Elemental analysis of Spanish moss using laser ablation inductively coupled plasma mass spectrometry

Analytica Chimica Acta 435 (2001) 309–318

Elemental analysis of Spanish moss using laser ablation inductively coupled plasma mass spectrometry Melody Bi, Emmet Austin, Ben W. Smith, James D. Winefordner∗ Department of Chemistry, University of Florida, Gainesville, FL 32611, USA Received 9 August 2000; received in revised form 31 January 2001; accepted 1 February 2001

Abstract NIST archival leaf standards are used as matrix matched standards for reliable quantitative elemental analysis of Spanish moss samples by LA (laser ablation)–ICP (inductively coupled plasma)–MS (mass spectrometry). Mixed standards are used to produce at least three data points for each calibration curve. The results obtained are compared to that obtained from microwave digestion ICP–MS/AES. For most of the elements studied, the results for the two methods agreed. Iron (Fe) is exceptional in that a discrepancy of ca. 10-fold in results occurred using the two methods. This is believed to have been caused by an interference of ArOH+ at mass 57. Standard addition was also studied and results showed that it is an effective method when matrix-matched standards are not available. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Spanish moss; LA–ICP–MS

1. Introduction Tillandsla usneoides L., family Bromeliaceae, commonly called Spanish moss, is an epiphyte which festoons the trees in swamps and hammocks south from Virginia to Florida and west to Texas, and further southward throughout northern South America. The plant possesses no functional internal conducting system or cuticle, and water absorption occurs over the whole surface of the plant [1]. Therefore, these mosses can be used as biomonitors for direct monitoring of wet and dry deposits from the atmosphere. By observing and measuring the changes of a bioindicator, a conclusion as to the kind of pollution (e.g. a heavy metal), its source and possibly its extent can be drawn [2]. Since biomonitoring using mosses was first ∗ Corresponding author. Tel.: +1-352-392-0556; fax: +1-352-392-4651. E-mail address: [email protected] (J.D. Winefordner).

introduced in 1968 by Rühling and Tyler [3], the use of mosses, lichens and barks for monitoring of heavy metal deposition from the atmosphere has found wide application [4–7]. A number of analytical tools have been used to determine elemental concentrations in mosses [8–10]. The most often used techniques include ICP–AES and electrothermal atomization AAS following the digestion of moss samples. Spanish moss is considered a difficult-to-digest sample containing a variety of matrix constituents including organic compounds [11]. Strong oxidizing agents with various acid mixtures as well as high pressure and temperature conditions are necessary for complete digestion. Microwave digestion (MD) is also widely used [12–19]. A problem associated with the determination of trace elements using any bomb digestion approach includes the risk of losing elements because of activation of the pressure relief mechanism of the vessel during digestion. Blank interference, which is mainly due to the contamination of the membrane filter

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as well as containers, may become an important factor during the preparation of samples. For samples of varied compositions and low elemental concentrations, a memory effect from previously digested samples may cause unreliable analytical results [20]. The acid residue also may affect the reliability of the analysis. Direct analysis of Spanish moss samples has not been reported previously. The use of pulsed laser ablation sample introduction for ICP–MS enables analysis of solid materials to be performed directly, without sample dissolution [21–24]. Although the relative concentration sensitivity of LA–ICP–MS is poorer compared to solution ICP–MS, because much less material is injected into the ICP, trace detection with submicrogram per gram (ppm) sensitivity in the bulk is routinely achieved. Because the ablation yield varies with material properties, such as reflectivity, thermal conductivity, and melting and boiling points, it is important to obtain matrix-matched standards that contain all the elements of interest [25]. There have been a few reports of full quantitative analysis by LA–ICP–MS using matched standards and multi-point calibration plots for each element of interest [26–30], e.g. using glass standard reference materials (SRMs) for the analysis of minerals [30]. This paper reports on use of LA–ICP–MS for the direct analysis of Spanish moss samples using NIST leaf SRMs as standards for calibration. The standard addition method is also studied. The results are compared to those obtained from MD–ICP–MS. The sampling strategy, instrumental parameters, fractionation effects are also studied to characterize the strengths and limitations of this approach. 2. Experimental 2.1. Instrumentation The experiments were performed with a Finnigan MAT (San Jose, CA, USA) SOLA ICP–MS and Finnigan MAT System 266 laser ablation accessory. The SOLA ICP–MS has a quadruple mass analyzer. Typical operating conditions for the ICP–MS are listed in Table 1. A combined flow of nebulized solution and carrier gas from the ablation chamber was introduced to the ICP–MS for all sample measurements. Plasma

