confirmation of sulfamethazine, sulfathiazole

DRUGS, COSMETICS, FORENSIC SCIENCES

Confirmation of Sulfamethazine, Sulfathiazole, andSulfadimethoxine Residues in Condensed Milk and Soft-CheeseProducts by Liquid Chromatography/Tandem MassSpectrometry

SUSAN B. CLARK, SHERRI B. TURNIPSEED,1

and MARK R. MADSON

U.S. Food and Drug Administration, PO Box 25087, Denver, CO 80225

JEFFREY A. HURLBUT

Western Washington University, Chemistry Department, MS-9150, Bellingham, WA 98225

LAURA R. KUCK2

University of Colorado, Boulder, CO 80309

JOHN N. SOFOS

Colorado State University, Department of Animal Sciences, 1171 Campus Delivery, Ft. Collins, CO 80523

A liquid chromatography/tandem mass

spectrometry method (LC/MS/MS) is described for

the simultaneous detection of 3 sulfonamide drug

residues at 1.25 ppb in condensed milk and

soft-cheese products. The 3 sulfonamide drugs of

interest are sulfathiazole (STZ), sulfamethazine

(SMZ), and sulfadimethoxine (SDM). The method

includes extraction of the product with phosphate

buffer, centrifugation of the diluted product, and

application of a portion of the extract onto a

polymeric solid-phase extraction cartridge. The

cartridge is washed with water, and the

sulfonamides are eluted with methanol. After

evaporation, the residue is dissolved in 0.1%

formic acid solution, and the solution is filtered

before analysis by LC/MS/MS. The LC/MS/MS

program involved a series of time-scheduled

selected-reaction monitoring transitions. The

transitions of MH+

to the common product ions at

m/z 156, 108, and 92 were monitored for each

residue. In addition, SMZ and SDM had a fourth

significant and unique product ion transition that

could be measured. Validation was performed with

control and fortified-control condensed bovine

milk with 2.5, 5, and 10 ppb sulfonamides. This

method was applied to imported flavored and

unflavored condensed milk and cream cheese

bars. The presence of STZ and SMZ residues was

confirmed in 3 out of 6 products.

The presence of sulfonamide drug residues in milk and

milk products is a continuing health issue because

recent studies have shown that one or more members of

this family are suspected carcinogens (1, 2). Sulfonamides are

extensively prescribed in veterinary medicine for treatment of

various bacterial infections (at both therapeutic and

prophylactic levels). Thus, any misuse or lack of adherence to

withdrawal times may result in the presence of illegal residues

in milk. Drug residues in food products are of concern to

consumers because they may compromise the human immune

system (pharmacological effects), cause an allergic reaction in

sensitive individuals, or contribute to the development of

antibiotic-resistant pathogenic bacterial strains (3). The U.S.

Food and Drug Administration’s Center for Food Safety and

Applied Nutrition (CFSAN) has established a 10 ppb safe

level for sulfachloropyridazine, sulfadiazine, sulfamerazine,

sulfamethazine (SMZ), sulfamethizole, sulfanilamide,

sulfapyridine, sulfaquinoxaline, and sulfathiazole (STZ), and

a 10 ppb tolerance level for sulfadimethoxine (SDM) in raw

milk (4). Because raw milk is not routinely screened for

sulfonamides before processing, methodology is needed for

processed milk products.

Historically these analytes were isolated from milk and

other food matrixes, derivatized, and then analyzed by gas

chromatography/mass spectrometry (GC/MS; 5) to confirm

their identities after determination by liquid chromatography

with UV detection (LC/UV; 6–8). Most recent methods for

sulfonamide residues use liquid chromatography/mass

spectrometry (LC/MS) with various ionization techniques

including thermospray (9), atmospheric pressure chemical

ionization (10, 11), and electrospray (12–19). There are

examples in the literature of LC/MS monitoring of these

residues in milk (9, 10, 12–15), as well as eggs (14–16),

honey (17, 18), and beef tissue (11, 19). One method

describes the ability to screen, quantitate, and confirm 21

sulfonamide residues in milk by using electrospray liquid

736 CLARK ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 88, NO. 3, 2005

Received August 27, 2004. Accepted by SG November 16, 2004.1

Author to whom correspondence should be addressed; e-mail:[email protected].

