Use of multiple irradiations and Reference Materials as comparators in quality control of neutron activation analysis data of biological samples

doi:10.21203/rs.3.rs-1565819/v1

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Use of multiple irradiations and Reference Materials as comparators in quality control of neutron activation analysis data of biological samples

https://doi.org/10.21203/rs.3.rs-1565819/v1

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We propose that several different reactor irradiation times followed by assaying of activity for differential counting periods may be employed for quality control (QC) of neutron activation analysis (NAA) data of biological samples. It is also recommended that three to four Reference Materials (RMs) of similar matrix but from different agencies such as National Institute of Standards and Technology (NIST, USA), International Atomic Energy Agency (IAEA, Vienna), Institute of Nuclear Chemistry and Technology (INCT, Poland), National Institute of environmental Studies (NIES, Japan) including a synthetic multielemental primary standard should always be analyzed simultaneously along with the samples. Finally, the mean ± σ values so obtained may be considered as more reliable after statistical analysis. We report our data for 20 elements in two candidate RMs Corn flour (INCT-CF-3) and Soya Bean flour (INCT-SBF-4) from the INCT, Poland where Z-score values are in reasonable range of certified values.

Multiple irradiation periods

Standard Reference Materials

Synthetic primary standard

Corn Flour (CF-3)

Soya Bean Flour (SBF-4)

Z-score analysis

Quality control (QC) of analytical data comprise of a set of experimental and statistical procedures designed to test, systematically and continually, if the measurement process is in full control during all the steps of analysis and can produce reliable data with confidence. International Atomic Energy Agency (IAEA) have issued a TECDOC 1350 dealing with development of Reference Materials (RM) as quality control materials and for quality assurance (QA) in neutron activation analysis (NAA) [1]. Bode [2] summarized his experiences gained at the Delft University of Technology in quality management on the basis of which his laboratory for NAA at Interfaculty Reactor Institute was accredited for its quality systems for compliance with the accreditation criteria following ISO-25 guide. Emons et al [3] have outlined the role of Reference Materials (RMs) in analytical QA with special emphasis on trace element analysis of foodstuff and discussed major influences on the uncertainty of certified values. Byrne [4] suggested the need of different modes of NAA (Instrumental NAA and Radiochemical NAA) to provide independent data sets in the same laboratory thus allowing internal validation or crosschecking. However, it is an established fact that RNAA, though more cumbersome and time consuming, gives more reliable data for a single element only after suppression of interferences. Hence, in our view this approach does not seem to be practical and hence not being used so frequently. Further, Einax and Reichenbacher [5] have discussed some statistical solutions such as homogeneity of the variance, simplification of the linear regression function including validation of the mathematical model to meet QA challenge. Recently Dybczynski [6] demonstrated unique role of NAA for analytical QA and its contribution to the certification of candidate RMs during last few decades. It has been emphasized that NAA plays vital role in determining the accuracy of analytical results in the trace element analysis (TEA) of inorganic elements in a variety of matrices. The quality of analytical data also depends on the minimization of errors at each stage of NAA methodology where one should have full knowledge and control of contamination, for discovering ways to minimize the run-to-run variation in analytical practice, and for improving the sample-to-blank ratio.

Standard/Certified RMs are considered as the best tools for harmonizing the results and providing QC of any analytical methodology. RMs have distinct advantage of wide acceptance and form the basis for inter-comparison of measurement systems for testing data produced under diverse conditions and by various laboratories around the world using similar or different techniques [7]. It is also recommended that one may prepare his own synthetic multielemental standard, also called primary standard, in the absence of a suitable in-house comparator standard. Similarly, statistical analysis of data such as box plot, Z-score or U-score have also been considered as tools for checking reliability of an analytical measurement as these express divergence of the experimental results and indicate how far and in what direction results deviate from the mean of its distribution [7].

