations, and the average intensities of five blanks from bare films were subtracted for background correction at the analysis wavelength. Quantitative results were obtained for comparison from discharges found earlier to be analytically useful. These discharges use high circuit inductance with no magnetic field (12) and low circuit inductance with an external magnetic field (14).
RESULTS AND DISCUSSION Magnesium Determinations. A variety of plant and animal materials including bovine liver, oyster tissue, citrus leaves, orchard leaves, and tomato leaves were chosen for studies comparing the Mg atomization characteristics with and without an external magnetic field. The Mg neutral-atom line a t 277.98 nm was chosen as the analytical line because photographic spectra of the samples indicated minimum interferences a t this wavelength. Figures 1-5 show intensity versus time profiles for NBS samples (solid lines), MgO powder standards (broken lines) and continuum background intensity (dashed lines). In each
u
,
I
0
0.2
I
I
I
0.4
0.6
0.0
I -0
Time, ms
Figure 4. Radiation intensity-time profiles for 105-pH discharges with no magnetic field (a) and with a magnetic field (b). The Mg I 277.9nm line was monitored from 100-pg samples of orchard leaves (soli lines) and MgO powder standards containing the same amount of Mg (broken lines). Dashed lines are background intensity. The discharge current waveform (c) is shown as a time reference.
figure, the top group labeled (a) was made with 105-pH discharges (see Table I) but with no external magnetic field. For these plots, the inductor had been removed from around the discharge chamber. The plots labeled (b) were made with the inductor placed around the chamber. This results in a time-varying magnetic field, the strength of which is proportional to the value of the discharge current (waveforms labeled c). The peak field strength of 3.7 kG occurs a t the time of the first current peak or about 0.04 ms after the start of the discharge. Results for bovine liver are shown in Figure 1. Without the magnetic field (a), the traces from the sample and standard do not coincide. The sample trace (solid line) is at all points higher than that of the refractory MgO standard (bp 3600 "C), suggesting that a high value would be obtained for Mg content in bovine liver if no magnetic field is used. Line-to-background ratios are largely less than unity. When the magnetic field
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ANALYTICAL CMMISTRY. VOL. 60, NO. 17. SEPTEMBER 1. 1988 r
n
Au
ci
SEI
After
1
0
0.2
0.4
0.6
0.8
1.0
T i m e . ms
5. RadhtkfI hnensny-lime profibs fa 105-11H disdrarges wtth no Wgnenc field (a) and wim a magnew field (b). The Mg I 277.9-nm Rne was moniiwed from 100-,AQ samples of tomato leaves (sdii lines) and MgO powder standards m i n i n g Um same a m n t of Mg (broken lines). Dashed lines are background intensity. The discharge current waveform (c) is shown as a time reference.
is present (b), significantly closer coincidence of sample and standard traces is observed, suggesting that better analytical results could be expected with the magnetically altered plasma and that vaporization of even refractory particles may be sufficiently rapid that these materials may be useful as standards. Results for oyster tissue are shown in Figure 2. The oyster tissue sample contains about 131 ng of Mg, about twice as much as found in bovine liver. Significant differences in the emission intensity trace without an external magnetic field (a) exist between these two sample types and masses of Mg. First, while emission intensity for bovine liver appears to peak during the second current half-cycle, the intensity peak for the oyster tissue appears in the first current half-cycle. Intensity-time curves for oyster tissue and the MgO standard (broken line) coincide closely during the first current half-cycle and then diverge rapidly, with the curve for the MgO rising above that for oyster tissue during the second half-cycle. It is possible that more time is needed to vaporize the greater mass of MgO corresponding to the 131 ng of Mg in oyster tissue. As in the case of bovine liver, line-to-background ratios are mostly less than unity. With application of an external magnetic field (b), coalescence of the MgO and oyster tissue emission profiles is greatly improved, as are line-to-background ratios. This suggests that magnetic tailoring may reduce effects arising