Approaching Standardless Quantitative Elemental Analysis of Solids

Oct 22, 2018 - Among various ionization sources for mass spectrometry, microsecond pulsed glow discharge (MP-GD) and buffer-gas-assisted laser ...
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Technical Note

Approaching Standardless Quantitative Elemental Analysis of Solids: Microsecond Pulsed Glow Discharge and Buffer-GasAssisted Laser Ionization Time-of-Flight Mass Spectrometry Le Hang, Zhouyi Xu, Zhibin Yin, and Wei Hang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b03398 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 23, 2018

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Analytical Chemistry

Technical Note

Approaching Standardless Quantitative Elemental Analysis of Solids: Microsecond Pulsed Glow Discharge and Buffer-Gas-Assisted Laser Ionization Time-of-Flight Mass Spectrometry Le Hanga, Zhouyi Xua, Zhibin Yina, Wei Hanga,b* a

Department of Chemistry, MOE Key Lab of Spectrochemical Analysis & Instrumentation, College of Chemistry and Chemical

Engineering, Xiamen University, Xiamen, 361005, China b

State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361005, China

ABSTRACT: Among various ionization sources for mass spectrometry, microsecond pulsed glow discharge (MP-GD) and buffer-gasassisted laser ionization (BGA-LI) sources have the potential for direct quantitative elemental analysis of solids without the requirement of standard reference materials. The analytical potential of these two ionization sources has been evaluated by coupling them to an orthogonal time-of-flight mass spectrometer (O-TOFMS). A straightforward method was proposed to achieve the quantitative result: if a spectrum contains little interference and elemental peak currents are proportional to their concentrations, then the molar concentration of each element is equal to its ion current proportion in the total ion current. Two series of metal standards were applied for the evaluation. Explicit spectra with little interference can be acquired by both techniques. The interferences contribute to only a very small portion to the total ion current for MP-GD-MS and BGA-LI-MS; therefore, their influence on the quantitative result can be ignored. All metal elements can be determined quite accurately by the proposed quantitation method, while gaps exist for nonmetal elements due to the high ionization potentials or gas species interference. Between the two techniques, BGA-LI-MS offers a more accurate quantitative result, primarily due to its higher plasma temperature.

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Direct solid analysis is of utmost importance in the field of modern metallurgical and manufacturing industries.1-3 To date, a variety of characterization techniques have been developed to acquire the chemical information of solid materials4-6. Among them, mass spectrometry is well known for its accuracy and sensitivity.7-9 The most common mass spectrometric methods include secondary ion mass spectrometry (SIMS),10,11 spark source mass spectrometry (SSMS),12,13 glow discharge mass spectrometry (GDMS),14,15 laser ionization mass spectrometry (LIMS)16,17 and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS).18,19 Compared with conventional solution-based analytical methods, the direct solid analysis MS techniques offer crucial advantages, requiring little or no sample pretreatment, minimal sample consumption, and fast analysis speed. Typically, spectra from MS techniques for direct solid analysis suffer from spectral and gas species interferences. Furthermore, standard reference materials are required for quantitative analysis, which can be problematic when searching for suitable solid standards that must be matrix matched and must contain all necessary analyte elements with proper concentrations,20,21 especially for unknown samples. If a spectrum contains little interference and if elemental peak currents are proportional to their concentrations, then the molar concentration of each element can be simply acquired by its ion current divided by the total ion current, giving a straightforward and standardless quantitative method. It is well known that spectra from regular GDMS suffers from gas species interferences.22,23 However, when the glow discharge is microsecond pulsed, interferences can be greatly reduced technically because most of the gas species ions are generated by electron impact ionization during the pulse-on regime, while the majority of the analyte ions are generated in the after-glow regime by Penning ionization.23,24 The ion source in traditional LIMS is operated under high vacuum, in which multiply charged ions could create severe spectral interferences. With the introduction of a buffer-gas-assisted source, multiply charged ion interferences can be reduced 2-3 orders of magnitude via 3-body recombination.21 LIMS is also capable of analyzing non-conductors and can offer high higher lateral resolution compared with regular GDMS.16,25,26 Using MP-GD and buffer-gas-assisted laser ionization (BGA-LI) sources, the interference current has been kept at a minimum,21 which could be ignored compared with the total ion current. Furthermore, both techniques have quite uniform sensitivity for most elements,24,27 therefore, the standardless quantitative analysis can be approached. Many studies have been carried out to investigate MP-GD-MS or BGA-LI-MS in various aspects. In this paper, we only focus on the standardless quantitative capability of these two techniques. Because both of the sources are operated in the low vacuum regime, it is possible to couple them to the same multiple vacuum stage MS. Both ion sources have a short-pulsed nature, such that a time-of-flight mass spectrometer (TOFMS) would be their ideal mass analyzer. It has to be mentioned that TOF mass analyzer has slightly poorer quantification capability than those static mass analyzer; and intense matrix peaks may affect the precise readout of neighborhood peak intensities.28 In this study, MP-GD and BGA-LI sources were coupled to the same orthogonal TOFMS. Two series of standard reference materials were used to evaluate the two sources for their capabilities of standardless quantitative analysis. The operating conditions of both sources and the TOFMS were optimized in terms of sensitivity and interference, respectively. Differences in spectral interferences and their influences are discussed, as well as the potential for standardless quantitative analysis capabilities of the two sources were demonstrated.

