Determination of Arsenic and Selenium in Food using a Microwave

JFCA 990814
BRR RS JAYASHREE
JOURNAL OF FOOD COMPOSITION AND ANALYSIS 00,
000—000 (1999)
Article No. jfca.1999.0814
Available online at http://www.idealibrary.com on
Determination of Arsenic and Selenium in Food using a
Microwave Digestion–Dry Ash Preparation and Flow Injection
Hydride Generation Atomic Absorption Spectrometry
William R. Mindak and Scott P. Dolan
Elemental Research Branch (HFS-338), Center for Food Safety and Applied Nutrition, U.S. Food and Drug
Administration, 200 C Street, SW, Washington, DC 20204, U.S.A.
A flow injection hydride generation atomic absorption spectrometry method was developed for
determination of total arsenic and selenium content in food. The availability of arsenic and
selenium for hydride generation was ensured by using a combination microwave digestion—dry
ash preparation that destroyed the organic matrix including refractory organometallic compounds present in some foods. Hydroxylamine was used to enhance arsenic sensitivity by
approximately 15%. The ratio of peak area to peak height was used to identify irregular peak
profiles and potential interference. Method validation was performed with 21 foods and nine
reference materials. Arsenic fortification recovery ranged from 96 to 105% in foods. Selenium
fortification recovery ranged from 88 to 107% in foods. Inorganic species were used for
fortification. Nine food-related reference materials were analyzed with satisfactory results.
Estimated quantitation limits for a 1 g test portion were 0)01 mg/kg for arsenic and 0)02 mg/kg
for selenium. Extensive quality control (QC) measures were included to support method
performance.
1999 Academic Press
Key ¼ords: arsenic; selenium; hydride; atomic absorption; food.
INTRODUCTION
Selenium is present in a wide variety of foods usually as seleno-amino acids (e.g.,
selenomethionine) or various selenoproteins (Combs and Combs, 1986). Dietary
arsenic is derived mainly from products of marine origin such as seaweed, fish,
mollusks and crustaceans (Anke et al., 1997). A wide variety of organoarsenic compounds occur in food sources including arsenobetaine, arsenocholine, arsenosugars
and various methylated species (Le et al., 1994). Although arsenobetaine and arsenocholine are usually associated with marine species, they have recently been
identified in mushrooms (Kuehnelt et al., 1997).
Debate over method selection for arsenic analysis can be traced in the literature
for over 150 years. Fresenius pointed out problems with then-current methods such
as false positives from antimony and insensitivity to some arsenic species (Fresenius, 1845). Hydride generation atomic absorption spectrometry (HG-AAS) is an
established technique for food analysis because of its great sensitivity, the availability of instrumentation and relative freedom from interference (Fiorino et al., 1976;
To whom correspondence and reprint requests should be addressed Fax: (202) 205-4422. E-mail:
[email protected].
0889—1575/99/000000#00 $30.00/0
1999 Academic Press
JFCA 990814 BRR RS JAYASHREE
2
MINDAK AND DOLAN
Tinggi et al., 1992; Fedorov et al., 1997). Flow injection (FI) coupled to HG-AAS has
several advantages over batch systems such as lower reagent consumption, higher
tolerance to interfering elements, and automation (Welz and Stauss, 1993; Fernandez
et al., 1993).
Nitric acid digestion is used extensively as a means of mineralization for trace
element analysis. Unfortunately, some refractory organometallic compounds
resist HNO oxidation and do not react with borohydride-reducing agent causing
a low biased result. Lan et al. (1994) reported that sulphuric acid in addition
to HNO was necessary for total recovery of selenium from various fish tissues,
even with microwave-assisted heating under pressure. As early as 1937, Cassil
(1937) found low arsenic recovery when employing nitric acid alone or in combination with sulphuric acid to prepare products such as shrimp, tobacco and cod liver oil
for hydride generation. Complete recovery was possible if a perchloric acid oxidation
step was included. Welz and Melcher (1985) determined that including perchloric
acid in addition to nitric acid in a PTFE bomb was necessary for recovery of
arsenic and selenium in marine food. Incomplete destruction of organic matter
can also cause foaming in the reaction cell or gas—liquid separator of an FI system
(Lloyd et al., 1982). Foaming can cause irregular absorbance peaks, erratic results,
contamination and sensitivity loss. Complete destruction of organic matter is imperative when working with the small dimensions employed with FI systems (Welz et al.,
1993a).