from analyte metal mass and volatility differences between samples and standards. Further, it suggests that better analytical accuracy can be expected with magnetic tailoring. Figures 3-5 give intensity-time profiles for botanical materials. In these plant materials, Mg concentrations are much higher than in the animal tissues studied. For the 1oO-fig sample size used in these studies, samples of citrus leaves, orchard leaves, and tomato leaves contained 581.628, and 690 ng, respectively. The citrus leaf (Figure 3, solid lines) and MpO (broken lines) emission profiles show little coincidence when no magnetic field is used (a). The Mg intensity from MgO powder peaks in the second current half-cycle, as was the case for the lower Mg mass in the bovine liver samples (Figure 2a). The Mg intensity from citrus leaves, however, decays regularly after
mm
Flgwa 6. Au Mor. CI K a . and SEI photographs of residues from 5-jtL droplets of 14 mgA WlI) in WI, aqueous soluliin deposited on Au
films. the first current half-cycle. This is in contrast to the waveform for bovine liver. When an external magnetic field is applied, as in Figure 3b, emission intensities from the leaf sample and the MgO standard very nearly overlap after the first current half-cycle, indicating that magnetic tailoring of the plasma should yield relatively g d analytical accuracy. In Figure 4, results for orchard leaves are shown. The Mg emission patterns from samples and standards, shown in Figure 4a, are similar to the patterns observed for citrus leaves in untailored plasmas, although emission from orchard leaf samples relative to MgO is lower than in the case with citrus leaves, suggesting a difference in the vaporization characteristics of the two sample types. When magnetic plasma tailoring is used, as shown in Figure 4b. sample and standard curves are much more similar than they are without the magnetic field, but the coalescence is not as good as for the citrus leaf samples. This suggests somewhat pcmrer analytical accuracy for Mg determinations in orchard leaves. Tomato leafsample results are shown in Figure 5. The most striking departure from previous emission patterns in discharges without external magnetic fields appears in Figure 5a. Here emission curves from the samples and standards are quite similar after the first current half-cycle. Acceptably accurate analytical results could probahly be obtained for tomato leaves with proper time-gated intensity integration. The considerable differences ohserved for the emission profiles from the three relatively similar botanical materials indicate that, in the absence of an external magnetic field, relative atomization rates are quite different in these plasmas. With the magnetic field present, the emission profiles for the tomato leaf sample and the MgO standard are quite similar for almost the entire duration of the discharge, indicating that acceptable analytical accuracy can be obtained without the need for time-gated intensity integration. Further, linetobackground ratios are improved, as is the case for the other plant and animal samples. From the traces displayed in Figures 1-5, it is reasonable to conclude that the magnetically altered plasma, used with MgO powder standards, is better suited to Mg determinations in the sample types studied here than is the plasma without the magnetic field. While the use of solution standards may be more convenient than powder standards, solution residues proved hard to vaporize. Figure 6 shows photomicrographs of a Au film surface before and after Vaporization in a magnetically altered plasma. Five-microliter drops of 14 mg/L
ANALYTICAL CHEMISTRY, VOL. 60, NO. 17, SEPTEMBER 1, 1988
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Table 11. Magnesium Content of NBS Materials
NBS material
NBS value,bng/100 wg
bovine liver SRM 1577a oyster tissue SRM 1566 citrus leaves SRM 1572 orchard leaves SRM 1571 tomato leaves SRM 1573
59.9 f 3.3 131 f 9.2 581 f 30.0 628 f 29.7 690"
time gate, ms
high inductance, no B field % error found: ng/100 pg 94.6 f 46.2 253 f 63 676 f 168 942 f 243 713 f 12.6 1.8-4.7
+57.9 +93.1 +16.3 +34.8 +1.1
low inductance,dwith B field found,' ng/ 100 pg % error 67.1 f 30.2 110 f 20 572 f 46 507 f 75 644 f 44
+12.1 -16.1 -1.5 -19.2 -6.6
0-1.8
Not certified. * f values reported by NBS. f values are estimates of standard deviation based on five determinations. dlog-log slope, 0.86;
correlation coefficient, 0.993.