In the present work we report our data on a duplicate set of synthetic primary multielemental standards containing As, Co, Cr, Fe, Mn, Hg and Zn prepared in our laboratory. We have determined Na and K in three biological RMs; Citrus Leaves SRM 1572 (NIST, USA), Mixed Human Diet H-9 (IAEA, Vienna) and Bowen’s Kale (UK) by using different irradiation periods of 15 m, 2 h, 6 h, and 1 d and compared our data with the certified values. We also present our data for 20 elements (Al, Br, Ca, Cl, Co, Cr, Cs, Eu, Fe, K, Mg, Mn, Na, P, Rb, Sb, Sc, Sn, Th, and Zn) for two candidate RMs Corn Flour (CF-3) and Soya Bean Flour (SBF-4) developed at INCT, Poland.

Sample collection and preparation

During past few decades we have analyzed a large number of Standard/Certified Reference Materials (S/CRMs) of biological, environmental and geological matrices acquired from different agencies. Some of the biological S/CRMs of plant origin but derived from the National Institute of Standards and Technology (USA), International Atomic Energy Agency (Vienna), Institute of Nuclear Chemistry and Technology (Poland) and National Institute of Environmental Studies (NIES), Japan are listed in Table 1. Bowen’s Kale, the first biological standard developed by Prof. H J M Bowen, University of Reading (UK) was sent to us as a gift. Some RMs were gifted by Prof R S Dybczynski INCT, Poland. All the samples were kept in oven at 100 ^oC for 2 h before weighing. For short irradiation of up to few hours, ~50 mg sample was weighed and packed in Alkathene. For longer duration of irradiation of ≥1d, however, ~100 mg sample was packed in aluminum foil. For 1-2 m irradiation in pneumatic carrier facility (PCF) of Dhruva, high density polypropylene (HDPP) rabbits were provided by the reactor division of the Bhabha Atomic Research center (BARC), Mumbai. Synthetic primary multielemental standards were prepared in a clean glove box by depositing aqueous acidic solution (prepared in dil HCl) of 1-15 µg amounts of AnalaR/GR/HP grade salts (assay 99.9% or better) on a Whatman filter paper No 42 circular disc (diameter 3-5 mm) using E Merck 5L79657 transferpette. It was handled with plastic tweezers, dried under Infrafil lamp and packed in aluminum foil. Composition of a typical synthetic standard is given in Table 2.

Irradiation and counting

The samples along with comparator RMs were irradiated at high Cd ratio position (high thermal to fast neutron ratio) in APSARA/CIRUS/Dhruva reactors at the BARC, Mumbai. Since all the samples along with RMs were irradiated in a small packet (3 cm long x 1.5 cm diameter), vertical or horizontal flux variations were likely to be minimal or insignificant. Care was taken so that in each packet at least 3 RMs and one primary synthetic multielemental standard were taken as comparator standards. For 1 or 2 m irradiations, pneumatic carrier facility (PCF) of Dhruva was used allowing short transfer time (~ 6 s) and convenient access to high flux irradiation position enabling minimal loss of activity thereby ensuring better reproducibility. Besides PCF of Dhruva, 5 m irradiation in APSARA was carried out followed by counting at the reactor site and later at the Radiochemistry Division of BARC. For long lived radionuclides, 1-3 d irradiations were carried out in CIRUS/APSARA reactor followed by counting at our laboratories at Nagpur University /IIT Roorkee. A schedule of irradiation, delay and counting times including radionuclides identified in each case is listed in Table 3. Overall samples were counted for up to delay after 2 months. Counting of ³²P (β^-) activity using GM counter and an Al absorber of 27 mg cm^-2 was carried out as per procedure developed in our laboratory [8].

Soon after irradiation and suitable cooling, the container is cut open and samples unpacked. Surface of the Alkathene/aluminum packed samples was decontaminated by swiping with acetone-soaked cotton/tissue paper. These were then mounted on a Perspex sheet with suitable geometry. After initial counting, samples were also unwrapped and transferred (recovery 80-95%) in butter paper packets (1 cm x 1 cm) in a glove box to avoid contamination of radionuclide impurities in Al-foil. All the samples were then counted using high resolution gamma spectrometry in our laboratories at Nagpur University/IIT Roorkee as described in our earlier publications [9, 10].