Environmental regulations and safety concerns restrict the use of perchloric acid in
some laboratories. Dry ashing is an alternative to perchloric acid for oxidation of
refractory organic compounds (Yban ez et al., 1992). Dry ashing is simple and most
food laboratories have the required equipment. Arsenic and selenium, however, are
subject to loss by volatilization if test portions are allowed to char or ignite during
ashing. Magnesium nitrate and magnesium oxide are commonly employed as ashing
aids to prevent volatilization (May, 1982; Cervera et al., 1994). To prevent loss during
dry ashing. Brumbaugh and Walther (1989) oxidized potentially volatile analyte
species with a nitric acid predigestion. Foster and Sumar (1996) eliminated foaming
that occurred during open-vessel wet oxidation by utilizing a nitric acid microwave
digestion before dry ashing.
The aim of this research was to develop a robust method for the determination of
total arsenic and total selenium in a variety of foods. An initial preparation was to be
performed using microwave-assisted nitric acid decomposition. Perchloric acid had to
be avoided because of environmental restrictions. The resulting solution was to be
shared with other analytical techniques.
Method Summary
The method described here was based on generation of volatile analyte hydride
and subsequent measurement by atomic absorption spectrometry. Initial mineralization was performed with a nitric acid microwave digestion. An aliquot of microwave digest was subjected to dry ashing — utilizing magnesium nitrate and
magnesium oxide as ashing aids. The resulting ash was dissolved in 6 M hydrochloric
acid and diluted to volume. An aliquot of dissolved ash was diluted and analyzed for
selenium. Another aliquot of dissolved ash was treated with hydroxylamine hydrochloride and potassium iodide/ascorbic acid solutions and was subsequently analyzed
for arsenic.
Analytes were determined by using an automated FI system to inject standards and
test solutions into a 10% HCl matrix stream. Sodium borohydride solution (0)2%)
was injected to reduce analytes to their volatile hydrides.
JFCA 990814 BRR RS JAYASHREE
3
DETERMINATION OF ARSENIC AND SELENIUM
MATERIALS AND METHOD
Reagents (ACS reagent grade unless otherwise specified)
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
Hydrochloric acid, HCl, concentrated (sp. gr. 1)18), trace metals grade.
Nitric acid, HNO , concentrated (sp. gr. 1)41), high purity double-distilled grade.
Nitric acid, HNO , concentrated (sp. gr. 1)41).
Reagent water. All references to water in this method refer to ASTM type I water.
Arsenic solution, 1000 lg As/ml (commercially available solution).
Selenium solution, 1000 lg Se>/ml, prepared from SeO (99)999%).
Hydroxylamine hydrochloride, NH OH ) HCl.
50% (m/v) hydroxylamine hydrochloride solution.
Potassium iodide, KI.
Ascorbic acid, powder.
KI/ascorbic acid reducing solution,—25% KI/20% ascorbic acid solution (m/v).
Sodium hydroxide, NaOH, pellets.
Sodium borohydride, NaBH , pellets, 98% purity.
Sodium borohydride solution, 0)2% NaBH /0)05% NaOH (m/v).
Magnesium nitrate, Mg(NO ) : 6H O.
Magnesium oxide, MgO, powder.
Ashing aid, 2% MgO suspension in 20% Mg(NO ) ) 6H O solution (m/v).
Sampling
Analytical samples were reserve portions from the Food and Drug Administration’s
(FDA) Total Diet Study, market basket 94-4 (Pennington et al., 1996). Each analytical
sample was a composite of field samples prepared for consumption. Foods were
processed to obtain a representative aliquot by grinding, blending, etc., and stored in
pre-cleaned plastic containers (FDA, 1993).
Sample Preparation
CEM Corp Heavy Duty Vessels (HDV) were used in a CEM Corp model MDS 2000
microwave digestion system. Prior to each use, digestion vessels were acid-cleaned
with 10 ml of reagent-grade HNO under microwave conditions listed in Table 1.