2Author's current address: U.S. Food and Drug Administration, PO Box

25087, Denver, CO 80225.

chromatography/tandem mass spectrometry (LC/MS/MS) on a

triple-quadrupole instrument (12). In another paper, the authors

describe the use an electrospray triple-quadrupole LC/MS

instrument and take advantage of the selectivity gained by

performing selected-reaction monitoring (SRM) experiments

to limit the cleanup to a filtration of a small sample of milk (13).

With a single-quadrupole instrument, LC/MS was used to

determine sulfonamide residues in milk and eggs after isolation of

the residues with matrix solid-phase dispersion techniques (15).

Our laboratory has recently developed an LC/MS/MS method

using triple-quadrupole instrumentation with SRM for the

determination and confirmation of 8 sulfonamides of regulatory

interest in milk (20).

Isolating animal drug residues from cheese or other

processed milk products presents unique challenges. There

are very few published analytical methods for monitoring

residues in cheese (21–23). A review of these methods

indicates that residues, including ivermectin in buffalo and

albendazole in cattle, partition into the cheese products from

milk obtained from lactating animals that have been

administered these drugs. The extraction methods used in our

laboratory for the analysis of raw or whole milk for

sulfonamides (6–8) are not applicable to the analysis of

condensed milk or cream cheese products because the organic

solvents used for the extraction process congeal condensed

milk and cheese products into a gel-like mass, thereby limiting

their extraction potential. No other methods were found for

the screening or analysis of condensed milk for sulfonamide

residues. This paper describes a new extraction procedure

optimized for these difficult matrixes and its use in

conjunction with the LC/MS/MS method developed for these

residues in milk (20). Although the instrumental method is

capable of monitoring 8 sulfonamides, this method focuses on

the 3 residues most likely to be found in processed milk

products: SMZ, STZ, and SDM.

Experimental

Apparatus

(a) Liquid chromatograph/mass spectrometer.—

Thermofinnigan TSQ Quantum triple-quadrupole mass

spectrometer coupled to a Thermofinnigan Surveyor LC/MS

pump and autosampler. XCaliber V.3 software was used to

obtain data (Thermoelectron Corp., San Jose, CA).

(b) LC column.—YMC ODS-AQ, 120 �, 2 � 100 mm,

3 �m (Part No. AQ 12S031002 WT, Waters Corp., Milford,

MA).

(c) Glassware.—50 mL nondisposable polypropylene

tube with polypropylene screw closure (Cat. No. 21009-386,

VWR International, Inc., Aurora, CO); disposable

polypropylene 15 and 50 mL centrifuge tubes (Cat. Nos.

21008-918 and 21008-951, respectively, VWR International,

Inc.); 2 mL LC sample vials with screw caps (Waters Corp.).

(d) Centrifuge.—International Equipment Co. (IEC)

B-22M refrigerated centrifuge with rotor No. 876, and IEC

DPR-6000, or equivalent.

(e) Pipettors.—Eppendorf variable (20 to 250 �L ± 0.8%)

volume (Brinkmann Instruments, Inc., Westbury, NY), or

equivalent, and variable volume pipettor, 1–5 mL, and

accompanying pipet tips (Cat. Nos. 53499-605 and

53503-826, VWR International, Inc.).

(f) Solid-phase extraction (SPE) cartridge—Oasis®

HLB

3 cc (60 mg) extraction cartridge (Cat. No. WAT094226,

Waters Corp.).

(g) SPE extraction manifold.—Cat. No. WAT200677

(Waters Corp.), or equivalent.

(h) Reservoir.—60 cc cartridge accessory (Cat. No.

WAT024659, Waters Corp.), or equivalent.

(i) pH meter.—Optimized with pH 4 and 7 buffers

(Mettler).

(j) Nitrogen evaporator.—Meyer N-Evap analytical

evaporator Model 111, or equivalent, with water bath set at

50� ± 5�C (Organomation Associates, Inc., South Berlin,

MA).

(k) Syringe filter.—Acrodisc LC 13 mm syringe filter

with 0.45 �m PVDF membrane (Cat. No. 4457, Gelman

Laboratory, Ann Arbor, MI), or equivalent.

(l) Disposable syringe.—3 mL, latex-free “luer-lok”

syringe (Cat. No. 309585, VWR International, Inc.).

Reagents and Solutions

(a) Deionized water.—18.2 M��cm (Millipore, Bedford,

MA).

