научная статья по теме SIMPLE AND RAPID DETERMINATION OF PHOSPHORUS IN MEAT SAMPLES BY WD-XRF METHOD Химия

Текст научной статьи на тему «SIMPLE AND RAPID DETERMINATION OF PHOSPHORUS IN MEAT SAMPLES BY WD-XRF METHOD»

ЖУРНАЛ АНАЛИТИЧЕСКОЙ ХИМИИ, 2010, том 65, № 4, с. 388-393

ОРИГИНАЛЬНЫЕ СТАТЬИ =

УДК 543

SIMPLE AND RAPID DETERMINATION OF PHOSPHORUS IN MEAT SAMPLES BY WD-XRF METHOD

© 2010 A. Jastrzebska, M. Cichosz, E. Sz l yk

Faculty of Chemistry, Nicolaus Copernicus University 7 Gagarin Str., 87-100 Torun, Poland Received 27.11.2008; in final form 03.09.2009

The wavelength dispersive X-ray fluorescence (WD-XRF) method for phosphorus determination in meat samples has been described. The effects ofsample pretreatment on the XRF analysis have been discussed. The phosphorus content determined in meat samples ranged from 603 ± 6 to 613 ± 19 mg /100 g dry mass (d.m.), depending on the sample preparation technique. The meat samples spiked with phosphates have been used for the calibration procedure. The accuracy was determined against a number of certified reference materials (SMRD 2000, RF 8414, NIST-1568A and NIST-1549), and recovery was assayed using the standard addition procedure. The proposed method has been compared with the standard spectrophotometric method (PN-ISO 13730, 1999) of total phosphorus determination. The sample pretreatment procedure has been reduced to minimum. The presented results suggest the WD-XRF method can be an alternative to the spectrophotometric analysis.

The X-ray fluorescence (XRF) method offers a possibility of quantitative determination of some analytes in the solid samples. This method is convenient for determination of such elements, for which the reliable wet chemical methods are unavailable, as well as for analysis of non-metallic specimens. However, in case of trace analysis, it is difficult to detect a given element with this technique [1]. On the other hand, XRF provides an alternative multi-element analytical tool for routine non-destructive analysis of many materials requiring minimum sample preparation [2, 3]. A review of the technical progress in XRF methodology, application and sample preparation was published by Sza-loki, et al. [4]. The XRF applications presented in literature indicate that this method has been rarely applied to food samples analysis mainly due to its low sensitivity [2—9].

In contrast, the advantages of the proposed method include: simplicity of the analytical procedure, short analysis time and simultaneous determination of many elements in more complicated matrices. To the best of our knowledge, there are no reports on phosphorus determination in meat samples by the XRF method. In this paper, the determination of total phosphorus content in meat samples by the WD-XRF method has been described and compared to the standard UV-Vis method of meat analysis. The latter technique is time consuming due to the sample preparation (mineralization or extraction procedure).

Therefore, in this paper two simple procedures of sample preparation have been described. During the first one meat samples were dried, mineralized and mixed with boric acid, while upon the second one they were dried at 105°C, ground and sieved prior to analy-

sis. The reduction of matrix effects was achieved by usage of certified reference materials and the method of internal standards addition. The validation of proposed method was performed in three ways: the analysis of spiked meat samples, the comparison of obtained results with the official method (PN-ISO 13730, 1999), and by the analysis of certified reference materials.

EXPERIMENTAL

Reagents and certified reference material. Pyrophosphate (K4P2O7) and tripolyphosphates (Na5P3O10, Na3P3O9) were purchased from Sigma Aldrich, whereas KH2PO4, HCl, NH4VO3, (NH4)6Mo7O24, H3BO3, HNO3 and H2O2 were from POCH (Gliwice, Poland). All reagents were of analytical grade. The phosphate standard solution, KH2PO4 (traceable to SRM from NIST) was supplied by Merck. Redistilled water was used in all solutions preparation (specific conductivity <10 ^S).

SMRD 2000 (fresh pork), RF 8414 (Bovine Muscle Powder), NIST-1568A (Rice Flour), NIST-1549 (Non-fat Milk Powder), CTA-OTL-1 (Oriental tobacco leaves), CTA-VTL-2 (Virginia Tobacco leaves), INCT-TL-1 (Tea Leaves) were supplied by Promo-chem (Poland). SRM 2690, SRM1633b and ASCRM010 were purchased from National Institute of Standards & Technology (USA), whereas BCR No 176 from Commission of the European Communities, Community Bureau of Reference.

Apparatus. The X-ray analyses were performed using a Spectroscan-V, Spectron-Optel Inc. Russian spectrometer with a crystal-analyzer (LiF(200); LiF(220); PET; RbAP; ML(44E)), X-ray tube with Cu anode and sampler handler. The method of WD-XRF

CPS

2000 -

1500 -

1000 -

500 -

2000 3000 4000 5000

X, mA

6000

7000

Fig. 1. XRF spectrum of SRM 2690 (40kV, 4 mA, 2s, PET, wavelength range 1805-7305 mA).

was employed for phosphorus determination with PET analytical crystal at 40 kV voltage and 4.0 mA current (parameters recommended by the producer). The detection limit for phosphorus was 5 x 10—4% (boric acid matrix) with apparatus error 0.5%.