Analytical portion masses are listed in Table 2. Analytical portions were removed with
an air displacement pipetter or Teflon spatula into a tared, clean vessel liner. Nine
milliliters high purity HNO was added and a digestion was performed according to
TABLE 1
Microwave digestion programs
Cleaning
program
Stage
Power (% of 630 W)
Max. pressure (psi)
Max. temperature (°C)
Time at max. (min)
Run time (min)
1
100
600
200
6 : 00
10 : 00
2
0
20
20
5 : 00
30 : 00
Digestion
1
55
85
130
3 : 00
—
2
65
200
150
3 : 00
10 : 00
program
3
100
450
180
3 : 00
10 : 00
4
100
600
200
3 : 00
10 : 00
5
0
20
20
5 : 00
10: 00
JFCA 990814 BRR RS JAYASHREE
4
MINDAK AND DOLAN
TABLE 2
Typical analytical portions
Food
Beef, strained junior
Beer
Bread, white
Broccoli
Cheddar cheese
Corn
Egg, soft boiled
Evaporated milk
Fruit flavored cereal
Haddock, cooked
Lemonade, reconstituted
Mayonnaise
Peanut butter
Pear
Pancakes
Analytical
2)2
6)7
1)2
9)5
0)88
2)7
1)7
2)3
0)84
1)7
7)7
0)41
0)59
5)0
1)4
Food portion
Bacon, cooked
Prune juice, bottled
Radish
Spaghetti w/ meat balls
Sweet potato, baked
Tuna, canned in oil
NIST SRM 1548
NIST RM 8415
NIST SRM 1577a
NRCC DORM-1
NRCC TORT-1
NIST SRM 1568a
NIST SRM 1566
NIES 6
NRCC DOLT-1
Analytical
Material
0)58
4)4
16
2)8
2)2
0)96
0)70
0)66
0)63
0)81
0)66
0)91
0)75
0)76
0)70
Mean (n"3)
NIST"National Institute of Standards and Technology (U.S.A).
NRCC"National Research Council Canada.
NIES"National Institute for Environmental Studies (Japan).
the microwave program listed in Table 1. Digests were diluted to 50 ml. Twenty
milliliters of digest were pipetted into a 20 ml glass beaker containing 2 ml ashing aid
and carefully heated on a hotplate until dry. Beakers were then placed in a muffle
furnace and ashed according to the following program: ramp to 150°C for 1 h, hold at
150°C for 1 h, ramp to 450°C for 3 h, and hold at 450°C for 8 h. The ashed analytical
portions were dissolved by adding 4)5 ml of 6 M HCl, covering the beakers with
a Teflon watch glass, and heating to 90°C. Temperature was maintained at 90°C for
20 min to ensure reduction of Se> to Se>. The ash solution was allowed to cool to
room temperature and then diluted to 10 ml. Arsenic test solutions were prepared by
pipetting 5 ml dissolved ash, 2 ml 50% hydroxylamine hydrochloride, and 0)4 ml
KI/ascorbic acid solution into a 10 ml volumetric flask and diluting to 10 ml. Arsenic
analysis proceeded after waiting at least 1 h to ensure complete reduction of As> to
As>. The remaining 5 ml dissolved ash solution was diluted to 10 ml and analyzed
for selenium.
Determination
Analyses were performed with a Perkin—Elmer model 5100PC AA spectrometer.
FIAS 400 flow injection system, AS90 autosampler and Electrodeless Discharge
Lamp (EDL) System II lamp power supply. Instrumental parameters were: As Type
2 EDL operated at a current of 300 mA; Se Type 2 EDL operated at a current of
210 mA; As wavelength 193)7 nm; Se wavelength 196)0 nm; As slit width 0)7 nm; Se slit
width 2)0 nm; peak area mode with 15-s integration; 2 replicate injections; 900°C cell
temperature; pump one operated on 100 rpm; pump two operated on 120 rpm; 500 ll
injection loop; 110-mm reaction coil; 1000-mm stripper coil; 80 ml/min argon flow; 8 s
wash; 5 s prefill; 9 s fill; 15-s read; and 0)5 mm autosampler probe diameter. Inside
surfaces of the gas/liquid separator was polished with optical rouge to improve
droplet shedding. Outside surfaces of the gas/liquid separator were polished to
JFCA 990814 BRR RS JAYASHREE
DETERMINATION OF ARSENIC AND SELENIUM
5
improve viewing of phase separation. Sodium borohydride solution was prepared
daily and filtered through fine porosity (2)5-lm retention) filter paper. Instrument
sensitivity was determined by analyzing a 5 ng/ml standard and calculating characteristic mass (m ). The m control limit was 100$20% of the mean value calculated
over three months. Instrument stability was determined by analyzing five replicates of
a 1-ng/ml standard and calculating the standard deviation. To ensure stable instrument operation, the control limit for the absorbance signals was (5% of relative
standard deviation (RSD).
After demonstrating proper sensitivity and stability, standardization was performed by using two replicate injections each of standardization blank and five
standards (0)5, 1, 2, 5 and 10 ng/ml). Arsenic standards and blank contained 10% (v/v)
HCl, 10% (m/v) hydroxylamine hydrochloride, 1% KI (m/v) and 0)8% (m/v) ascorbic
acid. Selenium standards and blank contained 10% (v/v) HCl. The nonlinear curve-fit
algorithm supplied by the instrument manufacturer was used to convert integrated
absorbance (peak area) to concentration units.
