Pittcon Conference Poster Presentation

Analysis of Food Grains using Automated Block Digestion
Michael A. Rutzke, Ph.D. <[email protected]> Cornell University, School of Integrative Plant Science
Suhas Narkhede <[email protected]>, Nicolas McLeod <[email protected]> Questron Technologies Corp.
Abstract
High-throughput trace metal analysis in food grains is key in regulating their respective
accepted levels. Sample preparation, in particular acid digestion, consumes the largest
amount of time during these analyses.1 Latest techniques such as microwave digestion have
significantly reduced digestion times but are limited to small sample sizes and are labor
intensive. Automated open vessel digestions are capable of handling large sample sizes and
dispensing accurate amounts of reagent while increasing analyst efficiency. Herein, we
describe the use of automated block digesters and automated dilution work-stations,
approximately 84 samples could be processed in 3.5 hours using minimal labour. Herein, we
describe the nitric/perchloric digestion procedure used in the analysis of various food grains,
as well as reporting the recoveries of elements; measured by ICP-AES. The system can
process 84 samples in 210 minutes with an average of 2.5 minutes per sample.
Introduction
Cereal grains represent an important food class consumed by all living organisms on a daily
basis. Important micronutrients within these grains such as Cu, Fe, Mn, and Zn are essential
for physiological processes. Ensuring trace level amounts of micronutrients obtained from
rice, wheat or maize is imperative as deficiencies in Fe, Zn and I are on the rise.2
Alternatively, the presence of toxic heavy metals present concerns, as short or long term
exposure can lead to adverse effects including nervous system and/or organ damage.3
The analysis of food grains and other agricultural products by automated open vessels
requires the dispensing of accurate amounts of caustic reagents (nitric and perchloric acids)
without contributing any additional contamination than what is present in the original
reagents. The analytical process needs to be fast, efficient, have minimal cost and not be
labor intensive. The Vulcan 84 automated digestion system can process about 320 samples
in one work day with minimal labor. The all-plastic design ensures contamination caused by
incorporating metallic parts is kept to a minimum. Herein, we describe a nitric/perchloric
acid digestion procedure used and report recovery’s of elements measured by ICP-AES.
of 0.021 mL was found for an average value of 7.99 mL with a maximum deviation of 0.037
mL from the theoretical value of 8 mL.
The results show that 80% of the dispensing is within 0.3 % which constitutes a class A
standard.
Section B - Food Grain Analysis
Reagents and Equipment
All digestions were performed using Vulcan84
automated digestion system provided by Questron
Technologies Corp. Canada. Acid reagents used for
digestion include Omnitrace nitric acid from EMD
Millipore Corporation an affiliate of Merck KGaA
Darmstadt Germany and 70% environmental grade
perchloric acid from GFS CHEMICALS, Columbus
Ohio. 18 MΩ water was used for all dilutions
performed. ICP analysis was performed using
Spectro Arcos axial viewed ICP-OES.
Vulcan 84 Automated Digestion System
Section A - Accuracy of Dispensing
In an effort to determine the accuracy of dispensing by automated digestion system; Vulcan,
a batch of 42 positions was processed. A tray loaded with preweighed 42, 50 mL vials was
placed in the system. A program was run to dispense 8 mL of water to each position.
Afterwards, each vial was re-weighed to calculate the water dispensed. A standard deviation
Table 1 - Accuracy Determination in Vulcan Dispensing.
Total Number of
Average Dispensing
Samples
Theoretical Dispensing Value
(mL)
42
8
7.990
Volume (mL)
Standard Deviation
0.021
The results for the two sets of NIST samples are shown below in Table 3. The values for the
micronutrients of Cu, Fe, Mn, and Zn all fall within the accepted certified values for each of
the processed samples indicating a complete digestion obtained by the developed method
using the automated digestion system,.
NIST Wheat Flour 1567b
Approximately 0.50 grams of grain sample was weighed a 50 ml Teflon container. 20 µg of
yttrium was pipetted into each container to be used as an internal standard. The containers
were placed into the Vulcan 84 automated digestion system.
5.0 ml of nitric acid and 2.5 ml of perchloric acid were simultaneously added to each vial.