EXPERIMENTAL SECTION Samples. Standard reference materials with two different matrices were applied for this study, including four NIST iron-based standards (SRM 1762a, 1763a, 1764a, and 1766) and four NIST copper-based alloy standards (SRM 1112, 1114, 1116, and 1117). These samples were cut into disks of 6 mm in diameter and 1 mm in thickness. Instrument and Analytical Procedures. The study was carried out in an in-house-built O-TOFMS system (Figure 1). The ions originating from the source were extracted from the plasma through a skimmer into the transmission stage. A set of electrostatic lenses were mounted in the transmission stage to focus and guide the ions through a slit (1×4 mm) into the TOF mass analyzer stage. 2

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Analytical Chemistry

A Faraday cup was mounted behind the repelling region. When an ion package entered the repelling region, an HV pulse train was applied to the repelling plate to push ions into the mass analyzer. The signal from the microchannel plates was recorded using a digital storage oscilloscope (44Xs, LeCroy).

Figure 1. Schematic diagram of (A) TOFMS system, (B) MP-GD and (C) BGA-LI ionization source

The MP-GD source (Figure 1B) consists of an HV pulse power supply (SY5017, Chengdu Senyuan technology Co. LTD), a copper sample holder, a cylindrical ceramic shield, a 3-dimensional transition stage, and a source chamber. The sample was stuck to the sample holder with silver glue (Silver Conductive Paint, Structure Probe, Inc.). Ultrahigh purity Argon (Ar, 99.999%, Linde Gas Southeast Xiamen Ltd.) was employed as the buffer gas in the source chamber, which was controlled by a gas flow meter. The distance between the sample and skimmer was 6 mm. After a series of tests, the GD parameters were optimized (Table 1) to sustain a stable and high instantaneous power MP-GD. The BGA-LI source (Figure 1C) consists of a pulsed Nd:YAG laser (Penny-100, ZY laser technology Co.), a sample holder mounted on the 3-dimensional transition stage, and a source chamber. Ultrahigh purity helium (He, 99.999%, Linde Gas Southeast Xiamen Ltd.) was applied as the buffer gas. Sample disks were adhered onto the sample holder with carbon tape. A laser power of 4.5 mJ was chosen for the experiment, according to previous work.29 Table 1 summarizes the detailed optimized conditions of the sources, transmission system, and TOFMS system. From the parameters of the transmission and TOFMS system, it can be speculated that the ion behaviors of the two low-pressure sources are quite similar. The biggest difference is the delay between the source pulse and repelling pulse train. The other difference is the number of pulses in the pulse train. The purpose of the repelling pulse train is to push only the analyte ions into the TOF mass analyzer. For MP-GD, it is known that gas species interference is generated in the pulse-on regime, while analytical ions are generated in the after-glow regime by Penning ionization.30,31 Therefore, the repelling pulse train was delayed by 250 μs to avoid pushing the gas species ions into the TOF. With its ion temporal profile of ~1300 μs in the repelling region (Figure 2A), 30 pulses with a 35 μs interval in the pulse train was enough to sample the stretched MP-GD ion package into the TOF mass analyzer. While for one laser shot, the ion temporal profile in the repelling region was approximately 650 μs (Figure 2B). Therefore, a pulse train of 700 μs in length (20 pulses with 35 μs 3

