在使用startAfter进行分页查询时，需要同时指定orderBy条件和startAfter的值。下面是一个示例代码，用于按照时间戳倒序排列并分页查询文章列表：

import { Component } from '@angular/core';
import { AngularFirestore, AngularFirestoreCollection } from '@angular/fire/firestore';
import { Observable } from 'rxjs';

@Component({
  selector: 'app-article-list',
  template: `
    

      

        {{ article.title }}
      
    
    Load More
  `,
})
export class ArticleListComponent {
  private limit = 10;
  private lastVisible: any = null;
  private articlesCollection: AngularFirestoreCollection;
  public articles: Observable;

  constructor(private afs: AngularFirestore) {
    this.articlesCollection = afs.collection('articles', ref => ref.orderBy('timestamp', 'desc').limit(this.limit));
    this.articles = this.articlesCollection.valueChanges({idField: 'id'});
  }

  loadMore() {
    this.articlesCollection = this.afs.collection('articles', ref =>
      ref
        .orderBy('timestamp', 'desc')
        .startAfter(this.lastVisible)
        .limit(this.limit)
    );
    this.articles = this.articlesCollection.valueChanges({ idField: 'id' });
    this.articles.subscribe(data => {
      const lastItem = data[data.length - 1];
      this.lastVisible = lastItem ? lastItem.timestamp : null;
    });
  }
}

在上述代码中，使用startAfter进行分页查询时，指定了orderBy条件（按照时间戳倒序排列）和startAfter的值（上一次查询的最后一条数据的时间戳）。在loadMore方法中会根据上一次查询的结果来更新lastVisible的值，以便下一次查询时使用。