Đang chuẩn bị liên kết để tải về tài liệu:
HPLC for Food Analysis phần 9

Không đóng trình duyệt đến khi xuất hiện nút TẢI XUỐNG Tải xuống

Lấy những yếu tố này vào tài khoản, LOD được định nghĩa là 2 đến 3 lần so với mức độ tiếng ồn. LOQ được định nghĩa là 10 đến 20 lần mức độ tiếng ồn. Một hệ thống phát hiện tia cực tím có thể được sử dụng để đo lường số lượng xuống đến 500 pg mỗi tiêm. | Abundance I 1 Aldicarb sulfoxide 2 Aldicarb sulfone 3 Methomyl 4 3-hydroxy-carbofuran 4 c 5 Aldicarb 0 10 Time min 20 Figure 72 Carbamate analysis and as they shrink analyte ions are desorbed. The ions are transported to the mass analyzer through a series of vacuum stages and ion-focusing elements. Electrospray ionization can produce multiply charged ions of macromolecular analytes such as proteins and peptides. Because mass analyzers separate ions based on mass-to-charge ratio m z lower-cost mass spectrometers with mass ranges of several thousand m z can be used to analyze compounds in excess of 150 000 daltons. The primary use of electrospray has been the analysis of compounds of higher molecular weight. However this technique also has been applied successfully to small polar molecules. Fig. 72 shows a separation of carbamate pesticides using electrospray. Figure 73 APCI LC MS interface APCI also can be used to analyze moderate polarity analytes. As in electrospray APCI ionization occurs at atmospheric pressure via a chemical ionization process see figure73 . 103 Abundance 100000I - M 80000 60000- 40000 20000 603 639 987 I-----I----------------I----------- m 400 600 800 1000 Figure 74 Mass spectrum of the fatty acid triolein C18 1 cis -9 molecular weight 884.781 molecular formula C57H104O6 APCI requires some compound volatility and is less suitable for highly thermally labile compounds. Figure 74 shows a typical triglyceride mass spectrum. Both the degree of unsaturation and the length of the fatty acid side chains can be determined from the M NH4 ion which corresponds to mass M 18. In-source CID experiments also can be helpful in determining the fatty acid composition of chromatographic peaks. Full-scan methods allow easy identification at the low nanogram level. If more precise quantitation is required selected ion mode SIM can be used to obtain detection limits at the low picogram level. Polar and semipolar compounds up to 150 000 daltons can be analyzed. .