The tubes were lowered into the block and heated to 110° C. The ramp rate was 40 minutes.
The temperature was held at 110° C for 60 minutes. The temperature was then raised to 180°
C over 20 minutes. The temperature was held at 180° C for 15 minutes.
The tubes were raised up and an additional 1.0 ml of nitric acid was added. The tubes were
lowered back into the blocks and heated for 20 minutes at 180° C.
The tubes were raised up and allowed to cool. 20.0 ml of 18 MΩ water was then added.
Contamination Control
In order to determine the degree of contamination throughout the digestion process, two
sets of blanks were prepared. 1) A reagent blank to determine the contamination attributed
by the acid reagents, 2) A method blank containing dispensed reagents and run through the
digestion process followed by workup in the Vulcan system.
For the reagent blank, 5.0 mL of nitric acid and 2.5 mL of perchloric acid were added
manually from the bottle to three 50 mL Teflon vials. The vials were allowed to sit for 2
minutes and was capped.
For the method blank, three positions were treated using the digestion method above
without any sample added to the vial.
Table 2- ICP analysis of reagent and method blank.
Al 396.152
As 189.042
B 249.773
Ba 455.4_2
Be 313.042
Ca 317.933
Cd 214.4_2
Co 228.616
Cr 205.618
Cr 267.716
Cu 224.700
Fe 259.941
K 766.491
Mg 279.079
Mn 257.611
Mo 202.095
Na 588.995
Ni 231.604
P 213.618
Pb 220.353
S 182.034
Sc 361.384
Se 196.090
Si 288.158
Zn 202.613
Na 589.592
Ca 393.366
1567b.
Table 3 - Trace metal analysis of NIST certified reference materials Wheat Flour 1567b and Rice 1568b
Digestion Method
Element/Wavelength
Questron Technologies
Processed Average Vulcan 84 Unprocessed Average Reagent
blank (ppm)
Blank (ppm)
0.001718
0.00428
-0.003185
-0.0045
0.004518
0.01187
-0.000021
0.0002
0.001892
0.002335
0.007973
0.02011
0.000080
0.00071
0.000073
0.00033
-0.000497
0.00054
0.000160
0.001305
-0.000661
0.000415
0.001686
0.003395
0.267539
0.240325
0.004951
0.013265
0.000112
0.000575
-0.000124
0.000755
0.034067
0.038775
0.000765
0.000835
-0.009255
-0.00872
-0.001302
0.00125
-0.002333
0.00372
0.001131
0.003505
-0.093853
-0.10355
0.055473
0.064095
-0.000308
0.00888
0.028603
0.032285
0.008803
0.0167
ICP Detection limits (ppm)
0.00245
0.0045
0.0015
0.000055
0.00048
0.0033
0.00044
0.000513
0.0011
0.00241
0.0011
0.00653
0.00653
0.018
0.00014
0.0019
0.0023
0.0011
0.0027
0.006
0.0045
0.000068
0.028
0.0018
0.00026
0.00226
0.000058
NIST Wheat Rice 1568b
Element and Analyzed
Certified
Certified
Wavelength Value (ppm) Value (ppm) range (ppm)
Element and Analyzed
Certified
Wavelength Value (ppm) Value (ppm)
Cd 214.438
Al 396.152
As 189.042
Ca 317.933
Cu 224.700
Fe 259.941
K 766.491
Mg 279.079
Mn 257.611
Mo 202.095
Na 589.592
P 213.618
Pb 220.353
S 182.034
Se 196.090
Zn 202.613
Cd 214.438
Al 396.152
As 189.042
Ca 317.933
Cu 224.700
Fe 259.941
K 766.491
Mg 279.079
Mn 257.611
Mo 202.095
Na 589.592
P 213.618
Pb 220.353
S 182.034
Se 196.090
Zn 202.613
0.032
6.070
-0.174
190.000
1.984
17.773
1141.625
407.046
8.444
0.247
18.429
1435.189
0.040
1564.308
1.128
10.995
0.032
190
2.10
18.3
1360
8.5
8
1.1
10.6
.025 - .039
180 - 200
1.9 - 2.3
17.3 - 19.3
1320 - 1400
8.0 - 9.0