The standardization curve was verified by analyzing a standardization blank and
a 5 lg/ml standard prepared from a source independent of standards. Control limits
for standardization blank and standard was $3 times LOD and 100$10% of
expected value, respectively. Instrument stability was verified by analyzing a standardization blank and a 5 ng/ml standard after every 10 test solutions and at the end
of an analytical run. Test solutions with absorbance above the highest standard were
diluted with standardization blank solution.
The ratio of peak area absorbance to peak height absorbance (A/H) was used as
a quantitative means to identify peak profiles significantly different from standards.
Test solutions were diluted by a factor of two or more with standardization blank if
their A/H ratio differed by more than 20% from the mean standard A/H ratio or if
visual inspection indicated an irregular peak.
RESULTS AND DISCUSSION
General
The microwave digestion was adapted from a procedure used to prepare test portions
for analysis by inductively coupled plasma atomic emission spectrometry (ICP-AES)
(Dolan, 1998). Sharing digests with ICP-AES reduces sample preparation time for
a multi-technique analysis scheme. Direct analysis of microwave digests was unsuccessful because some refractory compounds did not oxidize completely even under the
harsh conditions of microwave digestion (200°C, 600 psi). Arsenic recoveries were less
than 20% for National Institute of Standards and Technology standard reference
material (NIST SRM) 1566 (Oyster Tissue). National Research Council Canada
(NRCC) DORM-1 (dogfish muscle) and NRCC DORM-1 (lobster hepatopancreas)
and only 42% for NIST 1568 (rice flour). Selenium analysis was not attempted on the
microwave digests because of the poor arsenic recovery. The high concentration of
HNO in the microwave digests also presented several problems with analysis such as
poor sensitivity and oxidation of iodide to iodine. Several researchers have reported
interference with HG-AAS analysis when high concentrations of nitrates, nitrites or
nitric acid were present (Pettersson et al., 1986; Voth-Beach and Shrader, 1986; Tinggi
et al., 1992).
While investigating reagents for destroying unreacted HNO in the microwave
digest, hydroxylamine was found to enhance arsenic sensitive by approximately 15%.
As hydroxylamine increased from 0 to 10% in standards m decreased from 48 to
JFCA 990814 BRR RS JAYASHREE
6
MINDAK AND DOLAN
40 pg. Sample digestion test solutions behaved similarly. No contribution of arsenic
was observed in the blank at the level prescribed.
A portion of the microwave digest was dry ashed to eliminate the HNO and
organic matrix. Dry ashing was performed in small beakers without problems,
because most organic compounds were destroyed during microwave digestion. Inexpensive, reusable glass beakers performed satisfactorily and eliminated the need for
expensive quartz beakers. Magnesium from the ash aid had no effect on blank
absorbance and sensitivity when peak area was used for quantitation. Thus, matrix
matching with magnesium nitrate was not required for standards. However, magnesium in great excess of levels recommended can interfere with HG-AAS (Reamer and
Veillon, 1981; Yban ez et al., 1992; Cervera et al., 1994).
A heating step was incorporated during ash dissolution to reduce Se> to Se>,
which is necessary for hydride formation. Hydrochloric acid was chosen for selenium
reduction over many other options because of matrix compatibility, ease of use and
effectiveness (Bye, 1983), Potassium iodide was utilized to reduce As> to As>, which
is preferred for hydride formation.
Stripper coil lengths of 300, 600 and 1000 mm did not significantly affect arsenic
sensitivity. The 1000-mm coil resulted in highest sensitivity for selenium. Burguera et
al. (1996) also found that longer stripper coils aided selenium sensitivity.
Stability studies indicated test solutions were stable for five days or until solutions
started to turn brown from oxidation of iodide to iodine. Adjustments of argon flow
and gas—liquid separator drain pump rate were critical for optimum performance.
Draining the separator too rapidly resulted in reduced sensitivity because of insufficient analyte hydride residence time. Foaming was not observed in the separator,
therefore, antifoaming agents were not necessary. Better precision was obtained by
using integrated absorbance rather than peak height. Contamination control was
verified by analysis of method blanks. Method blank results averaged LOD so no
blank correction was performed.
This method can be extended to foods not listed in Table 2, but recovery and
interference properties must be evaluated. Of particular importance is the maximum
safe analytical portion for microwave digestion, which depends on food composition
(Dolan, 1998). There is danger of the microwave vessel bursting from excessive
pressure if a very large quantity of food mass is used. High fat foods (mayonnaise,
butter, etc.) should be limited to a relatively small analytical portion. Dry foods
(cereal, bread, etc.) should be limited to smaller portions compared to high moisture
foods (radish, pear, etc.). The minimum analytical portion to insure a representative
sampling depends on the degree of analyte homogeneity.