We have analyzed a large number of SRMs of different matrices derived from several agencies such as NIST (USA), IAEA (Vienna), INCT (Poland) and NIES (Japan) including synthetic multielemental standard along with other samples of medicinal herbs [8], herbal formulations [10] dust particulates from major Indian metropolitan cities [11]. In this study we present our results on the analysis of two synthetic multielemental standards along with our results for Na and K in 3 RMs of biological origin by four different irradiation periods. Further, we present our data on 20 elements for two proposed candidate RMs CF-3 and SBF-4 obtained from INCT, Poland as part of our inter comparison study.

Synthetic multielemental standard

In order to check the accuracy and precision in our NAA measurements, synthetic multielemental primary standards for selected elements were always irradiated and their elemental concentrations were calculated using results RMs from NIST (USA) and IAEA (Vienna) as comparator standards or using a duplicate standard. We had prepared two such standards I and II as mentioned above and results for the same are listed in Table 2. Also included in this table are errors calculated for both the standards. It is observed that most values are comparable within < ±10% of the actual amounts taken except for Hg in Standard-I and As in Standard-II where observed concentrations are higher by +12.7% and +14.0% respectively. This is a reasonably good agreement in our view though further care was taken in cases of Hg and As. Hence we can assume that our preparation methodology of primary standards is reasonably reliable. Based on these observations, it is presumed that our further measurements should also be in good agreement and reliable.

Different irradiation periods

In order to cross check on our INAA procedures and measurements we had selected two elements, Na and K that are of nutritional importance in dietary samples and these yield short-lived radionuclides ²⁴Na (t_½ = 15 h) and ⁴²K (t_½ = 12.4 h) and have sufficiently large cross section of 0.5 b and 1.3 b respectively. Three RMs of biological importance, Citrus leaves -SRM 1572 [12], Mixed Human Diet, H -9 [13] and Bowen’s Kale [14, 15] derived from different agencies of different countries NIST (USA), IAEA (Vienna) and UK respectively were selected. These were irradiated for different irradiation periods of 15 m, 2 h, 6 h and 1 d in Dhruva and CIRUS reactors. Activity of ²⁴Na and ⁴²K in all three SRMs along with comparator RMs were measured at the Radiochemistry Division of BARC, Mumbai. Elemental concentrations so calculated are listed in Table 4. A comparison of our data on average of all four concentrations ± σ obtained from four irradiation measurements with those of certified values shows very good agreement. In fact, the errors listed in last column are in the range of +0.2 to -2.7% only. Therefore, we propose that this approach could be followed for other elements as well so as to get more reliable and accurate analytical data of elements in a sample.

Intercomparison studies of CF-3 and SBF-4

Based on the quality of our INAA data, we were invited to participate in intercomparison studies of two candidate reference materials (RMs) Corn Flour (INCT-CF-3) and Soya Bean Flour (INCT-SBF-4) developed by Prof R S Dybczynski at INCT, Poland. Moisture contents of the two RMs were reported to be 8.77 ± 0.22% and 5.44 ± 0.07% respectively [16]. CF-3 was prepared from corn grown in Poland according to Polish standard PN-A-74205. It was sieved through 250 µm nylon sieves. SBF-4 was prepared from soya bean grown in India and sieved through 150 µm nylon sieves. Approximately 50 kg each of both flour samples were stored in polyethylene bags and examined by optical spectroscopy whereby Martin’s diameter was found to be 25 µm and 50 µm respectively [16]. Further, both the flour samples were homogenized by rotating in three directions and sterilized by electron beam irradiation for longer shelf life. In this case four SRMs of Rice Flour-SRM 1568a [17], and Wheat Flour-SRM 1567 [18] from NIST (USA), Whey Powder (IAEA-155) [19] from IAEA (Vienna) and Rice Flour no 10a from NIES. Japan [20] along with a primary synthetic multielemental standard (containing As, Cr, Fe, Co, Zn, Hg and Se) were used as comparator standards. Elemental concentrations were considered only if these were satisfactory (< ±10% of certified value) and finally mean ± σ were calculated for 20 elements on dry weight basis and these are listed in Table 5. In the meantime two candidate RMs have become Certified and their Certified/Information values [16] are also listed in Table 5. Two RMs were certified on the basis of worldwide inter laboratory comparison study where 92 laboratories from 19 countries had participated. Our elemental data for Eu and Sn may be considered for information only.