Table 1 Typical ICP–MS operating conditions Rf power (W) Coolant gas flow rate (l/min) Auxiliary gas flow rate (l/min) Nebulizer gas flow rate (l/min) Ablation chamber flow rate (l/min) Solution uptake rate (ml/min)

1200 15 0.9 0.65 0.35 1.0

Scan conditions ICP–MS Detector Scan range per isotope (amu) Number of passes Number of channels per amu Dwell time per channel (ms)

Electron multiplier 0.25 128 8 4

Laser Energy per shot (mJ) Repetition rate (Hz) Rastering speed (␮m/s)

∼4 5 15

conditions were maintained the same for the analysis of all the standards and samples. The laser was a Nd:YAG with a 266 nm output. It operated at 5 Hz with a typical pulse energy of ca. 4 mJ and a pulse width of 8 ns. The laser ablation system was modified by using a separate computer to control the x–y–z translation stage, which allowed for the translation of the sample at approximately 15 ␮m/s while the laser was repetitively fired. Typical analysis times were 80–100 s (signals measured over 400–500 laser shots). In general, sample translation during laser ablation provided more representative sampling of the surface; however, it also resulted in a larger mass removal rate and consequently higher sensitivities [31]. A photodiode, connected to a chart recorder, was used to monitor the laser output energy. A quartz plate was placed at a 45◦ angle to the laser beam between the source and the ablation chamber, deflecting ca. 4% of the laser beam to a UV filter and photodiode. The laser power was manually adjusted to maintain a constant photodiode signal. 2.2. Samples The NIST archival leaf SRMs were used as approximate matrix-matched standards for the elemental analysis of Spanish moss. These reference materials included NIST SRM 1515 (Apple leaves), NIST SRM 1547 (Peach leaves), NIST SRM 1570 (Spinach

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leaves) and NIST SRM 1573 (Tomato leaves). These standards were obtained in powder form. Mixed standards were also prepared to produce appropriate concentrations for the calibration curve. They were prepared by weighing out different portions from two standards and mixing in a Spex (Metuchen, NJ, USA) Mixer/Mill Model 8000 for 30 min to ensure homogeneity. The NIST standards were dried in a dissicator for 5 days before pressing into pellets for use. All Spanish moss samples were collected with plastic gloves and were placed in plastic storage bags for transport to the laboratory. The samples were then rinsed to remove wind-blown particles of dust and soil. Milli-Q double deionized water was used in the rinsing, and all samples were handled wearing plastic gloves. After rinsing, the wet moss samples

311

were dried in an oven at 110◦ C for 4 h. The samples were crushed in a ceramic mill for 25–30 min, reducing them to fine homogeneous powder well-suited to scooping and weighing. The powder was sealed in clean 60 ml polyethylene bottles for storage prior to pressing pellets and making measurements. The NIST SRM standards and Spanish moss samples were pressed into pellets without a binder at a pressure of 35 MPa (ca. 5000 psi). The samples for standard addition analysis were prepared by adding 200, 300, 500 and 1000 ␮l of 1 ppm Mn or Pb standard solution into each ca. 0.5 g portion of powder. The samples were dried at 110◦ C in the oven for several hours, transferred into plastic vials, mixed for 30 min, and pressed into pellets without a binder at a pressure of 35 MPa (ca. 5000 psi).

Fig. 1. Calibration curves of Mn, Ca, Zn and Fe using electron multiplier detector.