Method Performance and »alidation
The limits of detection (LOD) was determined by analysis of 17 method blanks and
calculated as 3 times the standard deviation estimate. Arsenic and selenium LODs
were 0)05 and 0)09 ng/ml, respectively. The limit of quantitation (LOQ) was calculated
as 10 times the standard deviation estimate for 17 method blanks. LOQs in food
depends on the mass of the analytical portion and number of dilutions required
(Tables 3 & 4). Based on a 1-g analytical portion. LOQs in food were estimated as
0)01 mg/kg for arsenic and 0)02 mg/kg for selenium.
Arsenic findings are summarized in Table 3. The results ranged from less than LOQ
to 5)7 mg/kg. Arsenic was determined at levels greater than LOD but less than LOQ
in the foods reported as containing less than LOQ. Selenium findings are summarized
in Table 4. Results ranged from less than LOQ to 0)66 mg/kg. Selenium was determined at levels greater than LOD but less than LOQ in pear, lemonade, prune juice
JFCA 990814 BRR RS JAYASHREE
7
DETERMINATION OF ARSENIC AND SELENIUM
TABLE 3
Arsenic recovery and concentration of selected foods
Food
Lemonade, reconstituted
Corn
Pear, canned
Prune juice, bottled
Evaporated milk
Beef, strained junior
Egg, soft boiled
Cheddar cheese
Fruit flavored cereal
Bacon, cooked
Mayonnaise
Broccoli
Spaghetti w/ meat balls
Pancakes
Beer
Radish
White bread
Sweet potato, baked
Peanut butter
Tuna, canned in oil
Haddock, cooked
LOQ
(mg/kg)
Conc
0)001
0)003
0)002
0)002
0)004
0.004
0.005
0)010
0)011
0)0216
0)02
0)001
0)003
0)006
0)002
0)001
0)008
0)004
0)02
0.03
0)1
(0)001
(0)003
(0)002
(0)002
(0)004
(0.004
(0.005
(0)010
(0)011
(0)0216
(0)02
0)003
0)004
0)007
0)007
0)007
0)009
0)013
0)022
1.0
5)7
RSD
(mg/kg)
Fortification
(%)
Recovery
(mg/kg)
—
—
—
—
—
—
—
—
—
—
—
4
7
5
0)6
15
26
4
6
8
3
0)029
0)060
0)032
0)033
0)068
0.062
0.093
0)191
0)188
0)296
0)410
0)030
0)050
0)108
0)059
0)037
0)135
0)098
0)307
1.66
8)47
100
105
98
96
100
98
96
101
104
98
102
102
97
100
98
98
100
105
98
98
96
Limit of quantitation.
Mean (n"3).
Mean (n"2).
TABLE 4
Selenium recovery and concentration of selected foods
Food
Pear, canned
Lemonade, reconstituted
Prune juice, bottled
Sweet potato, baked
Broccoli
Beer
Corn
Radish
Beef, strained junior
Evaporated milk
Mayonnaise
Fruit flavored cereal
Spaghetti w/ meat balls
Peanut butter
Pancakes
White bread
Cheddar cheese
Bacon, cooked
Egg, soft boiled
Haddock, cooked
Tuna, canned in oil
Limit of quantitation.
Mean (n"3).
Mean (n"2).
LOQ
(mg/kg)
Conc
(mg/kg)
RSD
(%)
Fortification
(mg/kg)
Recovery
(%)
0)003
0)002
0)004
0)008
0)002
0)002
0)006
0)001
0)008
0)007
0)04
0)02
0)006
0)03
0)01
0)014
0)02
0)03
0)02
0)04
0)03
(0)003
(0)002
(0)004
(0)008
0)009
0)010
0)011
0)016
0)040
0)050
0)060
0)086
0)12
0)17
0)17
0)18
0)23
0.37
0)42
0)54
0)66
—
—
—
—
3
0
7
13
10
2
10
3
7
11
5
4
1
16
2
2
6
0)032
0)029
0)033
0)098
0)030
0)059
0)060
0)037
0)082
0)112
0)410
0)188
0)166
0)307
0)359
0)225
0)318
0)494
0)618
0)848
0)832
88
104
100
107
105
100
101
102
96
103
104
99
96
98
96
98
95
101
99
98
101
JFCA 990814 BRR RS JAYASHREE
8
MINDAK AND DOLAN
and sweet potato. Analytical portions and consequently LOD ranged from 0)41 g for
mayonnaise to 16 g for radish. Recovery was assessed in foods by analysis of 21
fortified foods. Test portions and fortified test portions were prepared and analyzed in
triplicate. Fortification of foods was performed before microwave digestion using
solutions of As> and Se>. Recovery was within control limits of 100$20%.