Statistical analyses

As a part of statistical analysis, Z-score values [7] were calculated for those elements (14 for SBF-4 and 10 for CF-3) where certified values were available as listed in Table 5. A plot of Z-score values in both the candidate RMs is shown in Fig 1. No Z scores were calculated for information values. It is observed that for SBF-4, Z-scores of 8 elements out of 14 are within ± 2.0 but for CF-3 only four out of 10 are within ± 2.0. However, Z scores of 4 out of 14 elements in case of SBF-4 and 4 out of 10 in case of CF-3 are within the range of ± 5.0 Therefore, we can say that 12 out of 14 elements in SBF-4 and 8 out of 10 in CF-3 are within Z score values of ± 5.0. Overall the Fig 1 shows that concentrations for most elements besides Fe, Mg, and Mn for SBF-4 and Br, Mn, P, Rb and Zn lie between ±3.0 suggesting that our data to be within 95% confidence limit. Further, our data in Table 5 also show that %RSD for all the elements in both the candidate RMs is <10% to indicate high precision of our measurements. However, concentrations of Cr, Na and Sc in SBF-4 are higher ~25%. This could be perhaps because these are reported as information values. For CF-3, our data for Cl, Cs, K, Sb and Sc are within +15%. In general, our data are on the higher side of the reported certified values as it is clear from Z-score plots. A cursory look of Fig 1 shows that most of our data suggest positive bias and only a few points are on the negative side. Some elements such as Eu, Sb and Sn have no certified or even informative value, and these are reported for comparison only.

During last three decades we have extensively employed INAA for the determination of minor, trace and toxic elements in a large number of biological samples of plant origin such as Ayurvedic Indian medicinal herbs [9] and herbal formulations [10], Indian vegetarian dietary constituents of cereals, vegetables and spices [21], Indian chewing tobacco products [22], Arjuna bark an Indian herbal heart tonic [23], and turmeric powder-an Indian spice for forensic studies [24] where multiple RM comparators have been used for data validation and found it quite useful. Our method is simple and less cumbersome that can be used very easily.

INAA has been recognised as a suitable analytical technique for data validation of minor and trace elements in RMs of biological origin. However, it is essential to control various sources of errors in each step of INAA including sampling, packing, irradiation and counting of activity before we calculate elemental concentrations. In order to get quality data, it is proposed that more than two RMs of similar matrix but derived from different agencies along with a primary synthetic multielemental standard be analysed simultaneously along with the sample. Also, some elements such as Na, K, P etc should be irradiated for different time periods and then mean ± σ be calculated. Our data for INCT-CF-3 and INCT-SBF-4, candidate RMs from INCT, Poland were found in good agreement with the certified/informative values. Also, Z-score plots may be considered as means of statistical analysis of NAA data. Concept of multiple RMs may be used in other analytical methodologies using comparator method.

Author’s contribution: Prof A N Garg- Conceptualisation, planning, project management, supervision, editing and reviewing.

Dr Vivek Singh- Sample collection, preparation for irradiation, experimental work at BARC, Mumbai, data compilation, statistical analysis and draft writing.

Dr R P Choudhury- Sample preparation for irradiation, experimental work at BARC, Mumbai, data compilation, statistical analysis and draft writing.

Acknowledgements: Grateful thanks are due to BRNS (DAE, Government of India) for grants-in- aid at RTM. Nagpur University, Nagpur (37/13/91-G) and Indian Institute of Technology, Roorkee (2000/37/5/BRNS). RPC is thankful to MHRD for grant of research fellowship. Our sincere thanks to Drs SB Manohar, AVR Reddy and R Acharya for providing irradiation and counting facilities at the Radiochemistry Division, BARC, Mumbai.