6.5 - 9.5
.9 - 1.3
9.6 - 11.6
Table 4 -Trace metal analysis of oat, corn and wheat samples (set of 6 ) using ICP-OES
Oat
Corn
Element &
STDEV
Avg. (ppm)
STDEV
Avg. (ppm)
Wavelength
0.02
0.00
Cd 214.438
0.00
0.00
6.08
0.92
Al 396.152
3.45
0.68
6.73
0.12
Ba 455.404
0.01
0.01
848.88
18.71
Ca 317.933
29.89
1.09
-0.09
0.02
Co 228.616
-0.12
0.02
2.43
0.41
Cr 267.716
0.13
0.02
2.85
0.08
Cu 224.700
1.21
0.07
42.51
2.27
Fe 259.941
14.66
0.73
3222.81
55.16
K 766.491
3041.39
159.51
1044.98
17.33
Mg 279.079
907.08
53.35
61.89
1.27
Mn 257.611
4.18
0.24
0.06
0.02
Mo 202.095
-0.09
0.02
18.25
0.79
Na 589.592
0.97
0.16
2.51
0.19
Ni 231.604
0.16
0.03
2924.66
74.32
P 213.618
2450.07
149.60
-0.02
0.04
Pb 220.353
0.02
0.04
1102.96
22.97
S 182.034
820.37
12.75
0.02
0.03
Se 196.090
0.15
0.03
21.82
0.52
Zn 202.613
14.1155
0.77864
0.02
7.58
0.10
120.56
2.22
7.06
1049.77
490.75
19.64
1.33
6.68
1549.99
0.04
1069.75
0.68
18.87
0.0224
0.285
118.4
2.35
7.42
1282
559
19.2
1.451
6.74
1530
1200
0.365
19.42
Certified
range
(ppm)
.021-.0237
.271 - .299
115.3-121.5
2.19 - 2.51
6.98 - 7.86
1271 - 1293
549 - 569
17.4 -21
1.403 - 1.499
6.55 - 6.93
1490 - 1570
1190 - 1210
.336 - .394
19.16 - 19.16
Wheat
Avg. (ppm)
STDEV
0.09
4.03
4.32
304.01
-0.11
0.28
3.89
27.71
3174.53
1105.83
37.28
0.11
4.98
0.24
3339.91
-0.02
1174.04
0.12
26.97
0.003
0.570
0.165
6.978
0.019
0.020
0.105
1.282
85.677
38.740
2.084
0.031
0.472
0.011
106.325
0.050
26.509
0.035
1.118
The results for the analysis of multiple grain samples have accepted range of standard
deviation indicating excellent reproducibility of digestion carried using automated digestion
system.
Conclusion
From the results it was seen that the developed method for the digestion of grain samples
is reliable with excellent reproducibility. The use of automated digestion system have also
resulted in generous time saving for lab technician of up to 60 to 65%.
Based on the values obtained from ICP analysis, all elements analyzed fall below the ICP
detection limit. This implies Vulcan system does not contribute any contamination error for
determining trace metal content in grain analysis.
References
Based on the values obtained from ICP analysis, all elements analyzed fall below the ICP
detection limit. This implies the Vulcan system does not contribute and also prevents any
contamination from surroundings, in determining trace metal content in grain analysis.
1. Korn, M. G. A. et al. Appl. Spectrosc. Rev. 2008, 43, 67-92.
2. Teklić, T.; Lončarić, Z.; Kovačević, V.; Singh, B. R. Food and Energy Security 2013, 2(2), 81-95.
3. Singh, R.; Gautam, N.; Mishra, A.; Gupta, R. Indian J. Pharmacol. 2011, 43(3), 246–253.
Sample Processing
Authors Contact
Three sets of cereal grains including oats, corn and wheat were digested using the above
method and analyzed by ICP-OES. To determine the reliability of the analytical method
used, two standard certified samples were run; 1) NIST Rice 1568b, 2) NIST Wheat Flour
Questron Technologies Corp. 6660 Kennedy Road, Unit 14A, Mississauga, ON
www.QTechCorp.com; [email protected]
M5P 2X8, Canada; 1-844-363-1223,
Cornell University, G 36 Robert W. Holley Center for Agriculture and Health Ithaca, NY 14850, United States; (607) 255-2455