Mg(I1) as MgClz in aqueous solution were placed on the film and evaporated to dryness. Column Au shows an Au M a X-ray map before and after the discharge. Column C1 shows C1 K a maps, and column SEI shows secondary electron images of droplet residues. Some residue is clearly visible after the discharge in the SEI image. The C1 map indicates that the residue is a chloride salt. Gold may be seen under the residue after the discharge. When powder suspension standards are used, no such residues are observed and the Au film is completely scavenged from the substrate surface. The effect of film loading with salt crusts of relatively high mass has been reported elsewhere (18). It is clear from Figures 1-5 that for the discharge parameters used here, the plasma is of significant analytical utility only when the magnetic field is present. Goldberg and Sacks (12) showed earlier that increasing the circuit inductance to about 1200 p H results in improved atomization characteristics of the plasma with no magnetic field present. By delaying the measurement interval for 1.8 ms after the start of the discharge, they were able to reduce particle size effects and obtain reasonable analytical accuracy. Table I1 offers a comparison of Mg determination results obtained from 1200-pH discharges with no magnetic field and 105-pH discharges with the magnetic field present. For all determinations, standards were 10-pm Mg particles suspended in 2-propanol. Sample masses of 100 pg were suspended in 50 p L of 2-propanol. The Mg 277.98-nm neutral-atom line was used for analysis. Without the magnetic field, all relative errors are positive and range from 1.1% for tomato leaves to 93.1% for oyster tissue. For four of the five NBS materials, errors are reduced dramatically with the magnetically altered discharge and range in absolute value from 1.5% for citrus leaves to 19.2% for orchard leaves. These results are consistent with the vaporization profiles for these samples with the two plasma types. Also note that with the magnetic field, intensity integration can begin at the start of the discharge; without the field, it is necessary to wait about 1.8 ms after the start of the discharge in order for particle size effects to be reduced to acceptable values. Manganese Determinations. Intensity versus time profiles were obtained for the Mn 259.37-nm ion line in NBS pine needles, orchard leaves, and tomato leaves. In the latter two materials, the Mn concentration is about an order of magnitude lower than the Mg concentration. For this reason, solution standards were used. The amount of Mn on each film was too small to allow reproducible use of 10-pm powder standards. Results were compared for Mn(N03)2and MnS04 aqueous solution standards. In Figure 7, results are shown for pine needle samples on Ag films. Again, the plots labeled (a), (b), and (c) represent intensity-time traces with no magnetic field, intensity-time traces with the magnetic field, and the discharge current, respectively. The solid line intensity-time plots are for the NBS material. Broken line plots labeled N and S are for the M I I ( N O ~ )and ~ the MnSO, standards, respectively, and the
Time, ms
Figure 7. Radiation intensity-time profiles for 105-pH discharges with no magnetic field (a) and with a magnetic fieM (b). The Mn I 1 259.4-nm line was monitored from 100-pg samples of pine needles (solid lines) and aqueous residues of MnSO, (broken lines marked S) and Mn(NO,), (broken lines marked N) containing the same amount of Mn deposited on Ag films. Dashed lines are background intensity. The discharge current waveform (c) is shown as a time reference.
dashed line plots are for continuum background. With no magnetic field, the traces for the NBS material and the two standards are all quite different. Intensity values for the nitrate standard are significantly lower than values from the sulfate standard during the entire duration of the discharge. During the first current half-cycle of the discharge, peak intensity values are observed for the pine needles and for the sulfate standard. Radiation intensity from the nitrate standard, however, is much lower. A second large peak for the sulfate standard is seen in the third current half-cycle. Clearly, neither standard would be suitable for Mn determination in this sample. With the magnetic field present, the curves for the sulfate standard and the NBS material are fairly similar after the first current half-cycle. Once more, however, the nitrate curve is quite low in comparison to the others. This is probably the result of acid erosion of the Ag films by the nitrate solutions. Physical damage to the film surfaces was visible after nitrate solutions were dried. The pH of the nitrate standards was in the 1-2 range, while the pH of the sulfate standards was in the 5-6 range. Acid erosion of Ag films can retard efficient vaporization of solution sample residues (16). Figure 8 shows a similar study using Au films. Again, the traces labeled S and N are for the sulfate and the nitrate standards, respectively. Without the magnetic field, there is no analytically useful coalescence of either the nitrate or the sulfate trace with that of the pine needle sample trace.
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 17, SEPTEMBER 1, 1988
Table 111. Manganese Content of NBS Materials NBS material
NBS value," ng/100 pg
orchard leaves SRM 1571 tomato leaves SRM 1573 pine needles SRM 1575
23.8 f 0.7 67.5 f 1.5
high inductance, no B field found,bng/100 pg 70 error 7.1 f 2.2 24.8 f 5.0 86.5 f 12.0
9.1
low inductance, with B field
-22.0 +4.2 f28.1
found,*ng/100 pg
% error
9.0 f 3.6C3e 26.0 f 4.Sdse 77.5 f 17.5cvf
-1.1 +9.2 +14.8
film material time gate, ms
Ag
1.8--4.7
'0.174-1.8 d0.256-1.8
standard
MnSO, solution standard additions
'MnSO, solution calibration curve 'Mnln(N0A calibration curve
f values reported bv NBS.