Absorption spectra (for total phosphorus determination) were recorded with a Helios a — UNICAM spectrophotometer in a 1-cm quartz cell, against reagents blank.

Analytical procedure. Samples preparation for XRF analyses. Fresh pork meat samples, purchased at a local market, were stored in the freezer and used for analyses. Prior to the analyses, bulk meat samples were minced and homogenized with a plate of 3 mm diameter holes. This method is similar to the procedure of solid samples preparation for XRF, which involves grinding and pelletizing at high pressure [10]. Fresh meat samples were dried at 105°C to the constant weight, crushed in a ball mill (made from silicon carbide) and pressed through the sieve (0.4 mm diameter). Then, laboratory meat samples were mineralized by dry ashing at 450°C in a furnace. The obtained ashes were mixed with boric acid (1 : 10, sieve <0.4 mm), pressed into cuvettes, packed in an aluminum holder with sample cells, and analyzed by the XRF method.

This procedure enables reduction of the background intensity [11].

In order to check the impact of sample preparation procedure on phosphorus level, the second set of meat samples was dried at 105°C, pulverized and sieved (particle size <0.4 mm). Obtained dry powdered samples were pressed into cuvettes and sample holder and analyzed. All samples were counted for 60 s and the calibration curve technique was employed to quantify their phosphorus content.

The internal standard addition procedure was employed for each phosphate salt added to the meat samples. The following phosphates: KH2PO4, K4P2O7, Na5P3O10, Na3P3O9 were used as single standards by mixing with the fresh meat in the laboratory. In this way, four different calibration series of the phosphates were prepared. Such laboratory meat samples were then treated in the same way as fresh meat.

WD-XRF analyses. Qualitative analysis was performed using the external standard SRM 2690 for P-K analytical line at 6157 mA (Fig. 1).

Due to the limited availability of meat standards and non-sample destruction properties of X-ray spec-trometry, the standard ashes with the certified phosphorus content were used for the calibration graph. The following standard ashes were applied: BCR No

390

JASTRZ ÇBSKA и др.

CPS 120

80 -

40 -

(A)

CPS

120

(B)

60

d a Рч

1 p. г од СЗ и \ г^лД J 1 3 La,

CPS 180

100

20

5000 5600

Wavelength, mA (C)

6200

5600

в

Ph

«

m CQ.

Ь 1

i or 1 i

, JIV .ллЛ ^^ L v-^/AI Л r-s-J . l

CPS

140

80

20

6200 Wavelength, mA (D)

6800

5600 6200

Wavelength, mA

6600

4500 5500

Wavelength, mA

7000

Fig. 2. The XRF spectra of SRM 2690 (A), BCRNo 176 (B), ASCRM010 (C), SRM1633b (D) (40kV, 4 mA, 2s, PET).

176 (12.7 g P2O5/kg), SRM 2690 (0.52 ± 0.01% P), SRM1633b (2300 mg P/kg) and ASCRM010 (0.91% P2O5). The regression analysis was applied (using intensity of the Ka X-ray lines) by the Spectroscan V-708 program, available within the instrument software. The XRF spectra of the certified ashes are presented at Fig. 2.

The calibration curve expressed as y = (b ± Sb)x + + (a ± Sa), where Sb, Sa are standard deviations of slope and intercept, was found resulting in the regression equation of y = (0.0179 ± 0.0003)x - (0.2688 ± ± 0.0005), with the determination coefficient R2 = = 0.9986. The detection (DL) and quantification limits (QL) were calculated according to Miller and Miller [12], the obtained results being as follows: DL = 0.80 g P/100g and QL = 2.68 g P/100g.

Reference method (PN-ISO 13730, 1999). The phosphorus contents of the samples were determined spectrophotometrically by a molybdate-vanadate method (PN-ISO 13730, 1999) of total phosphorus determination. The calibration curve was measured for 6 working standards (from 4 to 15 mg P/L) pre-

pared from the standard solution of KH2PO4 (NIST, MERCK), resulting in the regression equation of y = (0.0361 ± 0.0038)x - (0.0026 ± 0.0018), with the coefficient of determination R2 = 0.9987.

RESULTS AND DISCUSSION

The choice of calibration standard is a critical parameter due to the minimization of the matrix absorption. Therefore, the SRM ashes were applied and compared with ashed meat samples fluxed with boric acid. As a new non-destructive procedure of meat samples preparation for XRF, drying at 105°C and sample sieving (grain size <0.4 mm) is proposed. The XRF and spectrophotometric method results (calculated as mg P/100g dry mass, d.m.) are listed in Table 1.

The data in Table 1 exhibit a slight difference (10 mg P/100g d.m.) between the phosphorus content in the meat samples prepared with and without ashing. Hence, a small impact of the sample preparation procedure is observed. However, the coefficient of variation (CV < 1%) noted for the ashing procedure indi-

cates good precision, which can be related to good homogeneity and grain size of the powdered samples and additional mixing with boric acid [12]. Omote et al. [13] discussed the relation between the grain size of plant samples and the X-ra

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