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Fig. 2. Calibration curves of Na and K using Faraday detector

3. Results and discussion 3.1. Elemental analysis of Spanish moss using NIST leaf SRMs as matrix matched standards The elements chosen for this study were present and detectable in both leaf standard materials as well as in the moss samples. These elements were: Na, Ca, K, Fe, Mn and Zn. The calibration curve prepared using standards covered the range of sample concentrations. This was true for most of these elements, since the leaf standards and Spanish moss are essentially similar materials having similar concentrations of these

elements. Mixed standards were used to produce at least three data points for each calibration curve. The calibration curves for the five elements measured are shown in Figs. 1 and 2. The calibration curves for most elements except K show reasonable linearity. The results obtained by LA–ICP–MS for the five moss samples were compared to those obtained from microwave digestion ICP–MS (Table 2). The results showed good agreement for Na, Ca, Zn and Mn obtained by the two methods. However, discrepancies of ca.10-fold occurred for Fe. The MD–ICP–MS results were compared with those obtained from MD–ICP–AES (Table 3). Good

Table 2 Comparison of elemental analysis by microwave digestion (MD) ICP–MS and LA–ICP–MS #

Ca(43) (%)

Mn(55) (ppm)

Fe (57) ppm

Zn (66) (ppm)

Na (23) (ppm)

LA–ICP–MS MD–ICP–MS

6

2.17 ± 0.13 2.38 ± 0.06

29.2 ± 3.0 39.2 ± 0.8

165 ± 11 1634 ± 18

62.0 ± 1.7 74.0 ± 1.7

874 ± 54 477 ± 27

LA–ICP–MS MD–ICP–MS

7

1.72 ± 0.021 1.55 ± 0.11

28.0 ± 1.7 31.6 ± 1.7

130 ± 10 427 ± 12

86.3 ± 6.9 79.3 ± 4.2

5967 ± 214 1976 ± 56

LA–ICP–MS MD–ICP–MS

8

1.21 ± 0.06 1.33 ± 0.05

30.8 ± 1.1 34.6 ± 1.3

89 ± 10 461 ± 18

67.8 ± 4.8 58.8 ± 2.6

7549 ± 1426 1714 ± 23

LA–ICP–MS MD–ICP–MS

10

1.19 ± 0.04 0.83 ± 0.03

68.5 ± 2.8 37.3 ± 2.8

69.3 ± 5.3 422 ± 8

73.6 ± 4.5 52.6 ± 2.8

2691 ± 120 1160 ± 44

LA–ICP–MS MD–ICP–MS

11

3.7 ± 0.2 2.9 ± 0.3

44.8 ± 5.2 33.5 ± 0.6

174 ± 9 844 ± 19

134.5 ± 6.6 122.8 ± 4.4

2080 ± 135 1423 ± 15

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313

Table 3 Comparison of elemental analysis by microwave digestion (MD) ICP–AES and (MD)ICP–MS #

Ca(43) (%)

Mn(55) (ppm)

Fe(57) (ppm)

Zn(66) (ppm)

Na(23) (ppm)

Pb(208) (ppm)