(b) Organic solvents.—High-purity chromatographic and

spectrophotometric grade methanol, acetone, and absolute

ethanol (Burdick & Jackson, Muskegon, MI), or equivalent.

(c) Formic acid.—Baker Analyzed (Cat. No. P285-500,

VWR International, Inc.).

(d) Formic acid solution.—0.1%. Pipet 0.1 mL formic

acid into a 100 mL graduated cylinder. Bring to volume with

deionized water.

(e) Potassium dihydrogen phosphate.—Certified ACS

(Cat. No. P285-500, Fisher Scientific, Pittsburgh, PA).

(f) Potassium dihydrogen phosphate solution.—0.50M,

pH 6.0. Accurately weigh 68 g potassium dihydrogen

phosphate into a 500 mL flask. Dissolve crystals in ca 300 mL

water. Adjust pH to 6.0 ± 0.2 with 10 and 1N NaOH, and

dilute to 1.0 L.

(g) Potassium dihydrogen phosphate solution.—0.25M,

pH 6. Dilute 0.50M, pH 6, potassium dihydrogen phosphate

solution 1:1 with water.

Standard Preparation

(a) Stock standard.—500 �g/mL. All sulfonamide

standards (sulfachloropyridazine, sulfadiazine, sulfamerazine,

SMZ, sulfapyridine, sulfaquinoxaline, STZ, and SDM) were

obtained from the United States Pharmacopeia. Accurately

weigh 5 mg of each standard into a 10 mL volumetric flask,

dissolve in methanol, and dilute to volume with methanol.

(b) Fortification/intermediate mixed standard.—500 ng/mL.

Transfer 0.100 mL aliquot of each individual standard to a 100

mL volumetric flask, and dilute to volume with water.

CLARK ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 88, NO. 3, 2005 737

(c) LC/MS/MS standard.—Standards ranging from 0.5 to

25 pg/�L were prepared for the calibration curve by

transferring aliquots of the 500 ng/mL mixed standard to a

5.0 mL volumetric flask and diluting to volume with 0.1%

formic acid solution. For example, a 5.0 pg/�L (ng/mL)

standard is prepared by transferring a 50 �L aliquot of the

mixed intermediate standard and diluting to 5 mL. Note that

the initial sample size is 10 g, that only one-fourth of the initial

extract is carried forward, and that the final volume is 1 mL;

therefore, the level of residues in the final extract obtained

from analysis of a control product spiked at 2.5 ppb would be

approximately equivalent to that in a 6.25 pg/�L (ng/mL)

standard (assuming 100% recovery). Working LC/MS/MS

standards are stable for �1 week.

Sample Extraction

For condensed milk and cream cheese products and

controls, accurately weigh 10.0 g product into a 50 mL

disposable graduated polypropylene centrifuge tube.

Centrifuge at 1000 relative centrifugal force (RCF) for 1 min

at 5�C to concentrate product at the bottom of the tube. Add

0.5M, pH 6, potassium dihydrogen phosphate solution to the

20 mL mark on the tube. Cap tube and vigorously mix

contents of tube, using a Vortex mixer for 20 s or until all of

the sample is incorporated into solution (cream cheese

products may require mixing with a glass rod to initially

incorporate sample into extraction solution). Shake tube

gently for an additional 2 min. Pour contents into a 50 mL


Table 1. SRM LC/MS/MS program

Time segment, min Scan event m/z

1: 0–4.62a

1 (sulfadiazine) 251�156, 108, 92, 96

2 (sulfathiazole) 256�156, 108, 92

2: 4.62–5.12 1 (sulfathiazole) 256�156, 108, 92

2 (sulfapyridine) 250�156, 108, 92, 184

3: 5.12–5.84 1 (sulfapyridine) 250�156, 108, 92, 184

2 (sulfamerazine) 265�156, 108, 92, 172

4: 5.84–6.06 1 (sulfamerazine) 265�156, 108, 92, 172

5: 6.06–7.06 1 (sulfamethazine) 279�156, 108, 92, 124

6: 7.06–8.63 1 (sulfachloropyridazine) 285�156, 108, 92

7: 8.63–10.5 1 (sulfaquinoxaline) 301�156, 108, 92, 145

2 (sulfadimethoxine) 311�156, 108, 92, 245

a Other LC/MS/MS conditions are described in the text. Segment times need to be adjusted occasionally.