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interval) was enough to sample the stretched ion package in the repelling region. Five parallel tests were performed for each sample in each ion source.

Figure 2. Typical ion temporal profile in the repelling region for the (A) MP-GD and (B) BGA-LI source using SRM 1762a sample.

Table 1. Operating parameters of BGA-LI and MP-GD sources and the O-TOFMS system Source BGA-LI

MP-GD

Ambient gas

Helium

Ambient gas

Argon

Chamber pressure

600 Pa

Chamber pressure

600 Pa

Energy

4.5 mJ

Pulse voltage

-3000 V

Duration

5 ns

Pulse width

10 μs

Laser frequency

10 Hz

Pulse frequency

10 Hz

Wavelength

532 nm

Pulse current

100 mA

Crater diameter

50 μm

Pulse energy

3 mJ

Laser irradiation

10

2

4.6 × 10 W/cm

Transmission System Skimmer

60 V

Skimmer

60 V

L1

-165 V

L1

-250 V

L2

-150 V

L2

-120 V

L3

0V

L3

0V

L4

-122 V

L4

-123 V

Slit

-54 V

Slit

-48 V

Repelling pulse magnitude

-460 V

Repelling pulse magnitude

-460 V

Delay of the pulse train

0 μs

Delay of the pulse train

250 μs

Number of pulses in the pulse train

20

Number of pulses in the pulse train

30

Pulse interval in the pulse train

35 μs

Pulse interval in the pulse train

35 μs

Acceleration potential

-2000 V

Acceleration potential

-2000 V

Steering plate potential

-2585 V

Steering plate potential

-2590 V

O-TOFMS

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Analytical Chemistry

RESULTS AND DISCUSSION Gas species and spectral Interferences. Figure 3 shows the mass spectra of SRM 1762a with the MP-GD and BGA-LI source (with red lines showing full intensities). It is obvious that the spectrum with BGA-LI shows less interference than the MP-GD source.

Figure 3. The mass spectra of SRM 1762a obtained with the (A) MP-GD and (B) BGA-LI source, with accumulation of 1000 pulsed events in the sources. Red lines show full intensity spectra. “*” indicates interference peaks or elemental peaks with great interferences.

It is well known that the buffer gas (Ar and gas species) in the MP-GD source are ionized, primarily by collisions with electrons during the pulse-on regime, which are thermalized very quickly after the termination of the GD pulse. Sputtered species are mainly ionized by Penning collisions with metastable Ar. The production of metastable Ar atoms at the after-glow is due to the recombination of Ar+ with electrons30,32 and their long lifetime yields an important production of analyte ions in the after-glow. The differential behavior of the gas ions and sputtered species ions in time allows the use of an appropriate delay between the pulsed GD and the repelling pulse train to eliminate most of the interferences from gas species. However, the interference cannot be completely reduced, owing to the inevitable spatial and energy distribution of ions. The most intense interferences in the MP-GD spectrum are caused by the Ar buffer gas, such as Ar+, ArH+, Ar2+, and Ar2+. Impurities in the Ar gas and residue gas in the source chamber introduce gas species interferences, while matrix oxides can be easily observed. However, with the sampling strategy of the OTOFMS, these interferences contribute to only 4.9% of the total ion current. For the BGA-LI source, the number of ions generated is strongly dependent on the laser intensity and ambient conditions. After absorption of a high-irradiance laser, the sample is evaporated and ionized, creating a dense plasma plume above the sample surface. 5