477 ± 27 510 ± 52.9

1.31 ± 0.11 –

ICP–MS ICP–AES

6

2.38 ± 0.06 3.0 ± 0.0

39.2 ± 0.82 38.7 ± 0.46

1634 ± 18 1600 ± 40

74.0 ± 1.7 78 ± 1.3

ICP–MS ICP–AES

7

1.55 ± 0.11 1.35 ± 0.003

31.6 ± 1.72 28.2 ± 0.465

427 ± 12 413 ± 5

79.3 ± 4.2 80.6 ± 0.97

1976 ± 56 1800 ± 115

0.22 ± 0.03 –

ICP–MS ICP–AES

8

1.33 ± 0.06 1.16 ± 0.007

34.6 ± 1.13 36.0 ± 3.6

461 ± 18 490 ± 29

58.8 ± 2.6 63.2 ± 3.1

1714 ± 23 1510 ± 372

3.12 ± 0.02 –

ICP–MS ICP–AES

10

0.83 ± 0.039 0.6 ± 0.002

37.3 ± 2.8 38.8 ± 0.49

422 ± 8 437 ± 18

52.6 ± 2.8 48.3 ± 2.1

1160 ± 44 940 ± 111

25.4 ± 0.4 –

ICP–MS ICP–AES

11

2.9 ± 0.3 2.54 ± 0.004

33.5 ± 0.6 34.61 ± 0.32

844 ± 19 820 ± 22

122.8 ± 4.4 125.1 ± 2.2

1423 ± 15 1646 ± 144

23.8 ± 1.6 –

agreement for all elements measured including Fe was obtained for the two techniques. Fe has three isotopes, mass 54, 56 and 57. Mass 54 and 56 overlap with the major interference from ArN+ and ArO+ , which leaves mass 57 as the only isotope to be measured. The above results indicate that the interference from ArOH+ at mass 57 is more prevalent when using laser ablation for sampling because botanical materials have a high content of organic compounds which would produce H and O during the ablation process.

The increase of H and O would result in formation of ArOH+ in the plasma. Since ArOH+ cannot be corrected by background subtraction during the laser ablation process, poor results for Fe were obtained using LA–ICP–MS. Theoretically, the results obtained from LA–ICP–MS should be higher than the solution ICP–MS if the interference from ArOH+ affect the standards only. However, the samples were also interfered by ArOH+ ; the LA–ICP–MS measurements were both higher or lower than the solution ICP–MS.

Fig. 3. SEM image of leaf SRM 1515 pellet surface.

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Therefore, LA–ICP–MS is not a good tool for measuring Fe for moss samples. When solution samples were measured by ICP–MS, because of the stability of ICP–MS, the ArOH+ formed from Ar and OH from H2 O in solution could be easily corrected by background subtraction. The calibration curve for K showed curvature (Fig. 2). Because the concentration of K is high (ca. 2%), the linear dynamic range has been exceeded. In general, the results obtained indicate that leaf standards can be used for elemental analysis of Spanish moss. A serious limitation of the LA–ICP–MS analysis is the need to have appropriate concentrations of each element of interest in the standard. In the case of Pb, the concentrations in the three leaf standards are below 0.1 ppm and so, they are not detectable by LA– ICP–MS. The standard addition method was evaluated to solve this problem and will be discussed below. The difference in results obtained by LA–ICP–MS with MD–ICP–MS/AES is a result of several causes. Scanning electron microscopy (SEM) images of the surface of leaf standard and moss sample prior to the laser ablation are shown in Figs. 3 and 4. The surface appearance in the SEM are somewhat different for the leaf and moss samples. This difference can be caused by the different conditions of sample preparation, including the process of sample collection,

treatment and the pressing of pellets. Fractionation and matrix effects can also affect the results obtained by LA–ICP–MS analysis to some extent. 3.2. Study of effect of laser fluence at the surface of the sample on fractionation It is critical in this study, that constant sensitivity be obtained for each element to be measured in all samples. A change of laser fluence on the surface of the sample when ablating different samples would cause a change of sensitivity of different elements which would result in poor linearity of the calibration curve. The change of relative sensitivity factor (RSFs) for different elements resulting from a change in laser fluence was investigated. The laser fluence at the surface of the sample could be changed either by changing the flash lamp voltage or by changing laser focal area by changing the focusing distance of the laser beam. The RSFs were measured when the flash lamp voltage was adjusted to produce laser output energies of ca. 6, 7 and 8 mJ (Fig. 5). The RSFs were measured when the laser was focused below, on the surface or above the surface of the sample, a total range of 1.6 mm, which produced laser spot sizes of ca. 40, 50 and 60 ␮m in diameter. The RSFs changed somewhat when the laser fluence was changed for most elements while a

Fig. 4. SEM image of moss # 11 pellet surface.