AU
* f values are estimates of standard deviation based on five determinations.
a
V
I
0
W I
0.2
0.4
0.6
1
I
0.0
0
0.2
0.4
0.6
0.0
T i m e , ins
Figure 8. Radiation intensity -time profiles for 105-pH discharges with no magnetic field (a)and with a magnetic field (b). The Mn I 1 259.4-nm line was monitored from 100-pg samples of pine needles (solid lines) and aqueous residues of MnSO, (broken lines marked S) and Mn(NO,), (broken lines marked N) containing the same amount of Mn deposited on Au films. Dashed lines are background intensity. The discharge current waveform (c) is shown as a time reference, with time in milliseconds.
Flgure 9. Radiation intensity-time profiles for 105-pH discharges with no magnetic field (a) and with a magnetic fiikl (b). The Mn I 1 259.4-nm line was monitored from 100-pg samples of tomato leaves (solid lines) and aqueous residues of MnSO, (broken lines marked S) and Mn(NO,), (broken lines marked N) containing the same amount of Mn deposited on Au films. Dashed lines are background intensity. The discharge current waveform (c) is shown as a time reference.
However, after the first half-cycle of the discharge, the sulfate and nitrate traces are fairly similar. With the magnetic field present, the nitrate standard and the pine needle curves are quite similar after the first half-cycle, and line-to-background values are considerably enhanced. For the sulfate standards, the peak intensity value occurs in the second half-cycle. This may be the result of the more refractory nature of the MnS04, which melts at 700 "C and decomposes at 850 "C, relative to Mn(NO&, which melts a t 26 "C and boils a t 129.4 "C. Previous studies (12,16)have indicated that powder samples are dislodged from the film during the discharge and interact more strongly with the plasma than solution residue samples, which form crusts that lie in the relatively cool boundary layer near the substrate surface. Larger samples form thicker crusts that require longer to ablate from the substrate surface. Pine needles contain a higher Mn concentration than the other NBS materials studied with 67.5 ng of Mn on the film surface. Relatively slow heat transfer to the large sulfate residue may explain why the peak intensity from the sulfate standard occurs in the second discharge half-cycle. The analytical implications of the data in Figures 7 and 8 are clear; good analytical accuracy for Mn determinations in NBS pine needles should be possible if nitrate standards are used on Au films and if intensity integration is delayed until
after the first half-cycle of the discharge. This is consistent with Albers' results with other NBS materials (14). Intensity-time traces for Mn in tomato leaves are shown in Figure 9. Gold thin films were used. With no magnetic field (a), intensity values for the nitrate and the sulfate standards are much lower than for the NBS material. Again, this is consistent with the more difficult vaporization of the solution residue relative to a dispersed powder sample. When the magnetic field is present, the sulfate standard and the tomato leaf sample curves follow each other very closely after the first three quarters of a discharge cycle, suggesting that proper time-gating should result in good analytical accuracy. Line-to-background ratios also are significantly larger with the magnetic field. A similar study for NBS orchard leaves is shown in Figure 10. In this case, each sample and standard contained only 9.1 ng of Mn. Without the magnetic field, line-to-background ratios are very low, and while the sulfate standard performs better than the nitrate, neither standard appears very useful. With the magnetic field, a dramatic increase in line-tobackground ratios is observed, and the sulfate standard appears to perform very well after the first discharge half-cycle. Table I11 compares Mn determinations in orchard leaves, tomato leaves, and pine needles using 1200-pH discharges with no magnetic field and 105-pH discharges with the magnetic
ANALYTICAL CHEMISTRY, VOL. 60,
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Figure 10. Radiation intensity-time profiles for 105 pH discharges with no magnetic field (a) and with a magnetic fiekl (b). The Mn I1 259.4-nm line was monitored from 100-pg samples of orchard leaves (soli lines) and aqueous residues of MnSO, (broken lines marked S) and Mn(N03), (broken lines marked N) containing the same amount of Mn deposRed on Au films. Dashed lines are background intensity. The discharge current waveform (c) is shown as a time reference.