Conflict of Interest: Authors declare no conflict of interest

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Table 1. Standard Reference Materials of botanical origin analysed in this study

Name of the Standard	Identification No.	Source
Apple Leaves	SRM-1515	NIST USA
Cabbage	IAEA-359	IAEA Vienna
Oriented Tobacco Leaves	CTA-OTL-1	INCT Poland
Peach Leaves	SRM-1547	NIST USA
Citrus Leaves	SRM-1572	NIST USA
Pine Needles	SRM-1575	NIST USA
Virginia Tobacco Leaves	CTA-VTL-2	INCT Poland
Mixed Polish Herbs	MPH-2	INCT Poland
Wheat flour	SRM-1567	NIST USA
Rice flour	SRM-1568a	NIST USA
Whey Powder	IAEA-155	IAEA Vienna
Rice flour	No 10a	NIES Japan
Mixed Human Diet	H-9	IAEA, Vienna
Kale	Bowen’s Kale	Prof. H.J.M. Bowen, UK
Tea Leaves	No 3	NIES, Japan

Table 2. Data validation using multielemental synthetic primary standards prepared in our laboratory

Element	Standard-I Concentration (µg/mL)	Error (%)	Standard-II Concentration (µg/mL)	Error (%)
As	0.68±0.05 (0.7)	-2.9	0.57±0.01 (0.5)	+14.0
Co	7.98±0.6 (7.5)	+6.4	5.51±0.04 (5)	+10.2
Cr	0.93±0.02 (1)	-7.0	1.53±0.05 (1.5)	+2.0
Fe	14.7±0.03 (15)	-2.0	11.7±0.02 (12)	-2.5
Hg	1.69±0.02 (1.5)	+12.7	1.87±0.02 (2.0)	-6.5
Mn	5.23±0.06 (5.0)	+4.6	7.67±0.11 (8.0)	-4.1
Se	0.38±0.01 (0.4)	-5.0	0.23±0.02 (0.25)	-8.0
Zn	2.37±0.04 (2.5)	-5.2	1.85±0.04 (2)	-7.5

Values in parentheses are the amounts taken using E Merck 5L79657 transferpette

Table 3. Irradiation, delay and counting schedule used in three reactors in India

Reactor

(Irradiation time)

Delay

Counting

Radionuclide identified

Dhruva/CIRUS

(1 m/ 2 m)

3 m

10 m

20 m

2 h

1 d

20 d

60 s

100 s

300 s

100 s

4000 s

2000 s

²⁸Al, ⁴⁹Ca, ²⁷Mg, ⁵²V

³⁸Cl, ⁵⁶Mn

⁵⁶Mn, ²⁴Na, ⁴²K, ¹⁹⁸Au

²⁴Na, ⁴²K, ⁸²Br, ⁷⁶As, ¹⁴⁰La

³²P (β^-)

APSARA

(5 m)

5 m

50 s

100 s

300 s

²⁸Al, ⁴⁹Ca, ²⁷Mg

³⁸Cl, ⁵⁶Mn, ²⁴Na, ⁴²K

Dhruva/CIRUS

(1 d/ 3 d)

10 d

12 d

20 d

25 d

40 d

1 h

2 h

6 h

1 h

12 h

¹⁴⁰La, ²³²Th, ²³³Pa, ⁸⁶Rb, ⁸²Br

⁷⁵Se,¹²⁴Sb,⁵¹Cr, ¹³¹Ba,¹⁴¹Ce

²⁰³Hg, ¹⁸¹Hf, ⁵⁹Fe, ⁶⁰Co, ⁴⁸Sc, ¹⁵²Eu

³²P (β^-)

⁶⁵Zn, ¹³⁴Cs, ⁶⁰Co, ¹⁵²Eu

Table 4. Concentrations of Na and K in standards (in µg/g) as obtained from different irradiation periods of 15 m, 2 h, 6 h and 1 d.

Standard

Certified

Value

Duration of irradiation

Average ± σ

Error (%)

Sodium (Na)

15m

2 h

6 h

1 d

Bowen’s Kale

2394±617

2316±170

2352±163

2374±177

2305±155

2337±28

-2.3

Citrus Leaves

SRM1572

160±20

163±14

162±18

156±9

166±18

162±4

1.3

Mixed Human Diet (H9)

8100±630

8170±233

8186±248

7965±168

8150±197

8118±89

+0.2

Potassium (K)

Bowen’s Kale

24630±1218

23837±915

24419±1673

24090±970

23460±1370

23950±350

-2.7

Citrus Leaves

SRM1572

18200±600

18362±1763

18162±850

18242±1317

18500±1700

18320±130

0.7

Mixed Human Diet (H9)

8300±870

8318±167

8418±193

8103±279

7832±144

8166±226

-1.6

Table 5. Comparison of our data with the certified/informative values for proposed References Materials (SBF-4 and CF-3) by INCT, Poland.