Figure 1. Combined ion chromatograms for the mixed standard containing 8 sulfonamides (12.5 pg/�L).

polypropylene tube, and centrifuge at 20 000 RCF for 15 min

at 5�C.

For fortified products, accurately weigh well-stirred

control condensed milk or cream cheese (free of sulfonamide

residues) into a 50 mL polypropylene graduated centrifuge

tube. Centrifuge at 1000 RCF for 1 min at 5�C to concentrate

product at the bottom of tube. Add 0.025 and 0.050 mL mixed

standard solution containing the 3 sulfonamides of interest

(500 ng/mL in water), for 1.25 and 2.5 ppb fortified levels,

respectively, to individual 10.0 g control portions and mix on a

Vortex mixer to incorporate. Bring to 20 mL mark on the tube

with 0.5M, pH 6, potassium dihydrogen phosphate solution.

Cap tube and vigorously mix contents of tube, using a Vortex

mixer for 20 s or until all of the sample is incorporated into

solution. Shake tube gently for an additional 3 min; shaking

too vigorously may cause excessive emulsification. Pour

contents into individual 50 mL polypropylene tubes, and

centrifuge at 20 000 RCF for 15 min at 5�C.

Push fat layer aside, and pipet 5.0 ± 0.1 mL defatted milk

product from each tube, using a variable volume pipettor into

a 15 mL disposable tube, and dilute with 5 ± 0.2 mL water

prior to application onto a prepared SPE cartridge. (To prepare

SPE cartridge, wash cartridge with 6 mL methanol, followed

by 6 mL water, followed by 6 mL 0.25M, pH 6, dihydrogen


Table 2. Example confirmation data for 1 day’s analyses

Compound

Retention time, min

Transition

Average relative abundance, %

Standardsa Fortified samplesb Standardsa Fortifed samplesb

STZ 4.52 4.51 256�156 39 39

256�108 88 89

256�92 100 100

SMZ 6.39 6.38 279�156 50 54

279�108 80 80

251�92 75 73

251�124 100 100

SDM 9.05 9.05 311�156 100 100

311�108 35 34

311�92 26 28

311�245 11 13

a Average obtained for 6 standards ranging from 1 to 12.5 pg/�L.b Average (n = 7) obtained for condensed milk extracts fortified at 5 and 10 ppb.

Figure 2. Combined ion chromatograms for control condensed milk.

phosphate buffer; drain by gravity; and allow ca 2 mL buffer

to remain atop bed prior to application of sample extract.)

Apply sample extract to cartridge, and let extract drain

through column with ca 2–4 psi of vacuum. Wash cartridge

with 3 column-volumes of water. Apply 20 psi vacuum for

5 min to dry column.

Elute sulfonamide residues from column with 6 mL

methanol. Evaporate methanol extraction solution to dryness,

using an N-evaporator in a 50�C water bath. Wash sides of

tube with absolute ethanol (ca 1 mL), and evaporate to

dryness.

Add 1.0 mL 0.1% formic acid solution to centrifuge tube.

Vigorously mix contents of tube, using a Vortex mixer.

Centrifuge tube for 3 min, using clinical centrifuge at

approximately 80 RCF at room temperature. Filter the lower

aqueous extract through a 0.45 or 0.2 �m Acrodisc PVDF

filter into an LC vial for analysis.

LC/MS/MS Analysis

The LC/MS/MS conditions are optimized by tuning with

solutions of sulfonamides. This is done by flowing 10 ng/�L

solutions of individual sulfonamides into the mass

spectrometer with a syringe pump at 10 �L/min while 0.1%

formic acid solution–acetonitrile (75 + 25) at 250 �L/min is

added via a T-union. The combined stream is introduced into

the electrospray interface. The source parameters are

optimized by monitoring the MS and MS/MS spectra of

sulfamethazine. The responses of the remaining sulfonamides

are then evaluated. SRM MS/MS is performed on the

protonated molecular ions for each of the analytes by using the

following general parameters: source voltage = 4.8 kV;

capillary temperature = 300°C; sheath gas (nitrogen) = 41

(arbitrary) units; auxiliary gas (nitrogen) = 7 (arbitrary) units;

Q1 peak width = 0.9 amu; Q3 peak width = 0.7 amu; collision

gas = 1.2 torr argon; collision energy = 25; peak width =

0.3 amu; and scan time = 0.15 s. A metal needle sample kit

(Thermoelectron Cat. No. 95000-00951) is installed on the

electrospray source; the orientation of the spray to the orifice

is set at the second notch (ca 62� offset). Source

collision-induced dissociation is not used. A time-scheduled

SRM program originally designed to monitor for

8 sulfonamides is used and is described in Table 1. Times may

need to be adjusted as the LC column ages or is replaced.