field present. Samples of 100 pg suspended in 50 pL of 2propanol were used. The analysis line was the Mn 259.37-nm ion line. Again note that the 1200-pH discharges were used previously with time-gated integration for the direct determination of some trace elements in NBS reference materials (12). When the 1200-pH discharges were used with no magnetic field, reliable calibration curves could not be prepared because of very poor correlation coefficients and large standard deviations. Thus, standard additions of aqueous MnS04 were used with better results. Satisfactory calibration curves with aqueous standards were generated by using the 105-pH discharge with the magnetic field. Concentration ranges for the analytical curves for orchard leaf, tomato leaf, and pine needle samples were 0.1-1.0,0.2-1.0, and 0.5-5.0 pg/g, respectively. Relative errors for Mn determinations in pine needles and orchard leaves were dramatically reduced by use of the magnetically altered discharges. While the relative error increased for the tomato leaves, the value of +9.2% is still very satisfactory for a rapid, direct single-channel analytical method. Correlation coefficients for orchard leaves, tomato leaves, and pine needles were 0.976, 0.991, and 0.996, respectively, while slopes of log-log plots of intensity vs concentration were 0.91, 1.09, and 0.65, respectively.
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The analytical data presented in this report suggest that the use of magnetically altered plasmas from electrically vaporized thin films is very useful for the rapid, direct determination of trace and minor components in a variety of biological materials. Powder standards, even of refractory MgO, yield vaporization profiles close to those of complex biological materials. Powder standards are preferred since they can be used with either Ag or Au films, and the particular compound used should be relatively unimportant. In addition, time-gated intensity integration does not appear to be necessary with powder standards. If powder standards can be prepared from materials of smaller particle size, improved sampling statistics should permit the use of powder standards at lower analyte concentrations. When analyte concentrations are too low for powder standards, solution standards may be quite satisfactory, but Au films are preferred, and time-gated integration may be necessary.
ACKNOWLEDGMENT Photomicrographs were taken by D. Buckner of the Electron Microbeam Analysis Laboratory of the University of Michigan, Ann Arbor. Registry No. Mg, 7439-95-4; Mn, 7439-96-5; MgO, 1309-48-4; Mn(NO&, 10377-66-9;MnS04, 7785-87-7.
LITERATURE CITED (1) Scheeline, A.; Coleman, D. M. Anal. Chem. 1987, 59,1185A-1196A. (2) Mohamed. N.; McCurdy. D. L.; Wichman, M. D.; Fry, R. C.; O’Reilly, J. E. ADD/. SDeCtfOSC. 1985. 39. 979-983. Hoen{g, M.: Van Hoeyweghen, P. Anal. Chem. 1986, 58, 2614-2617. Rettberg,T. M.;Holcombe, J. A. Anal. Chem. 1988. 56,1462-1467. McCurdy, D. L.; Wichman, M. D.; Fry, R. C. Appl. Spectrosc. 1985, 39,984-988. Sugimae, A.; Barnes, R. M. Anal. Chem. 1986, 56,765-789. Shao, Y.; Horlick, G. Appl. Spectrosc. 1986, 40,386-393. CobboM, D. G. Appl. Spectrosc. 1986, 40, 1242-1244. Nixon, D. E.; Fassel, V. A.; Kniseley, R. N. Anal. Chem. 1974, 46, 210-2 13. Ng, K. C.; Caruso, J. A. Appl. Spectrosc. 1985, 39,719-726. Ono, A.; Saeki, M.; Chiba, K. Appl. Spectrosc. 1987, 41,970-976. Goldberg, J.; Sacks, R. Anal. Chem. 1982, 54,2179-2185. Albers, D.; Sacks, R. Spectrochim. Acta, Part B 1986, 418, 391-402. Albers, D.; Sacks, R. Anal. Chem. 1987, 59,593-597. Clark, E. M.; Sacks, R. D. Spectrochim. Acta. Part B 1980, 358, 47 1-488. Brewer, S.W.; Sacks, R. D. Anal. Chem. 1985, 57, 724-729. Swan, J. M.; Sacks, R. D. Appl. Spectrosc. 1985, 39, 704-710. Brewer, S. W.; Fischer, P. T.; Sacks, R. D. Anal. Chem. 1985, 57, 2399-2403.
RECEIVED for review February 5, 1988. Accepted April 18, 1988. This work was supported by the National Science Foundation through Grant CHE 8411290 and by the Graduate School of Eastern Michign University through the Research and Sabbatical Leaves Program.