Element	INCT-SBF-4				INCT-CF-3
Element	This work	RSD (%)	Error %	Z-score (p)	This work	RSD (%)	Error %	Z-score (p)
Al (mg/kg)	47.7±1.7 (45.5±3.7)	3.56	+4.84	+0.60 (0.73)	17.1±1.0 [12]	5.85	+42.5	-
Br (mg/kg)	2.38±0.21 (2.40±0.17)	8.82	-0.83	-0.12 (0.55)	0.75±0.06 (0.39±0.05)	8.0	+92.3	+7.2 (1)
Ca(g/kg)	2.71±0.21 (2.47±0.17)	7.7	+9.72	+1.41 (0.92)	ND (40)	-	-	-
Cl (mg/kg)	73±6 (64.5±4.7)	8.21	+13.2	+1.81 (0.96)	368±15 (397±33)	4.08	-7.30	-0.88 (0.81)
Co(µg/kg)	90.1±2.1 (95.6±5.8)	2.33	-5.75	-0.95 (0.83)	13±1 [16]	7.70	-18.8	-
Cr(mg/kg)	0.29±0.02 [0.23]	6.90	+23.1	-	0.27±0.02 [0.14]	7.41	+92.9	-
Cs(µg/kg)	137±5 (129.1±4.3)	3.65	+6.11	+1.84 (0.97)	3.58±0.12 [4]	3.35	-10.5	-
Eu(µg/kg)	1.72±0.10 -	5.81	-	-	2.51±0.04 -	1.59	-	-
Fe(mg/kg)	112±3 (90.8±4.0)	2.68	+23.3	+5.3 (1.0)	33.0±1.4 (32.0±1.4)	4.24	+3.13	+0.71 (0.76)
K(g/kg)	26.6±2.2 (24.2±0.83)	8.27	+9.91	+2.41 (0.99)	3.50±0.30 (3.16±0.12)	8.57	+10.8	+2.83 (0.99)
Mg(g/kg)	3.34±0.17 (3.00±0.08)	5.09	+11.3	+4.25 (1)	1.18±0.06 (1.07±0.04)	5.08	+10.3	+2.75 (0.99)
Mn(mg/kg)	43±3 (32.3±1.1)	6.98	+30.0	+9.73 (1)	5.83±0.13 (4.98±0.22)	2.23	+17.1	+3.86 (1)
Na(mg/kg)	6.73±0.32 [5.5]	4.75	+22.3	-	6.9±0.6 [4.4]	8.70	+56.8	-
P(g/kg)	6.11±0.29 (6.56±0.35)	4.75	-6.86	-1.29 (0.90)	3.54±0.23 (2.83±0.10)	6.50	+25.1	+7.1 (1)
Rb(mg/kg)	26.1±1.4 (31.7±1.7)	5.36	-17.7	3.29 (1)	1.08±0.09 (0.91±0.04)	8.33	+18.9	+4.25 (1)
Sb(µg/kg)	11.7±0.3 -	2.56	-	-	9.7±0.4 [11]	4.12	-11.8	-
Sc(µg/kg)	8.74±0.53 [7]	6.06	+24.9	-	1.87±0.07 (2.13±0.32)	3.74	-10.8	-0.81 (0.79)
Sn(µg/kg)	2.19±0.15 -	6.85	-	-	3.41±0.15 -	4.40	-	-
Th(µg/kg)	7.9±0.4 (7.08±0.82)	5.06	+11.6	+1.0 (0.84)	7.71±0.24 -	3.11	-	-
Zn(mg/kg)	45.1±4.1 (52.3±1.3)	9.09	-13.8	-5.54 (1.0)	25.0±1.7 (20.1±0.8)	6.80	+25.4	+6.13 (1)

*Values in parenthesis ( ) are certified and those in [ ] are information values as in ref. (Polkwska-Morenko et al, 2007)

No competing interests reported.

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Editorial decision: Major revision
17 May, 2022
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17 Apr, 2022

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Use of multiple irradiations and Reference Materials as comparators in quality control of neutron activation analysis data of biological samples

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