Arsenic recovery ranges from 96 to 105%. Selenium recovery ranged from 88 to
107%.
Method precision was assessed as the relative standard deviation (RSD) of triplicate test portion results of reference materials (RMs), food and fortified
food. Except for white bread, precision was within control limits of 20% RSD when
analyte level was greater than LOQ. Findings are summarized in Tables 3—5. Typical
short term instrumental precision expressed as RSD of two replicate test solution
injections was less than 2% with control limits of 10% RSD for values greater than 10
times LOD.
The method was evaluated by using nine reference materials (RMs) that
were prepared and analyzed in triplicate. Findings are summarized in Table 5.
Recoveries were 100$14% of the certified value or within certified value of uncertainty, which ever was greater. Arsenic results were within certified value uncertainty
except for NIST SRM 1577a (bovine liver), which was recovered at 114%.
Selenium results were within certified value of uncertainty except for that in NRCC
DORM-1 (dogfish muscle) and NIST 1548 (total diet) in which recoveries were 113
and 111%, respectively.
Interferences
Three general types of interferences can be present with HG-AAS analysis: spectral,
memory, and matrix. Spectral interference is rare with hydride generation because
analyte is volatilized and separated from test solution matrix. Appropriate rinse and
cycle times eliminates simple memory effects leading to false positive signals. Matrix
components can inhibit analyte-hydride formation or alter analyte-hydride reaction
kinetics. Matrix components can also cause foaming and large bubble formation in
the gas—liquid separator resulting in analyte transport problems. High concentration
of other hydride-forming elements can cause suppression or distortion of analyte
signal, multiple absorption peaks or induce a memory interference requiring cleaning
of various flow injection components (Welz and Stauss, 1993). A complete mineralization procedure, such as dry ashing, destroys organic compounds and eliminates
source of interference. Optimized acid/reductant ratio improves analyte hydride
generation. Although transition metals can suppress analyte hydride generation (Welz
and Shubert-Jacobs, 1986) they are not expected to be present in most foods at levels
that will interfere. Possible exceptions where elevated concentrations of transition
metals might be encountered would be vitamin and mineral fortified foods and organ
meats.
Ideally, the absorbance peak shape of a food test solution should be the same or
highly similar to those of standardization solutions. Irregular peak profiles indicate
interference or other problems and can lead to inaccurate results. If peak profiles are
similar, results calculated by using peak height should be equivalent to those calculated using peak area. Peak area and peak height calculated concentrations were
not equivalent for test solutions with an irregular absorbance peak. This difference
increased as the A/H ratio of irregular peaks deviated from the mean standards A/H
ratio. Maintaining test solution A/H ratios within 20% of standard’s A/H ratios and
using peak area is recommended for accurate results. The A/H ratio remains constant
when certain interferences are absent but increases slightly where response starts to
nv
0)01
0)047$0)006
17)7$2)1
24)6$2)2
0)29$0)03
13)4$1)9
9)2$0)5
10)1$1)4
0)111
0)015
0)054
18)6
25)5
0)32
13)5
9)69
10)6
Mean
L
(mg/kg)
2
35
6
2
5
4
2
1
2
RSD
(%)
Arsenic result
0)245$0)005
1)39$0)17
0)71$0)07
1)62$0)12
6)88$0)47
0)38$0)04
2)1$0)5
(1)5)
7)34$0)42
Selenium
cert. value$uncert.
(mg/kg)
0)273
1)49
0)77
1)83
6)71
0)40
2)43
1)78
7)60
Mean
L
(mg/kg)
2
3
2
2
2
2
2
1
1
RSD
(%)
Selenium result
NIST"National Institute of Standards and Technology (USA); NRCC"National Research Council Canada; NIES"National Institute for
Environmental Studies (Japan).
nv"no value provided by certifying organization.
non-certified information value.
n"2.
NIST SRM 1548 (total diet)
NIST RM 8415 (whole egg powder)
NIST 1577a (bovine liver)
NRCC DORM-1 (dogfish muscle)
NRCC TORT-1 (lobster hepatopancreas)
NIST SRM 1568a (rice flour)
NIST SRM 1566 (oyster tissue)
NIES 6 (mussel)
NRCC DOLT-1 (dogfish liver)
Reference material
Arsenic
cert. value$uncert.
(mg/kg)
Reference material results
TABLE 5
JFCA 990814 BRR RS JAYASHREE
DETERMINATION OF ARSENIC AND SELENIUM
9
JFCA 990814 BRR RS JAYASHREE
10
MINDAK AND DOLAN
deviate from linearity. Ratios start to decrease when analyte concentration is less
than LOQ. Although irregular peaks can be visually discerned, calculating the
A/H ratio permits quantifying irregularity instead of relying on a subjective
observation.