The LC program is an acetonitrile–0.1% formic acid

solution gradient with a mobile phase flow rate of 250 �L/min.

The chromatographic gradient increases the percentage of

acetonitrile from 5 to 50% in the first 10 min of the program.

The LC column is then washed with 90% acetonitrile for 2 min

and is re-equilibrated at 5% acetonitrile for 3 min for a total run

time of 15 min. A divert valve is used, and the LC effluent is

introduced into the mass spectrometer from 1 to 10.25 min. The

column is held at 55�C, and the sample tray is maintained at

5�C. Injections of 20 �L are made, and a needle/syringe flush

and wash of 2000 �L methanol is used.

The treatment of the data varies, depending on whether

qualitative (screening/confirmation) or quantitative data are

being evaluated. For qualitative assessment, individual

ion-transition chromatograms are generated, and the resulting

chromatographic peaks are integrated. Relative abundances are

calculated from these peak areas and compared to

contemporary standards. The area counts for the most abundant

ion transition for each residue are used for quantitation.

Results and Discussion

The chloroform–acetone (1 + 1) extractant used by

Smedley and Weber (6) and Perez et al. (1), and the methylene

chloride extractant used by Clark et al. (7) for milk were found

unsuitable for this analysis because condensed milk and

cream cheese congealed into an impervious gel, which


Figure 3. Combined ion chromatograms for control condensed milk fortified with STZ, SMZ, and SDM at 5 ppb.

inhibited the extraction of any sulfonamide residues.

However, an SPE cleanup using aqueous extracts and C18

columns proved to be successful. This method was validated

by using store-bought condensed milk as the control matrix

for both condensed milk and cream cheese products.

The qualitative confirmation of sulfonamides in

milk-based products is based on the unique mass spectral

characteristics of these compounds. Sulfonamide residues

respond well when an electrospray interface in the

positive-ion mode is used. The protonated [MH]+

ion was the

base peak for all sulfonamide compounds. A

triple-quadrupole instrument using multiple SRM transitions

provided optimum sensitivity as well as enough data to meet

qualitative confirmation criteria. The drugs belonging to the

sulfonamide class have a common base and, therefore, have

several product ions in common, including m/z 156, 108, and

92. SRM transitions of the protonated molecular ion to these

product ions were monitored for all residues. In addition,

several drugs have additional prominent ions in their

product-ion spectra. These ions were identified and their

transitions were also monitored. Time segments (Table 1)

were established so that only 1 or 2 residues would be

monitored at any given time. This was done to increase

sensitivity and to make sure that no residues would be missed

if the retention times shifted somewhat.

The LC/MS/MS program was originally developed to

monitor 8 residues in whole milk (20), and therefore it

contains additional MS parameters for other sulfonamide

residues that were not evaluated in condensed milk and soft

cheese. Although the method was validated only for the

3 sulfonamides of primary interest in processed milk products

(SMZ, STZ, and SDM), the MS acquisition program

continued to monitor for all 8 drugs. By doing this, the method

could also serve as a screen for the other sulfonamide residues

that might be found unexpectedly. Figure 1 displays the

LC/MS/MS SRM combined ion chromatograms

(chromatograms representing the sum of SRM transitions) for

a 12.5 ng/mL standard mix of all 8 sulfonamides.

In order for a drug residue to be positively confirmed, it

must meet certain criteria that have been outlined in general

guidelines (24). These criteria are (i) the ion transitions

monitored for each residue must be present at a

signal-to-noise ratio (S/N) of >10, and the relative abundances

of the integrated peaks for each transition must match those

observed in an external standard by ±10% (for example, if the

relative abundance of an ion transition is 40% in the standard,

the relative abundance must be between 30 and 50% in the

sample for positive confirmation), and (ii) the retention time

should be ± 5% of that obtained for external standards run on

the same day. Representative data for 1 day’s analyses,

indicating how this method satisfied these criteria, are shown

in Table 2.