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315

Fig. 5. Fractionation study of NIST leaf standard 1515, RSFs change vs. change of voltage of laser system.

dramatic change is seen for Fe due to the interference from ArOH+ (Fig. 6). Maintaining a constant laser fluence on all of the samples is important for minimizing fractionation. In order to minimize fractionation, it was necessary to maintain the laser output energy constant for all measurements and to keep the same pellet thickness so that the laser focal area was maintained the same for all measurements. 3.3. Standard addition method The standard addition curves for the determination of Mn in moss 10 and 11 are shown in Fig. 7. The measured LA–ICP–MS intensities for the analyte, Mn, and for the internal standard, Ca in this case, were used to generate the calibration curve. Ca(43) was used to normalize the measured Mn intensities to correct for variations in the amount of laser-ablated particles reaching the ICP torch during the standard addition

procedure, using the expression [32] Ii (Xn ) = Ii (X) + I0 (X) [1 − Ii (IS)/I0 (IS)] where Ii (Xn ) denotes the normalized analyte intensity, plotted on the standard addition curve for i = 1–4 standard additions, Ii (X) the intensity for the analyte (e.g. Mn) in each measurement, Ii (IS) the intensity for the internal standard (e.g. Ca) in each measurement for i = 1–4 standard additions, and I0 (X) and I0 (IS) the intensity for the analyte and internal standard in the sample prior to any standard additions. When Ca was used as the internal standard to correct for the mass removal during ablation, the relative standard deviation for the analyte elements were below 6%. Using this normalization approach, no prior knowledge of the concentration of the matrix component in the standard or samples was required. The concentrations of Mn determined by standard addition for moss 10 and 11 were 41.4 ± 3.7 ppm (37.3 ± 2.8 ppm from MD–ICP–MS) and 34.0 ± 2.6 ppm (33.5 ± 0.6 ppm

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Fig. 6. Fractionation study of NIST leaf standard 1515, RSFs change vs. change of laser beam spot size (␮m in diameter).

from MD–ICP–MS), respectively. Although sample preparation time was increased, standard additions possessed several advantages. Strict matrix-matching was achieved and the concentration of the internal standard was not required. In general, the standard

addition method is well-suited for the analysis of moss samples and other particulate samples, since an element can be homogeneously spiked into the sample of interest. Also, using the standard addition method can bypass the problem of obtaining the proper

Fig. 7. Calibration curves of Mn in standard addition analysis: a) moss # 10; b) moss # 11.

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concentrations of certain elements in the standards. This can be important for monitoring certain elements in Spanish moss samples. For example, in the case of lead, the concentrations in the three SRM standards are below 0.1 ppm which cannot be detected by LA–ICP–MS. However, the standard addition method combined with the advantages of high sensitivity of LA–ICP–MS and the use of internal standardization to correct for the fluctuation of laser output energy gave good results. The concentration of Pb in Spanish mosses 10 and 11 measured by LA–ICP–MS were 35.6 ± 3.2 and 46.1 ± 2.8 ppm, respectively. The MD–ICP–MS of Pb in these two samples were 25.4 ± 0.4 and 23.8 ± 1.6 ppm, respectively. The calibration curves for Pb in moss 10 and 11 using the standard addition method are shown in Fig. 8.

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To explain the discrepancy between LA–ICP–MS and MD–ICP–MS results of Pb measurements, it is known that the loss of elements is a common drawback of microwave digestion. Fractionation during the laser ablation process may result in a preferential increase or decrease in the sensitivity of certain elements. In general, LA–ICP–MS is a semi-quantitative analysis tool. The errors from both methods may be responsible for the discrepancy in the results for Pb. 4. Conclusions The use of NIST leaf standards for elemental analysis of Spanish samples using LA–ICP–MS proved to be feasible for most of the elements studied. The results are compared with those obtained from MD–ICP–MS/AES. Good agreement is obtained for Mn, Zn Na and Ca. The poor agreement for Fe was caused by the interference of ArOH+ in LA–ICP–MS measurements. The standard addition method is demonstrated to be reliable for elemental analysis when matrix-matched standards are not available. Acknowledgements This work was supported by a Texaco Fellowship. We thank Mr. Kent Perkins of the University of Florida Herbarium for helpful advice regarding Spanish mosses. References

Fig. 8. Calibration curves of Pb in standard addition analysis: a) moss # 10; b) moss # 11.

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