Typical A/H ratios for arsenic and selenium standards ranged between 3)5 and 4.
The only irregular selenium peaks encountered were for test solutions from livertype RMs (bovine liver lobster hepatopancreas and dogfish liver). Diluting these
test solutions until the selenium concentration was 2—5 ng/ml resulted in regular
shaped peaks and an A/H ratio within 20% of the mean standards value. The
analytical challenge presented by liver samples is probably due to their relatively
high concentration of transition metals, such as copper and nickel, which can interfere
with HG-AAS (Welz and Shubert-Jacobs, 1986). Beer was the only food with arsenic
greater than the LOQ that had irregular peak shapes as indicated by a low A/H
ratio of 2)2. Diluting by a factor of 2 resulted in normal peaks. Cheese, beer,
lemonade, prune juice, spaghetti and pear fortified test solutions resulted in arsenic
A/H ratios of 2)6—2)9 and thus narrow peaks. These fortified test solutions were
diluted by a factor of 2—4 before arsenic analysis resulting in improved A/H ratio and
fortification recovery.
Quality Control
Extensive quality control (QC) guidelines are needed to ensure high quality, reliable data. QC documentation is valuable to data end-users to assess quality and is
essential when data are used to support litigation or enforce regulations. Various QC
measures have been illustrated in this method including instrument performance
(sensitivity, stability and precision), matrix validation, matrix recoveries, RM recoveries, standardization verification, potential interference identification, and contamination control. Use of QC measures is imperative to ensure reliable data and to
substantiate proper instrument performance. The analyst must decide an appropriate
QC criteria limits to meet data quality objectives.
CONCLUSION
Combining microwave digestion, dry ashing and HG-AAS has been shown to be
effective for arsenic and selenium analysis of food. The combined preparation procedure permits quantification of total arsenic and selenium, including refractory
organometallic compounds present in some foods. Problems with charring and
resulting analyte losses were eliminated by dry ashing a portion of microwave digest
instead of a portion of undigested food. Perchloric acid was not required, which
eliminated hazards and special equipment requirements associated with its use. The
ratio of peak area to peak height can be used to identify irregular peak profiles and
potential interference. Quality controls throughout the method ensure high-quality
data.
ACKNOWLEDGEMENTS
The authors would like to thank J. Kevin Cline of FDA’s Kansas City District Laboratory (Lenexa, KS) for
providing homogenized food samples and James A. Easterling of FDA’s Center for Food Safety and
Applied Nutrition for polishing the phase separator.
JFCA 990814 BRR RS JAYASHREE
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11
REFERENCES
Anke, M., Glei, M., Arnhold, W., Drobner, C., and Seifert, M. (1997). Arsenic. In Handbook of Nutritionally
Essential Mineral Elements (B. L. O’Dell, and R. A. Sunde, Eds.), Chap. 23, pp. 631—639. Marcel Dekker,
New York.
Brumbaugh, W. G., and Walther, M. J. (1989). Determination of arsenic, selenium in whole fish by
continuous-flow hydride generation atomic absorption spectrometry. J. Assoc. Off. Anal. Chem. 72,
484—486.
Burguera, J. L., Carrero, P., Burguera, M., Rondon, C., Brunetto, M. R., and Gallignani, M. (1996). Flow
injection for the determination of Se(IV) and Se(VI) by hydride generation atomic absorption spectrometry with microwave oven on-line prereduction of Se(VI) to Se(IV). Spectrochim. Acta, Part B 51,
1837—1847.
Bye, R. (1983). Critical examination of some common reagents for reducing selenium species in chemical
analysis. ¹alanta 30, 993—996.
Cassil, C. C. (1937). Report on arsenic. J. Assoc. Off. Agri. Chem. 20, 171—178.
Cervera, M. L., Lopez, J. C., and Montoro, R. (1994). Determination of arsenic in orange juice by dry ashing
hydride generation atomic absorption spectrometry. Microchem. J. 49, 20—26.
Combs, G. F., Jr., and Combs, S. B. (1986). In ¹he Role of Selenium in Nutrition, Chap. 2, pp. 15—40; Chap. 6,
pp. 206—249. Academic Press, Orlando.
Dolan, S. P. (1998). U.S. Food and Drug Administration, personal communication.
Fedorov, P. N., Ryabchuk, G. N., and Zverev, A. V. (1997). Comparison of hydride generation and graphite
furnace atomic absorption spectrometry for the determination of arsenic in food. Spectrochim. Acta, Part
B 52, 1517—1523.