Validation was performed with control and

fortified-control pasteurized bovine condensed milk at 2.5, 5,

10, and 25 ppb sulfonamide residue levels. All samples

fortified at those levels met the qualitative confirmation

criteria as described above. When milk was fortified at a lower

level (1.25 ppb), STZ and SMZ were confirmed in 2 of

3 extracts, and SDM was confirmed in all 3 samples tested.

Reagent blanks (n = 2) and control condensed milk samples (n

= 7) were negative for sulfonamide residues. Combined ion

chromatograms for control condensed milk (Figure 2) and for

condensed milk fortified with the 3 sulfonamides at 5 ppb

(Figure 3) are shown. Retention times for the 3 sulfonamides

were 4.5, 6.4, and 9.05 min for STZ, SMZ, and SDM,

respectively.

Although the method is qualitative, recovery data were

collected and are provided in Table 3. A standard curve was

used to determine the relative recoveries of each sulfonamide.

The standard curve included mixed standard levels of 50, 25,

12.5, 5, and 2.5 pg/�L for STZ, SMZ, and STZ prepared in

0.1% formic acid solution. The response was linear in this

range, with correlation coefficients (R2) of >0.995 for all


Table 3. Recovery data for fortified-control condensed

milk

Test Added, ppb

Recovery, %

STZ SMZ SDM

1 10 91 134 98

2 10 82 123 89

3 10 95 138 113

4 10 97 130 104

5 10 99 135 110

Avg. 92.8 132.0 102.8

RSD, % 7.2 4.4 9.4

1 5 69 76 60

2 5 83 83 75

3 5 78 81 71

4 5 86 85 76

5 5 70 92 78

Avg. 77.2 83.4 72.0

RSD, % 9.8 7.0 10.0

1 2.5 70 84 59

2 2.5 112 76 62

3 2.5 88 89 79

4 2.5 126 90 83

5 2.5 74 78 62

Avg. 94.0 83.4 69.0

RSD, % 25.8 7.6 16.1

1 1.25 81 96 67

2 1.25 88 117 84

3 1.25 94 116 96

4 1.25 96 126 119

5 1.25 90 123 113

Avg. 89.8 115.6 95.8

RSD, % 6.51 10.1 22.2

residues. Recoveries at the 5 ppb level were 77.2, 83.4, and

72.0% for STZ, SMZ, and SDM, respectively. Relative

standard deviations (RSDs) ranged from 7.0 to 10.0% at

5 ppb. Based on the data shown in Table 3, the method’s

quantitative detection level was determined to be 1.25 ppb for

these 3 sulfonamides. A limited recovery study of the other

residues that can be detected by using this method was

performed with samples fortified at 2.5 ppb. The recoveries of

sulfadiazine, sulfapyridine, sulfamerazine, and

sulfachloropyridazine were all >50%, with lower recoveries

of sulfaquinoxaline at approximately 35%. Because all

8 residues could be confirmed at the 2.5 ppb fortification

level, this method was found to be appropriate for screening

and confirmation of all sulfonamides tested at this level.

This method was also tested on 6 imported condensed milk

and cream cheese products, many of which contain extra

flavorings and additives (e.g., cocoa, pineapple, and raisins).

SMZ was confirmed in an unflavored condensed milk sample


Figure 5. Individual ion-transition chromatograms for SMZ residues in (A) condensed milk fortified at 5 ppb and(B) a pineapple-flavored condensed milk sample.

Figure 4. Combined ion chromatograms for a sample consisting of sweet cheese spread with raisins.

and in a cheese spread with raisins (Figure 4). SMZ and STZ

were found and confirmed in a pineapple-flavored condensed

milk product. The individual SRM ion chromatograms for

SMZ in condensed milk fortified at 5 ppb and in the

pineapple-flavored condensed milk sample are shown in

Figure 5.

In conclusion, the sensitivity of the developed LC/MS/MS

technique allows for the detection and confirmation of

multiple sulfonamide residues in 2 very difficult matrixes at

low ppb levels. The extraction and cleanup of the samples are

simple and rapid, and require minimal sample preparation

time. Analysis using the LC/MS/MS method allows sensitive

and selective detection of various sulfonamide residues in

processed cheese products.

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