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Because the volume of the laser plasma is much smaller than that of the glow discharge,30,33 contaminants in the buffer gas and residue gas in the source interfere with the spectrum to a smaller extent. Collisional dissociation is also effective in reducing the number of polyatomic ions with buffer gas in the source chamber. Multiply charged ions are the major spectral interference in the laser plasma at high vacuum condition. Through the charge reduction process of 3-body recombination21 in the BGA-LI source, the amount of multiply charged ions is greatly reduced, but peaks of doubly charged matrix ions are still observable (Figure 3B). Overall, these interferences contribute to only ~0.7% of the total ion current. Standardless quantitative analysis of samples with Fe matrix. As illustrated in the introduction section, if a spectrum contains little interference and elemental peak currents are proportional to their concentrations, then the molar concentration of each element can be simply acquired by its ion current divided by the total ion current. Therefore, the following expression enables quantitative determination of the mass fraction of elements: 𝑤𝑖 =

𝐼𝑖𝑗𝑀𝑖𝑗/𝑅𝑖𝑗 ∑ 𝐼𝑘𝑀𝑘 𝑘

(1)

where 𝑤𝑖 represents the mass fraction (element concentration) of element 𝑖, 𝐼𝑖𝑗 represents the peak intensity of isotope 𝑗 of element 𝑖, 𝑀𝑖𝑗 represents the molar mass of isotope of element 𝑖, 𝑅𝑖𝑗 represents the isotope ratio of isotope 𝑗, and ∑𝑘𝐼𝑘𝑀𝑘 represents the sum of all peak intensities multiplied by their corresponding mass weight, including all interference peaks. Basically, an isotope with a relatively high isotope ratio and little interference will be chosen for calculating the specific element concentration. Therefore, all samples in this study were treated as unknown samples. None of the certified values of reference materials involved in Equ. 1 for the calculation of concentrations of different elements. Table 2 shows the measured concentrations of all elements in the Fe matrix samples. While data in the table are trivial to read, a figure was plotted to show the certified concentrations vs. measured concentrations (Figure 4). In this figure, solid or half-solid symbols represent metal elements, and empty symbols represent nonmetal elements. The measured concentrations of the metal elements are close to their certified concentrations; and the values from BGA-LI are closer to the certified values than those from MPGD. Most of the values for the nonmetal elements deviated from their certified values due to the gas species interference (for C, N, and Si) or the low ion yields caused by the high ionization potentials (for P, S, and As). Overall, except for the nonmetal gas species that severely interfered (N and S), deviations for most of the elements are within one-order of magnitude for both sources. The measured concentrations for metal elements from BGA-LI are very close to their certified values, approaching standardless quantitative analysis.

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Analytical Chemistry

Table 2. Results of averaged measured concentrations for elements in Fe matrix samples with MP-GD and BGA-LI sources Certified conc. (μg/g)

Measured conc. for MP-GD* (μg/g)

Measured conc. for BGA-LI* (μg/g)

1762a

1763a

1764a

1766

1762a

1763a

1764a

1766

1762a

1763a

1764a

1766

B

49

54

10

1

8

9

2

0

31

34

6

1

C(12)

3370

2030

5920

150

137

153

244

157

913

1606

1035

484

N(14)

-

44

23

33

55

40

110

126

5380

11642

8404

8770

Al

690

430

90

120

1142

757

465

561

608

497

287

292

Si(28)

3500

6300

570

100

1454

5917

2003

928

1422

2533

1046

641

P

330

120

200

20

213

119

304

136

123

58

71

8

S(34)

300

230

120

24

10810

6080

16513

4890

118

97

51

9

Ti(48)

950

3100

280

5

294

868

215

192

1004

3343

334

4

V(51)

2000

3000

1060

90

405

641

213

21

2848

3910

1503

89

Cr(52)

9200

5000

14800

240

13332

9012

27148

380

9190

5195

17589

200

Mn(55)

20000

15800

12100

670

20647

17298

14562

1002

26896

20339

16097

807

Fe(56)

940929

950192

954582

997997

882798

844599

877095

937518

931973

926813

933654

980767

Co(59)