Fernandez, M. G. C., Palacios, M. A., and Camara, C. (1993). Flow-injection and continuous-flow systems
for the determination of Se(IV) and Se(VI) by hydride generation atomic absorption spectrometry with
on-line prereduction of Se(VI) to Se(IV). Anal. Chim. Acta 283, 386—392.
Fresenius, R. (1845). On an improved method for the detection and quantitative determination of arsenic.
Mem. Proc. Chem. Soc. ¸ondon 2, 129—134.
Fiorino, J. A., Jones, J. J., and Capar, S. G. (1976). Sequential determination of arsenic, selenium, antimony,
and tellurium in foods via rapid hydride evolution and atomic absorption spectrometry. Anal. Chem. 48,
120—125.
Food and Drug Administration (1993). ¹otal Diet Study—Sample Preparation: Standard Operating Procedure KCXI; Rev. 1, US FDA, Kansas City District Laboratory, Lenexa, KS.
Foster, L. H., and Sumar, S. (1996). Selenium concentration in soya based milks and infant formulae
available in the United Kingdom. Food Chem. 56, 93—98.
Kuehnelt, D., Goessler, W., Schlagenhaufen, C., and Irgolic, K. (1997). Arsenic Compounds in terrestrial
organisms III: Arsenic compounds in Formica sp. from an old arsenic site. Appl. Organomet. Chem. 11,
859—867.
Lan, W. G., Wong, M. K., and Sin, Y. M. (1994). Comparison of four microwave digestion methods for the
determination of selenium in fish tissue by using hydride generation atomic absorption spectrometry.
¹alanta 41, 195—200.
Le, S. X. C., Cullen, W. R., and Reimer, K. J. (1994). Speciation of arsenic compounds in some marine
organisms. Environ. Sci. ¹echnol. 28, 1598—1604.
Lloyd, B., Holt, P., Delves, H. T. (1982). Determination of selenium in biological samples by hydride
generation atomic absorption spectroscopy. Analyst 107, 927—933.
May, T. W. (1982). Recovery of endogenous selenium from fish tissues by open system dry ashing. J. Assoc.
Off. Anal. Chem. 65, 1140—1145.
Pennington, J. A. T., Capar, S. G., Parfitt, C. H., and Edwards, C. W. (1996). History of the food and drug
administration’s total diet study (Part II), 1987 to 1993. J. AOAC Int. 79, 163—170.
Pettersson, J., Hansson, L., and Olin, A. (1986). Comparison of four digestion methods for the determination of selenium in bovine liver by hydride generation atomic absorption spectrometry in a flow system.
¹alanta 33, 249—254.
Reamer, D. C., and Veillon, C. (1981). Preparation of biological materials for determination of the selenium
hydride generation atomic absorption spectrometry. Anal. Chem. 53, 1192—1195.
Tinggi, U., Reilly, C., and Patterson, C. M. (1992). Determination of selenium in foodstuffs using spectrofluorometry and hydride generation atomic absorption spectrometry. J. Food Compos. Anal. 5, 269—280.
Voth-Beach, L. M., and Shrader, D. E. (1986). Reduction of interference in the determination of arsenic and
selenium by hydride generation. Spectroscopy 1, 60—65.
JFCA 990814 BRR RS JAYASHREE
12
MINDAK AND DOLAN
Welz, B., and Melcher, M. (1985). Decomposition of marine biological tissues for determination of arsenic,
selenium, and mercury using hydride generation and cold vapor atomic absorption spectrometries. Anal.
Chem. 57, 427—431.
Wlez, B., and Schubert-Jacobs, M. (1986). Mechanisms of transition metal interferences in hydride
generation atomic absorption spectrometry. J. Anal. At. Spectrometry 1, 23—27.
Welz, B., He, Y., and Sperling, M. (1993a). Flow injection on-line acid digestion and pre-reduction of arsenic
for hydride generation atomic absorption spectrometry—a feasibility study. ¹alanta 40, 1917—1926.
Welz, B., and Stauss, P. (1993b). Interferences from hydride-forming elements on selenium in hydride
generation atomic absorption spectrometry with a heated quartz tube atomizer. Spectrochim. Acta, Part
B 48, 951—976.
Yban ez, N., Cervera, M. L., and Montoro, R. (1992). Determination of arsenic in dry ashed seafood
products by hydride generation atomic absorption spectrometry and a critical comparative study with
platform furnace Zeeman-effect atomic absorption spectrometry and inductively coupled plasma atomic
emission spectrometry. Anal. Chim. Acta 258, 61—71.

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