620

950

100

20

541

780

162

31

551

978

97

18

Ni(60)

11500

5100

2020

210

9023

4866

2709

223

10573

5106

1913

165

Cu(63)

1200

430

5100

150

1068

513

6800

219

1133

388

5845

123

As(75)

180

550

100

35

37

72

27

12

27

134

16

6

Zr(90)

290

440

15

-

114

216

14

0

290

498

11

0

Nb(93)

700

1000

420

50

111

186

123

14

748

1341

477

42

Mo(98)

3500

5000

2000

35

3701

5570

1658

177

4351

7863

2502

47

Ag(109) -

-

-

5

37

47

50

89

0

0

0

4

Sn(120) 460

110

200

10

686

158

431

19

367

81

169

8

Sb(121) -

-

-

5

0

0

0

10

0

0

0

5

Ta(181) 210

120

290

-

38

21

72

0

102

87

123

0

Pb(208) -

-

-

30

0

0

0

21

0

0

0

38

Figure 4. Certified concentrations vs. measured concentrations for elements in Fe matrix samples.

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Standardless quantitative analysis of samples with Cu matrix. Standardless quantitative experiments were further carried out using a series of Cu-based NIST samples, with a typical spectrum shown in Figure 5. Similar interferences can be observed as those in the Febased sample (Figure 3). The MP-GD source generated more interference peaks (with “*” indicated in Figure 5) than the BGA-LI source. The interferences contribute to only 1.4% and 0.2% of the total ion current for MP-GD and BGA-LI, respectively. Therefore, their influence on the quantitative result (according to Equ. 1) can be ignored.

Figure 5. The mass spectra of SRM 1112 obtained with the (A) MP-GD and (B) BGA-LI source with accumulation of 1000 pulsed events in the sources. Red lines show full intensity spectra. “*” indicates interference peaks or elemental peaks with great interferences.

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Figure 6. Certified concentrations vs. measured concentrations for elements in samples of Cu matrix.

Since this series of samples contains fewer elements than the previous series, it is quite clear (Figure 6) that the measured concentrations for metal elements are close to their certified concentrations, and the values from BGA-LI are closer to the certified values than those from MP-GD. Since there is only one nonmetal element (P) in this series of samples, the measured concentrations of P are lower than the certified values. However, the values from BGA-LI are closer to the certified values than those from MP-GD. Since nonmetal elements have high ionization potentials, they are not fully ionized in MP-GD and BGA-LI sources. The laser irradiance in the experiment generated a plasma with a higher temperature (12000 K)34,35 than that of MP-GD (~2000 K),36 therefore, the ion yield in BGA-LI would be higher than that of the MP-GD source. Another phenomenon worthy of mention is that the background noise in the BGA-LI spectra is lower than that of MP-GD (Figure 3 and 5), which makes the limits of detection (LODs) for BGA-LI about one-order of magnitude lower than that of MP-GD (μg/g level for most elements).

CONCLUSION MP-GD and BGA-LI have many aspects that are worthy of investigation, which cannot be illustrated in one paper. In this study, we focus only on the standardless quantitative capability of these two techniques. A straightforward method was proposed to achieve the quantitative result: the molar concentration of each element is equal to its ion current divided by the total ion current. Explicit spectra with little interference can be acquired with the pulse train sampling technique. The results from the two series of metal standards indicate that the interferences contribute to only a very small portion of the total ion current for MP-GD-MS and BGA-LIMS; therefore, their influence on the quantitative result can be ignored. All metal elements can be determined quite accurately by the proposed quantitation method. For nonmetal elements, due to gas species interference or high ionization potentials, their measured values would deviate from their certified values. Between these two techniques, BGA-LI-MS offers a more accurate quantitative result. Considering the difficulty of finding solid standard reference materials, particularly for unknown samples, these two standardless analysis techniques can play an important role for rapid determination of elements in solids, especially when a semiquantitative result can be accepted.

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ACKNOWLEDGMENTS The authors gratefully thank the Natural Science Foundation of China (21427813) for financial support of this work.

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